Self-propagating high-temperature synthesis of ZrO_2 incorporated Gd_2Ti_2O_7 pyrochlore

In this research, Zr-doped Gd_2Ti_2O_7 pyrochlores, with the composition of Gd_2(Ti_(1-x)Zr_x)_2O_7, were firstly synthesized by self-propagating high-temperature synthesis plus quick pressing(SHS/QP) using CuO as the oxidant and Ti as the reductant. To improve the radiation resistance of titanate–pyrochlore, up to 35 at% Zr was incorporated to substitute the Ti site of Gd_2Ti_2O_7 pyrochlore(Gd_2(Ti_(0.75)Zr_(0.35))_2O_7). XRD and SEM microstructural characterizations showed the formation of a composite ceramic with the major pyrochlore phase and the minor Cu phase. The generated temperature of samples decreased from 1702to 1011 ℃ with increasing Zr content. The effects of sintering temperature and pressure time on phase composition and microstructure were systematically studied. Besides, the influence of thermal transmission on the whole combustion process was also explored. The pyrochlore-based waste form possessed high bulk density of 6.25 g/cm^3 and Vickers hardness of 10.81 GPa. The MCC-1 leaching test showed the normalized elemental leaching rates(42d) of Cu, Gd, and Zr are 1.27×10^(-2), 1.33×10^(-3), and 8.44×10^(-7)g·m^(-2)·d^(-1), respectively.

Combustion synthesis and characterizations of Sm2O3 doped zirconolite-rich waste forms with CuO as oxidant

Zirconolite is one of the most important matrix materials for nuclear waste immobilization.In this study,Sm was employed as the surrogate of trivalent actinides.Sm-doped zirconolite-rich waste forms were readily prepared by combustion synthesis(CS)using CuO as the oxidant.Two different schemes with or without Al2O3 as the charge compensator were carried out simultaneously.The results demonstrate that Al2O3 addition results in complex phase composition.On the other hand,the samples without Al2O3 addition show desirable products of zirconolite and pyrochlore.The EDX analysis shows that Sm is concurrently incorporated into the Ca and Zr sites of zirconolite,which transforms to pyrochlore structure with high Sm2O3 contents.The aqueous durability of representative Sm-doped sample(Sm-0.6)was investigated.The 42 days normalized leaching rate of Sm is as low as 6.41×10-7 g/(m2 d),which exhibits high durability of Sm-doped sample.