Transition mechanism and thermokinetic analysis of Sr0.5Zr2(PO4)3–Ce0.5Sm0.5PO4 composite ceramics for nuclear waste forms based on in-situ synthesis

In this study, the novelly designed Sr 0.5 Zr 2 (PO 4 ) 3 –Ce 0.5 Sm 0.5 PO 4 composite ceramics were utilized as nuclear waste form to simultaneously immobilize simulated fission products Sr and actinide nuclides. The phase transition mechanism, thermokinetics and densification behavior of Sr 0.5 Zr 2 (PO 4 ) 3 –Ce 0.5 Sm 0.5 PO 4 composite ceramics based on in-situ synthesis were systemically analyzed by XRD, in-situ XPS, TG-DSC, SEM and density measurements. The results demonstrate that the formation of Ce 0.5 Sm 0.5 PO 4 solid solution phase preceded that of Sr 0.5 Zr 2 (PO 4 ) 3 phase, of which there is a valence change of Ce from Ce 4+ to Ce 3+ before 1023 K. Additionally, as for the exothermic crystallization enthalpy, change of exothermic crystallization enthalpy and activation energy, the values of Sr 0.5 Zr 2 (PO 4 ) 3 and Ce 0.5 Sm 0.5 PO 4 phases in composite ceramics are 286.17 – 515.61 mJ, 19.11 – 35.38 J/g, 702.45 kJ/mol and 145.21 – 232.44 mJ, 9.26 – 15.44 J/g, 450.04 kJ/mol, respectively. It is then indicated that the formation of Ce 0.5 Sm 0.5 PO 4 phase is definitely more favorable than that of Sr 0.5 Zr 2 (PO 4 ) 3 phase. Simultaneously, the relative densities of Sr 0.5 Zr 2 (PO 4 ) 3 –Ce 0.5 Sm 0.5 PO 4 composite ceramics are more than 95 % when the sintering temperature exceeds 1273 K.