Oxidation behavior of R4Si2O7N2 (J-phase; R = Y and rare-earth element) has been investigated by thermometric measurements in air. All R4Si2O7N2, of which the powder samples are prepared by gas-pressured sintering method under 1 MPa of N2 gas, are oxidized to R2SiO5 with an exothermic reaction in air under an ambient pressure. The oxidation temperature to produce the final oxide increases from c.a. 930 to 1060 °C with decreasing the ionic radii of the rare earth elements.
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