• Volume 9, Issue 3

      August 1987,   pages  159-247

    • Synthesis and properties of manganese-doped barium titanate

      L M Arunachalam D Chakravorty E C Subbarao

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      An oxalate route for the production of BaTiO3 has been modified to incorporate Mn (upto 2%) as a dopant and the reaction sequence has been studied. The resulting Mn-doped BaTiO3 exhibits a dielectric constant with an increasing insensitivity to temperature and applied a.c. field as the Mn content is increased. These samples possess a high electrical resistivity even after treatment in hydrogen at 1100°C and are therefore suitable as dielectric for multilayer capacitors with base metal electrodes.

    • Dielectric behaviour of ZnO-based ceramic semiconductors

      Subhash C Kashyap K L Chopra Bharat Bhushan

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      The effect of composition, sintering parameters, frequency and temperature on the dielectric parameters of ZnO-based ceramic semiconductors (cersems) having small amounts of Bi2O3, Sb2O3, CoO, MnO2, La2O3 and/or Cr2O3 has been investigated. The unusually high dielectric constant of these composites, arising due to a two-phase microstructure has been explained on the basis of a depletion layer model. The agreement in values of barrier height and donor concentration calculated fromC−2V plot, Hall measurement and I-V characteristics of these cersems supports the validity of barrier and depletion layer models. The depletion and inter-granular layers are estimated to be nearly 102 nm and 1–2 nm respectively. The observed variation of dielectric constant/capacitance with sintering parameters and temperature of measurement has also been explained on the basis of simplified microstructure and depletion layers. The loss peak (fmax) observed at 300 kHz remains practically unaltered with change in composition and sintering parameters. The observed dielectric dispersion in the range 102−106 Hz, exhibiting multiple relaxation times and activation energy of relaxation process as 0·36 eV, has been explained on the basis of Debye-type relaxation process originating due to trapping/detrapping and possibly due to scattering of carriers in the depletion regions.

    • Electrical resistivity of holmium hydride films

      M S Rahman Khan

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      The electrical resistivity and temperature coefficient of resistance of polycrystalline holmium hydride films, grown on glass slides, have been measured in vacuo. Experimental results show that the resistivity decreases with decreasing film thickness. The resistivity of 200 Å holmium dihydride film is less than that of the parent metal. The observed behaviour of the thickness dependence of the resistivity has been explained in terms of the structural phase change.

    • Electrical conductivity and transition temperature of NbN thin films

      C S Menon V S Pankajakshan

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      Niobium nitride thin films are grown using reactive RF sputtering technique for four different partial nitrogen pressures in argon atmosphere. The superconducting transition temperature of the films has been measured. The films exhibit a negative temperature coefficient of resistivity. The electrical characterization of the films has been carried out and the conductivity measured between room temperature and liquid nitrogen temperature. This is fitted usingTP law.

    • Optical properties of coloured transparent alumina gel monolith glasses

      V Saraswati G V Ramarao

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      Optical properties of transparent alumina prepared through gel process at room temperature and coloured with dopants have been studied. The absorption bands, molar absorptivity, effect of concentration and the dependence of oxidation state of copper on the copper content and water content are similar to the random network glasses. The ultraviolet absorption edge at lower energies confirm the disordered nature of the transition phase aluminasγ andδ. The absorption bands in the visible suggest the octahedral symmetry of oxygen around aluminum.

    • Magnetic susceptibility measurements of LHP and LNP crystals

      C C Desai M S V Ramana

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      Ferroelectric single crystals such as lead hydrogen phosphate (LHP) and dihydrated lead nitrate phosphate (LNP) have been derived from the diffusion of lead nitrate into the set gel containing orthophosphoric acid. By employing Faraday’s Gouy balance technique, the magnetic susceptibilities of LHP and LNP have been determined. It has been observed that (i) the gram and molar susceptibilities are independent of magnetic field, (ii)χM of LNP materials are greater than that of LHP materials and (iii) the positive values of magnetic susceptibilities suggest that LHP and LNP are paramagnetic materials.

    • Crystallization behaviour of Metglas 2826 MB (Fe40Ni38Mo4B18)

      V S Raja Kishore S Ranganathan

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      Differential scanning calorimetry, x-ray diffraction and transmission electron microscopy have been employed to determine the thermal stability and identify the crystalline phases on devitrification of Metglas 2826 MB. The glass crystallizes intoγ-FeNiMo and fcc (FeNi)23B6 with activation energies of 270 and 375 kJ mol−1 respectively. The reactions are primary and polymorphic in nature. The influence of Mo towards crystallization of Fe40Ni40B20 has been to enhance the formation of the fcc (FeNi)23B6 phase in preference to orthorhombic (FeNi)3B phase and to raise the thermal stability of the amorphous state.

    • Preparation and properties of native oxide film anodically grown on Hg0·8Cd0·2Te

      F R Chavda G Tiwari S C Gupta

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      The process for forming the anodic oxide on Hg0·8Cd0·2Te using a constant voltage d.c. source is studied. The electrolyte is composed of 0·1 N KOH, 90% ethylene glycol and 10% water. Good, uniform, transparent and reproducible films are obtained when the pH of the solution is adjusted between 7 and 8 by using oxalic acid. The anodic oxide film shows an anti-reflection effect. HCl and NH4OH etch the oxide well and give a smooth polished surface of Hg0·8Cd0·2Te.

    • Use of energy dispersive method with tungsten carbide opposed anvil high pressure set-up

      A K Singh S Usha Devi

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      The use of energy dispersive method has been discussed to record the X-ray diffraction data from a tungsten carbide opposed anvil high pressure set-up. It is pointed out that a proper collimation of the incident and the diffracted beams can eliminate the scattering from the gasket material. This results in an appreciable improvement in the quality of the recorded diffraction data. The choice of the various parameters has been discussed. The data on sodium chloride are presented in support of this analysis.

    • Positron annihilation in Zr-0·5 Nb alloy

      D D Upadhyaya R V Muraleedharan

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      The measurement of coincidence counting rate at the peak of the angular correlation curve shows a considerable difference between the recovery behaviour of cold-worked Zr-0·5 Nb alloy from that reported for pure zirconium. This difference in behaviour illustrates the point-defect activity during the course of isochronal annealing in this alloy. The recovery spectrum can be understood in terms of the vacancy-solute entrapment upto about 250°C. In the next stage the release and clustering effects of vacancies are indicated prior to their elimination. The study thus demonstrates the effect of alloying element on the defect mobility and associated point defect interactions in Zr.

    • A new formalism for representation of heat capacities of metals

      J P Hajra

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      The applicability of a function involving geometrical progression of temperature in interpreting the heat capacities of metals has been studied. The constants of the function have been described in terms of vibrational, electronic and magnetic contributions to heat capacities. The equation may be useful in representing heat capacity of metals.

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