• Volume 18, Issue 7

      November 1995,   pages  831-936

    • Tooth—A natural biocomposite

      Saraswati Venkataraman

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      The inorganic content of human tooth is predominantly hydroxyapatite. The content varies in enamel and dentin. A reversible thermal transformation at 157 C is observed and has correspondence in the dielectric measurements. The dental decay is related to the piezoelectric property of tooth, which is similar to a polar glassceramic. The toughness of tooth is higher than hotpressed hydroxyapatite and owes it to the microstructure signifying the importance of a weak, pliable interface. The hardness is graded, higher for enamel in the crown and very much lower for dentin, to suit their function and environment. The relevance of biomimics in designing materials for advanced performance is discussed.

    • Alumina-glassceramic composites

      Saraswati Venkataraman

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      Alumina-glassceramic composites show good densification and uniformly distributed fine-sized particle microstructure owing to the liquid phase of the glassceramic at the sintering temperatures. The flexure strength decreases with increase in the glassceramic content. This is attributed to the dissolution of alumina at the interface resulting in a changed crystalline phase, glass coating around the matrix grains and the difference in thermal expansion. These aspects have been confirmed through expansion coefficient measurements.

    • Static and impact fatigue behaviour of borosilicate glass

      Soumen Maity B K Sarkar

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      Failure of a borosilicate glass as a result of repeated impact has been studied. Impact fatigue study was conducted in an improved pendulum type repeated impact apparatus specially designed and fabricated for determining single and repeated impact strength. For elimination of the effect of humidity, repeated impact tests were carried out under liquid nitrogen. Quasi-static measurements were determined under four-point bending. Using a square waveform as applicable to the present impact tests and fracture mechanics interpretation, the number of cycles to failure during impact fatigue tests were predicted from quasi-static fatigue measurements. It has been shown that repeated impact loading has a deleterious effect on the failure cycles compared to slow stressing. The role of an added mechanical effect during repeated impacts has been suggested in controlling the cyclic fatigue behaviour.

    • Liquid phase epitaxial growth of pure and doped GaSb layers: morphological evolution and native defects

      P S Dutta H L Bhat Vikram Kumar

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      Undoped and Te-doped gallium antimonide (GaSb) layers have been grown on GaSb bulk substrates by the liquid phase epitaxial technique from Ga-rich and Sb-rich melts. The nucleation morphology of the grown layers has been studied as a function of growth temperature and substrate orientation. MOS structures have been fabricated on the epilayers to evaluate the native defect content in the grown layers from theC-V characteristics. Layers grown from antimony rich melts always exhibitp-type conductivity. In contrast, a type conversion fromp- ton- was observed in layers grown from gallium rich melts below 400 C. The electron mobility of undopedn-type layers grown from Ga-rich melts and tellurium doped layers grown from Sb- and Ga-rich solutions has been evaluated.

    • A comparative study on CdSe synthesized at low temperature

      H Padmanabha Sarma V Subramanian N Rangarajan K R Murali

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      CdSe is a well-known semiconductor which finds applications as photoconductors, photovoltaic cells and other optoelectronic devices. In this paper structural studies and carrier concentration measurements of vacuum evaporated CdSe films obtained by using CdSe synthesized by simple precipitation technique are presented.

    • Thermal expansion studies of substituted CTP derivatives

      A K Ganguli K P Jayadevan

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      Several new Na, Y and Zr substituted derivatives of Ca0·5Ti2(PO4)3 (CTP) have been synthesized. These derivatives retain the hexagonal structure of the parent (CTP) compound with minor changes in lattice parameters. Linear thermal expansion coefficients(α) have been obtained using a high sensitivity dilatometer.

    • Effect of doping concentration on normal state resistivity of La2 −x (Ba, Sr)x CuO4 superconductors

      Dinesh Varshney R K Singh

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      Based on free electron layered electron gas model of quasi two dimensional CuO2 layers in La(Ba/Sr)CuO superconductors a model potentialV(q) is developed earlier with the electron-electron and electron-phonon interactions. The model approach facilitates the dielectric functions and the dispersion relations of 2D acoustic phonon and plasmon modes. We have then worked out the coupling strength (γ) linking electrons to the 2D acoustic phonon mode (ħω_) from the residue at the pole ofV(q). Furthermore, the scattering time (τe−ph) during electron-phonon interaction (EPI) for this simplified system is also estimated. The contribution to the normal state in plane resistivity due to EPI is then evaluated. Finally, the variations ofτ andρ is studied with the doping concentration (x) and temperature (T) and the results obtained by us show reasonably good agreement with the available experimental data.

    • Nucleation and growth study of copper thin films on different substrates and wetting layers by metal-organic chemical vapour deposition

      A Dutta J Goswami S A Shivashankar

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      Chemical vapour deposition of copper thin films on different diffusion barrier/adhesion promoter layers have been studied. Copper thin films were grown in low pressure CVD reactor, using Cu(dpm)2 as precursor and argon as carrier gas. Growth rates, film adhesion to the substrate, and surface morphology were studied in detail.

    • Studies on spray pyrolyzed molybdenum trioxide thin films

      P S Patil R S Patil

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      Semiconducting molybdenum trioxide thin films have been prepared by employing simple and inexpensive spray pyrolysis technique. Films are found to be polycrystalline in nature with hexagonal phase. Optical band gap energy (direct) and room temperature electrical resistivity are of the order of 2·9 eV and 108 ohm-cm, respectively. These films exhibit cathodic electrochromism.

    • (PEG)xNH4ClO4: a new polymeric fast proton conductor

      Nader Binesh S V Bhat

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      A new polymer electrolyte (PEG)x NH4ClO4(x = 5, 10, 15, 20) has been prepared that shows protonic conduction. The room temperature conductivities are of the order of 10−7S/cm, and increase with decrease in salt concentration. NMR line width studies indicate fairly low glass transition temperatures of the polymer salt complexes.

    • Synthesis and properties of rare earth doped lamp phosphors

      S Ekambaram K C Patil

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      Red, blue and green emitting lamp phosphors such as Eu3+ doped Y2O3 (red phosphor), Eu2+ doped Ba0·64Al12O18·64, BaMgAl10O17 and BaMg2Al16O27 (blue phosphors) and Ce0·67Tb0·33MgAl11O19 and Eu2+, Mn2+ doped BaMgAl10O17 (green phosphors) have been prepared by the combustion of the corresponding metal nitrates (oxidizer) and oxalyl dihydrazide/urea/carbohydrazide (fuel) mixtures at 400°–500°C within 5 min. The formation of these phosphors has been confirmed by their characteristic powder X-ray diffraction patterns and fluorescence spectra. The phosphors showed characteristic emission bands at 611 nm (red emission), 430–450 nm (blue emission) and 515–540 nm (green emission). The fine-particle nature of the combustion derived phosphors has been investigated using powder density, particle size and BET surface area measurements.

    • SrB4O7:Bi2VO5·5 — A novel nanocomposite

      M V Shankar G N Subbanna K B R Varma

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      Glass composites of strontium tetraborate, SrB4O7(SBO) with bismuth vanadate, Bi2VO5·5 (BiV) of the composition (1 −x) SBO-x BiV (0 ≤x ≤ 0·75), have been synthesized. X-ray powder diffraction and electron microscopy indicate as-quenched composites to be amorphous and the annealed samples showed the presence of nanometer sized particles of BiV dispersed in the glassy matrix of SBO. The dielectric constant of these composites increases with increase in the volume fraction of BiV, at 300 K. The measured dielectric constant of the composite very nearly obeys the Maxwell’s relationship. Optical transmission studies confirm a steady shift in the optical absorption edge towards longer wavelengths with increase inx.

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