Etching studies on {100} faces of MgO crystals in H3PO4 acids

The effect of temperature (20–100°C) and concentration of H₃PO₄ etching solutions on the etching behaviour of {100} faces of MgO crystals in investigated. It is observed that the formation of etch pits and pyramids and the values of dissolution rate depend on the etching conditions. The density of the pyramids in 33.08 N H₃PO₄ solution was found to be maximum at 20°C. From the plots of logarithm of dissolution rate versus inverse of absolute temperature, the values of activation energy and pre-exponential factor for the dissolution process are computed. It is found that for H₃PO₄ concentrations upto 22.05 N the value of the activation energy remains constant but that of pre-exponential factor increases with acid concentration. At higher concentrations, the activation energy decreases. The results are systematically described and discussed.     

Current-controlled growth,segregation and amphoteric behavior of Si IN GaAs from Si-doped solutions

An unusual behavior of the growth kinetics and the segregation of Si during current-controlled LPE of GaAs is reported. The growth velocity (for a fiven current density) decreased by about two orders of magnitude from a value higher to a value smaller than that found in undoped solutions as the growth temperature decreased from 975 to 850°C; at 825°C dissolution of the substrate took place and a reversed current polarity was required for growth. At the same temperature range the growth velocity from undoped solutions decreased by a factor of only 3. Similarly, a two orders of magnitude decrease of the silicon segregation coefficient was observed for the same temperature change compared to changes by a factor or two in the thermally grown layers. For a given growth temperature (900°C) a change in conductivity from p- to n-type took place in the grown layers as the current density was increased. These findings were accounted for with a qualitative model based on the presence in the solution of different charged complexes containing silicon with different electromigrating characteristics. According to this model, the behavior of Si as p- and as n-type dopant (amphoteric dopant) in GaAs is related to the existence in the solution of the different charged complexes containing silicon.     

Thermal etch pits on calcite cleavages

Thermal etch pits are observed on calcite cleavages when the latter are kept at 520°C to 560°C in atmosphere for some hours. Boundaries of the above etch pits are circular, hexagonal and rhombic and they are dependent upon the temperature of etching. Different etch pit morphology is observed in chemical etching on calcite cleavages. It is conjectured that origin of thermal etch pits are different from that of chemical etch pits.     

Influence of selenization temperature on synthesis,morphology, and properties of nanostructured CoSe2 films

Nanostructured cobalt selenide (CoSe₂) thin films were deposited on a glass substrate using the selenization of Co films at different selenization temperatures (300 °C, 400 °C, and 500 °C) in a pure Se vapor for two hours. The morphology and structure of the as‐deposited films shows that the film morphology and crystallinity are affected by the selenization temperature. Increasing the selenization temperature from 300 °C to 400 °C and 500 °C results in a change in the surface and cross sectional morphology. At 300 °C, the Co films have an almost amorphous structure, while at temperatures of 400 and 500 °C, the Co films have a crystalline nanostructure with bilayered morphology. Optical analyses of the CoSe₂ films at 500 °C show a large absorption (α > 1.0 × 10⁵ cm⁻¹) and a direct band gap (∼1.0 eV).     

Investigation of the structure of nanocrystalline refractory oxides by X-ray diffraction,electron microscopy,and atomic force microscopy

The structures of nanocrystalline fibrous powders of refractory oxides have been investigated by different methods: determination of coherent-scattering regions, scanning electron microscopy (SEM), transmission electron microscopy (TEM), and atomic-force microscopy (AFM). The sizes of nanograins of different crystalline phases of refractory metal oxides have been determined during the formation of these nanograins and the dynamics of their growth during heat treatment in the temperature range 600–1600°C has been studied. The data on the structure of nanocrystalline refractory oxide powders, obtained by different methods, are in good agreement. According to the data on coherent-scattering regions, the sizes of the ZrO₂ (Y₂O₃) and Al₂O₃ grains formed are in the range 4–6 nm, and the particle sizes determined according to the TEM and AFM data are in the ranges 5–7 and 2–10 nm, respectively. SEM analysis made it possible to investigate the dynamics of nanoparticle growth at temperatures above 1000°C and establish the limiting temperatures of their consolidation in fibers.

     

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.     

Study of the Electrophysical Properties of Lithium Metaniobate Single Crystals with Deliberately Introduced Dopants

The effect of the dopants Ti, V, Cr. Mn, Ni, Ho, Er, Tm deliberately introduced into the melt of Czochralski grown lithium niobate (LiNbO₃) crystals on their electrophysical properties — electric conductivity, dielectric permittivity and loss tangent — has been studied. The characteristics were measured at temperatures up to 1200°C. The mechanism of dopant introduction into the crystalline lattice and the nature of electric conductivity of LiNbO₃ at varius temperatures are discussed.     

An X‐ray powder diffraction study of the microstructural evolution on heating 3:2 and 2:1 mullite single‐phase gels

Single‐phase gels with compositions 3Al₂O₃·2SiO₂ and 2Al₂O₃·SiO₂ were prepared by gelling mixtures of aluminium nitrate and tetraethylorthosilicate. Gels were fast heated at different temperatures between 900°C and 1600°C. The phase transformation and microstructural changes of both mullite precursor gels over the temperature range were followed by X‐ray powder diffraction (XRD), lattice parameter determination (LP), and scanning and transmission electron microscopies (SEM and TEM). The distribution of crystallite sizes and strains were determined by linewidth refinements of X‐ray diffraction patterns using the integral breadth method of Langford and the Warren‐Averbach analysis. XRD of both heated gels showed the formation of crystalline mullite single phase. Some amount of glassy phase coexisted with mullites at low temperatures, i. e. below 900°C. The compositional range of mullites formed on heating gels at temperatures between 900°C and 1600°C was dependent on the starting nominal composition of gels. SEM and TEM micrographs of both heated gels below 1200°C showed the formation of small, discrete, prismatic, well‐shaped nanocrystals in a very ordered arrangement. The size of these nanocrystals was dependant on the nominal composition of gels and increased on rising the heating temperature of gel precursors. The microstructural features obtained from linewidth refinement results of X‐ray diffraction patterns also allowed to suggest the formation of prismatic a little elongated nanocrystals at temperatures below 1200°C. Microstrain values were small and only displayed a relatively significant value for mullites processed at 900°C. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Orientational order parameter determination of the liquid crystal: 4 propyl paraethoxy phenyl cyclohexyl carboxylate by X-ray studies

The compound 4 propyl paraethoxy phenyl cyclohexyl carboxylate (code name D302) is a member of a homologous series and exhibits liquid crystalline (nematic) behavior in the temperature range 48°C to 78°C. An experiment has been set up for conducting X-ray studies of liquid crystal samples at various temperatures. From X-ray studies conducted at different temperatures on D302, the apparent molecular length and inter-molecular distance and their temperature dependence have been determined. The variation of the orientational-order parameter <P₂>_xray has been determined from the intensities of the scanned X-ray photographs taken at different temperatures. The results have been compared with the<P₂>_opt values obtained by us from birefringence studies.     

Effect of Li2CO3 on formation temperature of hBN by modified O'Connor model

This study is interested in the effect of lithium carbonate on the formation of hexagonal boron nitride (hBN) by means of the available experimental methods including TGA, XRD, FTIR, SEM and HR‐TEM. hBN samples were synthesized at the 1450 °C with different molar ratios of lithium carbonate by modified O'Connor routine. The crystalline hBN formation tended to improve with the increment of the Li₂CO₃ concentration level (especially after more 20 %). The dopant quantity decreased the residual stresses due to the presence of possible relaxation mechanisms along with the nanocrystal structure, even favored by XRD experimental findings regarding the enhancement of crystal plane alignments, crystallite sizes and lattice parameters. As for the FTIR surveys, the Li₂CO₃ foreign impurities strengthened more and more the covalent bonds between boron and nitrogen atoms. At the same time, the samples with 40 % lithium carbonate were annealed at the varied temperatures of 1000, 1150, 1300 and 1450 °C to determine the optimum annealing temperature. The XRD+FTIR investigations indicated that the degree of hexagonality improved with the increased annealing temperature. Similarly, the surface morphology confirmed not only the formation of regularity and flaky hexagonal BN structures, but also the strengthening of covalent bonds between the atoms.     

Influence of TiO2 and ZnO nanoparticles on orientational order parameter of LC compounds – An optical study

In the present paper ZnO and TiO₂ nano particles are dispersed in Cyano-biphenyl liquid crystalline compounds (n?=?7, 12). The thermal polarizing microscopy and differential scanning calorimetry techniques are employed to measure the transition temperatures. The Nematic transition temperatures are decreased by 2.34?°C and 1.53?°C in Heptylbiphenyl, 1.07?°C and 1.12?°C in dodecylbiphenyl compounds due to the dispersion of nanoparticles. The refractive indices and Newton’s rings methods are exploited to measure birefringence at different temperature in nematic phase. The orientational order parameters are estimated by finding the birefringence in perfect order. The orientational order parameter is increased in nanoparticles dispersed liquid crystals when compare to pure liquid crystalline compounds.     

Electrical properties of nitrogen implanted GaSe single crystal

N‐implantation to GaSe single crystals was carried out perpendicular to c‐axis with ion beam of 6 × 10¹⁵ ions/cm² dose having energy values 30 keV and 60 keV. Temperature dependent electrical conductivities and Hall mobilities of implanted samples were measured along the layer in the temperature range of 100‐320 K. It was observed that N‐implantation decreases the resistivity values down to 10³ Ω‐cm depending on the annealing temperature, from the room temperature resistivity values of as‐grown samples lying in the range 10⁶‐10⁷ Ω‐cm. The temperature dependent conductivities exhibits two regions (100‐190 and 200‐320 K) with the activation energies of 234‐267 meV and 26‐74 meV, for the annealing temperatures of 500 and 700 °C, respectively. The temperature dependence of Hall mobility for the sample annealed at 500 °C shows abrupt increase and decrease as the ambient temperature increases. The analysis of the mobility‐temperature dependence in the studied temperature range showed that impurity scattering and lattice scattering mechanisms are effective at different temperature regions with high temperature exponent. Annealing of the samples at 700 °C shifted impurity scattering mechanism toward higher temperature regions. In order to obtain the information about the defect produced by N‐implantation, the carrier density was analyzed by using single donor‐single acceptor model. We found acceptor ionization energy as E_a = 450 meV, and acceptor and donor concentration as 1.3 × 10¹³ and N_d = 3.5 × 10¹⁰ cm⁻³, respectively. (© 2003 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Influence of synthesis temperature on the structural characteristics of mesoporous silica

The characteristics of mesoporous silica prepared at different temperatures and the behavior of this system relating to the microencapsulation of a model drug were investigated. The preparation of mesoporous materials was initiated with the dissolution of a surfactant in distilled water and strong acid medium. After this, tetraethyl orthosilicate was added under agitation. The mixture was heated for 24 h at the synthesis temperature (60 °C, 80 °C, 100 °C and 130 °C) under static conditions. The surfactant was removed by calcination, which was carried out by increasing the temperature to 550 °C for 5 h. Atenolol was used as a model drug to study the kinetics of drug delivery. It could be observed that aging materials at higher temperatures presents no microporosity, and this influences the control of the release of the model drug.     

Studies on configuration of dislocations and dissolution of cleavage steps of magnesium orthosilicate crystals

(010) cleavages of magnesium orthosilicate crystals have been etched in concentrated hydrochloric acid vapour at room temperature, in the melts of potassium and sodium hydroxides at 400°C, and also in the aqueous solutions of KOH, NaOH, and their mixtures at boiling points. The configuration of isolated dislocations have been described. Bent, branched, stepped and the inclined nature of dislocation lines have been investigated. The effect of dissolution ledges on the size and morphology of etch pits has been discussed. The implications are discussed.     

Solubility of tin dioxide in soda-lime silicate melts

In the present work, the solubility of tin dioxide is assessed as a function of time, temperature and basicity in simple ternary glasses: NC3S, NC4S, NC5S and NC6S (N: Na₂O, C: CaO, S: SiO₂). An increase of silica contents in the glass composition leads indeed to a decrease of the glass basicity. First, a kinetic study of the dissolution has been performed. Consequently, the solubility limits of tin dioxide have been determined after 2 h of heat treatment: this duration is long enough to reach the dissolution equilibrium, and short enough to limit the sodium oxide losses in the melt at high temperatures. Nevertheless the specific case of the most acid glass has been underlined, as its higher viscosity implies longer heating times. At equilibrium state, SnO₂ solubility depends on the temperature (Arrhenius law) and on the glass basicity. In the 1200 °C–1400 °C temperature range, in these soda–lime glasses, the solubility of tin dioxide is between 1.3 and 2.1 at.% Sn and the temperature dependence of solubility exhibits a single mechanism of dissolution. Furthermore, the basicity dependence of the solubilization process is also discussed, and the presence of another oxidation state of tin (Sn^II) is thus proposed.     

Structure and properties of niobium-doped potassium titanyl phosphate crystals

A series of potassium titanyl phosphate crystals, KTiOPO₄, with various concentrations of niobium dopant has been grown, and some of their physical properties and structural characteristics have been studied. The incorporation of a small amount of niobium results in considerable changes in the electrical conductivity of KTP: Nb crystals and the temperature of the ferroelectric phase transition. Thus, the presence of 3–4 at. % of niobium results in an increase of conductivity by more than an order of magnitude, whereas T _C decreases from 930 to 620°C. The X-ray diffraction study of the crystals has been performed at room temperature; the neutron diffraction analysis was made at temperatures of 20, 330, and 730°C. It was revealed that two crystallographically independent positions are statistically (by 90%) occupied by potassium cations, which results in the concentration of potassium atoms in the structure higher than it was expected from the condition of preservation of crystal electroneutrality. At high niobium concentrations, the monoclinic compound of the composition K₂TiNb₂P₂O₃ is formed.     

On the influence of the pre-ageing temperature on the reversion behaviour of an Al-4.5 at.% Zn-2.0 at.% Mg alloy

The reversion behaviour of an Al-Zn(4.5)-Mg(2.0) alloy was investigated by SAXS in dependence of the pre-ageing temperature, T_pre, (ranging between 60°C and 100°C) and the reversion temperature, T_rev, (120°C till 200°C) starting with precipitates having a radius of (1.2 ± 0.1) nm and (1.5 ± 0.1) nm, respectively. During the reversion treatment applied up to T_rev = 160°C three stages could be distinctly distinguished, namely the dissolution of unstable zones, the growth of the stable zones on the expense of the dissolved one, and at last the coarsening of the precipitates by the OSTWALD-ripening process, where the structure changes become independent of the pre-history (start radius). The change of T_pre from 60°C to 100°C does not influence the structure changes going on at T_rev, that means between 60°C and 100°C the same type of G.P. zones grows.     

Electron microscopic study of the crystallization kinetics of the amorphous alloy Pd80Si20

Amorphous Pd₈₀Si₂₀ foils, prepared by the rapidly rotating mill device method, were quenched from the liquid state, temperatures of the melt being 1100, 1500 and 1600°C. The foils were linearly heated at a rate of 1.5 °C/min over a temperature range of 20–600°C. The structural changes were observed by methods of optical microscopy, transmission electron microscopy and electron diffraction. The beginning of crystallization of the amorphous phase is affected by the temperature of the melt. The crystallization of the amorphous phase was terminated at a temperature of 390–400°C. Silicides Pd₉Si₂ and Pd₃Si, the morphology of which varied with temperature, were formed during crystallization.     

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.     

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.