In this work, the effects of starting oxide powders with different-scale particle sizes on the synthesis of gadolinium zirconate pyrochlore (Gd 2 Zr 2 O 7 , GZO) and its physical properties were studied. Micron Gd 2 O 3 (μG), micron ZrO 2 (μZ), nano Gd 2 O 3 (nG), and nano ZrO 2 (nZ) powders were used. GZO ceramics were prepared by employing solid-state reactive sintering at 1300 °C, 1400 °C, 1500 °C and 1600 °C with mixed powders of different sizes (μGμZ, μGnZ, nGμZ and nGnZ). X-ray diffraction and Raman analyses of the ceramics revealed that nG has a more significant impact on the crystallization process than nZ. All ceramics synthesized with different sized oxide powders crystallized into pyrochlore phases except for those synthesized with μGnZ mixed powders, which resulted in a fluorite phase. The results indicated that decreasing the particle size of only ZrO 2 to synthesize pyrochlore-phase Gd 2 Zr 2 O 7 with high crystallinity may not be effective. Samples obtained at 1500 °C were further analyzed. Scanning electron microscopy results revealed that all four ceramics have a non-homogeneous grain size and that the average grain size ranges from 5.40 to 8.30 μm. In addition, the density and Vickers hardness measurements showed that the use of nanopowders significantly improves the mechanical properties.
Summary As a rapid sintering method, spark plasma sintering (SPS) process has been widely applied to immobilize nuclear waste. In this work, trialkyl phosphine oxides (TRPO) were immobilized by the mixture of Gd 2 O 3 and ZrO 2 with SPS proceedings at 1650°C in 3 minutes. X-ray diffraction results present that the solid solubility is 10 to 20 wt% and all the sintered samples show a fluorite structure. Raman spectroscopy results show that the order degree of structure decreases as waste content increases. The lattice parameters decrease with the increasing doped waste content indicating the success of waste immobilization. Densities decrease for the sample with higher waste content which mainly caused by the existence of pores. Scanning electron microscope-energy dispersive X-ray spectroscopy and backscattered electron results perform the microstructure of the sintered sample. Furthermore, the leaching test shows that the sample has a considerable chemical stability. This study confirmed the possibility of rapid consolidation of Gd 2 Zr 2 O 7 bulk, dense ceramic with up to 10 ~ 20 wt% of immobilized TRPO waste. Highlights Multi-nuclides waste was rapidly immobilized by Gd 2 Zr 2 O 7 via SPS process. Phase transition happened with enhanced doping content. Density results decreased with the higher waste content. The order degree of structure decreased as waste content increased. The sintered matrix represented considerable chemical stability.
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