• Volume 11, Issue 4

      December 1988,   pages  269-346

    • Synthesis and single crystal growth of gallium phosphide by the liquid encapsulated vertical Bridgman technique

      K Govinda Rajan N V Chandra Shekar G V N Rao A J Singh R M Iyer

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      Gallium phosphide is a typical III–V compound semiconductor and is also an important electronic material. The synthesis and single crystal growth of this compound by melt methods is rendered very difficult because of the large phosphorus vapour pressure. A high pressure vessel with internal heating and a quartz reactor was first developed for the direct synthesis of gallium phosphide. The crystal growth was carried out in a second high pressure chamber rated for 100 bars gas pressure and equipped with the paraphernalia for crystal growth. Single crystals of gallium phosphide were grown from the polycrystalline starting material by the vertical Bridgman method and the vapour pressure problem was overcome by encapsulating the melt in a column of molten boric oxide. Both boron nitride and silica were employed as crucibles, and with the former, single crystal rods of 8–10 mm diameter and 10–15 mm length were obtained.

    • Preparation of single crystals of YBa2Cu3O7−x from nonstoichiometric melts

      N V Chandra Shekar J Suresh K Govinda Rajan G V N Rao

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      Studies on the single crystal growth of YBa2Cu3O7−x show that the growth conditions have not been optimised yet and they vary in many ways. Here we report the growth of single crystals of YBCO in the size range 0·5–1·2 mm from nonstoichiometric melts. We have made systematic variations in the flux composition (constituting CuO and BaCO3) in order to arrive at an optimum composition for consistently getting single crystals of size 0·5–1·2 mm. The tetragonal phase was confirmed by X-ray diffraction and single crystallinity by the Laue technique. Superconductivity was confirmed in oxygenannealed crystals.

    • Low temperature preparation ofβ-alumina by an alkoxide route

      M K Dongare A P B Sinha

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      β-alumina has been prepared by the thermal decomposition of a mixture of sodium and aluminium isopropoxides followed by heating up to 1000°C. It has been found necessary to use sodium isopropoxide in excess (25–30%) for the complete formation ofβ-alumina at 1000°C. On the other hand one obtains a mixture ofβ-alumina andα-alumina when the starting materials are taken in the stoichiometric ratio Na2O:11Al2O3. DTA, TG and DTG studies of a mixture of sodium and aluminium isopropoxide showedβ-alumina formation at 1000°C.

    • Deintercalation of potassium from K0·67 CoO2

      M K Dongare

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      The deintercalation properties of layered oxide K0·67 CoO2 with H2O have been investigated with respect to topotactic solvation and exchange processes. The layered oxide K0·67 CoO2 forms hydrated phases and undergoes K+/H+ exchange resulting in the formation of a new K0·15 H0·85 CoO2 compound. Thermal behaviour of these compounds has been investigated.

    • High characteristic temperature of 1·3 µm crescent buried heterostructure laser diodes

      Y K Su T L Chen

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      Two-step liquid phase epitaxy and preferential etching technique have been used to fabricate 1·3 µm InGaAsP/InP buried crescent laser. The p-n-p-n thristor-like structure is used as the blocking layers. The leakage current is very small even at high temperatures. The characteristic temperatureT0 at room temperature is as high as 79 K. This value is higher than those ever reported.

    • Energy gap studies on As-Sb-Se glasses

      K Nandakumar Jacob Philip

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      Variation of optical energy gapE0 with composition in several glasses belonging to the As-Sb-Se system have been studied. The photoacoustic technique, well-suited for highly absorbing samples, has been used for the measurements. Results reported on the AsxSb5Se95−x, AsxSb10Se90−x, AsxSb15Se85−x and AsxSb20Se80−x glasses of the As-Sb-Se family are explained on the basis of the chemically ordered network model.

    • Electrical studies on ceramic samples of Rb2Ti4O9, Rb2Ti5O11 and RbKTi4O9

      R Dawar Shri Pal S D Pandey

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      Electrical conductivity studies have been reported on Rb2Ti4O9, Rb2Ti5O11 and RbKTi4O9 in the temperature range 373 K to 750 K. Three distinct regions have been identified in the log(σT) versus 1000/T plots, the lowest temperature region being attributed to extrinsic defects, the intermediate one to exchange of alkali ions and the highest temperature region to thermally created defects. A phase transition is indicated for each of the three compounds. The plot of resistivity with temperature further indicates the occurrence of a positive temperature coefficient of resistivity in RbKTi4O9.

    • Dielectric properties of the system Ca1 −xLaxTi1 −xCoxO3

      Ch Durga Prasad H S Tewari Devendra Kumar Om Parkash

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      Dielectric behaviour of samples of the system Ca1 −xLaxTi1 −xCoxO3 withx ⩽ 0·20 has been studied in the temperature range 300–525 K as a function of frequency. The strong dispersion ofɛ andD observed in these materials indicate the significant contribution of interfacial polarisation to the observed dielectric properties. The interfacial polarisation arises due to the presence of microscopic chemical heterogeneities arising out of the slow diffusion-controlled solid state sintering process used for their preparation.

    • On the variation of a.c. susceptibility with temperature for some Cu-Li ferrites

      A B Naik S R Sawant S A Patil J I Powar

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      The a.c. susceptibility vs. temperature of the system (Li0·5Fe0·5)1−xCuxFe2O4, for 0⩽x⩽1·0 has been studied. While pure CuFe2O4 (x=1) shows a dramatic peak in susceptibility just before its Curie temperature (Tc), the same is supressed with the addition of a small amount of Li to the system. The variation of lattice parameter with Li content has also been investigated. Samples up tox=0·4 show tetragonal structure while those withx>0·4 are cubic. This change of structure can be explained on the basis of reduction ofdsp2 bonding of Cu ions on B-sites of the spinel lattice.

    • Low frequency internal friction spectra of Cu0·81 Pd0·19 alloy

      B Sobha Y V G S Murti

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      Results on the internal friction of disordered and partially ordered Cu0·81Pd0·19 alloy in the temperature range 160–300 K and frequency range 1–2 Hz are reported. All the peaks are classified into three groups. Two prominent peaks of the first group occurring between 180 and 200 K are observed only in completely disordered samples. A long annealing treatment removes these peaks. These peaks may be due to interaction of dislocations with preferentially clustered palladium atoms in the disordered alloy. A second group of peaks in the temperature range 210–270K is sensitive to the microstructure and are presumably related to dislocation-dislocation interactions. In partially ordered samples new peaks appear at between 210 and 285 K. Activation parameters of these peaks are presented.

    • A first report on fracture toughness ofbcc iron alloys as influenced by solutes: opposite effects of silicon and cobalt

      M Srinivas G Malakondaiah P Rama Rao

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      The effect of solutes on resistance to fracture of body centred cubic iron single-phase solid-solution alloys has been investigated. TheJ-integral method has been used for the measurement of ductile fracture toughness. TheJIC values so determined quantitatively indicate the extent of degradation in fracture toughness due to the addition of hardening solute silicon. Cobalt addition results in alloy softening. The measuredJIC values clearly demonstrate the toughening effect of cobalt addition as a solute, which result renders the case of Fe-Co solid-solution alloys interesting.

    • Chemical segregation of solute elements in ultrasonically gas atomized aluminium alloy powders

      R D K Misra A B Pandey V V Rama Rao Mahendra Kumar

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      A new experimental approach to the evaluation of chemical segregation of solute elements in ultrasonically gas atomized aluminium-alloy powders using X-ray spectral data of scanning electron microprobe analyser is described. The experimentally obtained chemical segregation data is compared with the conventional method of quantitative analysis and with theoretical predictions as determined from Scheil’s approach to the evaluation of elemental segregation during the solidification process. A comparison of experimental and theoretical predictions confirms the validity of the experimental approach in the estimation of solute segregation levels and also suggests that the solidification conditions considered for estimation of microchemical segregation can appropriately be applied to ultrasonically gas atomized powders.

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