Mechanism of Formation of Cadmium Oxalate Liesegang Rings

Qualitative and semiquantitative study of the Liesegang rings formed by the reaction of oxalic acid and cadmium chloride in the acidic range of pH in silica hydrogel is made. The behaviour of Jablczynski's space constant and Morce-Pierce's time constant at various temperatures, pH and concentration of outer electrolyte is studied. The possible mechanisms for the above observed behaviour are suggested. The deviations from Isemura's general obserations are interprerted by considering two particular aspects, viz., comparatively larger crystals constituting the rings and the irreversibility of inner and outer electrolytes.     

Characteristic Growth Features and Etching of InBi Single Crystals

The characteristic growth features observed on the top free surface of InBi single crystals grown by the zone-melting method are reported. These features are analysed by stereographic technique and a possible mechanism responsible for the appearance of the characteristic growth features is discussed. A new etchant capable of revealing dislocations inclined to the cleavage plane is also reported.     

Etching and Polishing Studies on MgO Crystals in Acids

The effect of temperature and concentration of a variety of inorganic and organic acids, on etch rates is investigated. It is observed that in HCl, CH₃COOH and C₂H₅COOH the value of activation energy of dissolution and that of the corresponding pre-exponential factor are sensitive to acid concentration. In H₃PO₄ and HCOOH, while the value of activation energy remains unchanged, only the pre-exponential factor changes. The concentration dependence of etch rates in HCl, CH₃COOH and C₂H₅COOH is different from that of H₃PO₄ and HCOOH. In the former acids maximum and minimum values are obtained on the etch rate versus concentration curves, but in the letter ones etch rate slowly increases with concentration. The influence of temperature on the maximum value of etch rate on the etch rate-concentration plots of HCl, and that of the degree of dissociation of the acids on etch rate are also studied. The observations are discussed, and important conclusions are enumerated.     

Etching Figures in Neodymium Gallate and Yttrium Aluminate Crystals

The dislocations existing in single crystals of neodymium gallate and yttrium aluminate grown by the Czochralski technique have been studied by means of etch pits. The data concerning their solubility in cases of different directions of a face orientation, various treatment temperatures and several enchant types are reported. The investigation of etch pits in the twinned YAlO₃ and NdGaO₃ crystals showed that twins are formed during a growth process. In the [110]‐pulled NdGaO₃ crystals the discrepancy between the twin and matrix parts of a crystal is accommodated by the dislocation congestion and the dislocation low‐angle boundaries whereas in [010]‐pulled YAlO₃ crystals the microcracks perform this function.     

Vickers Micromechanical Indentation Studies of Barium Oxalate Dihydrate Crystals

Vickers microhardness measurements have been carried out on gel grown single crystals of barium oxalate dihydrate. The dependence of hardness on indentation load has been described. Attempts have also been made to understand the cracks developed around the micro indentation. The material chipping off and median vents observed around indentation figures have been discussed. Also explained are the fracture at high indentation load. The average load independent hardness value is 58.27 kg · mm².     

Magnetic Moment and Susceptibility Measurements of Mixed Rare Earth Oxalate Crystals

The growth of double rare earth oxalate crystals with a general chemical formula AB(C₂O₄)₃ · 10 H₂O (where A and H are Nd, Pr and Sm) is reported. The variation of the magnetic moment of these crystals with an external field is studied and the magnetic susceptibility and the effective magnetic moment are calculated. The observed effective magnetic moment of the crystals are in good agreement with those calculated on theoretical grounds.     

Effect of Impurities on the Etching of Sodium Chloride Crystals

The results of a study of the effect of HgCl₂, ZnCl₂, PbCl₂ and CaCl₂ on the surface micro-morphology and kinetics of etching of {100} planes of NaCl crystals in methanol and ethanol are described and discussed. It was found that addition of an impurity to the solvent leads to the formation of contrasting dislocation etch pits, and that the overall dissolution rate in a solvent decreases with an increase in additive concentration. In the case of HgCl₂ impurity added to methanol terraced etch pits are observed, but their terracing behaviour diminishes with the increasing impurity concentration.     

Etching Studies on Potassium Dichromate (KBC) Crystals

An investigation of the revelation of dislocations in potassium dichromate (KBC) crystals by etching in water, alcohls, and in inorganic acids mixes with acetone and alcohols is carried out. It is observed that several solutions including water, alcohols and organic acids produce disslocation etch pits on various faces of KBC crystals. Observations of twining and dissolution anisotorpy of (001) faces are also presented. Etching of (001) cleavage faces of same crystals in an etchant composed from 1 part HNO₃ and 3 parts acetone indicates the possible growth of another phase at low temperatures.     

Investigations on Flux Grown Cadmium Sulphide Crystals

Observations on surface microtopography of flux grown CdS crystals are reported. Crystals resulting with cooling rate above 30 °C/hr are reported to be dendritic crystals, while those with cooling rate below 30 °C/hr are reported to be normal crystals like mineral greenockite as reported in standard books on mineralogy. Reasoning for dendritic growth is briefly discussed. Growth patterns and etch pits formation on them are interpreted. It is established that these crystals grow by two-dimensional nucleation mechanism and by spreading and piling of growth layers.     

Dislocation Etching Behaviour of Hydrothermal and Organic Acid Etchants on Natrolite Crystals

Studies on etching of (110) cleavages of natrolite crystals in organic acids (viz., succinic acid, formic acid, and phthalic acid) and stream under high hydrostatic pressures have been made. It is observed that on successive etching of matched cleavages in any of these organic etchants, the pits get bigger both in regard to depth as well as lateral dimensions. Study of matched cleavages etched in any of these etchants indicates 1:1 correspondence of point bottomed pits. Similar results are obtained when matched cleavages are etched by the hydrothermal method. In contrast to such results, it is observed that the etch pit density on a cleavage etched by the hydrothermal method is always higher when compared with its matched cleavage etched in any of the given organic acids. Through a series of experiments, it is found that while corresponding to every etch pit due to any of these organic acids there is a pit due to hydrothermal etching, the converse is not true. It is suggested that the hydrothermal etching is more reliable so far as dislocation density estimation on (101) cleavages of natrolite is concerned.     

Dissolution Kinetics and Etch Pit Morphology of CaF2 Single Crystals

The effect of temperature and concentration of HCl aqeous solutions on the etching behaviour on the face {111} of CaF₂ single crystals is investigated. It has been observed that the shape and the evolution of etch pits and the values of dissolution rate depend on the etching conditions. From the plots of logarithm of dissolution rate versus inverse of absolute temperature the values of activation energy and the pre-exponential factor for the dissolution process are computed. It has been found that the value of the activation energy and of the pre-exponential factor do not remain constant with acid concentration, but exhibit a minimum for the concentration of 6 N.     

Kinetics of Etching of Dislocations in Ferroelectric Ammonium Hydrogen Tartrate Single Crystals

Ferroelectric crystals of Ammonium Hydrogen Tartrate (AHT) have been synthesised using the controlled reaction between NH₄Cl and (CHOHCOOH)₂ by diffusion process in silica gel medium. (010) cleavages of AHT crystals have been etched in 1.0 M and 1.5 M solutions of SrCl₂ solution. The lateral and normal velocities of the growth of pits were measured at different temperatures. The time dependence of the growth of the pit dimensions is found to be linear, whereas the temperature dependence of the growth of pits is found to be exponential, viz. \documentclass{article}\pagestyle{empty}\begin{document}$ \[V = A\,\exp \left({\frac{E}{{kT}}} \right)\] $\end{document}. The dissolution parameters, e.g. the activation energies and the pre-exponential factors for dissolution along the surface and along the dislocation lines have been computed. It is observed that the activation energy (E₁) of dissolution along [001] direction is found to be greater than the activation energy (E_b) along [100] direction and the implications are discussed.     

Kinetics of Dissolution of Calcium Oxalate Calculi in Physiological Solutions Containing Hippuric Acid

An analysis of the thermodynamics and kinetics of crystal growth and dissolution of slightly soluble salts in physiological solutions in the presence of complexing ions is made, accounting for conditions typical in human urine. It is found that hippuric acid, a normal physiological constituent of urine, acts at increased concentrations as a dissolving agent with respect to calcium oxalate and calcium oxalate calculi. The kinetics of dissolution of crystalline calcium oxalate calculi (CaOx) in physiological solutions containing hippuric acid in different concentrations is studied, using the change in the Archimedean weight of samples, immersed in the solution. The analysis of the experimental results enables the determination of the increased solubility of CaOx in the presence of hippuric acid and to characterize quantitatively this substance as a new and very prospective dissolving agent of CaOx calculi in human urine. A discussion is also given on the possible effect of hippuric acid as a neutral regulator of CaOx-supersaturation and crystallization in human and mammal urine.     

On the strengthening of Magnesium Single Crystals by Cadmium

Solid solution hardening in single crystals of magnesium containing cadmium (up to 2.4 at.%) has been investigated in the temperature range 77 K to 295 K. A strong temperature dependence of the critical resolved shear stress, τ₀, is observed below 230 K, while above this temperature τ₀ is temperature independent. At all temperatures τ₀ is found to increase linearly with c^2/3, where c is the concentration (in atomic fractions) of cadmium as solute. The concentration dependence of τ₀ can be explained by the theory of LABUSCH .     

Growth Kinetics and Mechanism of TGS Crystals

The growth kinetics of TGS crystals was studied at high supersaturations under the Curie temperature. The kinetics data proved that the crystal growth was mainly controlled by BCF surface diffusion model. The continuous growth was fitted to the growth rate data of (110) face. Its edge energy, Jackson factor, activity energies, kinetic coefficients were calculated.     

Study of the Growth Morphology of Ammonium Oxalate Monohydrate Crystals Obtained from Aqueous Solutions

The experimental results of the growth morphology of ammonium oxalate monohydrate [(NH₄)₂C₂O₄ · H₂O; AO] single crystals obtained from aqueous solutions at 30 and 40 °C and supersaturation up to 9% are presented. The observations are compared with the theoretical morphology predicted by PBC analysis and Braivais-Donnay-Harker law.