Thermal Behaviour of Nanostructured ZrO2 after Heavy Ion Irradiation

As was shown in the past by XRD, TEM and RBS measurements, heavy ion irradiation of nanocrystalline ZrO2 samples using 4 MeV Kr + ions causes the formation of a tetragonal zirconia phase that is stable at room temperature [1]. This phase is employed in solid oxide fuel cells and oxygen sensors, but usually has to be stabilized by addition of a second phase, e.g. Y2O3, which is not necessary when using the process mentioned above. To investigate the thermal stability of tetragonal zirconia, a new series of zirconia samples has been prepared by means of the inert gas condensation technique (IGC) and sintered afterwards at temperatures of 900 or 1000 °C using forces of 12 up to 40 kN. This procedure resulted in a density higher than 5 g/cm corresponding to about 90% of the theoretical value. The samples were then irradiated by 4 MeV Kr ions with fluences from 2x10/cm to 2x10/cm and investigated by means of x-ray diffraction (XRD). The formation of the tetragonal ZrO2 phase, which was observed as well as the (common) monoclinic phase could be reproduced. The samples were heated at temperature steps of 100 or 200 °C up to 1300 °C and studied by means of XRD after each step. Apart from the thermal stability of the tetragonal phase we were also interested in the development of the grain sizes of both phases depending on the ionic fluence [2]. Samples that had been irradiated with a fluence of 1x10/cm or above showed a thermal stability of the tetragonal phase up to the sintering temperature (see Fig. 1), whereas samples irradiated with lower fluences showed only a residual amount of this phase of about 5 vol. % at temperatures of 400-500 °C. The same holds for samples irradiated with higher fluences above the sintering temperature. No significant change in the grain size of both phases could be observed. The grain size of the monoclinic ZrO2 phase was approximately 60 nm, and that of the tetragonal phase about 20 nm (see Fig. 2). The value of the sintering temperature and the compaction forces during the sintering process did not influence the thermal behaviour of both zirconia phases significantly. Apart from the fact that depending on the ionic fluence of the samples a thermal stability of the tetragonal phase could be achieved, an ageing stability of the tetragonal ZrO2 phase was also observed within a time period of several years.