T. V. Mani

Dependence of calcination conditions of boehmite on the alumina particulate characteristics and sinterability

Abstract Monohydroxy aluminium oxide, boehmite (AlOOH), has been prepared by hydrolysis of aluminium nitrate under controlled conditions of pH, concentration and temperature. The boehmite passes through stages consisting of different transition aluminas on calcination until the stable alumina is formed at around 1393 K. The different transition aluminas have been characterized by surface area analysis and X-ray diffraction (XRD). The sintering studies on compacted discs from the calcined aluminas carried out at 1873 K show the maximum density of above 98% theoretical density for boehmite calcined at 1273 K containing mixed phases of γ, δ, θ and α-aluminas. The study of the particle characteristics, compaction and sintered densities shows that it is possible to obtain high-density alumina with fine-grained microstructure starting from a powder with transition aluminas rather than pure α-alumina.

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.

Zone melting-refining in Bi-Pb-Sr-Ca-Cu-O superconductor prepared through the sol-gel method

A zone melting-refining technique, with a horizontal sample-holding pattern for incorporating grain orientation in sintered bismuth-based superconductor (BPSCCO) is presented. Acrylate precursor-derived BPSCCO powder was uniaxially compacted into strips of 60 mm×10 mm ×2mm and sintered at 845°C for 20 h to obtain 90% density. These strips were zone melted in a three-zone furnace which had an independent temperature control system for each zone. The grain orientation in the sample was studied by scanning electron microscopy and X-ray diffraction. In the case of a BPSCCO-Ag (5%) composite, the silver was seen to flow along the intergrannular boundaries of the BPSCCO grains. The present technique is an easy and fast method to introduce orientation of grains with large aspect ratio into sintered BPSCCO or Ag-BPSCCO composite under partial melting-refining conditions. The possibility of extending the present technique to silver-sheathed wire and tapes is also indicated.

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.

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.

Texture development in Bi-Sr-Ca-Cu-O (BPSCCO) and Ag-BPSCCO superconductor through zone melting-refining

A horizontal sample holding novel zone melting refining technique for incorporating grain orientation in sintered Bi-based superconductor (BPSCCO) strips is presented. A three zone furnace having independent temperature controls and a rate of zone movement of 30 mm/h was used. While a-b plane oriented grains appear in BPSCCO samples, in Ag-BPSCCO composites the ductile silver is seen to flow along the intergranular boundaries of BPSCCO grains.

Effect of solvent on formation characteristics of sol-gel aluminium titanate

Abstracts are not published in this journal

Sol–spray preparation, particulate characteristics, and sintering of alumina powders

Fine alumina powders of spherical morphology and narrow particle–size distribution have been synthesized by a technique that uses precipitation/peptization/spray drying of boehmite sol prepared from aluminum nitrate. The spray–dried powder was further washed with solvents of varying polarities, such as acetone, isopropanol, and tert-butanol. This post-spray-drying treatment changed the powders particle-size distribution, morphology, density, and compaction characteristics. Microstructure, dielectric properties, and effect of post-treatment on the boehmite-sol-derived alumina powders in reducing agglomeration are discussed.