Linear growth of two-phase regions during unmixing of NaCl-KCl crystals

Microscopic and flotation density investigations have been performed of unmixing in equimolar single crystals of NaCl-KCl at elevated temperatures. The surface nucleation is found to be characteristic of the decomposition process. The two-phase regions formed at the surface grow inward at a constant linear rate. The activation energy for the movement of the decomposition front is 0.9 eV at 180 to 260°C. The decomposition is anisotropic, <111> being the fastest unmixing direction. The decomposition zones nucleated on air bubbles inside the crystal tend to acquire a cube-like form.     

Block Lattices with Asymmetric Interaction Forces

A series of 2D block lattices constructed by identical building elements and having asymmetric interaction forces is proposed as a generalisation of the well‐known Kossel crystal. Different polymorph structures are created depending on the mutual orientation of the neighbour elements. The lattice energy for some of them is calculated at low temperatures using the repeatable step of blocks (monomers, dimers, quadrumers). A variety of “surface” structures (including alternating “surfaces”) is established. The corresponding coexisting equilibrium forms are determined following the method of Stranski and Kaischew. The thermodynamic stability of these polymorph structures is discussed. An orientation order/disorder surface transition is expected to appear as a novel phenomenon at high temperatures.     

Surface Roughness and Kinetics of Spontaneous Transformation of Negative Crystals

Spontaneous isothermal transformation of negative crystals (vapcur inclusions) at temperatures close to the melting point is experimentally studied. Linear dependence of the transformation rate on supersaturation is established. This result shows that the evaporating crystal surfaces of the inclusions are moleculary-rough. Surface self-diffusion has no appreciable contribution to the transformation process of the molecularly-rough surfaces of diphenyl.     

Surface Roughening and Surface Self-diffusion. A Monte Carlo Simulation

A model for atom diffusion on surface is proposed, the energy barrier for surface self-diffusion as well as the barrier for diffusion along surface steps being considered. Using a Monte Carlo method, the equilibrium structure of crystal surface is obtained and its effect on the surface self-diffusion is studied. The mass transfer surface self-diffusivity D_s and the mean diffusion distance λ are calculated for different temperatures T. A nonlinear Arrhenius plot (log D vs l/T) is obtained. The maximal deviation from linearity occurs at roughening temperature.     

Gross Dissolution of Natural Rhombohedral-shaped Calcite in Glacial Acetic Acid

Gross dissolution of natural rhombohedral-shaped calcite crystals in different concentrations of glacial acetic (0.3 to 3.4 M) diluted by distilled water is studied. Activation energy of reaction between calcite crystal and acetic acid is calculated and found constant. The preexponential factor is not constant with different concentrations. Weight loss per unit surface area versus concentration of acetic acid is plotted at various temperatures (30 to 55 °C). It is found that the maximum weight loss per unit surface area occurs with 2.5 M of glacial acetic acid for the above temperature range. The value of the pre-exponential factor is also maximum at 2.5 M of acetic acid. The maximum weight loss per unit surface area is explained with the help of ionic conductivity and viscosity of the etchant.     

Formation of crystalline phases by means of solid state reactions at the base of vitreous industrial slags

In comparison with the solid state synthesis from the oxide components melilites are formed from vitreous materials with added oxides at lower temperatures and with higher velocity of reaction resulting in larger and more idiomorphic crystals. The crystallization mechanism of the applied vitreous materials is important for the course of the formation of the texture during sintering depending on their chemical composition. In copper slag with polyphase composition the melilite forming reaction starts during the crystallization of pyroxene using the newly relased relictic glass phase in statu nascendi. Lateron, at higher temperatures the pyroxene also reacts with other components to form melilite. In phosphor furnace slag with stoichiometric composition the polymorphic transformation of calcium silicate (CS) is important for the formation of melilite. The change of the glass structure during the nucleation of wollastonite is the process initiating the reaction. Later at higher temperatures the polymorphic transformation of metastable pseudowollastonite accelerates the formation of melilite. It could be proved that the excess of free energy in glasses speeds up crystal forming reactions with admixture components in sintering processes. The mechanism of structural and phase change processes is reflected in the temperature dependence of reaction velocity.     

Spontaneous crystallization of potassium chloride from aqueous and aqueous‐ethanol solutions; Part 3: Model of the crystallization process

A model of spontaneous crystallization process was proposed. The model describes kinetics of the crystallization process after the end of the induction period. To test the model the published earlier data on crystallization and aggregation kinetics of potassium chloride at its spontaneous crystallization from supersaturated aqueous and aqueous‐ethanol solutions were used. It was found excellent coincidence of the experimental and theoretical data on concentration of the salt and the total number of crystals in solution at crystallization. Somewhat change for the worse was at the theoretical calculations of crystal size distribution at the end of the crystallization process. It indicated that the ways of calculation of size of crystals and their weight fraction in deposit were very approximate. The model allows predicting with satisfactory accuracy kinetics of crystallization using such general parameters of potassium chloride as the specific surface energy and the height of the nucleus‐bridges between crystals at coalescence. It needs further test of the model for other salts. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

On the Quantum Theoretical Calculation of Activation Energies for the Self-Diffusion of Single Atoms and the Diffusion of Adatoms on Metal Surfaces (I)

By application and modification of the equation given by A. Einstein for calculating the internal oscillation energy of crystals, the activation energies for the self-diffusion of single atoms on the metal surfaces of Rh, Pt, and for the surfaces diffusion of adatoms on metal surfaces were calculated on the basis of quantum theory from the surface Debye frequency and the bulk Debye frequency, neglecting the frequency distribution. A comparison of the activation energies calculated by quantum theory with the experimental values of the literature obtained by the field-ion microscope shows a relatively good agreement for the single crystal surfaces and temperature ranges. The calculation showed a strong increase of the activation energies for the self-diffusion at higher temperatures and a relatively great dependence on the respective crystal surface of the metal. Analogous results were also obtained for the surface diffusion of adatoms on the metals.     

Polymorphic Modifications of Chitosan

This work describes the analysis of the crystal structures of chitosan, its main polymorphic modifications, and its characteristic mutual chain packing and hydrogen bond systems in the crystalline regions of conformers. The analysis takes into consideration the crystal structures of chitosan complexes (salts) with organic and inorganic acids and their structural transformations. Notably, the transformation of the hydrated form of chitosan into anhydrous is found to be irreversible and occurs either at high temperatures or through a less stable form of hydrated salts. The interaction with polyanions during the formation of multilayer films can be considered as a way to form the anhydrous crystalline form of chitosan.     

Etching and Dissolution Mechanism of Cadmium Oxalate Trihydrate Crystals

Dislocation etching and dissolution of gel-grown cadmium oxalate trihydrate crystals are carried out using different etchants. The study revealed the existence of dislocation network in the body of the crystal and shallow dislocation loops in the substructure. Faster dissolution of the crystal at elevated temperatures revealed that the rate of dissolution in the initial part of the dissolution time is much lesser, which increases fast to attain a steady value. This observation is related to the available specific surface area of the crystal.     

Polymorphous transformation of quartz induced by mechanical treatment

Because of its strongly damaged structure (partially crystalline) mechanically activated α-quartz is not transformed to β-quartz. Rather, mechanically activated α-quartz is rapidly transformed to the cristobalite modification even at temperatures as low as 1200 °C. The acceleration and temperature shift of the quartz-cristobalite transformation is caused by mechanically induced defects. The activation energy of the transformation is thus reduced by about 50%.     

Emergence of regular meandered step structure in simulated growth of GaN(0001) surface

Step meandering during the growth of gallium nitride crystal is studied on using kinetic Monte Carlo method. Cause of instability is identified to be the particle advection caused by the step flow. Growth process is conducted in N-rich conditions and GaN(0001) surface kinetics is modeled by setting jump probabilities for Ga atoms adsorbed at the surface. We show that at low enough temperatures and relatively high external particle fluxes periodic regular pattern of meanders is created with its wavelength inversely proportional to the particle flux. An increase of the meander amplitude saturates after some period and further crystal grow is stationary, creating “finger-like” structure. For medium fluxes regular structure of meanders builds up for low or zero value of Schwoebel barrier. For higher fluxes wavelengths of meanders become shorter than the terrace width and they start to grow independently and finally transfer the surface to a rough structure. For very low fluxes or at relatively high temperatures steps move steadily remaining their initial shapes of straight, parallel lines.     

Part 1: Kinetics and mechanism of the crystallization process

The kinetics of spontaneous crystallization of sodium chloride from aqueous‐ethanol solutions were studied. During the crystallization the electrical conductance and optical transmission of the supersaturated solutions were measured automatically. For monitoring of the total surface of growing potassium chloride crystals at the crystallization the turbidimetric method was used. The growth rate and activation energy were determined. The crystal growth rate was proportional to supersaturation. When the volume fraction of ethanol in solution increased from 14.85 to 29.72%, the activation energy of the growth process did not change and was about 50 kJ· mol^‐1. Aggregation of the crystals was found. The aggregation kinetics of the crystals may be described approximately by the famous Smoluchowski equation for coagulation of colloidal particles. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

The effect of crystal surface roughness on impurity adsorption

The effect of crystal surface roughness on impurity adsorption was investigated in a fluidized bed crystallizer and in a batch crystallizer. The crystallisation of sucrose in pure and impure systems was the study subject. Calcium chloride was utilized as impurity in this work. The results show that the impurity adsorption is growth rate dependent and is strongly influenced by the crystal surface properties. Crystals with high surface roughness have lower impurity adsorption. Based on experimental evidences, a new theoretical model is proposed to quantify the surface roughness influence on the impurity adsorption, allowing, by operating at the more adequate supersaturation, to control the impurity transfer into crystals. The used impurity does not have a significant influence on the growth rates at the studied temperatures. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Influence of the surface on the properties of macro-and nanocrystals

The influence of the surface energy on the properties of macro-and nanocrystals has been considered. It is shown that the properties of small crystals (several microns and less) depend on their dimensions, which is explained by an increase of the number of surface atoms in the total number of all the atoms in a crystal. On the basis of the energy criterion of crystalline substance amorphization, a formula relating the surface energy of a crystal to its crystal-lattice parameters is derived. The surface energies of elemental crystals are calculated. It is assumed that anisotropy in contact-melting temperature of single crystals can be observed experimentally.     

Kinetics of a Single Crystal Growth from Supersaturated Liquid Solution. An Attempt on the Analytical Description of the State of Solution

The attempt was made at formulating a mathematical model describing the state of the supersaturated liquid solution circumfluenting a single crystal, as well as the transport processes occurring in that solution. As the result of a suitable computation procedure, numerical values of velocity components, concentrations, temperatures and the normal components of the mass flux in the vicinity of the crystal surface have been obtained.     

Yb∶YAl3(BO3)4晶体形貌与生长速度的关系

观察测量了不同生长速度(相应于不同降温速度)自发成核生长的Yb∶YAl3(BO3)4晶体形貌。粒度较大(>2mm)的晶体不管降温速度快慢形态都很简单,只发育六方柱｛1120｝和菱面体｛1011｝;粒度较小(<2mm)的晶体形态随降温速度增快而变复杂,发育一些罕见的高指数晶面。说明在生长速度较快的条件下,在晶体生长早期,一些高能面发育,在晶体生长后期已尖灭了,晶体生长的大部分时间是在低能面｛1120｝和｛1011｝上进行的。对比了不同生长条件下晶面的粗糙度,随着降温速度的增快,六方柱面｛1120｝和菱面体面｛1011｝由光滑变粗糙,顶面｛0001｝永远是粗糙的。从晶体结构上定性地探讨了3种晶面的杰克逊因子α及生长机理     

PHOTOISOMERISING EFFECTS IN NITROAZO FERROELECTRIC DYE GUEST HOST SYSTEMS

Abstract

The effect of a photoisomerising dye in a ferroelectric liquid crystal host has been investigated. A nitroazo dye incorporating the same three-siloxane unit as an organosiloxane ferroelectric host has been used to insure good miscibility and high dye concentrations. The effect of photoisomerisation by 300–400 nm UV of up to 430 mW. cm^?2 has been investigated in two mixtures with 25% and 50% molar weight of dye. The 25% mixture has an I-SmC*-crystal phase sequence, while the 50% mixture has I-SmA*-SmC*-crystal. Transition temperatures of both the mixtures are reduced by increased UV illumination. For the first mixture at a constant temperature, the spontaneous polarisation in the ferroelectric phase is reduced by UV illumination. For the second mixture both the spontaneous polarisation and tilt angle of the SmC* phase are reduced by UV illumination and the phase is changed from SmC* to SmA* at temperatures close to the transition. The change of tilt angle and observations of micrograph textures indicate that this is a macroscopic change of the material rather than any phase separation of the dye and host materials.