Abstract: Improving brittle behavior and mechanical properties is still a big challenge for high-temperature structural materials. By means of first-principles calculations, in this paper, we systematically investigate the effect of vacancy and oxygen occupation on the elastic properties and brittle-or-ductile behavior on Mo5Si3. Four vacancies (Si–Va1, Si–Va2, Mo–Va1, Mo–Va2) and oxygen occupation models (O–Mo1, O–Mo2, O–Si1, O–Si2) are selected for research. It is found that Mo–Va2 vacancy has the stronger structural stability in the ground state in comparison with other vacancies. Besides, the deformation resistance and hardness of the parent Mo5Si3 are weakened due to the introduction of different vacancy defects and oxygen occupation. The ratio of B/G indicates that oxygen atoms occupation and vacancy defects result in brittle-to-ductile transition for Mo5Si3. These vacancies and the oxygen atoms occupation change the localized hybridization between Mo–Si and Mo–Mo atoms. The weaker O–Mo bond is a contributing factor for the excellent ductile behavior in the O-Si2 model for Mo5Si3.

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Topics: Vacancy defect (59%), Electronic structure (55%)

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28 results found

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Abstract: Although the TM3Si silicides play an important role in transition metal silicides high-temperature materials, the hydrogenated mechanism of the cubic TM3Si is entirely unclear. To explore the hydrogenated mechanism, in this work, we apply the first-principles calculations to study the influence of hydrogen on the structural stability, mechanical and thermodynamic properties of TM3Si silicides. Based on the structural characteristic, four TM3Si silicides (Mo3Si, Nb3Si, Cr3Si and W3Si) and three hydrogen occupied models were considered. In particular, the influence of hydrogen on the stability, elastic properties, hardness, brittle-or-ductile behavior and Debye temperature of TM3Si was investigated. The result shows that the hydrogen is a thermodynamic stability in TM3Si because of the electronic interaction between hydrogen and TM3Si silicides. The thermodynamic stability of the hydrogenated TM3Si follows the order of Nb3Si

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Topics: Hydrogen (51%), Chemical stability (50%)

13 Citations

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Abstract: Although zirconium oxides (Zr–O) are representative functional materials, the wide band gap of ZrO2 limits its applications. Zirconium oxides with low concentration of oxygen maybe improve the electronic interaction in Zr–O system. However, the structure and physical properties of ZrO are not well understood. Here, the structural, electronic and optical properties of ZrO are studied by the first-principles calculations. We further study the influence of Ag and Cu on the electronic and optical properties of ZrO. The result shows that the ZrO shows better thermodynamically stale. Naturally, the structural stability of ZrO is attributed to the symmetrical Zr–O bonds. It is further found that the ZrO shows better electronic properties compared to the ZrO2. In addition, the ZrO exhibits strong ultraviolet behavior due to the role of Zr-4d state. Furthermore, the additive Ag and Cu would improve the electronic jump between the valence band and conduction band. In particular, it is found that the alloying elements of Ag and Cu improve the storage optical properties of ZrO from the analysis of energy loss function.

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Topics: Zirconium (51%)

11 Citations

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Abstract: In recent years, ternary silicides have attracted more and more attentions. Hf5BSi3 high temperature material has been synthesized. Nevertheless, the inherent brittleness affects its industrial applications. In present work, the effects of the vacancies on the structural stability, elastic constants and thermodynamic properties of Hf5BSi3 are clarified by first-principles calculations. The values of vacancy formation energy illustrate the structural stability of Hf5BSi3 with atomic vacancies. These vacancies weaken the shear and volume deformation resistance of Hf5BSi3, but well improve the brittle behavior of Hf5BSi3 with the larger values of B/G. The structural stability and mechanical behavior of Hf5BSi3 with vacancies are analyzed by the density of states and electron density difference. The thermal parameters exhibit the influence of the vacancies on the thermodynamic properties of Hf5BSi3 at elevated temperature.

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6 Citations

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Abstract: The ternary rare-earth sulfides ALnS2 have been widely applied in phosphors or scintillator materials. This study focused on systematically discussing the structural, electronic and optical properties of AGdS2 (A = Li, Na, K, Rb and Cs) ternary gadolinium sulfides using First-principles calculations for a better understanding of the physical properties of AGdS2. The ΔH values of all these sulfides reveal that they are phase stable, and LiGdS2 has the best phase stability. Electronic properties, including band structure, DOS, electron density difference and Mulliken population, provide that these sulfides are indirect band-gap semiconductors and have strong Gd–S covalent bonds and weak A-S ionic-covalent mixed bonds. The calculated absorption coefficients and reflectivity indicate that these AGdS2 sulfides are appropriate for the longer wavelength lasers. The anisotropy in optical properties for AGdS2 sulfides was studied through the polycrystalline and directional static dielectric constants e1(0) and static refractive indexes n(0), and the order of optical anisotropy can be obtained as LiGdS2 > NaGdS2 > KGdS2 > RbGdS2 > CsGdS2.

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Topics: Ternary operation (54%), Population (51%)

4 Citations

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Abstract: The magnetic properties and magnetocaloric effect of an antiferromagnetic/ferromagnetic (AFM/FM) BiFeO3/Co bilayer with mixed-spin (5/2, 3/2) have been studied based on Monte Carlo simulation. The magnetization, susceptibility, and critical temperature are investigated under various exchange couplings and an external magnetic field. In particular, the influence of exchange couplings and an external magnetic field on the magnetic entropy change, adiabatic temperature change, and the relative cooling power (RCP) are studied. The simulation results indicated that the decrease of the exchange coupling and the increase of external magnetic fields can cause an increase of magnetic entropy change, adiabatic temperature change, and RCP. In addition, the hysteresis loops of the system are presented for different exchange couplings and temperatures.

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Topics: Bilayer (58%), Monte Carlo method (54%)

4 Citations

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51 results found

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Abstract: We propose a simple analytic representation of the correlation energy ${\mathrm{\ensuremath{\varepsilon}}}_{\mathit{c}}$ for a uniform electron gas, as a function of density parameter ${\mathit{r}}_{\mathit{s}}$ and relative spin polarization \ensuremath{\zeta}. Within the random-phase approximation (RPA), this representation allows for the ${\mathit{r}}_{\mathit{s}}^{\mathrm{\ensuremath{-}}3/4}$ behavior as ${\mathit{r}}_{\mathit{s}}$\ensuremath{\rightarrow}\ensuremath{\infty}. Close agreement with numerical RPA values for ${\mathrm{\ensuremath{\varepsilon}}}_{\mathit{c}}$(${\mathit{r}}_{\mathit{s}}$,0), ${\mathrm{\ensuremath{\varepsilon}}}_{\mathit{c}}$(${\mathit{r}}_{\mathit{s}}$,1), and the spin stiffness ${\mathrm{\ensuremath{\alpha}}}_{\mathit{c}}$(${\mathit{r}}_{\mathit{s}}$)=${\mathrm{\ensuremath{\partial}}}^{2}$${\mathrm{\ensuremath{\varepsilon}}}_{\mathit{c}}$(${\mathit{r}}_{\mathit{s}}$, \ensuremath{\zeta}=0)/\ensuremath{\delta}${\mathrm{\ensuremath{\zeta}}}^{2}$, and recovery of the correct ${\mathit{r}}_{\mathit{s}}$ln${\mathit{r}}_{\mathit{s}}$ term for ${\mathit{r}}_{\mathit{s}}$\ensuremath{\rightarrow}0, indicate the appropriateness of the chosen analytic form. Beyond RPA, different parameters for the same analytic form are found by fitting to the Green's-function Monte Carlo data of Ceperley and Alder [Phys. Rev. Lett. 45, 566 (1980)], taking into account data uncertainties that have been ignored in earlier fits by Vosko, Wilk, and Nusair (VWN) [Can. J. Phys. 58, 1200 (1980)] or by Perdew and Zunger (PZ) [Phys. Rev. B 23, 5048 (1981)]. While we confirm the practical accuracy of the VWN and PZ representations, we eliminate some minor problems with these forms. We study the \ensuremath{\zeta}-dependent coefficients in the high- and low-density expansions, and the ${\mathit{r}}_{\mathit{s}}$-dependent spin susceptibility. We also present a conjecture for the exact low-density limit. The correlation potential ${\mathrm{\ensuremath{\mu}}}_{\mathit{c}}^{\mathrm{\ensuremath{\sigma}}}$(${\mathit{r}}_{\mathit{s}}$,\ensuremath{\zeta}) is evaluated for use in self-consistent density-functional calculations.

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Topics: Energy (signal processing) (50%)

19,831 Citations

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Abstract: A new approach to the construction of first-principles pseudopotentials is described. The method allows transferability to be improved systematically while holding the cutoff radius fixed, even for large cutoff radii. Novel features are that the pseudopotential itself becomes charge-state dependent, the usual norm-conservation constraint does not apply, and a generalized eigenproblem is introduced. The potentials have a separable form well suited for plane-wave solid-state calculations, and show promise for application to first-row and transition-metal systems.

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Topics: Quantum ESPRESSO (52%), Projector augmented wave method (50%)

17,261 Citations

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XCII. Relations between the elastic moduli and the plastic properties of polycrystalline pure metals

Abstract: Relations between the elastic and plastic properties of pure polycrystalline metals are discussed and a systematic relation between shear modulus, Burgers vector and plastic shear strength of metals possessing the same lattice structure is proposed. In addition reasons are given for believing that in a limited temperature range malleability is related to Poisson's ratio.

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Topics: Elastic modulus (56%), Shear modulus (56%), Pure shear (56%) ... read more

4,542 Citations

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Abstract: Below 1000°C the oxidation of nickel cannot be controlled by the diffusion of ions through the bulk crystal lattice of the pure oxide, because the measured oxidation rates are several orders of magnitude faster than would be predicted on this basis. Short-circuit diffusion through oxide grain boundaries or dislocations has usually been held responsible, but there has hitherto been no proper quantitative confirmation of this mechanism. We report measurements of the oxide scale thickness and oxide grain size as a function of time during the oxidation of high-purity nickel in the temperature range 500–800°C. All the oxidation experiments were carried out in pure oxygen at a pressure of one atmosphere. The measured parabolic oxidation rate constants have been compared with those calculated from grain boundary diffusion data obtained in our previous work, using a grain boundary diffusion model for the oxidation process. The quantitative agreement between measured and calculated oxidation rates shows c...

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Topics: Grain boundary diffusion coefficient (69%), Grain boundary (60%), Oxide (59%) ... read more

2,573 Citations

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Abstract: Transition metal silicides are promising advanced functional materials. However, the structure and relevant properties of Mo 5 Si 3 are not well understood. In this work, we investigate the crystal structure, elastic properties, Vickers hardness, elastic anisotropy, electronic and thermodynamic properties of Mo 5 Si 3 by using the first-principles calculations. Three structures: tetragonal, hexagonal and orthorhombic structures are considered. The calculated results show that those structures are thermodynamically stable. In particular, we firstly predict that Mo 5 Si 3 with hexagonal ( P 63 /mcm ) structure is a stable phase. The calculated electronic structure shows that Mo 5 Si 3 exhibits better electronic properties because of the charge overlap between Mo-4 d state and Si-3 p state near the Fermi level. Importantly, Mo 5 Si 3 shows the strong deformation resistance and high elastic stiffness in comparison to other TM 5 Si 3 . Mo 5 Si 3 with tetragonal structure has the smaller percentage anisotropy in compressibility and high percentage anisotropy in shear. We further find that the Debye temperature and heat capacity of tetragonal structure are larger than that of hexagonal structure. The high-temperature thermodynamic properties of Mo 5 Si 3 are attributed to the vibration of Si atom.

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Topics: Fermi level (56%), Tetragonal crystal system (54%), Electronic structure (54%) ... read more

81 Citations