QUALITATIVE AND QUANTITATIVE-EVALUATION OF SILICON-NITRIDE GRAIN-BOUNDARY PHASE BY CRYOGENIC SPECIFIC-HEAT MEASUREMENTS

Three types of Si3N4 have been hot isostatically pressed at 1700-degrees-C under 60 MPa for 1 h in a capsule. The quantity and quality (i.e., crystalline or glassy structure) of the grain-boundary phase of the three types of Si3N4 can be evaluated by measuring cryogenic specific heats. The specific heats of Si3N4 ceramics with Al2O3 and Y2O3 additives have been measured at different temperatures between 10 and 40 K. The temperature dependency of measured cryogenic specific heat provides quantitative and qualitative information of the grain-boundary phases. This method is very useful for evaluating small changes in the amount and crystalline structure of grain-boundary phases and can clarify different heat histories of the sintering processes in Si3N4 ceramics. This proposed method is nondestructive, and the sensitivity can be extremely high. This method eventually leads to a new quality control method of ceramics.