Thermal behaviour of solids strongly depends on their microstructural characteristics. In this work, UO₂ ceramics with a highly developed porosity network were obtained by sintering compacts made of multi-crystalline granules. The manufacturing conditions were tuned to obtain samples over a wide range of porosity. These ceramics exhibit, in addition to a quasi-spherical and finely distributed porosity, slender and “filamentary” pores located at the interfaces between granules, associated with open porosity [1].
The material microstructure was characterized using optical microscopy (2D imaging). Cross-sections of ceramics provide a statistically representative description of the microstructure. Once the microstructures imaged, several processing steps are necessary to clearly isolate the pores. The skeletonization allows a simple description of the inter-granules porosity as a network of “cracks”. By extracting relevant information, a geometrical description of the microstructure can be obtained (orientation, density, size distribution of inter-porous space). Five descriptors were selected: the length of the intercepts [2] which makes it possible to evaluate the concentration in cracks, the ratio between the intercepts calculated in the crack orientation direction and those calculated in the perpendicular direction which is useful to characterize the anisotropy, the radius of Matheron’s circles [3] which allows to approach the spatial distribution of the cracks, the length of the branches and the length of the primary branches which are used to give information on the dimensions of the cracks.
Thermal diffusivity measurements were performed at 50°C in various gaseous environments (vacuum, air, helium) and up to 500°C in argon/hydrogen. The microstructure description allows to understand the thermal behaviour of the ceramics investigated. As expected, the porosity network plays an important role in the degradation of the thermal conductivity. Ceramics may have different thermal conductivities in spite of a similar concentration of cracks. This demonstrates the importance of considering the orientation of the cracks. Moreover, crack clusters may explain the presence of large zones representing important barriers for the heat flux (“dead zones”). Smaller pore thickness plays an important role as well in lowering gaseous thermal conductivity due to the Knudsen effect.

References:
[1]       J. Meynard. Influence de la taille, de la morphologie et de la distribution spatiale des pores sur la conductivité thermique de céramiques UO₂, Thesis, Aix-Marseille Université, 2019.
[2]       H. Abrams. Grain size measurement by the intercept method. Metallography, 4(1):59-78, 1971 ISSN 0026-0800
[3]       G. Matheron. Eléments pour une théorie des milieux poreux. Paris: Masson, 1967.