CAUSES OF RESIDUAL POROSITY IN SINTERED URANIUM DIOXIDE

Abstract

The densification characteristics of a sample of uranium dioxide approximating in composition to UO_2.1 have been investigated as a function of compacting pressure, firing temperature and time, and sintering atmosphere. Fired densities achieved were found to be markedly dependent on the initial as-pressed density, and microscopic examination of pellets after different firing treatments showed that this behaviour was associated with internal shrinkage, leading to fissuring in pellets of low initial density. This imposed a limit on both densification and grain growth. Pellets having as-pressed densities of ∼ 50% of theoretical, which was sufficient to eliminate fissuring, sintered to densities of 0.98% of theoretical in 2 h at 1400° C in nitrogen and argon and then expanded. Subsequently shrinkage was resumed and was still continuing after 32 h sintering. A study of the pore-size distribution after various sintering times suggests that the expansion may be associated with diffusion of gas from small to large pores. Grain growth was found to obey a relation of the form D³-D₀³=K(t-t₀), D and D₀ being the average grain diameters at times t and t₀

Notes

* Manuscript received 14 June 1963. Contribution to a Symposium on “Residual Porosity in Powder Metallurgy” to be held in London on 21 and 22 October 1963.