• Volume 8, Issue 4

      October 1986,   pages  439-542

    • Liquid phase epitaxy growth of GaAs: Si by temperature difference method

      C C Wei Y K Su C C Chang S C Lu

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      The LPE growth of a horizontal sliding system by temperature difference method is used to grow single and multiple layers of GaAs compounds from dilute solution. The weight ratio of Si to Ga solvent is 10−4 wt%. The growth rate, surface morphology, carrier concentration and Hall mobility are studied. Relationship between the above properties and the growth temperature and temperature different (ΔT) is also discussed. In general, the present results appear quite consistent with the diffusion limited model. The growth rate can be precisely controlled. The stability of the solid-liquid interface can be obtained in the epilayer growth at a constant temperature of the system which can avoid the effect of constitutional supercooling. Under proper control, a perfect epilayer and multiple smooth layers can be obtained.

    • X-ray diffraction study of epitaxial zinc and cadmium films prepared by hot-wall technique

      A V Kulkarni R Pratap

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      Hot-wall technique greatly improves the quality of zinc and cadmium films deposited on glass substrate. At substrate temperature the growth of such films is well ordered, showing highly preferred orientation along c-axis. However, if the substrate temperature is increased beyond certain limit, we get polycrystalline growth of the films. This shows that the growth of zinc and cadmium films on glass substrate strongly depends on the substrate temperature.

    • Concept of thermodynamic capacity

      R Akila K T Jacob A K Shukla

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      The thermodynamic capacity of a species (Ci)in a homogeneous phase is defined as (∂ni/∂µiP, T, njwhereniis the total number of moles ofi per unit quantity of the system irrespective of the actual system chemistry andµiis its chemical potential. Based on this definition, the thermodynamic capacity of oxygen in non-reactive and reactive gas mixtures and in binary and ternary liquid solutions has been computed. For reactive gas mixtures containing stable chemical species which do not undergo significant dissociation such as CO + CO2, H2 + H2O and H2 + CO2, the capacity curves show a maximum at equimolar ratio and a minimum at higher oxygen potentials. If one of the chemical species partly dissociates as in the case of H2S in H2 + H2S mixtures or SO3 in SO2 + SO3 mixtures, capacity curves do not exhibit such maxima and minima, especially at high temperatures. It would be difficult to produce stable oxygen fugacities when the capacity has a low value, for example at compositions near the minimum. Oxygen capacities of non-ideal liquid solutions, Cu-O and Cu-O-Sn, and heterogeneous systems formed at saturation with the respective oxides are discussed.

    • Growth of low dislocation density single crystals of nickel

      S K Khanna K Govinda Rajan

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      Low dislocation density single crystals of nickel have been grown at high ambient pressure by the Czochralski method. X-ray Laue picture shows that the crystals are strain-free. The dislocation density was determined to be <103/cm2 by the etching procedure. It was found that the necking and cone regions are very critical in the dislocation introduction in the crystals. An increase in the ambient pressure used during the growth seems to aid the crystal quality.

    • Emission spectrographic technique for the quantitative determination of trace elements in granitic rocks

      A EL Bialy M Rasmy L A Guirguis W Moussa

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      An emission spectrographic technique was developed to estimate 16 trace elements in some samples of Egyptian granite. The detection limits were: 0·1 ppm for Pb, Ba, Mo, Cu, Cr, Yb and Ni, 0·3 ppm for Sn, Ga and Be, 1 ppm for Co, Sc and V, 3 ppm for Bi and Y and 10 ppm for La. The relative deviation of the two-thirds limits ranges between ± 1·5 and ±24·7.

    • Determination of oxygen to metal ratio of U-Pu mixed oxides by x-ray diffraction

      Ravi Verma P R Roy

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      The possibility of measuring the oxygen-metal ratio (O/M) of U-Pu mixed oxides by x-ray diffraction technique has been explored. In single-phase U-Pu mixed oxides, the lattice parameter vs O/M plots for different plutonium concentrations are drawn by interpolation of lattice parameter values between those of UO2, PuO2 and Pu2O3. These plots are then used for determining the O/Ms of unknown samples against their experimentally measured lattice parameter values. In two-phase mixed oxides, the mole fractions of the two phases are determined from the intensities of their selected diffraction lines. The O/M of the mixed oxide is then given by the mole average of the individual O/Ms of the two phases.

    • Kinetics of calcium molybdate crystallization

      N M Batra S K Arora

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      Kinetics of crystallization of calcium molybdate from unstirred molten solutions of lithium chloride of low to medium supersaturation in platinum crucibles by the process of continuous cooling at 5° C hr−1 from temperaturesT0=700 and 750°C are investigated. The crystal size measured by optical microscopy for different crystallization periods reveals that both crystal length and width generally increase with cooling period. The degree of crystallizationαt,also increases with cooling period, attaining a maximum of 0·90. The diffusion rate constants,KDlat 700 and 750°C are 0·0776 and 0·1138 respectively. The effect of variation of the crystallization temperature on the crystal size and their number is also studied.

    • Thermal transformation in a chrysotile asbestos

      A K Datta B K Samantaray S Bhattacherjee

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      Structural changes accompanying thermal transformation in a chrysotile asbestos sample of Indian origin upto a temperature of 900°C have been investigated by x-ray method. The changes in lattice parameters have been systematically measured by applying a least square refinement technique and the crystallite size and strain defects at different stages have been estimated by line profile analysis of the diffraction peaks. The present study also confirms the earlier results on stepwise transformation of chrysotile fibres. This sequence of transformation has been explained assuming two different types of crystallites as reported in kaolinite.

    • Effect of H+ ion irradiation on surface morphology of Fe40Ni40B20 metallic glass

      A J K Prasad Vakil Singh

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      The effect of irradiation by H+ ions on surface modifications of metallic glass Fe40Ni40B20, in the as-quenched, structurally related and crystallized state, has been studied. Swollen regions develop on the surface of the as-quenched and structurally relaxed specimens, whereas blisters form on the surface of the crystallized specimen, under identical conditions of irradiation. The results are explained in terms of the distribution of hydrogen in the amorphous and crystallized conditions.

    • Periodic crystallization of barium oxalate in silica hydrogel

      S M Dharma Prakash P Mohan Rao

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      The experimental conditions for obtaining periodic crystallization of barium oxalate in silica hydrogel are investigated. The Liesegang rings formed by the reaction of oxalic acid and barium chloride are studied. The dependence of velocity constant and the spacing coefficient on pH, concentrations of inner and outer electrolytes and temperature have also been studied and a possible mechanism for the behaviour is suggested. The deviations from Isemura’s general observations are interpreted by considering the microcrystals constituting the rings and the irreversibility of inner and outer electrolytes.

    • Studies on etching of gel-grown barium oxalate dihydrate crystals

      S M Dharmaprakash P Mohan Rao

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      Etch pits produced by selective etchants such as 1 M HCl, 1 M HNO3, 4 M BaCl2, 4 M NH4Cl and 4 M NH4Cl-1 M HCl solutions on the as-grown {110} faces of barium oxalate dihydrate crystals are illustrated and explained. The kinetics of etching is studied. From Arrhenius plots, the activation energy of etching and the pre-exponential factor are computed. An empirical equation governing the kinetics has been suggested.

    • Determination of molecular weight distribution of polyethylene terephthalate by gelpermeation chromatography

      K Sreenivasan Prabha D Nair

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      Gel permeation chromatography was used to determine the molecular weight distribution of polyethylene terephthalate. Chloroform was used as the mobile phase. The average molecular weights calculated from the chromatogram closely agree with the known values of the samples used in this experiment.

    • Space charge limited currents in chlorazole black E dye

      J P S Chauhan D C Dube S C Mathur

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      Space-charge-limited-currents have been studied in Chlorazole Black E (CBE) dye to understand the current-voltage relationship and the nature and distribution of traps present in the dye. Three distinct regions of voltage-current characteristics have been observed probably for the first time. The results have been discussed on the basis of carrier injection theory. There are non-uniformly distributed deep as well as shallow traps.

    • Microstrip losses with Al2O3 overlay

      K K Joshi C Dhanavantri S A Gangal R N Karekar

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      Microstriplines were coated with Al2O3 by chemical vapour deposition to get homogeneous thin film overlay. The paper reports the decrease in the total power loss in such microstrips. The frequency response from 2–6 GHz is studied for the overlay effect.

    • Thermoluminescence of alkali halides doped with alkaline earth impurities

      A R Lakshmanan S V Moharil

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