HYDROGEN DISSOLUTION IN AND RELEASE FROM NONMETALS .3. TETRAGONAL ZIRCONIA

The solubility and release kinetics of hydrogen in single-crystal zirconia were investigated by the infusion-extraction method. Equilibration in hydrogen at pressures between 1.4 and 11 atm (0.14 and 1.1 MPa) and at temperatures from 1300-degrees to 1600-degrees-C produced H/Zr ratios varying from 4 x 10(-5) to 1.3 x 10(-4). Dissolution was exothermic with an enthalpy of solution of 6.6 kcal/mol (28 kJ/mol). The solubility increased with the extent of substoichiometry of the oxide caused by exposure to hydrogen. The solubility behavior and the kinetics of release indicated that hydrogen in the solid forms defect clusters consisting of a hydrogen atom, two trivalent cations, and a doubly charged oxygen vacancy. The trivalent ions were either impurities or reduced zirconium ions. Rate constants for first-order detrapping were determined. Diffusion was shown not to be a rate-limiting process in hydrogen release.