THE THERMAL-DECOMPOSITION OF ZIRCONIUM SULFATE HYDRATE

Zr(SO4)2 . 5.5H2O decomposes in five steps to tetragonal ZrO2 at 800-degrees-C. Dehydration occurs with the loss of 0.5, then 3, then 1 and, in the last dehydration step, 1 molecule of water of hydration. Using a heating rate of 5-degrees-C min-1, Zr(SO4)2 decomposes to zirconia from 540-degrees-C. This decomposition step consists of at least three intermediate steps that can be derived from the determination of the activation energy values at different transformation degrees (alpha). Between 800 and 900-degrees-C tetragonal zirconia is transformed slowly into monoclinic zirconia which, on cooling to room temperature, remains in the monoclinic structural phase. DTA and DSC measurements showed that 2505 J per g Zr(SO4)2.5.5H2O is needed to obtain ZrO2 in the monoclinic structure at 900-degrees-C. To obtain tetragonal zirconia at 800-degrees-C, 2306 J per g of the initial compound is needed.