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The anisotropy of impact sensitivity and microscopic electron properties of the cyclotrimethylene trinitramine ([C.sub.3][H.sub.6][N.sub.6][O.sub.6]) (RDX) under shock loading are investigated in our work. The simulation is performed using molecular dynamic (MD) simulations in conjunction with multi-scale shock technique (MSST). By calculating the microscopic electronic properties and combining the thermodynamic properties, we predict that the metallization pressure of the RDX crystal is approximately 170 GPa under shock loading, which is slightly less than the metallization pressure under hydrostatic pressure. We also found that the microscopic electronic properties are related to the impact sensitivity. When the shock loading is along the z direction, the time of the transition from the insulating state to the metallization of the RDX crystal lags behind the shock loading along the x or y direction. Therefore, we predict that the RDX crystal has a lower sensitivity when the shock loading is along the z direction. Key words: sensitive, metallization pressure, electron properties. Dans le cadre des présents travaux, nous avons étudié l'anisotropie de la sensibilité à l'impact et des propriétés électroniques microscopiques de la cyclotriméthylènetrinitramine ([C.sub.3][H.sub.6][N.sub.6][O.sub.6]) (RDX) sous l'action d'une charge dynamique. Nous avons modélisé le système par des simulations de dynamique moléculaire combinées à une technique d'impact à échelles multiples.Encombinant les propriétés électroniques microscopiques théoriques aux propriétés thermodynamiques, nous avons calculé que, sous l'action d'une charge dynamique, la pression de métallisation du RDX cristallin était d'environ 170 GPa, ce qui est légèrement plus faible que la pression de métallisation sous pression hydrostatique. Nous avons également observé que les propriétés électroniques microscopiques sont liées à la sensibilité à l'impact. Lorsque la charge dynamique est appliquée dans la direction z, la transition du RDX cristallin de l'état isolant à la métallisation à la suite de l'impact est retardée par rapport aux directions x ou y. Par conséquent, nous sommes en mesure de prédire que le RDX cristallin, sous une charge dynamique appliquée dans la direction z, présenterait une sensibilité réduite. [Traduit par la Rédaction] Mots-clés : sensible, pression de métallisation, propriétés électroniques., Introduction The studies about high explosives (HEs) have been a topic of interest in military and industry fields. Their basic physical and chemical properties (1-18) have been investigated in recent [...]

The phase transition of the β-HMX crystal has been widely studied under high pressure, but the microscopic transition mechanism is not sufficiently understood. In this article, we perform a series of ab initio molecular dynamics simulations focusing on structure deformation and the corresponding vibration spectra resolution of β-HMX at 0–40 GPa. Several typical pressure-induced phase transition processes are confirmed by analyzing the chemical bond, dihedral angle, charge transfer, and IR and Raman spectra. The corresponding relationship between molecular structure and spectral signal is constructed through the partial spectra calculations of special functional groups within the HMX molecule. The anisotropic effects of different groups on the initial structural phase transition are uncovered. The equatorial C–N and axial N–N bonds have the largest compression ratio as pressure increases, which is the intrinsic factor for the initiation of structure transformation. The C–N molecular ring plays an important role in the entire phase transition process. In addition, the phase transition of β → ζ is also closely related to the deformation of NO2, while that of ζ → ε is induced by the axial N–NO2 group. Regarding the higher-pressure phase transition, the synergetic effect of N–NO2, CH2 groups, and molecular rings becomes more considerable. [ABSTRACT FROM AUTHOR]

For two-dimensional transition metal dichalcogenides, the thermoelectric properties of the material are affected by layer thickness and lattice strain. In this paper, we investigate the thermoelectric properties of a HfS2 bilayer under different biaxial tensile strains by first-principles calculations combined with Boltzmann equations. The presence of degenerate bands in the HfS2 bilayer and the absence of its monolayer results in the better thermoelectric performance of the HfS2 bilayer than its monolayer. Moreover, this strain increases the band degeneracy of the HfS2 bilayer even more, and the degenerate bands and stepped 2D density of states lead to a high power factor. In addition, the lattice strain increases the phonon scattering rate and reduces the phonon lifetime of the HfS2 bilayer, resulting in a decrease in the lattice thermal conductivity. Ultimately, we obtained a maximum ZT value of 1.76 for the unstrained HfS2 bilayer at the optimal doping concentration. At this time, its power factor and thermal conductivity are 53.01 mW/mK2 and 9.06 W/mK, respectively. When the strain reaches 3%, for the n-type doped HfS2 bilayer, the power factor and thermal conductivity are 69.87 mW/mK2 and 6.36 W/mK, respectively, and the maximum ZT value is 3.29. For the p-type doped HfS2 bilayer, the maximum ZT value appears at 6% strain, which is 1.83, at which the power factor and thermal conductivity are 13.81 mW/mK2 and 2.27 W/mK, respectively. [ABSTRACT FROM AUTHOR]

The structure, phase transition, elastic, and thermodynamic properties of Os[N.sub.2] have been studied via first-principles calculations. It is shown that the CoS[b.sub.2] structure is more stable than other structures. By the calculated H-P relations at 0 K, we found that the phase transition of Os[N.sub.2] from CoS[b.sub.2] structure to marcasite structure (ζ → δ) occurs at 16.8 GPa, while the phase transition pressure between pyrite structure and fluorite structure (γ → α) is 80 GPa. The results of obtained phase transitions are also confirmed by bond length, sound velocity, and thermal expansion coefficient under different pressures. The pressure dependences of the elastic constants, mechanical stability, and mechanical anisotropy of four structures of Os[N.sub.2] have been investigated by finding that the fluorite (pyrite and marcasite) structure Os[N.sub.2] is mechanically stable under hydrostatic pressure (up to 60 GPa). However, the monoclinic structure is mechanically unstable under pressure from 0 to 60 Gpa. The calculated elastic anisotropic factors show that Os[N.sub.2] possesses high elastic anisotropy under pressure. Moreover, the calculations on total density of states show that Os[N.sub.2] of different structures has a metallic character, in agreement with previous theoretical results. The thermodynamic properties and sound velocity under diverse pressures of Os[N.sub.2] of the four structures have been also investigated successfully. PACS Nos.: 31.15.A-, 36.40.Ei, 87.19.rd. Nous presentons une etude ab initio de la structure, de la transition de phase et des proprietes thermodynamiques et elastiques de Os[N.sub.2]. Nous montrons que la structure de type CoS[b.sub.2] est pus stable que les autres structures. A l'aide des relations H-P calculees, nous trouvons que la transition de phase de Os[N.sub.2] de la structure CoS[b.sub.2] vers une structure marcassite (ζ → δ) se produit a 16.8 GPa, alors que la pression de transition de phase entre les structures pyrite et fluorine (γ → α) est 80 GPa. Les resultats obtenus sur les transitions de phase sont confirmes par la longueur de liaison, la vitesse du son et le parametre d'expansion thermique sous differentes pressions. Nous analysons d'abord la dependance sur la pression de la constante electrique, de la stabilite mecanique et de l'anisotropie mecanique pour quatre structures de Os[N.sub.2], trouvant que la structure fluorine (pyrite et marcassite) de Os[N.sub.2] est mecaniquement stable sous pression hydrostatique (jusqu'il 60 GPa). Cependant, la structure monoclinique est mecaniquement instable sous des pressions de 0 a 60 GPa. Les facteurs d'anisotropie elastique calcules pour Os[N.sub.2] possedent une haute anisotropie elastique sous pression. De plus, le calcul de la densite totale d'etats de differentes structures montre que Os[N.sub.2] a un caractere metallique, en accord avec des resultats theoriques anterieurs. Nous investiguons avec succes les proprietes thermodynamiques et la vitesse du son sous diverses pressions pour quatre structures de Os[N.sub.2]. [Traduit par le Redaction], 1. Introduction Recently, transition metal nitrides have attracted special attention for their unique properties such as hardness, superconductivity, and durability [1-5]. Several experimental investigations have been performed to explore the [...]