Synthesis And Stability Of Gd₂Zr₂O₇ Ceramics By Flash Sintering

Flash sintering refers to a new FAST(field-assisted sintering technology)with high voltage and low current.Its application to ceramic immobilization of radioactive waste has attracted wide attention.Because of high radiation resistance and chemical stability,Gd2Zr2O7-pyrochlore is considered a promising candidate for high-level waste immobilization materials.In this study,with gadolinium oxide and zirconium oxide as raw materials,a homogeneous defective-fluorite phase Gd2Zr2O7 immobilization matrix was synthesized and consolidated at furnace temperatures around 1050±10? for only 30s.In addition,phase evolution in the flash sintered process,crystal structure,metallographic,microstructure,hardness,density,leaching resistance and ?-irradiation stability of the as-prepared samples were characterized by means of X-ray diffraction(XRD),scanning electron microscope(SEM),laser Raman spectroscopy,transmission electron microscope(TEM),etc.The results of this thesis are summarized as follows:(1)X-ray diffraction results show the phase evolution from the green body to the final Gd2Zr2O7 products during the flash sintering process,indicating the successful synthesis of a single defective-fluorite phase Gd2Zr2O7.This study prepared the Gd2Zr2O7 ceramics at 1050±10? under an applied electric field of 150 V/cm and an electric current density of 133 mA/mm2 for a holding time within 30s.As shown by the metallographic analysis of SEM,the grain size of flash sintered samples is enlarged with longer holding time.Nonetheless,the average grain size of the samples is still much smaller than that of the conventional sintered one.(2)The leaching results indicate that the elements of flash sintered samples have been preserved in low concentration in leaching solution at 90? and 28d.Specifically,the normalized leaching rates of Gd and Zr on the same day decrease with the increasing holding time of flash sintering as well as the increasing leaching time.The flash sintered samples exhibit no significant differences from blank reagent in terms of the pH value and electrical conductivity.The flash sintered samples show excellent leaching behavior(3)An annealing process could transform the Gd2Zr2O7 sample prepared by flash sintering from the defect fluorite phase into the pyrochlore phase.The X-ray diffraction results and laser Raman spectroscopy show that Gd2Zr2O7 changes from a pyrochlore structure to a defect fluorite structure after irradiated at different radiation dosages.The diffraction maxima of the pyrochlore superlattice disappear with the increasing ion fluences,leaving only the diffraction maxima of the fluorite structure.The peaks of Gd2Zr2O7 shift towards the low angle,indicating the increase in the lattice parameters and the interplanar space.The flash sintered samples show high radiation stability.To conclude,flash sintering can be used for preparing an immobilization matrix materls with homogeneous phase,compact structure and good performance.Compared with previous conventional high temperature solid-state reaction methods,the furnace temperature and time for flash sintering are lower and shorter,and it is a new approach to the preparation of the Zr-based immobilization materials or other ceramic immobilization materials.