Molecular flow and wall collision age distributions

The use of storage cells has become a standard technique for internal gas targets in conjunction with high energy storage rings. In case of spin-polarized hydrogen and deuterium gas targets the interaction of the injected atoms with the walls of the storage cell can lead to depolarization and recombination. Thus the number of wall collisions of the atoms in the target gas is important for modeling the processes of spin relaxation and recombination. It is shown in this article that the diffusion process of rarefied gases in long tubes or storage cells can be described with the help of the one-dimensional diffusion equation. Mathematical methods are presented that allow one to calculate collision age distributions (CAD) and their moments analytically. These methods provide a better understanding of the different aspects of diffusion than Monte Carlo calculations. Additionally it is shown that measurements of the atomic density or polarization of a gas sample taken from the center of the tube allow one to determine the possible range of the corresponding density weighted average values along the tube. The calculations are applied to the storage cell geometry of the HERMES internal polarized hydrogen and deuterium gas target. Received 9 July 2001 and Received in final form 18 September 2001     

Synthesis and Characterization of PZT Fibers via Sol-Gel

A fiber forming PZT gel containing 58.5 wt% PZT was synthesized by using zirconium-n-propylate, titanium-iso-propylate, lead acetate and butoxyethanol. Unseeded PZT gels and gels containing 0.5 wt% PZT perovskite seeds (Ø = 200–300 nm) could be extruded through a monofilament nozzle (Ø = 100 m) at pressures between 50 and 150 bar, whereas PZT gels, containing 1 and 2 wt% PZT particles, were pressed through the nozzle at higher pressures (200–300 bar). The microstructure of unseeded and seeded (0.5, 1, 2 wt% PZT) PZT fibers was characterized by SEM. Unseeded fibers had three different shells at 450°C: an external dense shell (approx. 200 nm thick), a middle shell consisting of a porous structure (1.5m thick) and the center of the fiber, characterized by a matrix containing globular particles. At 700°C, a 200–250 nm thick and dense external shell and a porous fiber interior were be observed. 2 wt% of PZT seeds was necessary to densify the fiber completely. The seeds were located in the center of each PZT perovskite rosette.     

节省便携系统功率的设计思考

蜂窝电话是迅速发展的一个市场领域，预测明年可增长到２．５亿部；到２００３年可达５亿部。面对如此巨大的市场，制造商们尽全力使产品完备无缺，用某些关键指标使产品优于竞争对手。节省便携系统中每个部分的功率至关重要。例如，若ＣＰＵ功耗为４Ｗ，则降低２５％可节省１Ｗ，这对于由一对ＡＡ电池供电的手持装置意义尤其重大。设计人员千方百计节省可能节省的每一ｍＷ，如果使键盘控制器耗电从１０ｍＡ降到８ｍＡ，则可节省６．６ｍＷ（使用３．３Ｖ电源）。在蜂窝电话和计算机中能节省功率的产品是大不相同的，尽管它们的目的都是延长…     

电子图书设计初探

高技术让我们生活得更舒适.所有这些新技术发明是对有用的老式技术的改进.以PDA为例,我个人认为,它的确超越了过时的记事簿.     

分立逻辑器件何去何从

分立逻辑功能合并的趋势随处可见，很久以前就出现了芯片组，而且这一技术还在发展。一般说来，设计师想极力摆脱分立逻辑，但历史经验告诉我们，人们将不可能完全摒弃逻辑器件。市场调研表明：前年，逻辑产品的全部有效市场额度（TAM）为24亿美元。如果按工艺技术划分的话，CMOS逻辑占18.7亿美元，其次是BiCMOS，为3.05亿美元，而TTL（包括最早的双极工艺）为3.09亿美元。有朝一日会实现集成化确实，只要有可能，分立逻辑功能已被并入在其它器件中，如母板上的芯片组中。但对某些功能，还是需要分立逻辑。一个重要原因是，随着速度和…     

值得开发的语音技术

从便携式方面讲，引入正常的语音输入功能不一定非要去掉键盘的使用，但会减少键盘操作,由此可使每一系统都设计成更紧凑的样式。在汽车应用方面，语音输入几乎成了“必需”的功能。对于电话号码的查寻和拨通、查找地址、查看日记等功能操作，驾驶人员无需让手臂离开方向盘便可轻易实现，使它成为理想的解决方案。研究指出，到2006年远程信息服务的潜在市场可能高达64亿美元。据称，某些最新的语音识别系统已经达到了对字串而非单个字词的识别水平。在远程信息系统中，某些识别功能可以放在服务器端实现，但大部分功能必须保留在自身设备中…     

A theoretical study of medium effects on the structure of the glycine analogue aminomethylphosphonic acid

alpha-Aminophosphonic acids are analogues of natural alpha-aminoacids and very promising agents for use in various pharmaceutical applications. However, in contrast to the numerous theoretical investigations on the structure of natural alpha-aminoacids, only very few studies on alpha-aminophosphonic acids have been performed. In the present work, we report a detailed investigation of the simplest compound, the glycine analogue aminomethylphosphonic acid (AMPA), by means of quantum mechanical calculations at the B3LYP/6-311++G(3df,2p)//B3LYP/6-31+G(d,p) and MP2/6-311++G(3df,2p)//B3LYP/6-31+G(d,p) levels. We focus on the structure of the neutral species looking at the evolution of non-ionized and ionized forms from gas phase to non-polar solvents and aqueous media. Continuum and discrete-continuum solvent models have been employed to account for the effects of the environment. The discussion is centered on: (1) the geometry and relative stability of possible conformers in gas phase and aqueous solution, (2) the free energy of tautomerization in different media, (3) the role of hydrogen bonds in liquid water, and (4) the free energy of transfer from water to a hydrophobic solvent such as cyclohexane. Systematic comparison between AMPA and Gly is performed. Though both systems exhibit many similarities, some important differences have also been found that may be explained, at least in part, by the higher acidity of phosphonic acids compared to carboxylic acids. In particular, in solvents lacking hydrogen-bond formation capability, Gly derivatives should mainly exist as non-ionized molecules while the equivalent AMPA derivatives should adopt a zwitterionic structure in media with dielectric constant above 10. This might have significant environmental or biological consequences that will need to be addressed.     

选择恰当的便携系统总线

在便携系统中,从点Ａ到点Ｂ传 送数据有很多方法。第一个要回答的问题是,点Ａ和点Ｂ表示什么？第二个要回答的是信息是从主处理器到存储器（或到电池供电子系统）或者是另外其他目的地？ 在数据从处理器（或微控制器）传输到存储器（如串行ＥＥＰＲＯＭ）的最普通实例中,典型的传输媒体有三种类型：串行外设接口（ＳＰＩ）、ＩｎｔｅｒＩＣ（Ｉ２Ｃ）和 ＭｉｃｒｏＷｉｒｅ。 串行接口 ＳＰＩ开始由Ｍｏｔｏｒｏｌａ公司开发,是设计作为该公司微控制器系列的接口。其后,ＳＰＩ已变成某种行业标准。 ＳＰＩ是一种同步串行数据协议,它支…     

便携系统的用户接口设计

有许多方法从一个计算机平台输入或输出信息。在大多数情况下是由应用来决定什么是最适合的用户接口方式。选择的范围可以是标准键盘或小型键盘、光笔、触摸屏,甚至是语音输入。还包括和机器集成在一起的点击式输入设备,如触摸垫（touch pad）、小型鼠标、点击棒（pointing stick）等。将来,也许会有其他更有趣的设备。决定选择这些方法的主要因素是成本和接口的方式。如果成本不是一个主要考虑的因素,那么,几乎所有的接口方式都可以被选用。语音识别应该是最难实现的一种输入方式,但如果不考虑成本,则可以极大地提高处理器的处理能…