液Zn对Fe_2Al_5涂层侵蚀的相变过程研究

本文对Fe2Al5涂层在液态Zn中的侵蚀相变过程进行了研究,实验结果表明:在侵蚀初期,靠界面张力平衡的作用,先发生热侵蚀,在试样表面形成热蚀沟,当热蚀沟达到一定程度后,液Zn与Fe2Al5相由不浸润变为浸润;同时Zn原子扩散进入Fe2Al5相并形成Fe2Al5-Znx固溶体相(η相);随着侵蚀时间的增加,发生在腐蚀界面上的液Zn对Fe2Al5涂层的侵蚀过程是恒温相变过程,相变过程使Fe2Al5涂层发生定向熔化.相变的驱动力来自于相成分的改变所引起的各相自由能的变化及各相间的相平衡的重新建立.     

Particle‐in‐cell simulation of the effect of curved magnetic field on wall bombardment and erosion in a hall thruster

A curved, convex towards the channel bottom magnetic field is an important feature of an advanced Hall thruster that allows confining the plasma flow in the channel center, reducing the divergence angle of the ejected ion beam, and improving the discharge performance. In this article, the discharge behaviour of a Hall thruster in magnetic fields with different degrees of curvature is simulated with a particle‐in‐cell numerical method, and the effect of curved magnetic field on the ion bombardment and wall erosion and the associated mechanisms are studied and analysed. The results show that, as the curvature of the magnetic field increases, the propellant ionization becomes more confined at the channel center, the potential drop inside the channel decreases, and the acceleration region shifts outside the channel, which lead to the attenuation of the ion energy bombarding the wall and the deviation of the bombardment angle from the optimal sputtering angle. Conversely, the ion flux bombarding the wall near the channel exit increases. Nevertheless, the bombardment energy and angle are the dominant factors for the wall erosion, and the wall erosion rate clearly decreases with the increasing curvature of the magnetic field. These findings are closely related to the behaviour of electron conduction under a curved magnetic field; the relevant mechanisms are clarified in this article.     

化学腐蚀对混凝土材料力学特性影响的试验研究

对不同水化学环境腐蚀过程中砂浆试样物理力学性质进行了试验研究,分析了溶液pH值、浓度及水化学成分对砂浆的腐蚀程度,探讨了砂浆试样的化学腐蚀机制。试验结果表明:在相对封闭的环境条件下,无论是哪种水化学溶液,随着化学侵蚀时间的加长,溶液的pH值均趋于碱性;同时,溶出的Ca2+、Mg2+离子具有累积性特点。在其他条件相同的情况下,水化学溶液的离子成分对砂浆试样的腐蚀程度各不相同。酸性条件下,Na2SO4溶液砂浆试样物理化学性质的影响要比NaHCO3溶液大;中性条件下,与纯净的蒸馏水相比,Na2SO4溶液对砂浆试样的腐蚀作用要强一些。     

Spatial variability of 2H and 18O composition of meteoric freshwater lakes in Scotland

Coastal regions, and in particular islands where precipitation from clouds formed out at sea occurs for the first time, are prime candidates for regions where ²H and ¹⁸O composition of precipitation will deviate significantly from the global mean geographic and physiographic trends of vapour-transport patterns. The results reported here are the outcome of a study that aimed to test this hypothesis by ‘isotopographically’ mapping the characteristic δ²H and δ¹⁸O signatures of Scottish freshwaters. The resulting isotope abundance landscapes or ‘isoscapes’ will underpin studies aiming to authenticate origin of Scottish produce but may also offer a baseline against which environmental changes could be assessed. Between April 2011 and May 2012 freshwater samples were collected from 127 different freshwater lochs and reservoirs across Scotland, and analysis results were compared to precipitation data provided by the British Geological Survey. Here we present the results of the ²H and ¹⁸O analyses of these water samples as well as the first detailed Scotland freshwater isoscapes with a grid resolution of about 5 × 5 km (0.05 degrees).     

含砂流对弯管段冲蚀的机理分析

管道的弯管段、闸阀等处常常面临含砂流动的冲蚀磨损而导致管壁减薄或泄漏。为研究弯管段冲蚀磨损的主要机制,进行了加入5%石英砂的固液两相管流循环试验。试验按砂粒粒径分组,主要观测布置在弯管段的一组碳钢贴片在2~3小时后的壁面冲蚀特征。用失重法计算各贴片的冲蚀速率,基于扫描电镜(SEM)研究了贴片表面的损伤形貌。冲蚀后的贴片表面既有犁沟、挤压凹坑及划痕,亦有圆形蚀坑。能谱分析(EDS)显示有一定量的铁氧化物,且不同位置处的贴片失重率有正有负,由此可见导致贴片表面破坏的机制既包括切削、挤压和二次冲击等含沙流流动的机械作用,也伴随着腐蚀作用。这两种机制在弯管各区有不同的体现,且受砂粒粒径的影响。用数值模拟的两相流冲蚀弯管段流场压力和颗粒分布特征做了验证。结果表明弯管段的壁面损伤机制在外拱壁面以颗粒冲击和切削为主,在内拱璧面则以氧化腐蚀为主。     

细颗粒泥沙净冲刷和输移的大涡模拟研究

在传统水沙输移数值模拟研究中一般采用雷诺时均模拟技术(Reynolds-averaged simulation,RANS).与RANS相比,大涡模拟技术(large eddy simulation,LES)能够更加精确反映细部流动结构,计算机的发展使得采用LES探讨水流和泥沙运动规律成为可能.本文尝试给出净冲刷条件下悬沙计算的边界条件,采用动态亚格子模式对循环槽道和长槽道中的水流运动和泥沙输移进行了三维大涡模拟研究.利用直接数值模拟(direct numerical simulation,DNS)结果对LES模型进行了率定,计算结果符合良好,在此基础上初步探讨了泥沙浓度、湍动强度和湍动通量等的分布特征.结果表明,净冲刷条件下输沙平衡时泥沙浓度符合Rouse公式分布,单向流动中泥沙浓度沿着流向逐渐增大.泥沙浓度湍动强度和湍动通量都在近底部达到最大值,沿着垂向迅速减小.湍动黏性系数和扩散系数基本上在水深中间处达到最大.湍动Schmidt数沿着水深方向不是常数,在近底部和自由水面附近较大,在水深中间处较小.     

泡沫镍/环氧树脂/碳化硅双连续复合材料的料浆冲蚀行为研究

设计并制备出一种颗粒增强型双连续三元复合材料--泡沫镍/环氧树脂/碳化硅双连续复合材料,以材料的总损失量为重点研究了其在含氯化钠料浆冲蚀条件下的损伤行为.结果表明:颗粒增强的泡沫镍/环氧树脂/碳化硅双连续三元复合材料的耐料浆冲蚀性能明显优于环氧树脂/碳化硅二元复合材料;不同孔径泡沫镍骨架的复合材料的冲蚀量随攻角变化出现了不同的峰值;合理匹配泡沫镍骨架的孔径和体密度,可制备出具有优异抗冲蚀性能的颗粒增强双连续复合材料.     

模拟酸雨胁迫与植物叶片光谱反射率特征间的关系

酸雨是一个严重的全球环境问题,中国亚热带地区的严重酸雨污染对森林生态系统的健康构成了潜在的威胁。文章研究了中国亚热带地区常见的6种常绿阔叶树种幼树在不同强度的模拟酸雨(pH 2.5, pH 4.0和pH 5.6)作用下,其叶片的叶绿素含量及可见光部分的光谱反射率的变化特征。结果表明,随模拟酸雨强度的增加,受试树种在pH 2.5和pH 4.0处理下的叶绿素含量均高于pH 5.6的处理;而可见光部分的光谱反射率则普遍低于pH 5.6的处理;pH 2.5和pH 4.0间的差异无显著规律。在经不同强度模拟酸雨作用后,各树种叶片在可见光部分的光谱反射率的差异主要集中在绿光反射峰及红边附近。受试树种对酸雨胁迫均表现出了一定的耐受性,较强的酸雨可在一定程度上对植物生长起到正的生物效应。     

Formation of excited states of atoms and ions in laser plasma from CuInS2

Emission spectra and the energy distribution of the excited-state population density of atoms and ions in erosion laser plasma from CuInS₂ with various crystal-structure orderings are analyzed. It is shown that increased ordering of the target crystal structure causes the excited-state energies of indium atoms generated in the laser erosion plume to increase and that sulfur atoms always emit only in transitions from highly excited states. The ratio of relative ion concentrations in the laser plasma plume is Cu⁺/In⁺/S⁺ = 0.3/0.08/2, which corresponds neither to the atomic ratio of Cu/In/S (1/1/2) in the target nor to the ratio of ionization energies. The results are explained by recombination processes for ions and by the atomization specifics of the CuInS₂ target exposed to long-wavelength radiation. The atomization consists essentially of dissociative processes expressed by CuInS₂ → CuInS + S and CuInS₂ → Cu + InS + S. The electron temperature of polycrystal (single-crystal) plasma at a distance of 1 mm from the target is 0.3 eV (0.4 eV) for atoms and 1.3 eV (2.7 eV) for ions and varies negligibly for plasma up to a distance of 7 mm from the target. __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 75, No. 2, pp. 217–223, March–April, 2008.     

关于跃移沙破坏生物结皮时的临界速度的理论分析

利用经典的弹性薄板与塑性理论探讨了生物结皮在单颗跃移沙粒碰撞作用下的破坏问 题,建立了一个联结跃移沙粒的临界撞击速度和结皮性质的解析关系式. 种类、数量、发展 阶段等生物效应主要通过结皮的杨氏模量、Possion比、屈服应力等宏观参量引入. 结果表 明,刚度大、强度低的生物结皮更容易被跃移沙粒所破坏. 给出的关系式将有助于进一 步研究风沙运动和结皮之间的相互作用.