Elasticity, stability, and ideal strength of beta-SiC in plane-wave-based ab initio calculations

On the basis of the pseudopotential plane-wave method and the local-density-functional theory, this paper studies energetics, stress-strain relation, stability, and ideal strength of beta-SiC under various loading modes, where uniform uniaxial extension and tension and biaxial proportional extension are considered along directions [001] and [111]. The lattice constant, elastic constants, and moduli of equilibrium state are calculated and the results agree well with the experimental data. As the four SI-C bonds along directions [111], [(1) over bar 11], [11(1) over bar] and [111] are not the same under the loading along [111], internal relaxation and the corresponding internal displacements must be considered. We find that, at the beginning of loading, the effect of internal displacement through the shuffle and glide plane diminishes the difference among the four Si-C bonds lengths, but will increase the difference at the subsequent loading, which will result in a crack nucleated on the {111} shuffle plane and a subsequently cleavage fracture. Thus the corresponding theoretical strength is 50.8 GPa, which agrees well with the recent experiment value, 53.4 GPa. However, with the loading along [001], internal relaxation is not important for tetragonal symmetry. Elastic constants during the uniaxial tension along [001] are calculated. Based on the stability analysis with stiffness coefficients, we find that the spinodal and Born instabilities are triggered almost at the same strain, which agrees with the previous molecular-dynamics simulation. During biaxial proportional extension, stress and strength vary proportionally with the biaxial loading ratio at the same longitudinal strain.

Calculations of longitudinal and transverse velocity structure functions using a vortex model of isotropic turbulence

The longitudinal structure function (LSF) and the transverse structure function (TSF) in isotropic turbulence are calculated using a vortex model. The vortex model is composed of the Rankine and Burgers vortices which have the exponential distributions in the vortex Reynolds number and vortex radii. This model exhibits a power law in the inertial range and satisfies the minimal condition of isotropy that the second-order exponent of the LSF in the inertial range is equal to that of the TSF. Also observed are differences between longitudinal and transverse structure functions caused by intermittency. These differences are related to their scaling differences which have been previously observed in experiments and numerical simulations.

Persistent photoconductivity in Hf-In-Zn-O thin film transistors

Passivated Hf-In-Zn-O (HIZO) thin film transistors suffer from a negative threshold voltage shift under visible light stress due to persistent photoconductivity (PPC). Ionization of oxygen vacancy sites is identified as the origin of the PPC following observations of its temperature- and wavelength-dependence. This is further corroborated by the photoluminescence spectrum of the HIZO. We also show that the gate voltage can control the decay of PPC in the dark, giving rise to a memory action. © 2010 American Institute of Physics.

Instability in threshold voltage and subthreshold behavior in Hf-In-Zn-O thin film transistors induced by bias-and light-stress

Electrical bias and light stressing followed by natural recovery of amorphous hafnium-indium-zinc-oxide (HIZO) thin film transistors with a silicon oxide/nitride dielectric stack reveals defect density changes, charge trapping and persistent photoconductivity (PPC). In the absence of light, the polarity of bias stress controls the magnitude and direction of the threshold voltage shift (Δ VT), while under light stress, VT consistently shifts negatively. In all cases, there was no significant change in field-effect mobility. Light stress gives rise to a PPC with wavelength-dependent recovery on time scale of days. We observe that the PPC becomes more pronounced at shorter wavelengths. © 2010 American Institute of Physics.

Low loss HF band SOI wine glass bulk mode capacitive square-plate resonator

This paper reports on the design and electrical characterization of a single crystal silicon micromechanical square-plate resonator. The microresonator has been excited in the anti-symmetrical wine glass mode at a resonant frequency of 5.166 MHz and exhibits an impressive quality factor (Q) of 3.7 × 106 at a pressure of 33 mtorr. The device has been fabricated in a commercial foundry process. An associated motional resistance of approximately 50 kΩ using a dc bias voltage of 60 V is measured for a transduction gap of 2 νm due to the ultra-high Q of the resonator. This result corresponds to a frequency-Q product of 1.9 × 1013, the highest reported for a fundamental mode single-crystal silicon resonator and on par with some of the best quartz crystal resonators. The results are indicative of the superior performance of silicon as a mechanical material, and show that the wine glass resonant mode is beneficial for achieving high quality factors allowed by the material limit. © 2009 IOP Publishing Ltd.

反应速率系数对HF化学激光器增益特性的影响

<正> 本文研究HF化学激光器中化学反应和碰撞弛豫过程速率系数不确定性对增益系数的影响。指出M=H的(V-T)过程最快,影响最大。

用二维简化模型计算连续波HF化学激光器特性

<正> 利用二维流体力学简化模型求解两股平行流的扩散混合和化学反应问题,得到连续波HF化学激光器谐振腔中气流混合界面,不同振动能级的HF活性分子浓度及小信号增益曲线。

扩散型连续波HF化学激光器小信号增益的计算

应用层流边界层二维简化模型计算了扩散型连续波HF化学激光器的小信号增益。研究了气流速度、温度、组份对增益的影响,得到一些有用的结果。计算结果与二维Navier-Stokes方程组的计算结果相符,但计算方法较简单。

反应速率系数对HF化学激光器增益特性的影响

本文研究了HF化学激光器中两股平行气流的速度比和动力学速率系数的不确定性对增益系数的影响。指出HF(V)-H是最快的碰撞弛豫过程,其速率系数值对估算的增益系数有较大影响。

连续波HF化学激光器计算浓度场的简化模型

介绍一个计算连续波HF化学激光器浓度场的简化模型。假设速度场与普通平板边界层速度场相似。温度场与浓度场是用联立能量与组份守恒方程以及相似的速度场求解得到的。求得了混合、化学反应区的界面曲线,并研究了气流速度、温度以及化学反应等对激发态HF浓度的影响。

应用在HF化学激光器上的超音速引射器

<正> 本文根据流体力学守恒方程,探讨采用气体引射技术将化学激光器的低总压、低腔压气流排入大气的问题,并提出一个具体方案。如果能够实现,对于化学激光器或其他类型高能激光器的探索研究将具有实际意义。被引射气流的计算压力为5托,温度为300K,气流速度 M=4,气体组份为40%H_2,60%He。引射气体采用高压的 N_2,H_2及高温的 H_2O。计算结果表明,利用常温 N_2作为引射气体效果不好。利用10大气压的 H_2可以引射光腔尾气。如果被引射气体预先扩压至70托,引射器混合段保持超音速气流,则引射气体的用量和被引射气体用量相同。利用 H_2O 作为引射气体,H_2O 温度最好超过1700K。计算发现,分子量较大的气体不易引射分子量较小的气体。混合段保持超音速气流时引射器出口总压力比混合段为亚音速时为高。扩压器的总压恢复系数对引射器的出口总压影响很大。本文给出一个超音速引射器设计方案,可作为参考.