Preparation of tetragonal zirconia microspheres as surrogate precursor for uranium nitride microspheres

•ZrO2 microspheres were prepared by internal gelation and heat treatment at 800 °C.•The room temperature-stabilized tetragonal zirconia phase was obtained without any doping ions.•The heat treated microsphere was nanocrystalline and free of cracks.•The crush strength of the microspheres was up to 122.5 ± 13.8 N/sphere.•The obtained microspheres can be a reliable surrogate precursor for UN microspheres. Zirconia (ZrO2) microspheres are of great interest as a surrogate precursor material for preparation of uranium nitride (UN) microspheres in High Temperature Reactor (HTR). However, the crystallite transformation of ZrO2 microspheres in cooling process causes cracking of the microspheres. In our study, crack-free nanocrystalline ZrO2 microspheres were obtained. The tetragonal phase was stabilized at room temperature without doping any stabilizing additives. Firstly, gel microspheres were prepared through internal gelation process. Subsequent heat treatment of gel microspheres in nitrogen was conducted. The effect of heat treatment was investigated by using simultaneous thermogravimetric analysis and differential scanning calorimetric analysis (TGA/DSC). By using X-ray diffraction (XRD), optical microscopy, and field-emission scanning electron microscopy (FE-SEM), the phase compositions, morphology and microstructures of the heat treated microspheres were characterized. The ZrO2 microspheres treated at 800 °C exhibit room-stabilized tetragonal zirconia phase, with uniform size of 650 ± 15 μm, good sphericity of 1.02 ± 0.01, and relatively high crush strength 122.5 ± 13.8 N/sphere. Finally, a preliminary nitridation was conducted. Those room-stabilized tetragonal zirconia microspheres can be used as a good surrogate precursor for subsequent preparation of nitride microspheres.