Influence of initial crystal structure and electrical pulsing on densification of nanocrystalline alumina powder
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Influence of initial crystal structure and electrical pulsing on densification of nanocrystalline alumina powder R. S. Mishra, S. H. Risbud, and A. K. Mukherjee Department of Chemical Engineering and Materials Science, University of California, Davis, California 95616 (Received 5 August 1996; accepted 12 August 1997)
The effect of the crystal structure of starting alumina powder and electric pulsing on the initial stages of densification has been studied in the temperature range of 1200– 1500 ±C. Multiple electric pulsing cycles enhance the densification significantly. The a-alumina powders consolidate more readily in comparison to g-alumina powders. A high density a-alumina specimen (.98% of theoretical density) was obtained at 1300 ±C in less than 10 min.
The benefits of electro-discharge compaction for metallic systems have been well documented (for example, see Refs. 1 and 2). The initial electro-discharge usually involves very high currents, to the order of several kA. Recently, researchers have used Plasma Activated Sintering (PAS) equipment3–12 for densification of a number of metallic and ceramic systems. The first stage during PAS consists of electric pulsing. Although the exact mechanism of enhanced consolidation during PAS is not clear, it is hypothesized that initial electric discharge generates a plasma which conditions the powder surface and leads to enhanced sintering kinetics. PAS has been successfully used to densify a number of ceramic materials, like AlN,3 Al2 O3 ,8,9 and Al2 O3 –TiO2 .10,11 Mishra et al.11 and Schneider et al.12 have suggested that the applicability of PAS might be linked with the dielectric properties of the powders. This suggestion is based on the correlation between the observed enhancement in densification during PAS and the dielectric constant. Higher dielectric constant of the powders means that powder surfaces can hold higher electric charges and because of the charge build-up the individual powders can be considered as capacitors. Subsequent electric discharge would ionize the gas in between the powders, thereby generating plasma. Risbud et al.9 observed full densification of MgO doped a-alumina at significantly lower temperatures and shorter time during PAS. On the other hand, Mishra et al.8,10,11 observed that although PAS led to higher sintering of g –Al2 O3 , comparable to microwave sintering,13 it was difficult to obtain near full density. This was partly due to the development of vermicular structure14 during the transformation of g –Al2 O3 to a –Al2 O3 . Messing and co-workers15–17 have used seeding to enhance trans86
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J. Mater. Res., Vol. 13, No. 1, Jan 1998
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formation and densification of g –Al2 O3 . In this paper, we report the results of a follow-up research on PAS of alumina to determine the influence of (a) the electric pulsing cycle on densification, and (b) the starting crystal structure of the alumina powders, i.e., a and g. The details of plasma activat
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