Effect of Grain Size on the Thermal Expansion of Isotropic Synthetic Graphites

Abstract

The relationship is analyzed between the thermal expansion coefficient of synthetic graphites and the grain size of their filler. It is shown by examining a large number of commercial graphites of different classes that their thermal expansion coefficient increases from 2 × 10−6 to (7–8) × 10−6 K−1 as the grain size decreases from 3000 to 1 µm. The strength of synthetic graphites also increases with increasing grain size and correlates with thermal expansion. The likely reason for the increase in thermal expansion coefficient is the better contact between neighboring microvolumes of the material. A model is proposed for the thermal expansion of synthetic graphites which considers microvolumes 0.1 to 0.5 µm in size, with a thermal expansion coefficient on the order of 8.3 × 10−6 K−1, only a fraction of the microvolumes being involved in thermal-expansion transfer. The fraction of such microvolumes decreases with increasing grain size.