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"The radiation response of mesoporous nanocrystalline zirconia thin films"
Ayelén M. Manzini, Martin A. Alurralde, Gustavo Giménez and Vittorio Luca
J. Nucl. Mater. 482 (2016) 175-186
The next generation of nuclear systems will require materials capable of withstanding hostile chemical, physical and radiation environments over long time-frames. Aside from its chemical and physical stability, crystalline zirconia is one of the most radiation tolerant materials known. Here we report the first ever study of the radiation response of nanocrystalline and mesoporous zirconia and Ce3+-stabilized nanocrystalline zirconia (Ce0.1Zr0.9O2) thin films supported on silicon wafers. Zirconia films prepared using the block copolymer Brij-58 as the template had a thickness of around 60-80 nm. In the absence of a stabilizing trivalent cation they consisted of monoclinic and tetragonal zirconia nanocrystals with diameters in the range 8-10 nm. Films stabilized with Ce3+ contained only the tetragonal phase. The thin films were irradiated with iodine ions of energies of 70 MeV and 132 keV at low fluences (1013 - 1014 cm-2) corresponding to doses of 0.002 and 1.73 dpa respectively, and at 180 keV and high fluences (2 × 1016 cm-2) corresponding to 82.4 dpa. The influence of heavy ion irradiation on the nanocrystalline structure was monitored through Rietveld analysis of grazing incidence X-ray diffraction (GIXRD) patterns recorded at angles close to the critical angle to ensure minimum contribution to the diffraction pattern from the substrate. Irradiation of the mesoporous nanocrystalline zirconia thin films with 70 MeV iodine ions, for which electronic energy loss is dominant, resulted in slight changes in phase composition and virtually no change in crystallographic parameters as determined by Rietveld analysis. Iodine ion bombardment in the nuclear energy loss regime (132-180 keV) at low fluences did not provoke significant changes in phase composition or crystallographic parameters. However, at 180 keV and high fluences the monoclinic phase was totally eliminated from the GIXRD pattern of films prepared at both 350 and 500 °C implying either a monoclinic-to-tetragonal or a monoclinic-to-amorphous transition. This irradiation at very high doses resulted in film shrinkage and a loss of mesopore ordering but little or no degradation of the crystallinity of the tetragonal phase. A small increase in the crystalline domain size of the tZrO2 phase was noted in these films. In contrast, single-phase Ce3+-stabilized tetragonal nanocrystalline zirconia mesoporous films prepared at 350 °C suffered considerable loss of crystalline order when irradiated at 82 dpa. This loss of crystallinity was less pronounced in films heated to 500 °C. The loss of crystallinity of the tetragonal phase in the Ce-stabilized tetragonal zirconia thin films w.r.t. the tetragonal phase in the unstabilized films was attributed to the oxygen vacancies introduced in the latter due to the need for charge compensation.
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