Dislocation Generation in Dislocation-Free Germanium

A dislocation structure is studied in germanium single crystals grown in a regime of minimum temperature gradients at the crystallization front and low supercooling. The investigations show that the distortion of the flat crystallization front arising during crystal droplet detachment in the completion growth stage results in the dislocation generation in the lower parts of dislocation-free single crystals. The dislocations are generated at the phase boundary and propagate in the thermoplasticity zone. __________ Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 5, pp. 21–24, May, 2005.     

Growth of polymer crystals during annealing

Observations by transmission electron microscopy are reported on the processes involved in polymer crystal growth during annealing. The observations suggest that crystal growth occurs by two processes. One process involves the melting of those regions of the crystals in which the melting point is lower than the annealing temperature. The polymer melt due to the melting process gradually becomes incorporated into the unmolten crystals, resulting in crystal growth. The alternative process is solid-state crystal growth by the migration of the amorphous region between crystallites.     

Defect structure in EuS single crystals grown from the melt

The defect structure in EuS single crystals grown form the melt is studied by etch pitting, scanning and high-voltage electron microscopy. Circular and square etch pits and a second phase in the shape of thin hexagonal platelets are observed by etching. Microprobe analysis indicates the platelets to consist of Eu metal. In the transmission electron microscope, smoothly curved dislocations and helical dislocations, small dislocation loops and inclusions associated with dislocations are observed. The possible origin of the detected dislocation structure is considered with reference to climb and glide processes occurring during cooling down the grown crystals. The results corroborate the glide geometry of the NaCl lattice for EuS. On leave from Institute of Physics, Academic Sinica, Peking, VR China     

Effect of solid solution strengthening by iridium on the nucleation of dislocations in a molybdenum single crystal during nanoindentation

The nucleation of dislocations in single crystals of molybdenum and a Mo-1.5 at % Ir solid solution has been investigated by nanoindentation. In the curve of indentation of a Berkovich indenter into the single crystals, an abrupt transition from elastic to plastic deformation has been observed at a depth of 20–40 nm due to the nucleation of dislocations in the initially dislocation-free region under the contact. Alloying of molybdenum by iridium results in a twofold increase in shear stresses under which dislocations nucleated in the contact. Therefore, the solid solution impurity of iridium in molybdenum leads to an increase not only in the plastic strain resistance (to an increase in the hardness) but also in the elastic shear stresses under which dislocations are generated (homogeneously or heterogeneously) in the contact. The latter effect cannot be explained only by an increase in the elastic moduli because of its smallness; however, it is determined to a large extent by a higher degree of perfection of the solid solution crystal as compared to unalloyed molybdenum.     

Diffusive fluxes associated with interfacial defeet motion and interaction

The diffusional flux associated with the motion of interfacial defects is described by an equation expressed in terms of the topological parameters which characterise defects, namely their Burgers vectors and step heights, the defect velocity and the concentration of each atomic species in the two adjacent crystals. This expression demonstrates that glide/climb behaviour of grain boundary defects is analogous to motion of dislocations in single crystals; climb motion results if a component of b is perpendicular to the interface plane. However, the situation is more complex in the case of interphase interface defects, but the present approach, which considers the step and dislocation portions of defects separately, enables a straightforward analysis. Several examples are illustrated to show the various possibilities, such as climb motion even when b is parallel to the interface, and glide motion when b is not. The latter case arises in martensitic transformation where the existence of an invariant-plane-strain relation at the interface leads to equal and opposite fluxes to the step and dislocation portions of transformation defects so that overall the motion is diffusionless.Interfacial processes involve the motion and interaction of defects. The present analysis facilitates the consideration of diffusive fluxes associated with defect interaction since the step and dislocation portions can be treated independently. A general expression is derived for the total flux arising, and a particular case, the interaction of transformation dislocations with crystal dislocations which have reached the interface during lattice-invariant deformation in martensite formation, is considered.     

Determination of a pseudo-binary SrSe–Ga2Se3 phase diagram and single crystal growth of SrGa2Se4 compounds

We have investigated chemical reaction processes and thermal characteristics of IIa–III₂–VI₄ compounds in order to grow their single bulk crystals. Up to now, single crystals of Ca and Sr thiogallates have been successfully grown by the melt growth method based on their pseudo-binary phase diagrams. Here, a similar diagram of the SrSe–Ga₂Se₃ system has been constructed for the first time, where a eutectic reaction is found in the range of excess Ga₂Se₃ concentration, and it is shown that the SrGa₂Se₄ compound has a congruent melting point (1110 °C) suitable for the melt growth. A single crystal is grown from the melt by the horizontal Bridgman method. A trial is also made to grow a high-quality single crystal of CaGa₂S₄ already known as being grown easily.     

Model of ideal relaxation of thermoelastic stresses in the course of growth of single crystals

A simple dislocation model is proposed for relaxation of thermoelastic stresses generated during the growth of single crystals from a melt. This model does not require a solution of the kinetic equations for dislocations involved in relaxation and makes it possible to obtain the lower estimate of the dislocation density in the bulk of a grown crystal.     

Transmission electron microscopy of GaN columnar nanostructures grown by molecular beam epitaxy

The GaN columnar crystals of nanometric sizes have been grown by molecular beam epitaxy with high-frequency plasma initiation of nitrogen discharge. The types and distribution of defects in these nanostructures on the (0001) sapphire substrates are studied by transmission electron microscopy (TEM). It is revealed that inversion domains begin to form almost at the interface irrespective of the presence of an initial low-temperature buffer layer. The critical diameter of dislocation-free columns, their density, and mean sizes are determined. It is shown that the low-temperature buffer layer affects the density of dislocations, their spatial distribution, and the mean sizes of columns. The nanosizes of grown crystals suggest a further use of these crystals and the growth method for producing molecular-beam epitaxial quantum-size objects (quantum dots and wires) in a promising AlGaInN system.     

磷酸铝(AlPO4)晶体中缺陷的X射线形貌研究

用X射线衍射形貌法研究了AlPO₄晶体中的微观缺陷。在所研究的晶体中,主要晶体缺陷是生长层,沉淀物和位错。位错密度在晶体表面附近最大,晶体中部较低。位错主要起源于热应力和由沉淀物或生长层所造成的晶格畸变。多数位错的柏氏矢量是b=(a+c)<1123>型,部分的是b=a[2110]。分析了晶体缺陷与生长条件之间的关系。控制生长过程中的温度波动,特别是晶体出炉时的冷却速度,对提高晶体完美性是重要的。 关键词：     

金属片加热区熔法单晶生长及应用

在比较现有的各种溶体生长方法的基础上,针对氕物晶体生长的特点,提出了适合均匀生长非固液同成份氧化物晶体的金属片加热区溶法,对该方法的装置、优缺点以及国内外研究现状作了详细的介绍,报道并讨论了应用铂片加热区溶法生长化学计量比铌酸锂晶体的试验及结果。     

On the problem of the consistency of the high-temperature precipitation model with the classical nucleation theory

The adequacy of the model of high-temperature precipitation in dislocation-free silicon single crystals to the classical theory of nucleation and growth of second-phase particles in solids has been considered. It has been shown that the introduction and consideration of thermal conditions of crystal growth in the initial equations of the classical nucleation theory make it possible to explain the precipitation processes occurring in the high-temperature range and thus extend the theoretical basis of the application of the classical nucleation theory. According to the model of high-temperature precipitation, the smallest critical radius of oxygen and carbon precipitates is observed in the vicinity of the crystallization front. Cooling of the crystal is accompanied by the growth and coalescence of precipitates. During heat treatments, the nucleation of precipitates starts at low temperatures, whereas the growth and coalescence of precipitates occur with an increase in the temperature. It has been assumed that the high-temperature precipitation of impurities can determine the overall kinetics of defect formation in other dislocation-free single crystals of semiconductors and metals.     

Dislocations and mechanical properties of icosahedral quasicrystals

In this article we interpret the mechanical properties of icosahedral quasicrystals with the dislocation theory. After having defined the concept of dislocation in a periodic crystal, we extend this notion to quasicrystals in the 6-dimensional space. We show that perfect dislocations and imperfect dislocations trailing a phason fault can be defined and observed in transmission electron microscopy (TEM). In-situ straining TEM experiments at high temperature show that dislocations move solely by climb, a non-conservative motion-requiring diffusion. This behavior at variance with that of crystals which deform mainly by glide is explained by the atypical nature of the atomic structure of icosahedral quasicrystals.     

Molecular dynamics simulation of the surface properties of nanocrystalline uranium dioxide

The molecular dynamics method is used to simulate the nanosized UO₂ crystals. The phase-transition temperatures are calculated for the nanosized crystals of the uranium dioxide. It is demonstrated that the melting point and the temperature of the transition to the superionic state (melting of the anion sublattice) of the crystals decrease with decreasing sizes. In particular, melting point (T _m ～ 2300 K) for the cubic nanocrystal with a size of 3.3 nm is lower than the melting point of the single crystal by almost 1000 K. The calculated surface energies are in agreement with the experimental results. The dependence of the surface energy on the size of the UO₂ nanocrystals is obtained. The effect of the nanocrystal temperature on the surface energy is studied. The temperature dependence of the thickness of the melt layer is obtained in the framework of the model of the heterogeneous melting. The parameters and dependences can be used for the further analysis of the microstructure properties of nuclear fuel in working systems.     

Morphology,Crystallization, and Melting of Single Crystals and Thin Films of Star‐branched Polyesters with Poly(ϵ‐caprolactone) Arms as Revealed by Atomic Force Microscopy

The morphology and thermal stability of different sectors in solution‐ and melt‐grown crystals of star‐branched polyesters with poly(?‐caprolactone) (PCL) arms, and of a reference linear PCL, have been studied by tapping‐mode atomic‐force microscopy (AFM). Real‐time monitoring of melt‐crystallization in thin films of star‐branched and linear PCL has been performed using hot‐stage AFM. A striated fold surface was observed in both solution‐ and melt‐grown crystals of both star‐branched and linear PCL. The presence of striations in the melt‐grown crystals proved that this structure was genuine and not due to the collapse of tent‐shaped crystals. The crystals of the star‐branched polymers had smoother fold surfaces, which can be explained by the presence of dendritic cores close to the fold surfaces. The single crystals of linear PCL grown from solution showed earlier melting in the {100} sectors than in the {110} sectors, whereas no such sectorial dependence of the melting was found in the solution‐grown crystals of the star‐branched polymers. The proximity of the dendritic cores to the fold surface yields at least one amorphous PCL repeating unit next to the dendritic core and more nonadjacent and less sharp chain folding than in linear PCL single crystals; this evidently erased the difference in thermal stability between the {110} and {100} sectors. Melt‐crystallization in thin polymer films at 53–55°C showed 4 times faster crystal growth along b than along a, and more irregular crystals with niches on the lateral faces in star‐branched PCL than in linear PCL. Crystal growth rate was strictly constant with time. Multilayer crystals with central screw dislocation (growing with or without reorientation of the b–axis) and twisting were observed in both classes of polymers.     

The velocity of acoustic phonons in crystals with edge dislocations and point impurities

The velocity of longitudinal long-wave phonons in crystals with a small number of randomly distributed static edge dislocations was shown to be the same as in ideal crystals. The correction to the velocity of phonons is determined by their interaction with the field of lattice deformations caused by the presence of dislocations. The field of deformations of dislocation displacements is long-range. In addition, these deformations experience sign reversal when the sign of the coordinate perpendicular to the dislocation line and Burgers vector changes. It follows that a crystal contracts or expands in this direction because of the interaction of phonons with dislocations as the defect deformation sign alters. Such an anisotropic volume (and density) change over the whole crystal should however be balanced somehow. For this reason, the density of samples with edge dislocations should remain constant, and the correction δc to the velocity of phonons should be zero. The procedure for averaging that gives δc = 0 is based on the independence of this macro value from the orientation of phonon momemtum incident on randomly distributed dislocations, the unaveraged correction being a diverging value because of the long-range character of the field of defect deformations. For comparison, the correction to the velocity of phonons in crystals with point impurities was calculated. Lattice deformation caused by such defects is isotropic. For this reason, the mean density of crystals with point impurities and the velocity of phonons in them are different from those in ideal crystals. All calculations were performed using the Dyson equation derived on the basis of the Keldysh diagram technique.     

Heterophase dislocations — An approach towards interpreting high temperature grain boundary behavior

When the conditions of full thermodynamic equilibrium are added to the conventional linear elastic theory of dislocations a hybrid concept of “heterophase” dislocations arises. As developed here, heterophase dislocation theory provides a self-consistent method for calculating the core configuration, energy, and stress-strain field which minimize the thermodynamic potential of a dislocated crystal. Simple grain boundaries in crystals near the melting point are treated as constrained arrays of heterophase misfit dislocations with a liquid-like core phase similar to, but not identical with the properties of ordinary liquid phase. Measurements of the properties and behavior of {011&#x0304;} tilt boundaries in bismuth crystals near the melting point show that the new theory accounts accurately for the energetic dependence on tilt misorientation in the range 0° to 6°, and then deviates from the observed dependence. The new theory also permits prediction of a structural transition in grain boundaries which is extremely sensitive to the density of misfit dislocations. This transition, predicted to occur in bismuth at a tilt misorientation of 15° was also confirmed by hot-stage electron microscopy.     

Dynamic properties of dislocations in silicon wafers heat-treated at low temperatures

The specific features of the dislocation motion in dislocation-free silicon wafers (single crystals are grown by the Czochralski method) heat-treated at 450 and 650°C have been investigated. It is found that the low-temperature treatment of silicon wafers with an oxygen content of (7–8)×10¹⁷ cm^?3 substantially affects the dynamic properties of dislocations generated into silicon wafers during their four-point bending and brings about an increase in the starting stresses of the onset of the dislocation motion. A characteristic spatial inhomogeneity is observed in the generation and propagation of dislocations from indentations upon the bending of heat-treated wafers. The reasons for the regularities revealed are discussed.     

A unified theory for brittle and ductile shear mode fracture

A unified theory captures both brittle and ductile fracture. The fracture toughness is proportional to the applied stress squared and the length of the crack. For purely brittle solids, this criterion is equivalent to Griffith's theory. In other cases, it provides a theoretical basis for the Irwin-Orowan formula. For purely ductile solids, the theory makes direct contact with the Bilby-Cottrell-Swinden model. The toughness is highest in ductile materials because the shielding dislocations in the plastic zone provide additional resistance to crack growth. This resistance is the force opposing dislocation motion, and the Peach-Koehler force overcomes it. A dislocation-free zone separates the plastic zone from and the tip of the crack. The dislocation-free zone is finite because molecular forces responsible for the cohesion of the surfaces near the crack tip are not negligible. At the point of crack growth, the length of the dislocation-free zone is constant and the shielding dislocations advance in concert. As in Griffith's theory, the crack is in unstable equilibrium. The theory shows that a dimensionless variable controls the elastoplastic behaviour. A relationship for the size of the dislocation-free zone is derived in terms of the macroscopic and microscopic parameters that govern the fracture.     

Melting of a dust crystal with defects

The effect of defects on the melting of a two-layer dust Wigner crystal in the electrode layer of a radio-frequency gas discharge is investigated by Langevin molecular dynamics. Two types of defects have been included in the consideration: (a) point defects and dislocations and (b) particles levitating above and below a two-layer crystal (so-called strong defects). It is shown that local melting of a two-layer crystal found experimentally is explained by the occurrence of strong defects located above the crystal.