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Short‐term (<30 h) oxidation of CeO2‐doped hot‐pressed Si3N4, studied at 773 to 1623 K in flowing dry air, resulted in parabolic‐weight gain curves and in oxidation‐rate constants nearly independent of the amount of grain‐boundary second phase. Exolution of CeO2 crystals occurs from the oxide layer; their morphology depends on oxidation temperature. Either a direct redox reaction between CeO2 and Si3N4 or solution of Si3N4 in the glassy silica‐rich oxide layer, followed by its oxidation by dissolved oxygen, have been proposed as possible oxidation mechanisms, both appearing strongly dependent on the low solubility of cerium oxides hi silicate melts. The value of the activation energy for oxidation of 385 kj·mol−1 suggests additive and impurity migration from the bulk to the Si3N4/oxide reaction interface as the most probable rate‐limiting step.

Superconducting Bi-Pb-Sr-Ca-Cu-O ceramics were sintered by the hot-pressing technique. The densification mechanism was investigated at 800°C and 850°C and a densification phenomenology has been proposed. Samples hot pressed at 850°C showed high density and T c≈110 K.

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