3D Raman mapping of uniaxially loaded 6H‐SiC crystals

Raman spectroscopy is used to investigate the three‐dimensional stress distribution in 6H‐silicon carbide (SiC) specimens subjected to stresses up to 3.7 GPa along the c‐axis. Specifically, the relative Raman shift of the longitudinal optic phonon of 6H‐SiC is used to evaluate the local stress across the bulk crystal. For this purpose, an anvil device with opposed 6H‐SiC and sapphire specimens was used. After subjecting the anvils to uniaxial load, several series of two‐dimensional Raman maps were registered at different depths in the 6H‐SiC anvil. The analysis of the Raman spectra reveals an exponential decay of the stress as a function of the depth. A novel phenomenological Grüneisen‐like model is introduced here to account for such observation. On the contrary, the in‐plane stress analysis shows a radial Gaussian‐like distribution regardless the depth, a distinct behavior that is attributed to the appearance of shear stress components. The suitability of both models and their applicability to other materials are discussed, along with some future directions. Copyright © 2013 John Wiley & Sons, Ltd.     

A theoretical conformational study on the structural parameters involved in the ring strain of exo-unsaturated four-membered heterocycles,Y = CCH2CH2X

The effect of ring-puckering angle on the structural parameters (bond lengths and angles) involved in the ring strain of a series of four-membered heterocycles (1–16) was theoretically demonstrated by using the ab initio methods MP2 and HF, and the DFT methods PBE1PBE, B3LYP, SVWN5 with 6-31+G(d,p) as basis set. The results revealed that the bonds within the ring (C–X and C–C) are the most sensitive to puckering angle changes. The variation of the C–X and C=Y bond lengths as function of puckering angle are determined by a balance between the 1,3 repulsive interactions and the electronic nature of the heteroatoms X and Y. Particularly, for azetidines and phosphetanes, the C–X and C=Y bond lengths exhibit a major increase at axial conformations. In general, the C–C bond length decreases with the puckering angle for all heterocycles. While the heteroatom–H bonds (in the ring skeleton) are very sensitive to geometric changes, exhibiting an increasing behaviour for equatorial conformations and a decreasing behaviour for axial conformations highly puckered (? > ?20°). The C–X–C angle decreases monotonically with the puckering angle, increasing the Baeyer strain on the studied molecules. Finally, all methods predicted a similar behaviour for the studied parameters as function of the puckering angle, although some smaller differences in the predictions of their respective values, especially at HF level, were observed.     

Effect of uniaxial strain on the Drude weight of graphene

We study the dependence on the strength and orientation of applied uniaxial strain of the Drude weight in the conductivity of graphene. We find a non-monotonic dependence on strain, which may be related to the proximity to several strain-induced electronic topological transitions. Given the relation between the Drude weight and the long-wavelength plasmon frequency, such a strain dependence can be evidenced by infrared spectroscopy measurements.     

铁电薄膜漏电流的应变调控

基于密度泛函理论的第一性原理并结合非平衡格林函数, 探讨了应变对 BaTiO₃ 铁电薄膜漏电流的影响规律.研究表明,压应变能有效地抑制BaTiO₃ 铁电薄膜漏电流, 特别是当压应变为4%时,其漏电流相对无应变状态降低了近10 倍.通过考察体系的透射系数和电子态密度发现: 一方面压应变状态下铁电隧道结的透射几率要比张应变时小,特别是在费米面附近;另一方面随着张应变过渡至压应变时,价带的位置逐渐向低能区移动而导带向高能区移动,导致了其带隙的增大, 从而有效抑制了漏电流. 本研究为寻找抑制铁电薄膜漏电流,提高铁电薄膜及铁电存储器的性能提供合适的方法. 关键词： 铁电薄膜 双轴应变 漏电流 第一性原理     

SiN插入层对GaN外延膜应力和光学质量的影响

研究了MOCVD系统中原位SiN插入层对GaN薄膜应力和光学性质的影响。采用SiN插入层后,GaN薄膜的裂纹数量大大减少,薄膜所承受的张应力得到了一定的释放。同时,GaN薄膜的缺陷密度降低一倍,晶体质量得到了极大的改善。研究表明,位错密度的降低在GaN薄膜中留存较大的残余应力,补偿了降温过程中所引入的张应力。同样,随着SiN插入层的应用,低温PL谱的半峰宽降低,薄膜光学质量提高。最后研究了PL谱发光峰与应力的关系,得到了一个-13.8的线性系数。     

Algorithmic tangent modulus at finite strains based on multiplicative decomposition简

The algorithmic tangent modulus at finite strains in current configuration plays an important role in the nonlinear finite element method. In this work, the exact tensorial forms of the algorithmic tangent modulus at finite strains are derived in the principal space and their corresponding matrix expressions are also presented. The algorithmic tangent modulus consists of two terms. The first term depends on a specific yield surface, while the second term is independent of the specific yield surface. The elastoplastic matrix in the principal space associated with the specific yield surface is derived by the logarithmic strains in terms of the local multiplicative decomposition. The Drucker-Prager yield function of elastoplastic material is used as a numerical example to verify the present algorithmic tangent modulus at finite strains.     

Constitutive modeling of isotropic hyperelastic materials in an exponential framework using a self-contained approach

In this paper, an exponential framework for strain energy density functions of elastomers and soft biological tissues is proposed. Based on this framework and using a self-contained approach that is different from a guesswork or combination viewpoint, a set strain energy density functions in terms of the first and second strain invariants is rebuilt. Among the constructed options for strain energy density, a new exponential and mathematically justified model is examined. This model benefits from the existence of second strain invariant, simplicity, stability of parameters, and the state of being accurate. This model can capture strain softening, strain hardening and is able to differentiate between various deformation-state dependent responses of elastomers and soft tissues undergoing finite deformation. The model has two material parameters and the mathematical formulation is simple to render the possibility of numerical implementations. In order to investigate the appropriateness of the proposed model in comparison to other hyperelastic models, several experimental data for incompressible isotropic materials (elastomers) such as VHB 4905 (polyacrylate rubber), two various silicone rubbers, synthetic rubber neoprene, two different natural rubbers, b186 rubber (a carbon black-filled rubber), Yeoh vulcanizate rubber, and finally porcine liver tissue (a very soft biological tissue) are examined. The results demonstrate that the proposed model provides an acceptable prediction of the behavior of elastomers and soft tissues under large deformation for different applied loading states.     

Wrinkled membrane element based on the wrinkling potential

For a membrane structure, wrinkles have an important effect on its mechanical behaviors. Wrinkling level characterizes the development of wrinkles and reflects the performance of a membrane in its service, and it plays a very significant role in the wrinkling analysis. The shell elements combined with the stability theory would be an ideal solution to the wrinkling problem. However, this approach requires a dense mesh and the computation is very time consuming. Also the wrinkling parameters are very sensitive to the size of shell elements. Existing wrinkling models based on membrane elements are derived from the Tension Field Theory which are incapable of describing fully the wrinkling behaviors.A new wrinkling model adopting the wrinkling strain as a measure of the wrinkling level is proposed in this paper to address these issues. According to the analogy between the wrinkling strain and the elasto-plastic strain, a wrinkling potential surface is assumed to exist and its normal direction defines the direction of the wrinkling strain tensor by virtue of the flow rule. Based on the consistent condition of the wrinkling potential surface, a modified constitutive tensor is obtained. To avoid the switching of the wrinkling state in the numerical solution, a new wrinkling criterion is proposed, in which the predominant influence of the previous state is included. Besides, a new approach to determine the wrinkling orientation is given to improve the efficiency of convergence in the slack region. The objectivity of the wrinkling coordinate frame is also demonstrated as an accompanying set of results. Finally, two benchmark problems are analyzed with the proposed wrinkling model, and their results are compared with those in the literatures. Results indicate that the proposed wrinkling model is valid and accurate to characterize the wrinkling level of a membrane and it exhibits efficient convergence even in the slack region.     

基于纤芯失配和光纤布拉格光栅实现温度和应变同时测量

基于纤芯失配理论,提出了一种多模单模多模（MSM）结构与光纤布拉格光栅（FBG）级联实现温度和应变同时测量的光纤传感器。利用MSM结构的干涉谱和FBG对温度和应变的不同响应灵敏度,实现了对温度、应变的同时测量。实验结果表明,在20 ℃~80 ℃的温度范围内,MSM结构的干涉谱和FBG的温度灵敏度分别为0.091 nm/℃和0.0102 nm/℃;在0～650 με的应变范围内,应变灵敏度分别为 -0.0013 nm/με和0.0012 nm/με。因此利用敏感矩阵,即可实现对温度和应变的同时测量,且温度和应变的最大测量误差分别为±0.2 ℃和±8.25 με。该结构灵敏度高,结构简单,且不易受电磁等干扰,实验结果具有良好的线性度,在工程领域应用前景良好。