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.     

Crystallization of Silver — Germanium Alloys under Microgravity II. Primary Crystallization

Results of a study on primary crystallization in hypoeutectic and hypereutectic alloys under different convection conditions during solidification are presented. The conditions of suppressed buoyancy driven convection are assumed in the microgravity samples. The intensity of convection was shown to affect the resulting microstructures significantly. The number and the size of primary dendrites in both hypoeutectic samples was found to be comparable. In the microgravity processed hypoeutectic sample the primary dendrites exhibit a greater tendency to branch compared with the ground-based sample. In eutectic samples, a small amount of primary solid solution, due to the existence of an asymmetric coupled zone, was observed. In hypereutectic samples the number of primary germanium crystals is higher in the space sample. The influence of convection on the nucleation of primary crystals thus depends on the composition of such samples. In the case of the primary solid solution the effect of convection on the growth stage of crystallization appears. On the other hand, in the case of primary germanium the intensity of convection affects mainly the nucleation stage of crystallization. The influence of convection on the growth is only secondary. The reasons for such a difference are based on the transport requirements during the nucleation stage.     

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.     

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.     

The review of possible interrelations between ionic conductivity, internal friction and the viscosity of glasses and glass forming melts within the framework of Maxwell equations

The review contains the results of application of Maxwell equations for mechanical relaxation and electrical conductivity, to the systematization of large amount of experimental data related to mechanical, viscous, and electrical properties of inorganic glasses and glass forming melts. The generalization of internal friction results shows the existence of characteristic values for the ratios of temperatures, responsible for α-, β- and ionic relaxations; they are independent on the frequency and chemical composition. This is the evidence for the main role of elastic deformations at various corpuscular processes and the existence of characteristic scale of activation barriers predetermined by local volumes of deformation. It is shown the possibility of very precise calculation of the temperature of “ionic” internal friction maximum for one-alkali oxide glasses directly from Maxwell equation and d.c. conductivity experimental data. The volumes of particles overcoming the potential barrier at viscous flow practically coincide with the results of direct structural determinations. The existence of universal relation between d.c. conductivity and viscosity for the extremely wide temperature intervals (Littleton relation) is proved for silicate and phosphate melts. The theory of this dependence is proposed. The results show the effectiveness of the attempts to unite the continual and discrete approaches within the framework of Maxwell equations to obtain the simplest understanding the mechanisms of different types of relaxation. The review comprises many Russian papers unknown in English scientific literature.     

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.     

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.     

TiAl金属间化合物/羟基磷灰石生物复合材料的制备及性能表征

首先采用机械合金化的方法制备出TiAl金属间化合物,用化学沉淀法制备出HA粉体,然后将二者球磨混合,经过真空热压烧结工艺制得TiAl/HA复合材料.力学性能检测结果表明,TiAl/HA复合材料的硬度比纯HA生物陶瓷要低,并且随TiAl质量分数的增加呈现降低趋势;复合材料的抗弯强度随着TiAl金属间化合物含量的增加呈上升趋势,含量为15;以下,其强度低于纯羟基磷灰石,含量处于15;到30;之间,其强度高于纯羟基磷灰石;断裂韧性高于纯HA生物陶瓷,最高可比纯羟基磷灰石提高78.7;,并且随着TiAl质量分数的增加呈上升趋势.X射线衍射结果表明复合材料中只含有TiAl金属间化合物和HA两种成分;断口扫描电镜照片显示,纯羟基磷灰石材料比较致密,而复合材料中存在一定量的气孔,其部分断裂区域显示出穿晶断裂的形态.     

Comment on ‘Post UV irradiation annealing of E′ centers in silica controlled by H2 diffusion’ by M. Cannas et al. [J. Non-Cryst. Solids 337 (2004) 9–14]

In the paper by Cannas et al. [M. Cannas, S. Costa, R. Boscaino, F.M. Gelardi, J. Non-Cryst. Solids 337 (2004) 9] the simplest version of the Waite theory of diffusion limited reactions was applied to the post irradiation annealing of E′ centers by molecular hydrogen diffusing in a silica matrix. This version of the Waite theory does not take into account limitations arising from the rate of the chemical process between reactants but assumes that the annealing kinetics is limited by the diffusion only. An attempt to fit the annealing rate determined experimentally to that calculated from this version of the Waite theory led to an unrealistically small value of the capture radius. To clarify this issue, a more advanced version of the Waite theory, which takes into account the viability of the chemical reaction between H2 and E′ centers, should be used. In this comment such a version of the Waite theory is outlined and applied to an interpretation of the experimental data reported in [M. Cannas, S. Costa, R. Boscaino, F.M. Gelardi, J. Non-Cryst. Solids 337 (2004) 9]. A rigorous argument of the hypothesis introduced in paper [M. Cannas, S. Costa, R. Boscaino, F.M. Gelardi, J. Non-Cryst. Solids 337 (2004) 9] that the annealing kinetics of E′ centers is not limited by the diffusion of H₂ molecules but is defined by the rate of the chemical reaction itself is presented.     

Texture and structure of nickel-molybdenum catalysts supported on alumina modified with sulfate and phosphate ions

A series of nickel-molibdenum catalysts supported on alumina modified with sulfate and phosphate ions was studied. The studies involved texture evaluation measurements through N₂ adsorption isotherms. Besides, IR spectroscopic and derivatographic studies were carried out. Results of the studies enable us to conclude that modification of the alumina support with the above ions leads to a significant decrease in its surface area and pore volume. Modification does not result in a change of the type of pores of the support, but in the change of their size dimention. The pores present on the surface of the catalyst have the shape of “ink-bottles”. Modification with phosphate ions improves the thermal stability of the catalyst, whereas sulfate ions present on the surface of the catalyst undergo decomposition upon heating of the sample. Phosphate ions inhibit formation of aluminum molybdate, and instead form polymeric layer structures of phosphate on the surface of catalysts. Sulfate ions occur in the form of surface aluminum sulfate.     

Modeling of decagonal quasicrystal lattice

The model of the reciprocal lattice of decagonal quasicrystal has been proposed. It is based on the multiplication of the basis sites' groups according to the proposed rules. This model was shown to provide the information not only on the reciprocal sites location, but also on the intensity of the appropriated diffraction reflections. The physical justification of the proposed model correctness has been provided on the base of projection of six-dimensional hyperlattice onto the three-dimensional space. The possibility of three-indices labeling of the diffraction reflections of decagonal quasicrystals has been exhibited.     

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.     

Detection of side-products on heavy metal cations loaded zeolites by XRD measurements

During the high loading of A, X, and Y zeolites with heavy metal cations by ion-exchange undesired hydroxides are formed at the crystal surface. The precipitation is caused by the alkaline reaction of the parent sodium modification of zeolites in aqueous solutions and the low solubility product of heavy metal hydroxides. During the calcination of the samples the hydroxides are transformed into oxides. Exemplified by zeolite ZnNaA, a stepwise exchange in strongly diluted salt solutions may hinder the formation of hydroxides/oxides. XRD measurements enable a direct monitoring also of smallest amounts of zinc hydroxide/zinc oxide. The framework adopted ZnO causes a change of the symmetry of the zeolite lattice. The side product does not influence the formation of the new crystalline phases during the calcination of ZnNaA.     

Topological Transformations in the Acoustic Spectrum of Anisotropic Plate under Asymmetric Perturbations

It is shown how small variations in the conditions for the propagation of waveguide acoustic modes in anisotropic plates lead to a topological transformation of the entire net of acoustic dispersion curves. When both the sagittal plane and midplane of a plate are simultaneously planes of symmetry, the acoustic dispersion curves of the plate form four independent sets with many intersection points. If the symmetry is violated as a result of perturbations, the intersection points disappear and the curves are “repulsed”; thus, the eigenmode spectrum of the plate radically changes its topology. Versions of such changes are revealed. The polarization of waves in the repulsion zones sharply changes at small segments of the corresponding dispersion branches. The field structure of the wave is analyzed for the case where the perturbation is elastic anisotropy or a piezoelectric effect. The features of transformations of the acoustic spectrum of anisotropic plates in all purely elastic and piezoelectric symmetry classes are analyzed.     

Certification of Reference Material with Special Emphasis on Porous Solids

Certified reference materials (CRMs) are very important for the reliability of the results of chemical analyses and, at the same time, constitute the basis of comparability of analytical data. The development of a new type of CRMs is demonstrated using the example of porous and finely dispersed solids having specific surface area, specific pore volume, pore volume distribution, and mean pore radius as their characteristic values. Many of the properties of reference materials to be certified, such as their elemental content, are established by means of measurement procedures relying on different and independent principles of measurement. With porous or finely dispersed solids, however, the characteristic values referring to pores strongly depend on the size of the probe molecules used, a fact which is due to the fractal character of these substrates. This is particularly true for the specific surface area or the pore volume. As a result, when applying the method of gas adsorption, the certified values refer to a specific probe gas, e.g. to nitrogen. The use of other gases yields different numerical values for the characteristic values to be certified. Generally, the pore system of a solid contains pores of irregular shape. Therefore, depending on the presence of e.g. cylinder or slit pores, different evaluation models must be applied. For the verification of model calculation of microporous solids, zeolites with their crystallographically defined voids are particularly well suited. They can serve as a reference point for the correctness of the model used.     

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.     

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     

Fundamental laws of fracture region of solids

The present paper reports that the deformation is statistically symmetric even in the fracture region of solids. The major factor in the fracture region of brittle solids is the creation of new surfaces. On the basis of the creation of new surfaces, four fundamental laws are derived for the fracture region of brittle solids. These laws are: (i) New surfaces are created only after a particular strain of the crystal. (ii) For the smaller values of compressive strain, the area of newly created surfaces is directly proportional to the square of the strain. (iii) The proportionality constant between the area of the newly created surfaces and the square of strain may be termed as fracture constant. Mathematically the fracture constant is the area of newly created surface for unit compressive strain of the crystal. (iv) For larger values of the compressive strain, the logarithm of newly created surfaces is directly proportional to the strain. The experimental verification of the laws of fracture region of solids are discussed. The value of fracture constant of certain solids are estimated and their significance is discussed.     

Phase transitions in solid and liquid silicon

It has been established that continuous variations of impurity concentration and temperature in silicon and other semiconductors give rise to phase transitions in the range of existence of solid and liquid solutions. The specific feature of these phase transformations is the occurrence of a phase transition proper in the range of the varying parameter of state, which is accompanied by a thermal effect. In such ranges, all the electrophysical and thermodynamic properties oscillate. It has also been shown that these oscillations result from the equilibrium self-organization of the material. The beginning of phase transitions with the change of the temperature depends on the concentration of defect in the initial material. The “driving force” of the phase transformations is the instability arising in the material with the variations in the temperature and the component concentrations.