• Volume 18, Issue 3

      June 1995,   pages  195-307

    • Bulk high-Tc magnetometer

      Neeraj Khare JR Buckley GB Donaldson

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      We report a study of a bulk YBCO magnetometer based on the detection of harmonics during nonlinear magnetization. For a low-DC field, the amplitude of the even harmonic appearing at the pickup coil depends linearly on the DC field intensity. The performance of the magnetometer depends on the quality of the sample, and the intensity and frequency of the AC field excitation. At higherHac, the fourth harmonic becomes more sensitive than the second harmonic. The hysteresis in the magnetometer for DC field measurement has been reported.

    • Cone formation on Ag surface bombarded by Ar at oblique incidence

      S R Bhattacharyya T K Chini D Basu

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      Sputter-induced topographical modifications of a silver surface bombarded by mass-analysed ions of 30 keV40Ar+ obtained from an electromagnetic isotope separator incident at an angle of 58° was studied. Analysis of the silver surface by scanning electron microscopy showed that most of the bombarded area was covered with features similar to corrugated terrace steps, sharp and fine cones, and pyramids. The underlying mechanism for formation of such features is discussed. The method of formation of such sharp cones produced by ion bombardment will increasingly find applications in the formation of fine tips required in scanning probe and field emission microscopy.

    • The role of active elements in Fe-Cr-Al alloys for heating element applications

      D V Satyanarayana M C Pandey

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      The role of active elements (zirconium, cerium and yttrium) in grain growth and its subsequent effect on room-temperature tensile fracture behaviour of Fe-Cr-Al alloys has been investigated. The alloys containing active elements exhibited improved resistance to grain growth. Yttrium-containing alloy retained higher room-temperature tensile ductility even after heating at higher temperatures because of its superior resistance to grain coarsening. All the alloys, with and without active elements, exhibited a sudden drop in room-temperature tensile ductility, with concommitant change in fracture mode from dimple rupture to cleavage, in a narrow band of 39–44µm grain size. The ductile-brittle transition temperatures (DBTT) of these alloys were found to be 150°C and 215°C for grain sizes 25µm and 100µm respectively. The accelerated life test showed that the alloys containing active elements exhibited longer lives.

    • Influence of ion-induced interfacial chemical reactivity on contact resistance

      S T Lakshmikumar

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      Contact resistance measurements of chromium contacts deposited by partially ionized beam deposition on transparent conducting indium tin oxide (ITO) were performed. These provide a direct experimental evidence of the influence of interfacial chemical interaction on the contact resistance. The interfacial reactivity is controlled by modifying the energy and flux of ionized chromium atoms deposited on ITO employing a specially designed partially ionized deposition system with very high ionization efficiency. The true contact resistivityρc is obtained by iteratively correcting the experimentally measured values for the finite sheet resistance of the ITO layer.ρc decreases linearly with the energy of the ionized chromium. Auger sputter profiling shows no structural modifications at the interface due to a change in the energy of the chromium atoms, confirming that the observed change in the contact resistivity is directly related to interfacial chemical bonding of the atoms with the oxygen atoms in the ITO leading to a local increase of carrier concentration and lower interfacial resistance.

    • Electrical and magnetic studies of iron (III) vanadate

      N Suresh Rao O G Palanna

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      Iron(III) vanadate (FeVO4) is an n-type semiconductor between 300 and 800 K. Electrical conduction in this phase occurs due to small deviation from oxygen stoichiometric composition. The mechanism of electrical transport is of a thermally activated hopping of charge carriers (electrons) on equivalent iron lattice sites. The FeVO4 obeys Curie-Weiss law between 80 and 300 K. The measured magnetic moment (μeff) of Fe3+ ion in FeVO4 is 5·270 BM at 298 K, which is lower than theμspin only value. The predominant exchange interactions are the weak 90° M-O-M superexchange and M-O-O-M super-super-exchange. The negative Weiss constantϑ=− 30 K of the phase indicated the possibility of an antiferromagnetic ordering of the iron(III) vanadate lattice. The IR absorption spectrum of FeVO4 gave bands at 990, 900, 825 and 725 cm−1 due to the presence of distorted VO4 polyhedra of the lattice.

    • An analysis of the electrical conductivity in BaSO4-added Ag2SO4 solid electrolyte system

      K Singh S M Pande S S Bhoga

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      The compositions (1 −x)Ag2SO4−(x)BaSO4, wherex=0·01 to 0·6, were prepared by slow cooling of the melt. The extent of the solid solubility of Ba2+ in Ag2SO4 was determined by X-ray powder diffraction and scanning electron microscopy. The bulk conductivity of each sample was obtained using a detailed impedance analysis. The partial substitution of Ba2+ results in the enhancement of conductivity in compliance with the classical aliovalent doping theory. A simplistic model based on lattice distortion (expansion) due to partial substitution of Ag+ by the bigger Ba2+ has been considered to explain enhanced conductivity. Beyond solid-solubility limit (5·27 mole%) the BaSO4-dispersed Ag2SO4 conductivity follows the usual trend seen in binary systems. An increase in conductivity in this case is discussed in the light of interfacial reactions and surface defect chemistry. The maximum conductivity in 20 mole% BaSO4 dispersed Ag2SO4 is due to percolation threshold.

    • Acoustical parameters of polyethylene glycol/water mixtures

      A Varada Rajulu P Mabu Sab

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      Ultrasonic velocity (v) and density (d) have been measured for polyethylene glycol/water mixtures at 30°C. The adiabatic compressibility (βad), molar compressibility (β), specific acoustic impendance (Z), Rao number (R) and van der Waals constant (b) have been computed. The variations ofv, d, βad,β, Z, R andb with mole ratio of water/ether group oxygen have been studied. The association between the components and the formation of tetrahydrate have been reported.

    • Thermal stability of electrochemically prepared polythiophene and polypyrrole

      F Mohammad P D Calvert N C Billingham

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      The degradation behaviour of electrochemically prepared polythiophene and polypyrrole has been studied by thermal gravimetric analysis technique. Studies on both the polymers show that they are more stable than polyacetylene but still undergo degradation reactions which involve two steps, viz. loss of dopant and then degradation of polymer backbone. The general features of degradation mechanisms are discussed.

    • Study of the effect of chopping on the adhesion of codeposited mixed thin films

      Vijaya Puri R K Puri

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      This paper reports adhesion of codeposited mixed films of Cu-Ag, Cu-Al, MgF2-ZnS and MgF2-cryolite and enhancement of adhesion by chopping technique. Results indicate that codeposited mixed films show higher adhesion than single films and chopping improves the adhesion further. The quenching of crystal growth seems to be more effective if two materials are codeposited and chopped. Chopping along with mixing increases the number of nucleation sites and decreases defects in the film. Decreasing defects and modifying the microstructure increases the adhesion of the films. Chopping also seems to increase oxygen affinity of both metallic and dielectric films for growth of interfacial bonding layer.

    • Photoconductivity in crystalline phthalocyanines

      Francis P Xavier Géorge J Goldsmith

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      The wavelength, temperature, time and intensity dependence of photocurrent of metal-free phthalocyanine (H2Pc) and copper phthalocyanine (CuPc) single crystals were investigated. The thermal activation energies in the dark are 0·5 and 0·6 eV for H2Pc and CuPc respectively and the corresponding photo-thermal activation energies are 0·3 and 0·2 eV. An energy level scheme for single crystals of H2Pc and CuPc is proposed which consists of two trapping levels and five narrow optically active valence bands. In H2Pc (CuPc), one trapping level at 0·5 eV (0·6 eV) above the valence band edge to which the charge carriers are thermally excited in the dark; and the other trapping level is at 0·3 eV (0·2 eV) below the conduction band edge where all the optical transitions terminate. In H2Pc(CuPc), the forbidden gap is 1·44 eV (1·34 eV) wide; the five valence bands are at the band edge, and 0·09 (0·22), 0·42 (0·63), 0·69 (0·90), 1·32 (2·17) eV below the band edge.

    • Photoconductivity in thin films of phthalocyanine

      Francis P Xavier George J Goldsmith

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      The photocurrent and electrolyte electromodulation (EEM) spectra of thin films of metal-free phthalocyanine (H2Pc) and of copper phthalocyanine (CuPc) were investigated. The modulation spectra yielded three distinct features around 1·61, 2·30 and 2·93 eV for H2Pc and around 1·63, 2·04 and 3·20 eV for CuPc. The spectral dependence maxima of photoconductivity correspond to the modulation spectra. These features are interpreted to indicate transitions at critical points, i.e. the existence of transitions between three valence bands, since Pc’s are p-type, and the lowest conduction band in Pc’s.

    • Photoconductivity of iodine-doped single crystals of phthalocyanine

      Francis P Xavier George J Goldsmith

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      Single crystals of metal-free phthalocyanine (H2Pc) and of copper phthalocyanine (CuPc) were grown in the presence of iodine vapour. The presence of iodine enhances the spectral dependence of photoconductivity of H2Pc in the visible region but of CuPc in the near-IR region. The dark current is decreased but the photocurrent is increased by one order of magnitude in iodine-doped H2Pc but in the case of iodine-doped CuPc both currents are increased by nearly three orders of magnitude. Introduction of iodine results in about one order of magnitude decrease in response time for both modifications. Thus the introduction of iodine into Pc crystals decreases the energy barrier for conduction and increases the drift mobility of charge carriers thereby enhancing the conductivity of the material.

    • Growth mechanism of NaBrO3 crystals from aqueous solutions

      V Surender K Kishan Rao

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      To study the growth mechanism of {111} faces of NaBrO3, crystals were grown at different supersaturations ranging from 2% to 8%. The growth mechanisms were investigated based on the growth rate versus supersaturation relation and from the surface features observed on {111} faces. The growth mechanism of these crystals appear to be due to 2D nucleation. The growth rate curve has been further investigated using Ohara and Reid equations. Polynucleation model in two-dimensional nucleation growth theory is suggested as the most possible growth mechanism for these crystals in the present supersaturation range.

    • Near ideal electrical switching in fast ion conducting glasses: Evidence for an electronic process with chemical origin

      B Vaidhyanathan S Asokan K J Rao

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      Bulk AgI based fast ion conducting (FIC) glasses have been prepared by a novel microwave technique. Electrical switching characteristics of these glasses have been investigated for the first time. It has been found that AgI based FIC glasses exhibit a current-controlled high speed memory electrical switching behaviour. SEM, EDAX and ESR investigations have been performed on the virgin and switched sampies to understand the nature of the conducting state. A chemical model is proposed to explain the switching behaviour of these glasses, which is consistent with the observed results.

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