Duplex grain structure development and pore growth have been investigated in sintered $UO_2$ by thermogravimetric analysis, optical microscopy, scanning electron microscopy and quantitative analyses of the grain and pore sizes.
A duplex grain structure consisting of coarse irregular grains (30μm) and fine regular grains (3μm) develops only when sintered under $CO_2$ atmosphere using the powders prepared by the AUC process. TGA studies on the O/U ratios during sintering and the grain size difference derived from the two different uranium diffusion coefficients in $U_4O_{9-z}$ and $UO_{2+x}$ phases indicate that $U_2O_{9-z}$ and the $UO_{2+x}$ phases do not play a dominant role in development of the duplex grain structure. Microstructural features suggest that coarse irregular grains develop abnormally. The reason for development of the duplex grain structure is discussed on the basis of inhomogeneity of the compact of the powders derived from the AUC process. The coarse irregular grain seems to develop in the undeformed particles where the property of particles (the aggregate with many micro-pores) is well preserved, whereas fine regular grains seem to develop in the deformed particles where the bonding between crystallites is mostly broken.
As sintering temperature and time increase, fine regular grains in a cluster grow predominantly while coarse irregular grains become regular without apparent grain growth. The transition from irregular to regular grains is attributed to the local boundary migration driven by various curvatures along the boundary of irregular grains. The duplex grain structure initially shows the bimodal grain size distribution (mean 3 and 30μm sizes), which then transits into the other bimodal distribution (mean 7 and 30μm sizes) and finally settles into the monomodal distribution (mean 30μm size). The transient bimodal distribution results from the preferential grain growth in the clusters of fine regular grains. The duplex grain structure is eventually transformed into the uniform structure consisting of coarse regular grains.
The sintered density decreases with sintering time at 1500℃ under $CO_2$ and $H_2$-$CO_2$ atmospheres and agrees with the calculated density from the pore size and number, which suggests that the density decrease is caused by the pore growth. In the duplex grain structure, the pores located on the boundaries show significant growth, but most of the isolated pores within coarse irregular grains are annihilated, with the remaining pores unaffected. In the uniform grain structure obtained in $H_2$-$CO_2$ atmosphere, the average pore size is proportional to the grain size. A linear relationship is found in the logarithmic plot between pore size and sintering time, in which the slope is higher in the specimens sintered in $H_2$-$CO_2$ than in $CO_2$. The pore coalescence, which is little ascribed to grain growth, occurs to a larger extent on boundaries after sintering for 40 h in $CO_2$ atmosphere. The possible mechanism of the pore coalescence is proposed under an assumption which requires no migration of the grain boundary.
The formation of $U_3O_8$ in sintered $UO_2$ pellets has been studied in ambient atmosphere at 900℃. The X-ray diffraction and SEM fractography identify layer of orthorhombic $U_3O_8$ and again layer of cubic $U_4O_9$ and $UO_{2+x}$. The $U_3O_8$ layer except for the nearest to the pellet surface has columnar grains perpendicular to the surface, but $U_4O_9$ and $UO_{2+x}$ seems to hold the morphology of $UO_2$ grains. The $U_3O_8$ layer on the pellet surface shows a random orientation of grains, but the columnar $U_3O_8$ shows the preferred orientation of crystalline planes parallel to the orthorhombic c-axis, which accompanies the most strain on th transition from cubic $UO_2$ to orthorhombic $U_3O_8$. It can be suggested that although $U_3O_8$ grains form on $UO_2$ pellets in a random orientation some grains that have c-axis parallel to the pellet surface grow inward preferentially in order to minimize the transition strain.