Persistence of polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/F) in Ya-Er Lake area, China

The concentrations of polychlorinayed dibenzo-p-dioxins and dibenzofurans (PCDD/F) in surface sediment, soil, human hair, acid fish muscle from Ya-Er Lake area, China, were analyzed. The results showed that there were very high concentrations of PCDD/F existing in these samples. The results also indicated that Ya-Er Lake, which received a large amount of waste water from a nearby chloroalkali plant, was heavily polluted by PCDD:F. The present study demonstrated that those congeners, which possess at least three chlorine atoms in the lateral position with a fourth chlorine atom in the neighborhood bond of the third single chlorine atom, such as 1,2,7,8-TCDF and 2,3.6,7-TCDF, were very resistant to biodegradation due to the "neighbor effect" of every two chlorine atoms. The present study suggested that human hair may be a suitable alternative bioindicator for detecting PCDD/F exposure. (C) 2001 Elsevier Science Ltd. All rights reserved.

High-density and narrow size-distribution InAs quantum dots formed by a modified two-step growth

We develop a modified two-step method of growing high-density and narrow size-distribution InAs/GaAs quantum dots (QDs) by molecular beam epitaxy. In the first step, high-density small InAs QDs are formed by optimizing the continuous deposition amount. In the second step, deposition is carried out with a long growth interruption for every 0.1 InAs monolayer. Atomic force microscope images show that the high-density (similar to 5.9x 10(10) CM-2) good size-uniformity InAs QDs are achieved. The strong intensity and narrow linewidth (27.7 meV) of the photoluminescence spectrum show that the QDs grown in this two-step method have a good optical quality.

High temperature operation of 5.5 mu m strain-compensated quantum cascaded lasers

We develop 5.5-mu m InxGa1-xAs/InyAl1-yAs strain-compensated quantum cascade lasers with InP and InGaAs cladding layers by using solid-source molecular-beam epitaxy. Pulse operation has been achieved up to 323 K (50 degrees C) for uncoated 20-mu m-wide and 2-mm-long devices. These devices display an output power of 36 mW with a duty cycle of 1% at room temperature. In continuous wave operation a record peak optical power of 10 mW per facet has been measured at 83 K.

research on user-centered design and recognition pen gestures

Pen-based user interface has become a hot research field in recent years. Pen gesture plays an important role in Pen-based user interfaces. But it’s difficult for UI designers to design, and for users to learn and use. In this purpose, we performed a research on user-centered design and recognition pen gestures. We performed a survey of 100 pen gestures in twelve famous pen-bases systems to find problems of pen gestures currently used. And we conducted a questionnaire to evaluate the matching degree between commands and pen gestures to discover the characteristics that a good pen gestures should have. Then cognition theories were applied to analyze the advantages of those characteristics in helping improving the learnability of pen gestures. From these, we analyzed the pen gesture recognition effect and presented some improvements on features selection in recognition algorithm of pen gestures. Finally we used a couple of psychology experiments to evaluate twelve pen gestures designed based on the research. It shows those gestures is better for user to learn and use. Research results of this paper can be used for designer as a primary principle to design pen gestures in pen-based systems.

黄土高原苜蓿草地在不同种植方式下的土壤水分变化

在长期定位试验的基础上,通过田间实地测定0~400 cm土壤含水量,分析和比较了不同种植方式下苜蓿草地土壤水分的变化。结果表明,连作苜蓿地、轮作苜蓿地的400 cm土层平均土壤含水量分别为10.6%和11.4%,均低于土壤稳定湿度,其干燥化指数为24.6%和37.2%,分别属强烈干燥化和严重干燥化,而小麦连作的干燥化指数为86.4%,属轻度干燥化。连作苜蓿地土壤干层最厚,400 cm处仍十分干燥,而轮作苜蓿地和连作小麦地到240 cm以下时,土壤水分开始有所恢复。连作苜蓿地和轮作苜蓿地通过降雨可恢复部分土壤水分,可恢复的土壤深度为40 cm和60 cm,而连作小麦地可达100 cm。不同施肥措施下连作苜蓿地土壤干燥化程度都很严重,施肥措施不是造成土壤干燥化的主要原因。轮作系统中不同轮作年限苜蓿地的土壤水分状况有一定的差异,但是均没有形成深厚的土壤干层。与连作苜蓿相比,轮作苜蓿不会大量消耗土壤深层水分而形成深厚的土壤干层,有利于土壤水分的可持续利用。

作为阻尼材料的聚醚/聚苯乙烯接枝及交联多嵌段共聚物的合成与表征

接枝及嵌段共聚物作为阻尼材料，人们已经做了一定的工作。但是，从接枝与嵌段共聚物的对比中，系统地研究合成方法及多种因素对阻尼性能的影响规律，尚未见报导。这个工作就是从事这方面的研究，认为这种研究对于阻尼材料的分子设计及材料的推广应用具有意义的。所造反的反应体系及合成方法是，由端羟基聚醚即端羟基聚四氢呋喃及端羟基环氧丙烷，与顺丁烯二酸酐反应，以N，N-二四基苄胺为催化剂，合成了不同分子量的聚四氢呋喃大分子单体、不同结构（线型、支化型）的聚环氧丙烷大分子单体和相应的遥爪低聚物。由这些大分子单体和遥爪低聚物与苯乙烯共聚，用过氧化环已酮一环烷酸钴为氧化、还原引发体系，合成了各种嵌段、接枝共聚物。用DSC研究上述大分子单体及遥抓苯乙烯的固化反应动力学，结果表明：反应级数约为2，反应活化能为60 ～ 100kJ/mol；频率因子的对数LnA为17.4 ～ 26.7(min~(-1))。由这些动力学参数及Arrhenius方程可预测适当的固化反应条件。我们用粘弹谱仪研究了嵌段及接枝共聚物的阻尼性能，对于接枝共聚物，阴尼峰随着聚醚含量的增加具有先下降后升高的变化趋势，升高的原因，是由于共聚物中支链节的相互缠结，增大了分子间相对运动时的摩擦所致。具有较长的支链或支链上带有侧基的共聚物，阻尼性能较好。嵌段共聚物的阻尼峰随着聚醚含量的增加，具有下降的趋势，聚醚起到内部增塑的作用聚醚上侧基的存在，也有助于阻尼性能的提高，这在比较聚四氢呋喃/聚苯乙烯嵌段共聚物与聚环氧丙烷/聚苯乙烯嵌段共聚物的阻尼性能时，可以观察到。我们还研究了不同添加剂对共聚物阻尼性能的影响，实验结果表明，环氧树脂作为添加剂时，使共聚物的阻尼性能得到提高；而丁腈羟作为添加剂时，仅使共聚物的阻尼峰变宽，而阻尼峰值变小。红外光谱跟踪反应过程及电镜照片表明，这些添加剂都不同程度地参与了固化反应。关于这些共聚物的力学性能，测定的结果表明，嵌段共聚物比较，前者的抗张强度较大，伸长率较小。在嵌段和接枝共聚中，聚醚含量增加，两者的抗张强度均呈下降的趋势，聚醚起到增塑的作用。为了评价材料的耐热性能，我们做了各共聚物的热重分析，（TG）实验，实验结果表明，各共聚物具有较好的热稳定性，尤以嵌段共聚物为好。加有添加剂时，有助于热稳定性能的提高。以PPO/PS嵌段及接枝共聚物为例；嵌段共聚物的分解温度为352 ℃; 接枝共聚物分解温度为323 ℃；加有丁腈羟添加剂后，接枝共聚物的分解温度为358 ℃。

INVESTIGATION OF BURIED ALN LAYERS FORMED BY NITROGEN IMPLANTATION INTO AL

The composition and microstructure of buried layers of AlN formed by high energy N+ ion implantation into polycrystalline Al have been determined. Both bulk and evaporated thin films of Al have been implanted with 100 and 200 keV N+ ions to doses of up to 1.8 x 10(18)/cm2. The layers have been characterised using SIMS, XTEM, X-ray diffraction, FTIR, RBS and in terms of their microhardness. It is found that, for doses greater than the critical dose, buried, polycrystalline AlN layers are formed with preferred (100) or (002) orientations, which are sample specific. With increasing dose the nitrogen concentration saturates at the value for stoichiometric AlN although the synthesised compound is found to be rich in oxygen.

Measurement of trace phosphorus in dichlorosilane by high-temperature hydrogen reduction gas chromatography

Dichlorosilane, a gas at normal temperature with a boiling point of 8.3 degrees C, is very difficult to sample and detect using conventional methods. We reduced phosphorus in dichlorosilane to PH3 by hydrogen at high temperature, then PH3 was separated from chlorosilanes by NaOH solution and from other hydrides by chromatographic absorption. Thus the problem of interference of chlorosilanes and other hydrides was overcome and PH, was measured by a double flame photometric detector at 526 nm. This method was sensitive, reliable and convenient and the sensitivity reached as low as 0.04 mu g/l.

Fabrication of Silicon Crystal-Facet-Dependent Nanostructures by Electron-Beam Lithography

Silicon crystal-facet-dependent nanostructures have been successfully fabricated on a (100)-oriented silicon-on-insulator wafer using electron-beam lithography and the silicon anisotropic wet etching technique. This technique takes ad-vantage of the large difference in etching properties for different crystallographic planes in alkaline solution. The mini-mum size of the trapezoidal top for those Si nanostructures can be reduced to less than 10nm. Scanning electron microscopy(SEM) and atomic force microscopy (AFM) observations indicate that the etched nanostructures have controllable shapes and smooth surfaces.