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Träfflista för sökning "AMNE:(NATURAL SCIENCES Physical Sciences) ;lar1:(hb)"

Sökning: AMNE:(NATURAL SCIENCES Physical Sciences) > Högskolan i Borås

  • Resultat 1-10 av 79
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2.
  • Börjesson, Anders, et al. (författare)
  • First Principles Studies of the Effect of Nickel Carbide Catalyst Composition on Carbon Nanotube Growth
  • 2010
  • Ingår i: The Journal of Physical Chemistry C. - : American Chemical Society. - 1932-7447 .- 1932-7455. ; 114:42, s. 18045-18050
  • Tidskriftsartikel (refereegranskat)abstract
    • Density functional theory calculations were used to investigate the stability of single-walled carbon nanotubes (CNTs) attached to nanoparticles. The total energies and the adhesion energies between the CNTs and the nanoparticles were calculated for systems where the nanoparticles were either pure Ni or Ni carbide. It was found that the adhesion between the CNT and a pure Ni cluster is stronger than between the same CNT and a Ni carbide cluster although the energy difference was small compared to the total adhesion energies. This adhesion strength implies that CNTs are likely to remain attached to both pure Ni and Ni carbide clusters and that either pure Ni or Ni carbide clusters may be docked onto the open CNT ends to achieve continued growth or electronic contacts between CNTs and electrode materials. The system with a CNT attached to a pure Ni cluster was found to be energetically favored compared to a system containing the same CNT attached to a Ni carbide. The difference in total energy implies that a CNT should act as a sink for C atoms dissolved in the Ni carbide cluster, which means that the dissolved C atoms will be drained from the cluster, yielding a pure metal in the zero Kelvin thermodynamic limit. It is argued that this draining procedure is likely to occur even if carbon is added to the cluster at a proper rate, for example, during CNT growth.
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3.
  • Meiling, Pär, 1960, et al. (författare)
  • Physical planning of transport of dangerous goods in urban context as information environment
  • 2021
  • Ingår i: The 18th Scandinavian Workshop on E-Government (SWEG) 2021.
  • Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
    • The purpose of this research in progress is to explore the process of physical planning related to transport of dangerous goods in the City of Gothenburg (CoG) in Sweden. We have identified actors involved, some of their conflicts of interest, and challenges discernable in the planning process and in external yet related activities. Such activities are transportation of dangerous goods itself, and its supervisory control. Qualitative data in terms of written documentation from the detailed plan for transport of dangerous goods in CoG, and transcriptions of interviews have been analyzed. The ontology of the planning process, in terms of internal and external actors, activities, documentation, including legal frameworks, and issues such as access to information, information sharing, and information quality, indicate that the process can be seen as an information environment (Magoulas and Pessi, 1998; Hugosson, Magoulas, and Pessi, 2011). In the next step, the authors intend to expand the research work, and to conceptualize the planning process as an information environment. Stimulated by the idea of possible similarities between the planning process and large architectural design projects, the information environment is planned to be studied as form, context, and temporality (Boland, et al., 2008 and 2020). The research relates to the zero vision in traffic and a safer society (Trafikutskottet, 1998), strategy for research and development - Investing in knowledge for a safer society (MSB 2019), SDG 11 Sustainable cities and communities, and to the Sendai Framework for Disaster Risk Reduction 2015-2030. This paper reports on research executed within the transdisciplinary research project Transparent information management and collaboration for increased safety in the transport of dangerous goods (TISS, 2016-2021).
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4.
  • Bengtsson, Anders, 1988, et al. (författare)
  • Counterterms in gravity in the light-front formulation and a D=2 conformal-like symmetry in gravity
  • 2013
  • Ingår i: Journal of High Energy Physics. - : Springer Science and Business Media LLC. - 1029-8479 .- 1126-6708. ; 58:5-6, s. 485-501
  • Tidskriftsartikel (refereegranskat)abstract
    • In this paper we discuss gravity in the light-front formulation (light-cone gauge) and show how possible counterterms arise. We find that Poincare invariance is not enough to find the three-point counterterms uniquely. Higher-spin fields can intrude and mimic three-point higher derivative gravity terms. To select the correct term we have to use the remaining reparametrization invariance that exists after the gauge choice. We finally sketch how the corresponding programme for N = 8 Supergravity should work.
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5.
  • Becker, Simon, et al. (författare)
  • Spectral characterization of magic angles in twisted bilayer graphene
  • 2021
  • Ingår i: Physical Review B. - 2469-9950. ; 103:16
  • Tidskriftsartikel (refereegranskat)abstract
    • Twisted bilayer graphene (TBG) has been experimentally observed to exhibit almost flat bands when the twisting occurs at certain magic angles. In this Letter we show that in the approximation of vanishing AA coupling, the magic angles (at which there exist entirely flat bands) are given as the eigenvalues of a non-Hermitian operator, and that all bands start squeezing exponentially fast as the angle θ tends to 0. In particular, as the interaction potential changes, the dynamics of magic angles involves the nonphysical complex eigenvalues. Using our new spectral characterization, we show that the equidistant scaling of inverse magic angles is special for the choice of tunneling potentials in the continuum model, and is not protected by symmetries. While we also show that the protection of zero-energy states holds in the continuum model as long as particle-hole symmetry is preserved, we observe that the existence of flat bands and the exponential squeezing are special properties of the chiral model.
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6.
  • Jiang, A., et al. (författare)
  • Theoretical study of the thermal behaviour of free and alumina-supported Fe-C nanoparticles
  • 2007
  • Ingår i: Physical Review B Condensed Matter. - : American Physical Society. - 0163-1829 .- 1095-3795. ; 75:20
  • Tidskriftsartikel (refereegranskat)abstract
    • The thermal behavior of free and alumina-supported iron-carbon nanoparticles is investigated via molecular-dynamics simulations, in which the effect of the substrate is treated with a simple Morse potential fitted to ab initio data. We observe that the presence of the substrate raises the melting temperature of medium and large Fe1-xCx nanoparticles (x=0-0.16, N=80-1000, nonmagic numbers) by 40-60 K; it also plays an important role in defining the ground state of smaller Fe nanoparticles (N=50-80). The main focus of our study is the investigation of Fe-C phase diagrams as a function of the nanoparticle size. We find that as the cluster size decreases in the 1.1-1.6-nm-diameter range, the eutectic point shifts significantly not only toward lower temperatures, as expected from the Gibbs-Thomson law, but also toward lower concentrations of C. The strong dependence of the maximum C solubility on the Fe-C cluster size may have important implications for the catalytic growth of carbon nanotubes by chemical-vapor deposition.
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7.
  • Mohsenzadeh, Abas, et al. (författare)
  • The Effect of Carbon Monoxide Co-Adsorption on Ni-Catalysed Water Dissociation
  • 2013
  • Ingår i: International Journal of Molecular Sciences. - : M D P I AG. - 1661-6596 .- 1422-0067. ; 14:12, s. 23301-23314
  • Tidskriftsartikel (refereegranskat)abstract
    • The effect of carbon monoxide (CO) co-adsorption on the dissociation of water on the Ni(111) surface has been studied using density functional theory. The structures of the adsorbed water molecule and of the transition state are changed by the presence of the CO molecule. The water O–H bond that is closest to the CO is lengthened compared to the structure in the absence of the CO, and the breaking O–H bond in the transition state structure has a larger imaginary frequency in the presence of CO. In addition, the distances between the Ni surface and H2O reactant and OH and H products decrease in the presence of the CO. The changes in structures and vibrational frequencies lead to a reaction energy that is 0.17 eV less exothermic in the presence of the CO, and an activation barrier that is 0.12 eV larger in the presence of the CO. At 463 K the water dissociation rate constant is an order of magnitude smaller in the presence of the CO. This reveals that far fewer water molecules will dissociate in the presence of CO under reaction conditions that are typical for the water-gas-shift reaction.
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8.
  • Tian, Bo-Xue, et al. (författare)
  • Catalytic Mechanism of Porphobilinogen Synthase : The Chemical Step Revisited by QM/MM Calculations
  • 2012
  • Ingår i: Journal of Physical Chemistry B. - : American Chemical Society. - 1520-6106 .- 1520-5207. ; 116:40, s. 12105-12112
  • Tidskriftsartikel (refereegranskat)abstract
    • Porphobilinogen synthase (PBGS) catalyzes the asymmetric condensation and cyclization of two 5-aminolevulinic acid (5-ALA) substrate molecules to give porphobilinogen (PBG). The chemical step of PBGS is herein revisited using QM/MM (ONIOM) calculations. Two different protonation states and several different mechanisms are considered. Previous mechanisms based on DFT-only calculations are shown unlikely to occur. According to these new calculations, the deprotonation step rather than ring closure is rate-limiting. Both the C–C bond formation first mechanism and the C–N bond formation first mechanism are possible, depending on how the A-site ALA binds to the enzyme. We furthermore propose that future work should focus on the substrate binding step rather than the enzymatic mechanism.
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9.
  • Anderson, Dan, 1943, et al. (författare)
  • Approximate solution of a Thomas-Fermi model equation for bulk self-gravitating stellar objects in two dimensions
  • 2017
  • Ingår i: European journal of physics. - : Institute of Physics (IOP). - 0143-0807 .- 1361-6404. ; 38, s. 1-8
  • Tidskriftsartikel (refereegranskat)abstract
    • Direct variational methods are used to find simple approximate solutions of the Thomas–Fermi equations describing the properties of self-gravitating radially symmetric stellar objects both in the non-relativistic and ultra-relativistic cases. The approximate solutions are compared and shown to be in good agreement with exact and numerically obtained solutions.
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10.
  • Rosenberg, C.-J., et al. (författare)
  • Diffusion of super-Gaussian profiles
  • 2007
  • Ingår i: European journal of physics. - : Institute of Physics Publishing, Bristol, BS1 6BE, United Kingdom. - 0143-0807 .- 1361-6404. ; 28, s. 45-50
  • Tidskriftsartikel (refereegranskat)
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