先进材料在三叉戟Ⅰ导弹上的应用

为降低三叉戟Ⅰ导弹的成本,提高可靠性,对其进行了深入的材料研究。结果使得大约65％的主要结构及辅助结构采用了复合材料。其中包括层压Sitka云杉木、纤维缠绕的发动机壳体、石墨环氧布和注模聚苯撑硫。由于应用了这些材料,三叉戟导弹的重量比北极星和海神导弹大为减轻。     

桑给巴尔恢复昔日的辉煌

桑给巴尔——斯坦利和利维斯顿在世时曾经在这里居住(见注解)。这里还曾经是东部非洲最大的港口,它的服务面向西延伸到现在的莫桑比克以及刚果盆地。但是,多年的殖民主义统治,加上后来这个国家的社会主义政权实行闭关锁国政策长达30年之久,随着阿拉伯-印度贸易的结束,这个坦桑尼亚海岸线之外的岛国成了人们记忆中若有若无的东西。不过事情正在发生变化。眼下,桑给巴尔全国上下齐心协力,争取恢复昔日的辉煌。科雷特·拉·格朗迈松写过一本关于桑给巴尔的书。他说:15年前,整个桑给巴尔城一片破败。政府没有钱修复,这里的海洋性气候又是那样潮湿,使得这里的一切糟糕到了无以复     

卷积码的有限响应输入序列及其码距特性

研究了卷积码的有限响应输入序列的特性和卷积码的误比特率的实用特性,提出了汉明距的一种解析求解法,利用这些特性和解析求解法,能够完全取代编码器设计中现有的计算机模拟方法,在搜索好码和计算误比特率方面具有明显的优越性,并可应用于Ｔｕｒｂｏ码中交织器的设计。     

Efficient delivery of meteorites to the Earth from a wide range of asteroid parent bodies

Almost all meteorites come from asteroids, but identifying their specific parent bodies, and modelling their transport to the Earth, has proved to be difficult. The usual model of delivery through orbital resonances with the major planets has recently been shown to deplete the supply of meteorites much too rapidly to explain either the observed flux at the Earth, or the length of time the meteorites have spent in space (as measured by cosmic-ray exposure ages). Independently, it has been found that a force arising from anisotropically emitted thermal radiation from asteroidal fragments (the 'Yarkovsky effect') influences the fragments' orbits in important ways. Here we report the results of a detailed model for the transport of meteorites to the Earth, which includes the Yarkovsky effect and collisional evolution of the asteroidal fragments. We find that the Yarkovsky effect significantly increases the efficiency of the delivery of meteorites to the Earth, while at the same time allowing a much wider range of asteroids to contribute to the flux of meteorites. Our model also reproduces the observed distribution of cosmic-ray exposure ages of stony meteorites.     

Coulomb-blockade transport in single-crystal organic thin-film transistors

Coulomb-blockade transport--whereby the Coulomb interaction between electrons can prohibit their transport around a circuit--occurs in systems in which both the tunnel resistance, Rb between neighbouring sites is large (>h/e2) and the charging energy, E(C) (E(C) = e2/2C, where C is the capacitance of the site), of an excess electron on a site is large compared to kT. (Here e is the charge of an electron, k is Boltzmann's constant, and h is Planck's constant.) The nature of the individual sites--metallic, superconducting, semiconducting or quantum dot--is to first order irrelevant for this phenomenon to be observed. Coulomb blockade has also been observed in two-dimensional arrays of normal-metal tunnel junctions, but the relatively large capacitances of these micrometre-sized metal islands results in a small charging energy, and so the effect can be seen only at extremely low temperatures. Here we demonstrate that organic thin-film transistors based on highly ordered molecular materials can, to first order, also be considered as an array of sites separated by tunnel resistances. And as a result of the sub-nanometre sizes of the sites (the individual molecules), and hence their small capacitances, the charging energy dominates at room temperature. Conductivity measurements as a function of both gate bias and temperature reveal the presence of thermally activated transport, consistent with the conventional model of Coulomb blockade.     

Reduction in the surface energy of liquid interfaces at short length scales

Liquid-vapour interfaces, particularly those involving water, are common in both natural and artificial environments. They were first described as regions of continuous variation of density, caused by density fluctuations within the bulk phases. In contrast, the more recent capillary-wave models assumes a step-like local density profile across the liquid-vapour interface, whose width is the result of the propagation of thermally excited capillary waves. The model has been validated for length scales of tenths of micrometres and larger, but the structure of liquid surfaces on submicrometre length scales--where the capillary theory is expected to break down--remains poorly understood. Here we report grazing-incidence X-ray scattering experiments that allow for a complete determination of the free surface structure and surface energy for water and a range of organic liquids. We observe a large decrease of up to 75% in the surface energy of submicrometre waves that cannot be explained by capillary theory, but is in accord with the effects arising from the non-locality of attractive intermolecule interactions as predicted by a recent density functional theory. Our data, and the results of comparable measurements on liquid solutions, metallic alloys, surfactants, lipids and wetting films should thus provide a stringent test for any new theories that attempt to describe the structure of liquid interfaces with nanometre-scale resolution.