• Volume 20, Issue 3

      June 1997,   pages  287-375

    • Chemical bath deposition of CdS thin films and their partial conversion to CdO on annealing

      K L Narayanan K P Vijayakumar K G M Nair G xxxV N Rao

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      CdS thin films prepared by chemical bath deposition technique are characterized using X-ray diffraction, optical absorption spectrometry and scanning electron microscopy. The results of the annealing studies on the films in flowing argon and air atmospheres are also presented in this paper. The resistivity has drastically reduced on annealing in flowing air which is attributed to the partial conversion of CdS to CdO phase.

    • Conduction mechanism of metal-free phthalocyanine single crystals as a function of temperature

      Ayona Simon Francis P Xavier

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      Metal-free phthalocyanine (H2Pc) single crystals grown by vacuum sublimation were investigated for their conductivity (both in dark and light). The investigations consisted of dark- and photo-current variations with (i) applied electric field and (ii) temperature. The applied electric field ranged from 0·8 kV/cm to 6 kV/cm. The temperature range was from 300°K to around 570°K. The crystals were found to be photoconductive. Based on activation energies calculated from photoconductivity due to temperature dependence, an energy level scheme for H2Pc single crystals is proposed. The model consists of two trapping levels within the forbidden gap — one at 0·4 eV below the conduction band edge from which electrons are thermally excited into the conduction band and the other acts as recombination centre at 0·3 eV above the valence band edge. The band gap is calculated to be 1·4 eV. Comparative study of the proposed model with that of earlier investigations on the same crystals of the H2Pc is in good agreement, thereby indicating that H2Pc is thermally stable even at relatively higher temperature as semiconductor.

    • Mixed conduction in charge transfer materials based on aromatic diamines and iodine having different mole ratios

      Ram Adhar Singh Vinod Kumar Singh

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      Mixed ionic and electronic conduction in charge transfer materials based on aromatic diamine electron donors (benzidine,o-tolidine (3,3′-dimethyl benzidine) andN,N′-diphenylbenzidine) with iodine in different mole ratios has been reported. The current-voltage and capacitance-voltage curves as a function of time and temperature have been obtained to determine the role of ionic conduction in electronic conductors based on charge transfer complexes. The compositional dependence of ionic transport numbers, diffusion parameters, ionic(σi) and electronic(σe) conductivities and thermal activation energies has been studied. Temperature and frequency dependence of AC impedance and related parameters have been studied to learn about the electrical conduction behaviour in these non-stoichiometric charge transfer materials. An electrochemical mechanism has been proposed to account for the ionic conduction in some of these complexes.

    • Conduction mechanism in anthracene as a function of temperature

      Jeena V Joseph Francis P Xavier

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      Conduction mechanism in anthracene single crystal grown by Bridgman method was carried out. The investigations consisted of dark- and photo-current variation with respect to (i) applied electric field and (ii) temperature. The applied electric field ranged from 0·5 to 2·5 kV/cm and the temperature range was between 300 K and 450 K. Photo and dark current variations with temperature indicate, based on activation energy determination, that a band model can be applied to the conduction process. The band gap is calculated to be 1·6 eV. The band model consists of a recombination centre 0·37 eV above the valence band edge and a trap level 0·55 eV below the conduction band edge to which electrons are first thermo-optically excited and then they are thermally excited into the conduction band.

    • Measurement of residual stresses in CFRP laminates by X-ray diffraction method

      C Balasingh Vandana Singh

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      The fabrication of CFRP laminates from prepregs involves curing at elevated temperatures. Residual stresses are set up due to the difference in thermal expansion coefficient between the matrix and the fibre. In this investigation, the X-ray diffraction method is used to measure the curing stresses in CFRP laminates by incorporating a very fine layer of aluminium particles during the lay up of the laminate. A calibration procedure is followed to correlate the strain in the crystalline particles, as measured by X-rays, with the composite strain and stress. Curing stresses measured by this technique are quite close to the value calculated from the differential coefficient of thermal expansion.

    • Knoop hardness studies on anthracene single crystals

      Nimisha Vaidya M J Joshi B S Shah D R Joshi

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      Knoop microhardness studies were carried out on anthracene single crystals. The hardness vs load plot shows two peaks, one at 5g and another at 17·5 g having hardness values 13·0 kg/mm2 and 11·4 kg/mm2 respectively. The present observation shows that the$$(20\bar 1)[010]$$ dislocations split into partials.

    • Influence of rolling schedule on characteristics of thermomechanically treated CuBe2 alloy

      S Nestorovich D Markovich D Guskovich

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      Thermomechanical treatment of CuBe2 alloy was performed as follows: quenching, primary ageing, cold rolling and secondary ageing at different temperatures. The cold rolling was carried out following two deformation schedules (with different intensity of deformation). The investigation results on influence of deformation intensity and secondary ageing temperature on hardness, electrical conductivity, structure and activation energy on thermomechanically treated CuBe2 alloy are presented.

      It was found that the deformation intensity had the greatest influence on hardness upon secondary ageing at 270°C. A hardness of 470 HV was attained by use of higher deformation intensity schedule while a hardness of 390 HV was attained with lower deformation intensity schedule.

    • Electron structure and activation energy of hydrogen in α-Zr using nonlinear response theory

      N Singh DK Avasthi A Tripathi

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      The electron structure of hydrogen in hcp Zr is calculated by using self-consistent nonlinear screening theory. The host-ion contribution is included through the spherical solid model potential (SSMP). The resulting charge density and scattering phase shifts are used to calculate the activation energy and residual resistivity of hydrogen in α-Zr matrix. The calculated activation energy 0·285 eV is found in reasonably good agreement with experimental value 0·3 eV. The estimated residual resistivity 0·53 μΩ cm/at% for Zr-H system using the scattering phase shifts agrees reasonably well with the observed value 0·27 μΩ cm/at%. The calculated configurational energy shows that hydrogen prefers tetrahedral(T)-sites over octahedral(O)-sites in α-Zr. The strong binding energy of electron-proton suggests that hydrogen forms zirconium hydride.

    • Mass spectrometry based evolved gas analysis system for thermal decomposition studies

      S Dash M Kamruddin A K Tyagi

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      An experimental facility for evolved gas analysis by mass spectrometry (EGA-MS) has been built in-house and extensively used to study the temperature programmed decomposition (TPD) of a number of inorganic solids. Fractional extent of reactionα acquired from real time multiple ion detection trend analysis mass spectra of gases released from thermally impressed specimen has been used to obtain functional transformf(α) of non-isothermal solid state kinetic rate expressions. The corresponding model integral functionsg(α) based on mechanisms like random nucleation, diffusion and interface motion have been used to establish kinetics control regimes for specific decomposition sequences. From ln[g(α)/T2] vs 1/T plots Arrhenius parameters like activation energy and pre-exponential factor could be determined. Signature of the rate controlling mechanism governing the gas release behaviour was found in the crystallographic transformation brought about by the temperature programme. This paper describes the scope and capabilities of our EGA-MS facility with typical results on temperature programmed decomposition of CuSO4·5H2O and AlNH4(SO4)2·12H2O.

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