o-C_8碳:新的超硬碳同素异形体（英文）

通过粒子群优化算法和密度泛函计算,证明了空间群为PMM A的正交晶系的碳同素异形体o-C_8是稳定的超硬相.声子谱计算表明,o-C_8碳相是动力学稳定的;体积压缩计算表明,它是体模量为298.6 GPa的高度不可压缩材料.o-C_8相是一种新型的密度为2.993 g/cm~3、维氏硬度为67.0 GPa的低密度超硬材料.     

强冲击压缩条件下g-C3N4向β-C3N4直接转化

 为了合成出理论预言的具有致密结构的超硬材料C₃N₄,运用二级轻气炮加载和冲击回收实验技术,以富含N的g-C₃N₄为前驱物,在40~65 GPa压力下完成了冲击合成实验。在低于51 GPa压力时,X射线衍射分析表明,在回收样品中未发现有新相生成,说明g-C₃N₄是稳定的;而在51~65 GPa范围内,回收样品中有新相生成,与理论计算结果对照发现,新相为β-C₃N₄相,且不含其它结晶相。证实利用冲击合成方法将g-C₃N₄直接转化为单纯β-C₃N₄是可能的,对纯净的超硬相碳氮化合物的合成研究具有参考意义。     

Mn掺杂LiFePO4的第一性原理研究

采用基于密度泛函理论的第一性原理方法, 计算了不同Mn掺杂浓度LiFe_1-xMn_xPO₄ (x=0,0.25,0.50,0.75) 的电子结构. 同时采用流变相辅助高温固相碳热还原法制备了LiFe_1-xMn_xPO₄ (x= 0,0.25,0.50,0.75) 材料. 理论计算表明: LiFePO₄具有E_g = 0.725 eV的带隙宽度, 为半导体材料. 通过Fe位掺杂25%的Mn离子可最大程度地 减小材料带隙宽度、降低Fe---O键及Li---O键键能, 进而提高材料的电子电导率及锂离子扩散速率. 实验结果亦表明, 当Mn掺杂量x=0.25时, 材料具有最优的电化学性能, 其具有约为158 mAh· g^-1的放电比容量以及551 Wh· kg^-1的能量密度. 理论计算与实验结果非常符合.     

高密度间苯二酚-甲醛碳气凝胶ICF靶的制备与吸附性能研究

 以间苯二酚和甲醛为前驱体,通过改进传统制备技术解决了高密度间苯二酚-甲醛(CRF)碳气凝胶制备过程中的龟裂问题,制备出了符合ICF实验需要的高密度CRF碳气凝胶材料。分别对CRF碳气凝胶的元素组成和物相组成进行了鉴定,采用自动吸附仪考察了CRF碳气凝胶对N₂和H₂的吸附性能。结果表明：该碳气凝胶是一种由C元素组成的类似石墨结构的非晶固态材料,结构均匀性好,具有良好的机械加工性能,比表面积达676 m²·g^-1,平均孔径为7.16 nm;氢吸附质量分数达2.28%,相应体积密度为17.83 kg·m^-3。     

同位旋非对称核物质性质与扩展的BHF方法(Ⅶ)中子星物质中质子和中子的1S0态超流性

利用Brueckner Hartree Fock和BCS理论方法,计算了β稳定中子星物质中处于¹S₀ 态的质子和中子的对关联能隙,着重研究和讨论了三体核力的影响.结果表明三体核力对β稳定中子星物质中¹S₀态中子超流性的影响相对较小,但对¹S₀态质子超流性具有重要影响,而且其效应随核子数密度增大而迅速增强.三体核力的主要作用是强烈地抑制了高密度β稳定中子星物质中的1S0态质子超流性.     

第一性原理研究hcp-C3碳体环材料的力学性质

为探索新型高强度材料, 使用第一性原理方法研究了hcp-C3碳体环材料的晶体结构、电子性质与力学性质. 结构计算与电子性质分析表明, 基于特殊的分子结构, 碳体环结构中出现了变形的sp³杂化形式. 这使得hcp-C3碳体环结构中力学特性具有明显的方向依赖性. 力学性质计算表明, 沿着[0001]晶向, 碳体环结构的弹性模量达到1033 GPa, 抗拉强度达到124.17 GPa, 抗压强度达到381.83 GPa, 沿[2110]晶向的抗压强度达到了458.34 GPa, 从而显示了hcp-C3碳体环材料优秀的力学性质. hcp-C3碳体环材料可作为新型的高强度材料而使用.     

BC5力学性质的第一性原理计算

采用平面波赝势密度泛函理论方法对0—60 GPa静水压下BC₅ 六角晶系P3m1和四方晶系I4m2结构的平衡态晶格常数、弹性常数、各向异性以及泊松比与Cauchy扰动进行了研究. 研究结果表明, BC₅的两种结构在高压下是稳定的, 且不可压缩性随着压强的增加而增大. 另外, 对其电子结构也进行了计算, 计算结果表明, BC₅存在一个较宽的带隙, 两种原子间有较强的共价杂化, 材料的性质主要由B的2p¹和C的2p²态电子共同决定. 压强对材料带隙和费米能级附近的态密度几乎没有影响, 只引起微小的漂移, 可推断其很好的高压稳定性.     

z切LiTaO3单晶的冲击相变研究

利用二级轻气炮加载下的冲击Hugoniot线(冲击波速度D-粒子速度u关系)和粒子速度剖面测量，结合基于密度泛函理论的平面波赝势计算研究了z切LiTaO₃单晶的高压相变.实验发现，D-u关系在u=0.95km/s附近出现明显拐折；实测波剖面中25.9 GPa和32.6 GPa时观测到弹-塑性双波结构，而终态压力为42.7 GPa和53.0 GPa时则为三波结构.上述结果都清楚地表明z切LiTaO₃单晶冲击相变的发生，相变起始压力约为37.9 GPa.同时，理论计算的菱形相(R3c对称群)压缩线与低压实验数据符合较好，而正交相(Pbnm对称群)压缩线则与扣除热压贡献的高压实验数据相符，由此推断z-切LiTaO₃的高压相为正交结构.从实验和理论上澄清了z切LiTaO₃的相变起始压力和高压相晶体结构的认识，研究工作亦对类似单晶材料的冲击相变研究有参考价值.     

密度渐变多层碳气凝胶靶型的制备

 以间苯二酚-甲醛为原料,结合自制活动式微模具成型工艺制备不同厚度和密度的碳气凝胶薄片,采用密度为10 mg·cm^-3的SiO₂溶胶为“粘合剂”,获得单元薄片厚度在100～580 μm,密度在50～400 mg·cm^-3范围内变化的5层密度渐变碳气凝胶靶型。重点研究了该特殊靶型内部C/SiO₂气凝胶层间界面情况。采用场发射扫描电镜（FESEM）,X射线相衬成像仪等对靶型整体结构及碳气凝胶单元薄片表面和内部微观结构进行了表征。结果表明：胶粘层SiO₂气凝胶厚度约为15 μm,厚度一致,远小于碳气凝胶层厚度且与碳气凝胶薄片的胶粘程度较好,界面平整,靶结构均匀。     

非平衡磁控溅射制备类石墨碳膜及性能研究

利用中频非平衡磁控溅射技术在单晶硅基底上沉积了类石墨碳膜, 采用Raman光谱、高分辨透射电子显微镜、原子力显微镜分析了薄膜微观结构和表面形貌; 采用纳米压痕仪和CSM摩擦磨损试验机测试了碳膜力学性能和摩擦学性能. 结果表明: 利用中频非平衡磁控溅射技术沉积的碳膜是一种以sp²键合碳为主、结构非晶、硬度适中、应力较低、表面粗糙度较大、摩擦性能优异的薄膜. 脉冲占空比对薄膜微观结构和性能有显著影响, 随着脉冲占空比的增大, Raman光谱D峰和G峰的强度比I_D/I_G先减小后增大, 而硬度随脉冲占空比的增大却呈现出相反的变化趋势, 即先增大后减小; 大气氛围中的摩擦性能测试表明, 本实验制备的薄膜具有优异的抗磨性能(～10^-11 cm³/N^-1. m^-1)和承载能力(～2.5 GPa). 随脉冲占空比的增大, 薄膜摩擦系数变化甚微而磨损率却呈现先显著减小后轻微增大的变化趋势. 类石墨碳膜优异的摩擦学性能主要归因于其独特的结构、较低的内应力及良好的结构稳定性.     

A new superhard carbon allotrope: Orthorhombic C20

A new superhard carbon orthorhombic allotrope oC20 is proposed, which exhibits distinct topologies including C4, C3 and two types of C6 carbon rings. The calculated elastic constants and phonon spectra reveal that oC20 is mechanically and dynamically stable at ambient pressure. The calculated electronic band structure of oC20 shows that it is an indirect band gap semiconductor with a band gap of 4.46 eV. The Vickers hardness of oC20 is 75 GPa. The calculated tensile and shear strength indicate that the weakest tensile strength is 64 GPa and the weakest shear strength is 48 GPa, which means oC20 is a potential superhard material.     

Structural,mechanical, and electronic properties of P21/m-Carbon✰

The structural, mechanical, and electronic properties of P2₁/m-carbon were comprehensively investigated by using first principles calculations. Our calculated structure parameters are in good agreement with the previous theoretical values. P2₁/m-carbon consists of 10 atoms in a unit cell and is made of an exclusively sp³ hybridized bonding network. The calculated phonon spectra and elastic constant verify that P2₁/m-carbon is dynamically and mechanically stable at ambient pressure. The analysis of the electronic band structure reveals that P2₁/m-carbon is an insulator with a band gap of 5.47 eV. It has a large bulk moduli of 398 GPa and a high shear moduli of 457 GPa. Further mechanical properties demonstrate that P2₁/m-carbon is prone to be ductile and possesses a high Vickers hardness value of 82.3 GPa. These values show that P2₁/m-carbon simultaneously possesses ultra-incompressible and the superhard property. Furthermore, the X-ray diffraction spectra is also theoretically simulated to provide more structure information for future experimental observations.     

High-throughput calculations screening for new direct band gap superhard carbon allotropes

High-throughput first principles calculations for 109 carbon allotropes were performed. The elastic constants and phonon calculations suggest that these new structures are mechanically and dynamically stable at ambient pressure. Seven direct band gap semiconductor carbon allotropes were uncovered. The Vickers hardness of all seven structures exceeds 40 GPa, indicating that these allotropes are potential superhard materials.     

Mechanical properties and hardness of new carbon-rich superhard C11N4 from first-principles investigations

The structural, mechanical properties and hardness of the new carbon-rich material C₁₁N₄ are studied by first-principles total energy calculations based on the density-functional theory. We use the empirical equations of state (EOS) to investigate the lattice properties and bulk modulus. It is found that the calculated lattice constants and bulk modulus are in good agreement with previous calculations. And the full set elastic constants are calculated using the stress-strain method. The Voigt-Reuss-Hill approximation is used to evaluate the mechanical moduli. The elastic constants show that the two phases of C₁₁N₄ are mechanically stable. The tetragonal-C₁₁N₄ (α-C₁₁N₄) exhibits larger mechanical moduli than the orthorhombic-C₁₁N₄ (β-C₁₁N₄). The mechanical anisotropy is calculated of several different anisotropic indexes and factors, such as universal anisotropic index (A^U), the percent anisotropy (A_G and A_B) and shear anisotropic factors (A₁, A₂ and A₃). Furthermore, the hardness of α-C₁₁N₄ and β-C₁₁N₄ are evaluated according to the intrinsic hardness calculation theory. α-C₁₁N₄ is predicted to be a superhard material with the Vickers hardness of 67.17 GPa, which is slightly higher than that of the cubic boron nitride. And the β-C₁₁N₄ is also a superhard material with the calculated Vickers hardness of 45.63 GPa. C₁₁N₄ can be considered as candidate superhard compounds.     

Full potential calculation of structural, elastic properties and high-pressure phase of binary noble metal carbide: ruthenium carbide

We present in this paper the results of an ab initio theoretical study within the local density approximation (LDA) to determine in rock-salt (B1), cesium chloride (B2), zinc-blende (B3), and tungsten carbide (WC) type structures, the structural, elastic constants, hardness properties and high-pressure phase of the noble metal carbide of ruthenium carbide (RuC).The ground state properties such as the equilibrium lattice constant, elastic constant, the bulk modulus, its pressure derivative, and the hardness in the four phases are determined and compared with available theoretical data. Only for the three phases B1, B3, and WC, is the RuC mechanically stable, while in the B2 phase it is unstable, but in B3 RuC is the most energetically favourable phase with the bulk modulus 263 GPa, and at sufficiently high pressure (P_t=19.2 GPa) the tungsten carbide (WC) structure would be favoured, where ReC-WC is meta-stable.The highest bulk modulus values in the B3, B2, and WC structures and the hardnesses of H(B3)=36.94 GPa, H(B1)=25.21 GPa, and H(WC)=25.30 GPa indicate that the RuC compound is a superhard material in B3, and is not superhard in B1 and WC structures compared with the H(diamond)=96 GPa.     

Origin of incompressibility and hardness from electronic and mechanical properties of hard material ruthenium diboride

We study structural, elastic, and electronic properties for three RuB₂ phases using ab initio total-energy calculations within the density functional theory. The orthorhombic and hexagonal structures are mechanically stable. More precisely, the orthorhombic is more stable than the hexagonal form. Results of bulk modulus, which are in good agreement with experimental data, show that the considered structures are potentially highly compressible materials. This is confirmed by the calculation of the hardness, indicating that RuB₂ is an ultracompressible material, but not a superhard material.     

Structural Transformations of Amorphous Carbon (Glassy Carbon) at High Shock Pressures

Amorphous carbon (glassy carbon) samples were shock compressed up to 80 GPa and temperatures up to 1700 K for several microseconds. Glassy carbon samples before and after an explosive action are analyzed by X-ray diffraction, electron microscopy, and electron-probe microanalysis. It is shown that as a result of microsecond shock pressure exposure, glassy carbon is compacted to ρ_CG ≈ 2.3(5) g/cm³ and is partly transformed into a graphite-like nanomaterial with a cellular structure. At the level of crystallites, the density of glassy carbon increases via a decrease in the interplanar spacings and an increase in the crystallite thickness and width. Spheres from 20 nm to 80 μm in diameter are found to be formed during shock-wave compression of glassy carbon in a copper container and high-temperature shock heating posteffects. Spheres 20 μm in diameter consist of a copper-rich core and a carbon shell.     

Mechanical properties,lattice thermal conductivity,infrared and Raman spectrum of the fullerite C24

Lightweight carbon materials with excellent thermal and mechanical properties have important applications in aerospace industry. In this study, the stability, mechanical properties, lattice thermal conductivity, electronic structure, infrared and Raman spectrum of sp³ hybridized low-density fullerite C₂₄ were investigated according to density functional theory (DFT) calculations. It was found that the fullerite C₂₄ was both thermodynamic and dynamic stable. Quasi-harmonic approximation and Grüneisen parameter calculations clarified why the fullerite C₂₄ had a positive thermal expansion coefficient at low temperature. The fullerite C₂₄ also exhibited excellent mechanical properties. Interestingly, the Vickers hardness of carbon allotropes was found to almost be linear proportional to the density of a carbon material. HSE06 electronic structure calculations showed that it was a semiconductor with direct bandgap of 2.56 eV. Anharmonic lattice dynamic calculations showed that its thermal conductivity was higher than semiconductor silicon. Besides, Raman and infrared active modes as well as the corresponding spectra were presented.     

Superhard F-carbon predicted by ab initio particle-swarm optimization methodology

A simple (5 + 6 + 7)-sp(3) carbon (denoted as F-carbon) with eight atoms per unit cell predicted by a newly developed ab initio particle-swarm optimization methodology on crystal structure prediction is proposed. F-carbon can be seen as the reconstruction of AA-stacked or 3R-graphite, and is energetically more stable than 2H-graphite beyond 13.9 GPa. Band structure and hardness calculations indicate that F-carbon is a transparent superhard carbon with a gap of 4.55 eV at 15 GPa and a hardness of 93.9 GPa at zero pressure. Compared with the previously proposed Bct-, M- and W-carbons, the simulative x-ray diffraction pattern of F-carbon also well matches the superhard intermediate phase of the experimentally cold-compressed graphite. The possible transition route and energy barrier were observed using the variable cell nudged elastic band method. Our simulations show that the cold compression of graphite can produce some reversible metastable carbons (e.g. M- and F-carbons) with energy barriers close to diamond or lonsdaleite.     

Prediction of a superhard material of ReN4 with a high shear modulus

Using first-principles calculations, this paper systematically investigates the structural, elastic, and electronic properties of ReN 4 . The calculated positive eigenvalues of the elastic constant matrix show that the orthorhombic P bca structure of ReN 4 is elastically stable. The calculated band structure indicates that ReN 4 is metallic. Compared with the synthesized superhard material WB 4 , it finds that ReN 4 exhibits larger bulk and shear moduli as well as a smaller Poisson’s ratio. In addition, the elastic constant c 44 of ReN 4 is larger than all the known 5d transition metal nitrides and borides. This combination of properties makes it an ideal candidate for a superhard material.