Č. Barta

Optical gaps from ‘mean’ bond energy in Ge1−x S x and Ge k Sb m S n non-crystalline solids

Abstract An attempt has been made to evaluate the optical gaps in Ge1-x S x and Ge k Sb m S n glassy systems using Mancas relation and the bond statistics of various models. The best agreement between calculated and experimental data is obtained for a chain-crossing model in the S-rich region and for a model due to Philipp in the metal-rich region. The change of the optical gap of Ge1-x S x thin films associated with the transition from as-deposited to annealed states is also investigated.

Crystallization of KNO3 under conditions of diffusive and convective regime

Abstract Influence of the nature of heat transfer on crystallization from a cooling solution has been investigated. Sodium polymethylacrylate (PMANa) was used to adjust the viscosity of the medium in the desired way. It was shown that the nature of heat transfer influences the crystallization kinetics and that the absence of convection results in enhanced supercooling. These findings corroborate our interpretation of space experiments /1/. The KNO 3 -H 2 O-PMANa system is suitable for a detailed analysis of the influence of zero gravity on the nucleation and crystallization processes.

Directional solidification of complex-forming eutectic melt of the PbCl2-AgCl dielectric system under conditions of zero, normal and increased gravity

Abstract First results in the area of studying the influence of gravity field on the degree of ordering of directionally solified eutectict are presented. As the model complex-forming melts, PbCl 2 and its eutectic with AgCl was chosen. Directional solidification of the mentioned eutectic was realised under conditions of zero, normal and increased gravity. It was shown that increasing level of gravity results in a marked improvement of the degree of spacial ordering of the lamelar structure.

A new module for the CSK-1 crystallizer: Thermographic DTA probe for the study of metastable phase equilibria under microgravity conditions

Abstract A special thermographic DTA probe was designed to complet the CSK-1 crystallizer for materials research in space. This equipment is desirable for a deeper understanding of the nature of the solidification under gravity-less conditions. It is shown that the new DTA probe in combination with a specially adapted measuring device ARP (GDR) is suitable for this purpose and the results of measurements are comparable with those obtained with commercial apparatures.

Annealing of Hg2Cl2 crystals — Project of the experiment

Abstract The creep of polycrystalline Hg 2 X 2 (where X = Cl, Br, J) under the terrestrial and space conditions is studied in the present experiment. The authors suppose that the creep of the Hg 2 X 2 cylindrical sample can be studied by the help of the theory of micropolar (non-Newtonian) fluids. In accordance with this theory we obtain the flux of fluid through any cross-section as functions of the radius, viscosity coefficients, the gradient of pressure and the gravity acceleration. In this paper a comparison of the theoretical results for normal and zero- gravity conditions is given. It is shown that the annealing of materials having a broad region of plasticity under microgravity conditions can lead to great improvement in optical as well as mechanical qualities of the crystal. Technical aspects of the experiment are also described.

Effect of gravity on the velocity of a crystallization front

Abstract The crystal growth from a liquid can be studied by methods of non-equilibrium thermodynamics of a discontinuity surface. These methods are used to derive the dependence of the crystalization velocity on transport and surface phenomena across and along an interface solid-liquid and on the acceleration of gravity.