Sintering behavior and thermal conductivity of graphite flakes/uranium dioxide composites

Abstract Graphite flakes (GFs) were introduced into UO2 via spark plasma sintering to improve its thermal conductivity (TC). The effect of GFs on the sintering behavior and TC of GFs/UO2 was systematically investigated. GFs/UO2 composites exhibited a highly oriented structure; thus, the radial TC was significantly higher than the axial TC. GFs/UO2 was crack-free below a sintering temperature of 1673 K. At higher temperatures, some cracks were formed owing to the reaction at the interface. Accordingly, the radial TC of GFs/UO2 reached its peak value at 1673 K. The radial TC presented a nearly linear relationship with the GF fraction. The radial TC of GFs/UO2 composites was averagely improved by 340% from 298 to 1273 K with 15 vol.% GFs addition. The radial TC continuously increased with the diameter of the GFs. In contrast, the volume fraction and diameter of the GFs had a relatively inconspicuous effect on the axial TC. This was attributed to the thermal resistance of the interface and deflection of GFs from the radial direction.