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  • 1.
    Berglund, Kim
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Shi, Yijun
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Friction and Wear of Self-Lubricating Materials for Hydropower Applications under Different Lubricating Conditions2017In: Lubricants, ISSN 2075-4442, Vol. 5, no 3, article id 24Article in journal (Refereed)
    Abstract [en]

    Self-lubricating bearings in hydropower applications are often lubricated with water under boundary lubricating conditions. Polyhydric alcohols replacing water have shown the potential to reduce both friction and wear. The objective of this work is, therefore, to evaluate the effect of a polyhydric alcohol-based environmentally-acceptable lubricant (EAL) on the friction and wear of self-lubricating materials for conformal contacts under boundary lubricating conditions. The lubricating properties of four commercially-available self-lubricating bearing materials were investigated under three different lubricating conditions: dry, water and a new polyhydric alcohol-based EAL. Bearing materials include one metallic composite and three polymer composites. A reciprocating motion test rig was used to evaluate the wear and friction properties. Surface analysis was performed by scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), and optical profilometry. Results show that the wear rate for the polymer composites is increased when water is present compared to dry operating conditions. The new polyhydric alcohol-based EAL substantially improves both friction and anti-wear performance of all four self-lubricating bearing materials compared to both dry and water conditions. Surface analysis indicates that the material transfer to the counter-surface is limited when the polyhydric alcohol-based EAL is used.

  • 2.
    Berglund, Kim
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Shi, Yijun
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Friction characteristics of environmentally adapted lubricants in boundary lubricated contacts2017Conference paper (Refereed)
  • 3.
    Berglund, Kim
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Shi, Yijun
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Performance of self‐lubricating materials for hydropower applications under different lubricating conditions2016In: Nordic Symposium on Tribology - NORDTRIB 2016, 2016Conference paper (Refereed)
  • 4.
    Björling, Marcus
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Bair, Scott
    Georgia Institute of Technology, Centre for High Pressure Rheology, G.W. Woodruff School of Mechanical Engineering, Atlanta, GA.
    Mu, Liwen
    Intelligent Composites Laboratory, Department of Chemical and Biomolecular Engineering, The University of Akron.
    Zhu, Jiahua
    Intelligent Composites Laboratory, Department of Chemical and Biomolecular Engineering, The University of Akron.
    Shi, Yijun
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Elastohydrodynamic performance of a bio-based, non-corrosive ionic liquid2017In: Applied Sciences: APPS, ISSN 1454-5101, E-ISSN 1454-5101, Vol. 17, no 10, article id 996Article in journal (Refereed)
    Abstract [en]

    To improve performance of machine components, lubrication is one of the most important factors. Especially for use in extreme environments, researchers look for other solutions rather than common lubricant base stocks like mineral oils or vegetable oils. One such example is ionic liquids. Ionic liquids have been defined as molten salts with melting points below 100 °C that are entirely ionic in nature, comprising both cationic and anionic species. The industrial use of ionic liquids is mostly as solvents, electrolytes, extractants and catalysts. In tribological applications, ionic liquids are mainly studied in boundary lubrication and in pure sliding contacts. In this work, the elastohydrodynamic performance of a bio-based, non-corrosive, [choline][L-proline] ionic liquid is evaluated in terms of pressure-viscosity response, film forming capability and friction. The results show a pressure-viscosity coefficient of below 8 GPa-1 at 25 °C, among the lowest reported for any ionic liquid. The ionic liquid generated up to 70% lower friction than a reference paraffin oil with a calculated difference in film thickness of 11%. It was also shown that this ionic liquid is very hygroscopic, which is believed to explain part of the low friction results, but also has to be considered in practical applications since the water content will influence the properties and thus the performance of the lubricant.

  • 5.
    Björling, Marcus
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Bair, Scott
    Georgia Institute of Technology, Centre for High Pressure Rheology, G.W. Woodruff School of Mechanical Engineering, Atlanta, GA 30332-0405, USA;.
    Mu, Liwen
    Intelligent Composites Laboratory, Department of Chemical and Biomolecular Engineering, The University of Akron, Akron, OH 44325, USA;.
    Zhu, Jiahua
    Intelligent Composites Laboratory, Department of Chemical and Biomolecular Engineering, The University of Akron, Akron, OH 44325, USA;.
    Shi, Yijun
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Elastohydrodynamic Performance of a Non-Corrosive Non-Protic Ionic Liquid2017Conference paper (Refereed)
    Abstract [en]

    Ionic liquids have been defined as molten salts with melting points below 100 degrees C that are entirely ionic in nature, comprising both cationic and anionic species. The industrial use of ionic liquids is mostly as solvents, electrolytes, extractants and catalysts. In tribological applications, Ionic liquids are mainly studied in boundary lubrication and in pure sliding contacts. In this work, the elastohydrodynamic performance of a non-corrosive, non-protonic ionic liquid is studied to assess the feasibility to use this kind of ionic liquid in machine components such as gears, rolling bearings and cam followers. This study includes ball on disc friction experiments in rolling sliding full film elastohydrodynamic lubrication at high slide to roll ratios, as well as film thickness measurements with optical interferometry. A commercially available paraffin oil has been used as a reference.

  • 6.
    Björling, Marcus
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Shi, Yijun
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    DLC and Glycerol: Superlubricity in Rolling/Sliding Elastohydrodynamic Lubrication2019In: Tribology letters, ISSN 1023-8883, E-ISSN 1573-2711, Vol. 67, no 1, article id 23Article in journal (Refereed)
    Abstract [en]

    Low friction is one of the most important parameters for the development of machine components and machines with high efficiency. Many of the common machine components of today such as gears, rolling element bearings and cam-followers are defined by their non-conformal contacts leading to high-contact pressures, typically 1–4 GPa. The lubrication of such contacts is usually called elastohydrodynamic lubrication (EHL). Diamond-like carbon (DLC) coatings and glycerol have individually been shown to produce low friction in boundary, mixed and full film lubrication. A few studies have been conducted using both glycerol and DLC-coated surfaces to achieve even lower friction in pure sliding boundary-lubricated contacts. However, the literature is lacking studies of how the combination of glycerol and DLC performs in non-conformal rolling/sliding contacts where many common machine components operate. Such a study is presented in this article where a ball-on-disc test rig is used to investigate the performance of the combination of DLC and glycerol at pressures up to 1.95 GPa at various entrainment speeds and slide-to-roll ratios. The investigation shows that the DLC-glycerol combination provides very low friction values, in some cases, below the superlubricity threshold of 0.01, possibly shown for the first time at such high pressure in a non-conformal rolling/sliding contact. The low friction mechanism in full film lubrication is a combination of the low pressure-viscosity and high temperature-viscosity sensitivity of glycerol in combination with thermal insulation of the DLC coating and is presented as thermally assisted liquid superlubricity.

  • 7.
    Chen, Jun
    et al.
    College of Science, Nanjing Forestry University, Nanjing, P. R. China .
    Li, Fanzhu
    Key Lab Beijing City Preparat & Proc Novel Polyme, State Key Lab Organ Inorgan Composites, Beijing University of Chemical Technology, Beijing, P. R. China.
    Luo, Yanlong
    College of Science, Nanjing Forestry University, Nanjing, P. R. China .
    Shi, Yijun
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Ma, Xiaofeng
    College of Science, Nanjing Forestry University, Nanjing, P. R. China .
    Zhang, Meng
    Institute of Chemical Industry of Forestry Products, CAF, Nanjing, P. R. China.
    Boukhvalov, D. W.
    College of Science, Nanjing Forestry University, Nanjing, P. R. China.
    Luo, Zhenyang
    College of Science, Nanjing Forestry University, Nanjing, P. R. China.
    A self-healing elastomer based on an intrinsic non-covalent cross-linking mechanism2019In: Journal of Materials Chemistry A, ISSN 2050-7488, Vol. 7, no 25, p. 15207-15214Article in journal (Refereed)
    Abstract [en]

    Synthesis and comprehensive examination of a polyurethane (urea) elastomer that self-heals based on intrinsic dynamic non-covalent bonds (van der Waals and hydrogen) are reported. The dynamic non-covalent bonds include hydrogen bonds and van der Waals forces. The difference in the previous approach in which hydrogen bond self-healing materials introduced dense quadruple hydrogen bonds at the ends or branched chains poly(propylene carbonate) (PPC) diol was used as the soft segment of the polyurethane (urea) material, and strong van der Waals forces were provided by the large number of carbonyl groups in its main chain; hydrogen bonds were formed by urethane bonds, urea bonds, and the carbonyl groups on PPC. The mechanical properties and healing efficiency of the self-healing polyurethane (urea) elastomer were studied. In situtemperature-dependent infrared and low-field nuclear magnetic resonance (LNMR) measurements were combined with molecular dynamics simulations to investigate the self-healing mechanisms. The results of the studies on the self-healing polyurethane demonstrate that the dynamic cross-linking between hydrogen bonds and van der Waals forces is the basic driving force for the self-healing ability of the material, and temperature is the key factor that affects hydrogen bonding and van der Waals forces. The effect of crystallization on the self-healing ability of the material was also studied. The molecular dynamics simulation results also demonstrate interplay between van der Waals forces and hydrogen bonds at different temperatures.

  • 8.
    Chen, Long
    et al.
    Department of Chemical and Biomolecular Engineering, The University of Akron, Intelligent Composites Laboratory, Department of Chemical and Biomolecular Engineering, The University of Akron.
    Ji, Tuo
    Intelligent Composites Laboratory, Department of Chemical and Biomolecular Engineering, The University of Akron.
    Mu, Liwen
    State Key Laboratory of Materials-Oriented Chemical Engineering, Nanjing University of Technology, Intelligent Composites Laboratory, Department of Chemical and Biomolecular Engineering, The University of Akron.
    Shi, Yijun
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Brisbin, Logan
    Intelligent Composites Laboratory, Department of Chemical and Biomolecular Engineering, The University of Akron.
    Guo, Zhanhu
    Department of Chemical and Biomolecular Engineering, University of Tennessee, Knoxville.
    Khan, Mohammel A.
    Department of Physics and Astronomy, Louisiana State University, Baton Rouge.
    Young, David P.
    Department of Physics and Astronomy, Louisiana State University, Baton Rouge.
    Zhu, Jiahua
    State Key Laboratory of Materials-Oriented Chemical Engineering, Nanjing University of Technology, Intelligent Composites Laboratory, Department of Chemical and Biomolecular Engineering, The University of Akron.
    Facile synthesis of mesoporous carbon nanocomposites from natural biomass for efficient dye adsorption and selective heavy metal removal2016In: RSC Advances, ISSN 2046-2069, E-ISSN 2046-2069, Vol. 6, no 3, p. 2259-2269Article in journal (Refereed)
    Abstract [en]

    Mesoporous carbon with embedded iron carbide nanoparticles was successfully synthesized via a facile impregnation–carbonization method. A green biomass resource, cotton fabric, was used as a carbon precursor and an iron precursor was implanted to create mesopores through a catalytic graphitization reaction. The pore structure of the nanocomposites can be tuned by adjusting the iron precursor loadings and the embedded iron carbide nanoparticles serve as an active component for magnetic separation after adsorption. The microstructure of the nanocomposites was carefully investigated by various characterization techniques including electron microscopy, X-ray diffraction, surface analyzer, magnetic property analyzer and etc. The newly created mesopores are demonstrated as a critical component to enhance the adsorption capacity of organic dyes and embedded iron carbide nanoparticles are responsible for the selective removal of heavy metal ions (Zn2+, Cu2+, Ni2+, Cr6+ and Pb2+). Isotherm adsorption, kinetic study at three different temperatures (25, 45 and 65 °C) and cycling retention tests were performed to understand the adsorptive behavior of the nanocomposites with organic dyes (methylene blue and methyl orange). Together with the preferable removal of more toxic heavy metal species (Cr6+ and Pb2+), these mesoporous nanocomposites show promising applications in pollutant removal from water. The facile material preparation allows convenient scale-up manufacturing with low cost and minimum environmental impact.

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  • 9.
    Chen, Long
    et al.
    Intelligent Composites Laboratory, Department of Chemical and Biomolecular Engineering, The University of Akron.
    Ji, Tuo
    Intelligent Composites Laboratory, Department of Chemical and Biomolecular Engineering, The University of Akron.
    Mu, Liwen
    Intelligent Composites Laboratory, Department of Chemical and Biomolecular Engineering, The University of Akron.
    Shi, Yijun
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Wang, Huaiyuan
    College of Chemistry and Chemical Engineering, Northeast Petroleum University, Daqing.
    Zhu, Jiahua
    Intelligent Composites Laboratory, Department of Chemical and Biomolecular Engineering, The University of Akron.
    Pore size dependent molecular adsorption of cationic dye in biomass derived hierarchically porous carbon2017In: Journal of Environmental Management, ISSN 0301-4797, E-ISSN 1095-8630, Vol. 196, p. 168-177Article in journal (Refereed)
    Abstract [en]

    Hierarchically porous carbon adsorbents were successfully fabricated from different biomass resources (softwood, hardwood, bamboo and cotton) by a facile two-step process, i.e. carbonization in nitrogen and thermal oxidation in air. Without involving any toxic/corrosive chemicals, large surface area of up to 890 m2/g was achieved, which is comparable to commercial activated carbon. The porous carbons with various surface area and pore size were used as adsorbents to investigate the pore size dependent adsorption phenomenon. Based on the density functional theory, effective (E-SSA) and ineffective surface area (InE-SSA) was calculated considering the geometry of used probing adsorbate. It was demonstrated that the adsorption capacity strongly depends on E-SSA instead of total surface area. Moreover, a regression model was developed to quantify the adsorption capacities contributed from E-SSA and InE-SSA, respectively. The applicability of this model has been verified by satisfactory prediction results on porous carbons prepared in this work as well as commercial activated carbon. Revealing the pore size dependent adsorption behavior in these biomass derived porous carbon adsorbents will help to design more effective materials (either from biomass or other carbon resources) targeting to specific adsorption applications.

  • 10.
    Fisher, Trevor R.
    et al.
    School of Chemical, Biological and Materials Engineering, University of Oklahoma, Norman, Oklahoma, US.
    Zhou, Guobing
    School of Chemical, Biological and Materials Engineering, University of Oklahoma, Norman, Oklahoma, US.
    Shi, Yijun
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Huang, Liangliang
    School of Chemical, Biological and Materials Engineering, University of Oklahoma, Norman, Oklahoma, US.
    How does hydrogen bond network analysis reveal the golden ratio of water–glycerol mixtures?2020In: Physical Chemistry, Chemical Physics - PCCP, ISSN 1463-9076, E-ISSN 1463-9084, Vol. 22, no 5, p. 2887-2907Article in journal (Refereed)
    Abstract [en]

    Properties of water–glycerol mixtures depend closely on the water/glycerol ratio. Around the 30 mol% glycerol concentration mark, the so-called golden ratio of water–glycerol mixtures, several of the mixture's properties have observed maxima or minima, without a clear fundamental explanation. In this work, a series of molecular dynamics simulations have been performed over a wide range of water–glycerol concentrations to analyze the intermolecular hydrogen bond (H-bond) network. The collected values from simulations are justified from both a probabilistic model of H-bonding and from observing the dynamic behavior of each type of H-bonds. The populations of H-bonds that exist at a given concentration of glycerol are largely governed by the probability of one oxygen atom randomly associating with another oxygen atom. However, the H-bonds that glycerol oxygen can form are dependent on the H-bonds that are formed by the other intramolecular glycerol oxygen. Based on the dynamic analysis of each type of H-bonds, there are deviations from randomly associating with another oxygen. Water preferentially donates a hydrogen to a glycerol than to another water molecule. Yet, glycerol has a near-equal likelihood for donating a hydrogen to either another glycerol or a water. This has an effect of increasing the number of H-bonds between water and glycerol molecules and decreasing H-bonds between two water molecules. A maximum contribution of H-bonds between water and glycerol occurs around 30 mol% glycerol which is a concentration where several of the mixture's properties have an observed maxima or minima.

  • 11.
    Hua, Jing
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Björling, Marcus
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Grahn, Mattias
    Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Chemical Engineering.
    Larsson, Roland
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Shi, Yijun
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    A smart friction control strategy enabled by CO2 absorption and desorption2019In: Scientific Reports, ISSN 2045-2322, E-ISSN 2045-2322, Vol. 9, no 1, article id 13262Article in journal (Refereed)
    Abstract [en]

    Intelligent control of friction is an attractive but challenging topic and it has rarely been investigated for full size engineering applications. In this work, it is instigated if it would be possible to adjust friction by controlling viscosity in a lubricated contact. By exploiting the ability to adjust the viscosity of the switchable ionic liquids, 1,8-Diazabicyclo (5.4.0) undec-7-ene (DBU)/ glycerol mixture via the addition of CO2, the friction could be controlled in the elastohydrodynamic lubrication (EHL) regime. The friction decreased with increasing the amount of CO2 to the lubricant and increased after partial releasing CO2. As CO2 was absorbed by the liquid, the viscosity of the liquid increased which resulted in that the film thickness increased. At the same time the pressure-viscosity coefficient decreased with the addition of CO2. When CO2 was released again the friction increased and it was thus possible to control friction by adding or removing CO2.

  • 12.
    Hua, Jing
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Björling, Marcus
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Larsson, Roland
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Shi, Yijun
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Controllable Friction of Green Ionic Liquids via Environmental Humidity2020In: Advanced Engineering Materials, ISSN 1438-1656, E-ISSN 1527-2648, Vol. 22, no 5, article id 1901253Article in journal (Refereed)
    Abstract [en]

    Intelligent control of friction is an attractive but challenging topic. In this work, it is investigated if it would be possible to adjust friction in a lubricated contact by controlling environmental humidity. By exploiting the ability to adjust the environmental humidity by various saturated salt solutions, friction behavior of contacts lubricated with Choline l‐Proline ([Cho][Pro]) is modulated in a wide range of relative humidity (RH). The friction increases when the environmental humidity is increased and decreases when water is partially evaporated to a lower RH. It is thus possible to control friction by environmental humidity. The addition of water in ionic liquids (ILs) causes a decrease in viscosity, but as the tests are calculated to be performed in boundary lubrication the viscosity change is not the main factor for the change in friction. The friction sensitivity of RH can be explained by the effect of adhesion on the water uptake from humid air by [Cho][Pro]. Furthermore, the reversible changes of H‐bond types determined by the water content could be another explanation to the altered friction.

  • 13.
    Hua, Jing
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Shi, Yijun
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Non-corrosive Green Lubricant With Dissolved Lignin in Ionic Liquids Behave as Ideal Lubricants for Steel-DLC Applications2019In: Frontiers in Chemistry, E-ISSN 2296-2646, Vol. 7, article id 857Article in journal (Refereed)
    Abstract [en]

    Diamond-like carbon (DLC)–steel contacts become more and more popular in the industry now. Since the surface chemical properties of DLC are quite different from those of iron, traditional formulated lubricants have problems to form tribo-chemical films, which are effective to improve the tribological performance for steel-steel contacts, on the surface of DLC. Thus, new lubricants formulation strategies are needed to be considered for steel-DLC applications. A kind of green lubricant (lignin-[Choline][L-Proline] (L-[CH][Pro])) without any traditional tribo-chemical active element, i.e., free of P, S, B, etc., was studied in this paper for the steel-DLC contact. To find the difference between this new ILs and the traditional lubricants, a commercially available fully formulated lubricant was used as a reference. An Optimol SRV-III oscillating friction and wear tester was used to evaluate the tribological performance. Three different kinds of commercially available DLC coatings (Tribobond 40(Cr + a-C:H:W), Tribobond 43 [(Cr+) a-C:H), and Tribobond 44(a-C:Cr)] were investigated. The results show that the ILs exhibit an obviously lower friction coefficient than that of the traditional commercially available fully formulated lubricant. Among those three DLC coatings, the (Cr+) a-C:H DLC coating exhibits the biggest improvement of wear resistance lubricated with the new ILs than that of the commercially available fully formulated lubricant. It's expected that its excellent tribological properties are attributed to the affinity of the ILs to the metal surface and the strength of the ionic liquids interactions by hydrogen bonding. Thus, forming strong physical adsorption strategy, instead of forming chemical tribo-films, is recommended to enhance the lubricating performance of lubricants for DLC.

  • 14.
    Huang, Xianzhu
    et al.
    State Key Laboratory of Materials-Oriented Chemical Engineering, Nanjing Tech University, Nanjing .
    Wu, Jian
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Lu, Xiaohua
    Key Laboratory of Material and Chemical Engineering, Nanjing Tech University, Nanjing .
    Feng, Xin
    State Key Laboratory of Materials-Oriented Chemical Engineering, Nanjing Tech University, Nanjing .
    Shi, Yijun
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Tribological Properties of Porous PEEK Composites Containing Ionic Liquid under Dry Friction Condition2017In: Lubricants, ISSN 2075-4442, Vol. 5, no 2, article id 19Article in journal (Refereed)
    Abstract [en]

    NaCl particles were added into Polyetheretherketone (PEEK) and its composites to produce porous PEEK-based materials by washing NaCl away after the high-temperature compression molding process. After that, an ionic liquid was added into the porous materials under vacuum condition. Carbon fibers (CF), as reinforcement, and PTFE, as an internal solid lubricant, were employed to prepare PEEK composites. Tribological properties under dry friction condition were studied on a ring-on-disc tribo-meter. The influence of CF and PTFE on tribological properties was carefully investigated. The results indicated that, in comparison with traditional PEEK composites (CF/PTFE/PEEK), the porous PEEK composites containing ionic liquid showed much better tribological properties. It is found that CF can help PEEK form effective pores to suck in the ionic liquid resulting in a better tribological performance. CF reinforced porous PEEK containing ionic liquid (p-CF/PEEK + IL) demonstrated the lowest friction coefficient (27% of CF/PTFE/PEEK) and the lowest wear loss (only 0.9% of CF/PTFE/PEEK). Long time tribological test revealed that the wear mass loss comes from the running-in period, while its wear is negligible after this period. It is also found that the addition of PTFE has a negative influence on the tribological behaviors, especially under high sliding velocity and applied load

  • 15.
    Ji, Tuo
    et al.
    Intelligent Composites Laboratory, Department of Chemical and Biomolecular Engineering, The University of Akron.
    Chen, Long
    Department of Chemical and Biomolecular Engineering, The University of Akron, Intelligent Composites Laboratory, Department of Chemical and Biomolecular Engineering, The University of Akron.
    Mu, Liwen
    Intelligent Composites Laboratory, Department of Chemical and Biomolecular Engineering, The University of Akron.
    Yuan, Ruixia
    Intelligent Composites Laboratory, Department of Chemical and Biomolecular Engineering, The University of Akron.
    Knoblauch, Michael
    Intelligent Composites Laboratory, Department of Chemical and Biomolecular Engineering, The University of Akron.
    Bao, Forrest Sheng
    College of Chemistry and Chemical Engineering, Northeast Petroleum University, Daqing.
    Shi, Yijun
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Wang, Huaiyan
    Department of Electrical and Computer Engineering, The University of Akron.
    Zhu, Jiahua
    State Key Laboratory of Materials-Oriented Chemical Engineering, Nanjing University of Technology, Intelligent Composites Laboratory, Department of Chemical and Biomolecular Engineering, The University of Akron.
    Green Processing of Plant Biomass into Mesoporous Carbon as Catalyst Support2016In: Chemical Engineering Journal, ISSN 1385-8947, E-ISSN 1873-3212, Vol. 295, p. 301-308Article in journal (Refereed)
    Abstract [en]

    Four different plant biomass, bamboo, cotton, soft wood and hard wood, were utilized as carbon precursors to fabricate porous carbon catalyst supports via a chemical free approach. Large surface area with unique mesoporous structure was successfully created in the carbon, which made them suitable for catalyst support. After decorating silver nanoparticles onto these carbon supports, nitroaromatics reduction reactions were performed to evaluate the catalyst activity. Results indicate that chemical composition and surface groups of carbon supports determine the metal catalyst nucleation/growth while the porous microstructure of support affects the mass transport of reactant/product across the liquid/catalyst interface. Among the four selected biomass, porous carbon manufactured from soft wood acquires the highest average pore size, pore volume, mesopore volume fraction and best catalytic activity after decorating silver nanoparticles. This work not only presents an environmental benign process that converts natural biomass into effective porous carbon catalyst supports, but also offers a comprehensive understanding of biomass structure/composition relating to their suitability as catalyst support.

  • 16.
    Lan, Xiaoyu
    et al.
    College of Science, Nanjing Forestry University, Nanjing, P. R. China.
    Ma, Xiaofeng
    College of Science, Nanjing Forestry University, Nanjing, P. R. China.
    Wang, Lei
    College of Science, Nanjing Forestry University, Nanjing, P. R. China.
    Shi, Yijun
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Gu, Qun
    Department of Chemistry, Edinboro University of Pennsylvania, Edinboro, United States.
    Wu, Linlin
    College of Materials Science and Engineering, Nanjing Tech University, Nanjing, P. R. China.
    Gu, Xiaoli
    College of Chemical Engineering, Nanjing Forestry University, Nanjing, P. R. China.
    Luo, Zhenyang
    College of Science, Nanjing Forestry University, Nanjing , P. R. China.
    Self-Assembly of Diblock Copolymers Containing Thermo- and Photoresponsive Lower Critical Solution Temperature Phase Behavior Polymer with Tunable Assembly Temperature in an Ionic Liquid Mixture2019In: ACS Omega, E-ISSN 2470-1343, Vol. 4, no 6, p. 11229-11236Article in journal (Refereed)
    Abstract [en]

    This work prepared a type of diblock copolymer with thermo- and photosensitivity in ionic liquids (ILs). P(N,N-dimethylacrylamide) (compatible with ILs) was prepared as one segment, while butyl acrylate (BA) and 4-phenylazophenylmethacrylate (AzoMA) were copolymerized as another segment P(AzoMA-r-BA) with stimuli responsiveness. The diblock copolymer showed tunable lower critical micellization temperature (LCMT) in two mixed imidazole ionic liquids. The value of LCMT depends on not only the conformation status of the azo group in copolymers but also the azo group content in copolymers and mixed ratio of ionic liquids. Based on this tunable LCMT, photoinduced micellization/demicellization can be achieved near room temperature by alternate irradiation with visible and ultraviolet light, and it is totally reversible.

  • 17.
    Li, Jiahui
    et al.
    College of Chemical Engineering, State Key Laboratory of Materials-oriented Chemical Engineering, Nanjing Tech University, Nanjing .
    Zhu, Yudan
    College of Chemical Engineering, State Key Laboratory of Materials-oriented Chemical Engineering, Nanjing Tech University, Nanjing .
    Zhang, Yumeng
    College of Chemical Engineering, State Key Laboratory of Materials-Oriented Chemical Engineering, Nanjing Tech University.
    Gao, Qingwei
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Energy Science. College of Chemical Engineering, State Key Laboratory of Materials-Oriented Chemical Engineering, Nanjing Tech University.
    Zhu, Wei
    College of Chemical Engineering, State Key Laboratory of Materials-oriented Chemical Engineering, Jiangsu National Synergetic Innovation Center for Advanced Materials (SICAM), Nanjing Tech University.
    Lu, Xiaohua
    Key Laboratory of Material and Chemical Engineering, Nanjing Tech University, Nanjing .
    Shi, Yijun
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Extra low friction coefficient caused by the formation of a solid-like layer: A new lubrication mechanism found through molecular simulation of the lubrication of MoS2 nanoslits2018In: Chinese Journal of Chemical Engineering, ISSN 1004-9541, E-ISSN 2210-321X, Vol. 26, no 12, p. 2412-2419Article in journal (Refereed)
    Abstract [en]

    Monolayer molybdenum disulfide (MoS2) is a novel two-dimensional material that exhibits potential application in lubrication technology. In this work, molecular dynamics was used to investigate the lubrication behaviour of different polar fluid molecules (i.e., water, methanol and decane) confined in monolayer MoS2 nanoslits. The pore width effect (i.e., 1.2, 1.6 and 2.0 nm) was also evaluated. Results revealed that decane molecules exhibited good lubricating performance compared to the other two kinds of molecules. The friction coefficient followed the order of decane < methanol < water, and decreased evidently as the slit width increased, except for decane. Analysis of the spatial distribution and mobility of different confined fluid molecules showed that a solid-like layer was formed near the slit wall. This phenomenon led to the extra low friction coefficient of confined decane molecules

  • 18.
    Li, Yiling
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Xi, Yinhu
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Shi, Yijun
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Estimation of rolling friction coefficients in a tribosystem using optical measurements2018In: Industrial Lubrication and Tribology, ISSN 0036-8792, E-ISSN 1758-5775, Vol. 70, no 4, p. 680-686Article in journal (Refereed)
    Abstract [en]

    Purpose

    This paper presents a method to measure the rolling friction coefficient in an easy and fast way. The aim is to measure the rolling friction coefficient between a small steel ball and a cylindrical aluminum surface.

    Design/methodology/approach

    An analytical model of the tribosystem of a freely rolling ball and a cylindrical surface is established. The rolling friction coefficient is evaluated from images recorded by a high-speed camera. The coefficient between a 1.58 mm diameter steel ball and a cylindrical aluminum surface is measured. A background subtraction algorithm is used to determine the position of the small steel ball.

    Findings

    The angular positions of the ball are predicted using the analytical model, and good agreement is found between the experimental and theoretical results.

    Originality/value

    An optical method for evaluating the rolling friction coefficient is presented, and the value of this coefficient between a small steel ball and a cylindrical aluminum surface is evaluated.

  • 19.
    Ma, Xiaofeng
    et al.
    College of Science, Nanjing Forestry University, Nanjing, P. R. China. Institute of Polymer Materials, Nanjing Forestry University, Nanjing, P. R. China..
    Lan, Xiaoyu
    College of Science, Nanjing Forestry University, Nanjing, P. R. China.
    Wu, Linlin
    College of Materials Science and Engineering, Nanjing Tech University, Nanjing, P. R. China.
    Wang, Lei
    Institute of Polymer Materials, Nanjing Forestry University, Nanjing, P. R. China.
    Gu, Qun
    Chemistry Department, Edinboro University of Pennsylvania, Edinboro PA, USA.
    Shi, Yijun
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Gu, Xiaoli
    College of Chemical Engineering, Nanjing Forestry University, Nanjing, P. R. China.
    Luo, Zhenyang
    College of Science, Nanjing Forestry University, Nanjing, P. R. China. Institute of Polymer Materials, Nanjing Forestry University, Nanjing, P. R. China..
    Photo-induced Actuator using Temperature and Light Dual Responsive Azobenzene Containing Ion Gel in Ionic Liquid2020In: European Polymer Journal, ISSN 0014-3057, E-ISSN 1873-1945, Vol. 123, article id 109446Article in journal (Refereed)
    Abstract [en]

    A series of well-defined random copolymers comprising butyl acrylate (BA) and 4-phenylazophenylmethacrylate (AzoMA) (P(AzoMA-r-BA)) are prepared successfully by reversible addition fragmentation chain transfer (RAFT) polymerization. P(AzoMA-r-BA)s show LCST-type phase transition in 1-butyl-3-methylimidazolium bis(trifluoromethanesulfone)amide ([C4mim][NTf2]). LCST depends on the photoisomerization state of azobenzene, as well as on the AzoMA composition in the random copolymers. LCST of (P(cis-AzoMA-r-BA) is significantly higher than that of P(trans-AzoMA-r-BA), because cis-AzoMA and trans-AzoMA behave as solvato-philic and solvato-phobic comonomer, respectively. At a bistable temperature, photo-induced phase separation is completely reversible. Secondly, based on this phenomenon, a thermo- / photo- responsive ion gel (BA-AzoMA ion gel) can be prepared by free radical polymerization of BA and AzoMA using ethylene glycol dimethacrylate (EGDMA) as crosslinker in [C4mim][NTf2]. BA-AzoMA ion gel shows high temperature contraction and low temperature expansion behavior, due to LCST-type phase behavior of polymer system consisting of AzoMA and BA. Contraction temperature of BA-AzoMA ion gel depends on photoisomerization state of the azobenzene group in polymers. At a bistable temperature, photo-induced expansion-contraction is reversible. Finally, a Photo-induced actuator can be realized using BA-AzoMA ion gel at suitable temperatures, featuring reversible bending by alternate irradiating with visible and UV light. The bending behavior is also demonstrated.

  • 20.
    Mu, Liwen
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements. State Key Laboratory of Materials‐Oriented Chemical Engineering, Nanjing Tech University, Nanjing, 210009 P. R. China.
    Cao, Danyang
    State Key Laboratory of Materials‐Oriented Chemical Engineering, Nanjing Tech University, Nanjing, 210009 P. R. China.
    Zhuang, Wei
    State Key Laboratory of Materials‐Oriented Chemical Engineering, Nanjing Tech University, Nanjing, 210009 P. R. China. College of Biotechnology and Pharmaceutical Engineering, National Engineering Technique Research Center for Biotechnology, Nanjing Tech University, No. 30, Puzhu South Road, Nanjing, 211816 China.
    Yu, Qiangliang
    State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Lanzhou, 730000 China.
    Cai, Meirong
    State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Lanzhou, 730000 China.
    Shi, Yijun
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Stable Dispersed Zeolitic Imidazolate Framework/Graphene Oxide Nanocomposites in Ionic Liquids Resulting in High Lubricating Performance2020In: Advanced Materials Interfaces, ISSN 2196-7350, Vol. 7, no 9, article id 1902194Article in journal (Refereed)
    Abstract [en]

    The “block‐on‐plate” nanocomposite structure material is realized with the zeolitic imidazolate framework (ZIF‐67) on graphene oxide (GO) with surface oxygen‐rich functional groups. Scanning electron microscopy, X‐ray photoelectron spectroscopy, X‐ray diffractograms, fourier transform infrared spectroscopy, and ZIF‐67/GO demonstrate a high capability of promoting the lubricating performance of [Choline][Proline] ([CH][P]) bio‐ionic liquid. Specifically, the wear volume of the disc by [CH][P]‐0.1 ZIF‐67/GO is only 52% of the one lubricated by pure [CH][P]. Moreover, ZIF‐67/GO shows a long time, high dispersion stability in [CH][P]. In addition, the combination of GO nanosheets and ZIF‐67 can make ZIF‐67/GO have better polishing effect between the steels during the friction process, which can effectively form a robust lubrication layer and improves overall lubricating properties. As a result, the interfacial lubrication can be significantly improved by these newly developed [CH][P]‐ZIF‐67/GO lubricants. This work offers a new promising application for ZIF‐67 nanocomposites with GO in advanced lubrication systems. This work also provides a new strategy of producing lubricants containing nanoparticles with high dispersion stability, which may solve the biggest challenge in this field.

  • 21.
    Mu, Liwen
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements. Intelligent Composites Laboratory, Department of Chemical and Biomolecular Engineering, The University of Akron.
    He, Jian
    Intelligent Composites Laboratory, Department of Chemical and Biomolecular Engineering, The University of Akron.
    Li, Yifan
    Intelligent Composites Laboratory, Department of Chemical and Biomolecular Engineering, The University of Akron.
    Ji, Tuo
    Intelligent Composites Laboratory, Department of Chemical and Biomolecular Engineering, The University of Akron.
    Mehra, Nitin
    Intelligent Composites Laboratory, Department of Chemical and Biomolecular Engineering, The University of Akron.
    Shi, Yijun
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Zhu, Jiahua
    Intelligent Composites Laboratory, Department of Chemical and Biomolecular Engineering, The University of Akron.
    Molecular Origin of Efficient Phonon Transfer in Modulated Polymer Blends: Effect of Hydrogen Bonding on Polymer Coil Size and Assembled Microstructure2017In: The Journal of Physical Chemistry C, ISSN 1932-7447, E-ISSN 1932-7455, Vol. 121, no 26, p. 14204-14212Article in journal (Refereed)
    Abstract [en]

    Molecular level engineering of polymer or polymer blends has been recently demonstrated effective strategy to regulate thermal conductivity. Such materials are of great interest to meet critical requirements of transparent, light weight, flexible, etc for thermal management in electronic applications. In this work, modulated polymer blends with poly(vinyl alcohol) (PVA) and biopolymers (lignin, gelatin) were designed and significantly enhanced thermal conductivity was achieved by tuning the intermolecular interaction among polymer components. The hydrogen bond interaction has been revealed as the major driving force that affects the polymer coil dimension in aqueous solution, the microstructure of coil-coil interaction in solid film and thus the thermal conduction. A solid relationship across molecular level interaction to macro-scale thermal conduction is constructed via careful characterization of the coil size in liquid phase and assembled microstructure in solid phase. Appropriate integration of biopolymers and PVA is essential to achieve synergistic effect. Specifically, thermal conductivity of polymer blend with 10% lignin and 10% G90 in PVA reaches 0.71 W/m·K, which is 184% enhancement as compared to pure PVA. This work reveals the fundamental molecular origin of polymer blends in association with thermal conductivity and has great potential to guide molecular engineering for superior physicochemical properties.

  • 22.
    Mu, Liwen
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Ji, Tuo
    Intelligent Composites Laboratory, Department of Chemical and Biomolecular Engineering, The University of Akron.
    Chen, Long
    Intelligent Composites Laboratory, Department of Chemical and Biomolecular Engineering, The University of Akron.
    Mehra, Nitin
    Intelligent Composites Laboratory, Department of Chemical and Biomolecular Engineering, The University of Akron.
    Shi, Yijun
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Zhu, Jiahua
    Intelligent Composites Laboratory, Department of Chemical and Biomolecular Engineering, The University of Akron.
    Paving the Thermal Highway with Self-Organized Nanocrystals in Transparent Polymer Composites2016In: ACS Applied Materials and Interfaces, ISSN 1944-8244, E-ISSN 1944-8252, Vol. 8, no 42, p. 29080-29087Article in journal (Refereed)
    Abstract [en]

    Phonon transfer is greatly scattered in traditional polymer composites due to the unpaired phonon frequency at the polymer/filler interface. A key innovation of this work is to build continuous crystal network by self-organization and utilize it as “thermal highway” that circumvents the long-existing interfacial thermal barrier issue in traditional composites. By tuning the molecular diffusion rate of dicarboxylic acids (oxalic acid, malonic acid, and succinic acid), different crystal structures including skeletal, dendrite, diffusion-limited aggregates, and spherulite were synthesized in PVA film. These continuous crystal structures benefit the efficient phonon transfer in the composites with minimized interfacial scattering and lead to a significant thermal conductivity enhancement of up to 180% compared to that of pure polymer. Moreover, the transparent feature of these composite films provides additional benefits in display applications. The post heat treatment effect on the thermal conductivity of the composite films shows a time-dependent behavior. These uniquely structured polymer/crystal composites are expected to generate significant impacts in thermal management applications.

  • 23.
    Mu, Liwen
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements. The University of Akron, Akron, USA.
    Ma, Xiaofeng
    Nanjing Forestry University, Nanjing, PR China.
    Guo, Xiaojing
    Chinese Academy of Sciences, Shanghai, PR China.
    Chen, Minjiao
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Ji, Tuo
    The University of Akron, Akron, USA.
    Hua, Jing
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Zhu, Jiahua
    The University of Akron, Akron, USA.
    Shi, Yijun
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Structural strategies to design bio-ionic liquid: Tuning molecular interaction with lignin for enhanced lubrication2019In: Journal of Molecular Liquids, ISSN 0167-7322, E-ISSN 1873-3166, Vol. 280, p. 49-57Article in journal (Refereed)
    Abstract [en]

    Lignin strengthened ionic liquids (ILs) have shown high potential to be used as high performance green lubricants. Strengthened lignin-ILs molecular interaction is an effective approach to improve their lubrication properties. The molecular interactions of ILs’ cation and anion containing different functional groups with lignin and efficiency on the lubricating properties have rarely been studied yet. In this work, a series of novel green lubricants with dissolved lignin in [Choline][Amino Acid] ([CH][AA]), [Tetramethylammonium][Glycine] ([N 1111 ][Gly]) and [Tetrabutylammonium][Glycine] ([N 4444 ][Gly]) ILs have been synthesized and their tribological properties were systematically investigated. The longer alkyl chain in cation without reciprocal H-bond interaction between ILs’ cation and anion has the positive effect on the anti-wear properties. In addition, the less steric effect and more negative natural charges of amino acid anion synergistically contribute to the stronger H-bond interaction between lignin and choline base ILs, which enhances lubrication film strength and thus resulting in the better tribological property of ILs/lignin green lubricants. Specifically, the wear volume loss of the steel disc lubricated by [N 4444 ][Gly] with the addition of 15% lignin is only 12% of the one lubricated by pure [N 4444 ][Gly]. This work presents a method to tune molecular interaction between lignin and ILs via the structural design of ILs’ cation and anion, which are revealed as the key factor that bridges the individual components and improves overall lubricating properties. 

  • 24.
    Mu, Liwen
    et al.
    State Key Laboratory of Materials-Oriented Chemical Engineering, Nanjing University of Technology, Intelligent Composites Laboratory, Department of Chemical and Biomolecular Engineering, The University of Akron.
    Shi, Yijun
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Chen, Long
    Department of Chemical and Biomolecular Engineering, The University of Akron, Intelligent Composites Laboratory, Department of Chemical and Biomolecular Engineering, The University of Akron.
    Ji, Tuo
    Intelligent Composites Laboratory, Department of Chemical and Biomolecular Engineering, The University of Akron.
    Yuan, Ruixia
    Intelligent Composites Laboratory, Department of Chemical and Biomolecular Engineering, The University of Akron.
    Wang, Huaiyuan
    School of Chemistry & Chemical Engineering, Northeast Petroleum University, College of Chemistry and Chemical Engineering, Northeast Petroleum University, Daqing.
    Zhu, Jiahua
    State Key Laboratory of Materials-Oriented Chemical Engineering, Nanjing University of Technology, Intelligent Composites Laboratory, Department of Chemical and Biomolecular Engineering, The University of Akron.
    [N-Methyl-2-pyrrolidone][C1–C4 carboxylic acid]: novel solvent system with exceptional lignin solubility2015In: Chemical Communications, ISSN 1359-7345, E-ISSN 1364-548X, Vol. 51, no 70, p. 13554-13557Article in journal (Refereed)
    Abstract [en]

    Novel solvent systems composed of N-methyl-2-pyrrolidone and C1–C4 carboxylic acid exhibit unique physicochemical properties, e.g. large polarity, low viscosity and excellent hydrogen bonding capacity, which have demonstrated excellent lignin solubility that outperforms conventional solvents and ionic liquids.

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  • 25.
    Mu, Liwen
    et al.
    State Key Laboratory of Materials-Oriented Chemical Engineering, Nanjing University of Technology.
    Shi, Yijun
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Feng, Xin
    State Key Laboratory of Materials-Oriented Chemical Engineering, Nanjing University of Technology.
    Lu, Xiaohua
    State Key Laboratory of Materials-Oriented Chemical Engineering, Nanjing University of Technology.
    Non-equilibrium thermodynamics analysis and its application for polymer composites design in tribological fields2014In: Huagong Xuebao/Journal of Chemical Industry and Engineering, ISSN 0438-1157, Vol. 65, no 5, p. 1629-1635Article in journal (Refereed)
    Abstract [en]

    The heat generation and transfer process of self-lubricating materials and friction materials was quantitatively described from the fundamental principle of non-equilibrium thermodynamics. The effect of friction coefficient and thermal conductivity on the contact temperature of materials was studied, and the finding could guide the design of polymer composites. For the self-lubricating materials, the frictional heat generation and transfer process was assumed to be two processes in series. The frictional heat generated was the key controlling factor for stable operation of the friction system. For the friction materials, frictional heat transfer process was assumed to be heat transfer and heat distribution, two processes in parallel, and it was required to reduce thermal conductivity of the brake pads in order to lower contact surface temperature.

  • 26.
    Mu, Liwen
    et al.
    State Key Laboratory of Materials-Oriented Chemical Engineering, Nanjing University of Technology, Intelligent Composites Laboratory, Department of Chemical and Biomolecular Engineering, The University of Akron.
    Shi, Yijun
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Guo, Xiaojing
    Shanghai Institute of Applied Physics, Chinese Academy of Sciences.
    Ji, Tuo
    Intelligent Composites Laboratory, Department of Chemical and Biomolecular Engineering, The University of Akron.
    Chen, Long
    Department of Chemical and Biomolecular Engineering, The University of Akron.
    Yuan, Ruixia
    Intelligent Composites Laboratory, Department of Chemical and Biomolecular Engineering, The University of Akron.
    Brisbin, Logan
    Intelligent Composites Laboratory, Department of Chemical and Biomolecular Engineering, The University of Akron.
    Wang, Huaiyuan
    School of Chemistry & Chemical Engineering, Northeast Petroleum University.
    Zhu, Jiahua
    State Key Laboratory of Materials-Oriented Chemical Engineering, Nanjing University of Technology, Intelligent Composites Laboratory, Department of Chemical and Biomolecular Engineering, The University of Akron.
    Non-corrosive green lubricants: strengthened lignin–[choline][amino acid] ionic liquids interaction via reciprocal hydrogen bonding2015In: RSC Advances, ISSN 2046-2069, E-ISSN 2046-2069, Vol. 5, no 81, p. 66067-66072Article in journal (Refereed)
    Abstract [en]

    A series of novel green lubricants with dissolved lignin in [choline][amino acid] ([CH][AA]) ionic liquids (ILs) have been synthesized in this work. The effect of lignin on the thermal and tribological properties of the lignin/[CH][AA] lubricants was systematically investigated by means of thermogravimetric analysis, differential scanning calorimetry, and a friction and wear tester. The lignin in [CH][AA] has been demonstrated to be an effective additive to improve thermal stability, reduce the wear rates and stabilize the friction coefficients of lignin/[CH][AA] lubricants. Density function theory calculations on the electronic structure of [CH][AA] ILs reveal the atomic natural charge of ILs and their hydrogen bonding capability with lignin. Moreover, these green lubricants show excellent anti-corrosive properties against commercial aluminum and iron boards. The strong physical adsorption of [CH][AA] ILs onto the steel surface and the reciprocal hydrogen bonding between [CH][AA] ILs and lignin synergistically contribute to the enhanced lubrication film strength and thus the tribological properties of these new lubricants. This work provides a new perspective on utilizing complete bio-products in advanced tribological lubrication systems. In addition, this will open a new application venue for lignin to improve product value in lignocellulosic biomass utilization.

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  • 27.
    Mu, Liwen
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements. Intelligent Composites Laboratory, Department of Chemical and Biomolecular Engineering, The University of Akron.
    Shi, Yijun
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements. State Key Laboratory of Materials-Oriented Chemical Engineering, Nanjing Tech University, China.
    Guo, Xiaojing
    Shanghai Institute of Applied Physics, Chinese Academy of Sciences.
    Wu, Jian
    State Key Laboratory of Materials-Oriented Chemical Engineering, Nanjing University of Technology.
    Ji, Tuo
    Intelligent Composites Laboratory, Department of Chemical and Biomolecular Engineering, The University of Akron.
    Chen, Long
    Department of Chemical and Biomolecular Engineering, The University of Akron, Intelligent Composites Laboratory, Department of Chemical and Biomolecular Engineering, The University of Akron.
    Feng, Xin
    State Key Laboratory of Materials-Oriented Chemical Engineering, College of Chemistry and Chemical Engineering, Nanjing University of Technology, Nanjing University of Chemical Technology, State Key Laboratory of Materials-Oriented Chemical Engineering, Nanjing University of Technology.
    Lu, Xiaohua
    State Key Laboratory of Materials-Oriented Chemical Engineering, Nanjing University of Technology, College of Chemistry and Chemical Engineering, Nanjing University of Technology.
    Hua, Jing
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Zhu, Jiahua
    State Key Laboratory of Materials-Oriented Chemical Engineering, Nanjing University of Technology.
    Enriching Heteroelements in Lignin as Lubricating Additives for Bioionic Liquids2016In: A C S Sustainable Chemistry & Engineering, ISSN 2168-0485, Vol. 4, no 7, p. 3877-3887Article in journal (Refereed)
    Abstract [en]

    Depolymerization and modification of lignin have been achieved simultaneously in a one-pot chemical reaction. Two heteroelement-rich modifiers, imidazol-1-yl phosphonic dichloride and 1H-1,2,4-triazol-1-yl phosphonic dichloride, were selected to react with lignin in this work. The modified lignin (m-lignin) is demonstrated as an effective lubricating additive for [choline][amino acid] ([CH][AA]) bioionic liquids. Different characterization techniques have been utilized to study the lignin depolymerization, reaction between lignin and modifiers and m-lignin/[CH][AA] interaction. The effect of the molecular structure of the modifiers on the rheological and tribological properties of m-lignin/[CH][AA] lubricants was systematically investigated. Density function theory is used to calculate the electronic structure of lignin, m-lignin, and [CH][AA]. The atomic natural charge analysis revealed the most negative charge on nitrogen bonded to a phosphorus atom and the strongest capability of forming hydrogen bonding with [CH][AA]. The introduced nitrogen and phosphorus elements not only increase the hydrogen bonding density in m-lignin/[CH][AA] but also enhance the polarity of the m-lignin, both of which facilitate a strong adhesion of lubricant on a metal surface and thus promote lubrication. A larger fraction of heteroatom groups in m-lignin contributes to a better lubrication property of these lubricants

  • 28.
    Mu, Liwen
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements. Intelligent Composites Laboratory, Department of Chemical and Biomolecular Engineering, The University of Akron.
    Shi, Yijun
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Guo, Xiaojing
    Shanghai Institute of Applied Physics, Chinese Academy of Sciences, Shanghai .
    Zhuang, Wei
    College of Biotechnology and Pharmaceutical Engineering, National Engineering Technique Research Center for Biotechnology, Nanjing Tech University.
    Chen, Long
    Intelligent Composites Laboratory, Department of Chemical and Biomolecular Engineering, The University of Akron.
    Ji, Tuo
    Intelligent Composites Laboratory, Department of Chemical and Biomolecular Engineering, The University of Akron.
    Hua, Jing
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Wang, Huaiyuan
    College of Chemistry and Chemical Engineering, Northeast Petroleum University, Daqing.
    Zhu, Jiahua
    Intelligent Composites Laboratory, Department of Chemical and Biomolecular Engineering, The University of Akron.
    Grafting Heteroelement-Rich Groups on Graphene Oxide: Tuning Polarity and Molecular Interaction with Bio-Ionic Liquid for Enhanced Lubrication2017In: Journal of Colloid and Interface Science, ISSN 0021-9797, E-ISSN 1095-7103, Vol. 498, p. 47-54Article in journal (Refereed)
    Abstract [en]

    Two different heteroelement-rich molecules have been successfully grafted on graphene oxide (GO) sheets which were then used as lubricant additives in bio-ionic liquid. The grafting was processed with reactions between GO sheets and synthesized heteroelement-rich molecules (Imidazol-1-yl phosphonic dichloride and 1H-1,2,4-triazol-1-yl phosphonic dichloride, respectively). The modified GO (m-GO) was added into [Choline][Proline] ([CH][P]) bio-ionic liquid, and has been demonstrated effective additive in promoting lubrication. Different characterization techniques have been utilized to study the reaction between GO and the two modifiers. The effect of molecular structure of the modifiers on the rheological and tribological properties of m-GO/[CH][P] lubricants was systematically investigated. Both theoretical calculation and experimental results demonstrated that the introduced heteroelement-rich groups are beneficial to increase the robustness of lubrication film by intensified hydrogen bonding and enhance the lubricant/friction surface adhesion by increased polarity of the m-GO. As a result, the interfacial lubrication could be significantly improved by these newly developed m-GO/[CH][P] lubricants.

  • 29.
    Mu, Liwen
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements. Intelligent Composites Laboratory, Department of Chemical and Biomolecular Engineering, The University of Akron.
    Shi, Yijun
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Hua, Jing
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Zhuang, Wei
    College of Biotechnology and Pharmaceutical Engineering, National Engineering Technique Research Center for Biotechnology, Nanjing Tech University.
    Zhu, Jiahua
    Intelligent Composites Laboratory, Department of Chemical and Biomolecular Engineering, The University of Akron.
    Engineering Hydrogen Bonding Interaction and Charge Separation in Bio-polymers for Green Lubrication2017In: Journal of Physical Chemistry B, ISSN 1520-6106, E-ISSN 1520-5207, Vol. 121, no 22, p. 5669-5678Article in journal (Refereed)
    Abstract [en]

    Synthetic additives are widely used in lubricants nowadays to upgrade lubrication properties. The potential of integrating sustainable components in modern lubricants has rarely been studied yet. In this work, two sustainable resources lignin and gelatin have been synergistically incorporated into ethylene glycol (EG), and their tribological properties were systematically investigated. The abundant hydrogen bonding sites in lignin and gelatin as well as their interchain interaction via hydrogen bonding play the dominating roles in tuning the physicochemical properties of the mixture and improving lubricating properties. Moreover, the synergistic combination of lignin and gelatin induces charge separation of gelatin that enables its preferable adsorption on the friction surface through electrostatic force and forms a robust lubrication layer. This layer will be strengthened by lignin through the interpolymer chain hydrogen bonding. At an optimized lignin:gelatin mass ratio of 1:1 and 19 wt % loading of each in EG, the friction coefficient can be greatly stabilized and the wear loss was reduced by 89% compared to pure EG. This work presents a unique synergistic phenomenon between gelatin and lignin, where hydrogen bonding and change separation are revealed as the key factor that bridges the individual components and improves overall lubricating properties.

  • 30.
    Mu, Liwen
    et al.
    State Key Laboratory of Materials-Oriented Chemical Engineering, Nanjing University of Technology, Intelligent Composites Laboratory, Department of Chemical and Biomolecular Engineering, The University of Akron.
    Shi, Yijun
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Ji, Tuo
    Intelligent Composites Laboratory, Department of Chemical and Biomolecular Engineering, The University of Akron.
    Chen, Long
    Department of Chemical and Biomolecular Engineering, The University of Akron.
    Yuan, Ruixia
    Intelligent Composites Laboratory, Department of Chemical and Biomolecular Engineering, The University of Akron.
    Wang, Huaiyuan
    School of Chemistry & Chemical Engineering, Northeast Petroleum University, College of Chemistry and Chemical Engineering, Northeast Petroleum University, Daqing.
    Zhu, Jiahua
    State Key Laboratory of Materials-Oriented Chemical Engineering, Nanjing University of Technology, Intelligent Composites Laboratory, Department of Chemical and Biomolecular Engineering, The University of Akron.
    Ionic Grease Lubricants: Protic [Triethanolamine][Oleic acid] and Aprotic [Choline][Oleic acid]2016In: ACS Applied Materials and Interfaces, ISSN 1944-8244, E-ISSN 1944-8252, Vol. 8, no 7, p. 4977-4984Article in journal (Refereed)
    Abstract [en]

    Ionic liquid lubricants or lubricant additives have been studied intensively over past decades. However, ionic grease serving as lubricant has rarely been investigated so far. In this work, novel protic [Triethanolamine][Oleic acid] and aprotic [Choline][Oleic acid] ionic greases are successfully synthesized. These ionic greases can be directly used as lubricants without adding thickener or other additives. Their distinct thermal and rheological properties are investigated and well correlated to their tribological properties. It is revealed that aprotic ionic grease shows superior temperature and pressure tolerant lubrication properties than protic ionic grease. The lubrication mechanism is studies and it reveals that strong physical adsorption of ionic grease onto friction surface plays a dominating role for promoted lubrication instead of tribo-chemical film formation.

  • 31.
    Mu, Liwen
    et al.
    State Key Laboratory of Materials-Oriented Chemical Engineering, Nanjing University of Technology, Intelligent Composites Laboratory, Department of Chemical and Biomolecular Engineering, The University of Akron.
    Shi, Yijun
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Wang, Huaiyuan
    School of Chemistry & Chemical Engineering, Northeast Petroleum University, College of Chemistry and Chemical Engineering, Northeast Petroleum University, Daqing.
    Zhu, Jiahua
    State Key Laboratory of Materials-Oriented Chemical Engineering, Nanjing University of Technology, Intelligent Composites Laboratory, Department of Chemical and Biomolecular Engineering, The University of Akron.
    Lignin in Ethylene Glycol and Poly(ethylene glycol): Fortified Lubricants with Internal Hydrogen Bonding2016In: A C S Sustainable Chemistry & Engineering, ISSN 2168-0485, Vol. 4, no 3, p. 1840-1849Article in journal (Refereed)
    Abstract [en]

    Lignin, one of the most naturally abundant polymers, has been successfully incorporated into ethylene glycol (EG) and poly(ethylene glycol) (PEG) in this work and fortified lubricating properties were achieved in EG/lignin and PEG/lignin. The molecular interaction between lignin and EG (or PEG) has been revealed as hydrogen bonding, which serves as the dominating factor that determines the thermal, rheological, and tribological properties of the mixed systems of EG/lignin and PEG/lignin. The physicochemical properties of the mixed lubricants are tightly related to the state of internal hydrogen bonding (EG–EG, PEG–PEG, EG–lignin, PEG–lignin, and lignin–lignin) and are well correlated to their lubrication properties. Generally, larger lignin fractions lead to better lubricating performance in both EG and PEG systems. Lignin liquefaction in PEG has been addressed by catalytic degradation with the presence of sulfuric acid, which was then neutralized by triethanolamine for lubricant development. Lignin in PEG significantly improves the lubricating property at higher pressure conditions, where a wear reduction of 94.6% was observed. Lignin fortified EG and PEG based lubricants show outstanding noncorrosive characteristic to the mostly used metal materials such as aluminum and iron.

  • 32.
    Mu, Liwen
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements. Intelligent Composites Laboratory, Department of Chemical and Biomolecular Engineering, The University of Akron, Akron, OH, USA.
    Wu, Jian
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Matsakas, Leonidas
    Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Chemical Engineering.
    Chen, Minjiao
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Rova, Ulrika
    Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Chemical Engineering.
    Christakopoulos, Paul
    Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Chemical Engineering.
    Zhu, Jiahua
    Intelligent Composites Laboratory, Department of Chemical and Biomolecular Engineering, The University of Akron, Akron, OH, USA.
    Shi, Yijun
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Two important factors of selecting lignin as efficient lubricating additives in poly (ethylene glycol): Hydrogen bond and molecular weight2019In: International Journal of Biological Macromolecules, ISSN 0141-8130, E-ISSN 1879-0003, Vol. 129, p. 564-570Article in journal (Refereed)
    Abstract [en]

    Lignin, one of the most abundant natural polymers, has been successfully used as an effective lubricant additive with high value. The chemical structure of lignin is very diverse and strongly affected by both the source of lignin (i.e. plant species) and the lignin extraction process. In this work, a series of lignin from different biomass sources (hard or soft wood) and extraction process (organosolv with or without acid catalyst) has been successfully incorporated into poly(ethylene glycol) (PEG) and fortified lubricating properties were achieved. The effects of different lignin on the rheological, thermal and tribological properties of the lignin/EG lubricants were systematically investigated by different characterization techniques. Lignin in PEG significantly improves the lubricating property, where a wear reduction of 93.8% was observed. The thermal and lubrication properties of the PEG lubricants filled with different kinds of lignin are tightly related to the synergistic state of hydrogen bonding and molecular weight distribution. Lignin with broader molecular weight distribution and higher hydroxyl content shows better adhesion on metal surfaces and strengthened lubricating film, which could be used as the efficient lubricating additives. This work provides a criterion for selecting appropriate lignin as the efficient lubricant additive and accelerates the application of lignin.

  • 33.
    Mu, Liwen
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements. Intelligent Composites Laboratory, Department of Chemical and Biomolecular Engineering, The University of Akron.
    Wu, Jian
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Matsakas, Leonidas
    Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Chemical Engineering.
    Chen, Minjiao
    Vahidi, Alireza
    Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Chemical Engineering.
    Grahn, Mattias
    Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Chemical Engineering.
    Rova, Ulrika
    Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Chemical Engineering.
    Christakopoulos, Paul
    Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Chemical Engineering.
    Zhu, Jiahua
    Intelligent Composites Laboratory, Department of Chemical and Biomolecular Engineering, The University of Akron.
    Shi, Yijun
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Lignin from Hardwood and Softwood Biomass as a Lubricating Additive to Ethylene Glycol2018In: Molecules, ISSN 1420-3049, E-ISSN 1420-3049, Vol. 23, no 3, article id 537Article in journal (Refereed)
    Abstract [en]

    Ethylene glycol (EG)-based lubricant was prepared with dissolved organosolv lignin from birch wood (BL) and softwood (SL) biomass. The effects of different lignin types on the rheological, thermal, and tribological properties of the lignin/EG lubricants were comprehensively investigated by various characterization techniques. Dissolving organosolv lignin in EG results in outstanding lubricating properties. Specifically, the wear volume of the disc by EG-44BL is only 8.9% of that lubricated by pure EG. The enhanced anti-wear property of the EG/lignin system could be attributed to the formation of a robust lubrication film and the strong adhesion of the lubricant on the contacting metal surface due to the presence of a dense hydrogen bonding (H-bonding) network. The lubricating performance of EG-BL outperforms EG-SL, which could be attributed to the denser H-bonding sites in BL and its broader molecular weight distribution. The disc wear loss of EG-44BL is only 45.7% of that lubricated by EG-44SL. Overall, H-bonding is the major contributor to the different tribological properties of BL and SL in EG-based lubricants.

  • 34.
    Mu, Liwen
    et al.
    State Key Laboratory of Materials-Oriented Chemical Engineering, Nanjing University of Technology, School of Materials Engineering, Nanjing Institute of Technology.
    Zhu, Jiahua
    State Key Laboratory of Materials-Oriented Chemical Engineering, Nanjing University of Technology, School of Materials Engineering, Nanjing Institute of Technology.
    Fan, Jingdeng
    School of Materials Engineering, Nanjing Institute of Technology.
    Zhou, Zhongxin
    School of Materials Engineering, Nanjing Institute of Technology.
    Shi, Yijun
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Feng, Xin
    State Key Laboratory of Materials-Oriented Chemical Engineering, College of Chemistry and Chemical Engineering, Nanjing University of Technology, Nanjing University of Chemical Technology, State Key Laboratory of Materials-Oriented Chemical Engineering, Nanjing University of Technology.
    Wang, Huaiyuan
    School of Chemistry & Chemical Engineering, Northeast Petroleum University.
    Lu, Xiaohua
    State Key Laboratory of Materials-Oriented Chemical Engineering, Nanjing University of Technology, College of Chemistry and Chemical Engineering, Nanjing University of Technology.
    Self-Lubricating Polytetrafluoroethylene/Polyimide Blends Reinforced with Zinc Oxide Nanoparticles2015In: Journal of Nanomaterials, ISSN 1687-4110, E-ISSN 1687-4129, article id 545307Article in journal (Refereed)
    Abstract [en]

    ZnO nanoparticle reinforced polytetrafluoroethylene/polyimide (PTFE/PI) nanocomposites were prepared and their corresponding tribological and mechanical properties were studied in this work. The influences of ZnO loading, sliding load, and velocity on the tribological properties of ZnO/PTFE/PI nanocomposites were systematically investigated. Results reveal that nanocomposites reinforced with 3 wt% ZnO exhibit the optimal tribological and mechanical properties. Specifically, the wear loss decreased by 20% after incorporating 3 wt% ZnO compared to unfilled PTFE/PI. Meanwhile, the impact strength, tensile strength, and elongation-at-break of 3 wt% ZnO/PTFE/PI nanocomposite are enhanced by 85, 5, and 10% compared to pure PTFE/PI blend. Microstructure investigation reveals that ZnO nanoparticles facilitate the formation of continuous, uniform, and smooth transfer film and thus reduce the adhesive wear of PTFE/PI.

  • 35.
    Patel, Alok
    et al.
    Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Chemical Engineering.
    Mu, Liwen
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Shi, Yijun
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Rova, Ulrika
    Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Chemical Engineering.
    Christakopoulos, Paul
    Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Chemical Engineering.
    Matsakas, Leonidas
    Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Chemical Engineering.
    A novel biorefinery approach aimed at vegetarians reduces the dependency on marine fish stocks for obtaining squalene and DHA2020In: ACS Sustainable Chemistry & Engineering, E-ISSN 2168-0485Article in journal (Refereed)
    Abstract [en]

    Squalene and docosahexaenoic acid (DHA) have gained substantial market share as dietary supplements and vital nutraceuticals due to their beneficial effects on human health. Marine fish are the main commercial source of these nutraceuticals, but a growing global demand, issues of sustainability, and an expanding vegan and vegetarian population has prompted the search for alternatives. Oils obtained from oleaginous microorganisms such as microalgae, diatoms, certain fungi, and thraustochytrids are alternatives to fish oils for omega-3 fatty acids. Among these, DHA is now being mined from thraustochytrids due to its highest proportion in their lipids, however, this strategy is not cost-effective. One way to offset such elevated production costs is to simultaneously extract other high value-added biological products from these oleaginous microorganisms. Here, we propose a novel biorefinery process based on single-step purification of squalene from total lipids extracted from an oleaginous thraustochytrid cultivated on non-edible forest biomass. To render the process economically feasible and sustainable, additional squalene-free lipids were exploited for enrichment of DHA; whereas leftover lipids generated as by-product during the process were tested as a novel biolubricant.

  • 36. Roy, Amit
    et al.
    Mu, Liwen
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Shi, Yijun
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Tribological properties of polyimide coating filled with carbon nanotube at elevated temperatures2020In: Polymer Composites, ISSN 0272-8397, E-ISSN 1548-0569Article in journal (Refereed)
    Abstract [en]

    This work mainly focuses on the development and characterization of polyimide (PI) based composite coatings filled with multi‐walled carbon nanotubes (MWCNT) on a steel substrate. The tribological properties of PI/MWCNT composite coatings at different temperatures ranging from room temperature (RT) to 200°C were investigated. Also, the influence of MWCNT and thermal & mechanical properties of PI composites coatings were measured. The addition of MWCNT could reduce the friction coefficient as well as wear volume at elevated temperatures. The PI/3MWCNT composites coatings show better wear‐resistant properties as compared to other composites. Also, the PI/MWCNT composites showed improved mechanical properties such as micro‐hardness and nanoindentation. Formation of an organic‐inorganic structure due to the inclusion of MWCNT, enhance the mechanical properties and reduce the thermo‐degradation of composites coatings. The glass transition temperature (Tg) of the PI/MWCNT composites was improved as the MWCNT content increases from 0.1 to 5 wt%.

  • 37.
    Roy, Amit
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Mu, Liwen
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Shi, Yijun
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Tribological properties of polyimide-graphene composite coatings at elevated temperatures2020In: Progress in organic coatings, ISSN 0300-9440, E-ISSN 1873-331X, Vol. 142, article id 105602Article in journal (Refereed)
    Abstract [en]

    This paper focuses on developing a polyamic acid (PAA) of polyimide (PI) and PI filled with graphene (PI/GP) on 100CR6 ESU hardened steel. The tribological testing result of the pure PI and PI/GP against 100Cr6 steel ball at room temperature (RT) and elevated temperatures (50 °C, 100 °C, 150 °C, 200 °C) exhibits that the addition of GP clearly enhances the friction reduction and wear-resistant properties of PI, due to the excellent lubricating properties of GP. The 3 wt % of GP content in the PI is the optimum loading for the lowest friction and wear at RT and elevated temperatures. The SEM analysis of the worn surfaces showed that the high wear-resistant of the PI/GP composite is due to the generation of the loose particles from the coating surfaces which could be embedded on the friction pairs and rolled over there as a lubrication film. Also, at 60 % weight residue the PI/3 G P showed the high decomposition temperature.

  • 38.
    Shao, Jiajia
    et al.
    CAS Center for Excellence in Nanoscience, Beijing Key Laboratory of Micro-nano Energy and Sensor, Beijing Institute of Nanoenergy and Nanosystems, Chinese Academy of Sciences, Beijing, China.College of Nanoscience and Technology, University of Chinese Academy of Sciences, Beijing, People's Republic of China.
    Willatzen, Morten
    CAS Center for Excellence in Nanoscience, Beijing Key Laboratory of Micro-nano Energy and Sensor, Beijing Institute of Nanoenergy and Nanosystems, Chinese Academy of Sciences, Beijing, China.College of Nanoscience and Technology, University of Chinese Academy of Sciences, Beijing, People's Republic of China.
    Shi, Yijun
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Wang, Zhong Lin
    CAS Center for Excellence in Nanoscience, Beijing Key Laboratory of Micro-nano Energy and Sensor, Beijing Institute of Nanoenergy and Nanosystems, Chinese Academy of Sciences, Beijing, China.College of Nanoscience and Technology, University of Chinese Academy of Sciences, Beijing, People's Republic of China.School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, GA, United States.
    3D mathematical model of contact-separation and single-electrode mode triboelectric nanogenerators2019In: Nano Energy, ISSN 2211-2855, E-ISSN 2211-3282, Vol. 60, p. 630-640Article in journal (Refereed)
    Abstract [en]

    Based on a set of finite-sized charged planes (FSCP), a simple time-dependent three-dimensional spatial model for the electric potential and electric field in an inhomogeneous medium composed of dielectric materials and metal contacts is proposed and used to assert triboelectric nanogenerator operation. Solving the problem of FSCP makes the three-dimensional spatial model relevant for practical TENG applications and allow for accurate and reliable results. Connecting the metal contacts to an electric resistance, Kirchhoff's law is used to derive a first-order time-dependent differential equation for the mobile charges on the metal contacts and the displacement current. Specially, the displacement current (Maxwell's displacement current) in a TENG equals to the conduction current in the external circuit is obtained. We then consider two important types of triboelectric nanogenerators: the contact-separation (CS) mode and the single-electrode (SEC) mode. A forced movement of the dielectric materials and/or the metal contacts leads to currents flowing in the system and a time-varying electrical potential, and therefore the generation of electrical power. Then, new and more accurate capacitance equations for CS and SEC modes of TENGs are extracted. Several examples of energy harvesting scenarios are finally analyzed.

  • 39.
    Shetty, Pramod
    et al.
    Luleå University of Technology.
    Mu, Liwen
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Shi, Yijun
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Polyelectrolyte Cellulose Gel with PEG/Water: Toward Fully Green Lubricating Grease2020In: Carbohydrate Polymers, ISSN 0144-8617, E-ISSN 1879-1344, Vol. 230, article id 115670Article in journal (Refereed)
    Abstract [en]

    Developing a fully green lubricant is an urgent need due to the growing consciousness of environmental protection and dwindling resources. In this work, fully green gel lubricants were developed out of cellulose derivatives as gelator and mixture of water and poly(ethylene glycol) 200 (PEG 200) as the base fluid. The non-ionic hydroxyethyl cellulose (HEC) and anionic sodium carboxymethyl cellulose (NaCMC) were chosen to understand the effect of ionic and non-ionic gelators on the thermal, rheological and the tribological properties of the gel lubricant. HEC or NaCMC is demonstrated as effective additive to reduce wear, stabilize friction coefficient and enhance the thermal stability of developed lubricants. It is shown that anionic gelator will result in producing lower friction and wear in comparison to non-ionic gelator, which may be attributed to the possible tribo-film formation due to the negative charge in the NaCMC molecules and its larger molecular weight.

  • 40.
    Shi, Yijun
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Tribological properties of CF/Gr/PTFE bilayer composites2013Conference paper (Refereed)
  • 41.
    Shi, Yijun
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Larsson, Roland
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Non-corrosive and Biomaterials Protic Ionic Liquids with High Lubricating Performance2016In: Tribology letters, ISSN 1023-8883, E-ISSN 1573-2711, Vol. 63, no 1, article id 1Article in journal (Refereed)
    Abstract [en]

    Achieving non-corrosive and green ionic liquids is a big challenge for the tribologist. A kind of biomaterials-based protic ionic liquids (PILs) was synthesized in this paper. Rapeseed oil, group 1 mineral oil and one commercially available fully formulated gear oil were as used reference to study the property of the synthesized PILs. The copper strip standard corrosion test was employed to study the anti-corrosion property. The boundary lubrication and elastohydrodynamic lubrication performance of the lubricants was studied on an Optimol SRV-III oscillating friction and wear tester, and a WAM (model 11) ball-on-disk test rig, respectively. The lubricating mechanism of the synthesized PILs was also discussed in this paper

  • 42.
    Shi, Yijun
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Minami, Ichiro
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Grahn, Mattias
    Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Chemical Engineering.
    Björling, Marcus
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Larsson, Roland
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Boundary and elastohydrodynamic lubrication studies of glycerol aqueous solutions as green lubricants2014In: Tribology International, ISSN 0301-679X, E-ISSN 1879-2464, Vol. 69, p. 39-45Article in journal (Refereed)
    Abstract [en]

    In this paper, the boundary and elastohydrodynamic lubricating behaviour of glycerol and its aqueous solutions are discussed in both rolling and sliding contacts with a view on assessing the use of glycerol as a green lubricant. To understand the lubricating mechanism, the film thickness of glycerol and its aqueous solutions were studied at different velocities. The results show that the viscosity of glycerol can be controlled for a wide range by adding different amounts of water. The lubricating behaviour of glycerol in all lubricating regimes can be improved by adding water. The results suggest that glycerol aqueous solutions have great potential to replace rapeseed oils as environmentally friendly base oils in several applications.

  • 43.
    Shi, Yijun
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Mu, Liwen
    State Key Laboratory of Materials-Oriented Chemical Engineering, Nanjing University of Technology.
    Feng, Xin
    State Key Laboratory of Materials-Oriented Chemical Engineering, Nanjing University of Technology.
    Lu, Xiaohua
    State Key Laboratory of Materials-Oriented Chemical Engineering, Nanjing University of Technology.
    Friction and wear behavior of CF/PTFE composites lubricated by choline chloride ionic liquids2013In: Tribology letters, ISSN 1023-8883, E-ISSN 1573-2711, Vol. 49, no 2, p. 413-420Article in journal (Refereed)
    Abstract [en]

    The use of ionic liquids (ILs) as lubricants has received increasing attention in recent years. The use of ILs, however, is limited by the corrosion problem and their potential toxic property. Here we present the results of our initial study on the tribological properties of carbon fiber (CF)-filled polytetrafluoroethylene (PTFE) composites, which have an excellent chemical resistance property, lubricated by choline chloride ILs. The difference between choline chloride ILs and water and hydraulic oil as lubricants was studied at the same time, as was the effect of the anion on the lubricating property of choline chloride ILs. The worn surface and transfer film of CF/PTFE composites were studied by scanning electron microscopy. Our results indicate that the lubricating property of choline chloride ILs is much better than that of water and hydraulic oil. The friction coefficient and wear rate of CF/PTFE composites lubricated with ILs were approximately 60 and 50 % lower than those under the dry friction condition. Among the three kinds of ILs tested, the best tribological properties of the CF/PTFE composites were found for those sliding in the mixture of 1,2-propanediol and choline chloride. The worn surface and transfer film of CF/PTFE composites were also much smoother than those under the dry friction, water lubrication, and hydraulic oil lubrication conditions.

  • 44.
    Shi, Yijun
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Vadivel, Hari
    Berglund, Kim
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Studying the tribological properties of PEEK composites lubricated by different lubricants for hydropower turbine application2016In: Nordic Symposium on Tribology - NORDTRIB 2016, 2016Conference paper (Refereed)
  • 45.
    Shu, Yudan
    et al.
    College of Chemical Engineering, State Key Laboratory of Materials-Oriented Chemical Engineering, Nanjing Tech University.
    Zhang, Yumeng
    College of Chemical Engineering, State Key Laboratory of Materials-Oriented Chemical Engineering, Nanjing Tech University.
    Shi, Yijun
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Lu, Xuaihua
    College of Chemical Engineering, State Key Laboratory of Materials-Oriented Chemical Engineering, Nanjing Tech University.
    Li, Jiahui
    College of Chemical Engineering, State Key Laboratory of Materials-Oriented Chemical Engineering, Nanjing Tech University.
    Lu, Linghong
    College of Chemical Engineering, State Key Laboratory of Materials-Oriented Chemical Engineering, Nanjing Tech University.
    Lubrication Behavior of Water Molecules Confined in TiO2 Nanoslits: A Molecular Dynamics Study2016In: Journal of Chemical and Engineering Data, ISSN 0021-9568, E-ISSN 1520-5134, Vol. 61, no 12, p. 4023-4030Article in journal (Refereed)
    Abstract [en]

    Titanium (Ti) metal has been widely used in orthopedic implants, such as knee replacements and fracture fixation devices, where water is the base fluid of the lubricant. In this work, a series of nonequilibrium molecular dynamics have been carried out to investigate the microstructure and lubrication of water molecules confined in TiO2 nanoslits under shearing. The effects of varying slit gap widths (0.8, 1.2, 1.6, and 2.0 nm) and shear velocities (200, 100, 50, and 10 m/s) on the friction coefficients between TiO2 and water molecules were evaluated to shed light on the role of the confined water molecules on lubrication. Simulation results showed that the friction coefficient decreased as the slit width increased. Detailed analysis of water molecules microstructure revealed that water molecules confined in the slits were layered. Typically, all the water molecules in Layer 1 and some water molecules in Layer 2 could reach the sliding velocity of the wall, which were in agreement with the reported mobility of water molecules absorbed on TiO2 nanoparticles via nuclear magnetic resonance. As the width of slit gap increased, the average lifetime of the H-bonds between water molecules within and beyond Layer 1 reduced and the amount of free water increased accordingly, which caused a decrease in the friction coefficient. This understanding can be used to explain at the molecular scale the observation in our previous atomic force microscope experiment in which the higher roughness in TiO2 reflected a lower friction coefficient. 

  • 46.
    Wang, Hongdong
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements. State Key Laboratory of Tribology, Tsinghua University, Beijing, China. Applied Materials Division, Argonne National Laboratory, Argonne, Illinois, United States..
    Liu, Yuhong
    State Key Laboratory of Tribology, Tsinghua University, Beijing, China. .
    Guo, Fangmin
    X-ray Science Division, Argonne National Laboratory, Argonne, Illinois, United States .
    Sheng, Huaping
    Center for Nanoscale Materials, Argonne National Laboratory, Argonne, Illinois, United States.
    Xia, Kailun
    Department of Chemistry and Center for Nano and Micro Mechanics, Tsinghua University, Beijing, China.
    Liu, Wenrui
    State Key Laboratory of Tribology, Tsinghua University, Beijing, China.
    Wen, Jianguo
    Center for Nanoscale Materials, Argonne National Laboratory, Argonne, Illinois, United States.
    Shi, Yijun
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Erdemir, Ali
    Applied Materials Division, Argonne National Laboratory, Argonne, Illinois, United States.
    Luo, Jianbin
    State Key Laboratory of Tribology, Tsinghua University, Beijing, China.
    Catalytically Active Oil-based Lubricant Additives Enabled by Calcining Ni-Al Layered Double Hydroxides2020In: Journal of Physical Chemistry Letters, ISSN 1948-7185, E-ISSN 1948-7185, Vol. 11, p. 113-120Article in journal (Refereed)
    Abstract [en]

    Layered double hydroxides (LDHs) have lately been hailed as robust lubricant additives for improving tribological properties and as ideal catalysts for synthesizing carbon-based nanomaterials. In this paper, in situ analytical tools are used to track the evolution of the crystal structure and chemical composition of LDHs during calcination. Nickel oxide and elemental nickel can be produced by calcining NiAl-LDH in air (LDH-C-Air) and argon (LDH-C-Ar), respectively. For the base oil with 1 wt % LDH-C-Air, negligible wear can be detected even after a 2 h friction test under a severe contact pressure (∼637 MPa). A relatively thick tribofilm (∼60 nm) with a better mechanical property is formed, which protects the solid surface from severe wear. In addition, the possible formed carbon debris may also prevent the direct collision of asperities and effectively improve the wear resistance. This work provides a unique vision for the application of calcined LDHs with the combination of catalysis and tribology.

  • 47.
    Wang, Hongdong
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements. State Key Laboratory of Tribology, Tsinghua University, Beijing, China. Applied Materials Division, Argonne National Laboratory, Argonne, Illinois , United States.
    Liu, Yuhong
    State Key Laboratory of Tribology, Tsinghua University, Beijing, China.
    Liu, Wenrui
    State Key Laboratory of Tribology, Tsinghua University, Beijing, China.
    Wang, Kunpeng
    State Key Laboratory of Tribology, Tsinghua University, Beijing, China.
    Li, Jinjin
    State Key Laboratory of Tribology, Tsinghua University, Beijing, China.
    Ma, Tianbao
    State Key Laboratory of Tribology, Tsinghua University, Beijing, China.
    Levent Eryilmaz, Osman
    Applied Materials Division, Argonne National Laboratory, Argonne, Illinois, United States.
    Shi, Yijun
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Erdemir, Ali
    Applied Materials Division, Argonne National Laboratory, Argonne, Illinois, United States.
    Luo, Jianbin
    State Key Laboratory of Tribology, Tsinghua University, Beijing, China.
    Superlubricity of Polyalkylene Glycol Aqueous Solutions Enabled by Ultrathin Layered Double Hydroxide Nanosheets2019In: ACS Applied Materials and Interfaces, ISSN 1944-8244, E-ISSN 1944-8252, Vol. 11, no 22, p. 20249-20256Article in journal (Refereed)
    Abstract [en]

    It was previously proved that the existence of a large amount of hydrogen ions in water-based lubricants can easily lead to a superlubric state; however, it was also shown that these hydrogen ions could cause severe corrosion. As part of a large family of attractive clays, layered double hydroxides (LDHs) possess excellent tribological properties in water-based lubrication systems. In the present work, two different kinds of LDHs are dispersed in polyalkylene glycol (PAG) aqueous solutions, in two distinct forms: ultrathin nanosheets (ULDH-NS) of ca. 60 nm wide and ca. 1 nm thick (single or double layer) and nanoparticles (LDH-NP) of ca. 19.73 nm wide and ca. 8.68 nm thick. We find that the addition of ULDH-NS greatly shortens (as much as 85%) the running-in period prior to reaching the superlubricity regime and increases the ultimate load-bearing capacity by about four times. As compared to the fluid film thickness of the lubricating PAG solution, their ultrathin longitudinal dimension will not impair or influence the fluid film coverage in the contact zone. The analysis of sliding solid surfaces and the atomic force microscope microscale friction test demonstrate that the adsorption of ULDH-NS enables the sliding solid surfaces to be polished and protected because of their relatively weak interlayer interaction and increased adhesion effect. Owing to their superior tribological properties as lubricant additives, ultrathin LDH nanosheets hold great potential for enabling liquid superlubricity in industrial applications in the future.

  • 48.
    Wu, Jian
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements. State Key Laboratory of Materials-oriented Chemical Engineering, Nanjing Tech University.
    Huang, Xianzhu
    State Key Laboratory of Materials-Oriented Chemical Engineering, Nanjing Tech University, Nanjing .
    Berglund, Kim
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Lu, Xiaohua
    Key Laboratory of Material and Chemical Engineering, Nanjing Tech University, Nanjing .
    Feng, Xin
    State Key Laboratory of Materials-Oriented Chemical Engineering, Nanjing Tech University, Nanjing .
    Larsson, Roland
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Shi, Yijun
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    CuO nanosheets produced in graphene oxide solution: An excellent anti-wear additive for self-lubricating polymer composites2018In: Composites Science And Technology, ISSN 0266-3538, E-ISSN 1879-1050, Vol. 162, p. 86-92Article in journal (Refereed)
    Abstract [en]

    In the present work, graphene oxide is used as template to produce CuO nanosheets, which solves aggregation and dispersion problems of CuO nanosheets resulting in improved lubricating performance. SEM and AFM studies show that CuO nanosheets are present in fusiform flake shape with a thickness, width and length of around 13, 400 and 1000 nm, respectively. CuO nanosheets were added to the carbon fibers reinforced Polytetrafluoroethylene (CF/PTFE) to study their lubricating performance. It is interesting, from fractured surfaces of composites, to find that CuO nanosheets enhance the interface properties between carbon fibers and PTFE. The wear resistance property of CF/PTFE is remarkably improved after filling CuO nanosheets. For example, the wear rate is reduced by 51% after filling 1.5 wt % CuO nanosheets. The wear resistance improvement effect of CuO nanosheets is much better than that of commercial CuO nanogranules and CuO nanorods. Worn surfaces and counter-surfaces studying indicates that CuO nanosheets can not only prevent the rubbed-off of PTFE or the detachment of CF, but also improve the properties of transfer films, which greatly reduce the adhesive wear and abrasive wear.

  • 49.
    Wu, Jian
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements. State Key Laboratory of Materials-Oriented Chemical Engineering, Nanjing University of Technology.
    Lu, Xiaohua
    State Key Laboratory of Materials-Oriented Chemical Engineering, Nanjing University of Technology, College of Chemistry and Chemical Engineering, Nanjing University of Technology.
    Feng, Xin
    State Key Laboratory of Materials-Oriented Chemical Engineering, College of Chemistry and Chemical Engineering, Nanjing University of Technology, Nanjing University of Chemical Technology, State Key Laboratory of Materials-Oriented Chemical Engineering, Nanjing University of Technology.
    Shi, Yijun
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Halogen-free ionic liquids as excellent lubricants for PEEK-stainless steel contacts at elevated temperatures2016In: Tribology International, ISSN 0301-679X, E-ISSN 1879-2464, Vol. 104, p. 1-9Article in journal (Refereed)
    Abstract [en]

    Advanced polymers sliding against steel under ionic liquids (ILs) has great potential applications. However, halogen-containing ILs (h-ILs) often induce high corrosive wear to steel. In this work, halogen-free ILs (hf-ILs) were used as neat lubricants to investigate the performance of poly(ether-ether-ketone)-AISI304 stainless steel (PEEK-304ss) contacts at elevated temperatures. The results indicate that hf-ILs with short alkyl chains show excellent anti-corrosion and antiwear properties. The wear rates of 304ss lubricated by hf-ILs at 160 °C are 3–71% of those by h-ILs. The friction coefficients of PEEK-304ss lubricated by hf-ILs are just 13–33% of those by h-ILs. The excellent performance of hf-ILs can be attributed to the reason that hf-ILs will not generate any halogen containing acid during the test

  • 50.
    Wu, Jian
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements. Nanjing Tech University, Nanjing, China.
    Mu, Liwen
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Feng, Xin
    Nanjing Tech University, Nanjing, China.
    Lu, Xiaohua
    Nanjing Tech University, Nanjing, China.
    Larsson, Roland
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Shi, Yijun
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Poly(alkylimidazolium bis(trifluoromethylsulfonyl) imide)-Based Polymerized Ionic Liquids: A Potential  High-Performance Lubricating Grease2019In: Advanced Materials Interfaces, ISSN 2196-7350, Vol. 6, no 5, article id 1801796Article in journal (Refereed)
    Abstract [en]

    Polymers prepared from ionic liquids are widely called polymerized ionic liquids (PILs). Compared to monocationic and dicationic ILs, PILs have higher molecular weights, charge, and greater intermolecular interactions, which make PILs have a higher possibility to generate better lubricity. PILs of poly‐alkylimidazolium bis(trifluoromethylsulfonyl)imide (PImC6NTf2) is studied herein. Dicationic ILs of 1,1′‐(pentane‐1,5‐diyl)‐bis(3‐butylimidazolium) bis(trifluoromethylsulfonyl)imide (BIm5‐(NTf2)2) is used as additive to decrease the crystallization temperature of PImC6NTf2. Lubricity of PImC6NTf2 and PImC6NTf2+BIm5‐(NTf2)2, as well as BIm5‐(NTf2)2 for comparison is evaluated under severe conditions, i.e., 3.0 to 3.5 GPa and 200 °C. The rheological study suggests that PImC6NTf2 can be classified into grease. Tribological test results show that PImC6NTf2 has much better antiwear property than BIm5‐(NTf2)2, especially at 3.5 GPa. Adding 4% BIm5‐(NTf2)2 to PImC6NTf2 is able to reduce friction under high pressure. At 200 °C, PImC6NTf2 exhibits excellent lubricity. The mixture of 96%PImC6NTf2+4%BIm5‐(NTf2)2 shows even better antiwear property than neat PImC6NTf2 and exhibits the highest friction reducing property among the ILs at 200 °C. It is speculated that the robust strength of PILs and strong adhesion between PILs and solids are key factors in achieving the excellent antiwear property.

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