接触热阻的蒙特卡罗法模拟

 用蒙特卡罗随机模拟法计算了固体接触界面间的接触热阻。取服从高斯分布的随机数作为粗糙峰高度，分别对每个粗糙峰计算出单点接触热阻，再并联所有单点接触热阻得到总的接触热阻。修正了“积分法”的理论结果且与发表的文献数据相符。     

基于相对光谱强度的非接触式LED结温测量法

基于一体化封装高导热铝板,利用蓝光芯片及常用YAG荧光粉,制备了大功率白光LED,并研究了其在不同结温下的光谱变化规律。发现白光LED辐射光谱在波长485 nm处辐射强度具有极小值,并且此波长的辐射强度与LED结温存在良好的线性关系,以此为依据给出了该波长辐射强度与结温的关系公式,测量了LED结温,并与正向压降法及光谱法的测量结果进行对比。实验结果显示：所提出的结温测量方法与正向压降法测量结果差距不超过2 ℃,该方法保持了正向压降法的结温测量较为准确的优点,克服了光谱法的光谱漂移过小,对测试结果带来较大误差的缺点,同样也具有光谱法的实用性强、高效直观、非接触测量、不破坏灯具结构的优点。     

表面InGaN厚度对GaN基发光二极管特性的影响

通过调整GaN基发光二极管(LED)表面InGaN层的厚度，发现在20 mA电流驱动下，LED器件的正向压降有明显差距.本文考虑了极化效应的影响，通过求解InGaN/GaN三角形势阱内二维空穴气浓度以及空穴隧穿概率的变化，求得了表面InGaN层厚度不同时器件正向压降的变化趋势，发现理论结果与实验结果有很好的吻合.同时得到了获得最低正向压降的表面InGaN厚度.     

水-胶凝原油两相流压降特性研究

压降是影响胶凝原油水力悬浮输送系统安全运行的主要参数。本文运用量纲分析原理建立水-胶凝原油两相流压降计算关联式,并通过试验的方法确定出其四种典型流型的压降计算模型。利用该模型对压降的计算值与测试值进行比较,相对误差在17%以内。分析了含水率和混合物流速对压降的影响,结果表明:含水率对压降的影响存在一个转折点,转折点之前,对于同种流型,定流速时压降随含水率的增加而降低;转折点之后压降是流速的单值函数。     

CFETR TF导体的管内压降分析

目前管内电缆导体(CICC)压降的实验结果都是高空隙率(～35%)下测得的,而CFETR纵向场(Toroidal Field,TF)导体的设计偏向于较小的空隙率(～29%),压降实验数据方面的短缺,使得需要重新考虑目前Katheder关系式能否用于CFETR TF的压降估算。在CFETR TF导体的压降分析中,分别采用了修正的Katheder关系式和多孔介质模型对导体的压降进行比较,获得了更为保守的压降预测。     

集成电路中的物理问题讲座 第五讲 金属-半导体接触的电流机制

在金属-半导体界面附近一般存在势垒,称为肖特基势垒.基于这一结构的二极管称为肖特基势垒二极管或肖特基二极管. 由于肖特基二极管较p-n结有更低的正向压降,所以在双极型集成电路中用它作限饱和二极管,以显著提高电路的速度.金属半导体接触也是金属-半导体场效应晶体管(MES-FET)及有关集成电路的基础。金属-半导体欧姆接触则广泛用于形成低阻接触. 关于金属-半导体接触界面势垒的形成问题,在本刊 11卷(1982)8期中陈克铭已经作了介绍,这里不再重复.本文着重讨论金属-半导体接触的电流. 金属-半导体接触的电流通常由多数载流子越过势垒形…     

微尺度平行多通道内制冷剂流动沸腾压降特性

在热流密度3.54 W·cm~(-2)到40.7 W·cm~(-2),雷诺数由278到6502,出口干度0到1的工况范围内对R134a在微尺度单通道和多通道内流动沸腾压降特性进行了可视化实验研究。在相同工况时,单通道压降小于多通道压降;随热流密度和出口干度的增加,多通道压降增长范围小于单通道压降增长范围,通过可视观察,多通道内流动沸腾不稳定流动现象较单通道更为明显,并且多通道平均压降和实时压降波动范围更大。结合可视实验观察,多通道内压降高于单通道压降的主要原因为不稳定流动现象造成,并对产生不稳定流动的因素和造成压降升高的原因进行了分析。     

过载条件下流动沸腾压降振荡的建模与仿真研究

流动沸腾系统中,压降振荡是系统不稳定性的主要型式之一。过载条件下流动沸腾压降振荡缺乏研究。本文采用数值仿真方法,对过载条件下管内流动沸腾的压降振荡特性进行了研究。建立了不同重力条件下压降振荡计算的数学模型,基于此,对过载条件下R134a在2.168 mm水平管内的压降振荡进行了仿真分析,得出了1.41 g、3.16 g(g=9.8 m/s^2)过载条件下的压降振荡特性及其引起的流量振荡、流体温度振荡和壁温振荡,并与对常重力(1 g)下的压降振荡特性进行了对比。结果表明,随着重力增加,流动特性N曲线的负斜率段缩短;一定条件下,当过载增加时,系统从稳定状态趋于不稳定状态。     

特高含水期油气水混输管道压降计算方法研究

在油田开发后期,产出液的含水率已超过80％,集输管道内介质的流动属于特高含水油气水混合流动。准确的油气水混输管道压降计算,对油田集输系统运行管理有着重要的意义。利用井口到计量间的现有设施,对特高含水期油气水管道的液量、气量、含水率、温度及压降进行了测试。介绍了Baker冲击流压降计算模型,用实测数据对Baker模型进行了修正。计算与测试数据对比分析结果表明,所给的修正压降模型适合特高含水采油期油气水三相冲击流水平管道压降计算。     

波纹参数对板式换热器传热和压降影响研究

建立冷热双流道模型,研究了不同波纹参数对其传热和压降性能的影响,分析了传热机理。结果表明,板式换热器的传热和压降性能主要受流体流动形式和触点密度的影响。波纹倾角β=70°时传热效果最好但压降较大,波纹深度对传热性能的影响效果比波纹倾角和波纹节距大8%和15%左右,波纹倾角对压降性能的影响大于波纹节距。     

Cell membrane voltage during electrical cell fusion calculated by re-expansion method

In electrical cell fusion, two cells are first brought into contact by dielectrophoresis, and then a pulsed voltage is applied to induce reversible membrane breakdown at the contact point, by which the membranes of the two cells are reconnected to form a fusant cell. The prediction of the membrane voltage is a crucial issue for high fusion yield, however, its mathematical expression is known only for the case of an isolated cell in a uniform external field. In this paper, we employ the re-expansion method for the transient field analysis of such a multiple cell system. Each cell is modeled by an infinitesimally thin spherical insulating membrane in conducting media, on which accumulation of free charge occurs when an external field is applied. It is shown that the system has two time constants: (a) that governed by the conductivity and the permittivity of the media and (b) that of charging the membrane capacitance through the conducting media, and that the former is far shorter than the latter. Hence, the time variation due to the former is neglected to obtain a simplified expression for the membrane voltage. By expanding the potential into Legendre harmonic components and relating the coefficients for each cell based on the re-expansion method, a differential equation governing the membrane voltage buildup is obtained. The numerical calculation is performed for the axisymmetric case of two cells in contact, to which a step-wise voltage is applied. It is found that the maximum membrane voltage occurs initially at the contact point, but when the steady state is reached, it moves to the ends of the cell pair, and might lead to unsuccessful fusion. The analysis suggests that high-yield fusion may be achieved by an application of shorter pulse, or of a non-uniform field to concentrate the voltage drop at the contact point.     

Limiting phenomena for the spreading of water on polymer films by electrowetting

This paper is about fundamental limitations in electrowetting, used as a tool for spreading water solutions on hydrophobic surfaces, like the surface of a polymer film. Up to which point can an electric voltage decrease the contact angle? The first limitation comes when using pure water, above a threshold voltage, little droplets are emitted at the perimeter of the mother drop. We present an analysis of the drop contour line stability, involving competition between electrostatic and capillary forces, which is compatible with observations. The use of salted water solutions suppresses this instability, then one faces a second limitation: the evolution of the contact angle saturates before complete wetting. We show that this saturation is caused by ionisation of the air in the vicinity of the drop edge. We analyse the luminescence induced by gas ionization and measure the related electrical discharges. We explain how air ionization suppresses the driving force for electrowetting and how it induces the formation of an hydrophillic ring around the drop.     

Superconductivity in ropes of single-walled carbon nanotubes

We report measurements on ropes of single-walled carbon nanotubes (SWNT) in low-resistance contact to nonsuperconducting (normal) metallic pads, at low voltage and at temperatures down to 70 mK. In one sample, we find a 2 orders of magnitude resistance drop below 0.55 K, which is destroyed by a magnetic field of the order of 1 T, or by a dc current greater than 2.5 microA. These features strongly suggest the existence of superconductivity in ropes of SWNT.     

Remarkable room‐temperature magnetoresistance in silicon strip devices

In this paper, we report remarkable room‐temperature magnetoresistance (MR) in silicon strip devices. Saturating and non‐saturating MRs can be realized based on the measuring configurations with one top contact and ten top contacts, respectively. Using the one‐top‐contact measurement, a saturating MR ratio of ～400% is obtained at an applied voltage of only 1.0 V when the magnetic field is larger than 0.8 T. While the non‐saturating MR is achieved in the ten‐top‐contact measurement and the MR ratio is only ～155% for the 1.0 V applied voltage even under the magnetic field of 1.2 T. The differences for MR ratio values and change trends in these two measuring configurations are attributed to the enhanced Hall electric field with the increase of the top contact number.     

Novel fast-switching LIGBT with P-buried layer and partial SOI

A novel silicon-on-insulator lateral insulated gate bipolar transistor(SOI LIGBT)is proposed in this paper.The proposed device has a P-type buried layer and a partial-SOI layer,which is called the BPSOI-LIGBT.Due to the electric field modulation effect generated by the P-type buried layer and the partial-SOI layer,the proposed structure generates two new peaks in the surface electric field distribution,which can achieve a smaller device size with a higher breakdown voltage.The smaller size of the device is beneficial to the fast switching.The simulation shows that under the same size,the breakdown voltage of the BPSOI LIGBT is 26%higher than that of the conventional partial-SOI LIGBT(PSOI LIGBT),and 84%higher than the traditional SOI LIGBT.When the forward voltage drop is 2.05 V,the turn-off time of the BPSOI LIGBT is 71%shorter than that of the traditional SOI LIGBT.Therefore,the proposed BPSOI LIGBT has a better forward voltage drop and turn-off time trade-off than the traditional SOI LIGBT.In addition,the BPSOI LIGBT effectively relieves the self-heating effect of the traditional SOI LIGBT.     

Voltage of the vacuum arc with a ring anode in an axial magneticfield

The plasma jet focusing and voltage distribution in the interelectrode gap of a vacuum arc with a ring anode and subjected to an axial magnetic field were studied theoretically. A two-dimensional model was developed based on the free plasma jet expansion into vacuum, and the steady-state solution of the fully ionized plasma in the hydrodynamic approximation was analyzed. It was found that the imposition of an axial magnetic field reduces the radial expansion of the plasma jet. The characteristic jet angle decreases from about 40° in the zero magnetic field case and approaches a value of about 20° with a 0.02 T magnetic field. The arc voltage consisting of the cathode drop, the plasma voltage drop, and anode sheath drop increased, with the imposition of a magnetic field, and decreased with the anode length. The model was compared to experimental measurements of the vacuum arc voltage behavior in an axial magnetic field, and good agreement was found     

New GaN Schottky barrier diode employing a trench on AlGaN/GaN heterostructure

A new GaN Schottky barrier diode employing a trench structure, which is proposed and fabricated, successfully decreases a forward voltage drop without sacrificing any other electric characteristics. The trench is located in the middle of Schottky contact during a mesa etch. The Schottky metal of Pt/Mo/Ti/Au is e-gun evaporated on the 300 nm-deep trench as well as the surface of the proposed GaN Schottky barrier diode. The trench forms the vertical Au Schottky contact and lateral Pt Schottky contact due to the evaporation sequence of Schottky metal. The forward voltage drops of the proposed diode and conventional one are 0.73 V and 1.25 V respectively because the metal work function (5.15 eV) of the vertical Au Schottky contact is considerably less than that of the lateral Pt Schottky contact (5.65 eV). The proposed diode exhibits the low on-resistance of 1.58 mΩ cm² while the conventional one exhibits 8.20 mΩ cm² due to the decrease of a forward voltage drop.     

Study on both discharge and charging of the earthed atomizing corona discharge and the influence of magnetic fields on them

In the paper, the influences of water flux on both discharge current and onset voltage were studied. Both charging and capturing particles of atomizing corona discharges were investigated when the magnetic field was used or not. The charge number of droplets and their sizes were calculated after some parameters were measured by Millikan oil drop instrument. In addition, the capturing ability of atomizing corona discharge pre-charger with magnetic field was compared with the traditional pre-charger. Eventually, the charging mechanism of atomizing corona discharge with magnetic field was analyzed through the above-mentioned experimentation and comparison. The result shows that the smallest onset voltage will appear with water flow increase in the atomizing corona discharge, and that the ion concentration between electrodes is the highest in the atomizing corona discharge charger with magnetic field than any other pre-charger, which is conducive for charging dust particles. Hence the new pre-charging technique is promising for capturing fine aerosol particles in electrostatic precipitators.     

Lateral vibration of a water drop and its motion on a vibrating surface

The resonant modes of sessile water drops on a hydrophobic substrate subjected to a small-amplitude lateral vibration are investigated using computational fluid dynamic (CFD) modeling. As the substrate is vibrated laterally, its momentum diffuses within the Stokes layer of the drop. Above the Stokes layer, the competition between the inertial and Laplace forces causes the formation of capillary waves on the surface of the drop. In the first part of this paper, the resonant states of water drops are illustrated by investigating the velocity profile and the hydrostatic force using a 3d simulation of the Navier-Stokes equation. The simulation also allows an estimation of the contact angle variation on both sides of the drop. In the second part of the paper, we investigate the effect of vibration on a water drop in contact with a vertical plate. Here, as the plate vibrates parallel to gravity, the contact line oscillates. Each oscillation is, however, rectified by hysteresis, thus inducing a ratcheting motion to the water droplet vertically downward. Maximum rectification occurs at the resonant states of the drop. A comparison between the frequency-dependent motion of these drops and the variation of contact angles on their both sides is made. The paper ends with a discussion on the movements of the drops on a horizontal hydrophobic surface subjected to an asymmetric vibration.     

The debate on the dependence of apparent contact angles on drop contact area or three-phase contact line: A review

A sessile drop is an isolated drop which has been deposited on a solid substrate where the wetted area is limited by the three-phase contact line and characterized by contact angle, contact radius and drop height. Although, wetting has been studied using contact angles of drops on solids for more than 200 years, the question remains unanswered: Is wetting of a rough and chemically heterogeneous surface controlled by the interactions within the solid/liquid contact area beneath the droplet or only at the three-phase contact line? After the publications of Pease in 1945, Extrand in 1997, 2003 and Gao and McCarthy in 2007 and 2009, it was proposed that advancing, receding contact angles, and contact angle hysteresis of rough and chemically heterogeneous surfaces are determined by interactions of the liquid and the solid at the three-phase contact line alone and the interfacial area within the contact perimeter is irrelevant. As a consequence of this statement, the well-known Wenzel (1934) and Cassie (1945) equations which were derived using the contact area approach are proposed to be invalid and should be abandoned. A hot debate started in the field of surface science after 2007, between the three-phase contact line and interfacial contact area approach defenders. This paper presents a review of the published articles on contact angles and summarizes the views of the both sides. After presenting a brief history of the contact angles and their measurement methods, we discussed the basic contact angle theory and applications of contact angles on the characterization of flat, rough and micropatterned superhydrophobic surfaces. The weak and strong sides of both three-phase contact line and contact area approaches were discussed in detail and some practical conclusions were drawn.