Strong circular photogalvanic effect in ZnO epitaxial films

We report a strong circular photogalvanic effect (CPGE) in ZnO epitaxial films under interband excitation. It is observed that CPGE current is as large as 100 nA/W in ZnO, which is about one order in magnitude higher than that in InN film while the CPGE currents in GaN films are not detectable. The possible reasons for the above observations are the strong spin orbit coupling in ZnO or the inversed valence band structure of ZnO.

Evaluation of thermal radiation dependent performance of GaSb thermophotovoltaic cell based on an analytical absorption coefficient model

Applying the model dielectric function method, we have expressed the absorption coefficient of GaSb analytically at room temperature relating to the contribution of various critical points of its electronic band structure. The calculated absorption spectrum shows good agreement with the reported experimental data obtained by spectral ellipsometry on nominally undoped sample. Based on this analytical absorption spectrum, we have qualitatively evaluated the response of active absorbing layer structure and its photoelectric conversion properties of GaSb thermophotovoltaic device on the perturbation of external thermal radiation induced by the varying radiator temperature or emissivity. Our calculation has demonstrated that desirable thickness to achieve the maximum conversion efficiency should be decreased with the increment of radiator temperature and the performance degradation brought by any structure deviation from its optimal one would be stronger meanwhile. For the popular radiator temperature, no more than 1500 K in a real solar thermophotovoltaic system, and typical doping profile in GaSb cell, a reasonable absorbing layer structure parameter should be controlled within 100-300 nm for the emitter while 3000-5000 nm for the base.

Seed bubbles stabilize flow and heat transfer in parallel microchannels

Seed bubbles are generated on microheaters located at the microchannel upstream and driven by a pulse voltage signal, to improve flow and heat transfer performance in microchannels. The present study investigates how seed bubbles stabilize flow and heat transfer in micro-boiling systems. For the forced convection flow, when heat flux at the wall surface is continuously increased, flow instability is self-sustained in microchannels with large oscillation amplitudes and long periods. Introduction of seed bubbles in time sequence improves flow and heat transfer performance significantly. Low frequency (similar to 10 Hz) seed bubbles not only decrease oscillation amplitudes of pressure drops, fluid inlet and outlet temperatures and heating surface temperatures, but also shorten oscillation cycle periods. High frequency (similar to 100 Hz or high) seed bubbles completely suppress the flow instability and the heat transfer system displays stable parameters of pressure drops, fluid inlet and outlet temperatures and heating surface temperatures. Flow visualizations show that a quasi-stable boundary interface from spheric bubble to elongated bubble is maintained in a very narrow distance range at any time. The seed bubble technique almost does not increase the pressure drop across microsystems, which is thoroughly different from those reported in the literature. The higher the seed bubble frequency, the more decreased heating surface temperatures are. A saturation seed bubble frequency of 1000-2000 Hz can be reached, at which heat transfer enhancement attains the maximum degree, inferring a complete thermal equilibrium of vapor and liquid phases in microchannels. Benefits of the seed bubble technique are the stabilization of flow and heat transfer, decreasing heating surface temperatures and improving temperature uniformity of the heating surface.

Numerical simulations of interrupted and conventional microchannel heat sinks

We provide three-dimensional numerical simulations of conjugate heat transfer in conventional and the newly proposed interrupted microchannel heat sinks. The new microchannel heat sink consists of a set of separated zones adjoining shortened parallel microchannels and transverse microchambers. Multi-channel effect, physical property variations, and axial thermal conduction are considered. It is found that flow rate variations in different channels can be neglected, while heat received by different channels accounts for 2% deviations from the averaged value when the heat flux at the back surface of the silicon chip reaches 100 W/cm(2). The computed hydraulic and thermal boundary layers are redeveloping in each separated zone due to shortened flow length for the interrupted microchannel heat sink. The periodic thermal developing flow is responsible for the significant heat transfer enhancement. Two effects influence pressure drops across the newly proposed microchannel heat sink. The first one is the pressure recovery effect in the microchamber, while the second one is the head loss when liquid leaves the microchamber and enters the next zone. The first effect compensates or suppresses the second one, leading to similar or decreased pressure drop than that for the conventional microchannel heat sink, with the fluid Prandtl number larger than unity.

Effect of pulse heating parameters on the microscale bubble dynamics at a microheater surface

We study the effects of pulse heating parameters on the micro bubble behavior of a platinum microheater (100 mu m x 20 mu m) immersed in a methanol pool. The experiment covers the heat fluxes of 10-37 MW/m(2) and pulse frequencies of 25-500 Hz. The boiling incipience is initiated at the superheat limit of methanol, corresponding to the homogeneous nucleation. Three types of micro boiling patterns are identified. The first type is named as the bubble explosion and regrowth, consisting of a violent explosive boiling and shrinking, followed by a slower bubble regrowth and subsequent shrinking, occurring at lower heat fluxes. The second type, named as the bubble breakup and attraction, consists of the violent explosive boiling, bubble breakup and emission, bubble attraction and coalescence process, occurring at higher heat fluxes than those of the first type. The third type, named as the bubble size oscillation and large bubble formation, involves the initial explosive boiling, followed by a short periodic bubble growth and shrinking. Then the bubble continues to increase its size, until a constant bubble size is reached which is larger than the microheater length. 

ZnO thin film photoconductive ultraviolet detector with fast photoresponse

C-axis preferred oriented ZnO thin films were prepared on quartz substrates by RF sputtering. Photoconductive ultraviolet detector with planar interdigital electrodes was fabricated on ZnO thin film by the lift off technique. Linear I-V characteristic was observed under dark or 365 nm UV light illumination and has obvious difference. The photoresponsivity of 365 nm at 5 V bias is 18 A/W. The response time measure set mainly contains KrF excimer laser with the pulse width of 30 ns and the oscillograph with the bandwidth of 200 MHz. The result shows fast photoresponse with a rise time of 100 ns and fall time of 1.5 mu s. (c) 2005 Elsevier B.V. All rights reserved.

Simultaneous multiwavelength observations of sagittarius A

We observed Sgr A* using the Very Large Array (VLA) and the Giant Metrewave Radio Telescope (GMRT) at multiple centimeter and millimeter wavelengths on 2003 June 17. The measured flux densities of Sgr A*, together with those obtained from the Submillimeter Array (SMA) and the Keck II 10 m telescope on the same date, are used to construct a simultaneous spectrum of Sgr A* from 90 cm to 3.8 mu m. The simultaneous spectrum shows a spectral break at about 3.6 cm, a possible signature of synchrotron self-absorption of the strong radio outburst that occurred near epoch 2003 July 17. At 90 cm, the flux density of Sgr A* is 0.22 +/- 0.06 Jy, suggesting a sharp decrease in flux density at wavelengths longer than 47 cm. The spectrum at long cm wavelengths appears to be consistent with free-free absorption by a screen of ionized gas with a cutoff similar to 100 cm. This cutoff wavelength appears to be three times longer than that of similar to 30 cm suggested by Davies, Walsh, & Booth based on observations in 1974 and 1975. Our analysis suggests that the flux densities of Sgr A* at wavelengths longer than 30 cm could be attenuated and modulated by stellar winds from massive stars close to Sgr A*.

稀土-聚合物纳米复合材料的制备与表征

纳米材料以其特有的优异性能成为下一代信息技术、能源技术和生物医学技术的重要基础。纳米材料的研究是一个多学科交叉的领域，受到各国科学家及政府的极大重视，成为当前科学研究的热点前沿领域之一。近年来，无机纳米微粒与聚合物复合而成的无机-有机纳米复合材料可能成为集无机、有机、纳米等诸多材料的优良特性于一身的新型功能材料，倍受各国研究者的关注，对无机-有 机纳米复合材料的研究构成了当前纳米材料领域的新热点。我们在当今科学研究的前沿领域内，通过大量文献调研，选择稀土化合物为 目标化合物，开展了研究工作。采用新方法制备了ZnS、ZnS:Eu、ZnS:Mn、ZnS:Mn,Eu和CeO_2纳米微粒，制备了稀土硫化物、CeO_2纳米微粒与聚苯乙烯的纳米复合材料，对所制备的材料进行了表征，研究了反应过程及材料的发光特性，取得了一些创新性结果。1．首次采用固相法在接近室温和室温条件下，分别以硫代乙酰胺(TAA)和硫化钠为原料与醋酸锌进行反应，成功地制备出ZnS纳米微粒，通过XRD、TEM、SPS、荧光光谱等对样品进行了表征，通过差热分析对反应机理进行了研究，得到以下结论：(1)．ZnS纳米微粒具有立方面心结构，为闪锌矿型ZnS。发现TAA与醋酸锌通过低温固相法制备ZnS纳米微粒的最佳温度为100℃，低于此温度反应进行得不完全。提高反应温度将使ZnS纳米微粒的尺寸增大。(2)．100℃下合成的ZnS纳米微粒的晶粒粒径为3.2 nm，TEM观察到的每个ZnS纳米微粒是由更小的ZnS晶粒构成的多晶颗粒，平均颗粒尺寸为40 nm左右。(3)．TAA与醋酸锌反应的历程几乎同时经历了以下几个过程，其中包括TAA的熔融，TAA的分解并产生硫化氢，产生的硫化氢与醋酸锌反应生成醋酸及ZnS，醋酸的气化等，ZnS纳米微粒在上述过程中得以迅速生成，产生的气体物质起到了控制颗粒进一步生长的作用。2．首次采用低温固相法制备出掺杂的ZnS纳米微粒，ZnS:Eu、ZnS:Mn和ZnS:Mn，Eu，研究了制备条件对粒径及发光的影响。研究表明，灼烧温度对掺杂的ZnS纳米微粒的尺寸影响较大，温度升高粒径随之增大：灼烧时间及掺杂浓度对粒径影响很小。荧光光谱中出现了掺杂离子的特征发光，增大掺杂离子浓度及提高灼烧温度可使发光增强。3．首次发现ZnS:Mn纳米粒子经紫外光照射后出现了荧光增强现象。无论是波长为254 nm还是365 nm的紫外光照射后都可以观察到上述现象，并且发现在一定时间范围内，发光亮度随着照射时间的延长而增强。产生这种现象的原因可能是纳米微粒的表面态得到改善，使无辐射通道减少，因而辐射强度提高。4．首次采用表面光电压谱对ZnS纳米微粒进行了表征，观察到表面光电压谱发生了蓝移，ZnS纳米微粒有丰富的表面态。荧光光谱中观察到产生于表面态的陷阱荧光，并发生蓝移。5．采用W/O微乳液法，选用多种非离子型表面活性剂，成功地制备出粒径很小、颗粒均匀的CeO_2纳米晶，对制备过程中的影响因素(如灼烧温度、水与表面活性剂的摩尔比、Ce~(3+)的浓度、灼烧时间、表面活性剂种类等)进行了详细研究。结果如下：(1)．灼烧温度是影响CeO_2纳米微粒尺寸及形貌的关键因素。找到制备纯净的Ce02纳米晶的最佳温度为500℃，低于此温度CeO_2为无定形态，高于此温度则CeO_2纳米微粒的形貌规整但粒径增大。CeO_2纳米微粒的晶格畸变率随着灼烧温度的升高和粒径的增大而减小。(2)．W(水与表面活性剂的摩尔比)、Ce~(3+)浓度、灼烧时间、表面活性剂种类等也对CeO_2纳米微粒的尺寸有影响，但其影响小于灼烧温度的影响。(3)．首次在高分辨条件下通过TEM观察到CeO_2纳米晶的条纹相结构。6．首次将CeO_2纳米微粒与聚苯乙烯复合，成功地制备出CeO_2聚苯乙烯纳米复合材料，对材料进行了表征。(1)．CeO_2/聚苯乙烯纳米复合材料的IR光谱中有Ce-O键的振动吸收，且比通常CeO_2的Ce-O键的振动吸收向高能方向移动，说明CeO_2粒径较小。(2)．XPS谱中Ce的3d_(5/2)和3d_(3/2)结合能较CeO_2的标准谱向高能方向发生了移动，表明CeO_2与表面活性剂及聚苯乙烯之间存在着某种化学键作用。7．首次采用原位合成法制备出稀土硫化物聚苯乙烯纳米复合材料，并通过荧光光谱、磁性、光电子能谱、透射电镜等手段对它们进行了表征。研究了该复合材料的磁性质、荧光光谱、XPS谱、微结构与掺稀土聚苯乙烯相比发生的变化，分析了产生上述变化的原因。(1)．在掺Eu聚苯乙烯复合材料的激发光谱和发射光谱中可见Eu~(3+)的激发和发射峰。Eu~(3+)离子处于对称性很低的聚合物环境中，发射光谱中所出现的~5D_0 → ~7F_J(J = 0，l，2，3，4)跃迁的发射峰中~7F_l和~7F_2等能级解除简并发生了能级劈裂。随着Eu浓度增大，Eu~(3+)离子的发射峰强度增加。(2)．稀土硫化物聚苯乙烯纳米复合材料的发射光谱中在400 nm左右有宽带发射，且经紫外光照射后其发光强度降低，这是由于H_2S与Eu~(3+)发生而生成的不很稳定的Eu~(2+)经紫外光照射后被氧化成更为稳定的Eu~(3+)。(3)．稀土硫化物聚苯乙烯纳米复合材料和掺Eu聚苯乙烯复合材料相比，在荧光光谱、磁性、XPS谱和微结构等方面存在较大的差别。研究表明，在稀土硫化物聚苯乙烯复合材料中有较多的Eu~(3+)离子处于无反演对称性的格位，使得它的发射光谱在615 nm处的发光峰为最强；稀土硫化物聚苯乙烯纳米复合材料的饱和磁化强度较掺Eu聚苯乙烯复合材料的低；稀土硫化物聚苯乙烯纳米复合材料的球粒尺寸明显大于掺Eu聚苯乙烯复合材料的球粒尺寸，远大于纯聚苯乙烯的球粒尺寸。

液/液界面上可质子化的药物转移反应的研究

近年来，电荷在两互不相溶电解质溶液界面上的电化学转移过程从实验和理理论上都引起了人们的极大兴趣，这主要是由于它与许多重要的生物和化学体系密切相关，如相转移催化、化学传感器、药理学中的药物释放以及模拟生物膜等。液／液界面常被看作人工膜和生物膜的简单模型，渺液界面电分析化学的主要研究对象是界面上的电荷转移反应以及伴随发生的相关的化学反应。本文简要回顾了油液界面电分析化学研究的发展历程，其双电层结构模型，油液界面电分析化学及可质子化的药物在油液界面上转移的基本理论。用三电极系统和电化学方法研究了可质子化的药物在水/1,2m-二氯乙烷界面上转移反应的机理，以及相比和溶液的pH值对药物转移反应的影响。主要结果如下： 1．用三电极系统研究了相比（r＝0.0004一1和1～2500）对可质子化的药物吡啶在水／1，2-二氯乙烷界面（W／DCE）上的转移反应的影响，实验结果用离子分配图表示出来。结果表明：在实验范围内，随着相比的增大，吡啶以中性分子形式存在的区域增大，质子化吡啶的转移式电位降低。2．设计了一利特定的电化学池，可使水相和有机相的体积减小到100μL以下。将含有支持电解质的水溶液滴在银／氯化银电极上，然后将1，2-二氯乙烷溶液覆盖在水溶液的表面，再与有机相参比电极和对电极构成常规的三电极系统，研究了相比为1行：附吩时可质子化药物阿米替林、苯海拉明和苯海索在水／1，2-二氯乙烷界面（W／DCE）上的转移反应，确定了它们的亲油性及转移反应的机理。

CA-100萃取稀土及二价金属的热力学和动力学研究

该文系统研究了一种新型有机羧酸类萃取剂仲壬基苯氧基乙酸(CA-100)对稀土及其杂质的萃取热力学和动力学规律,并通过协同萃取、双溶剂萃取、加入络合剂等手段对CA-100萃取体系进行改善,为该萃取剂在工业上的应用打下基础,具体的研究内容如下:1.研究了CA-100对于稀土及Zn,Cd,Cu,Co,Ni,Mn,Mg等金属元素的萃取热力学规律,计算了金属间分离系数,获得了萃取平衡方程式,考察了反萃性能及稀释剂和甲庚醇的加入对萃取的影响.研究发现该萃取剂可用于Sc同其它稀土的分离及某些金属对的分离,在很多方面优于环烷酸体系.2.探讨了CA-100与—盐基磷(膦)酸类萃取剂对锌和镉的协同萃取,研究了协同萃取机理,确定了协萃配合物的组成.3.研究了在络合试剂的存在下CA-100萃取重稀土的行为及Y同重稀土的分离情况.4.用恒流层界面池研究了CA-100萃取Y,Yb,La的萃取动力学,考察了各因素对萃取速率的影响,获得了萃取速率方程,探讨了动力学机理.5.探讨了各种因素对CA-100界面活性的影响.6.在上述热力学和动力学研究基础上,进行了CA-100从混合稀土溶液中富集和纯化Sc的工艺模拟实验.