High-temperature X-ray powder diffraction analysis of selected ceramic mixtures

The utility of ultra-high-temperature (2100 °C) X-ray powder diffraction technique for investigating the high-temperature phase relationships of two pseudobinary mixtures, Al 2 O 3 /Y 2 O 3 and AlN/SiO 2 , is described. The in situ analysis to 1600 °C was carried out using a platinum holder and a Pt/Rh thermocouple; whereas analysis beyond 1600 °C was performed with a tungsten holder, extremely pure, oxygen-free inert gas environment, and an opticalpyrometer to monitor temperatures up to 2100 °C. The solid-state interaction between Al 2 O 3 and Y 2 O 3 commenced with the formation of Al 2 Y 4 O 9 (YAM), a yttria rich compound, at 1300 °C followed by AlYO 3 (YAP) and Al 5 Y 3 O 12 (YAG) at 1400 °C. Further heating to 1500 °C and above resulted in the increased concentration of YAG at the expense of the other two phases, followed by melting of the entire sample at 2050 °C. The only phase formed during cooling (from the molten state) was YAG. AlN and SiO 2 did not react with each other, in an inert atmosphere, and they remained as separate, discrete phases even at 1600 °C. In the presence of oxygen, they reacted to form mullite and cristobalite, or SIALON depending on the partial pressure of oxygen.