超硬B-C-N材料的电子结构、硬度和光学性质的第一性原理计算

本文基于密度泛函理论(DFT)的第一性原理平面波超软赝势方法计算了z-BC₂N和z-B₂CN的4种晶体结构的电子结构、硬度和光学性质。结果表明,z-BC₂N(2)为直接带隙半导体,其禁带宽度2.449 eV,z-BC₂N(1)为间接宽带隙半导体,其禁带宽度为3.381 eV,而z-B₂CN(1)和z-B₂CN(2)为导体;硬度结果显示z-BC₂N(1)、z-BC₂N(2)和z-B₂CN(1)为超硬材料。最后通过计算z-BC₂N基本光学函数与光子能量的关系表征了其光学性质。分析结果表明,z-BC₂N结构可以用作良好的耐磨材料和窗口耐热材料。     

高压相立方氮化物的硬度预测

硬度是一个复杂的物理量,用第一性原理难以描述,我们基于固体硬度等于单位面积上所有键对压头的抵抗力之和的观点,从化学键理论出发定义了物质的硬度.本文利用复杂晶体的化学键理论计算了立方氮化物高压相的化学键参数,结果表明这些氮化物具有高的共价成键特性.利用硬度的化学键理论预测了立方氮化物高压相的硬度,通过与实验值的比较说明了结果的合理性.     

KDP晶体中铝取代钾点缺陷第一性原理计算

为了明确Al3+在KDP晶体生长过程中对光学性质和力学性质的具体影响,采用第一性原理计算程序包VASP软件计算并分析了Al取代K对KDP晶体的晶体结构、电子能态密度和光学性质,并同理想KDP晶体进行对比研究.结果表明,KDP晶体中Al取代K的缺陷形成能为0.974 eV,并且Al替位K点缺陷引起的晶格畸变非常微弱,缺陷比较容易形成. Al取代K后晶体能带中价带顶附近的态密度发生了变化,并且带隙中存在缺陷能级,取代后KDP晶体的带隙宽度减小为4.37 eV,缺陷增加了KDP晶体对可见到紫外波段的光子吸收,影响KDP晶体光学质量及其激光损伤性能.计算力学性质发现,Al替位掺杂KDP晶体比理想KDP晶体的杨氏模量增加了,这会减弱晶体抗激光损伤能力.     

第一性原理研究硅酸钇晶体的电子结构和物理性质

基于第一性原理利用CASTEP软件系统地模拟计算了理想的单斜硅酸钇晶体的电子结构、光学、力学和热力学等物理性质.光学性质的计算结果表现出了与硅酸钇结构一致的各向异性,比较得到的力学性质与实验结果基本吻合,表明硅酸钇是一种良好的韧性与耐磨性材料.运用线性响应的方法确定了声子态密度,得出其热力学性质,如等容比热、熵和德拜温度,得到的热容值与实验值符合较好.     

激光晶体的化学键和折射率

利用介电描述的化学键理论求得８种激光基质晶体的化学键参数和晶体的折射率，计算的折射率和实验测量值符合较好。     

理论研究Stone-Wales缺陷和C掺杂对手性BN纳米带的带隙调控

通过第一性原理密度泛函理论的方法,研究了Stone-Wales 缺陷和C掺杂对手性BN纳米带的带隙调控.结果表明,Stone-Wales 缺陷使得BN纳米带的价带顶(VBM)和导带底(CBM)的占据态发生变化,从而引入了缺陷能级降低了带隙,但Stone-Wales 缺陷的个数对带隙的大小影响不明显.电子结构计算表明,带Stone-Wales 缺陷的BN纳米带的缺陷能级主要是由VBM附近形成N-N原子的类π键轨道和CBM附近形成B-B原子的类σ键分布决定.通过在带Stone-Wales 缺陷的BN纳米带中引入C掺杂改变杂质能级的分布,在VBM附近形成了C-C原子的类σ键轨道和CBM附近形成了C-B原子的类σ键,这样可以进一步降低BN纳米带的带隙,拓展了BN纳米带的应用.     

单粒子在金刚石(001)表面吸附与迁移的第一性原理计算

为了探究纳米金刚石复合薄膜中界面相粒子的微观行为,采用第一性原理方法计算了碳、硅单粒子在清洁金刚石(001)表面的吸附作用与迁移行为.包括C、Si粒子在金刚石(001)面四个高对称位置的构型总能和吸附能以及其在金刚石(001)表面的迁移激活能.结果表明:最稳定的构型是沿(001)生长方向沉积粒子与表面层两粒子相接,且C、Si粒子迁移激活能分别为2.824 eV、0.475 eV.两激活能的差异表明:添加Si能显著促进碳粒子的扩散并形成更加致密的纳米金刚石复合薄膜.     

Hardness Anisotropy of Rhombohedral Crystals of Calcite. Variation of Hardness with Orientation

The variation of hardness of rhombohedral single crystals of calcite with orientation of longer diagonal of Knoop indenter with respect to direction [100] on a cleavage plane (100) is studied. The Knoop hardness number H which has a constant value in the range of high loads (40–80 g) varies with orientation A and quenching temperature. By following phenomenological approach, an empirical relation √HA = mA + C is developed from these studies. The effects of crystal anisotropy and thermal treatment of specimens after indentation are also discussed.     

Quench Hardness Anisotropy of Rhombohedral Crystals: Calcite

Variation of microhardness with orientation of indenter (A) and quenching temperature T_q is studied on freshly cleaved surfaces {100} of natural rhombohedral crystals of calcite by producing Knoop indentations for various applied loads. The relation between hardness H_A and quenching temperatures for the high loads (40–80 g) is graphically established. It also depends on the crystal orientation. The anisotropic indices are also determined.     

Quench Hardness Anisotropy of Rhombohedral Crystals. Sodium Nitrate

The effects of quenching temperature and orientation of Knoop indenter on microhardness of cleavage planes of synthetic sodium nitrate crystals are studied for the range of applied loads (20–160 g) in the high load region. Phenomenological approach was followed to obtain an empirical relation connecting Knoop hardness number, orientation, and quenching temperature. Anisotropic indices are also determined. The implications are discussed.     

Effect of Various Impurities on the Hardness of NaCl Crystals

The hardness of NaCl crystal in the presence of mono, divalent and polyvalent ions were measured. Measurements were made in the indentation load range from 5 x 10^‐3 to 20 x 10^‐3 N. The measured data showed that there is an indentation size effect. Classical Meyer's law was used for the characterization of crystal hardness f NaCl. The Meyer index was found to be smaller than 2 indicating brittle material characteristic. The PRS model was also used for the determination of the load‐independent microhardness value. It was found that the crystal hardness of NaCl is chancing depending on the type of impurity and the concentration.     

Hardness of Crystals with NaCl Structure and the Significance of the GILMAN-CHIN Parameter

The microhardness of PbTe, PbSe, EuS and EuSe has been determined. The microhardness vs lattice constant plot for chalcogenides is a curve similar to the one for alkali halides. The Gilman-Chin parameter (H/C₄₄) has been calculated for a large number of crytals with NaCl structure. There is no unique value representing the entire group of crystals, the values ranging from 0.01 to 0.20. The Gilman-Chin parameter broadly correlates with the effective ionic charge (e) indicating that it is a measure of the ionicity. Empirically, it is observed that the product (H/C₄₄) · (e)² is approximately constant.     

Structural Properties of o-Sulphobenzoimide in Complex Crystals

The saccharin (o-sulphobenzoimide) crystal is synthesized and its crystal structure has also been determined. Saccharin, as a ligand, can be coordinated with a metal ion in different modes. The variations of the saccharin molecular structure in different coordination environments are studied. As the saccharin molecule takes part as ligand or anion, its molecule structure will take some obvious changes. The factors of these changes may mostly be due to the coordination environment and hydrogen bond. The effects of different coordinated metal ion and different coordination mode are very small.     

Calculation of Birefringence Changes in Anthracene and Phenanthrene Crystals

Birefringence changes due to thermal expansion have been calculated for anthracene and phenanthrene. The method assumes that the effective molecular polarizability calculated at room temperature is constant in the molecular axis system for small changes of molecular orientation and separation. For anthracene, the calculations reproduce the observed birefringence change quantitatively. For phenanthrene, two thirds of the observed change is due to thermal expansion, the rest to structural alterations at the 70°C phase transition; transition to a Pa structure is ruled out. Birefringence calculations could be used to test other high-temperature structures for phenanthrene which might be suggested by lattice-energy calculations.     

Nonlocal Continuum Theory of Liquid Crystals

A nonlocal continuum theory of liquid crystals is constructed to explain and predict the physical behavior of liquid crystals under long range intermolecular forces Balance laws consist of conservation of mass and mocroinertia, balance of momenta and energy. Constitutive equations are given for the equibilibirium and non-equilibirium parts of the stress, couple strees, free energy, entropy an nonlocal body force and couple. Thermodynamic restrinctions and material frame-indifference are studied. The theory is valid for liquid crystals having arbitrary shapes (inertia), Passage is made to the thread-like molecuels and to local theory. Applications are considered to two-dimensional problmes, steady, plane shear flows and disperison of twist waves.