Structural transition of Ti3SiC2 under high hydrostatic pressure: A first-principles study

We theoretically studied the phase transformation, electronic and elastic properties of Ti₃SiC₂ ceramic by using the pseudopotential plane-wave method within the density functional theory. Our results demonstrate that there exists a structural phase transition from α‐Ti₃SiC₂ to β‐Ti₃SiC₂ under pressure up to 384 GPa, and α‐Ti₃SiC₂ is the most stable phase at zero pressure. The calculated electronic band structure and density of states reveal the metallic behavior for the polymorphs of Ti₃SiC₂. The mechanical stability of α‐Ti₃SiC₂ at zero pressure is confirmed by the elastic constants, and is analyzed in terms of electronic level. By analyzing the ratio between bulk and shear moduli, we conclude that α‐Ti₃SiC₂ is brittle in nature.     

THREE-DIMENSIONAL ANALYSIS OF A MAGNETOELECTROELASTIC HALF-SPACE UNDER AXISYMMETRIC TEMPERATURE LOADING 1)

考虑力-电-磁-热等多场耦合作用, 基于线性理论给出了磁-电-弹性半空间在表面轴对称温度载荷作用下的热-磁-电-弹性分析, 并得到了问题的解析解. 利用Hankel 积分变换法求解了磁-电-弹性材料中的热传导及控制方程, 讨论了在磁-电-弹性半空间在边界表面上作用局部热载荷时的混合边值问题, 利用积分变换和积分方程技术, 通过在边界表面上施加应力自由及磁-电开路条件, 推导得到了磁-电-弹性半空间中位移、电势及磁势的积分形式的表达式. 获得了磁-电-弹性半空间中温度场的解析表达式并且给出了应力, 电位移和磁通量的解析解. 数值计算结果表明温度载荷对磁-电-弹性场的分布有显著影响. 当温度载荷作用的圆域半径增大时, 最大正应力发生的位置会远离半无限大体的边界; 反之当温度载荷作用的圆域半径减小时, 最大应力发生的位置会靠近半无限大体的边界. 电场和磁场在温度载荷作用的圆域内在边界表面附近有明显的强化, 而磁-电-弹性场强化区域的强化程度跟温度载荷的大小和作用区域大小相关. 本研究的相关结果对智能材料和结构在热载荷作用下的设计和制造具有指导意义.     

First-principles study of phase transition and elastic properties of ScSb and YSb compounds

The phase transition of ScSb and YSb from the NaCl-type (B1) structure to the CsCl-type (B2) structure is investigated by the ab initio plane-wave pseudopotential density functional theory method. It is found that the pressures for transition from the B1 structure to the B2 structure obtained from the equal enthalpies are 38.3 and 32.1 GPa for ScSb and YSb, respectively. From the variations of elastic constants with pressure, we find that the B1 phase of ScSb and YSb compounds are unstable when applied pressures are larger than 46.3 and 64.2 GPa, respectively. Moreover, the detailed volume changes during phase transition are analyzed.     

Process and device characteristics of self-assembled metal nano-crystal EEPROM

Metal nanocrystals self-assembled on gate tunneling oxide can be used to replace the conventional Si/Ge nanocrystals as the floating gate in EEPROM cells. We have demonstrated the successful use of Au and W with their respective process dependence and self-assembly characteristics. The new material options can potentially enhance the applicability and functionality of the nanocrystal EEPROM device. Implications on process integration, in particular the control oxide growth and overall thermal budget, are examined by microscopy, gate current injection and channel mobility monitoring. Charging by hot-carrier injection and control gate tunneling have both been observed by shifts in I–V characteristics. The electrostatic behavior of metal nanocrystals is similar to that of Si nanocrystals in terms of the asymmetrical threshold voltage on source–drain reversal after hot-carrier injection and the Coulomb blockade effects. The electrodynamic behavior is expected to be quite different due to the density of states, but further study is required for quantitative analysis.     

延性材料的动态崩落研究

采用Ｇｒａｄｙ发展的一种动态崩落的一般描述方法，考虑了Ｈｅｒｔｚｂｅｒｇ对崩落过程的塑性修正，借助于不同的基体材料的等效应力假定，给出了考虑粘性（应变率）效应的强冲击载荷下延性材料的动态崩落尺寸与孔隙度的定量关系。这对动态断裂的细观描述提供了一个新的途径。     

First-principles study of structural, electronic and elastic properties of single crystal CuZr

Investigations into the structural, electronic and elastic properties of intermetallic CuZr compound had been conducted by the plane-wave pseudopotential method. The calculated lattice constant was consistent well with the experimental value. The absence of band gap and finite value of the density of states (DOS) at the Fermi level reveal the metallic behavior of CuZr crystal, and Zr 4d states give rise to the electrical conductivity. The calculated elastic constants for single crystal CuZr at zero pressure obey the cubic mechanical stability condition, which indicates that the cubic CuZr crystal is mechanical stable at zero pressure. By analyzing the ratio between the bulk and shear moduli, we conclude that CuZr crystal is ductile in nature. The present theoretical investigations might give prediction to polycrystalline CuZr system.     

Pre-kinking of a moving crack in a magnetoelectroelastic material under in-plane loading

A constant moving crack in a magnetoelectroelastic material under in-plane mechanical, electric and magnetic loading is studied for impermeable crack surface boundary conditions. Fourier transform is employed to reduce the mixed boundary value problem of the crack to dual integral equations, which are solved exactly. Steady-state asymptotic fields near the crack tip are obtained in closed form and the corresponding field intensity factors are expressed explicitly. The crack speed influences the singular field distribution around the crack tip and the effects of electric and magnetic loading on the crack tip fields are discussed. The crack kinking phenomena is investigated using the maximum hoop stress intensity factor criterion. The magnitude of the maximum hoop stress intensity factor tends to increase as the crack speed increases.     

Influences of strains on the formation of the quasi-Dirac cone and the Landau levels in black phosphorus

A tunable tight-binding (TB) Hamiltonian, which can be tuned continuously by applying the strains, is proposed based on the two tight-binding Hamiltonian models used to describle the pristine and compressed black phosphorus (BP). This tunable TB Hamiltonian should be of fundamental significance in two aspects: i) A general tunable TB Hamiltonian making it possible to study the BP more conveniently and deeply; ii) An extension of the study of the strain effect from the monolayer phosphorene to the multilayer phosphorene and even the bulk BP in the presence of both the in-plane and out-of-plane strains. Furthermore, based on the tunable TB Hamiltonian we can find the strain-induced quasi-Dirac cone, and clearly disclose how the strains affect the formation of the quasi-Dirac cone. Then we investigate the Landau levels (LLs) of BP under different magnetic fields, uncovering the linear, square-root, or complex dependence of the LLs on the LL number or the magnetic field. Moreover, the double-step and triple-step transitions of the LLs are found to be induced by the strains for the BP in the in-plane and out-of-plane magnetic fields. This research successfully incorporates the out-of-plane strain effect into the tunable TB Hamiltonian, enabling the comprehensive investigation of the strain-correlated effect in BP.     

Transient response of a piezoelectric ceramic with two coplanar cracks under electromechanical impact

The transient response of two coplanar cracks in a piezoelectric ceramic under antiplane mechanical and inplane electric impacting loads is investigated in the present paper. Laplace and Fourier transforms are used to reduce the mixed boundary value problems to Cauchy-type singular integral equations in Laplace transform domain, which are solved numerically. The dynamic stress and electric displacement factors are obtained as the functions of time and geometry parameters. The present study shows that the presence of the dynamic electric field will impede or enhance the propagation of the crack in piezoelectric ceramics at different stages of the dynamic electromechanical load. Moreover, the electromechanical response is greatly affected by the ratio of the space of the cracks and the crack length.     

压电介质损伤、断裂力学研究的现状

压电介质的损伤与断裂力学是现代固体力学的重要课题．本文简要地综述了压电介质损伤与断裂力学研究的现状,集中讨论了：（１）裂纹面电边界条件的不同模型及其求解的结果;（２）宏观连续力学与细观力学用于压电介质的损伤与断裂的静力学分析;（３）压电介质动态断裂分析的某些新结果．文末,指出了今后在压电介质损伤与断裂研究的某些有吸引力的研究方向