Abstract
Nanoindentation has been employed to probe the mechanical properties [indentation hardness (H) and elastic modulus (E)] of radiation-damaged pyrochlore before and after step-wise thermal annealing up to 900 K. Three natural U and/or Th containing samples with increasing degree of disorder have been investigated (i.e., Panda Hill: 1.8 wt% ThO2, maximum life-time alpha-decay event dose ~1.6 x 1018 α-decay g-1; Blue River: 11.9 wt% UO2, maximum life-time alpha-decay event dose ~115.4 x 1018 α-decay g-1; and Miass: 7.2 wt% ThO2, maximum life-time alpha-decay event dose ~23.1 x 1018 α-decay g-1). Complementary investigations by photoluminescence and Raman spectroscopy and in-situ annealing transmission electron microscopy (TEM) enabled to follow the structural evolution. Therefore, a comprehensive understanding of the thermally induced structural reorganization process was obtained. Recrystallization has found to start above 500 K in the pyrochlores. Due to the increasing structural order a general hardening of the mechanical properties was observed. Miass pyrochlore (highest degree of structural damage of the investigated samples) reaches a polycrystalline state after annealing. While, lesser damaged, but also highly disordered Blue River pyrochlore (containing small preserved crystalline domains) has found to transform into a single crystal. The recrystallization of both pyrochlore samples was followed by in-situ TEM at 800 and 750 K, respectively.
Highlights
Phases with pyrochlore structure (A2B2O7) display in all their chemical varieties a multitude of technically important characteristics, such as, catalytic abilities, ferromagnetism, luminescence, giant magnetoresistance, and piezoelectricity [e.g., 1–5]
Step-wise annealing causes a steady increase in hardness and elastic modulus from 600 to 700 K reaching values of ~12.2 GPa and ~203 GPa, respectively
Step-wise annealing of the Blue River sample up to 600 K indicates a very slightly increasing trend of H and E (Tables 1,2, Figs. 2,3)
Summary
Phases with pyrochlore structure (A2B2O7) display in all their chemical varieties (around 450 synthetic compounds are currently known) a multitude of technically important characteristics, such as, catalytic abilities, ferromagnetism, luminescence, giant magnetoresistance, and piezoelectricity [e.g., 1–5]. The incorporation of radiogenic elements leads to a structural damage and amorphization process (metamictization), mainly caused by the α-decay of the unstable nuclei, which has been described in detail in literature [24,25,30,31]. During such an event an α-particle and a heavy recoil nucleus are generated. The radiation-damaged state is metastable and thermal treatment can lead (at least partially) to structural reorganization In pyrochlore, this recrystallization process has found to be accompanied by weight loss and an energy release that decreases with increasing crystallinity [36]
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