Size effects under plastic deformation of microcrystals and nanocrystals

A theoretical analysis of size effects in plastically deformed crystals with transverse sizes in micro and nanometer ranges has been performed in the framework of the dislocation-kinetic approach. The analysis is based on the evolution equation of the dislocation density in these crystals and takes into account the generation of dislocations from surface dislocation sources and the escape of dislocations from the crystal through the crystal surface. It has been established that the generation of dislocations from the sources leads to a strong strain hardening of the crystal and that the escape of dislocations through the crystal surface results in a fast equilibration of these two kinetic processes. As a result, there occurs a strong “exhaustion” of strain hardening of thin crystals at the early stage of their plastic deformation in accordance with experiments. According to the theory, the flow stresses σ and transverse sizes D of microcrystals and nanocrystals are related by the expressions σ ∼ D ⁻ⁿ (n = 0.625–1.0), which are in agreement with the experiment.     

Quantum corrections to 2D hole conductivity in a quantum well on (10\bar 10)

The anomalous positive magnetoresistance of a two-dimensional (2D) layer on Te is analyzed, and it is shown that this phenomenon can be described within the weak localization theory taking into account the specific features of the band spectrum symmetry of a 2D layer on this tellurium surface and the associated peculiarities in the phase relaxation processes, as well as the existence of several 2D subbands. The main parameters of the theory have been determined, and it has been found that this orientation of the 2D layer is characterized by an extremely high probability of intersubband transitions in elastic scattering, which makes this case qualitatively different from the 2D hole system on the (0001) surface studied previously. The phenomenon is attributed to the difference in the character of electronic states between these crystal interfaces, namely, dangling covalent bonds in the chains making up the tellurium crystal on the (0001) surface, and the disruption of the weaker interchain Van der Waals bonds on the face representing the cleavage surface. A conclusion is drawn that the wave-state phase-relaxation time in inelastic processes is dominated by electron-electron scattering. Fiz. Tverd. Tela (St. Petersburg) 40, 1554–1558 (August 1998)     

Surface size effects under plastic deformation of microcrystals and nanocrystals

The size effects associated with the crystal surface as an effective sink for moving dislocations in a thin crystal and as a barrier for these dislocations in the presence of a high-strength film or a special hardened layer on the surface, which favor the accumulation of dislocations in the crystal, have been considered theoretically in terms of the kinetic equation for the density of dislocations concentrated in the crystal in the critical lengths of single-ended (unipolar) dislocation sources. The theoretical results have been illustrated by the experimental data available in the literature for microcrystals and nanocrystals of copper and aluminum. It has been found in accordance with these data that the dependence of the yield stress ??_2% of the crystal on the crystal transverse size D has the form ??_2% ?? D ^?0.75 when there is a free crystal surface for the escape ofthe dislocations and ??_2% ?? D ^?0.5 when there is a high-strength layer on the lateral surface of the crystal..     

Evolution of dislocation structure during deformation of γ-irradiated LiF crystals

The effect of γ irradiation on the mechanical characteristics and dislocation structure of slip bands in LiF crystals is studied at doses D⩽7.3×10⁸ R. Irradiation causes a substantial increase (up to a factor of 30) in the yield stress τ _y of the crystals, with τ _y ∝ D ^0.4 in the first approximation. The deformation shear increases in the slip bands of irradiated crystals, as do the densities of the screw and edge dislocation components, while the dislocation mean free paths decrease. Irradiation also raises the probability of twinning cross slip for screw dislocations. The observed effects are assumed to be related to the formation of a different kind of defects in the irradiated crystals, primarily clusters of implanted atoms. Fiz. Tverd. Tela (St. Petersburg) 39, 1072–1075 (June 1997)     

Generalized stacking fault energy surfaces in the molecular crystal αRDX

The generalized stacking fault (GSF) energy surfaces in the organic energetic molecular crystal, hexahydro-1,3,5-trinitro-s-triazine (RDX), were studied through atomistic simulations. Using a fully flexible molecular potential in highly damped molecular dynamics simulations, we determined quenched 0?K GSF energy surfaces and structures for a set of planes in the α-polymorph RDX crystal and subsequently compare predictions of slip or cleavage with available experimental observations. To account for the steric contributions and elastic shearing due to the presence of flexible molecules, a modified calculation procedure for the GSF energy surface is proposed that enables the distinction of elastic shear energy from the energy associated with the interfacial displacement discontinuity at the slip plane. Comparisons of the unstable stacking fault energy with the surface energy are used to differentiate cleavage planes from likely slip planes, and the calculations are found to be largely in agreement with available experimental data.     

Orientational phase transition and the solvation force in a nematic liquid crystal confined between inhomogeneous substrates

A nematic liquid crystal confined between two identical flat solid substrates, with an alternating stripe pattern of planar and homeotropic anchoring, is studied in the framework of the Frank-Oseen theory. By means of numerical minimization of the free energy functional we study the effect of the sample thickness D on the location of the phase transition between a uniform alignment, either planar or homeotropic, and a distorted nematic texture. The solvation force f due to distortions of the nematic director is also studied. It is found that f is always attractive, and for D small compared to the periodicity of the surface structure it exhibits two distinct asymptotic behaviors: f ∼ - D ^-1/2 or f ∼ - D ^-1, depending on the relation between D and the extrapolation lengths. Received 12 November 2002 Published online: 16 April 2003     

Tm:Y2SiO5晶体的生长和光谱性质研究

采用中频感应提拉法生长了高质量的Tm:Y₂SiO₅(Tm:YSO)晶体,测定了晶体的晶格常数和分凝系数.运用劳厄照相法确定了单斜晶系Tm:YSO晶体的三个偏振轴〈010〉,D₁和D₂,在室温下测量了三个偏振轴方向的吸收光谱、荧光光谱和荧光寿命,计算了晶体吸收峰的吸收线宽和吸收截面.研究发现,相对于其他两个偏振轴方向,D₁方向在790nm处出现较强的吸收峰, 关键词： 2SiO₅')" href="#">Tm:Y₂SiO₅ 单斜晶系 吸收光谱 荧光光谱     

Stability and diffusion of surface clusters

Using kinetic Monte Carlo simulations and a bond-counting ansatz, thermal stability and diffusion of an adatom island on a crystal surface are studied. At low temperatures, the diffusion constant D is found to decrease for a wide range of island sizes like , where is close to one, N being the number of adatoms in the cluster. By heating up the surface, the system undergoes a phase transition above which the island disappears. Characteristics of that transition are discussed. Received 20 January 1999     

Berkovich nanoindentation study of monocrystalline tungsten: a crystal plasticity study of surface pile-up deformation

Abstract

In this paper, it was investigated whether Berkovich indentation test with a triangular-based pyramidal imprint would exhibit the same surface pile-up deformation behaviour as in Vickers or spherical indentation tests. The characteristic correlation between the pile-up patterns of monocrystalline tungsten and the geometry of slip systems was examined both experimentally and computationally. Surface pile-up patterns for three different crystallographic orientations of specimens with corresponding rotational crystal symmetry were characterised. In addition, the effect of the varying azimuthal orientation of the indenter on the pile-up patterns was also discussed. Predictions from finite element simulation based on the crystal plasticity theory are also presented and compared with the measured results. It was found that the surface pile-up patterns of Berkovich indentation did not necessarily reflect the rotational crystal symmetry of tungsten single crystal specimens. The pile-up patterns were affected by the variation of the indenter’s azimuthal orientation. The height of the pile-up hillocks was often highly non-uniform even on the same surface plane indicating strong influence of slip geometry leading to the plastic anisotropy.     

On Perpendicular and Tilted Chains in Lamellar Crystals

Chain tilt and surface disorder were investigated in end-deuterated long n-alkane C₁₂D₂₅C₁₉₂H₃₈₄C₁₂HD₂₄ crystallized from solution and in n-alkane C₁₆₂H₃₂₆ crystallized from melt. Small-angle X-ray scattering and infrared spectroscopy were employed. Extended-chain crystals of C₁₂D₂₅C₁₉₂H₃₈₄C₁₂HD₂₄ as-grown from solution have the molecular axis perpendicular to the lamellar surface, but when heated, around 90°C, they start tilting relative to the layer normal. The tilt increases gradually to reach 35° just below the melting point. C₁₆₂H₃₂₆ crystallized from the melt at small supercoolings has chains tilted at 35° at the outset, as found previously for all melt-crystallized long alkanes and polyethylene. However, for the first time in long alkanes, it is found that when molten C₁₆₂H₃₂₆ is supercooled to ΔT≥10 K, crystals with perpendicular chains form. At still larger ΔT, the chains are once-folded, with a mixed population of tilted and perpendicular chain crystals. The use of Davydov splitting of the CH₂ and CD₂ bending vibration of the end-labelled alkane C₁₂D₂₅C₁₉₂H₃₈₄C₁₂HD₂₄ allows independent IR probing of molecular disorder at the deuterated surface and in the hydrogenous crystal interior. The initially small CD₂ splitting and the presence of an additional singlet component indicate a rough surface in as-grown crystals, with considerable longitudinal interchain disorder. It is estimated that about 10% of chains are displaced by up to a dozen C-atoms. The increase in splitting and decrease in absorbance of the singlet, which occur on annealing at progressively higher temperatures, are evidence of steady improvement in translational surface order, occurring simultaneously with increasing chain tilt angle. From the above evidence, it is concluded that the absence of tilt in as-grown crystals is not the result of high surface order, as in the case of shorter odd n-alkanes, but rather of a high frozen-in longitudinal disorder with chain ends or folds protruding out of or sunken beneath the crystal surface. It is also concluded that chain tilt only becomes necessary as the crystal surface becomes translationally more ordered and the crystal–amorphous interface sharpens. The effect of chain tilt on the Davydov splitting is addressed briefly.     

Non-Maxwell slippage induced by surface roughness for microscale gas flow: a molecular dynamics simulation

Rarefied gas flows in rough microchannels are investigated by non-equilibrium molecular dynamics simulations. The surface roughness is modelled by an array of triangular modules. The Maxwell slip model is found to break down due to the surface roughness for gas flows in microchannels with large surface roughness. Non-Maxwell slippage shows that the slip length is smaller than that predicted by the Maxwell model and is nonlinearly related to the mean free path. For larger surface roughness and smaller Knudsen number, the non-Maxwell effect becomes more pronounced. The boundary conditions, generally including velocity slip, no-slip and negative slip, depend not only on the Knudsen number but also on the surface roughness. Simulation results show that A/λ?≈?1 is a good criterion to validate the no-slip boundary condition and A/λ?>?0.3 can be a criterion to judge the occurrence of non-Maxwell slippage, where A is the surface roughness size and?λ?is the mean free path of gas molecules. The permeability enhanced by the surface roughness may be responsible for the roughness-induced non-Maxwell slippage.     

γ-TiAl金属间化合物面缺陷能的分子动力学研究

运用分子动力学方法，对γ-TiAl金属间化合物的面缺陷能(层错能和孪晶能)进行了研究. 计算得到γ-TiAl不同滑移系(或孪生系)的整体堆垛层错能曲线，结果表明，γ-TiAl较一般fcc晶体结构的金属可动滑移系(孪生系)的数量减少，在外界条件下呈脆性. 研究孪生系(1/6)〈112〉{111}的弛豫的整体堆垛层错(GSF)能和整体孪晶(GTF)能曲线，对不稳定层错能γ_usf、稳定层错能γ_sf和不稳定孪晶能γ_usf值进行分析，可以预知， γ-TiAl的主要变形机理为孪生系(1/6)〈112〉{111}的孪生和普通滑移系(1/6)〈110〉{111}的滑移，以及超滑移系(1/2)〈011〉{111}的滑移. 关键词： γ-TiAl')" href="#">γ-TiAl 堆垛层错能 孪晶能 分子动力学     

Quasiholes in a MgZnO/ZnO Heterojunction as Vacancions

Recombination of two-dimensional electrons of a low density in a MgZnO/ZnO heterojunction with localized valence-band holes is considered. It is suggested that quasiholes appearing in the process of photoluminescence of strongly interacting two-dimensional electrons should be considered as vacancion quasiparticles in a quantum Wigner crystal. Vacancions formed upon the removal of an electron from the crystal are delocalized owing to the tunneling effect. The vacancion energies E(k) form a band of width D that depends on the probability of vacancy tunneling. The width D corresponds to the width of the photoluminescence band of the two-dimensional electron system. The shape of the photoluminescence band of the Wigner crystal is obtained using the tight-binding approximation for the vacancion dispersion relation E(k) is compared with experimental results.

     

Isotope exchange studies of the oxidation and reduction of SrTiO3 single crystal surfaces by water and hydrogen

Thermal desorption studies of chemisorbed D₂ and D₂O on a reduced SrTiO₃(111) surface reveal that D₂ causes the reduction of the crystal, whereas D₂O causes its oxidation. Thermal desorption of H₂¹⁸O indicates that there is a 15% exchange between the oxygen in the adsorbed water molecules and the lattice oxygen.     

Modeling of Surface Diffusion in hcp Zr and Ti

The statics and dynamics of vacancies and adatoms on different surface orientations in two hcp materials are studied by using static relaxation techniques and many-body potentials. Formation and migration energies and entropies as well as attempt frequencies are evaluated and used in the random walk approach to obtain correlation factors and diffusivities. It is found that the main features of surface diffusion are dominated by jumps on and between a few atomic layers, so that a consistent comparison between the two mechanisms is feasible. The activation energies and the diffusivities for different environments, namely, bulk Q _b, D _b, symmetric grain boundaries Q _gb, D _gb, and surfaces, Q _s, D _s, calculated using the same simulation technique and interatomic potentials, fulfil the expected relationships Q _s < Q _gb < Q _b and D _s > D _gb > D _b. It is also found that generally adatoms are faster surface diffusers than vacancies.     

界面缺陷态密度与衬底电阻率取值对硅异质结光伏电池性能的影响

在异质结前界面缺陷态密度Dit1和异质结背界面缺陷态密度Dit2均取不同值时,对p型单晶硅(c-Si(p))为衬底的硅异质结太阳电池的衬底电阻率ρ与电池性能的关系进行了数值研究.结果表明:衬底电阻率的最优值ρop取决于前界面缺陷态密度Dit1,且ρop随着Dit1的增大而增大;当ρρop时,背界面缺陷态密度Dit2对衬底电阻率的可取值范围具有较大影响,Dit2越大衬底电阻率的可取值范围越小.     

Relation between the activation parameters of low-temperature slip and the mean-square amplitude of atomic vibrations in cubic metals

The available data on various activation parameters of low-temperature slip in 9 body-centred cubic and 5 face-centred cubic elements have been examined as a function of a single microscopic parameter, namely mean-square amplitude of atomic vibrations <u ²>, specific to the material. It is found that for a given crystal structure, the microscopic parameters of the unit activation process of yielding, e.g. the initial length of dislocation segment, the critical height of the kink-pair nucleated, the associated activation volume, the binding energy per interatomic spacing along the glide dislocation on the slip plane etc. correlate well with the mean-square amplitude of atomic vibrations <u ²> through a power regression formula.     

Computer simulation study of a flexible-rigid-flexible model for liquid crystals

Small molecules that form liquid crystals typically consist of a rigid core with flexible tails on one end or on both ends. To date, most computer simulation studies have used completely rigid models such as hard spherocylinders: cylinders, characterized by their length/diameter ratio L/D, with hemispherical end caps. We have studied a model consisting of spherocylinders with L/D = 4, with a flexible tail attached to each end. The tails are ‘ideal’ in the sense that they have no volume. Using Monte Carlo simulations the phase behaviour of this model was studied and, for comparison, the behaviour of hard spherocylinders with L/D = 4 without tails was studied as well. The addition of the tails is found to stabilize the smectic-A phase at a lower pressure, and the nematic phase disappears. In the smectic-A and crystal phases, the smectic layers are further apart when tails are added. The structure of the layers and the smectic-A–crystal transition pressure change only a little. For both models close to melting the crystal consists of ordered layers, but there is almost no correlation between particle positions in neighbouring layers. In fact, the layer coupling is so weak that in a long simulation the layers are found to glide over each other. As the pressure is increased the crystal gradually becomes more ordered and the crystalline layers ultimately ‘lock’ into place.     

A study of the electronic excitation of he from grazing incidence ion-surface interaction

The neutralization of He⁺ at grazing incidence from a Ni(111) surface leads to excited states. The light emission from the 3d ³ D and 3d ¹ D states is studied as a function of the beam energy and angular parameters. The observed circular polarization dependence on the crystallographic surface directions is interpreted in terms of the Brillouin zone of the solid.