Profile-sapphire and its structural perfection

The paper is concerned with the growing of sapphire of various profiles and with the investigation of its structural perfection. It is shown that the kind of the crystallographic forms and the dislocation structure essentially depends on the degree of overheating of the melt. Near the surface of profiled crystals wide dislocation-free zones (upto 200 μm) may be obtained. A dependence of the length of the mosaic block boundary on the rate of growing can be observed. A high ratio of the area of the radiating surface to the crystal volume gives the possibility of a rapid growing of profile crystals.     

Problems and prospects of development of nanomembrane technology

The latest Russian and foreign studies devoted to the theoretical and experimental investigation of the processes of formation of porous structure of track nanomembranes, the methods for studying their properties, and the application of track membranes to synthesis of nanostructures by a template method are reviewed. The results of the investigations carried out in these promising directions at the Department of Membrane Technologies of the Shubnikov Institute of Crystallography, Russian Academy of Sciences, are also presented. On the basis of the theoretical investigations of the mechanism of interaction of high-energy ions with a solid, it is shown that the theory of instantaneous explosion in a track region must be carefully verified. The results of the experimental investigation of the structure of tracks of high-energy ions in a polymer and the mechanism of pore formation in the stage of chemical etching of tracks showed that the track structure is inhomogeneous in the radial direction. It is shown that the surface of polyethylene terephthalate track membranes becomes negatively charged in neutral and alkaline solutions, and the value of this charge depends on the pore diameter. Good prospects of application of secondary metal structures formed on the basis of track membranes in mass spectroscopy are demonstrated experimentally.     

Verfahren und Vorrichtung zur Bestimmung von Krümmungen einkristalliner,scheibenförmiger Körper

With the production of semiconductor devices the problem of bending of monocrystalline slices occurs, these being an expression of the reduction of the crystal perfection, increasing with each working step during the technological process. Thus the determination of the bend can give an information on the influence of each working step on the internal stresses of the crystalline body. A procedure is described which takes advantage of the Bragg reflection of an X-ray beam in the lattice planes of the crystal, in order to determine the inclination of the plane. A design is presented based on this procedure permitting a high degree of automation.     

Structural study of sol-gel silicate glasses by IR and Raman spectroscopies

A study of the structure and bonding configuration of sol-gel silicate glasses by Raman and infrared spectroscopies is presented. Moreover, a review of the Raman lines and infrared bands assignment, the identification of the non-bridging silicon-oxygen groups and the ring structures are also demonstrated. The evolution of the changes of the bonding configuration in the composition and the stabilization temperature of the bioactive glasses is discussed in terms of the structural and textural characteristics of the glasses. Raman and infrared analyses contribute to the improvement in understanding of the local symmetry for sol-gel silicate glasses. infrared spectroscopy has allowed to identify the vibration bands of the hydroxyl groups associated with various configurations of the terminal silanol bonds on the glass surface and the free molecular water in the glass matrix. Raman analysis has provided an alternative method of quantifying the network connectivity grade and predicting the textural properties of the sol-gel silicate glasses.     

Diffusion of intrinsic defects in dielectric and semiconductor crystals with impurities

This paper reports on the results of detailed theoretical investigations into the diffusion of intrinsic defects in impurity crystals doped with mixed-valence ions. The special case of diffusion stimulated by variations in the redox properties of the atmosphere at the crystal boundary during high-temperature annealing is analyzed. The major consideration is given to the following fundamental problems: (i) the dynamics of valence transitions and the structure of the chemical reaction zone, (ii) the possibility of determining the type of chemical reaction at the crystal-atmosphere interface and the type of diffusing defects, (iii) the effect of dilatation mechanical stresses arising in the reaction zone on the reaction-zone structure and on the dynamics of diffusion processes, and (iv) the determination of the diffusion parameters of intrinsic defects and the constants of their interaction with impurity centers.     

On the Fit between Precipitates and Matrix in Al–Zn Alloys

The quality of the fit between the precipitates occurring in Al–Zn alloys and the matrix of the Al host is considered taking into account the positions of the atoms in the lattice planes forming the possible faces of the precipitates and the neighboured lattice planes of the matrix. The quality of the fit depends on the number of (pseudo-)coincidence sites of both planes, the density of Al atoms in the matrix plane and the misfit in the spacings of the atoms in the adjoining planes in consideration. The effect of the misfit is taken into account by means of two different models. Based upon the obtained results many of the experimentally observed features about the preferred shapes and orientations of the faces of the growing precipitates can be explained. It turned out that in both of the applied models most probably the effect of the density of the coincidence sites on the interfacial energy is underestimated.     

Investigation of the formation and growth processes of silicon carbide monocrystals

The formation and growth processes of SiC monocrystals from vapour phase by sublimation method have been investigated. It is shown that the peaks of the evaporation and growth whiskers are the centres of SiC monocrystals formation. One of the mechanisms of development of the whisker into SiC platelet is established. It is shown that on the naturally mirror like crystal face either the edge of the crystal or the screw dislocation, situated on the crystal edge, is the source of the steps. On the stepped crystal face the root of the crystal is the source of the steps.     

Brittle fracture of single crystals and active structure of cleavage surfaces

By the method of vacuum decoration at an electronmicroscopic scale with the use of lowangle electron diffraction the dependence of brittle fracture of alkali halide crystals and of the active structure of cleavage surfaces on the character of ordered distribution of point defects in crystals has been studied. The occurrence of cleavage steps reflects the presence in crystal volume, where brittle fracture took place, of a high degree of periodicity of point defects mainly in one crystallographic direction. The smooth areas of the cleavage surface reflect the existence in these places of a periodic distribution of point defects in two mutually perpendicular crystallographic directions. Coincidence of periods between the cleavage steps with the periods in the arrangement of point defects on smooth areas of the cleavage surface allows the steps to be regarded not only as elements of the geometric microrelief but primarily as electrically active linear elements of the crystal surface, reflecting the linear arrangement of point defects. Together with cleavage steps exhibiting a stable in time activity there arise cleavage steps with an extremely high, but rather shortlived activity being the consequence of the brittle fracture process proper. The electrically active linear elements of the point defect lattice – linear elements of the potential relief of the surface – play a most important role in heterogeneous processes, in particular crystallization processes.     

Study of inclusions and causes of their formation during growth of barium metaborate crystals

The complex study of barium metaborate crystals has been undertaken with the aim to determine the conditions of their growth with the minimum density of inclusions. It is shown that almost all the inclusions are multiphase formations of the melt type, and, in addition to the mixture of the main components of the quasieutectic composition, they can also be enriched with gas-forming impurities not removed during barium metaborate synthesis. It is shown that the amount and the composition of melt-like inclusions in crystals depends on the removal of by-products of the reactions used for barium metaborate synthesis from the system, the patterns of the convective flows in the flux, the character of its flow under the interface, the axial temperature gradient in the solution, and the temperature fluctuations at the crystallization front.     

Reactor problems in modified chemical vapour deposition (i). the collapse of quartz glass tubes

In the case of the so-called “Modified Chemical Vapour Deposition” for the preparation of optical fiber preforms, the quartz glass reactor is identical with the substrate. Therefore, the knowledge of reactor properties such as the viscous flow of the tubes (collapse) and the heat transport through the wall are especially important for a successful control of the process. As a first step, an analysis is presented for the calculation of the viscous flow of the tubes under the real conditions of dynamic heating by a traversing flame. The experimental determination of the material parameters surface tension and viscosity by collapsing experiments yields an approach to the problem of the temperature distribution in the tube wall.     

Supramolecular structure formation of Langmuir-Blodgett films of comblike precursor and polyimide

The surface structure of Langmuir-Blodgett films of a comblike polyimide precursor—a rigid-chain polyamic acid alkylamine salt bearing multichains of tertiary amine—and films of the corresponding polyimide were studied by atomic force microscopy (AFM). An analysis of the images of the surface of three-layer films revealed a domain structure. It was found that the Langmuir-Blodgett film formation of the precursor occurs as a result of the layer-by-layer deposition of two-dimensional domains (composed of polyamic acid salt molecules on the water surface) onto a substrate. The formation of domains in a monolayer is associated with the chemical structure of the precursor, to be more precise, with the rigidity of the main chain and the presence of closely spaced aliphatic side chains in the polymer chain, whose total cross-section area is close to the surface area of the projection onto the plane of the repeating unit of the main chain. Polyimide films inherit the domain structure of the precursor films; the inhomogeneity of the film thickness substantially decreases, whereas the domain size and character of their distribution in the film remain unchanged.     

Change of the Normal Rate of Hills Growth on the (001) Face of TGS Single Crystal Under Constant Crystallization Conditions

Considerable change (up to 100% in one of the cases) of the normal growth rate of hills on the (001) face of the TGS crystal at strictly constant supersaturation and temperature is registered experimentally. On the basis of the spatial disposition of the dislocations, which form the source of the steps, a model of a growth centre (called herein non-parallel centre of growth) is proposed. The model is qualitatively concordant with the experimental data. It is shown that the inconstancy of the normal rate of growth at constant supersaturation and temperature is an immanent characteristic of the non-parallel centre of growth.     

Three-dimensionality of space in the structure of the periodic table of chemical elements

The effect of the dimension of the 3D homogeneous and isotropic Euclidean space, and the electron spin on the self-organization of the electron systems of atoms of chemical elements is considered. It is shown that the finite dimension of space creates the possibility of periodicity in the structure of an electron cloud, while the value of the dimension determines the number of stable systems of electrons at different levels of the periodic table of chemical elements and some characteristics of the systems. The conditions for the stability of systems of electrons and the electron system of an atom as a whole are considered. On the basis of the results obtained, comparison with other hierarchical systems (nanostructures and biological structures) is performed.     

Liquid phase epitaxy of AlxGa1–xAs-GaAs heterostructures

This paper is concerned with the investigation of the peculiarities of the application of liquid phase epitaxy for obtaining multilayer structures with heterojunctions in the AlAs–GaAs system. Segregational depletion of Al in liquid and in solid phases is characteristic of this system. It is proved that supercooling of the substrate and numerous inclusions of Al allow to decrease this segregational depletion of Al and to obtain layers of nearly constant composition up to 600 μm in thickness. A new technique of crystallization of structures from a limited volume of melt with the change of solutions by way of compulsory squeezing out of the foregoing solution by the following one is put forward with the view to perfect the planarity of multilayer structures with abrupt heterojunctions and to improve the complete change of solutions on a substrate. The results of the study of various methods of controlling the value and direction of the Al concentration gradient are listed. It has been proved that structures with fluent increase of Al concentration during the crystallization process and also structures with alteration in the gradient direction of Al concentration may be obtained by way of artificial slowing down or acceleration of the process of mixing up solutions of various compositions.     

Calculation of volume fraction of phase growing by a diffusion-type law

The problem of determining the volume fraction of a phase is considered for the case where the rate of nucleus growth is a decreasing function of its radius. The solution is obtained within the framework of the geometrical-probabilistic method suggested earlier. The procedure of successive approximations is described, which allows one to determine the volume fraction of a phase with the required accuracy. The errors arising in the calculation of the volume fraction of a phase from the Kolmogorov formula are estimated analytically. As an example of numerical estimates, the case of the diffusion growth mechanism is considered. It is shown that in the three-dimensional space, this error lies within 0.01 irrespective of the initial parameters of the problem.     

New Transport Mechanisms for Conducting Polymers: The Role of Disorder

Abstract

A unified view of the metallic state in polyaniline and heavily-doped polacetylene is presented. We first consider a single randomly-protonated strand of the emeraldine form of polyaniline. We show explicitly that the disorder inherent in this system is described by the random-dimer model of Dunlap, Wu and Phillips¹. The random dimer model is simply a tight-binding model for a binary alloy in which pairs of lattice sites are assigned one of two values at random. The random dimer model is shown to possess a narrow band of conducting states that can ultimately lead to dramatic increases in the conductivity if the Fermi level is appropriately tuned. It is demonstrated explicitly that the location in the energy band of the conducting states of the random-dimer model for polyaniline coincides with recent calculations of the location of the Fermi level in the protonated form of the polymer^2,3. We argue then that the random-dimer model is capable of explaining the insulator-metal transition in polyaniline. In the context of polyacetylene, we propose a ‘dirty metal’ picture for the metallic state which is consistent with both the closing of the band gap and the existence of a band of extended states. Our model is based on the observation that a random distribution of solitons closes the band gap at ～5-6% doping and is formally equivalent to the random dimer model. The location of the extended states in the disordered soliton lattice is computed. It is shown that because these states lie in the vicinity of the band edge, they are capable of explaining the sudden onset of the experimentally-observed Pauli susceptibility and the subsequent depinning of the solitons in the metallic state. We close by noting that because the random dimer model applies to any lattice in which the defects are extended and possess a plane of symmetry, it quite generally describes any polymeric system in which the stable defects are solitonic or bi-polaronic in nature     

Theoretical calculation of optical properties of crystals with statistical distribution of atoms over crystallographic positions based on the model of point dipoles

Principal refractive indices and orientations of indicatrices of eleven minerals characterized by the statistical distribution of some ions over the crystallographic positions spaced by forbidden distances are calculated theoretically on the basis of the model of point dipoles. The calculated optical signs of the minerals of the medium crystallographic systems and the orientation of the indicatrices of the orthorhombic and monoclinic minerals agree with the experimentally determined values.     

Nematic-isotropic phase transition in polar liquid crystals. 1. Statistical theory

The theory of transition from the nematic to the isotropic phase for liquid crystals in the system of rodlike particles with large longitudinal dipoles has been developed with due regard for the equilibrium between monomers and antiparallel molecular pairs—dimers. The order parameters of monomers and dimers are determined as well as the dimer fraction. It is shown that, in accordance with the results obtained earlier, for low values of dipole moments, the temperature of the nematic-isotropic phase transition increases with an increase of the dipole moment. However, for large dipoles, the transition temperature starts decreasing with an increase of the dipole moment because of higher dimer concentration. This provides the interpretation of the recent computer simulation, which showed a destabilization of the nematic phase in the system of rigid rods with pronounced central dipoles. The temperature dependence of the dimer fraction is also studied. The qualitative relation between the sign of the jump in the dimer fraction at the transition point and the effect of dimerization on the transition temperature are established.