Effects of Various Rare Earth Sesquioxide Additives on Grain Growth in Millimeter-Wave Sintered Silicon Nitride Ceramics

Masayuki HIROTA  Maria-Cecilia VALECILLOS  Manuel E. BRITO  Kiyoshi HIRAO  Motohiro TORIYAMA  

IEICE TRANSACTIONS on Electronics   Vol.E86-C   No.12   pp.2462-2468
Publication Date: 2003/12/01
Online ISSN: 
Print ISSN: 0916-8516
Type of Manuscript: Special Section PAPER (Special Issue on Recent Trends on Microwave and Millimeter Wave Application Technology)
Category: Millimeter-Wave Heating
silicon nitride,  millimeter-wave,  microstructure,  sintering,  grain growth,  

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Using various rare earth sesquioxides as additives, silicon nitride (Si3N4) samples were sintered at 1700 for 4 h by millimeter-wave heating performed in an applicator fed by a 28 GHz Gyrotron source under a nitrogen pressure of 0.1 MPa. A comparative study of densification, grain growth behavior and mechanical properties of silicon nitride fabricated by millimeter-wave and conventional sintering was carried out. Bulk densities were measured by Archimedes' technique. Except for the Eu2O3 containing sample, all samples were densified to relative densities of above 97.0%. Microstructure of the specimens was analyzed by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). To investigate quantitatively the effect of millimeter-wave heating on grain growth, image analysis was carried out for grains in the specimens. Fracture toughness was determined by the indentation-fracture method (IF method) in accordance with Japan Industrial Standards (JIS). Fully dense millimeter-wave sintered silicon nitride presenting a bimodal microstructure exhibited higher values of fracture toughness than materials processed by conventional heating techniques. Results indicate that millimeter-wave sintering is more effective in enhancing the grain growth and in producing the bimodal microstructure than conventional heating. It was also confirmed that localized runaway in temperature, depending upon the sintering additives, can occur under millimeter-wave heating.