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  • 1.
    Berezovsky, Vladimir
    et al.
    Department of Applied Mathematics and High-performance ComputingM.V.Lomonosov Northern (Arctic) Federal University, Arkhangelsk.
    Öberg, Sven
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Computational study of the CO adsorption and diffusion in zeolites: validating the Reed–Ehrlich model2018In: Adsorption, ISSN 0929-5607, E-ISSN 1572-8757, Vol. 24, no 4, p. 403-413Article in journal (Refereed)
    Abstract [en]

    Molecular simulations have been employed to explore at the microscopic scale the adsorption of CO in zeolites (MFI, CHA and DDR). On the basis of classical force fields, grand canonical Monte Carlo simulations are performed to predict the adsorption properties (isotherms) of these types of zeolites up to high pressure. Subsequent careful analysis yields details the microscopic mechanism in play, along the whole adsorption process, together with a considering of the arrangements of CO in MFI at high pressure. This work also summarizes an approach which uses single component diffusion data in prediction of multicomponent diffusion.

  • 2.
    Hedman, Daniel
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    A Theoretical Study: The Connection between Stability of Single-Walled Carbon Nanotubes and Observed Products2017Licentiate thesis, comprehensive summary (Other academic)
    Abstract [en]

    Over the past 20 years’ researchers have tried to utilize the remarkable properties of single-walled carbon nanotubes (SWCNTs) to create new high-tech materials and devices, such as strong light-weight composites, efficient electrical wires and super-fast transistors. But the mass production of these materials and devices are still hampered by the poor uniformity of the produced SWCNTs. These are hollow cylindrical tubes of carbon where the atomic structure of the tube wall consists of just a single atomic layer of carbon atoms arranged in a hexagonal grid. For a SWCNT the orientation of the hexagonal grid making up the tube wall is what determines its properties, this orientation is known as the chirality of a SWCNT. As an example, tubes with certain chiralities will be electrically conductive while others having different chiralities will be semiconducting.

    Today’s large scale methods for producing SWCNTs, commonly known as growth of SWCNTs, gives products with a large spread of different chiralities. A mixture of chiralities will give products with a mixture of different properties. This is one of the major problems holding back the use of SWCNTs in future materials and devices. The ultimate goal is to achieve growth where the resulting product is uniform, meaning that all of the SWCNTs have the same chirality, a process termed chirality-specific growth. To achieve chirality-specific growth of SWCNTs requires us to obtain a better fundamental understanding about how they grow, both from an experimental and a theoretical point of view.

    This work focuses on theoretical studies of SWCNT properties and how they relate to the growth process, thereby giving us vital new information about how SWCNTs grow and taking us ever closer to achieving the ultimate goal of chirality-specific growth. In this thesis, an introduction to the field is given and the current state of the art experiments focusing on chirality-specific growth of SWCNTs are presented. A brief review of the current theoretical works and computer simulations related to growth of SWCNTs is also presented. The results presented in this thesis are obtained using first principle density functional theory. The first study shows a correlation between the stability of SWCNT-fragments and the observed products from experiments. Calculations confirm that in 84% of the investigated cases the chirality of experimental products matches the chirality of the most stable SWCNT-fragments (within 0.2 eV). Further theoretical calculations also reveal a previously unknown link between the stability of SWCNT-fragments and their length. The calculations show that at specific SWCNT-fragment lengths the most stable chirality changes. Thus, introducing the concept of a switching length for SWCNT stability. How these new results link to the existing understanding of SWCNT growth is discussed at the end of the thesis.

  • 3.
    Hedman, Daniel
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Linking Stability of Single-Walled Carbon Nanotubes with Growth Products2017Conference paper (Other academic)
    Abstract [en]

    Many of the envisioned products and technologies using single-walled carbon nanotubes (SWCNTs) are only possible with a uniform product. Thus, control over the chirality during catalytical chemical vapor deposition (CCVD) growth of SWCNTs is necessary. Our highlighted works1,2 focuses on stabilities of SWCNTs and how that relates to growth, in order to reach the ultimate goal of chirality-specific growth. In ref.1 density functional theory (DFT) has been used to calculate the stability of SWCNT-fragments of all chiralities in the (n+m) = 8 to 18 series. The fragment stabilities are compare to the chiralities of actual CCVD products from all properly analysed experiments to date. The results show that in 84% of the cases the experimental products represent chiralities among the most stable SWCNT-fragments (within 0.2 eV) from the calculations. The analysed products from growth experiments show that diameters of SWCNTs are governed by the well-known relation to the size of the catalytic particle and that the specific chirality of SWCNT products are strongly dependent on the stability of the tubes within each series, suggesting thermodynamic control at the early stage of growth. Analysis of the relative energy show that for the lower series 8 to 10, zigzag SWCNTs are the most stable and for the higher series 11 to 18 the most stable chirality changes from zigzag to armchair. This switch in stability between armchair and zigzag chiralities is studied further in ref.2, where DFT was used to calculate the stability of armchair and zigzag SWCNTs and graphene nanoribbons (GNRs) of different lengths. The calculations show that the stability of armchair and zigzag tubes has different linear dependence with regard to their length, with switches in the most stable chirality occurring at specific lengths for each SWCNT-series. These dependencies are explained by competing edge and curvature energies. Within each series armchair nanotubes are the most stable at short lengths, while zigzag nanotubes are the most stable at long lengths, this sheds new light into why armchair and near-armchair tubes are the dominant product from CCVD growth, if templating is not used. Paradoxically, the stability of armchair nanotubes at short lengths favors their growth although zigzag nanotubes are more stable at long lengths, resulting in the production of the least stable SWCNTs.

  • 4.
    Hedman, Daniel
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    On the Stability of Single-Walled Carbon Nanotubes and how it relates to Growth2017In: CCTN17: 12th International Symposium on Computational Challenges and Tools for Nanotubes, 2017Conference paper (Other academic)
    Abstract [en]

    Many envisioned products and technologies using single-walled carbon nanotubes (SWCNTs) are only possible with a uniform product. Thus, control over the chirality during catalytical chemical vapor deposition (CCVD) growth is necessary. Our highlighted works [1,2] focuses on stabilities of SWCNTs and how they relate to growth. In ref. [1] density functional theory (DFT) is used to calculate the stability of SWCNT-fragments of all chiralities in the 8-18 series. The fragment stabilities are compare with chiralities from actual CCVD products. The results show that 84% of the experimental products represent chiralities among the most stable SWCNT-fragments (within 0.2 eV) from the calculations. The analyzed products from growth experiments show that the chirality of SWCNT products are strongly dependent on the stability of the tubes within each series, suggesting thermodynamic control at the early stage of growth. Analysis of the relative energy show that for lower series 8-10, zigzag SWCNTs are the most stable and for higher series 11-18 the most stable chirality changes from zigzag to armchair. This switch in stability is studied further in ref. [2], where DFT is used to calculate the stability of armchair and zigzag SWCNTs and graphene nanoribbons of different lengths. The calculations show that the stability of armchair and zigzag tubes have different linear dependence with regards to their length, with switches in the most stable chirality occurring at specific lengths for each SWCNT-series. These dependencies are explained by competing edge and curvature energies. Within each series armchair nanotubes are most stable at short lengths, while zigzag nanotubes are most stable at long lengths. This sheds new light into why armchair and near-armchair tubes are dominant products from CCVD growth.

    [1] D. Hedman, H.R Barzegar, A. Rosen, T. Wågberg, J.A Larsson, Sci. Rep., 2015, 5, 16850. [2] D. Hedman, J.A. Larsson, Carbon, 2017, 116, 443.

  • 5.
    Hedman, Daniel
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Single-Walled Carbon Nanotubes: A theoretical study of stability, growth and properties2019Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    Since their discovery over 25 years ago, scientists have explored the remarkable properties of single-walled carbon nanotubes (SWCNTs) for use in high-tech materials and devices, such as strong light-weight composites, efficient electrical wires, supercapacitors and high-speed transistors. However, the mass production of such materials and devices is still limited by the capability of producing uniform high-quality SWCNTs. The properties of a SWCNT are determined by the orientation of the hexagonal grid of carbon atoms constituting the tube wall, this is known as the chirality of the SWCNT.

    Today's large-scale methods for producing SWCNTs, commonly known as growth, give products with a large spread of different chiralities. A mixture of chiralities give products with a mixture of different properties. This is one of the major obstacles preventing large-scale use of SWCNTs in future materials and devices. The goal is to achieve growth where the resulting product is uniform, meaning that all SWCNTs have the same chirality, a process termed chirality-specific growth. To achieve this requires a deep fundamental understanding of how SWCNTs grow, both from an experimental and a theoretical perspective.

    This work focuses on theoretical studies of SWCNTs and their growth mechanisms. With the goal of achieving a deeper understanding of how chirality arises during growth and how to control it. Thus, taking us ever closer to the ultimate goal of achieving chirality-specific growth. In this thesis, an introduction to the field is given and the current research questions are stated. Followed by chapters on carbon nanomaterials, SWCNTs and computational physics. A review of the state-of-the-art experimental and theoretical works relating to chirality specific growth is also given.

    The results presented in this thesis are obtained using first principle density functional theory calculations. Results show that the stability of short SWCNT-fragments can be linked to the products observed in experiments. In 84% of the investigate cases, the chirality of experimental products matches the chirality of the most stable SWCNT-fragments (within 0.2 eV). Further studies also reveal a previously unknown link between the stability of SWCNT-fragments and their length. Calculations show that at specific lengths the most stable chirality changes. Thus, introducing the concept of a switching length for SWCNT stabilities.

    This newly found property of SWCNTs is used in combination with previously published works to create a state-of-the-art analytical model to investigate growth of SWCNTs any temperature. Results from the model show that the most stable chirality obtained is dependent on the diameter, length of the SWCNT, the growth temperature and the composition of the catalyst. Finally, a detailed study on the ability of catalyst metals to sustain SWCNT growth points to Pt as an interesting candidate to achieve growth of rarely seen chiralities. The new knowledge gained from these results takes us even closer to achieving chirality-specific growth.

  • 6.
    Magnusson, Jens
    Luleå University of Technology, Department of Engineering Sciences and Mathematics.
    Training Neural Network Potentials for Atomistic Calculations on Carbon Materials: An initial study on diamond structures2018Independent thesis Advanced level (professional degree), 20 credits / 30 HE creditsStudent thesis
    Abstract [en]

    Machine Learning (ML) and especially implementations of  Neural Networks (NNs) is growing in popularity  across numerous application areas. One of which is the use of a trained NN as an interatomic potential in Atomistic Simulations (AS), a NN applied in this manner is referred to as a Neural Network Potential (NNP).

    A well established method of atomistic calculations is the use of the first principle Density Functional Theory (DFT). DFT can very precisely model properties of nanomaterials, but for large systems of atoms DFT is not a feasible method because of its heavy computational load. The use of NNPs enables accurate simulations of big systems of atoms with a reasonable low computational cost.

    Previous work by students at Luleå University of Technology (LTU) where NNs were trained on fullerenes and carbon nanotubes (CNTs) demonstrated promising results. The NNs trained by the use of Atomistic Machine-Learning Package (AMP) managed to predict energies with considerable accuracy (100 meV/atom), but the force predictions were problematic and did not reach desired accuracy (the force RMSE reached was 6 eV/Å). Attempts made to run AS such as Molecular Dynamics (MD) and Geometry Optimization (GO) were unsuccessful, likely due to a poor representation of forces. 

    This work aims to improve the performance of NNs on carbon materials by studying diamond structures using AMP, such that working AS can be achieved.This was done in two stages, first a feasibility study was made to find appropriate hyperparameters. Moreover a study was made, where NNs was trained with the hyperparameters found. Two types of feature mapping descriptors were considered here, Gaussian and Zernike.The NNs trained was used as NNPs to perform MD and GO simulations as a mean of evaluation. The NNPs were also used to calculate the phonon dispersion curve of diamond.The trained NNPs in this work managed to perform AS and calculate the phonon dispersion curve with varying success. The best performing NN trained on 333 super-cells of diamond reached an accuracy of 120 meV/atom when predicting energies, and 640 meV/Å predicting forces. A NNP trained with Gaussian descriptors turned out to be 10 times faster than the reference simulation done with DFT, compared while performing a single step in a GO. The phonon dispersion curve produced by the Gaussian NNP displayed a striking resemblance to the reference produced by using DFT. Phonon dispersion curves produced by the Zernike NNP was distorted and involved a great deal of imaginary frequencies, but the correct amplitude was reached.The Gaussian NNPs trained in this work turned out to be faster and better in almost all regards compared to the Zernike alternative. The only time Zernike outperformed Gaussian descriptors were in the total energy reached in a GO simulation applying the NNPs from the study. Compared to DFT results the Zernike error was 0.26 eV (0.05%) and the Gaussian error was 0.855 eV (0.17%). MD simulations where the NNPs was used worked well for the Gaussian variant but not for the Zernike.With the AS up and running (at least for the Gaussian NNP) the following step is either to improve the performance on diamond structures. Or to include more carbon materials in the studies such as CNT and fullerenes.

1 - 6 of 6
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