Microstructural and chemical stability of Y-ZrO2 reinforced beta''-alumina in molten sodium sulfide and sulfur

The stability of beta''-alumina reinforced with in vol% of tetragonal partially stabilized 3 mol% Y2O3-ZrO2 (3Y-ZrO2) and with 10 vol% of cubic 8 mol% Y2O3-ZrO2 (8Y-ZrO2) in molten sulfur or molten Na2S4 has been examined using scanning electron microscopy (SEM) X-ray diffraction (XRD) and electron probe microanalysis (EPMA) both before and after immersion at 350 degrees C. Tetragonal partially stabilized 3 mol% Y2O3-ZrO2 was destabilized when reinforced into beta''-alumina and immersed in molten Na2S4. Destabilization without incorporation into beta''-alumina or using molten S as the immersion medium was minor. EPMA analyses indicated that the presence of beta''-alumina enhanced zirconia destabilization in that beta''-alumina can react with the molten corrodants to form corrosion products which are known corrosion agents for the leaching of Y2O3 from partially stabilized 3Y-ZrO2. From XRD analyses, changing from partially stabilized 3Y-ZrO2 to cubic 8Y-ZrO2 in the composite increased resistance against phase destabilization. EPMA analyses revealed that the depletion was almost halted for cubic 8Y-ZrO2 suggesting that the change in the zirconia phase used had reduced the chemical reactivity between Y2O3 and the corrodants. In order to avoid depletion destabilization of zirconia in beta''-alumina, corrosion resistance can be increased by reducing chemical reactivity by using fully stabilizing zirconia. In addition, partially stabilized tetragonal zirconia may still be considered for use if a less reactive stabilizer such as CeO2 is used.