Abstract
The reactions of ZrC or ZrN powder and H2O have been studied at 190°-400°C under 100MPa. The reaction of ZrC with H2O above 240°C for 3h yielded monoclinic ZrO2 and CH4, while the reaction of ZrN above 190°C for 3h yielded monoclinic ZrO2, NH3 and H2. The oxidation rate calculated from the wegiht gain or the integrated peak intensity of the X-ray diffraction was compared with various model functions. A nucleation-controlled model was the best fitted one, while a core-shrinking model fitted as well. This suggested that the reaction was controlled by phase boundary reactions. TEM observation showed that the ultrafine particles about 10nm nucleated on the surface of ZrN or ZrC grains of about 1-3μm. These ZrO2 fine particles did not from a protective layer on ZrN or ZrC particles, but formed clusters of particles because of large volume expansions about 40% during the oxidation. An Arrhenius plot of the rate constants gave apparent activation energies of 99kJ/mol and 78kJ/mol for the oxidation of ZrC and ZrN by H2O, respectively, on the basis of the nucleation-controllod model. The difference of about 20kJ/mol in the activation energies for ZrC and ZrN migth be explained by the difference in the formation energy between the C-H bonding and N-H bonding.
