Plasma activated sintering of additive-free AlN powders to near-theoretical density in 5 minutes
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A1N powders (particle size = 0.44 ± 0.08 /xm) containing no deliberate sintering additives were consolidated to near theoretical density in 5 min at 2003 K (1730 °C) using a Plasma Activated Sintering (PAS) process. PAS is a novel consolidation method that combines a very short time at high temperature with pressure application in a plasma environment. The in situ cleaning ability of powder particle during plasma activated densification leads to enhanced particle sinterability. The densities of undoped A1N specimens that were PAS consolidated at 2003 K for 5 min under 50 MPa pressure ranged from 97.5 to 99.3% of theoretical. The initial submicron particle size of A1N powders was retained in the final microstructure that consisted of polycrystalline grains with an average size of —0.77 ± 0 . 1
New materials and processes that are manufacturefriendly as well as cost competitive are being sought constantly in an era of sophisticated high quality materials processing for electronics, aerospace, and chemical applications. In the field of ceramics and powder metallurgy, powder consolidation provides a highly versatile near net shape forming method for a wide variety of engineering components with minimal materials wastage and low machining costs. At present, most powder consolidation in industry is done by sintering or hot pressing, with hot isostatic pressing (HIP) gaining much recent popularity for ceramics and refractory materials. Despite the adequacy of these processes for materials production in the past, a rapid consolidation processing method that can be transferred to the manufacturing sector would represent a major advance in sophisticated materials processing technology. Rapid heating and densification of ceramics dates back to the late 1960's when Bennett et al.1 reported plasma sintering of ceramic oxides. Johnson and co-workers have reported a number of investigations in recent years2"7 using plasma sintering of mainly alumina based ceramics. As discussed in an overview by Johnson,2 microstructural coarsening during the very early stages of sintering (by surface diffusion) reduces the driving force for continued densification and increases diffusion distances. Use of rapid heating rates minimizes the surface diffusion problem and enhances densification rates.2^10 We report here the successful densification of A1N powders, of interest in electronic substrate production, by Plasma Activated Sintering (PAS). This process was recently developed by Sodick, Ltd. in Japan and commercialized by Matsushita Inc., for producing hard magnets.11 The important features of the PAS technique and experimental results leading to the achievement of J. Mater. Res., Vol. 7, No. 10, Oct 1992
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near-theoretical density in A1N after 5 min of consolidation at 1730 °C are presented in this communication. The PAS process is schematically depicted in Fig. 1. In this process, plasma is generated among powder particles by instantaneous high electric pulsed power application. This p
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