Properties of graphite composites based on natural and synthetic graphite powders and a phenolic novolac binder

Abstract

Model graphite composites, similar to those used in nuclear applications as encasement material in fuel pebbles, were prepared by uniaxial cold compression moulding. They contained natural flake graphite, synthetic graphite and 20 wt.% phenolic novolac resin binder. The materials were carbonised at 900 C in a nitrogen atmosphere and then annealed at 1800 C in helium atmosphere. The X-ray diffraction studies showed that the graphite in these composites had hexagonal crystal structure after annealing. Raman spectroscopy revealed the presence of the structurally disordered phase derived from the carbonised resin. Optical microscopy revealed a flake-like microstructure for composites containing mainly natural graphite and needle-coke like particles for composites containing mainly synthetic graphite. The composites featured anisotropic property behaviour as the particles were partially aligned in a direction perpendicular to the compression direction. Thermogravimetric analysis studies showed that the annealed graphite composites were stable in air to 650 C. The linear thermal expansion coefficients measured by thermomechanical analysis (20–600 C) in the direction of pressing were in the range 5– 9 10 6 K 1 and in the range 1.2–2 10 6 K 1 in the direction normal to pressing. The thermal conductivity of the composites were measured using Xenon flash method from 100 to 1000 C and the values ranged from 19 to 30Wm 1 K 1.