| 751. |
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| 752. |
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| 753. |
- Kwon, S. K., et al.
(författare)
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Surface energy and stress release by layer relaxation
- 2005
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 72:23
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Tidskriftsartikel (refereegranskat)abstract
- The surface energy (gamma) and surface stress (tau) for semi-infinite close-packed surfaces of 4d transition metals have been calculated using ab initio total-energy methods. The moderate agreement between the present and former theoretical data for tau indicates the high level of numerical difficulty associated with such calculations. For the most close-packed surfaces, the present unrelaxed tau values follow the typical trend characteristic for the cohesive energy in nonmagnetic transition-metal series, whereas the relaxed tau values group around similar to 1 mJ/m(2), obtained for Y, Zr, and Ag, and similar to 3 mJ/m(2), calculated for Nb, Mo, Tc, Ru, Rh, and Pd. We have found that the average surface energy reduction upon layer relaxation is around 4%. At the same time, a large part of the surface stress is released during the surface relaxation process. To explain the observed behaviors, we have established a simple relationship, which connects the variations of gamma and tau to the layer relaxation. This relation reveals the principal factors determining the difference between the surface energy and stress release rates at 4d transition-metal surfaces.
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| 754. |
- Kwon, S. K., et al.
(författare)
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Surface Energy and Stress Release by Layer Relaxation
- 2005
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Ingår i: Physical Review B. ; 72, s. 235423-
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Tidskriftsartikel (refereegranskat)abstract
- The influence of carbon impurities on the properties of iron phases (bcc, hcp, dhcp, fcc) has been studied using the first-principles projector augmented-wave (PAW) method for a wide pressure range. It is shown that the presence of ~6 at. % of interstitial carbon has a little effect on the calculated structural sequence of the iron phases under high pressure. The bcc → hcp transition both for pure iron and iron containing carbon takes place around 9 GPa. According to the enthalpies comparison, the solubility of carbon into the iron solid is decreased by high pressure. The coexistence of iron carbide (Fe3C) + pure hcp Fe is most stable phase at high pressure compared with other phases. Based on the analysis of the pressure-density dependences for Fe3C and hcp Fe, we suggest that there might be some fraction of iron carbide present in the core.
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| 755. |
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| 756. |
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| 757. |
- Li, Chun-Mei, et al.
(författare)
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Temperature dependence of elastic properties of Ni(2+x)Mn(1-x)Ga and Ni(2)Mn(Ga(1-x)Al(x)) from first principles
- 2011
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 84:17, s. 174117-
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Tidskriftsartikel (refereegranskat)abstract
- The temperature dependence of the elastic properties of Ni(2+x)Mn(1-x)Ga and Ni(2)Mn(Ga(1-x)Al(x)) (x = 0.0, 0.1, and 0.2) random alloys are investigated by using the first-principles exact muffin-tin orbitals method. At 0 K, the calculated equilibrium parameters in both cubic L2(1) and nonmodulated tetragonal beta'''-Ni(2)MnGa are in good agreement with the available experimental data and other theoretical results. Separating the thermal effects into single electron excitation, volume expansion, phonon smearing, and magnetic terms, we find that phonon smearing gives the dominating positive tetragonal elastic constant (C') versus temperature (T) slope for the cubic phase. For Ni(2+x)Mn(1-x)Ga, the competition between the negative alloying effect (partial derivative C'/partial derivative x < 0) and the positive temperature effect (partial derivative C'/partial derivative T > 0) leads to nearly constant C'[x, T(M)(x)] at the martensitic transition temperature T(M)(x). For Ni(2)Mn(Ga(1-x)Al(x)), where both partial derivative C'/partial derivative x and partial derivative C'/partial derivative T are positive, however, due to the significantly decrease of T(M)(x), the critical C'[x, T(M)(x)] slightly decreases with Al doping. Furthermore, it is demonstrated that both the composition and the temperature dependence of C' are indispensable to get a reasonable theoretical T(M)(x).
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| 758. |
- Li, F., et al.
(författare)
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Hydrogen storage behavior in C15 Laves phase compound TiCr2 by first principles
- 2009
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Ingår i: Journal of Applied Physics. - : AIP Publishing. - 0021-8979 .- 1089-7550. ; 105:4
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Tidskriftsartikel (refereegranskat)abstract
- Hydrogen storage in TiCr2 alloy of AB(2) type with C15 Laves phase structure has been studied using density functional theory and plane-wave pseudopotential technique. The H atom interacts stronger with Cr atoms than with Ti and tends to occupy the g (2A2B) site due to the largest interstitial space. The g sites together with e (1A3B) sites form a three-dimensional clathratelike network, providing a diffusion pathway for H atoms, and the energy barriers for the H diffusion along these paths were computed. Interaction between two interstitial H atoms inside the TiCr2 lattice can be considered as a screened Coulomb repulsion, which seems to control the maximum hydrogen content in the metal hydrides. The absorption energies and electronic structures of metal hydrides TiCr2Hx with x=0.5-12 have been computed, and the theoretical maximum hydrogen storage capacity (between 2.6 and 3.8 wt %) agrees with experiments. The effects of the lattice expansion due to hydrogen absorption of different amounts have also been investigated.
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| 759. |
- Li, F., et al.
(författare)
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Improving hydrogen storage properties of covalent organic frameworks by substitutional doping
- 2010
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Ingår i: International journal of hydrogen energy. - : Elsevier BV. - 0360-3199 .- 1879-3487. ; 35:1, s. 266-271
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Tidskriftsartikel (refereegranskat)abstract
- We proposed a possible way of promoting the binding of H-2 molecules on covalent organic frameworks crystals via substituting the bridge C2O2B rings with different metal-participated rings, which can naturally avoid the clustering of metal atoms. First-principles calculations on both crystalline phase and molecular fragments show that the H-2 binding energy can be enhanced by a factor of four with regard to the undoped crystal, i.e. reaching about 10 kJ/mol. Grand canonical Monte Carlo simulations further confirm that such substitutional doping would improve the room temperature hydrogen storage capacity by a factor of two to three.
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| 760. |
- Li, Sa, 1975-
(författare)
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Materials Design from ab initio Calculations
- 2004
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- This thesis presents a theoretical study of bulk materials using ab initio methods based on the density functional theory (DFT).Crystallographic structural phase transformations and phase stability for 5f-dioxides, ABO3 perovskites, and ABO4 compounds have been extensively studied. Different approaches such as static total energy calculations, elastic stability and dynamical stability (phonon calculations) criteria have been used to determine the phase stability. As a special case, the lattice dynamics of solid Xe has been studied as a function of pressure.Dielectric functions and optical constants have been calculated for solar energy cell system CuIn1-xGaxSe2 with concentrations x=0, 0.25, 0.5 and 1.0 as well as for C60, PbWO4 and δ-AlOOH. The absorption coefficient provides information about the optimum solar energy conversion efficiency. We have derived absorption coefficients for a number of compounds. Comparisons between the calculated and experimental dielectric functions and absorption coefficients have been made.The main part of this thesis focuses on the nanolayered ternary compounds M N+1AXN (MAX), where N = 1, 2 or 3, M is an early transition metal, A is an A-group (mostly IIIA and IVA) element, and X is either C and/or N. These ternary carbides and nitrides combine unusual properties of both metals and ceramics. They exhibit high hardness, but fully reversible plasticity, and negligible thermoelectric power. These excellent properties make the MAX phases another new class of materials with versatile technological applications. Our work presents a systematic study of the electronic, bonding, elastic and optical properties of the MAX phases. A new MAX phase-Ti4SiC3, is calculated to be stable, and at the same time also been synthesized by experimentalists. Surface energy calculations have also been performed for the (0001) surface of the Ti-Si-C system. The general relations between the electronic structure and materials properties of the MAX phases have been elaborated in the thesis.
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