H. K. Varma

Sol-spray technique for fine-grained ceria particles

Abstract Submicron-range cerium oxide powders of narrow size distribution have been prepared by spray-drying of a sol derived from cerium nitrate. Cerium hydroxide is first precipitated by taking care to keep cerium in the Ce3+ state and later peptized to a stable sol at pH 3. The stability of the sol at different pH values was studied by nephelometry. Thermal-decomposition characteristics of spray-dried cerium hydroxide, the formation of the CeO2 phase, and the physical properties of calcined ceria, including particle size and distribution, are reported. A crystallite size as small as 100 A could be obtained.

Thermal decomposition of citrate precursor for 1-2-3 high-Tc superconductor

On a utilise des citrates comme precurseurs pour la preparation de ceramiques supraconductrices YBaCuO par le procede sol-gel. On etudie les phases formees lors du traitement thermique et la microstructure du materiau fritte

Synthesis of zinc oxide varistors through microwave-derived precursor

Abstract A rapid microwave decomposition method has been reported for the preparation of ZnO varistor precursors which on subsequent heat treatment for short period results in non-ohmic properties. Thus a mixture of nitrates of zinc and praseodymium along with minor additives such as cobalt, chromium and potassium when subjected to microwaves of 2.45 GHz in a domestic oven for 8 min, resulted in the decomposition of nitrates and formation of the precursor oxide mixtures. The precursor powder consists of easily friable agglomerates of size about 5 μm, having the major phase as ZnO. Such powder on further compaction at about 150 MPa and sintering in the range of 1200–1350 °C, resulted in high density (> 95%) varistors. The electrical measurements of the varistor sample showed nonlinearity coefficient (∝) above 30. Electrical field ( E ) vs current density ( J ) measurements showed breakdown fields of 135 V/mm for Pr 2 O 3 -doped varistors. The fractograph of the sintered varistor sample indicates an average grain size of 6–8 μm.

Cerium oxide powder with increased surface area for catalyst support

This letter presents the characteristics of thermally treated cerium oxide powder prepared by the above method according to the requirements for automotive catalyst support

Influence of the precipitation medium on the properties of fine alumina powders

Fine alumina powders have been synthesised by a homogeneous precipitation of aluminium sulphate by urea in water-glycerol, water-ethylene glycol and in pure water, followed by thermal decomposition of the hydroxide at 600°C. Alumina powder was also prepared by the conventional ammonia method for comparison. The powders were characterised by their particle size distribution, BET surface area, tap density and particle morphology (SEM). Point of zero charge of the powder was determined by the potentiometric method. The surface areas ranged from 43 m2/g to 105·5 m2/g, the particle size less than 0·5 micron from 50% to 12% and the point of zero charge from 7·05 to 7·85. The precipitation medium has been shown to have profound influence on the particle characteristics as well as on the surface charge of fine aluminium oxide powders.

Zone refining of sintered, microwave derived YBCO superconductors

Post-sintering treatments, such as zone melting under a thermal gradient, have been conducted on sintered YBCO tape cast films. YBCO precursor powder was derived through decomposition of a mixture of nitrates of cations in a microwave oven for ~ 4 min. The resulting powder was characterized and made into thin sheets by tape casting and then sintered at 945 °C for 5 h. The sintered tapes were subjected to repeated zone refining operations at the relatively high speed of ~ 30 mm h−1. A microstructure having uniformly orientated grains in thea-b plane throughout the bulk of the sample was obtained by three repeated zone refining operations. Details of precursor preparation, microwave processing and its advantages, zone refining conditions and microstructural features are presented in this paper.

Non-linearity in rare earth doped zinc oxide varistor prepared by flash combustion method

Abstract A novel processing technique involving flash combustion of nitrate-urea mixtures has been reported for praseodymium doped zinc oxide containing K 2 O in the range 0·0–0·5 mol%. The starting average powder particle size was 2 μm with a tap density of 1250 kg/m 3 . The non-linear behaviour of the prepared zinc oxide varistor was examined in the field range 200–650 V/mm. The breakdown voltage was seen to attain a maximum value of about 600 V/mm in compositions containing 0·3 mol% K 2 O. The variation of breakdown voltage is explained on the basis of the microstructure and K 2 O content.

Silver-YBCO composite derived from citrate gel

Silver-YBCO composite powders containing up to 50% by weight of silver have been prepared from nitrate mixtures as citrate felt followed by thermal decomposition. These powders possess a bulk density of 2 g cm-3, an average particle size of 3 mu m and near spherical morphology. Consolidated pellets of these powders when sintered at 915 degrees C in air showed a density around 95% theoretical. The XRD of the sintered composite has shown the orthorhombic YBCO phase, while the microstructure and EDAX showed a composite nature. Furthermore, the composite sample still retains the superconductivity on sudden cooling from 915 degrees C in air. The electrical resistivity is shown to decrease with increasing silver content in the composite.

Microstructure Development in Repeated Zone Refining of Microwave-Derived YBCO Bulk Compacts

YBCO precursor particulates have been prepared using microwave decomposition of a mixture of yttrium, barium and copper nitrates in a microwave oven having the frequency 2.45 GHz and 600 W power, within as short as 240 seconds. This precursor, upon further compaction into strips of 10 mm×60 mm×1 mm and sintering at 940°C for 5 h, attained 90% density. Such strips are subjected to repeated zone refining operations in a zone refining system having a three-zone furnace at relatively high speeds of 30 mm/h. The development of microstructure in such samples from the as-sintered condition through the various steps has been presented. A microstructure having uniformly orinted grains in the a-b plane throughout the bulk of the sample could be obtained by three repeated zone refining operations. The possibility of introducing a faster movement of the melting zone compared to earlier reports, is highlighted.

Gelation characteristics of aluminium titanate precursor sols in different solvent media

Abstract Stable sol containing boehmite (AlOOH) and titanium butoxide in 1:1 Al 2 O 3 TiO 2 stoichiometry was prepared in the presence of acetate ions. The progress of gelation was studied by nephelometry. Gelation was carried out in 2:1 water-isopropanol and water-dioxane mixtures having different polarities. The time required for the formation of aluminium titanate precursor gel is considerably reduced in the case of water-isopropanol and water-dioxane systems. All the gels have been spray-dried to spherical powders which were further characterised by thermal decomposition, particle-size distribution and surface morphology. The TGA curves were identical while there was considerable variation in the particle size and morphology.