混合制冷剂泄漏试验研究与模拟分析

1研究背景目前在制冷和空调系统中，绝大多数采用混合制冷剂作为R502与HCFC－22等的替代物。但混合物的特殊问题，即在贮存、使用时因泄漏而引起的成分变化、特别是成分变化而导致制冷剂可燃性或系统性能的变化，引起了人们的普遍重视。英国I.C.I公司对R407C（HFC－32／125／134a）系统进行测试表明，由于泄漏后成分变化而引起的性能变化在10％以内。日本DAIKIN公司研究HFC－32／134a混合物时发现：其临界可燃配比为56Wt％，即HFC－32重量比超过56％时可燃；使用名义配比30／70wt％的HFC－32／134a混合物并不可燃，但发生泄漏后…     

Guided-Wave Two-Dimensional Acousto-Optic Scanner Using Proton-Exchanged Lithium Niobate Waveguide

Two-dimensional scanning of a 0.6328 mum guided-light beam has been realized using noncolinear acousto-optic (AO) coplanar Bragg diffraction together with colinear AO guided-mode to substrate radiation-mode conversion in a Z-cut Xpropagation LiNbO3 proton-exchanged (PE) waveguide. The two surface acoustic (SAW) waves utilized are at the center frequencies of 500 and 200 MHz, propagating in the Y and X axes, respectively. Two-dimensional scanning of approximately 720 resolvable light beam spots, namely, 18 40 (horizontal vertical) scanning, has been demonstrated using a light beam of 1.0 mm aperture. The total number of resolvable beam spots can be greatly increased from 720 by simply utilizing SAW transducers of larger bandwidth and a light beam of greater aperture. It should also be possible to significantly increase the diffraction efficiency from 3 % by optimizing the parameters of the PE waveguide and the SAWs.     

Acoustic field imaging by light reflection at the free surface of a liquid

In this paper we present a novel technique for acoustic field imaging. This technique is based on reflection of a collimated laser beam at the free surface of a liquid. The reflected beam becomes phase modulated by the acoustic wave as in acoustical holographic systems. We do not use a reference acoustical beam for holographic reconstruction but we observe this phase modulation using dark-field techniques. It gives a measurement of the acoustic field power as a function of the position. The authors have built an optical imaging system and carried out experiments with piezocomposite transducers. The technique presented in this work is able to give fast quantitative information about the performance of individual ceramic rods of the piezocomposite.     

复合漏电模型建立及阶梯场板GaN肖特基势垒二极管设计

准垂直GaN肖特基势垒二极管(SBD)因其低成本和高电流传输能力而备受关注.但其主要问题在于无法很好地估计器件的反向特性,从而影响二极管的设计.本文考虑了GaN材料的缺陷以及多种漏电机制,建立了复合漏电模型,对准垂直Ga N SBD的特性进行了模拟,仿真结果与实验结果吻合.基于此所提模型设计出具有高击穿电压的阶梯型场板结构准垂直GaN SBD.根据漏电流、温度和电场在反向电压下的相关性,分析了漏电机制和器件耐压特性,设计的阶梯型场板结构准垂直GaN SBD的Baliga优值BFOM达到73.81 MW/cm~2.     

Pitch Characteristics of Homosexual Males

Objective

To investigate the common stereotype that homosexual males show pitch patterns that mirror those of heterosexual females.

Study Design

Static group comparison.

Method

Comparison of speaking fundamental frequency and pitch variation of 30 homosexual males, 56 heterosexual age-matched males, and 54 age-matched heterosexual females as demonstrated in a sample of read speech.

Results

In the homosexual males, average fundamental frequency and pitch variation were significantly higher than in the heterosexual males but also significantly lower than in the heterosexual females.

Conclusions

Results do not confirm the stereotype that gay male speech mirrors the patterns of women’s speech with respect to pitch characteristics. It would seem that the pitch patterns of gay male speakers constitute an example of sociophonetic variation.     

Confining medium and absorptive overlay: Their effects on a laser-induced shock wave

In this paper the characteristics (such as amplitude, width) of a laser-induced shock wave under confining conditions is studied. For engineering applications, a physical study of this method is useful in order to optimize this technique. We have first introduced a new pressure gauge – PVDF (polyvenyliden fluoride) gauge with short rise time and wide linear response range. Experimentally, by measuring the generated pressures under different confining materials, the relationship between the pressures and the acoustic impedance of confining materials, is illustrated, which somewhat agrees with the theoretical calculation. We have also found that under confining conditions laser-induced shock waves persist longer than a laser pulse. Then, the effects of black paint overlay (absorptive overlay) is studied. We experimentally point out that a black paint overlay placed before an irradiated target can greatly increase the generated pressure under any confining material in our experiments for its beneficial effect on the plasma-generating process. To our surprise, comparing the impulse ( ), which the shock wave induced under absorptive overlay executes on the target, to that induced under no black paint overlay, the increase ratio is approximately equal.     

A novel Si-rich SiN bilayer passivation with thin-barrier AlGaN/GaN HEMTs for high performance millimeter-wave applications

We demonstrate a novel Si-rich SiN bilayer passivation technology for AlGaN/GaN high electron mobility transistors (HEMTs) with thin-barrier to minimize surface leakage current to enhance the breakdown voltage. The bilayer SiN with 20-nm Si-rich SiN and 100-nm Si$_{3}$N$_{4}$ was deposited by plasma-enhanced chemical vapor deposition (PECVD) after removing 20-nm SiO$_{2}$ pre-deposition layer. Compared to traditional Si$_{3}$N$_{4}$ passivation for thin-barrier AlGaN/GaN HEMTs, Si-rich SiN bilayer passivation can suppress the current collapse ratio from 18.54% to 8.40%. However, Si-rich bilayer passivation leads to a severer surface leakage current, so that it has a low breakdown voltage. The 20-nm SiO$_{2}$ pre-deposition layer can protect the surface of HEMTs in fabrication process and decrease Ga-O bonds, resulting in a lower surface leakage current. In contrast to passivating Si-rich SiN directly, devices with the novel Si-rich SiN bilayer passivation increase the breakdown voltage from 29 V to 85 V. Radio frequency (RF) small-signal characteristics show that HEMTs with the novel bilayer SiN passivation leads to $f_{\rm T}/f_{\rm max}$ of 68 GHz/102 GHz. At 30 GHz and $V_{\rm DS} = 20$ V, devices achieve a maximum $P_{\rm out}$ of 5.2 W/mm and a peak power-added efficiency (PAE) of 42.2%. These results indicate that HEMTs with the novel bilayer SiN passivation can have potential applications in the millimeter-wave range.     

气幕对潜艇声屏蔽作用的数值计算

本文利用惠更斯-菲涅尔原理和虚源方法建立了均匀海水中障碍物后声传播损失的计算模型,对模型进行了数值实验验证,并与海上试验数据对比,得到了较为合理的结果。     

应用声有限元素法模拟坐席吸声低谷效应

本文给出了应用声有限元素法模拟坐席吸声低谷效应的计算模型。该模型首先在时间域计算房间脉冲响应,再通过傅立叶变换得到声压频谱特性。将有限元分析结果与已有的模型实验及大厅实测数据相比较,证明该方法是研究坐席吸声低谷效应的有效手段。     

An Extension of the Effective Index Method to Analyze Leakage Losses in Rib Type Waveguides

An extension of the effective index method is proposed as a tool to investigate leakage losses in two-dimensional waveguides. The above method, referred to as the Extended Effective Index Method (EEIM), utilizes the Transfer Matrix Technique (TMT) for the calculation of the complex propagation constants. The results show that the EEIM can be applied to conventional rib leaky waveguides as well as to rib ARROW leaky waveguides. The results for rib ARROW waveguides show excellent agreement when compared to those obtained with Finite Element Method.