尼龙有结节菱形网片的水动力系数

为了提高网片水动力系数的计算精度,本文通过动水槽实验对5种网片的水动力学性能进行测试。测试的参数包括水流对网片的冲角、流速、网片的水平缩结系数ET等,网片的水动力学升力系数CL、阻力系数Cd、升阻系数比K等。结果表明:(1)当雷诺数Re小于1 500,冲角小于45°时,阻力系数呈现先增后减的趋势;而冲角大于45°时,阻力系数呈减小的趋势。雷诺数Re大于2 800时,阻力系数基本趋于稳定。(2)升阻系数比K最大值出现在20°附近;在30°~90°之间,K值呈减小趋势;(3)升阻力系数随d/a的增大先增大后减小;(4)升阻力系数随线面积系数的增大而减小;(5)通过多元非线性拟合得出升阻力系数的经验公式,拟合度较好。本实验经验公式为拖网阻力的计算提供了依据。     

The mechanism of diamond nucleation from energetic species

A model for diamond nucleation by energetic species (for example, bias-enhanced nucleation) is proposed. It involves spontaneous bulk nucleation of a diamond embryo cluster in a dense, amorphous carbon hydrogenated matrix; stabilization of the cluster by favorable boundary conditions of nucleation sites and hydrogen termination; and ion bombardment-induced growth through a preferential displacement mechanism. The model is substantiated by density functional tight-binding molecular dynamics simulations and an experimental study of the structure of bias-enhanced and ion beam-nucleated films. The model is also applicable to the nucleation of other materials by energetic species, such as cubic boron nitride.     

Melting of diamond

Radiation from a Q-switched YAG laser, focused on the (100) face of a single crystal diamond anvil in a high-pressure diamond cell, caused a portion of the diamond anvil face to melt. Potassium bromide mixed with graphite was under pressure between the anvils when melting occurred. The diamond surface melted at pressures greater than approximately 120 kilobars and graphitized at lower pressures. Evidence for the melting and graphitization of the diamond was obtained by optical and scanning electron microscopy.     

Strength of diamond

The yield strength of diamond is measured under a pressure of 10 gigapascals at temperatures up to 1550 degrees C by the analysis of x-ray peak shapes on diamond diffraction lines in a powdered sample as a function of pressure and temperature. At room temperature, the diamond crystals exhibit elastic behavior with increasing pressure. Significant ductile deformation is observed only at temperatures above 1000 degrees C at this pressure. The differential yield strength of diamond decreases with temperature from 16 gigapascals at 1100 degrees C to 4 gigapascals at 1550 degrees C. Transmission electron microscopy observations on the recovered sample indicate that the dominant deformation mechanism under high pressure and temperature is crystal plasticity.     

The genesis and collapse of third millennium north mesopotamian civilization

Archaeological and soil-stratigraphic data define the origin, growth, and collapse of Subir, the third millennium rain-fed agriculture civilization of northern Mesopotamia on the Habur Plains of Syria. At 2200 B. C., a marked increase in aridity and wind circulation, subsequent to a volcanic eruption, induced a considerable degradation of land-use conditions. After four centuries of urban life, this abrupt climatic change evidently caused abandonment of Tell Leilan, regional desertion, and collapse of the Akkadian empire based in southern Mesopotamia. Synchronous collapse in adjacent regions suggests that the impact of the abrupt climatic change was extensive.     

小菜蛾对阿维菌素的抗性形成规律

通过叶片浸药饲喂法用阿维菌素对室内敏感品系小菜蛾连续进行抗性汰选。经２６代选育,ＬＤ５０值由选育前的０．００２９μｇ／虫提高到０．３６２９μｇ／虫,抗性增加１２５．１倍。从整上抗性发展过程看,小菜蛾对阿维菌素的抗性形成呈现出先缓慢、后迅速、再缓慢、再迅速的阶梯式上升特点。     

蒙古绵羊胎儿卵巢发育的组织学研究

对37-99 d胎龄的蒙古绵羊胎儿卵巢进行组织学观察。结果表明,绵羊胎儿在37 d,可以从外观上分辨雌雄,雌性胎儿的性腺初步出现卵巢的组织学特征;37-51 d为卵原细胞的共质体期,卵原细胞数量增长加快;55~76 d卵巢中逐渐充满大量的合胞体样卵母细胞群;从58 d开始卵巢皮质深层出现少量的形态不典型的原始卵泡。胎龄73 d时,皮质深层出现真正的原始卵泡;81 d时皮质中层出现大量原始卵泡;99 d时皮质深层中可看到一些初级卵泡。与成体卵母细胞相比,胎儿时期的生殖细胞有活跃的增殖分裂特征;生殖细胞发育按卵原细胞增殖→卵母细胞分化→形成原始卵泡的步骤发育,在位置分布上由卵巢皮质外层逐渐向皮质里层分步发育。     

A reduction-pyrolysis-catalysis synthesis of diamond

Diamond powder was synthesized through a metallic reduction-pyrolysis-catalysis route with the reaction of carbon tetrachloride and sodium at 700 degreesC, in which the sodium was used as reductant and flux. This temperature is much lower than that of traditional methods. The x-ray powder diffraction patterns showed three strong peaks of diamond. The Raman spectrum showed a sharp peak at 1332 inverse centimeters, which is characteristic of diamond. Although the yield was only 2 percent, this method is a simple means of forming diamond.     

Short-lived radioactivity and magma genesis

Short-lived decay products of uranium and thorium have half-lives and chemistries sensitive to the processes and time scales of magma genesis, including partial melting in the mantle and magmatic differentiation in the crust. Radioactive disequilibrium between (238)U, (230)Th, and (226)Ra is widespread in volcanic rocks. These disequilibria and the isotopic composition of thorium depend especially on the extent and rate of melting as well as the presence and composition of vapor during melting. The duration of mantle melting may be several hundred millennia, whereas ascent times are a few decades to thousands of years. Differentiation of most magmas commonly occurs within a few millennia, but felsic ones can be tens of millennia old upon eruption.