• Volume 21, Issue 2

      April 1998,   pages  107-184

    • Crystalline alumina films prepared by nebulized spray pyrolysis

      P Murugavel Ita Benedict A R Raju

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      Crystalline alumina films have been successfully deposited on Si(100) and amorphous silica substrates by the nebulized spray pyrolysis technique. The surface morphology of the films has been studied by various microscopic techniques. The films exhibit satisfactory microhardness and frequency independent capacitance.

    • The epitaxial growth of Ag on Si(111)-(7 x 7) surface and its (√3x√3)-R30 surface phase transformation

      S M Shivaprasad Santanu Bera Y Aparna

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      Ag is adsorbed in ultra-high vacuum on to the (7x7) reconstructed Si(111) surface with submonolayer coverage control with a deposition rate of 3-3 x 1012 atoms/cm2/sec. The initial stages of growth and intermediate equilibrium phase formation are determined by using low energy electron diffraction (LEED) and X-ray photoelectron diffraction (XPD) for structural information, and auger electron spectroscopy (AES) and electron energy loss spectroscopy (EELS) for composition and interaction analyses. Room temperature (RT) adsorption results in the nearly epitaxial (1 x 1) surface phase growth in the simultaneous multi-layer growth mode. The quenching of the dangling bond states during adsorption is observed by monitoring thep-character of the Si LVV auger peak. For depositions carried out at high temperatures (HT), several plateaus in the auger uptake curve with the (√3 x √3)-R30° LEED structures are formed. It is observed that a minimum coverage of 0–33 monolayer (ML) is required for the formation of the (√3 x √3) phase and this phase causes the reappearance of thep-electron-related states that were quenched by 1.0 ML adsorption at RT. However the (√3 x √3) is observed for higher coverages (0.66 and 1.0 ML) also. The polar angle anisotropy of Si(2p) emission in XPD indicates the rearrangement of substrate Si atoms for the formation of the (√3 x √3) phase. The EELS data also shows relevant changes due to adsorption of Ag at RT and upon annealing. The results suggest the importance of controlled deposition parameters, the lack of which may have kept the determination of the nature and coverage of the (√3 x √3) surface phase unresolved in literature.

    • Growth of anthraquinone crystals by gel aided solution technique and their characterization

      K Rajendra Babu M Deepa C M K Nair V K Vaidyan

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      Anthraquinone, an oxidation product of anthracene, has been grown by gel aided solution technique employing sodium metasilicate. Needle shaped crystals were obtained in 0–5% (w/v) solution of anthraquinone for pH 5-5. The grown crystals were characterized by optical microscopy, SEM, IR, UV and XRD.

    • Growth of undoped and Te doped InSb crystals by vertical directional solidification technique

      D B Gadkari K B Lal B M Arora

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      We have successfully grown high mobility undoped and Te doped InSb crystals of size 10–12 mm dia. and 60 mm length under inert argon atmosphere in closed quartz ampoules, by vertical directional solidification (VDS) technique. The crystals showed predominantly (220) orientation along the growth axis. The surface defects, such as voids were reduced drastically by selecting proper lowering rate, rotational speed and cone angle of the ampoule. The high mobility and quality crystals were obtained with the ampoule conical angle less than 20°, lowering rate 5mm/h, and rotational speed 10 rpm.

    • Studies on synthesis of calcium ferrite-based bio glass ceramics

      B R Jagadish N Prabhu D Bahadur

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      The possibility of synthesizing a Ca-ferrite based biocompatible glass ceramic has been explored in the following two glass compositions: (i) 28Na2O-8CaO-3P2O5-llFe2O3-50SiO2, and (ii) 25Na2O-8CaO-3P2O5-20Fe2O3-41SiO2-3B2O3 (in weight ratio). The effect of simulated body fluid on the different glasses and glass ceramics was also investigated. While there is no direct evidence for apatite formation, the weight losses recorded and formation of a Si-rich layer at the surface appears to be an indication of onset of apatite formation. The rate of apatite formation is presumably retarded due to the presence of Al3+ (picked up from AL2O3 crucible). Ferromagnetic resonance experiments at 9.03 GHz demonstrate that these glass ceramics can possibly be used for microwave hyperthermia.

    • Electrical conduction mechanism in solution grown doped polyvinyl pyrrolidone films

      P K Khare S K Paliwal R Kuraria H L Vishwakarma Ashish Verma S K Jain

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      A detailed study of electrical conduction mechanism in bimetallized ferrocene-doped polyvinyl pyrrolidone films was carried out. The measurements were carried out on films of about 20 μm thick, in the field range of (2.0–8.0) x 104 V/cm at temperatures ranging from 363 to 423 K. An investigation of the effect of impurity such as ferrocene in the polymer matrix was undertaken. Lowering of activation energy and increase in current due to doping were observed. The results showed that the charge carriers were generated by field-assisted lowering of coulombic barriers at the traps and were conducted through the bulk of the material by a hopping process between the localized states by a Jonscher-Ansari modified Poole-Frenkel mechanism. The dependence of current and activation energy on the ferrocene concentration is explained on the basis of charge transfer type of interaction between dopant and polymeric material.

    • Electrical and optical properties of rf-sputtered CdTe films

      P C Sarmah A Rahman

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      In this paper some electrical and optical properties ofn-type CdTe films prepared by rf sputtering at 180 W power have been reported. For doping the films a number of pellets of pure Cd placed on the CdTe target were simultaneously sputtered with the target material to get Cd-doped CdTe films. The films after doping were foundn-type. Maximum doping concentration obtained this way was of the order of 1014 cm−3. XRF spectra of target material and the rf-sputtered films were found to be more or less similar. All the films were found to have large number of defects indicated by profound aging effect in the initial stages of aging. The films became stable for measurements after about 8–10 days. Activation energy and band gap found from the temperature dependence of dark conductivity were 0–5 eV and 1.43 eV respectively. Photoconductivity of the films was studied and the photoconductive rise time, decay time and the decay constants were determined from the photoconductive rise and decay curves at 500 Lx and 1000 Lx of intensity of illumination.

    • Hydrogen passivation studies in Pd/n-GaAs devices

      U P Singh

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      The effect of hydrogen on the current (I-V) and capacitance (C-V, TSCAP) has been studied for Pd/n-GaAs diodes. Hydrogenation has been found to improve ideality factor of the diode. ReverseC-V characteristics show that the number of shallow and deep donors is reduced on hydrogenation. The TSCAP measurement shows the presence of two donor states at ∼ 0.48 and 0.72 eV which is being partially passivated on hydrogenation. The likely origin of these states is discussed.

    • Conducting polymer nanocomposites with extremely low percolation threshold

      B M Mandal

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      Abstract. This article gives a brief review of our work on conducting polyaniline nanocomposites which exhibit extremely low percolation threshold, ca. <003 vol.%. The nanocomposites are essentially blends of nanoparticles of HCI doped polyaniline (PANI) with conventional polymers. The nanoparticles (<20 nm) were prepared by sonicating a suspension of sterically stabilized conoidal PANI particles in solutions of conventional polymers. The sonication process breaks down the stable colloid particles into nanoparticles. The latter are unstable and they aggregate fractally to yield nanocomposites with extremely low percolation threshold.

    • Infrared and chemical durability studies of some binary PbO and ZnO phosphate glasses and their corresponding sodium metaphosphate derivatives

      N Abd El Shafi S Ibrahim

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      Abstract. Structural conclusions are obtained for glasses in the binary PbO and ZnO phosphate systems by IR spectroscopy. Alterations in bands, frequencies and intensities as a function of chemical composition are interpreted and these indicate various types of interlink between Pb2+ and Zn2+ cations and the host network, especially, when they are incorporated as NWF’s. Moreover, the metaphosphate structure, theoretically possible to be achieved in the structure of glasses with equimolar compositions 50M(II)O·50P2O5 (mol%) (where M is Zn or Pb), is not confirmed by their spectral patterns. However, this structure is confirmed to be achieved in the structure of glasses with 20Na2O·30PbO·50P2O5 and 40Na2O·10ZnO·50P2O5 (mol%). The dissolution rates of most of the studied glasses were measured as a function of pH and variation in chemical composition. The results are discussed in light of the conclusions obtained from the IR study, structural model of phosphate glasses (Van Wazer) and the previously suggested mechanisms for dissolution of phosphate glasses.

    • Compaction behaviour of plaster of Paris dewatered- and air dried-Nigerian clay

      C S Nwobodo

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      The effects of compaction pressure on the green bulk density and porosity of Nigerian clay were examined over a range of pressure from 47-2 to 157-3 MPa. An exponential behaviour was found between pressed relative density and compaction pressure, the logarithm of relative density increasing with compaction pressure.

      An exponential behaviour was also found between compaction pressure and true porosity, the latter decreasing as the logarithm of compaction pressure increased. Finally, empirical equations relating to true porosity and compaction pressure and also the relationship between relative density and compaction pressure, are presented.

    • CCA teachability of slow dried three major bamboo species of Bangladesh

      A K Lahiry

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      Chromated copper arsenate (CCA) teachability tests on full cell pressure impregnated (with 2–3% CCA solution) and slow dried (six months air-drying under cover) bamboo block of three major bamboo species of Bangladesh revealed initial insignificant leaching of CCA within first week and no leaching in next week. Use of low concentration of CCA, release of particle form of CCA due to exposure of bamboo blocks by cutting and presence of water soluble extractives in bamboo might be the causes for initial leaching of CCA.

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