Porosity in graphites

Abstract

Graphite is a porous material with a complex porous structure. It is used as a moderator in nuclear reactors and as such undergoes radiolytic oxidation in carbon dioxi)de. -The main objective of this study is to measure, quantitatively, the changes in the pore shapes and sizes which occur during this radiolytic oxidation. Methods of pore measurement are assessed. Quantitative methods include photogrammetry which is the analysis of stereopairs (in this study, stereo-micrographs) and stereology which is the development of three-dimensional space from twodimensional sections made through a material. Microscopic methods including optical, scanning electron and high-resolution scanning electron microscopy are used to study pores of diameters from -500 Pm. to 5 nm. Optical and scanning electron microscopy coupled with the "Quantimet 720 Image Analysing Computer' proved to be the best techniques for the study. Optical microscopy coupled with the Quantimet 720 enables the size-distributions of macropores and their shape factors to be measured before and after oxidation. By studying the same area of polished surfaces of graphites, using optical microscopy and scanning electron microscopy, the optical texture can be correlated with the surface topography induced by gasification in carbon dioxide and air. Fizzures develop in directions parallel to the basal planes in flow-type anisotropy, but near-circular pores develop in positions of mozaics. However, similar forms of optical texture behave similarly when oxidised with molecular oxygen, carbon dioxide and atomic oxygen to comparable weight losses. A comparison of radiolytic oxidation and thermal oxidation, using scanning electron microscopy, of Lima graphite to about 9% weight loss, shows that during radiolytic oxidation pores between 10 pm and 20 Um develop evenly over the surface whereas during thermal oxidation, pores between 50 pm and 150 pm in diameter develop in some areas, while other areas remain non-porous. I Progressive polishing of graphites is a new approach to the study of porosity in three-dimensions. This is carried out using scanning electron microscopy. The surface is polished and overlapping scanning electron micrographs are taken to bui ld-up a photo-montage of each polished section. The pore. -shapes of each section can be drawn out on transparent sheets and placed on top of each other so enabling the tortuosity and the interconnections of the pores in three-dimensions to be measured.