• Volume 22, Issue 2

      April 1999,   pages  71-137

    • SOx solid state gas sensors: A review

      K Singh S S Bhoga

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      The performance of any solid state electrochemical gas sensor is always rated on its response time, thermodynamic stability, operating temperature, gas sensing ability, sensitivity and gas concentration range which is sensed. Here, we have reviewed the factors contributing towards a gradual development of electrochemical solid state SOx sensor in terms of a continuous tailoring of its two basic components, i.e. solid electrolyte and reference electrode with high ionic and mixed (ionic + electronic) conductivities, respectively.

    • Recovery characteristics of optical hydrogen sensor using Pd thin film: Behaviour of three-stage hydrogen desorption

      Yoshiki Okuhara Masasuke Takata

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      The recovery behaviour of optical hydrogen sensor using Pd film has been investigated. The change in reflectance with hydrogen desorption indicated distinctive characteristics with three-stage curve which depended on thickness of the Pd film. Moreover, the distinction became clearer with increasing number of absorption-desorption cycles. These behaviours were related to the change in pressure concentration isotherm.

    • Influence of metal particles on the reduction properties of ceria-based materials studied by TPR

      G Ranga Rao

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      Temperature programmed reduction (TPR) investigations on ceria-based catalytic materials are presented. Pure ceria shows two major reduction regions around 790 K and 1100 K due to surface capping oxygen ions and bulk oxygen ions. The extent of reduction in the low temperature region depends greatly on the surface area of the sample. The remnant reduction features that appear below 700 K are ascribed to O2− ions located at various low coordination sites on the oxide crystallites. The substitution of ZrO2 (40–60%) in CeO2 lattice decreases the overall reduction temperature of high surface area samples. Metal particles supported on low surface area CeO2-ZrO2 solid solutions induce bulk reduction of the support at low temperatures to a great extent. These observations provide a clue that the CeO2-based oxide supports, irrespective of their surface area, can perform as good oxygen exchangers in the presence of metal particles, contributing indirectly to the overall catalytic activity.

    • Comparative study of adatom induced relaxations and energetics for Si, Ge and carbon adsorption on a (2×1) Si(001) surface

      U P Wad A V Limaye M P Gokhale S B Ogale

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      Surface complexes involving silicon, germanium and carbon adatom on predefined adsorption sites of the 2×1 reconstructed Si(001) surface are investigated for their energetics and associated structural relaxations. Tersoff’s semi-empirical potential is used and the relaxations are obtained by employing the Monte Carlo simulated annealing technique. The results for the cases of Si, Ge and carbon adsorptions are compared. It is found that Si and Ge as adatoms on Si(001) 2×1 reconstructed surface behave in a similar fashion but the carbon adatom behaves in a markedly different way. Specifically, the carbon adatom induces a Si-C-Si chain configuration. It is also found that the adsorption sites between two dimer rows are the most favourable ones.

    • X-ray studies on microstructural characterization of tantalum substituted tungsten mixed molybdenum diselenide; W0·65Mo35−xTaxSe2 (0≤x≤0·35)

      Debashis Palit S K Srivastava B K Samantay Ray

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      The wide angle X-ray diffraction on W0·65Mo5−xTaxSe2 (0≤x≤0·35) compounds have been used for calculation of the crystallite size by method of variance and Fourier technique and for microstructural parameters purposes. The crystallite size showed the decreasing trend with increasing composition up tox=0·15 and after that it increased sharply. On the contrary, mean fractional change in interlayer spacing, fractions of the planes affected by defects, dislocation density, and root mean square strain showed the opposite trend. The crystallite size anisotropy and stacking fault probability showed 1:1 correspondence in their variation with composition. The radial distribution analysis have also been employed to find out the information about the inter atomic distances, coupling constants and mean square displacements for different pairs of atoms. These results have been interpreted in terms of difference in packing of layers in these compounds.

    • Open- and short-circuit thermally stimulated currents in ethyl cellulose (EC): polymethyl methacrylate (PMMA) blend

      P K Khare J M Keller S C Datt

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      Mechanisms of charge generation and its persistence in one and both-side vacuum-aluminized ethyl cellulose (EC):polymethyl methacrylate (PMMA) blend thermoelectrets, prepared under different fields (10, 25, 50 and 100 kV/cm) and temperatures (40, 60, 80 and 100°C), have been analysed using short- and open-circuit thermally stimulated depolarization current (TSDC) technique. The TSDCs were recorded by reheating the samples at a linear heating rate of 4°C/min. The TSDC thermograms of polyblends containing EC:PMMA in different weight ratio are, in general, characterized with two peaks in lower and higher temperature regions. However, the polarity of the peaks was found to be just opposite in short- and open-circuit TSDC measurements. Moreover, results on 97:3, 93:7 and 90:10 EC:PMMA polyblends indicated that the current increases with concentration of PMMA. The results indicate the existence of heterocharge due to dipole orientation and ionic charge drift together with the injection of charge carriers from electrodes with their subsequent localization in surface and bulk traps. Further, the chances of charge trapping in polyblends, at the interfaces are greater than in the individual polymers.

    • Effect of hydroxyl content on the physical properties of calcium metaphosphate glasses

      B Karmakar P Kundu Anup K Chaudhuri K Annapurna A Kumar R N Dwivedi

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      The hydroxyl (OH) content of calcium metaphosphate glasses has been controlled in the range 50–800 ppm by melting calcium dihydrogen phosphate in air, under vacuum and with fluoride addition. Density, refractive index and glass transition temperature of the glasses increase with decrease in OH content while the coefficient of thermal expansion remains almost unchanged. With gradual decrease in OH, the UV cutoff initially shifts towards shorter and finally towards longer wavelengths. IR spectroscopic study shows that the OH groups exist exclusively in the hydrogen bonded states. Correlations of the glass properties with OH content have been explained in terms of structural rearrangement leading to the change in P-O bond length and O-P-O/P-O-P bond angles of the PO4 tetrahedral units of (PO3)n chains. These changes are caused due to conversion of non-bridging oxygens (NBOs) of the H-bonded OH groups into bridging oxygens (BOs) during progress of dehydroxylation.

    • Rare earth fluorescence in NASICON type phosphate glass, Na3TiZnP3O12

      G Vijaya Prakash P Nachimuthu M Vithal R Jagannathan

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      The emission spectra of Pr3+, Eu3+ and Dy3+ doped in NASICON type phosphate glass, Na3TiZnP3O12 (NTZP) are studied. The dopant rare earth ions occupy sites with 8–9 coordination in a highly covalent environment. For Pr3+ ion, calculated and observed branching ratios for lasing transitions3P03H4,6 agree well and found to be 0·64 and 0·24, respectively. The emissions of Pr3+ show strong temperature dependence on account of Boltzmann population of the higher excited states at room temperature. The excitation spectrum of Eu3+ gives rise to phonon assisted side band for5D27F0 transition at higher energy side with a phonon energy maximum of 1022 cm−1 and an electron phonon coupling strength (g) of 0·018. The value of phonon energy maximum agrees with infrared spectral data. The results show that observation of high energy emissions in phosphate glasses require much higherg values. The red/orange and yellow/blue transitions of Eu3+ and Dy3+, respectively show that the Eu3+ occupy more distorted site than Dy3+.

    • Microscopic order parameter of two nematogenic compounds using X-rays

      A P Divya D Revannasiddaiah R Somashekar M S Madhava

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      X-ray Laue type diffraction patterns were recorded using the image plate system at different temperatures in the nematic phase for two nematogenic compounds. Micro-orientational order parameters have been computed using different methods. These are compared with the microscopic orientational order parameters obtained from refractive index data. Higher order parameters have also been computed and compared. Distribution function at various temperatures for the two nematogenic compounds from Deutsch method has been reported.

    • Design and fabrication of liquid phase epitaxy system

      R Venkataraghavan N K Udayashankar Blasius Victor Rodrigues K S R K Rao H L Bhat

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      The design and fabrication of a simple and versatile liquid phase epitaxial (LPE) system has been described. The present LPE system makes use of the horizontal multi-bin boat and slider arrangement which enables the growth of even multilayered structures. The growth chamber is heated by a single-zone resistive furnace precisely controlled through a Eurotherm 902P temperature programmer and controller. The vacuum manifolds and accessories are set up in such a way as to ensure high vacuum needed for growth experiments. The provision is also made to admit high purity gases like hydrogen or nitrogen into the growth chamber. The design has been kept simple without sacrificing the versatility and adaptability for novel growth experiments. The typical films grown by this LPE system are also presented.

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