Change search
Refine search result
1234567 1 - 50 of 8248
CiteExportLink to result list
Permanent link
Cite
Citation style
  • apa
  • harvard1
  • ieee
  • modern-language-association-8th-edition
  • vancouver
  • Other style
More styles
Language
  • de-DE
  • en-GB
  • en-US
  • fi-FI
  • nn-NO
  • nn-NB
  • sv-SE
  • Other locale
More languages
Output format
  • html
  • text
  • asciidoc
  • rtf
Rows per page
  • 5
  • 10
  • 20
  • 50
  • 100
  • 250
Sort
  • Standard (Relevance)
  • Author A-Ö
  • Author Ö-A
  • Title A-Ö
  • Title Ö-A
  • Publication type A-Ö
  • Publication type Ö-A
  • Issued (Oldest first)
  • Issued (Newest first)
  • Created (Oldest first)
  • Created (Newest first)
  • Last updated (Oldest first)
  • Last updated (Newest first)
  • Disputation date (earliest first)
  • Disputation date (latest first)
  • Standard (Relevance)
  • Author A-Ö
  • Author Ö-A
  • Title A-Ö
  • Title Ö-A
  • Publication type A-Ö
  • Publication type Ö-A
  • Issued (Oldest first)
  • Issued (Newest first)
  • Created (Oldest first)
  • Created (Newest first)
  • Last updated (Oldest first)
  • Last updated (Newest first)
  • Disputation date (earliest first)
  • Disputation date (latest first)
Select
The maximal number of hits you can export is 250. When you want to export more records please use the Create feeds function.
  • 1.
    Abadei, S.
    et al.
    Chalmers University of Technology, Department of Microelectronics.
    Gevorgian, S.
    Chalmers University of Technology, Department of Microelectronics.
    Kugler, V.
    Department of Physics, Linköping University.
    Helmersson, U.
    Department of Physics, Linköping University.
    Andreasson, Johanna
    Luleå University of Technology, Department of Engineering Sciences and Mathematics.
    Microwave properties of tunable capacitors basee on magnetron sputtered ferroelectric Na0.5K0.5NbO3 film on low and high resistivity silicon substrates2001In: Integrated Ferroelectrics, ISSN 1058-4587, E-ISSN 1607-8489, Vol. 39, no 1-4, p. 359-366Article in journal (Refereed)
    Abstract [en]

    In this work, small signal DC voltage dependent dielectric permittivity, loss tangent, and tuneability of magnetron sputtered epitaxial Na0.5K0.5NO3 films are studied experimentally. (100)-oriented Na0.5K0.5NbO3 films are deposited onto SiO2-buffered CMOS grade low resistivity (p = 10-20 cm) and high resistivity (p = 15-45 kcm) silicon substrates. Planar capacitors with 2 or 4 m gaps between electrodes have been fabricated on top of ferroelectric films. These devices have been characterized in the frequency range 1.0 MHz to 50 GHz at temperatures 30 - 300K. Na0.5K0.5NbO3/SiO2/Si structures on high resistivity silicon substrate exhibit C-V performances typical for Metal-Insulator- Semiconductor (MIS) capacitors. At low frequencies, f 1.0 GHz, the large tuneability and large losses are associated with the MIS structure, while at higher microwave frequencies the tuneability is mainly associated with the ferroelectric, film. At 1.0 MHz and room temperature, the tuneability of Na0.5K0.5NbO3/SiO2/Si structures more than 90%, reducing to 10-15 % at 50 GHz. The losses decrease with increasing the DC bias and frequency. A Q-factor more than 15 at 50 GHz is observed. The dielectric permittivity of the Na0.5K0.5NbO3 film is in the range 50-150 at frequencies 0.045-50 GHz. On low resistivity substrate the performance of Na0.5K0.5NbO3 films is completely screened by the high losses in silicon, and the tuneability is negligible

  • 2.
    Abdikalikova, Zamira
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mathematical Science.
    Compactness of embedding between Sobolev type spaces with multiweighted derivatives2009In: AIHT : Analysis, Inequalities and Homogenization Theory: Midnight sun conference in honor of Lars-Erik Persson, 2009Conference paper (Other academic)
    Abstract [en]

    We consider a new Sobolev type function space called the space with multiweighted derivatives. As basis for this space serves some differential operators containing weight functions. We establish necessary and sufficient conditions for the boundedness and compactness of the embedding between the spaces with multiweighted derivatives in different selections of weights.

  • 3.
    Abdikalikova, Zamira
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mathematical Science.
    Embedding theorems for spaces with multiweighted derivatives2007Licentiate thesis, comprehensive summary (Other academic)
    Abstract [en]

    This Licentiate Thesis consists of four chapters, which deal with a new Sobolev type function space called the space with multiweighted derivatives. This space is a generalization of the usual one dimensional Sobolev space. Chapter 1 is an introduction, where, in particular, the importance to study function spaces with weights is discussed and motivated. In Chapter 2 we consider and analyze some results of L. D. Kudryavtsev, where he investigated one dimensional Sobolev spaces. Moreover, in this chapter we present and prove analogous results by B. L. Baidel'dinov for generalized Sobolev spaces. These results are crucially for the proofs of the main results of this Licentiate Thesis. In Chapter 3 we prove some embedding theorems for these new generalized Sobolev spaces. The main results of Kudryavtsev and Baidel'dinov about characterization of the behavior of functions at a singularity take place in weak degeneration of spaces. However, with the help of our new embedding theorems we can extend these results to the case of strong degeneration. In Chapter 4 we prove some new estimates for each function in a Tchebychev system. In order to be able to study also compactness of the embeddings from Chapter 3 such estimates are crucial. I plan to study this question in detail in my further PhD studies.

  • 4.
    Abdikalikova, Zamira
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mathematical Science.
    Some new results concerning boundedness and compactness for embeddings between spaces with multiweighted derivatives2009Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    This Doctoral Thesis consists of five chapters, which deal with a new Sobolev type function space called the space with multiweighted derivatives. This space is a generalization of the usual one dimensional Sobolev space. As basis for this space serves some differential operators containing weight functions.Chapter 1 is an introduction, where, in particular, the importance to study function spaces with weights is discussed and motivated. In Chapter 2 we prove some new estimates for each function in a Tchebychev system. In order to be able to study compactness of the embeddings from Chapter 3 such estimates are crucial.In Chapter 3 we rewrite and present some results of L. D. Kudryavtsev, where he investigated one dimensional Sobolev spaces. Moreover, in this chapter we rewrite and discuss some analogous results by B. L. Baidel'dinov for generalized Sobolev spaces. These results are not available in the Western literatures in this way and they are crucial for the proofs of the main results in Chapter 4. In Chapter 4 we prove some embedding theorems for these new generalized Sobolev spaces. The main results of Kudryavtsev and Baidel'dinov about characterization of the behavior of functions at a singularity take place in weak degeneration of the spaces. However, with the help of our new embedding theorems we can extend theseresults to the case of strong degeneration.The main aim of Chapter 5 is to establish boundedness and compactness of the embedding considered in Chapter 4.In Chapter 4 basically only sufficient conditions for boundedness of this embedding were obtained. In Chapter 5 we obtain necessary and sufficient conditions for boundedness and compactness of this embedding and the main results are proved in a different way.

  • 5.
    Abdikalikova, Zamira
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mathematical Science.
    Kalybay, Aigerim
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mathematical Science.
    Summability of a Tchebysheff system of functions2007Report (Other academic)
  • 6.
    Abdikalikova, Zamira
    et al.
    L.N. Gumilyov Eurasian National University.
    Oinarov, Ryskul
    L.N. Gumilyov Eurasian National University.
    Persson, Lars-Erik
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mathematical Science.
    Boundedness and compactness of the embedding between spaces with multiweighted derivatives when 12011In: Czechoslovak Mathematical Journal, ISSN 0011-4642, E-ISSN 1572-9141, Vol. 61, no 1, p. 7-26Article in journal (Refereed)
    Abstract [en]

    We consider a new Sobolev type function space called the space with multiweighted derivatives W-p(n),(alpha) over bar, where (alpha) over bar = (alpha(0), alpha(1), ......, alpha(n)), alpha(i) is an element of R, i = 0, 1,......,n, and parallel to f parallel to W-p(n),((alpha) over bar) = parallel to D((alpha) over bar)(n)f parallel to(p) + Sigma(n-1) (i=0) vertical bar D((alpha) over bar)(i)f(1)vertical bar, D((alpha) over bar)(0)f(t) = t(alpha 0) f(t), d((alpha) over bar)(i)f(t) = t(alpha i) d/dt D-(alpha) over bar(i-1) f(t), i = 1, 2, ....., n. We establish necessary and sufficient conditions for the boundedness and compactness of the embedding W-p,(alpha) over bar(n) -> W-q,(beta) over bar,(m) when 1 <= q < p < infinity, 0 <= m < n

  • 7.
    Abdikalikova, Zamira
    et al.
    L.N. Gumilyov Eurasian National University.
    Oinarov, Ryskul
    L.N. Gumilyov Eurasian National University.
    Persson, Lars-Erik
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mathematical Science.
    Boundedness and compactness of the embedding between spaces with multiweighted derivatives when 1≤ q2009Report (Other academic)
  • 8.
    Abid, Fahim
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Energy Science.
    Busatto, Tatiano
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Energy Science.
    Rönnberg, Sarah
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Energy Science.
    Bollen, Math
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Energy Science.
    Intermodulation due to interaction of photovoltaic inverter and electric vehicle at supraharmonic range2016In: 2016 17th International Conference on Harmonics and Quality of Power, Piscataway. NJ, 2016, p. 685-690, article id 7783471Conference paper (Refereed)
    Abstract [en]

    Advent of power electronic switching is introducing more and more non-linear loads in the low voltage grid. Besides harmonic current generation in the frequency range below 2 kHz, these non-linear loads are also responsible for current emission in the range of 2 kHz to 150 kHz, commonly known as supraharmonic emission. Supraharmonic currents mainly flow between nearby appliances and heavily influence the overall emission of neighboring devices. This paper presents an analysis of supraharmonic interaction between a photovoltaic inverter and an electric vehicle. It has been noticed that intermodulation distortion arises as a result of interaction between different switching frequencies used by the devices. Later, additional household equipment were added to photovoltaic and electric vehicle to observe their effect on intermodulation distortion. All the measurements were conducted in a controlled laboratory environment imitating a domestic customer.

  • 9.
    Abiri, Olufunminiyi
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mechanics of Solid Materials.
    Non-local models in manufacturing simulations2016Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    Ductile fracture presents challenges with respect to material modelling andnumerical simulations of localization. The strain and damage localization maybe unwanted as it indicates a failure in the process or, as in the case ofmachining and cutting, a wanted phenomenon to be controlled. The latterrequires a higher accuracy regarding the modelling of the underlying coupledplastic and fracturing/damage behaviour of the material, metal in the currentcontext as well as the stability and robustness of the simulation procedure.This aim of this work is to develop, evaluate and implement formulations thatcan efficiently and reliably handle localization problems in machiningsimulations. The focus is on non-local models. The non-local models extendthe standard continuum mechanics theory by using non-local continuumtheory in order to achieve mesh independent results when simulating fractureor shear localization.The non-local damage model is implemented and various formulations areevaluated in a Matlab™ based finite element code. The chosen algorithm wasthen implemented in commercial software. The implementations remedy themesh sensitivity problem and gives convergent solution for metal cuttingsimulations with reasonable cost. The length scale associated with the nonlocalmodels are in the current context considered as a numericalregularization parameter. The model has been applied in machiningsimulations and compared with measurements from industry.Keywords: Finite element simulation; Non-local damage; Plasticity; Machining

  • 10.
    Abiri, Olufunminiyi
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mechanics of Solid Materials.
    Simplifications of non-local damage models2014Licentiate thesis, comprehensive summary (Other academic)
    Abstract [en]

    Ductile fracture presents challenges with respect to material modelling and numerical simulations of localization. The strain and damage localization may be unwanted as it indicates a failure in the process or, as in the case of machining and cutting, a wanted phenomenon to be controlled. The latter requires a higher accuracy regarding the modelling of the underlying coupled plastic and fracturing/damage behaviour of the material, metal in the current context as well as the robustness of the simulation procedure. The focus of this thesis is on efficient and reliable finite element solution of the localization problem through the non-local damage model. The non-local damage model extends the standard continuum mechanics theory by using non-local continuum theory in order to achieve mesh independent results when simulating fracture or shear localization. In this work, the non-local damage model and its various simplifications are evaluated in an in-house finite element code developed using Matlab™. The accuracy, robustness, efficiency and costs of the models are investigated and also compared to a general multi-length scale finite element formulation. A numerical study versus published data is used to demonstrate the validity of the model. The explicit non-local damage variant will be implemented in a commercial finite element code for use in machining simulation

  • 11.
    Abiri, Olufunminiyi
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mechanics of Solid Materials.
    Lindgren, Lars-Erik
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mechanics of Solid Materials.
    Non-local damage models in manufacturing simulations2013Conference paper (Refereed)
    Abstract [en]

    Localisation of deformation is a problem in several manufacturing processes. Machining is an exception where it is a wanted feature. However, it is always a problem in finite element modelling of these processes due to mesh sensitivity of the computed results. The remedy is to incorporate a length scale into the numerical formulations in order to achieve convergent solutions. Different simplifications in the implementation of a non-local damage model are evaluated with respect to temporal and spatial discretisation to show the effect of different approximations on accuracy and convergence.

  • 12.
    Abiri, Olufunminiyi
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mechanics of Solid Materials.
    Lindgren, Lars-Erik
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mechanics of Solid Materials.
    Non-local damage models in manufacturing simulations2015In: European journal of mechanics. A, Solids, ISSN 0997-7538, E-ISSN 1873-7285, Vol. 49, p. 548-560Article in journal (Refereed)
    Abstract [en]

    Localisation of deformation is a problem in several manufacturing processes. Machining is an exception where it is a wanted feature. However, it is always a problem in finite element modelling of these processes due to mesh sensitivity of the computed results. The remedy is to incorporate a length scale into the numerical formulations in order to achieve convergent solutions. Different simplifications in the implementation of a non-local damage model are evaluated with respect to temporal and spatial discretisation to show the effect of different approximations on accuracy and convergence.

  • 13.
    Abiri, Olufunminiyi
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mechanics of Solid Materials.
    Qin, Hao
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mechanics of Solid Materials.
    Lindgren, Lars-Erik
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mechanics of Solid Materials.
    Comparison of Multiresolution Continuum Theory and Nonlocal Dame model for use in Simulation of Manufacutring Processes2016In: International Journal for Multiscale Computational Engineering, ISSN 1543-1649, Vol. 14, no 1, p. 81-94Article in journal (Refereed)
    Abstract [en]

    Modelling and simulation of manufacturing processes may require the capability to account for localization behavior, often associated with damage/fracture. It may be unwanted localization indicating a failure in the process or, as in the case of machining and cutting, a wanted phenomenon to be controlled. The latter requires a higher accuracy regarding the modelling of the underlying physics, as well as the robustness of the simulation procedure. Two different approaches for achieving mesh-independent solutions are compared in this paper. They are the multiresolution continuum theory (MRCT) and nonlocal damage model. The MRCT theory is a general multilength-scale finite element formulation, while the nonlocal damage model is a specialized method using a weighted averaging of softening internal variables over a spatial neighborhood of the material point. Both approaches result in a converged finite element solution of the localization problem upon mesh refinement. This study compares the accuracy and robustness of their numerical schemes in implicit finite element codes for the plane strain shear deformation test case. Final remarks concerning ease of implementation of the methods in commercial finite element packages are also given.

  • 14.
    Abiri, Olufunminiyi
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mechanics of Solid Materials. University of Johannesburg, South Africa.
    Svoboda, Ales
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mechanics of Solid Materials.
    Lindgren, Lars-Erik
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mechanics of Solid Materials.
    Wedberg, Dan
    Controlling Thermal Softening Using Non-Local Temperature Field in Modelling2016In: Journal of Machining and Forming Technologies, ISSN 1947-4369, Vol. 8, no 1-2, p. 13-28Article in journal (Refereed)
    Abstract [en]

    One of the aims of this work is to show that thermal softening due to the reduced flow strength of a material with increasing temperature may cause chip serrations to form during machining. The other purpose, the main focus of the paper, is to demonstrate that a non-local temperature field can be used to control these serrations. The non-local temperature is a weighted average of the temperature field in the region surrounding an integration point. Its size is determined by a length scale. This length scale may be based on the physics of the process but is taken here as a regularization parameter.

  • 15.
    Abiri, Olufunminiyi
    et al.
    Institute of Intelligent Systems, University of Johannesburg.
    Wedberg, Dan
    AB Sandvik Coromant.
    Svoboda, Ales
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mechanics of Solid Materials.
    Lindgren, Lars-Erik
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mechanics of Solid Materials.
    Non-Local Modelling of Strain Softening in Machining Simulations2017In: IOP Conference Series: Materials Science and Engineering, ISSN 1757-8981, E-ISSN 1757-899X, Vol. 225, article id 012053Article in journal (Refereed)
    Abstract [en]

    Non-local damage model for strain softening in a machining simulation is presented in this paper. The coupled damage-plasticity model consists of a physically based dislocation density model and a damage model driven by plastic straining in combination with the stress state. The predicted chip serration is highly consistent with the measurement results. 

  • 16.
    Abrahamsson, Lars
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Energy Science.
    AC cables strengthening low frequency AC railway with purely active power loads2018Conference proceedings (editor) (Refereed)
    Abstract [en]

    Converter-fed railway power systems traditionally use high voltage AC overhead transmission lines in the railway grid for increased loadability. An increased resistance to overhead high voltage AC transmission lines, may motivate cables as one alternative for the future. The focus of this paper is to compare a non-strengthened system with two cable solutions for comparing loadability, voltages, and reactive powers for different levels of load scaling. The studies confirmed that the obstacle of reactive power produced in lowly utilized cables is, even if less significant for low frequency AC, still present. A simplified load model is used representing trains with VSC-converters and three-phase motors as purely active loads regardless of motoring or regenerating. A previous study has been done on thyristor-based trains. The voltage levels while regenerating are higher than in the thyristor-train case study. Besides that the loadability for motoring and regenerating is higher with modern VSC-trains.

  • 17.
    Abrahamsson, Lars
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Energy Science.
    AC cables strengthening railway low frequency AC power supply systems: a deepened study2017Conference paper (Refereed)
    Abstract [en]

    n railway power supply systems using ACfrequencies lower than the public grids of 50/60 Hz,high voltage AC overhead transmission lines in railwaygrid frequency are used as one measure of strengthen-ing the systems. An increased resistance to overheadhigh voltage AC transmission lines, may motivate ca-bles for future railway power systems.With the frequency of 50/60 Hz, reactive powerproduced in lowly utilized cables imposes an obstacle.For low frequency AC, this issue is less significant.Moreover, in converter-fed railways, no reactive powerwill leak into the feeding public grid.This paper studies AC cables in low-frequency ACrailway. Two reinforcement cable solutions are com-pared with no reinforcement. A simplified load model oftrains, with thyristor bridges and DC motors, is used

  • 18.
    Abrahamsson, Lars
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Energy Science.
    Administrativ bilaga till Slutrapport2017Report (Other academic)
  • 19.
    Abrahamsson, Lars
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Energy Science.
    Considering risks in power system operationand the consequences of different acceptedrisk levels2017Report (Other academic)
  • 20.
    Abrahamsson, Lars
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Energy Science.
    Optimal drift och dimensionering avbanmatningssystem: Slutrapport Elektraprojekt2016Report (Other academic)
  • 21.
    Abrahamsson, Lars
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Energy Science.
    Risker i drift av elkraftsystem och konsekvenser av olika accepterade risknivåer: Svensk kortversion2017Report (Other academic)
  • 22.
    Abrahamsson, Lars
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Energy Science.
    Slutrapport2017Report (Other academic)
  • 23.
    Abrahamsson, Lars
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Energy Science.
    Belea, Radu
    Atkins Sverige AB.
    Klerfors, Bertil
    Chalmers Technical University.
    Schütte, Thorsten
    STRI AB.
    Warner, Bruce
    ABB Switzerland Ltd.
    Modern methods for balancing of single phase loads when feeding a.c. Electrified railways2017In: Eb - Elektrische Bahnen, ISSN 0013-5437, Vol. 115, no 6-7, p. 378-384Article in journal (Refereed)
    Abstract [en]

    Conventional compensation circuits for phase balancing of single phase AC railways for 50 or 60 Hz have high cost for the associated transformers. A new version of the classical Steinmetz scheme, using three single phase transformers connected as a W with the phase angles -60°, 0° and +60°, can reuse the two standard single phase transformers of a V-connected feeding station with a third identical transformer added. Power factor compensation can easily be included. For neutral sections between different feeding systems, a neutral section converter is proposed

  • 24.
    Abrahamsson, Lars
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Energy Science.
    Serrano Jimenez, Daniel
    University Carlos III.
    Laury, John
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Energy Science.
    Bollen, Math
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Energy Science.
    AC cables strengthening railway low frequency AC power supplysystems2017In: ASME/IEEE 2017 Joint Rail Conference, ASME Press, 2017Conference paper (Refereed)
    Abstract [en]

    In present-day railway power supply systems using an AC frequency lower than the one in the public power system of 50/60 Hz, high voltage overhead transmission lines are used as one measure of strengthening the railway power supply system grids. This option may be economically beneficial, compared to strengthening the grid purely by increasing the density of converter stations or increasing the cross section areas of the overhead catenary wires. High voltage AC transmission lines in the railway power supply system allow larger distances between converter stations than would otherwise be possible for a given amount of train traffic. Moreover, the introduction of AC transmission lines implies reduced line losses and reduced voltage level fluctuations at the catenary for a given amount of train traffic. However, due to the increased public and government resistance for additional overhead high voltage AC transmission lines in general, different alternatives will be needed for the future improvements and strengthening of railway power systems. For a more sustainable transport sector, the share and amount of railway traffic needs to increase, in which case such a strengthening becomes inevitable. Earlier, usage of VSC-HVDC transmission cables has been proposed as one alternative to overhead AC transmission lines. One of the main benefits with VSC-HVDC transmission is that control of power flows in the railway power systems is easier and that less converter capacity may be needed. Technically, VSC-HVDC transmission for railway power systems is a competitive solution as it offers a large variety of control options. However, there might be other more economical alternatives reducing the overall impedance in the railway power system. In public power systems with the frequency of 50/60 Hz, an excess of reactive power production in lowly utilized cables imposes an obstacle in replacing overhead transmission lines with cables. In low frequency AC railway power system, the capacitive properties are less significant allowing longer cables compared to 50/60 Hz power systems. Moreover, in converter-fed railways, some kind of reactive compensation will automatically be applied during low-load. At each converter station, voltage control is already present following the railway operation tradition. Therefore, in this paper, we propose AC cables as a measure of strengthening low-frequency AC railway power systems. The paper compares the electrical performances of two alternative reinforcement cable solutions with the base case of no reinforcement. The options of disconnecting or toggling the cables at low load as well as the automatic reactive compensation by converter voltage control are considered. Losses and voltage levels are compared for the different solutions. Investment costs and other relevant issues are discussed.

  • 25.
    Abrahamsson, Lena
    et al.
    Luleå University of Technology, Department of Business Administration, Technology and Social Sciences, Human Work Science.
    Kaplan, Alexander
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Product and Production Development.
    Johansson, Jan
    Luleå University of Technology, Department of Business Administration, Technology and Social Sciences, Human Work Science.
    Rask, Kjell
    Luleå University of Technology, Department of Business Administration, Technology and Social Sciences, Human Work Science.
    Fältholm, Ylva
    Luleå University of Technology, Department of Business Administration, Technology and Social Sciences, Human Work Science.
    Kumar, Uday
    Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Operation, Maintenance and Acoustics.
    Bergquist, Bjarne
    Projekt: LUPO - globala länkar2010Other (Other (popular science, discussion, etc.))
  • 26.
    Abramovic, Shoshana
    et al.
    University of Haifa.
    Persson, Lars-Erik
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mathematical Science. University of Tromsø ; The Arctic University of Norway, Narvik.
    Fejer and Hermite–Hadamard Type Inequalitiesfor N-Quasiconvex Functions2017In: Mathematical notes of the Academy of Sciences of the USSR, ISSN 0001-4346, E-ISSN 1573-8876, Vol. 102, no 5-6, p. 599-609Article in journal (Refereed)
    Abstract [en]

    Abstract—Some new extensions and refinements of Hermite–Hadamard and Fejer type inequali-ties for functions which are N-quasiconvex are derived and discussed.

  • 27.
    Abramovich, S.
    et al.
    Department of Mathematics, University of Haifa, Haifa, Israel.
    Persson, Lars-Erik
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mathematical Science. UIT The Arctic University of Norway, Narvik, Norway.
    Extensions and Refinements of Fejer and Hermite–Hadamard Type Inequalities2018In: Mathematical Inequalities & Applications, ISSN 1331-4343, E-ISSN 1848-9966, Vol. 21, no 3, p. 759-772Article in journal (Refereed)
    Abstract [en]

    In this paper extensions and refinements of Hermite-Hadamard and Fejer type inequalities are derived including monotonicity of some functions related to the Fejer inequality and extensions for functions, which are 1-quasiconvex and for function with bounded second derivative. We deal also with Fejer inequalities in cases that p, the weight function in Fejer inequality, is not symmetric but monotone on [a, b] .

  • 28.
    Abramovich, Shosana
    et al.
    Department of Mathematics, University of Haifa.
    Persson, Lars-Erik
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mathematical Science.
    Some New Refined Hardy Type Inequalities with Breaking Points p = 2 or p = 32014In: Concrete Operators, Spectral Theory, Operators in Harmonic Analysis and Approximation: 22nd International Workshop in Operator Theory and its Applications, Sevilla, July 2011 / [ed] Manuel Cepedello Boiso; Håkan Hedenmalm; Marinus A. Kaashoek; Alfonso Montes Rodríguez; Sergei Treil, Basel: Encyclopedia of Global Archaeology/Springer Verlag, 2014, p. 1-10Conference paper (Refereed)
    Abstract [en]

    For usual Hardy type inequalities the natural “breaking point” (the parameter value where the inequality reverses) is p = 1. Recently, J. Oguntuase and L.-E. Persson proved a refined Hardy type inequality with breaking point at p = 2. In this paper we show that this refinement is not unique and can be replaced by another refined Hardy type inequality with breaking point at p = 2. Moreover, a new refined Hardy type inequality with breaking point at p = 3 is obtained. One key idea is to prove some new Jensen type inequalities related to convex or superquadratic funcions, which are also of independent interest

  • 29.
    Abramovich, Shoshana
    et al.
    Department of Mathematics, University of Haifa.
    Krulić, Kristina
    Faculty of Textile Technology, University of Zagreb.
    Pečarić, Josip
    Faculty of Textile Technology, University of Zagreb.
    Persson, Lars-Erik
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mathematical Science.
    Some new refined Hardy type inequalities with general kernels and measures2010In: Aequationes Mathematicae, ISSN 0001-9054, E-ISSN 1420-8903, Vol. 79, no 1-2, p. 157-172Article in journal (Refereed)
    Abstract [en]

    We state and prove some new refined Hardy type inequalities using the notation of superquadratic and subquadratic functions with an integral operator Ak defined by, where k: Ω1 × Ω2 is a general nonnegative kernel, (Ω1, μ1) and (Ω2, μ2) are measure spaces and,. The relations to other results of this type are discussed and, in particular, some new integral identities of independent interest are obtained.

  • 30.
    Abramovich, Shoshana
    et al.
    Department of Mathematics, University of Haifa.
    Persson, Lars-Erik
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mathematical Science.
    Inequalities for averages of quasiconvex and superquadratic functions2016In: Mathematical Inequalities & Applications, ISSN 1331-4343, E-ISSN 1848-9966, Vol. 19, no 2, p. 535-550Article in journal (Refereed)
    Abstract [en]

    For n ε ℤ+ we consider the difference Bn-1 (f)-Bn(f):= 1/an n-1/ηi=0 f(ai/an-1)-1/an+1 nηi=0f(ai/an) where the sequences{ai} and {ai-ai-1} are increasing. Some lower bounds are derived when f is 1-quasiconvex and when f is a closely related superquadratic function. In particular, by using some fairly new results concerning the so called "Jensen gap", these bounds can be compared. Some applications and related results about An-1 (f)-An(f):= 1/an n-1/ηi=0 f(ai/an-1)-1/an+1 nηi=0f(ai/an) are also included.

  • 31.
    Abramovich, Shoshana
    et al.
    Department of Mathematics, University of Haifa.
    Persson, Lars-Erik
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mathematical Science.
    Some new estimates of the ‘Jensen gap’2016In: Journal of inequalities and applications (Print), ISSN 1025-5834, E-ISSN 1029-242X, Vol. 2016, article id 39Article in journal (Refereed)
    Abstract [en]

    Let (μ,Ω) be a probability measure space. We consider the so-called ‘Jensen gap’ J(φ,μ,f)=∫ Ω φ(f(s))dμ(s)−φ(∫ Ω f(s)dμ(s)) for some classes of functions φ. Several new estimates and equalities are derived and compared with other results of this type. Especially the case when φ has a Taylor expansion is treated and the corresponding discrete results are pointed out.

  • 32.
    Abramovich, Shoshana
    et al.
    University of Haifa.
    Persson, Lars-Erik
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mathematical Science.
    Some new scales of refined Hardy type inequalities via functions related to superquadracity2013In: Mathematical Inequalities & Applications, ISSN 1331-4343, E-ISSN 1848-9966, Vol. 16, no 3, p. 679-695Article in journal (Refereed)
    Abstract [en]

    For the Hardy type inequalities the "breaking point" (=the point where the inequality reverses) is p = 1. Recently, J. Oguntoase and L. E. Persson proved a refined Hardy type inequality with a breaking point at p = 2. In this paper we prove a new scale of refined Hardy type inequality which can have a breaking point at any p ≥ 2. The technique is to first make some further investigations for superquadratic and superterzatic functions of independent interest, among which, a new Jensen type inequality is proved

  • 33.
    Abramovich, Shoshana
    et al.
    University of Haifa, Department of Mathematics.
    Persson, Lars-Erik
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mathematical Science.
    Pecaric, Josip
    University of Zagreb.
    Varosanec, Sanja
    University of Zagreb.
    General inequalities via isotonic subadditive functionals2007In: Mathematical Inequalities & Applications, ISSN 1331-4343, E-ISSN 1848-9966, Vol. 10, no 1, p. 15-28Article in journal (Refereed)
    Abstract [en]

    In this manuscript a number of general inequalities for isotonic subadditive functionals on a set of positive mappings are proved and applied. In particular, it is pointed out that these inequalities both unify and generalize some general forms of the Holder, Popoviciu, Minkowski, Bellman and Power mean inequalities. Also some refinements of some of these results are proved.

  • 34.
    Abramovich, Shoshana
    et al.
    University of Haifa.
    Persson, Lars-Erik
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mathematical Science.
    Samko, Natasha
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mathematical Science.
    On some new developments of Hardy-type inequalities2012In: 9th International Conference on Mathematical Problems in Engineering, Aerospace and Sciences: ICNPAA 2012 / [ed] Seenith Sivasundaram, Melville, NY: American Institute of Physics (AIP), 2012, p. 739-746Conference paper (Refereed)
    Abstract [en]

    In this paper we present and discuss some new developments of Hardy-type inequalities, namely to derive (a) Hardy-type inequalities via a convexity approach, (b) refined scales of Hardy-type inequalities with other “breaking points” than p = 1 via superquadratic and superterzatic functions, (c) scales of conditions to characterize modern forms of weighted Hardy-type inequalities.

  • 35.
    Abramovich, Shoshana
    et al.
    Department of Mathematics, University of Haifa.
    Persson, Lars-Erik
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mathematical Science.
    Samko, Natasha
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mathematical Science.
    On γ-quasiconvexity, superquadracity and two-sided reversed Jensen type inequalities2015In: Mathematical Inequalities & Applications, ISSN 1331-4343, E-ISSN 1848-9966, Vol. 18, no 2, p. 615-627Article in journal (Refereed)
    Abstract [en]

    In this paper we deal with γ -quasiconvex functions when −1γ 0, to derive sometwo-sided Jensen type inequalities. We also discuss some Jensen-Steffensen type inequalitiesfor 1-quasiconvex functions. We compare Jensen type inequalities for 1-quasiconvex functionswith Jensen type inequalities for superquadratic functions and we extend the result obtained forγ -quasiconvex functions to more general classes of functions.

  • 36.
    Abramovich, Shoshana
    et al.
    Department of Mathematics, University of Haifa.
    Persson, Lars-Erik
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mathematical Science.
    Samko, Natasha
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mathematical Science.
    Some new scales of refined Jensen and Hardy type inequalities2014In: Mathematical Inequalities & Applications, ISSN 1331-4343, E-ISSN 1848-9966, Vol. 17, no 3, p. 1105-1114Article in journal (Refereed)
    Abstract [en]

    Some scales of refined Jensen and Hardy type inequalities are derived and discussed. The key object in our technique is ? -quasiconvex functions K(x) defined by K(x)x-? =? (x) , where Φ is convex on [0,b) , 0 < b > ∞ and γ > 0.

  • 37.
    Abylayeva, Akbota
    Luleå University of Technology, Department of Engineering Sciences and Mathematics.
    Inequalities for some classes of Hardy type operators and compactness in weighted Lebesgue spaces2016Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    This PhD thesis is devoted to investigate weighted differential Hardy inequalities and Hardy-type inequalities with the kernel when the kernel has an integrable singularity, and also the additivity of the estimate of a Hardy type operator with a kernel.The thesis consists of seven papers (Papers 1, 2, 3, 4, 5, 6, 7) and an introduction where a review on the subject of the thesis is given. In Paper 1 weighted differential Hardy type inequalities are investigated on the set of compactly supported smooth functions, where necessary and sufficient conditions on the weight functions are established for which this inequality and two-sided estimates for the best constant hold. In Papers 2, 3, 4 a more general class of -order fractional integrationoperators are considered including the well-known classical Weyl, Riemann-Liouville, Erdelyi-Kober and Hadamard operators. Here 0 <  < 1. In Papers 2 and 3 the boundedness and compactness of two classes of such operators are investigated namely of Weyl and Riemann-Liouville type, respectively, in weighted Lebesgue spaces for 1 < p ≤ q < 1 and 0 < q < p < ∞. As applications some new results for the fractional integration operators of Weyl, Riemann-Liouville, Erdelyi-Kober and Hadamard are given and discussed.In Paper 4 the Riemann-Liouville type operator with variable upper limit is considered. The main results are proved by using a localization method equipped with the upper limit function and the kernel of the operator. In Papers 5 and 6 the Hardy operator with kernel is considered, where the kernel has a logarithmic singularity. The criteria of the boundedness and compactness of the operator in weighted Lebesgue spaces are given for 1 < p ≤ q < ∞ and 0 < q < p < ∞, respectively. In Paper 7 we investigated the weighted additive estimates for integral operators K+ and K¯ defined by

    K+ ƒ(x) := ∫ k(x,s) ƒ(s)ds,  K¯ ƒ(x) := ∫ k(x,s)ƒ(s)ds.

    It is assumed that the kernel k of the operators K+and K- belongs to the general Oinarov class. We derived the criteria for the validity of these addittive estimates when 1 ≤ p≤ q < ∞

  • 38.
    Abylayeva, Akbota M.
    et al.
    Department of Mechanics and Mathematics, L.N. Gumilyov Eurasian National University, Astana .
    Oinarov, Ryskul
    Department of Mechanics and Mathematics, L.N. Gumilyov Eurasian National University, Astana.
    Persson, Lars-Erik
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mathematical Science.
    Boundedness and compactness of a class of Hardy type operators2016In: Journal of inequalities and applications (Print), ISSN 1025-5834, E-ISSN 1029-242X, no 1, article id 324Article in journal (Refereed)
    Abstract [en]

    We establish characterizations of both boundedness and of compactness of a general class of fractional integral operators involving the Riemann-Liouville, Hadamard, and Erdelyi-Kober operators. In particular, these results imply new results in the theory of Hardy type inequalities. As applications both new and well-known results are pointed out.

  • 39.
    Abylayeva, Akbota M.
    et al.
    Department of Mechanics and Mathematics, L.N. Gumilyov Eurasian National University, Astana .
    Persson, Lars-Erik
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mathematical Science. UiT, Tromso.
    Hardy type inequalities and compactness of a class of integral operators with logarithmic singularities2018In: Mathematical Inequalities & Applications, ISSN 1331-4343, E-ISSN 1848-9966, Vol. 21, no 1, p. 201-215Article in journal (Refereed)
    Abstract [en]

    We establish criteria for both boundedness and compactness for some classes of integraloperators with logarithmic singularities in weighted Lebesgue spaces for cases 1 < p 6 q <¥ and 1 < q < p < ¥. As corollaries some corresponding new Hardy inequalities are pointedout.1

  • 40.
    Abylayeva, A.M.
    et al.
    L. N.Gumilev Eurasian National University, Khazakstan.
    Baiarystanov, A.O.
    L. N.Gumilev Eurasian National University, Khazakstan.
    Persson, Lars-Erik
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mathematical Science.
    Wall, Peter
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mathematical Science.
    Additive weighted Lp estimates of some classes of integral operators involving generalized Oinarov Kernels2017In: Journal of Mathematical Inequalities, ISSN 1846-579X, E-ISSN 1848-9575, Vol. 11, no 3, p. 683-694Article in journal (Refereed)
    Abstract [en]

    Abstract. Inequalities of the formkuK f kq 6C(kr f kp +kvH f kp) , f > 0,are considered, where K is an integral operator of Volterra type and H is the Hardy operator.Under some assumptions on the kernel K we give necessary and sufficient conditions for suchan inequality to hold.1

  • 41.
    Adam, Jonsson
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mathematical Science.
    On permutations and equity among generationsIn: Article in journal (Other academic)
    Abstract [en]

    In the literature on intergenerational justice, there is a widely held view that in order to live up to the utilitarian ideal of equal regard for all individuals, including those who have not yet been born, we must impose infinite permutation invariance conditions known as axioms of extended anonymity. I argue that such conditions do not bear relevance to the utilitarian ideal. I do so by showing that all examples that have been used to motivate extended anonymity axioms can be accounted for with an alternative principle that is supported independently.

  • 42. Adeleke, E.
    et al.
    Cizmesija, A.
    Oguntuase, James
    Persson, Lars-Erik
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mathematical Science.
    Pokaz, D.
    On a new class of Hardy-type inequalities2012In: Journal of inequalities and applications (Print), ISSN 1025-5834, E-ISSN 1029-242X, Vol. 2012, no 259Article in journal (Refereed)
    Abstract [en]

    In this paper, we generalize a Hardy-type inequality to the class of arbitrary non-negative functions bounded from below and above with a convex function multiplied with positive real constants. This enables us to obtain new generalizations of the classical integral Hardy's, Hardy-Hilbert's, Hardy-Littlewood-P\'{o}lya's and P\'{o}lya-Knopp's inequalities as well as of Godunova's and of some recently obtained inequalities in multidimensional settings. Finally, we apply a similar idea to functions bounded from below and above with a superquadratic function.

  • 43. Adey, J
    et al.
    Jones, R
    Palmer, D W
    Briddon, P R
    Öberg, Sven
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mathematical Science.
    Erratum: Degradation of Boron-Doped Czochralski-Grown Silicon Solar Cells [Phys. Rev. Lett., 93, 055504 (2004)]2004In: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 93, no 16, p. 169904-Article in journal (Other academic)
  • 44.
    Adey, J.
    et al.
    School of Physics, University of Exeter.
    Jones, R.
    School of Physics, University of Exeter.
    Palmer, D.W.
    School of Physics, University of Exeter.
    Briddon, P.R.
    School of Natural Science, University of Newcastle upon Tyne.
    Öberg, Sven
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mathematical Science.
    Degradation of boron-doped Czochralski-grown silicon solar cells2004In: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 93, no 5, p. 055504-1Article in journal (Refereed)
    Abstract [en]

    The formation mechanism and properties of the boron-oxygen center responsible for the degradation of Czochralski-grown Si(B) solar cells during operation is investigated using density functional calculations. We find that boron traps an oxygen dimer to form a bistable defect with a donor level in the upper half of the band gap. The activation energy for its dissociation is found to be 1.2 eV. The formation of the defect from mobile oxygen dimers, which are shown to migrate by a Bourgoin mechanism under minority carrier injection, has a calculated activation energy of 0.3 eV. These energies and the dependence of the generation rate of the recombination center on boron concentration are in good agreement with observations.

  • 45.
    Adey, J.
    et al.
    School of Physics, University of Exeter.
    Palmer, D.W,
    School of Physics, University of Exeter.
    Briddon, P.R.
    School of Natural Science, University of Newcastle upon Tyne.
    Öberg, Sven
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mathematical Science.
    Theory of boron-vacancy complexes in silicon2005In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 71, no 16, p. 165211-Article in journal (Refereed)
    Abstract [en]

    The substitutional boron-vacancy BsV complex in silicon is investigated using the local density functional theory. These theoretical results give an explanation of the experimentally reported, well established metastability of the boron-related defect observed in p-type silicon irradiated at low temperature and of the two hole transitions that are observed to be associated with one of the configurations of the metastable defect. BsV is found to have several stable configurations, depending on charge state. In the positive charge state the second nearest neighbor configuration with C1 symmetry is almost degenerate with the second nearest neighbor configuration that has C1h symmetry since the bond reconstruction is weakened by the removal of electrons from the center. A third nearest neighbor configuration of BsV has the lowest energy in the negative charge state. An assignment of the three energy levels associated with BsV is made. The experimentally observed Ev+0.31 eV and Ev+0.37 eV levels are related to the donor levels of second nearest neighbor BsV with C1 and C1h symmetry respectively. The observed Ev+0.11 eV level is assigned to the vertical donor level of the third nearest neighbor configuration. The boron-divacancy complex BsV2 is also studied and is found to be stable with a binding energy between V2 and Bs of around 0.2 eV. Its energy levels lie close to those of the V2. However, the defect is likely to be an important defect only in heavily doped material.

  • 46.
    Adjizian, J.J.
    et al.
    Department of Chemistry, University of Sussex, Falmer, Brighton.
    Latham, Chris
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mathematical Science.
    Öberg, Sven
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Briddon, P.R.
    Electrical, Electronic and Computer Engineering, University of Newcastle upon Tyne.
    Heggie, M.I.
    Department of Chemistry, University of Sussex, Falmer, Brighton.
    DFT study of the chemistry of sulfur in graphite, including interactions with defects, edges and folds2013In: Carbon, ISSN 0008-6223, E-ISSN 1873-3891, Vol. 62, p. 256-262Article in journal (Refereed)
    Abstract [en]

    Sulfur has several roles, desirable and undesirable, in graphitization. We perform density functional theory calculations within the local density approximation to define the structures and energetics of sulphur in graphite, including its interactions with point defects and edges, in order to understand its role in the later stages of graphitization. We find sulphur does not cross-link layers, except where there are defects. It reacts very strongly with vacancies in neighbouring layers to form a six coordinate split vacancy structure, analogous to that found in diamond. It is also highly stable at basal edge sites, where, as might be expected, the size and valency of sulfur can be easily accommodated. This suggests a role for sulphur in stabilizing graphene edges, and following from this, we show that sulfur dimers can open, i.e. unzip, folds in graphite rapidly and exothermically.

  • 47. Adlerborn, J
    et al.
    Björk, L
    Westman, Anna-Karin
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Wood Science and Engineering.
    Larker, H
    Particulate-reinforced ceramics densified by glass encapsulated HIP1991In: Hot isostatic pressing: Theory and applications / [ed] R J Schaefer; Melvin Linzer, Materials Park, Ohio: ASM International, 1991, p. 211-216Conference paper (Refereed)
  • 48.
    Adu, Cynthia
    et al.
    Manufacturing and Materials Department, Cranfield University.
    Berglund, Linn
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Oksman, Kristiina
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Eichhorn, Stephen J.
    Bristol Composites Institute (ACCIS), Queens Building, School of Engineering, Bristol University.
    Jolly, Mark
    Manufacturing and Materials Department, Cranfield University.
    Zhu, Chenchen
    Bristol Composites Institute (ACCIS), Queens Building, School of Engineering, Bristol University.
    Properties of cellulose nanofibre networks prepared from never-dried and dried paper mill sludge2018In: Journal of Cleaner Production, ISSN 0959-6526, E-ISSN 1879-1786, Vol. 197, no 1, p. 765-771Article in journal (Refereed)
    Abstract [en]

    Paper mills yield large volumes of sludge materials which pose an environmental and economic challenge for disposal, despite the fact that they could be a valuable source for cellulose nanofibres (CNF) production. The aim of the study was to evaluate the production process and properties of CNF prepared by mechanical fibrillation of never-dried and dried paper mill sludge (PMS). Atomic force microscopy (AFM) showed that average diameters for both never-dried and dried paper sludge nanofibres (PSNF) were less than 50 nm. The never-dried and dried sludge nanofibres showed no statistical significant difference (p > 0.05) in strength 92 MPa, and 85 MPa and modulus 11 GPa and 10 GPa. The study concludes that paper mill sludge can be used in a dried state for CNF production to reduce transportation and storage challenges posed on industrial scale.

  • 49.
    Aftab, A.
    et al.
    Faculty of Chemical and Energy Engineering, Universiti Teknologi Malaysia.
    Ismail, Abdul Razak
    Faculty of Chemical and Energy Engineering, Universiti Teknologi Malaysia.
    Ibupoto, Zafar
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Akeiber, Hussein J.
    Faculty of Mechanical Engineering, Universiti Teknologi Malaysia.
    Malghani, M.G.K.
    Department of Environmental Management and Policy, BUITEMS Quetta, Pakistan.
    Nanoparticles based drilling muds a solution to drill elevated temperature wells: a review2017In: Renewable & sustainable energy reviews, ISSN 1364-0321, E-ISSN 1879-0690, Vol. 76, p. 1301-1313Article in journal (Refereed)
    Abstract [en]

    Demand of the oil and gas energy is increasing very drastically. Conventional hydrocarbon reservoirs contain below the sealing cap rock (shale) and easily move towards wellbore are at the depletion stage. Therefore, drilling engineers in collaboration with mud engineers, geologists and geophysicists are looking for innovative materials to drill unconventional hydrocarbons reservoir which are distributed at the basin scale and cannot approach easily. Geo-thermal energy wells and most of unconventional reservoirs are occurred at high pressure high temperature (HPHT) conditions. Conventional micro-macro organic drilling mud additives with heat insulator in nature can minimize efficiency while drilling HPHT wells. Oil-based muds (OBM) are strictly restricted due to high toxic level and poor emulsion stability at HT. However, this review suggests that addition of macro size organic particles and inorganic nanoparticles can enhance rheological performance, reduce filtrate loss volume and improve shale inhibition characteristics of environmental friendly water-based mud (WBM). Despite an impressive amount of experimental work has been done over drilling additives and their effect over rheological and shale inhibition, taking into account their literature review are rare. In addition, there is no review work of the knowledge gained to date. This work will hope fully trigger further development and new research topics in the area of drilling muds system.

  • 50.
    Aguilar Sánchez, Andrea
    Luleå University of Technology, Department of Engineering Sciences and Mathematics.
    Different fiber combinations for enhancing properties of compression molded fiber composites2016Independent thesis Advanced level (degree of Master (Two Years)), 20 credits / 30 HE creditsStudent thesis
    Abstract [en]

    The present work involved the manufacturing and characterization of hybrid biocomposites from poly-lactic acid reinforced with cellulose derived fibers and synthetic high performing fibers, such as carbon and aramid fibers for improving the performance. The manufacturing and characterization of these composites were executed at Innventia AB in Stockholm and in the Division of Materials Science at Luleå University of Technology. A dynamic sheet former was used to obtain the pre-forms or mats. The final composite was obtained by compression molding and annealing process. The characterization of the composites was done by performing tensile test, fracture toughness test, differential scanning calorimetry, computed tomography and scanning electron microscopy. A study of the degradation of the poly-lactic acid in the composites was executed through size exclusion chromatography. Composite porosity was determined to be between 15-64%, depending on the composition. Composites containing aramid fibers showed higher values of strain at break, while composites containing carbon fibers showed higher values of tensile strength and young modulus. Synthetic fibers improved fracture toughness only when the fracture propagates in cross direction. In general, annealing process increased the degree of crystallinity of the composites. SEC results showed that samples lost around 23% and 94% of their molecular weight after compression molding and annealing process, which could have affected the mechanical behavior of the composite. Recommendations concerning the manufacturing of composites and suggestions for further experimental work are presented.

1234567 1 - 50 of 8248
CiteExportLink to result list
Permanent link
Cite
Citation style
  • apa
  • harvard1
  • ieee
  • modern-language-association-8th-edition
  • vancouver
  • Other style
More styles
Language
  • de-DE
  • en-GB
  • en-US
  • fi-FI
  • nn-NO
  • nn-NB
  • sv-SE
  • Other locale
More languages
Output format
  • html
  • text
  • asciidoc
  • rtf