EFFECTIVE CONDUCTIVITY OF POLYCRYSTALLINE GRAPHITE

An effective conductivity (thermal or electrical) is derived for polyc rystalline graphite using a very simple approach. The material is mode led as a series of links and junctions, the former representing the cr ystallies and the latter the intercrystallite region. The large anisot ropy of the principal conductivities permits definition of an excluded region in which conduction contributes negligibly to the overall ther mal/electrical transport. Two approximations are obtained for the valu e of the excluded region, which depend not only upon the principal con ductivities ratio, but also upon the crystallite shape anisotropy. Thi s model allows estimation of the effective conductivity of polycrystal line graphite from a knowledge of the principal conductivities ratio, the conductivity along the graphite basal plane, the microstructural f eatures (crystallite shape and size, number, and geometry of intercrys tallite contacts per crystallite), and the density. Applications to nu clear-grade graphites gave satisfactory agreement with experimentally measured electrical and thermal conductivities.