Low-frequency variable charge solitary waves in a collisionless dusty plasma with a Cairns-Gurevich ion velocity distribution

In this paper, the problem of large amplitude dust acoustic (DA) solitons has been addressed in a charge varying dusty plasma with ions following a Cairns-Gurevich distribution. Based on the orbit motion limited approach, the correct Cairns-Gurevich ion charging current is presented for the first time. The expression relating the variable dust charge to the plasma potential is given in terms of the Lambert function and we take advantage of this transcendental function to, carefully, analyse DA solitons in a charge varying dusty plasma with trapped nonthermal ions. Our results show that the spatial patterns of the variable charge solitary wave are significantly changed due to the presence of ion population modelled by the Cairns-Gurevich distribution. An addition of a small concentration of trapped nonthermal ions makes the solitary structure less spiky, grows the net negative charge residing on the dust grain surface, and contributes to the electron depletion. Finally, our investigation is extended to highlight the effect of the grain dust charge variation. We have shown that under certain conditions, the impact of dust charge fluctuation may furnish an alternate physical mechanism rasing anomalous dissipation, which becomes more strong and may predominate over the dispersion as the nonthermal character of ions following the Cairns-Gurevich distribution increases.     

Is the Collecting Surface of a Flat One-Sided Probe Constant when Measuring the Ion Distribution Function?

The correctness of the known plane single-ended probe method for measuring the anisotropic ion distribution functions in a gas-discharge plasma has been considered. Analysis has been performed for positive probe potentials relative to the plasma with magnitudes on the order of the mean ion energy, which as a rule is much lower than the mean electron energy. We have analyzed the dependence of the collection surface area of a plane probe on its potential in this range. The structure of the near-probe layer has been determined for an isotropic electron distribution function of the Maxwellian or Druvestein type and an anisotropic ion distribution function. These results are used to derive analytic relations for the correction to the second derivative of the probe current with respect to the plane probe potential. It has been shown that, when the ion distribution function is measured in a wide range of conditions in the gas-discharge plasma, when the approximation of a collisionless probe layer is applicable, and the probe does not perturb the plasma, the dependence of the collection surface area of the probe on the potential can be disregarded in this range.     

相分布和晶粒尺寸对Nd2Fe14B/ɑ-Fe纳米复合永磁材料矫顽力的影响

本文采用统计平均方法研究了软、硬磁性晶粒尺寸及相分布对Nd2Fe14B/α-Fe纳米复合永磁材料矫顽力的影响。计算结果表明：对于单相纳米硬磁材料，磁体矫顽力随着硬磁性晶粒尺寸的减小而降低；对于软、硬两磁性相组成的Nd2Fe14B/a-Fe纳米复合永磁材料，两相的随机分布将导致磁体矫顽力随硬磁性晶粒尺寸的减小呈现极大值。本文的计算结果还表明当硬磁性晶粒尺寸大于软磁性晶粒的最佳尺寸时(15nm)，具有多层膜结构的Nd2Fe14B/a-Fe纳米复合永磁材料将比两相随机分布时具有更大的矫顽力。     

考虑晶粒分布的多晶体材料超声散射统一理论

现有超声散射统一理论可通过多晶体材料的微观结构和力学特性,实现全频域范围内衰减和相速度的正演建模,但其忽略晶粒尺寸分布的影响,进而降低了正演模型的计算精度.本文对不均匀介质的波动方程进行二阶Keller近似,用全频域格林函数推导介质中的平均波;以截断对数正态分布描述晶粒分布,构建加权的空间相关函数;结合材料的弹性模量协方差,建立含晶粒分布的超声散射统一理论,揭示晶粒分布对超声散射的影响规律;制备304不锈钢试块并开展超声散射实验.结果表明考虑晶粒分布特性后,纵波衰减谱和相速度谱相对于实验结果的相异性降低约49%和64%,横波衰减谱和相速度谱相对于实验结果的相异性降低约12%和4%.可见,本文的统一理论模型能有效修正晶粒分布导致的衰减谱和相速度谱偏差,为晶粒分布反演评价提供理论基础.     

Bi2Te3: implications of the rhombohedral k-space texture on the evaluation of the in-plane/out-of-plane conductivity anisotropy

Different computational scheme for calculating surface integrals in anisotropic Brillouin zones are compared. An example of the transport distribution function (plasma frequency) of the thermoelectric material Bi(2)Te(3) near the band edges is discussed. The layered structure of the material together with the rhombohedral symmetry causes a strong anisotropy of the transport distribution function for the directions in the basal plane (in-plane) and perpendicular to the basal plane (out-of-plane). It is shown that a thorough reciprocal space integration is necessary to reproduce the in-plane/out-of-plane anisotropy. A quantitative comparison can be made at the band edges, where the transport anisotropy is given in terms of the anisotropic mass tensor.     

Grain boundary curvature and grain growth kinetics with particle pinning

Second-phase particles are used extensively in design of polycrystalline materials to control the grain size. According to Zener’s theory, a distribution of particles creates a pinning pressure on a moving grain boundary. As a result, a limiting grain size is observed, but the effect of pinning on the detail of grain growth kinetics is less known. The influence of the particles on the microstructure occurs in multiple length scales, established by particle radius and the grain size. In this article, we use a meso-scale phase-field model that simulates grain growth in the presence of a uniform pinning pressure. The curvature of the grain boundary network is measured to determine the driving pressure of grain growth in 2D and 3D systems. It was observed that the grain growth continues, even under conditions where the average driving pressure is smaller than the pinning pressure. The limiting grain size is reached when the maximum of driving pressure distribution in the structure is equal to the pinning pressure. This results in a limiting grain size, larger than the one predicted by conventional models, and further analysis shows consistency with experimental observations. A physical model is proposed for the kinetics of grain growth using parameters based on the curvature analysis of the grain boundaries. This model can describe the simulated grain growth kinetics.     

Molecular dynamics simulation of the melting of a twist Σ=5 grain boundary

The existence of a possible grain boundary disordering transition of the melting type in a =5 (001) twist boundary of aluminium bicrystal below the melting temperature was investigated using a constant pressure molecular dynamics simulation. The calculated melting temperature T _cm of the bulk Al is about 960 K. The total internal energy, the structure factor, and the pair distribution function were calculated at different layers across the grain boundary. The mean atomic volume, the grain boundary energy, and the thermal expansion coefficients were also calculated using the same simulation method. This simulation also allows us to image the grain boundary structure at different temperatures. The equilibrium grain boundary structure at 300 K retains the periodicity of the coincident site lattice, so that the lowest energy structure corresponds to the coincident site arrangement of the two ideal crystals. With increasing temperature, the total internal energy of the atoms for both the perfect crystal and the grain boundary increases, as do the number of layers in the grain boundary. The grain boundary core exists and the perfect crystal structure still exists outside the grain boundary at 0.9375 T _cm. However, two atomic layers of the equilibrium grain boundary structure at 0.9375 T _cm lose the coincident site lattice periodicity and attain a structure with liquid-like disorder. Therefore, partial melting of the grain boundary has occurred at the temperature above 0.9375 T _cm which is in agreement with the experimental results.     

Fractal structure of films deposited in a tokamak

The surface of amorphous films deposited in the T-10 tokamak was studied in a scanning tunnel microscope. The surface relief on a scale from 10 nm to 100 μm showed a stochastic surface topography and revealed a hierarchy of grains. The observed variety of irregular structures of the films was studied within the framework of the concept of scale invariance using the methods of fractal geometry and statistical physics. The experimental probability density distribution functions of the surface height variations are close in shape to the Cauchy distribution. The stochastic surface topography of the films is characterized by a Hurst parameter of H = 0.68–0.85, which is evidence of a nontrivial self-similarity of the film structure. The fractal character and porous structure of deposited irregular films must be considered as an important issue related to the accumulation of tritium in the ITER project. The process of film growth on the surface of tokamak components exposed to plasma has been treated within the framework of the general concept of inhomogeneous surface growth. A strong turbulence of the edge plasma in tokamaks can give rise to fluctuations in the incident flux of particles, which leads to the growth of fractal films with grain dimensions ranging from nano-to micrometer scale. The shape of the surface of some films found in the T-10 tokamak has been interpreted using a model of diffusion-limited aggregation (DLA). The growth of films according to the discrete DLA model was simulated using statistics of fluctuations observed in a turbulent edge plasma of the T-10 tokamak. The modified DLA model reproduces well the main features of the surface of some films deposited in tokamaks.     

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.     

Forces on a small grain in the nonlinear plasma wake of another

The transverse force on a spherical charged grain lying in the plasma wake of another grain is analyzed to assess the importance of ion-drag perturbation, in addition to the wake-potential-gradient. The ion-drag perturbation is intrinsically one order smaller than the wake-potential force in the ratio of grain size (r(p)) to Debye length (λ(De)). So ion-drag perturbation is important only in nonlinear wakes. Rigorous particle-in-cell calculations of the force are performed in the nonlinear regime with two interacting grains. It is found that even for quite large grains, r(p)/λ(De)=0.1, the force is dominated by the wake-potential gradient. The wake-potential structure can then help explain the preferred alignment of floating dust grains.     

Dependence of helium ion diffusion and drift characteristics in own gas on its temperature

The low-temperature gas discharge has a number of features which can appear in experimentswith dusty plasma. For example, at cryogenic temperatures of gas-discharge tube walls, the strong anisotropy of the velocity distribution function of ions takes place, which in turn can cause a significant change in dust structure properties. The dependences of the helium ion drift characteristics in a dc uniform electric field on the atomic temperature of own gas is analyzed.     

纳米镍粉体的晶格膨胀

采用阳极弧放电等离子体技术成功制备了各种不同晶粒尺寸的纳米镍粉体材料，并利用X射线衍射(XRD)、透射电子显微镜(TEM)和相应选区电子衍射(SAED)等测试手段对所制备的样品的晶体结构、晶格参数、形貌、粒度进行性能表征. 实验结果表明：阳极弧等离子体法制备的镍纳米粉的晶体结构与相应的块物质相同，为fcc结构的晶态. 不同晶粒尺寸的纳米镍粉体的晶格常数均大于完整单晶镍的晶格常数，晶格畸变表现为晶格膨胀. 晶格常数和晶胞体积随着晶粒尺寸的减小而增大，晶格畸变量与晶粒尺寸的倒数成正比. 镍纳米粉体的晶格膨胀主要是由于受到表面能和表面张力的作用而引起的，可以利用纳米晶体的热力学理论作定性解释. 关键词： 镍纳米粉体 晶格常数 晶格膨胀 晶胞体积     

Modelling study on the three-dimensional neutron depolarisation response of the evolving ferrite particle size distribution during the austenite–ferrite phase transformation in steels

The magnetic configuration of a ferromagnetic system with mono-disperse and poly-disperse distribution of magnetic particles with inter-particle interactions has been computed. The analysis is general in nature and applies to all systems containing magnetically interacting particles in a non-magnetic matrix, but has been applied to steel microstructures, consisting of a paramagnetic austenite phase and a ferromagnetic ferrite phase, as formed during the austenite-to-ferrite phase transformation in low-alloyed steels. The characteristics of the computational microstructures are linked to the correlation function and determinant of depolarisation matrix, which can be experimentally obtained in three-dimensional neutron depolarisation (3DND). By tuning the parameters in the model used to generate the microstructure, we studied the effect of the (magnetic) particle size distribution on the 3DND parameters. It is found that the magnetic particle size derived from 3DND data matches the microstructural grain size over a wide range of volume fractions and grain size distributions. A relationship between the correlation function and the relative width of the particle size distribution was proposed to accurately account for the width of the size distribution. This evaluation shows that 3DND experiments can provide unique in situ information on the austenite-to-ferrite phase transformation in steels.     

Linear and nonlinear dust lattice waves in plasma crystals

Linear and nonlinear studies of dust lattice waves in a dusty plasma crystal have been carried out on the basis of the Schrödinger equation which is deduced from Poisson's equation for small dust grain potentials. The spatial distribution of the potential in the dust-lattice includes the effect of the whole system of the dust particles. Such a self-consistent analysis gives a dispersion relation for the dust lattice wave, which is different from the expression found earlier. The frequency of the lattice oscillation increases considerably for large grain charges. Furthermore, it is found that an ideal lattice can only exist if the dusty plasma parameters satisfy a definite relationship between the dusty plasma Debye radius, the inter-grain separation, and the grain size. Finally, accounting for the weak nonlinearities, we also derive a Korteweg-de Vries (KdV) equation for the nonlinear dust lattice waves in the long wavelength approximation (kd≪1), where k is the wave number and d the inter-grain spacing.     

Grain-boundary sliding in elongated microstructures during diffusion creep

In diffusion creep, the contribution of grain-boundary sliding to the overall strain os can be evaluated in arbitrary polycrystals, if the angular distribution of grain boundaries is known. A os value of 0.5 is obtained for two-dimensional (2D) equiaxed microstructures consisting of regular hexagonal grains, equiaxed grains grown from a Voronoi structure or grains having a circular distribution of grain-boundary angles. The os value is also evaluated for uniaxially deformed 2D microstructures, both diffusionally and uniformly deformed. For the former, the deformed microstructure is obtained by the simulation of microstructural evolution in polycrystals with straight grain boundaries. The os value increases gradually with increasing or decreasing strain and is larger in the diffusionally deformed microstructures than in the uniformly deformed microstructures for a given grain aspect ratio. The os value for three-dimensional (3D) polycrystalline microstructures is also obtained from an ellipsoidal distribution of grain-boundary angles. The resultant os value is 0.60 for 3D equiaxed polycrystals and increases gradually with increasing strain.     

Superfluidlike motion of an absorbing body in a collisional plasma

Motion of a small charged absorbing body (micrograin) immersed in a stationary weakly ionized high pressure plasma environment is considered. It is shown that the total frictional (drag) force acting on the grain can be directed along its motion, causing the grain acceleration. At some velocity, the forces associated with different plasma components can balance each other, allowing free undamped superfluid motion of the grain. The conditions when such behavior can be realized and the possibility of a superconductive grain current are discussed in the context of complex (dusty) plasmas.     

Enhanced magnetization and suppressed current leakage in BiFeO3 ceramics prepared by spark plasma sintering of sol-gel derived nanoparticles

BiFeO₃ (BFO) ceramics of different grain size have been synthesized by spark plasma sintering of sol-gel derived nanoparticles. It was found that with decreasing grain size there occurs an enhancement in magnetization and a simultaneous suppression in current leakage. According to systematic materials characterization, the enhanced magnetization is attributed to the enriched grain boundaries where the missing structural order perturbs the spin helix structure of BFO and thus generates uncompensated spins, while the reduced current leakage is ascribed to fewer conduction paths provided by the compacted grain structure.     

具有非广延分布电子的碰撞等离子体磁鞘的结构

很多关于等离子体鞘层的研究工作都是基于电子满足经典的麦克斯韦速度分布函数,而等离子体中的粒子具有长程电磁相互作用,使用Tsallis提出的非广延分布来描述电子更为恰当.本文建立一个具有非广延分布电子的碰撞等离子体磁鞘模型,理论推导出受非广延参数q影响的玻姆判据,离子马赫数的下限数值会随着参数q的增大而减小.经过数值模拟,发现与具有麦克斯韦分布(q=1)电子的碰撞等离子体磁鞘对比,具有超广延分布(q<1)和亚广延分布(q>1)电子的碰撞等离子体磁鞘的结构各有不同,包括空间电势分布、离子电子密度分布、空间电荷密度分布.模拟结果显示非广延分布的参数q对碰撞等离子体磁鞘的结构具有不可忽略的影响.希望这些结论对相关的天体物理、等离子体边界问题的研究有参考价值.