The incorporat~on of Zr0 2 in Al~3 matrix was performed by conventional mechanical mixing and co-precipitation methods. The densification behaviour on sintering up to 1800°C and the resulting microstructural features of these ceramics have been studied. In either case no appreciable densification was noticed up to l600°C. The coprecipitated powder compacts containing 20% Zr0 2 after sintering at 1700°C achieved the density of 4.14 gm.cm-3 and open porosity about 3.0 %· Microstructural features showed a pronounced grain growth inhibition effect with increasing concentration of Zr02• Vacuum hot-pressing was found to be beneficial from the point of view of densification and mechanical properties of these compacts.

Alumina bodies have been prepared from commercially available, technical grade alumina. Effects of processing parameters and additions like clay, pyrophyllite and magnesium carbonate on densification have been studied in the range of 1500°-1800°C with a view to the utilization of these compositions for electrical applications.The bodies have been evaluated for microstructures and electrical properties at room temperature for 20 MHz. It was observed that alumina bodies containing 91-93% Al 2 0 3 densified to the extent of 3.4-3.5 gm.cm-3 with imperviousness at 1500° to 1600'C. Bodies containing pyrophyllite exhibited dissipation factor as low as 0.0002 along with dielectric constant (K), in the range 6-7, while bodies containing alumina over 97% had a dissipation factor as low as 0.0001 having K in the range 7-8 when densities of these bodies exceeded 3.6 gm.cm-• and open porosity decreased below 1 %. IntroductionEconomical industrial processing and fabrication of alumina ceramics demand the use of commercially available pure alumina as a raw material and lower sintering temperature. Hence, in the present work, influence of additions such as china clay and pyrophyllite to the extent of 10 parts by weight of body composition (> 93% Al 2 0 3 ) and that of comminution, on densification and electrical properties 1 -4 like dielectric constant, dissipation factor etc of alumina bodies have been studied. Magnesia addition in the form of MgC0 3 was made to the extent of 0.5 and 1.5% to control the grain growth. Experimental observations show that suitable grinding operation and magnesia addition help in densification of these batch compositions at comparatively lower temperature and also improve the electrical properties such as dielectric constant and dissipation factor to the level of acceptance.

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The influence of grinding process and minor additions like MgC03 and Ti0 2 on sintering behaviour of technical grade commercial alumina has been studied with a view to making dense and impervious alumina wares. The grinding process has significant influence on densification of these powders prepared by variation in the ratio of material and ball used in comminution for reduction of particle size as well as increasing surface activity of the powders. Powders prepared by this method showed rapid densification between 1500° and 1600°C while minor addition of Ti0 2 resulted in dense compacts of these powders (over 90% T.D.) at temperatures as low as 1400°C. Sintering treatment at higher temperature (up to 1800°C) did not indicate further densification; however, it helped in improving microstructural characteristics and resulted in translucency in these samples (imperviousness), indicating recrystallization of alumina.

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