纳米Si薄膜光学性质和弹性常数的研究

采用基于密度泛函理论的第一性原理的方法,对厚度为0.54～3.30nm纳米Si薄膜的电子结构、光学性质及弹性常数进行了计算。结果表明,纳米Si薄膜是直接带隙半导体材料;随着纳米Si薄膜厚度的减小,带隙逐渐增大;薄膜的光学吸收边发生蓝移,吸收带出现宽化现象;弹性常数、杨氏模量和泊松比呈现尺寸效应。     

Ce3Th合金高压相变的第一性原理研究

为探究Ce3 Th合金在高压下的相变规律及物理性质,本文利用基于密度泛函理论的虚晶近似(VCA)对Ce3 Th进行了系统的理论研究.通过对计算结果的分析,发现了Ce3 Th合金在压力下的相变规律.当压强增加到25 GPa附近,轴向比c/a突然从2跃迁到1.68左右,fcc结构开始向bct结构转变.同时,通过计算弹性性质...     

高压下Zr2AlC电子结构的第一性原理研究

通过基于密度泛函理论的第一性原理方法,计算了三元层状化合物Zr2AlC在不同压力下的电子结构。在0~40GPa的压力范围内,Zr2AlC沿c轴的压缩性较a轴更容易,晶格的内坐标z随压力的增加而增加。对能带结构和电子态密度分析表明,能带随着压力的增加将会展宽,费米能级处的电子态密度减少,金属性减弱。在Zr2AlC中还表现出一定共价键和离子键特性,其中共价性随压力的增加而增强。     

Ru-Cr下底层成分对Co-W磁性薄膜结构和磁性能的影响

用直流磁控溅射在300℃不同Ru-x%Cr(x=0,20,40)下底层沉积制备了Co-15%W(原子分数)磁性薄膜。用XRD表征了薄膜的精细晶体结构,分析了Co-W和Ru-Cr层的取向关系,估算了薄膜层的晶格常数、c轴分散角、fcc/hcp体积百分比和堆垛层错密度值。结果表明,随着Ru-Cr下底层Cr成分的增加Co-W磁性层和Ru-Cr下底层间的晶格错配度降低,hcp-Co的晶格常数比c/a随之减小。磁性能测试结果证实,由于Co-W磁性层中晶格常数比c/a的减小磁性层的磁各向异性能显著提高。     

法向载荷对紫铜的微米划痕测试的影响

使用Rockwell C金刚石圆锥压头对紫铜进行微米划痕实验,研究了法向载荷对样品的微米划痕测试的影响。结果表明:随着法向载荷的增大,压入深度和残余深度均线性增加,弹性恢复率线性减小;划痕宽度随压入深度的增加先非线性地增大,之后趋于线性增加。当法向载荷在0.08~0.11N的范围内时,摩擦力线性增大,摩擦系数趋于一个常数,摩擦机制为粘着摩擦;当法向载荷在0.11~17N的范围内时,摩擦力和摩擦系数非线性地增大,摩擦机制为犁沟摩擦;当法向载荷在17~28N的范围内时,摩擦系数趋于一个常数,摩擦力线性增大,摩擦机制为微切削。     

（001）应变对正交相Ca2P0.25Si0.75能带结构及光学性质的影响

为了研究(001)应变对正交相Ca2P0.25Si0.75能带结构及光学性质的影响,采用第一性原理贋势平面波方法对(001)应变下正交相Ca2P0.25Si0.75的能带结构及光学性质进行了模拟计算.计算结果表明:晶格(001)面发生100%~116%张应变时,带隙随着应变增加而减小;在晶格发生88%~100%压应变时,带隙随着张应变的增加而增加;84%~88%压应变时,带隙随着压应变的增加而减小.当施加应变后光学性质发生显著的变化:随着压应变的增加,静态介电常数、折射率逐渐减小,张应变则增大.施加压应变反射向高能方向偏移,施加张应变反射向低能方向偏移,但施加应变对反射区域的影响不显著.施压应变吸收谱、光电导率的变化与介电函数和折射率相反.综上所述,(001)应变改变了Ca2P0.25Si0.75的电子结构和光学常数,是调节Ca2P0.25Si0.75光电传输性能的有效手段.     

第一性原理研究α-Al2O3的电子结构、力学性质和德拜温度

采用基于密度泛函理论的平面波超软赝势的第一性原理方法,利用CASTEP软件计算了α-Al_2O_3的电子结构、力学性质和德拜温度。计算结果显示优化后的晶格常数与实验数据基本一致。能带结构图表明α-Al_2O_3具有典型的离子化合物特征,禁带宽度为5.964eV。计算的态密度表明α-Al_2O_3的下价带在-19.426~-15.370eV之间,态密度主要由O的2s轨道电子贡献;上价带在-7.178~0.711eV之间,态密度主要由O的2p轨道电子贡献;导带在5.698~14.977eV之间,态密度主要由Al的3s和3p轨道电子贡献。差分电荷密度分布表明α-Al_2O_3中存在离子键和共价键。力学性质的计算结果表明α-Al_2O_3的体积模量为224.885GPa,剪切模量为144.687GPa,弹性模量为357.411GPa,泊松比为0.235,各向异性指数为0.220;α-Al_2O_3剪切模量与体积模量的比值为0.643,表明α-Al_2O_3具有脆性。α-Al_2O_3的德拜温度为974.834K。     

Mn双掺间距对ZnO磁光性能的影响

采用自旋密度泛函理论(DFT)下的广义梯度近似(GGA)平面波超软赝势法构建了含氧空位以及不含氧空位的Mn双掺ZnO超胞模型,分析了电子自旋极化处理下两个磁性Mn离子之间的距离变化(分为沿c轴和沿垂直c轴方向变化)对ZnO的结构稳定性、磁性以及吸收光谱的影响。随着磁性离子距离逐渐增大,无氧空位掺杂体系ZnO超胞的形成能沿着c轴和垂直c轴都逐渐减小,而含氧空位掺杂体系ZnO超胞的形成能沿垂直c轴方向逐渐减小,沿c轴方向逐渐增大;含氧空位和不含氧空位ZnO体系的反铁磁性随着Mn-Mn间距增大而减弱,沿着c轴方向减弱更加明显,而且氧空位促进体系的反铁磁性减弱;不含氧空位组态沿c轴方向吸收光谱发生了红移现象,而沿垂直c轴方向吸收光谱发生了蓝移现象;但是含氧空位组态沿c轴和沿垂直c轴方向吸收光谱都发生了红移。     

单斜氧化铜薄膜的磁控溅射制备及表征北大核心CSCD

采用磁控溅射,通过氧氩比的调制在玻璃衬底上沉积了单斜结构的Cu O薄膜,并重点研究了氧氩比对薄膜微结构及光学吸收边的影响。研究表明随着氧氩比的增加,薄膜逐渐从多相向单相转变,薄膜表面逐渐趋于光滑和平整,Cu O<-111>择优取向逐渐减弱。晶轴上的晶格畸变结果显示氧氩比对晶轴a,b和c的影响不尽相同。a,b轴上呈现的压应力均先明显增大然后变得稳定,而c轴上则呈现了压应力向拉应力的转变。氧氩比对晶格常数的影响反应了a,b和c轴上晶粒生长的各向异性。Cu O薄膜为直接带隙半导体,其1.9 e V附近的光学吸收边随着氧氩比的增加而略微蓝移,吸收边的蓝移归结于晶粒的量子尺寸效应。     

热电材料CoSi和CrSi_2的晶格热膨胀性

利用动态高温X射线衍射技术分别对立方CoSi和六方CrSi2化合物在298~973 K温度范围内的晶格热膨胀性进行了研究。结果表明:化合物CoSi的点阵参数随温度升高呈线性增长关系,其平均线热膨胀系数aα和平均体热膨胀系数αV分别为1.14×10-5K-1和3.42×10-5K-1,两者之间符合立方晶系关系式,即3αa=αV;化合物CrSi2的点阵参数随温度升高而显著增大,其中沿a轴和c轴的平均线热膨胀系数及平均体热膨胀系数分别为αa=0.96×10-5K-1,cα=0.73×10-6K-1和αV=2.45×10-5K-1,三者之间符合六方晶系关系式,即2αa+cα=αV;化合物CrSi2沿a轴方向的线热膨胀系数远大于沿c轴方向的,呈较强的各向异性。     

Joining of SiC ceramic-based materials with ternary carbide Ti3SiC2

The joining of two pieces of SiC-based ceramic materials (SiC or C_f/SiC composite) was conducted using Ti₃SiC₂ as filler in vacuum in the joining temperatures range from 1200 °C to 1600 °C. The similar chemical reactions took place at the interface between Ti₃SiC₂ and SiC or C_f/SiC, and became more complete with joining temperature increases, and with the consequent increased joining strengths of the SiC and C_f/SiC joints. Based on the XRD and SEM analyses, it turns out that two reasons are most important for the high joining strengths of the SiC and C_f/SiC joints. One is the development of layered Ti₃SiC₂ ceramic, which has plasticity in nature and can contribute to thermal stress relaxation of the joints; the other is the chemical reactions between Ti₃SiC₂ and the base materials which result in good interface bonding.     

FP-LAPW study of the fundamental properties of the cubic spinel CdAl2O4

We have investigated the structural, elastic, electronic, optical and thermodynamic properties of the cubic spinel CdAl₂O₄ using accurate ab initio calculations. Computed equilibrium structural parameters are in good agreement with the available experimental data. Single-crystals elastic parameters are calculated for pressure up to 30 GPa using a conserving-volume total energy-strain method. Isotropic elastic parameters for ideal polycrystalline CdAl₂O₄ aggregates are computed in the framework of the Voigt-Reuss-Hill approximation. Result for band structure using the Engel-Vosko scheme of the GGA shows a significant improvement over the common GGA functionals. Optical spectra have been calculated for the energy range 0-30 eV. The peaks and structures in the optical spectra are assigned to interband transitions. Pressure dependence of the band gaps, static dielectric constant and static refractive index are also investigated. Pressure and thermal effects on some macroscopic properties are predicted using the quasi-harmonic Debye model.     

First-principles calculations of structural, electronic, and elastic properties of MgZrSi2O7

For the first time, the recently synthesized pyrochlore MgZrSi₂O₇ [J. Xu et al., Mater. Chem. Phys. 128 (2011) 410] has been analyzed using the first principles calculations. The electronic and elastic properties were predicted; in particular, the band gap is indirect and has the value of 6.75 eV. The bulk modulus equals to 186.51 ± 1.95 GPa. Anisotropy of elastic properties was analyzed by comparing the upper and lower estimates of the shear moduli. In addition, directional dependence of the Young's modulus was calculated and visualized; its value varies in the range from 249.7 GPa (along the a, b, c crystallographic axes) to 136.84 GPa (along the bisector direction in any of the ab, bc, ac planes).     

Effects of infiltration parameters on the synthesis of nano-laminated Ti3SiC2

In this paper, the nano-laminated Ti₃SiC₂ ceramics were fabricated by liquid silicon infiltration of gelcast porous titanium carbide (TiC) preforms. The phase compositions and microstructures of the synthesized samples at various infiltration times and temperatures were analyzed by the X-ray diffraction (XRD) technique and were observed by field emission scanning electron microscopy (FESEM). The results showed that the formed Ti₃SiC₂ decomposes to the TiC phase with the increase of infiltration time. It was found from the XRD patterns that the samples with an 88?wt% Ti₃SiC₂ MAX phase can be produced with infiltration at 1500°C for 1?h with 50 vol% solid loading and 10?wt% monomer content. It is found that the hardness and flexural strength of Ti₃SiC₂-based ceramic has been reduced with a decrease in SiC and TiC impurities and reach 5.8?GPa and 420?MPa, respectively, for the sample with 15?wt% impurity. The microstructure evaluation revealed that the purity and properties of samples were affected both through the gelcasting and infiltration parameters.     

Influence of N2 partial pressure on mechanical properties of (Ti,Al)N films deposited by reactive magnetron sputtering

The influence of the nitrogen partial pressure on the mechanical properties of (Ti,Al)N films deposited by DC reactive magnetron sputtering using a Ti-Al mosaic target at a substrate bias of −100 V is investigated. Nanoindentation tests reveal that with increasing N₂ partial pressure, the film hardness and elastic modulus increase initially and then decrease afterwards. The maximum hardness and elastic modulus are 43.4 GPa and 430.8 GPa, respectively. The trend is believed to stem from the variations in the grain size and preferential orientation of the crystals in the (Ti,Al)N films fabricated at varying N₂ partial pressure. The phenomenon is confirmed by results acquired using glancing angle X-ray diffraction (XRD) and energy dispersive X-ray spectroscopy (EDS).     

Structural, elastic, electronic, chemical bonding and thermodynamic properties of CaMg2N2 and SrMg2N2: First-principles calculations

We report first-principles density functional theory calculations of the structural, elastic, electronic, chemical bonding and thermodynamic properties of the ternary alkaline earth metal nitrides CaMg₂N₂ and SrMg₂N₂. The calculated equilibrium structural parameters agree well with the experimental findings. Single-crystal and polycrystalline elastic constants and some related properties under pressure effect have been predicted. Both compounds exhibit a striking elastic anisotropy and a ductile behavior. Electronic properties and chemical bonding nature have been studied throughout the band structure, density of states and charge distribution analyses. It is found that these two materials have a direct band gap (Γ-Γ) and a transition to an indirect gap (Γ-M) occurs at about 8.63 and 5.16 GPa in CaMg₂N₂ and SrMg₂N₂, respectively. The chemical bonding has a mixture covalent-ionic character. Thermal effects on some macroscopic properties are predicted using the quasi-harmonic Debye model.     

Electric response of Pb0.99[(Zr0.90Sn0.10)0.968Ti0.032]0.98Nb0.02O3 ceramics to the shock-wave-induced ferroelectric-to-antiferroelectric phase transition

Shock-wave-enforced ferroelectric (FE)-to-antiferroelectric (AFE) phase transition releases a large electrical polarization, having application in pulse power technology. In the present work, the depoling currents under shock wave compression were investigated in Pb_0.99[(Zr_0.90Sn_0.10)_0.968Ti_0.032]_0.98Nb_0.02O₃ (PZST) ceramics with composition close to the FE/AFE phase boundary. Shock wave was generated by gas-gun and propagated in a direction perpendicular to the remanent polarization. It was found that the shock pressure promoted the phase transition under the short-circuit condition. The shock pressure dependence of the released charge was associated with the evolution of FE-to-AFE phase transition. The onset of phase transition was about 0.40 GPa and complete transformation occurred at 1.23 GPa. However, the released charge decreased with increasing load resistance. The reason may be that the electric field suppresses the phase transition in uncompressed zone and/or shock induces conductivity in compressed zone. Results lay the foundation for application of PZST ceramics in shock-activated power supply.     

Structural and thermodynamic properties of antiperovskite SbNMg3

The full potential augmented plane wave plus local orbital (FP-LAPW + lo) method using the generalized-gradient approximation within the framework of density functional theory is applied to the study of the lattice constant, bulk modulus, pressure derivative of the bulk modulus and elastic constants of antiperovskite semiconductor SbNMg₃. The quasi-harmonic Debye model, in which the phononic effects are considered, is applied to the study of the thermodynamic properties. The temperature effect on the structural parameters, bulk modulus, thermal expansion coefficient, specific heats and Debye temperatures in the whole pressure range from 0 to 30 GPa and temperature range from 0 to 1200 K.     

Elastic modulus and hardness of CaTiO3, CaCu3Ti4O12 and CaTiO3/CaCu3Ti4O12 mixture

Three ceramic systems, CaTiO₃ (CTO), CaCu₃Ti₄O₁₂ (CCTO) and intermediate nonstoichiometric CaTiO₃/CaCu₃Ti₄O₁₂ mixtures (CTO.CCTO), were investigated and characterized. The ceramics were sintered at 1100 °C for 180 min. The surface morphology and structures were investigated by XRD and SEM. Elastic modulus and hardness of the surfaces were studied by instrumented indentation. It was observed that CCTO presented the higher mechanical properties (E = 256 GPa, hardness = 10.6 GPa), while CTO/CCTO mixture showed intermediate properties between CTO and CCTO.     

First-principles calculations of structural,elastic and electronic properties of TaB2 with AlB2 structure under high pressure

We present a systematic theoretical study for the structural, elastic and electronic properties of TaB₂ with AlB₂ structure under pressures ranging from 0 GPa to 120 GPa within the framework of density-functional theory in this paper. The results at zero pressure are in good agreement with available theoretical and experimental values. Our attention has been focused on high pressure behavior of TaB₂. The pressure dependence of structure, elastic constants, Debye temperature, and density of states (DOS) are successfully calculated and discussed.