The sintering and morphology of interconnected porosity in UO₂ powder compacts

Abstract

Uranium dioxide powder compacts of ∼46% green density were sintered in flowing hydrogen at temperatures between 1500 and 1700° C. On annealing, the compacts readily formed an interconnected system of pores stabilized by grain boundaries. The volume of open porosity decreased with an activation energy of 4.6 J mol⁻¹ at a rate controlled by grain growth. The grain-boundary migration removed the restraint on the porosity allowing shrinkage to commence. The compact surface area decreased with a higher activation energy of 6.0 J mol⁻¹. The mechanism proposed for the diminishing area was the smoothing of the faceted powder grains. Nucleation of atomic layers on the facets was shown to account for the high activation energy. The equilibrium shapes that may be adopted by interconnected porosity were calculated using a model in which simpler geometry was substituted for the real anticlastic surface curvature. The model demonstrated the stabilizing effect of increasing grain-boundary energy and the formation of closed pores.