Etching and kinetics of dissolution of Mo1−x WxSe2 single crystals

The dislocation etchant has been established for single crystals of Mo_1−xW_xSe₂ system. Effect of concentration of an etchant on the etch rate has been discussed. The morphology of etch pits is also described. The etch activation energies for all the compositions of these crystals for a particular concentration (7 N) of an etchant has been found out. The etch activation energies for a particular composition (x = 0.4) of crystals with different concentrations of an etchant have been investigated and the results have been discussed.     

Etching studies of tungstenite (WS2) crystals in acids

Single crystals of tungstenite (WS₂) have been grown by direct vapour transport or sublimation method. An etchant capable of revealing the sites of dislocations has been developed. The effects of etching time, etchant concentration and temperature on the selective etch rate have been worked out. It is seen that the etch rate is independent on time, but is influenced by the temperature and concentration of the etchant. The values of activation energy determined from the etch rate against temperature plots established that the process of etching in chromic acid is chemical reaction rate controlled. The effect of addition of HF to the above etchant has led to some further interesting results.     

Etch features in bismuth germanate (Bi4Ge3O12) single crystal

The three component mixture of HCl: HNO₃: CH₃COOH in the volumetric proportion of 3: 1: 4 was found to be a satisfactory etchant for various surfaces of Bi₄Ge₃O₁₂. The correlation between etch pits and dislocations was established by means of repeated etching and optical observations.     

Growth and Dislocation Etching of InBi0.8Sb0.2 Single Crystal

InBi_0.8Sb_0.2 single crystals have been grown by zone melting method. The freezing interface temperature gradient of 3 °C/cm has been found to yield the best quality crystals obtainable at growth velocity 1.0 cm/hr. Traingular features have been obtained on the free surface of the as grown crystal. A new dislocation etchant based on nitric acid has been found to give reproducible etch-pitting on the cleavage surface. Standard tests for a dislocation etchant have been carried out and results are reported.     

AES Investigation of Chemical Treatment Effect on CdTe and CdZnTe Surfaces

{211} polar surfaces of a Cd_0.96Zn_0.04Te chemically treated single crystal were investigated by Auger electron spectroscopy depth profiling. It was found that the use of a discriminatory “black-white” etchant causes the formation of a thick layer with cadmium having been severely depleted at both (211) A and (211) B faces. The thickness of this layer is larger by a factor of 2 for the matt black face then for the bright and reflecting face. AES investigation of chemical treatment influence on (100) CdTe epitaxial layer surface has been performed. It has been established that etching in Br/methanol solution removes S, Cl and O containing contaminants from the surface, but simultaneously forms a Cd-depleted layer.     

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.     

Growth and dissolution of BaFCl crystals

Single crystals of BaFCl have been growth by flux technique using BaF₂ and BaCl₂. Etching with formic acid revealed dislocation etch pits on (001) cleavage faces of the crystals, at room temperature. The influence of etching parameters such as undersaturation, temperature and concentration of poison in the etchant is studied. Decreasing the undersaturation of formic acid by reducing the percentage of water and increasing the temperature of the etchant were found not to have any effect on the morphology of etch pits. However, as the CdCl₂ poison concentration is gradually increased, the orientation of the pits change from 〈100〉 to 〈110〉 at high concentration.     

Growth and etching of calcite crystals

Single crystals of calcite (CaCO₃) have been grown by the method employed by GRUZENSKY , using an aquoeus solution of CaCl₂ and a solid (NH₄)₂SO₃, The chemical reaction takes place according to the following equation: CaCl₂ + (NH₄)₂SO₃ CaCO₃ + 2 NH₄Cl The crystals grown by this method are about 0.2 to 0.8 mm in edge dimensions. Synthetic calcite crystals have been cleaved along (100) planes and the cleavage surfaces have been studied by multiple beam interferometry. The interferograms have revealed that the cleavages are quite flat. The cleavage faces have also been chemically etched and the etch patterns studied optically. By etching a cleavage successively for three different periods it was found that the bottoms of the point-bottomed pits follow a linear etch path. By etching a cleavage pair, one face in one etchant and the other face in a different etchant and by comparing the etch patterns produced, before and after polishing a cleavage face it has been shown that the etch pits nucleate at the sites of dislocations in the crystal. The etch patterns have also been compared with those produced on the cleavage faces of natural crystals. The density of dislocations in the syntheitc calctie crystals was generally less than the density of dislocations in the natural calcite crystals. The implications have been discussed.     

Structure,Crystallization and Electrochemical Corrosion Behaviour of Amorphous Cu66Ti34 Alloy

The effect of thermally induced structural changes on electrochemical corrosion behaviour of amorphous Cu₆₆Ti₃₄ alloy is studied using electrochemical potentiodynamic polarization technique in conjunction with XRD, TEM and DTA. The heat treatment has been carried out at two temperatures: 300 °C — where the process of structural relaxation is only possible and 500 °C — where crystallization of the alloy occurs. As model corrosive media 1N H₂SO₄ and 1N HNO₃ solutions are used. The short range order of the amorphous samples is studied with the total pair correlation function. Some changes of the interatomic distances and a tendency to increasing of the density of the amorphous structure after low temperature heating are established. The crystallization leads to formation of Cu₃Ti₂, β-Cu₃Ti and Cu₂Ti crystalline phases. It has been found that structural relaxation may have a beneficial effect on the susceptibility of the amorphous alloy to passivation, while crystallization lowers considerably its corrosion resistance.     

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.     

Differential etching of chalcogenides for infrared photonic waveguide structures

Chemical etching rates for two different chalcogenide glass compositions, As₄₀S₆₀ and As₂₄S₃₈Se₃₈, were studied using sodium hydroxide based etchant solutions. Etching was performed using a variation of standard photolithographic masking and wet-etching techniques. Variations in etch rate with NaOH concentration and glass composition were observed. The depth of etch was characterized using an optical profilometer. Etch rate differences as large as three orders of magnitude between these two glasses were observed at low NaOH concentration (0.053 M). We present a single variable etch rate curve of etch depth per time (nm/s) versus NaOH overall solution concentration (in M) for these two different chalcogenide glasses. This technology shows promise for fabricating photonic structures and has potential applications in fabricating novel photonic bandgap structures that will function in the long-wave infrared (LWIR) regime.     

Properties of Surface Layers in Single Crystals of Calomel

Within the framework of the systematic study of differences in physical properties of the calomel (Hg₂Cl₂) single crystals before and after surface treatment, the differences in the microhardness, the residual stress in the surface layer, and the chemical etching rate were investigated on the (110) and (001) crystal faces. It has turned out that Beilby layer (B-layer) formed during surface treatment is softer, and, on the cleavage surface is harder than the underlying bulk material. The thickness of the B-layer in these samples ranged between 5 anf 7 m̈m. Further it has been found that a tensile residual stress s̀ = 14 Nm^-2 prevails in the surface layer of a polished (110) face to a depth of 70 m̈m and, after grinding, to a depth from 90 to 200 m̈m. On the other hand, a compressive stress has been found in the natural cleavage plane. Experiments on etching the Hg₂Cl₂ crystals with HCl + HNO₃ etching solution have shown that the rate of chemical etching is 2 to 4 times larger on the (110) face than on the (001) face. Differences are very strongly dependent on the experimental conditions and on the real structure of a given surface.     

Chemical durability of Na2OCaOSiO2 glasses in acid solutions

The resistance of Na₂OCaOSiO₂ glasses to acid solutions has been studied. The compositions studied were Na₂O · 4SiO₂, Na₂O · x CaO · (4?x) SiO₂ and a common electrode glass containing 22.63Na₂O and 5.58 CaO, mol%. The reaction was made at 40°C for about 3 h in 1N solutions of HCl, HNO₃ and H₂SO₄. Powdered glass samples were used and the reaction was followed by analyzing the solution for soda, lime and silica.The extraction rates of each constituent were measured. The effect of acid concentration was also studied for each glass using 10^?3–10N solutions of the three acids for a fixed time. The quantity of calcium extracted increased slowly at first with increasing calcium content in the glass, but rapidly when the lime content exceeded ≈10 mol%. Above this concentration, both calcium and sodium appears to pass into solution in the same proportion in which they are present in the glass. The extraction rate was found to depend on the type and concentration of the acid used, being least in H₂SO₄ and much higher and almost equal in both HCl and HNO₃.An attempt was made to correlate the results of decomposition of the soda-lime-silica glasses to their membrane potentials in acidic solutions.     

Growth and characterization of InxBi2−xTe3 single crystals

The V–VI group narrow band gap compounds are known to have important photoconductivity and thermoelectric properties. Among these, Bi₂Te₃ is the most potential material for thermoelectric devices having a direct band gap of 0.16 eV. There has been ample study reported on crystal growth and polycrystalline thin films of both pure and indium doped Bi₂Te₃ pertaining to its basic semiconducting, optoelectronic and thermoelectric properties. It has been shown that on exceeding certain limiting concentration of indium in Bi₂Te₃, the conductivity changes from p-type to n-type. However, there is hardly any work reported in literature on crystal growth, dislocation etching and optical band gap of In_xBi_2?xTe₃ (x=0.1, 0.2, 0.5) single crystals. The authors have grown their single crystals using the zone melting method. The freezing interface temperature gradient of 70 °C/ cm^?1 has been found to yield the best quality crystals obtainable at the growth rate of 0.4 cm/h. The as-grown crystals have been observed to exhibit certain typical features on their top free surfaces. The crystals have been characterized using XRD technique. A chemical dislocation etchant has been used for estimating perfection in terms of dislocation density in the crystals. The optical absorption was measured in the wave number range 500 to 4000 cm^?1. The transitions in all the cases were observed to be allowed direct type. The detailed results are reported in the paper.     

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.     

Growth,Structural and Microtopographical Studies of Calcium Iodate,Monohydrate Crystals Grown in Silica Gel

Single crystals of calcium iodate, monohydrate [Ca(IO₃)₂, H₂O] have been grown by gel technique. Prismatic, prismatic pyramidal, needle shaped and hopper crystals were obtained. These crystals were also grown by doping impurities such as copper and iron. Kinetics of growth parameters was investigated. Structural analysis was carried out by using X‐ray powder diffraction method. Microtopographical study of the habit faces, such as prismatic and pyramidal, of as grown crystals was carried out. Some surface structures are reported. These crystals were etched by various etchants and appropriate etchant is reported.     

H3PO4 — etching of {001}-faces of InP, (GaIn)P,GaP, and Ga(AsP)

This paper shows that hot H₃PO₄ is a suitable etchant for the production of dislocation etch pits on {001}-InP, (InGa)P, GaP, Ga(AsP), and {111}-GaP. The effects upon etch pit morphology of the misorientation of samples and the type of dislocations are investigated in detail.     

Investigation of structural transformations in pyridine nitrate at low temperatures and high pressures

The structural transformations in pyridine nitrate PyHNO₃ (C₅D₅NHNO₃) are investigated by neutron diffraction in the temperature range 16–300 K at normal pressure and in the high-pressure range 0–3.5 GPa at room temperature. A new high-pressure phase with a monoclinic structure (space group P2₁/c) is revealed in the PyHNO₃ compound at pressures P > P_tr ～ 1 GPa. The geometry of hydrogen bonds and the coordination of the PyH⁺ and NO ₃ ^? ions in the structure of the PyHNO₃ compound are studied as a function of the temperature and pressure.     

Potentiodynamic polarization studies on amorphous Zr46.75Ti8.25Cu7.5Ni10Be27.5, Zr65Cu17.5Ni10Al7.5, Zr67Ni33 and Ti60Ni40 in aqueous HNO3 solutions

Potentiodynamic polarization studies were carried out on virgin specimens of Zr-based bulk amorphous alloys Zr_46.75Ti_8.25Cu_7.5Ni₁₀Be_27.5 and Zr₆₅Cu_17.5Ni₁₀Al_7.5, and conventional-type binary amorphous alloys Zr₆₇Ni₃₃ and Ti₆₀Ni₄₀ in solutions of 0.2 M, 0.5 M and 1.0 M HNO₃ at room temperature. The values of the corrosion current density (I_corr) for the bulk amorphous alloy Zr_46.75Ti_8.25Cu_7.5Ni₁₀Be_27.5 were found to be comparable with those of Zr₆₅Cu_17.5Ni₁₀Al_7.5 in 0.2 M and 0.5 M HNO₃, but the value of I_corr for the former was almost three times more than that of the latter in 1.0 M HNO₃. In the case of conventional binary amorphous alloys, Ti₆₀Ni₄₀ showed lower value of I_corr as compared to Zr₆₇Ni₃₃ in 0.5 M and 1.0 M HNO₃ and a comparable value of I_corr in 0.2 M HNO₃. In general, the binary Ti₆₀Ni₄₀ displayed the best corrosion resistance among all the alloys in all the cases and the corrosion current density (I_corr) for all the alloys was found to increase with the increasing concentration of nitric acid. It is noticed that the bulk amorphous alloys do not possess superior corrosion resistance as compared to conventional binary amorphous alloys in aqueous HNO₃ solutions. The observed differences in their corrosion behavior are attributed to different alloy constituents and composition of the alloys investigated.     

Causes of the formation of etch pits of different size and morphology on {100} faces of CsI crystals

Causes of the formation of dislocation etch pits of different shape and size on {100} faces of cesium iodide crystals by an etchant composed from 25–35 mg/1 CuCl₂ · 2 H₂O in 96% ethanol are studied. It is shown that one of the reasons of the change in the morphology of etch pits is the segregation of excessive iodine at dislocations.