阶梯AlGaN外延新型Al0.25Ga0.75N/GaNHEMTs器件实验研究

本文报道了作者提出的阶梯AlGaN外延层新型AlGaN/GaN HEMTs结构的实验结果. 实验利用感应耦合等离子体刻蚀(ICP)刻蚀栅边缘的AlGaN外延层, 形成阶梯的AlGaN 外延层结构, 获得浓度分区的沟道2DEG, 使得阶梯AlGaN外延层边缘出现新的电场峰, 有效降低栅边缘的高峰电场, 从而优化了AlGaN/GaN HEMTs器件的表面电场分布. 实验获得了阈值电压-1.5 V的新型AlGaN/GaN HEMTs器件. 经过测试, 同样面积的器件击穿电压从传统结构的67 V提高到新结构的106 V, 提高了58%左右; 脉冲测试下电流崩塌量也比传统结构减少了30%左右, 电流崩塌效应得到了一定的缓解.     

可改变DNA构象的非离子糖基表面活性剂

通过Zeta电位及粒径分析考察发现随着体系中辛基葡萄糖多苷表面活性剂质量 农度的增加，DNA分子在溶液中的构趋于缩拢。通过DNA-C_8APG复合物的UV吸收及 CD谱图进一步考察了DNA二级结构变化。随后的扫描电子显微镜（SEM）照片也证实 DNA分子在水溶液中的构象缩拢。通过表面张力UV图谱分析，推测非离子糖基表面 活性剂与DNA分子复合物结合的可能结构是表面活性剂与DNA之间的疏水作用及 多羟基的糖类的亲水头基结构与DNA带负电的核酸磷酸骨架以氢健的方式结合。     

流体动力润滑油膜破裂的热力学失稳机理

分析了流体动力学润滑过程中的热量传递及对润滑剂流变特性的影响,得出了流体动力润滑油膜发生热力学失稳的条件,建立了描述润滑剂温度非牛顿效应的本构方程数值计算结果表明,由于温度的影响,流体动力润滑油膜存在最大承载能力;在临界状态,微小的扰动将会引起油膜失稳而丧失承载能力。初步揭示了流体动力学力润滑膜破失效的内在力学机制。     

SPH-based simulation of granular collapse on an inclined bed

The paper presents a numerical model for simulating a granular flow and its deposition on an inclined bed. A granular material is described as an elastic–plastic continuum and its constitutive law, namely Hooke's law, is discretized on the basis of the Smoothed Particle Hydrodynamics (SPH) method. In the equation of motion, however, the artificial viscosity, which is widely used in SPH, is not applied. The diffusive term derived from Hooke's law is introduced with a diffusion coefficient that varies depending on the stress and strain rate based on the Drucker–Prager yield function. The model is verified and validated through two numerical tests. It is shown that the basic elastic–perfectly plastic characteristics are reproduced with a simple shearing test. The effects of the diffusion coefficient and spatial resolution are investigated to show the validity of the model. In the simulation of the gravitational collapse of a granular column on an inclined bed, the performance of the model from the final deposition profile, the time history of the front position of the granular flow, the maximum runout distance, and the velocity profile are investigated for several cases of basal inclinations. The calculated results show good agreement with the experimental results.     

用逆压电极化模型对AlGaN/GaN 高电子迁移率晶体管电流崩塌现象的研究

通过自洽求解一维Poisson-Schrdinger方程,模拟了AlGaN/GaN高电子迁移率晶体管在工作时等效外电场对AlGaN/GaN异质结沟道处二维电子气(2DEG)浓度的影响.分析了逆压电极化效应的作用,从正-逆压电极化现象出发,提出了逆压电极化模型.计算结果显示：逆压电极化明显影响2DEG性质,当Al组分x=0.3,AlGaN层厚度为20 nm时,不考虑逆压电极化,2DEG浓度为1.53×10¹³cm^-2;当等效外电压分别为10和15V 关键词： AlGaN/GaN高电子迁移率晶体管 Poisson-Schrdinger方程 逆压电极化模型 电流崩塌     

Scattering experiments on polymer solutions

Laser light-scattering characterization of poly(tetrafluoroethylene) (PTFE) in a perfluoroalkane (perfluorotetracosane) or oligomers of poly(chlorotrifluoroethylene) at temperatures near the crystalline melting point of PTFE and studies of the coil-to-globule transition for polystyrene in cyclohexane are reviewed. Discussions on approach to solving some experimental problems are emphasized. © 1993 John Wiley & Sons, Inc.     

Dynamics of General Relativistic Spherically Symmetric Dust Thick Shells

We consider a spherical thick shell immersed in two different spherically symmetric space-times. Using the fact that the boundaries of the thick shell with two embedding space-times must be nonsingular hypersurfaces, we develop a scheme to obtain the underlying equation of motion for the thick shell in general. As a simple example, the equation of motion of a spherical dustlike shell in vacuum is obtained. To compare our formalism with the thin shell one, the dynamical equation of motion of the thick shell is then expanded to the first order of its thickness. It is easily seen that the thin shell limit of our dynamical equation is exactly that given in the literature for the dynamics of a thin shell. It turns out that the effect of thickness is to speed up the collapse of the shell.     

On the zero-curvature criterion for structures with material strengthening and geometrical non-linearity

A zero-curvature criterion is proposed to determine the practical collapse load. Six tests have been carried out on spherical pressure vessels with cylindrical nozzle. Results of these tests and other experimental data quoted from different references are discussed in order to compare the new criterion with various previously proposed criteria. It is concluded that: (1) the zero-curvature point is a characteristic point of the experimental curve and that the new criterion is an objective criterion without any artificial coefficient. (2) the deviation of the collapse load estimated by the new criterion is smaller than that estimated by other criteria.Supported by the Chinese National Foundation of Natural Science.     

Interaction of water with poly(vinyl methyl ether) in aqueous solution

Near-infrared, viscometric, and calorimetric measurements were made on aqueous poly(vinyl methyl ether) (PVME) solutions at temperatures between 15 and 43°C. We found a hydrogen-bonded structure of water around the polymer chain (a polymer-water complex), which is characterized by two distinct hydration numbers (i.e., 2.7 and 5.0 water molecules on each monomer unit of the chain) by analyzing the concentration dependence of endothermic enthalpies at a cloud point temperature, ca. 35°C. In particular, the 2.7 water-polymer complex has been suggested to be cooperatively formed by using data of the near-infrared (nir) absorption spectrum around 1930 nm. Furthermore, the peak-wavelength of the nir spectrum has been observed to change drastically at the cloud point when the temperature is raised. This can be interpreted as a cooperative collapse of the hydrogen-bonded water structure to free water, resulting in the aggregation of the polymer chains due to the exposure of their hydrophobic groups at the cloud point. © 1994 John Wiley & Sons, Inc.     

On the theory of the condensed states of heteropolymers

The collapse (globulization) of an ideal heteropolymer chain under the action of an external attractive field is considered. The problem of the collapse of different types of primary structures, including mobile, periodic, large-block, and statistical structures, is formulated. It is shown that for a random heteropolymer, the mathematical image of the globular state is the chain-length independence of the probability distribution of a random thermal distribution function of the end monomer coordinates. The free energy per monomer of a chain in a globular state and local densities of monomers of all types are shown to be a self-averaging quantities. An exactly solvable model is proposed for a globule formed by a statistical heteropolymer chain. In this model, different types of monomers are attracted to different centers by linear elastic forces with identical elastic constants. The modulus of elasticity is obtained for a heteropolymer globule with respect to the attraction of different types of monomers in different directions. It is shown that this modulus is higher for a short-periodic polymer than for a statistical one.