Highly pure Si3N4 powder without additives, sealed with a glass capsule using pure hBN powder as an insulation to be isolated from it, was treated by HIP under the condition that can be sintered and densified up to theoretical value. After HIP treatment, an oxygen content distribution in the inner parts of the glass capsule was analyzed. Oxygen content in sintered Si3N4 was increased, on the other hand in hBN was decreased, as compared with their starting materials. Both the oxygen contents changed remarkably on a boundary between them. A sintered Si3N4 had the least oxygen content in the center. Oxygen content in hBN near Si3N4 was constant, and on the other place in hBN it was correlated by a distance from the surface of Si3N4. Therefore, it was concluded that the permeation of oxygen into Si3N4 from impurities in hBN was dominant, and that supply of oxygen from glass of the capsule was almost negligible.
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