Atomic mobility in a ternary liquid Ga–In–Sn alloy of the eutectic composition

The nuclear spin–lattice relaxation and Knight shift of ⁷¹Ga, ⁶⁹Ga, and ¹¹⁵In nuclei in a ternary liquid gallium–indium–tin alloy of the eutectic composition, which was introduced into pores of an opal matrix and porous glasses with pore sizes of 18 and 7 nm, have been investigated and compared with those for the bulk melt. It has been found that longitudinal relaxation is accelerated and the Knight shift is decreased, depending on the size of pores. The correlation time of the atomic motion has been calculated for the nanostructured melt in porous matrices. It has been shown that the atomic mobility in the melt decreases with decreasing size of pores in the glasses.

     

Diffusion slowdown in the nanostructured liquid Ga‐Sn alloy

The diffusion of gallium in liquid Ga‐Sn alloy embedded into different porous silica matrices was studied by NMR. Spin relaxation was measured for two gallium isotopes, ⁷¹Ga and ⁶⁹Ga, at two magnetic fields. Pronounced rise of quadrupole contribution to relaxation was observed for the nanostructured alloy which increased with decreasing the pore size. The correlation time of atomic mobility was evaluated and found to be much larger than in the relevant bulk melt which evidenced a pronounced diffusion slowdown in the Ga‐Sn alloy under nanoconfinement. It is shown that the diffusion was slower by a factor of 30 for the alloy within 7 nm pores. The spectral densities of electric field gradients at zero frequency were found to double for the finest pores. The Knight shift was found to decrease but slightly for the nanostructured alloy.     

Spin-lattice relaxation enhancement in liquid gallium confined within nanoporous matrices

Nuclear spin relaxation for liquid gallium embedded into nanoporous matrices was found to accelerate remarkably compared to the bulk melt. NMR measurements on two gallium isotopes showed that the dominant mechanism of relaxation was changed from magnetic to quadrupolar and the relation rate depended on the Larmor frequency. The correlation time of electric field gradient fluctuations was estimated using data for quadrupolar relaxation contribution and was found to increase drastically compared to bulk, which corresponded to slowing down mobility in confined liquid gallium.     

Synthesis of silver nanocomposites by SCF impregnation of matrices of synthetic opal and Vycor glass by the Ag(hfac)COD precursor

Silver-containing nanocomposites were prepared by impregnating Vycor glass (a pore diameter of 4 nm) and synthesized opal matrices (an interstitial void size of 40 nm) with cyclooctadiene complex of silver hexafluoroacetylacetonate (Ag(hfac)COD), a silver precursor, dissolved in supercritical carbon dioxide and were examined by optical absorption spectroscopy, atomic force microscopy, and electron spin-resonance spectroscopy. It was demonstrated that the absorption spectra of Vycor glass and opal matrices impregnated with Ag(hfac)COD molecules and subjected to thermal treatment in air at temperatures above 50°C exhibit plasmon resonances characteristic of Ag nanoparticles at 420–430 nm. The peculiarities of the plasmon resonance band for both types of samples were attributed to the morphology of the pore space in which silver particles are formed. Paramagnetic Cu(hfac)₂ molecules (copper hexafluoroacetylacetonate) were used as a spectroscopic probe for estimating the distribution of the precursor in the pores of Vycor glass and opal matrices during supercritical fluid impregnation.     

Spatial Density Distributions and Correlations in a Quasi-one-Dimensional Polydisperse Granular Gas

By Monte Carlo simulations, the effect of the dispersion of particle size distribution on the spatial density distributions and correlations of a quasi one-dimensional polydisperse granular gas with fractal size distribution is investigated in the same inelasticity. The dispersive degree of the particle size distribution can be measured by a fractal dimension dr, and the smooth particles are constrained to move along a circle of length L, colliding inelastically with each other and thermalized by a viscosity heat bath. When the typical relaxation time τ of the driving Brownian process is longer than the mean collision time To, the system can reach a nonequilibrium steady state. The average energy of the system decays exponentially with time towards a stable asymptotic value, and the energy relaxation time τB to the steady state becomes shorter with increasing values of df. In the steady state, the spatial density distribution becomes more clusterized as df increases, which can be quantitatively characterized by statistical entropy of the system. Furthermore, the spatial correlation functions of density and velocities are found to be a power-law form for small separation distance of particles, and both of the correlations become stronger with the increase of df. Also, tile density clusterization is explained from the correlations.     

Formation of two-dimensional ordered magnetic nanolattices in opal structures

A method of forming a two-dimensional ordered superlattice of magnetic nanoparticles in close-packed opal structures of silica (SiO₂) spheres has been developed. Nickel nanopowder with an average particle size of about 70 nm is used as a source of magnetic particles. Atomic-force and magnetic-force microscopy studies show that all magnetic particles are located in the interstices of the opal lattice, while the magnetization vectors of neighboring nickel particles can have different magnitudes and directions.     

Defect formation processes and their influence on mechanical stresses in green glow gallium phosphide structures

Defect formation and their influence on internal mechanical stresses in epitaxial gallium phosphide layers grown from a gallium melt with the addition of finely-dispersed gallium nitride particles in an atmosphere of hydrogen with ammonia are investigated in this paper. It is established that an increase from 0.04 to 0.1 vol. % in the ammonia content in the gas mixture will result in growth in the quantity of defects, in particular, inclusions of the second phase, as well as internal mechanical stresses, while for an NH₃ content greater than 0.1 vol. % – in the formation of shallow cracks and stress relaxation. The dependence between the internal mechanical stresses, the volume fraction, and the dispersion of the GaN inclusions in gallium phosphide is shown. The results obtained are discussed within the framework of the proposed model.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 1, pp. 23–27, January, 1991.     

聚甲基丙烯酸甲酯/镓纳米复合物的动力学弛豫行为

通过原位自由基聚合方法制备了聚甲基丙烯酸甲酯(PMMA)以及含镓(Ga)质量分数为11.3%和13.5%的PMMA/Ga纳米复合材料.在玻璃化转变温度及以上温区,利用能量耗散技术研究PMMA/Ga纳米复合物的动力学弛豫行为,发现随着Ga含量的增加,复合物的α弛豫峰移向高温但峰高降低.此外,还定量研究了Ga含量对PMMA/Ga复合物的α' 弛豫的影响,并作出了相应的解释. 关键词： 相对能量耗散 玻璃化转变 力学弛豫     

Relaxation of an electrooptical effect in colloids induced by a field of short pulses

The possibility of studying the size distribution of particles suspended in a liquid irradiated with short pulses of strong electric fields, which almost do not change the properties of the disperse system is considered. The particle size is varied from 10 to 1000 nm. Relaxation dependences of the optically anisotropy induced by such pulses are investigated. The relaxation curves of the induced electrooptical effect are shown to behave similarly to the relaxation curves of the effect in the system with completely oriented particles. For aqueous polydisperse systems of diamond, graphite, and palygorskit, whose particles significantly differ in shape and physicochemical properties, the relaxation dependences of the electrooptical effect induced by fields of different duration and amplitude are measured. The correlation coefficients between the relaxation dependences are calculated. If the field is sufficiently strong, the correlation coefficients remain close to unity for all the three systems studied irrespective of the duration of the field pulse. This indicates that these curves are similar and that short powerful pulses can be used for determining the size distribution of particles in nanodisperse systems.     

Zr纳米粒子内HCP-BCC堆积结构转变的分子动力学模拟

本文采用基于嵌入原子势的分子动力学方法模拟了不同粒径的Zr纳米粒子在升温过程中HCP-BCC结构转变的路径。通过对粒子在升温过程中的势能差分曲线的计算,分别确定了小粒径、较大粒径和大粒径粒子的结构转变温度区间,并使用二分法进一步确定了转变温度点。然后借助对结构转变温度点处的形状因子、键对占比和原子堆积结构随弛豫时间的变化的模拟计算,确定了驰豫过程中堆积结构的演变过程。计算结果表明,小粒径粒子存在着多个结构转变温度点,并会在较低的结构转变温度点处出现多结构共存现象,其驰豫过程就是不同结构间相互竞争的过程。随着粒径的增大,在较高温度点处虽然仍会出现多结构共存,但粒子内的大部分原子堆积为BCC结构,并且在大粒径粒子内会出现明显的界面区。     

Change of the luminescence decay time for Lu<Subscript>2</Subscript>O<Subscript>3</Subscript>: Eu nanocrystals embedded in synthetic opal

The opal-Lu_1.86Eu_0.14O₃ composites have been prepared using the developed technique for synthesizing luminophor nanocrystals in pores of synthetic opal through coprecipitation from a solution. It has been demonstrated that the position of the photonic stop band in the reflection spectrum of the infiltrated opal depends on the diameter of its spheres, the volume fraction of the embedded luminophor, and the angle of detection of the signal. The excitation and photoluminescence spectra of the composites have been analyzed, and the lifetime of the ⁵ D ₀ excited state of Eu³⁺ ions has been examined. It has been revealed that the luminescence decay time for the luminophor increases by almost one order of magnitude with an increase in its content in opal pores. This effect has been attributed to the change in the nanocrystal size and to the decrease in the contribution from the surface nonradiative recombination in luminophor nanolayers of the composites.     

Stress investigations of thin amorphous Ge-Se-Ga films

Thin chalcogenide films from the systems (GeSe₄)_1-xGa_x and (GeSe₅)_1-xGa_x with gallium contents up to 20 at. % have been prepared by vacuum evaporation and their stress has been investigated by a cantilever technique. The addition of gallium to the Ge-Se matrix plays an important role in stress formation in the films: films without gallium possess negligible stress, while all gallium-containing films are under compressive stress. The increase of the gallium content leads to structural changes and an increase in the density, which results in higher stress values. For all films, stress reduction with time is observed due to spontaneous relaxation. Received: 2 October 2002 / Accepted: 22 November 2002 / Published online: 28 March 2003 RID="*" ID="*"Corresponding author. Fax: +49-561/8044-136, E-mail: popov@schottky.physik.uni-kassel.de     

The calculation of the viscosity from the autocorrelation function using molecular and atomic stress tensors

The stress-stress correlation function and the viscosity of a united-atom model of liquid decane are studied by equilibrium molecular dynamics simulation using two different formalisms for the stress tensor: the atomic and the molecular formalisms. The atomic and molecular correlation functions show dramatic difference in short-time behaviour. The integrals of the two correlation functions, however, become identical after a short transient period which is significantly shorter than the rotational relaxation time of the molecule. Both reach the same plateau value in a time period corresponding to this relaxation time. These results provide a convenient guide for the choice of the upper integral time limit in calculating the viscosity by the Green-Kubo formula.     

Electrical conductivity and superconductivity of ordered indium-opal nanocomposites

The electrical conductivity is measured experimentally and the parameters of the superconducting transition are determined in a regular spatial network of multiply connected submicron-sized indium grains embedded in voids of an ordered opal dielectric matrix. The In-opal nanocomposite was prepared by pressure injection of the molten metal into voids of opal samples. Arrays of In grains of different sizes were produced by properly varying the characteristic geometric sizes of the opal voids, which offered the possibility of observing quantitative and qualitative changes in the temperature dependence of electrical resistance and studying the size effects on the critical temperature and critical magnetic field in the In-opal nanocomposites. It was found that, as the coherence length becomes comparable to the size of the superconducting grains, the parameters of the superconducting transition in the nanocomposite increase sharply.     

Metal particle polarization

The effect of electron collisions with lattice atoms in metals on metal particle polarization in an ac electric field was analyzed. It was shown that, in contrast to collisionless (free) electron gas, an increase in the negative electronic permittivity with decreasing electric field frequency in relaxation metal particles is limited by the conductivity relaxation time. It was shown that the plasma frequency appears in relaxation metal only if the dielectric relaxation time is less than the free path time in metal.     

Intermediate-temperature embrittlement of Cu bicrystals with dispersed Fe particles

The temperature dependence of deformation and fracture behavior of orientation-controlled Cu bicrystals with dispersed Fe precipitate particles was investigated. Deformation and fracture behavior at an intermediatetemperature range showed clear aging-time dependency: With increase in aging time, grain-boundary fracture became more pronounced, although fracture strain remained practically the same. The grain-boundary character and strain rate also affected the mechanical properties: With increase in the misorientation angle between the two crystals and with decrease in strain rate, grain-boundary fracture started to occur more easily at lower temperatures associated with clearer intermediate-temperature embrittlement. These experimental results were analyzed by incorporating various phenomena such as grain-boundary sliding, dynamic precipitation, growth of grain-boundary particles, and stress relaxation induced by interfacial diffusion between grain-boundary particles and the Cu matrix.     

Variation of chaotic motion of silicon particles suspended in liquid under field

A decrease in the characteristic correlation time of light scattered by silicon particles in transformer oil and an increase in their effective diffusion coefficient when the particle suspension in oil is exposed to a dc electric field were experimentally shown. The current in the suspension did not exceed 100 nA and its temperature was unchanged. It was also shown that the shape of the correlation function of light scattered by particles changes, i.e., a Gaussian factor appears in it. The dependences of the exponential and Gaussian components on the electrode voltage are presented.     

用双稳态势场模型研究观点转变的驱动-响应关系

从物理学的视角看,群体观点演化实质可以看作是观点粒子状态变化的集体效应.本文考察在双稳态势中噪声诱导观点粒子的状态转变,利用加权拉盖尔完备正交函数法计算了时间关联函数和描述驱动-响应关系的弛豫时间.理论计算结果表明,噪声诱导作用存在一个临界值Dc,若噪声强度高于临界值,时间关联函数随关联时间呈指数型增加.结果还显示,存在弛豫时间随势垒纵横比/噪声强度变化取值趋于无穷的双奇异点现象.奇异点处无法实现观点粒子状态的转变.弛豫时间与势垒纵横比之间存在线性关系,预示着在双稳态势场中观点粒子受噪声驱动呈现类似牛顿第二定律的驱动-响应关系,而弛豫时间在这个关系中充当表征惯性质量的角色.     

Equations for filling factor estimation in opal matrix. Addendum to “Deep level emission of ZnO nanoparticles deposited inside UV opal”, [Opt. Commun. 259 (1) (2006) 378-384]

We consider two equations for the filling factor estimation of infiltrated zinc oxide (ZnO) in silica (SiO₂) opal and gallium nitride (GaN) in ZnO opal. The first equation is based on the effective medium approximation, while the second one—on Maxwell-Garnett approximation. The comparison between two filling factors shows that both equations can be equally used for the estimation of the quantity of infiltrated nanoparticles inside opal photonic crystal.