Design of a Resonant-Cavity-Enhanced p-i-n GaN/AlxGa1-xN Photodetector

A resonant-cavity-enhanced p-i-n photodetector has been designed and analyzed to operate at a wavelength of 360 nm based on the Al_xGa_1-xN material system. The novel approach has been adopted of using epitaxial AlN/Al_xGa_1-xN quarter-wave stacks as the distributed Bragg reflector that serves as the front mirror. An Al_xGa_1-xN absorptive filter layer is incorporated to suppress all but one resonant mode to ensure single, narrow-band operation. This device structure is projected to achieve wavelength selective, high speed, and high quantum efficiency operation in the ultraviolet. MOCVD-grown 6 1/2-pair AlN/Al_xGa_1-x     

Shear viscosity of polymer and surfactant monolayers

The surface shear viscosity of monolayers formed at the surface of water by adsorbed polyethyl- eneoxyde and by stearic acid is measured as a function of the surface pressure of the monolayer using a new surface viscometer. The principle of the viscometer is the measurement of the drag force on a circular disk undergoing a uniform translation at the water surface: a hydrodynamic model based on the lubrication approximation allows a calculation of the surface viscosities from the absolute measurement of the drag forces. Received: 26 August 1999     

Frozen capillary waves on glass surfaces: an AFM study

Using atomic force microscopy on silica and float glass surfaces, we give evidence that the roughness of melted glass surfaces can be quantitatively accounted for by frozen capillary waves. In this framework the height spatial correlations are shown to obey a logarithmic scaling law; the identification of this behaviour allows to estimate the ratio kT_F/πγ where k is the Boltzmann constant, γ the interface tension and T_F the temperature corresponding to the “freezing” of the capillary waves. Variations of interface tension and (to a lesser extent) temperatures of annealing treatments are shown to be directly measurable from a statistical analysis of the roughness spectrum of the glass surfaces.     

Wetting of aluminium and carbon interface during preparation of Al-Ti-C grain refiner under ultrasonic field

In the preparation of an Al-Ti-C grain refiner under an ultrasonic field, the mechanism of the wetting behaviour between Al and C was systematically investigated. The results demonstrated that the wetting behaviour was mainly dependent on the wetting of the Al melt on graphite under the ultrasonic field (physical wetting) and the formation and mass transfer of TiC (reactive wetting). The diffusion of Ti atoms and their adsorption around the graphite could contribute to the wetting of Al-C. TiC particles were formed under the high temperature caused by the cavitation effect, and they detached from the interface due to the sound pressure, which resulted in consistently sufficient contact on the wetting interface. Moreover, the wetting and spreading behaviour of the Al melt on graphite under an ultrasonic field were numerically simulated, strongly manifesting that the ultrasonic field could facilitate the wetting of the Al-C interface.     

气液界面微观结构研究的一种实验新方法

许多物理、化学和生物过程发生于气液界面,且受到局域微观结构的深远影响. 不同于液体的体相,分子取向是界面处微观结构突出的不对称特征之一. 本文综述了电子碰撞时间延迟质谱方法和近期取得的进展. 利用这个崭新的方法,不仅可以侦测得到利用小角度X射线或中子散射以及振动和频光谱方法获得的界面微观结构,同时也提供了一种研究气液界面处电子驱动化学反应的实验方案.     

Large impact in electrical properties of polypropylene by improving the filler-matrix interface effect in PP/PS blends

Inorganic nanoparticles are widely used to improve space charge behavior, DC breakdown strength and other electrical properties of polymer insulating materials, but the uniform distribution of inorganic nanofillers in matrix is difficult due to their agglomeration and bad compatibility with the polymeric matrix. In this paper, polypropylene (PP)/polystyrene (PS) blends were prepared to suppress space charge accumulation and improve DC breakdown strength. Polypropylene-g-polystyrene (PP-g-PS) graft copolymer was used as compatibilizer to improve the compatibility of PP matrix and PS filler. The evolution of microstructure of PP/PS blends were investigated by scanning electron microscope (SEM), the space charge distributions were measured by a pulsed electro-acoustic (PEA) system, and DC breakdown strength was also tested. The morphologies show that the size of PS particles reduced to 310 nm when the content of PP-g-PS graft copolymer increased to 24 wt%, and the interaction between PP matrix and PS particles enhanced. The presence of PS particles in all PP/PS blends suppressed the space charge accumulation compared to neat PP, but the DC breakdown strength in uncompatibilized blend was lower than neat PP. The increasing of content of PP-g-PS improved the DC breakdown strength with the maximum value of 408.9 kV/mm was obtained. This may attribute to excellent interface structure formed between PP matrix and PS particles.     

Understanding the effect of filler shape induced immobilized rubber on the interfacial and mechanical strength of rubber composites

Styrene butadiene rubber (SBR) composites with silica, halloysite nanotubes (HNTs) and montmorillonite (MMT) were prepared and the interfacial and mechanical properties were compared to understand the reinforcing behaviours of these fillers based on the results of SEM, DSC, DMA, etc. Due to the formation of interparticle domain, HNTs immobilized more rubber approaching their surface than silica and MMT. Interestingly, only tightly immobilized rubber chains made contribution to the enhancement of interfacial and mechanical strength of SBR composites. This was because the tightly immobilized rubber acted as a bridge in the filler-rubber interface and induced the formation of stretched rubber chains linked filler network when the composites were loaded in tension, while loosely immobilized rubber were easy to slip off from filler surface, causing the separation between filler and bulk rubber. Therefore, silica with more tightly immobilized rubber approaching its surface showed better reinforcing effect on rubber than HNTs and MMT.     

MnSb/GaSb(001)界面半金属特性的第一性原理研究

利用密度泛函理论计算六角NiAs型和立方闪锌矿型MnSb的电子和电磁特性,研究闪锌矿MnSb与GaSb(001)界面,并采用广义渐变近似计算了交换-相关项．闪锌矿结构的MnSb具有半金属铁磁特性,单分子磁矩4μB,MnSb/GaSb(001)界面同样具有半金属特性．界面中Mn原子的磁矩减小,界面中Sb原子磁矩等于两个相应界面磁矩的平均值．另外计算了能带排列在异质结中价带偏移大约1.25 eV.     

Anti-diffusion method for interface steepening in two-phase incompressible flow

In this paper, we present a method for obtaining sharp interfaces in two-phase incompressible flows by an anti-diffusion correction, that is applicable in a straight-forward fashion for the improvement of two-phase flow solution schemes typically employed in practical applications. The underlying discretization is based on the volume-of-fluid (VOF) interface-capturing method on unstructured meshes. The key idea is to steepen the interface, independently of the underlying volume-fraction transport equation, by solving a diffusion equation with reverse time, i.e. an anti-diffusion equation, after each advection time step of the volume fraction. As the solution of the anti-diffusion equation requires regularization, a limiter based on the directional derivative is developed for calculating the gradient of the volume fraction. This limiter ensures the boundedness of the volume fraction. In order to control the amount of anti-diffusion introduced by the correction algorithm we propose a suitable stopping criterion for interface steepening. The formulation of the limiter and the algorithm for solving the anti-diffusion equation are applicable to 3-dimensional unstructured meshes. Validation computations are performed for passive advection of an interface, for 2-dimensional and 3-dimensional rising-bubbles, and for a rising drop in a periodically constricted channel. The results demonstrate that sharp interfaces can be recovered reliably. They show that the accuracy is similar to or even better than that of level-set methods using comparable discretizations for the flow and the level-set evolution. Also, we observe a good agreement with experimental results for the rising drop where proper interface evolution requires accurate mass conservation.