The influence of ion bombardment on emission properties of small dust grains

Dust grains that are present in many plasma and vacuum systems and in the space usually carry a non-negligible charge. Their charging significantly depends on surface properties of the grain material. In cold plasma, charging is mainly given by electron attachment, nevertheless, when plasma becomes hot, other processes (secondary electron emission, field emission, etc.) take place. Emission properties of the grain surface could be modified by grain baking or by ion bombardment. Our study is carried out at the dust charging experiment dealing with a single dust grain electro-dynamically levitated in a 3D quadrupole trap. The grain can be exposed to the ion beam in the energy range of 100 eV–5 keV and to the electron beam in the energy range of 100 eV–10 keV. We have chosen He⁺ and Ar⁺ ions for the surface treatment and the observed influence on the surface properties is discussed in terms of secondary emission. A non-negligible shift of the secondary electron emission yield, as well as a change of energy distribution of secondary electrons, were measured after Ar⁺ bombardment. A preliminary study suggests that the effects of He⁺ and Ar⁺ are comparable.     

X荧光分析法测定地表灰尘和空调灰中重金属含量

使用X荧光分析法对地表灰尘和空调滤网灰中所含金属元素(Mn,Fe,Cu,Zn,Pb)做了定量分析.结果表明,5种元素在空调灰中的含量都大于其在地表灰尘中的含量,其中Cu,Zn,Pb 3种元素在地表灰尘和空调灰中的含量远远大于四川省表层土壤中对应的平均含量.通过对不同粒径组地表灰尘中重金属含量的分析表明,地表灰尘中五种元素的含量在测量范围内随灰尘直径的变化规律大致相同.     

Evolution of surface roughness of some metallic materials in cavitation erosion

The evolution of surface roughness of three common metallic materials (316L stainless steel, CP titanium, and brass) in ultrasonic vibratory cavitation tests was monitored using profilometric measurements. Three stages of roughness change, based on the rate of change of the mean surface roughness d(Ra)/dt, may be identified. In stage I (initial stage), Ra increases almost linearly with the test time; in stage II (transition stage), the rate decreases until stage III (steady-state stage) is reached, in which Ra remains unchanged. Concurrent measurements of mass loss in the ultrasonic cavitation test indicated that stage I approximately coincides with the incubation stage, stage II approximately coincides with the acceleration stage, and stage III approximately coincides with the maximum erosion rate stage as defined by ASTM Standard G 32. Compared with conventional mass loss measurements in assessing material degradation in cavitation erosion, surface roughness measurements provide an alternative and convenient method which possesses several advantages. In the first place, change in surface roughness provides information of material response before mass loss is detected. Secondly, there is no restriction of the size of the component, while weighing is suitable for small samples only. Thirdly, mass loss reflects erosion of the whole surface under cavitation attack, and the mean depth of penetration or erosion only gives an average loss, while in roughness measurement, damage in specific locations may be studied. The present study indicates that roughness measurement may constitute a practical method for monitoring damage in industrial ultrasonic cleaners.     

Effects of dust size distribution on nonlinear waves in a dusty plasma

For two-dimensional unmagnetized dusty plasmas with many different dust grain species, a Kadomtsev--Petviashvili (KP) equation, a modified KP (mKP) equation and a coupled KP(cKP) equation for small, but finite amplitude dust-acoustic solitary waves are obtained for different physical conditions respectively. The influence of an arbitrary dust size distribution described by a polynomial expressed function on the properties of dust-acoustic solitary waves is investigated numerically. How dust size distribution affects the sign and the magnitude of nonlinear coefficient A(D) of KP (mKP) equation is also discussed in detail. It is noted that whether a compressive or a rarefactive solitary wave exists depends on the dust size distribution in some dusty plasmas.     

Measurement and simulation of sand saltation movement under fluctuating wind in a natural field environment

Extensive research on the near surface movement of sand particles has focused on wind tunnel experiments, theoretical analysis, and numerical simulation of sand saltation under ideal and controllable conditions. Most field observations are results on the average rate of sand transport over some hours or the whole day. However, researchers found recently that the effect of turbulent characteristics of near surface wind in real atmospheric boundary layers on the sand transport rate was obvious. The turbulent characteristics would cause a significant discrepancy between field observation and simulation of sand transport rate. In this work, a field experiment in a real-time system was designed to synchronously measure physical quantities, such as fluctuating wind velocity in the near surface region, sand transport intensity, temperature, and humidity, with the frequency of 1 Hz, at two points on a homogeneous flat sand surface located in the Minqin area, which is between the edges of the Badain Jaran Desert and the Tengger Desert. The relationship between the saltation events and some physical properties, such as the fluctuating wind velocity, temperature, and humidity, was studied. On the basis of the field observation results, a numerical model was developed to simulate sand movement under the fluctuating wind. The overall features of the experimental measurements were reproduced by simulation.     

Distribution of charged lunar dust in the south polar region of the moon

Lunar dust is one of the most threatening problems confronting the return of human beings to the moon. In this work we studied the spatial distribution behavior of charged lunar dust in the solar wind plasma environment in the south polar region of the moon and considered the influence of a mini-crater using Spacecraft Plasma Interactions Software. The distribution of dust and plasma at low solar altitude angles of 20° and 0° was studied, and the spatial density of lunar dust was ~10^10.4 m^-3 and ~10^11.5 m^-3, respectively. This is because a higher surface potential will result in transportation of small dust particles and photoelectrons can also neutralize positively charged lunar dust. The dust density in the plasma void region created by a mini-crater with a 5 m high wall was studied. We obtained a quasi-neutral electric environment in the plasma void region of the mini-crater, and the dust density was about a magnitude lower than that in other regions. The dust risk to a spacesuit is much lower on the nightside than on the dayside, but there is severe charged lunar dust transport in the region between light and shade, which is dominated by the difference in surface and plasma potential caused by photoelectrons.     

等离子体鞘层中尘埃粒子的分布特性

采用柱槽状电极的流体模型，数值模拟了等离子体鞘层及鞘层中尘埃粒子的分布结构. 研究了尘埃粒子数、粒子大小、电极尺寸等因素对尘埃分布结构的影响. 研究表明：当等离子体密度较高时，鞘层较薄，反之鞘层较厚；当尘埃粒子数少时，尘埃分布形成一层结构，反之则形成多层结构；随电极尺寸的不同，尘埃粒子形成一些复杂而又有趣的结构. 关键词： 尘埃粒子 等离子体鞘层 电极     

表面粗糙度对固体内部超声背散射的影响

超声背散射法可通过多晶体金属内部的空间方差信号,实现微观结构参数的无损评价,但表面粗糙度对评价模型的精度及实用性存在显著影响.基于高斯声束理论推导垂直入射粗糙界面的纵波声场,以此研究声能的Wigner分布规律;在超声的波长远大于粗糙度的前提下,构造表面粗糙度修正系数,并建立粗糙界面的单次散射响应模型,揭示粗糙度对超声波背向散射的影响规律.用304不锈钢制备轮廓均方根值为0.159μm的光滑试块和25.722μm的粗糙试块开展超声背散射实验,结果表明模型在粗糙度修正前后均可实现光滑试块的晶粒尺寸有效评价,但未经修正的传统模型对粗糙试块的晶粒尺寸评价结果与金相法结果的相对误差高达-21.35%,而本模型的评价结果与金相法结果符合得很好,相对误差仅为1.35%.可见,本模型能有效补偿粗糙度引起的超声背散射信号衰减,从而提高晶粒尺寸无损评价的精度.     

The effect of negative ions on the dust acoustic wave in dusty electronegative plasma with positively charged dust grains

The effect of negative ions on the modulational instability properties of nonlinear dust acoustic (DA) waves in the electronegative dusty plasmas was investigated by considering Boltzmann-distributed electrons, negative ions, positive ions as well as positively charged dust grain under the ultraviolet irradiation. It is shown that the modulational instability properties of the DA waves were strongly affected by the temperature and proportion of negative ions. The modulational instability can occur only if the proportion of negative ions was smaller than critical value. The instability growth rate has a maximum value when the proportion of negative ions was a critical one in the unstable region. The effect of photoelectron generated by ultraviolet irradiation on the modulational instability of dust acoustic waves was also discussed by numerical method.     

Effect of pearlitic morphology with varying fineness on the cavitation erosion behavior of eutectoid rail steel

The present work discusses the effect of the pearlitic morphology with varying fineness on the cavitation erosion behavior of eutectoid rail steel. Cavitation erosion of three different types of the pearlitic steels (furnace-cooled, air-cooled, and forced-air-cooled) consisting of coarse, fine, and very fine microstructures were tested in 3.5% NaCl solution and compared with that of the as-received pearlitic rail steel. The variation in the mean depth of erosion (MDE) and mean depth erosion rate (MDER) with erosion time was analyzed. Furthermore, the cavitation erosion resistance of the as-received, the air-cooled, and the forced-air-cooled was found to be 1.03, 1.51, and 2.14 times better than the furnace-cooled pearlitic steel, respectively. It was concluded that the cavitation erosion resistance of the pearlitic steel increased with the increase in the fineness of the microstructure.     

合成孔径雷达反演海面风场变分模型分析

为考察合成孔径雷达反演海面风场变分模型精度,开展了误差分析试验.在背景场误差为极大值条件下分析场误差低于背景场误差,且随背景场风向增大呈周期性变化;在背景场误差逐渐变大条件下分析场误差逐渐增大,误差偏离方向与背景场误差偏离方向一致;在特定背景场条件下与直接反演模型相比,低风速时反演误差略高,中高风速时反演误差明显较低.总体来讲,变分模型误差小于背景场误差,风速反演误差小于1.60 m/s,风向误差小于17.15°.     

Resonant decay of a Langmuir wave in the presence of dust grains in a cylindrical plasma

The resonant parametric decay of a Langmuir wave into a backward propagating Langmuir wave and an ion acoustic (IA) wave is studied in a cylindrical dusty plasma. The analysis shows that the frequency of the IA mode decreases with the parameter δ_c (where δ_c is the ratio of the ion density to the electron density) for negatively charged dust grains. The growth rate of the resonance decay instability (RDI) and the threshold required for its onset also decrease with δ_c and are strongly dependent on the electron to ion temperature ratio for both positively and negatively charged dust grains. The results obtained also illustrate the dependence of the threshold of the resonance decay instability (μ_th) on the plasma cylinder radius.     

弱电离尘埃等离子体的介电张量研究

忽略磁场作用,通过求解含BGK碰撞项的Boltzmann方程和尘埃粒子充电方程导出了弱电离尘埃等离子体介电张量的表达式.证明了“冷”尘埃等离子体的纵向介电张量系数与横向介电张量系数相等.完善了弱电离尘埃等离子体电磁特性的理论模型. 关键词： 弱电离尘埃等离子体 Boltzmann方程 充电方程 介电张量     

基于降雨率的GMF+RAIN模型构建及在台风风场反演中的应用

首先选取2006年QuikSCAT散射计扫描的3次台风个例("珊珊"、"摩羯"及"象神"),分析其获取的台风区域雷达后向散射截面值(σ^o)在降雨影响下的分布特征.然后依据大气辐射传输理论,考虑降雨对雷达后向散射截面值的影响,引进两个不同形式的降雨辐射传输模型(简称为SY和AMSR模型),并将这两个降雨辐射传输模型分别与NSCAT2模型函数相结合,构建了适合降雨情况的地球物理模型函数(简称为GMF+RAIN).最后,在GMF+RAIN模型的基础上,采用二维变分结合多解方案     

高层大气风场洛伦兹光谱线型粒子辐射率探测研究

以高层大气中的极光谱线为被探测源,提出了一种高层大气(80—300km)风场洛伦兹光谱线型的粒子辐射率被动探测的新原理.采用该原理不仅可获得高层大气风场的速度、温度和压强信息,更重要的是还能同时探测到高层大气中辐射粒子体辐射率,解决了目前高层大气风场研究中只能探测高层大气风场速度、温度和压强却不能获知粒子的体辐射率的问题.给出了基于洛伦兹光谱线型的辐射源粒子体辐射率、大气风场速度、温度和压强的理论计算公式.采用计算机模拟对探测误差进行了分析,给出了体辐射率、风场速度、温度、压强的误差分布规律,为进一步丰富和完善高层大气风场探测提供了理论依据和实验基础,对航空航天、空间探测、环境保护、国家安全和国民经济建设都具有重要的科学意义和广阔的应用前景.     

Study on the surface erosion route to the fabrication of TiO2 hollow spheres

Three-dimensional (3D) architecture of TiO₂ hollow sphere has many excellent and interesting performances that attract significant attention nowadays. In this paper, a simple surface erosion approach to the fabrication of TiO₂ hollow spheres via the hydrothermal process has been developed. The morphologies and the phase were characterized by scanning electron microscopy (SEM) and X-ray diffractometer (XRD). The results indicate that the anatase-type TiO₂ hollow spheres with a diameter of ∼1 μm are successfully synthesized. The shell thickness of TiO₂ hollow spheres is ∼150 nm and the size of hollow cavity is ∼600 nm. By the control experiments, the influence of ammonium fluoride and hydrogen peroxide on the hollow spherical structures was studied. Hydrogen peroxide acts as both the oxidant and the bubble generator, ammonium fluoride is crucial for the erosion and dissolution of titanium, the detailed dissolution-crystallization mechanism for the formation of TiO₂ hollow spheres was also proposed.     

The effect of grain size distribution on the frequency dependence of the ultrasonic attenuation in polycrystalline materials

An expression has been derived for the effect of the grain size distribution in polycrystalline materials on the frequency dependence of the ultrasonic attenuation. It has been shown that two specimens with the same mean grain size can have significantly different ultrasonic attenuations if their grain size distributions are different and that no unique solution in terms of the grain size exists for a particular ultrasonic attenuation against frequency curve. Qualitative agreement has been found between the theory and some of the experimental data available.     

爆炸载荷下纳米晶铜晶粒度分布及影响因素研究

 采用爆炸动态加载使粗晶铜发生高应变率塑性大变形的方法制备了纳米晶铜。利用X射线衍射法对其晶粒度进行了检测,借助于LS-DYNA3D非线性有限元程序对试样变形过程进行了数值模拟,在此基础上对应变和应变率进行了统计,分析了宏、细观应变对晶粒细化程度的影响。结果表明：采用爆炸加载法可制备出纳米晶铜,平均晶粒度范围可有效控制在100 nm以内;爆炸加载过程中应变率高达10⁴ s^-1,应变的提高有利于晶粒细化;在爆炸加载方向晶粒度成不均匀分布。     

The study of the ionic wind blower with multi-needles/ring type electrodes disposed on inner wall of the cylindrical blower

Corona discharge is one of many methods that convert electrical power into mechanical force. It has been studied for various industrial fields because of its many advantages over conventional motor, such as its no moving parts, simpler structure, minimizing size and so on. In this paper, a discharge system with multiple corona electrodes disposed in a ring format, is studied by focusing on the electrical and mechanical characteristics. Effective ionic wind generation is due to the corona discharge which depends on electric field. Therefore, the electric field is affected by the voltage, discharge spacing, and distance between each corona electrodes.     

The influence of reverse energy transport on emission anisotropy in two-component viscous solutions

The effect of nonradiative reverse energy transport (NRET) in two donor-acceptor systems was studied experimentally. It was found that the NRET occurring in system I; rhodamine 6G (donor) and rhodamine B (acceptor), considerably lowers the emission anisotropy at medium and high concentrations. These results qualitatively confirm the predictions of the approximate theoretical approach of L. Kulak and C. Bojarski (see the preceding paper). In system II; rhodamine 6G (donor) and Nile Blue (acceptor), for which the NRET process does not occur, a good agreement with no-back-transport theory was obtained.