Microstructural and chemical stability of Y-ZrO₂ reinforced β″-alumina in molten sodium sulfide and sulfur

The stability of β″-alumina reinforced with 10 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°C. Tetragonal partially stabilized 3 mol % Y₂O₃-ZrO₂ was destabilized when reinforced into β″-alumina and immersed in molten Na₂S₄. Destabilization without incorporation into β″-alumina or using molten S as the immersion medium was minor. EPMA analyses indicated that the presence of β″-alumina enhanced zirconia destabilization in that β″-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 β″-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.