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.     

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.     

Gross dissolution of natural rhombohedral calcite crystal in nitric acid in dynamic etching condition

Gross dissolution of natural rhombohedral calcite crystal in concentrated nitric acid in dynamic etching condition is studied and interesting results are obtained. The gross dissolution is maximum at 6 molar concentration of nitric acid in dynamic etching condition which is similar to static etching condition. The activation energy and pre-exponential factor is calculated for different concentrations (range 3.0 to 7.5 molar) of nitric acid. The values of activation energy and pre-exponential factors increase as concentration increases in dynamic etching, and it is more than static etching. The higher values of gross dissolution, activation energy, and pre-exponential factor in dynamic etching condition is explained.     

Kinetics of dissolution of BaF2 crystals

Kinetics of dissolution of BaF₂ crystals in aqueous solutions of HNO₃ and HCl acids have been studied for the different acid concentrations in the temperature interval 20–100°C. It is established that this process is diffusion controlled. Activation energy is constant, but pre-exponential factor is dependent on the concentration of the acid.     

Gross dissolution of natural rhombohedral calcite crystal in nitric acid

Gross dissolution of natural calcite crystal in concentrated nitric acid give interesting results. The gross dissolution is maximum at 6 M concentration of HNO₃. The activation energy increases as the concentration of HNO₃ increases. Similarly pre-exponential factor is also increases as concentration increases. The combined effect of activation energy and pre-exponentail factor gives the peak at 6 M concentration of nitric acid.     

Kinetics of dissolution and calcium sulphate growth on {111} faces of calcium fluoride crystals etched in sulphuric acid

The effect of temperature (20–100°C) and concentration of H₂SO₄ etching solutions on the etching behaviour of {111} faces of calcium fluoride crystals is investigated. Whisker growth of calcium sulphate is observed. Temperature of the etchant also has an effect on their formation. The kinetics of dissolution at the sites of dislocations and general dissolution have been studied. Studies on induction period and its dependence on the concentration of the acid and temperature are also described. It is observed that: (i) at low and high acid concentrations, the dissolution is diffusion controlled while it is predominantly reaction-rate controlled in the intermediate concentration range and (ii) the growth rate of calcium sulphate whiskers decreases with time and their induction period decreases with increase in the temperature of the solution. The implications are discussed.     

Preparation of sapphire substrates for Gas phase GaN epitaxial processes

After mechanical treatment the surface of α-Al₂O₃ plates with orientation (0001) for GaN epitaxy was polished in a mixture of H₃PO₄ and H₂SO₄. The best mixture H₂SO₄/H₃PO₄ and the best temperature for polishing were chosen. — The quality of the polished substrates was determined by structural etching in KOH. The optimum structural etching time was 3 min. at 320°C. The thickness of the layer which should be removed after mechanical treatment and the dislocation density of the starting material for GaN epitaxy was determined.     

Effect of CuCl2 · 2 H2O additive in ethanol on dissolution and selective etching behaviour of CsI crystals

{100} faces of Kyropoulos grown CsI crystals had undergone a selective etching, and CsI spheres of ∼ 10 mm in diameter were subjected to dissolution at room temperature for 10 hrs. The results are: pure ethanol does not bring about any anisotropy in dissolution, addition of CuCl₂ · 2 H₂O leads to an anisotropy in dissolution of CsI, change in CuCl₂ · 2 H₂O concentration changes etch pit morphology, whereas the dissolution form does not depend on additive concentration. With increasing CuCl₂ · 2 H₂O etching rates at first increase, later decrease, dissolution rates linearly increase.     

Chemical etching of sapphire

The results on etching of sapphire substrates with different orientation are discussed. High temperature etching in hydrogen stream and etching in the mixture of H₃PO₄ and H₂SO₄ were used in experiments. The relation of etching rates for different sapphire orientations were established in both cases. The conditions of nonselective etching were determined.     

Ätzuntersuchungen an Verneuilspinellen

The quality of (100), (110), and (111) oriented spinel substrates in the composition range MgO × 3.3-3.5 Al₂O₃ is examined by chemical etching. The investigation shows that KHSO₄, H₃PO₄, and Na₂B₄O₇ cause dislocation etch pits on (111), and only KHSO₄ on (100) faces. The little rod-shaped defects revealed by etching on (100), (110), and (111) faces were found to be Al₂O₃ precipitates.     

Production of complex rubidium and cesium hydrogen sulfate‒phosphates

The solubility in the CsH₂PO₄?CsHSO₄?H₂O system at different temperatures (25, 50, and 75°C) is studied and the phase equilibria in the Rb₃H(SO₄)₂?RbH₂PO₄?H₂O system under isothermal conditions (at 25°C) are analyzed. The temperature and concentration conditions for forming Rb₂(HSO₄)(H₂PO₄), Rb₄(HSO₄)₃(H₂PO₄), Cs₄(HSO₄)₃(H₂PO₄), Cs₃(HSO₄)₂(H₂PO₄), Cs₂(HSO₄)(H₂PO₄), and Cs₆H(HSO₄)₃(H₂PO₄)₄ compounds (the latter has been obtained for the first time) are determined. The conditions for growing large single crystals of complex acid rubidium and cesium salts are found.     

Some characteristic features of formation of composite material based on KDP single crystal with incorporated Al2O3·nH2O nanoparticles

Composite material based on KDP (KH₂PO₄) crystal matrix with incorporated aluminum oxyhydroxide Al₂O₃·nH₂O nanoparticles is obtained and peculiarities of the formation of KDP:Al₂O₃·nH₂O composite structure are studied by selective etching, optical and scanning electron microscopy. Influence of the nanoparticles on the formation of defect subsystem is analyzed. The obtained material is shown to have a zonal structure with a period independent of the concentration of nanoparticles. By means of FTIR‐spectroscopy, interaction of nanoparticles with KH₂PO₄ solution is studied. A model of the capture of nanoparticles by the {100} KDP crystal face is proposed.     

Crystallization of Calcium and Magnesium Phosphates from Solutions of Medium and Low Concentrations

Calcium phosphates are precipitated at 25 °C from solutions of medium (Ca = P = 50 mM) and low (Ca = P = 10 mM) concentrations in the presence of magnesium. Experiments are also performed with solutions in which Ca + Mg = P = 50 mM and Ca + Mg = P = 10 mM. An amorphous calcium phosphate, Ca₃ (PO₄)₂ · nH₂O, and brushite, CaHPO₄ · 2 H₂O, are the phases first nucleated. The phases occurring one year later are brushite in the more concentrated solutions, hydroxyapatite (Ca₅OH(PO₄)₃) and whitlockite (Ca₉MgH(PO₄)₇) in the others. Octacalciumphosphate, Ca₈H₂ (PO₄)₆ · 5H₂O, occurs as transitory phase. The effects of concentration, pH, supersaturation and magnesium on the precipitation and evolution of calcium phosphates, and the conditions for phase stability are discussed.     

Investigations on the HCl gas-phase etching of differently doped and oriented gaas crystals

The influence of the orientation and dopant on the dissolution of GaAs surfaces in H₂–HCl gas mixtures has been investigated. For this purpose the etch rates, temperature regions of different etching behaviour and the corresponding activation energies were determined for temperatures ranging from 540 °C to 945 °C. The etch rate anisotropy is correlated with the three existing temperature regions of different dissolution mechanisms. The orientation-dependent transition temperatures are influenced by the dopant and can be related to the thermodynamic data of the dopant.     

Kinetics of etching of dislocations in magnesium orthosilicate single crystals

Cleavages of undoped, doped and natural magnesium orthosilicate crystals have been simultaneously etched in concentrated hydrochloric acid solution. The lateral and normal velocities of the growth of pits were then measured at different temperatures for the varieties of crystals. The time dependence of the growth of pit dimensions is found to be linear, while the temperature dependence of the growth of pits is found to be exponential, viz. V = A exp (− E/kT). The dissolution parameters, e. g. the activation energy (E) and the pre-exponential factor (A) for dissolution along the surface and along the dislocation lines have been computed. It is observed that: (1) the activation energy (E₁) of dissolution along [001] direction is found to be greater than the activation energy (E_b) along [100] direction, irrespective of whether the crystals are doped, undoped or natural, and (2) the doping of the crystal with manganese has lowered down the activation energy (E_d) to such an extent that it is less than E₁ and E_b. The implications are discussed.     

Growth of single crystals of NaH2PO4 (NaDP), NaH2PO4 · H2O (NaDP · H2O), and NaH2PO4 · 2H2O (NaDP · 2H2O) sodium dihydrogenphosphate based on the analysis of the Na2O-P2O5-H2O phase diagram

The crystallization conditions for the NaH₂PO₄, NaH₂PO₄ · H₂O, and NaH₂PO₄ · 2H₂O solid phases have been established from the analysis of the phase diagram of solubility of the ternary Na₂O-P₂O₅-H₂O system in the temperature range from 0 to 100°C. Based on these data, the methods for growing sodium dihydrogenphosphate single crystals of the above compositions are developed. The initial components for preparing mother solutions were H₃PO₄ and NaOH solutions taken in certain weight ratios. For the first time, NaDP, NaDP · H₂O, and NaDP · 2H₂O single crystals were grown on a seed by the method of temperature decrease. The habits of the NaDP and NaDP · H₂O single crystals are determined. __________ Translated from Kristallografiya, Vol. 47, No. 5, 2002, pp. 937–944. Original Russian Text Copyright ? 2002 by Soboleva, Voloshin.     

The small anomaly in the temperature dependence of the electrical conductivity of iron containing phosphate glasses

The dc conductivities, σ,of 2 CaO·K₂O·x Fe₂O₃·(5 - x)P₂O₅ glasses (1.4 ?x?2.2) [Fe³⁺/(Fe²⁺ + Fe³⁺)= constant (0.88±0.015)] have been measured in the temperature range from room temperature to 300°C. It has been found that the glass transition temperature increases rapidly with increasing Fe/P ratio and that mixed-valence hopping type electronic conduction takes place in these glasses and every log σ versus 1/T curve obtained shows a change in slope near 100°C, which produces a small increase, 0.14 (for x=2.2)?0.18 eV (for x=1.4), in activation energy at higher temperatures. Both the activation energy and pre-exponential factor for conduction display different dependences on the Fe/P ratio in the different temperature ranges on both sides of the anomaly temperature. These results are discussed in terms of the Mott's equation for small polaron hopping.     

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.     

Growth and shape control of orthorhombic Fe5(PO4)4(OH)3·2H2O single crystalline dendrites

Orthorhombic Fe₅(PO₄)₄(OH)₃·2H₂O single crystalline dendritic nanostructures have been synthesized by a facile and reproducible hydrothermal method without the aid of any surfactants. The influences of synthetic parameters, such as reaction time, temperature, the amount of H₂O₂ solution, pH values, and types of iron precursors, on the crystal structures and morphologies of the resulting products have been investigated. The formation process of Fe₅(PO₄)₄(OH)₃·2H₂O dendritic nanostructures is time dependent: amorphous FePO₄·nH₂O nanoparticles are formed firstly, and then Fe₅(PO₄)₄(OH)₃·2H₂O dendrites are assembled via a crystallization-orientation attachment process, accompanying a color change from yellow to green. The shapes and sizes of Fe₅(PO₄)₄(OH)₃·2H₂O products can be controlled by adjusting the amount of H₂O₂ solution, pH values, and types of iron precursors in the reaction system.     

Präparation und Charakterisierung von Granateinkristalloberflächen

The preparation of single crystal substrates of gadolinium gallium garnet as a typical example of a brittle material is described. The mechanical polishing with iron oxide and diamond as well as a mechano-chemical polishing procedure basing on SiO₂/H₃PO₄ are investigated. Working damage is investigated by X-ray topography, double crystal spectrometry and selective etching. Results are discussed in the frame of a brittle fracture model of the abrasion process.