Volume 11, Issue 1
September 1988, pages 1-85
pp 1-9 September 1988
The process of gelation through chemical polymerization is described for the preparation of transparent alumina. The nature of disorder in the transition phases of alumina is discussed using the results of XRD, IR, optical absorption and NMR. The microstructures in the monolithic porous xerogels are observed to be fractals arising due to instabilities while drying. Phase separation noticed in copper and chromium-doped xerogels is attributed to fluctuations in polymer density.
pp 11-19 September 1988
Optical properties of chromium-doped transparent alumina xerogels have been studied in the ultraviolet, visible and near infra-red regions. The coordination, valence state and symmetry of crystal field around the transition metal ion is found to change in different transition phases of the xerogel in accordance with the earlier conclusion that the metal ion penetrates into the alumina matrix and occupies aluminium sites. In boehmite and α-Al2O3 chromium is essentially in trivalent state, octahedrally coordinated, whereas in the transitionγ andδ phases tetrahedrally coordinated hexavalent state appears to be dominant. This agrees with the spinel structure expected for the transition phases.
pp 21-30 September 1988
Lead nitrate phosphate (LNP) single crystals were grown in silica hydrogel at an ambient temperature employing various nucleation control methods. The effect of concentration of feed solution, gel density, gel pH, ageing of gels and temperature on reduction of nucleation centres is reported. Kinetic studies of the nucleation and growth of LNP crystals and dentrites were studied. The crystals are characterized by X-ray diffraction, density measurements, magnetic susceptibility, mechanical strength measurements and dissolution behaviour.
pp 31-37 September 1988
Rubidium hydrogen tartrate single crystals were grown in silica hydrogel. Controlled reaction was employed between tartaric acid and rubidium chloride solutions by slow diffusion process in the gel medium. Colourless transparent crystals were grown at room temperature. The average density of crystals measured pyknometrically was 2·263 g cm−3. Thermal stability of the materials was studied. Chemical etching was employed to determine the perfection of these crystals.
pp 39-41 September 1988
The possibility of the formation of solid solution in the system Ca1 −xLaxTi1 −x CoxO3 forx⩽0·5 has been investigated. X-ray diffraction studies show that compositions withx=0·05, 0·1, 0·2, 0·3 and 0·5 prepared by the ceramic method are single-phase materials. All the compositions have a structure similar to CaTiO3 with a pseudo-cubic unit cell. Preliminary studies show that interfacial polarization contributes significantly to their dielectric constant.
pp 43-48 September 1988
Crystallographically oriented etch traces produced by selective etchant on (111) and (110) habit faces of dicalcium strontium propionate [Ca2Sr(C2H5CO2)6] crystals are attributed to the growth traces nucleated during the superficial growth of crystal. This view is supported by the absence of such etch traces on seized habit faces and from the observations of layer structure on the bottom faces. Mother liquid acting as an etchant produces circular terraced depressions on habit faces. These depressions originate at the sites of isolated impurity centres. Identical features such as circular etch structures bounded by cylindrical outer periphery produced on cooled faces are attributed to isolated domains.
pp 49-54 September 1988
Lanthanum-modified lead zirconate titanate (PZT) powder and volatilisable polymethylmethacrylate (PMM) polymer particles have been used for fabrication of porous sintered ceramics of interconnected porosity varying from 25% to 59%. Sintered ceramics are converted into piezoelectric PZT-polymer composites by incorporating silicone rubber elastomer followed by electroding and poling. Influence of the variation of PZT-PMM ratio and sintering temperatures on the open and closed porosity of the sintered ceramics as well as volume fraction PZT in the composites has been studied and correlated for the optimization of piezoelectric properties. The PZT-polymer composites possess low density, considerably high piezoelectric voltage coefficient and considerably lower ageing characteristics and are therefore considered suitable for designing highly sensitive hydrophone systems.
pp 55-61 September 1988
Dielectric constant variation with temperature and frequency is reported for barium-borate glass-bonded lead zirconate. Lowering of the relative permittivity of the ceramic is attributed to the presence of the glass.
pp 63-66 September 1988
The d.c. electric field effect on ferroelectric triglycine sulphate (TGS) and TGS-Se crystals at high temperature above the Curie temperature shows the formation of a solid state battery. The current and the electromotive force of the battery are measured and their changes observed near the Curie temperature. Even if the electrodes are short-circuited the emf recovers almost to the same value.
pp 67-73 September 1988
Partial substitution of Ce in place of Y has been achieved by taking care of charge compensation and compounds with the general formula Y1-2xCexBa2+xCu3O6·5+δ have been successfully prepared. X-ray powder diffraction analysis reveals that this substitution gives orthorhombic single phase for 0⩽x⩽0·3. TheTc remains nearly unchanged between 94-90 K.
pp 75-85 September 1988
Based on the physical interpretation of the linear equation of state (EOS) of dense solids under shock loading, which relates particle and shock speeds asUs=Cb+gUp, the EOS for porous solids has been developed and is expressed asUs*=ΨCb*+g*Up whereCb* andg* are effective bulk sound speed and effective inverse ultimate volume strain respectively. Ψ is a pore collapse function introduced specially to differentiate loading and unloading behaviour.Cb* andg* are derived theoretically whereas Ψ is established empirically as Ψ=f(Up,Cb). This EOS does not call for any experimentally established material constant to describe the effect of porosity. Also its ability to describe the unloading behaviour distinguishes it from the presently available equations of state.
Volume 42 | Issue 5
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