声结晶技术的研究进展

声结晶是应用超声场来影响控制结晶过程的技术,研究证明这是一种新型的结晶分离方法,由于声场具有促进成核,改善晶体性质的特点,使声结晶技术成为强化结晶过程的研究热点。文章阐述了结晶的原理,综述了超声场在结晶中的应用现状,分析和探讨了超声场对结晶过程影响的机理,提出了存在的问题并对今后的发展进行了展望。

超声对穿心莲内酯溶析结晶的影响

研究了超声波对穿心莲内酯溶析结晶过程的影响,考察不同超声功率对均相成核诱导期、介稳区、晶体粒度和晶形的影响。实验结果表明,超声波显著降低了结晶诱导期,诱导期随着超声功率的增加而缩短。分析了超声对均相成核影响的机理,在相同过饱和度条件下,诱导期缩短的主要原因是超声波提高了成核过程中的扩散系数,而表面张力受到声场的影响较小,基本保持不变。超声波引入溶析结晶体系后,晶体的粒度减小,在高功率超声作用下,溶液中产生了更多晶核降低了晶体生长的速度。经超声波作用的晶体外形发生变化,并且减少了晶体间的聚集。同时超声波的作用使介稳区宽度明显变窄,表明在超声场中过饱和溶液变得更不稳定。

Study of Cadmium-Doped Zinc Oxide Nanocrystals with Composition and Size Dependent Band Gaps

Cadrniurn-doped zinc oxide nanocrystals in the quantum confinement region have been firstly synthesized by a fast and facile sonochernical method. The alloyed structure of the nanocrystals is confirmed by X-ray diflraction, transmission electron rnicroscopy, and infrared analysis. With the increase of cadmium to zinc molar ratio from 0 to 2.0, the crystallite sizes of the samples decrease from 5.1 nrn to 2.6 nrn, and the band gaps of the samples show a red shift then a blue shift, and a red shift again. The variations of band gaps of the samples can be interpreted by the crystallite size and the composition. It is found that both the non-therrnal equilibrium environrnent established in the sonochernical reaction and the coordination ability of triethylene glycol solvent play crucial roles in the current preparation.

HBT's High Frequency Noise Modeling and Analysis

A T equivalent high frequency heterojunction bipolar transistor (HBT) noise model is reported.This model is derived from Hawkins noise model commonly used in Si BJT.The main modifications include the influence of the ideality factor,emitter resistance,intrinsic base collector capacitance,extrinsic base collector capacitance and other parasitic elements of HBT represented in equivalent circuit topology.In order to calculate accurate noise parameters from the equivalent circuit,the noise correlation matrix method is used to avoid any simplifications generated in circuit transformations and complex noise measurements.The analysis of the influence of the equivalent circuit elements on the minimum noise figure is reported,the results of analysis agree well with the physics explanations.By means of the formulae derived from device physics of HBT,the influence of device parameters on the minimum noise figure is also represented.

Fractal Interpretation of Ultrasonic Crystallization of Solutions

Experiments were performed on the crystallization of a CuSO4 solution upon the action of the temperature gradient with the forming of mono crystals three wedges crystal system （prisms）. We found that the fractal dimension of crystals equals 2.45, which is consistent with the literature data. Crystal growth is represented as the N-rd translation of each side of the crystal lattice with its own speed and with relation to the formation of similar structures--fractals. A mathematical model of ultrasonic crystallization of a CuSO4 solution was proposed. The model is based on the combined use of differential transport equations of momentum, mass, energy and sound waves and a method of similarity and dimensional analysis. The calculated formulas for the concentration of Ccr, the equivalent diameter of the formed crystals dcr and the intensity of internal energy source Ф, associated with the interaction of crystals with the hydro mechanical, heat and sound fields were obtained. Fractal interpretation of ultrasonic crystallization of the CuSO4 solution was made. It was found that on the growth of crystal size d^r directly affects translation N, i.e., an increase in the number of sets of crystals of infinitely small size e, correspond to the size of the crystal lattice. In turn, translation of crystals depends on the geometry of the crystallizer and the physical parameters of external force fields, acting on the CuSO4 solution. A connection of results of the mathematical modeling with the results of fractal analysis of the ultrasonic crystallization of solutions was established.

Elastic wave propagation and localization in band gap materials:a review

Band gap materials(i.e.phononic crystals) are the artificially periodic structures,which have the stop band characteristic for elastic waves.The elastic waves will be localized in phononic crystals with defects,which results in the energy being accumulated around the defects.As a result,it is important to analyze the wave propagation and localization in band gap materials,especially for the structures consisting of smart materials.For example,with the mechanical-electro and mechanical-electro-magneto coupling,the phononic crystals consisting of piezoelectric and magnetoelectroelastic materials can be applied widely.This sets the theoretical basis for the design of band gap materials with multi fields coupling.This paper reviews the recent development of the elastic wave propagation and localization in both ordered and disordered band gap materials.The discussion focuses on the stop band and localization characteristics of elastic waves.Analytical methods and important results are also presented.Finally,some problems for further studies are discussed.This work aims to present the basic properties of wave band gaps in phononic crystals and wave localization in disordered periodic structures(e.g.phononic crystals with definite and random defects and phononic quasicrystals).

Preparation of sound ribbons with submicrometer-grained microstructure on a Mg–Zn alloy

In this paper, a viable way to fabricate Mg alloy sound ribbons with ultra-fine-grained microstructure was presented. The hot-rolled and annealed Mg-0.4Zn （at%） alloy exhibited excellent rollability to form sound ribbons with submicrometer grains when subjected to one-pass cold rolling process. The more balanced multi-mode dislocation slips originated from the significant decrease of critical resolved shear stress for non-basal slip with the addition of solute Zn and the favorable crystallographic orientation were suggested to be responsible for the excellent cold rollability. The formation of ultra-fine-grained microstructure was attributed to low-temperature dynamic recrystallization occurring during the cold rolling process with large strain.

The Brillouin zones and band gaps of a two-dimensional phononic crystal with parallelogram lattice structure

We present a detailed theoretical study on the acoustic band structure of two-dimensional （2D） phononic crystal. The 2D pho- nonic crystal with parallelogram lattice structure is considered to be formed by rigid solid rods embedded in air. For the circu- lar rods, some of the extrema of the acoustic bands appear in the usual high-symmetry points and, in contrast, we find that some of them are located in other specific lines. For the case of elliptic rods, our results indicate that it is necessary to study the whole first Brillouin zone to obtain rightly the band structure and corresponding band gaps. Furthermore, we evaluate the first and second band gaps using the plane wave expansion method and find that these gaps can be tuned by adjusting the side lengths ratio R, inclined angle 0 and filling fraction F of the parallelogram lattice with circular rods. The results show that the largest value of the first band gap appears at θ=90° and F--0.7854. In contrast, the largest value of the second band gap is at θ=60° and F=0.9068. Our results indicate that the improvement of matching degree between scatterers and lattice pattern, ra- ther than the reduction of structural symmetry, is mainly responsible for the enhancement of the band gaps in the 2D phononic crystal.