Dimensional changes induced in pyrolytic carbon by high-temperature fast-neutron irradiation

Abstract

A variety of pyrolytic carbons with various densities, anisotropies, and microstructures, but with similar crystallite sizes, were irradiated at 600°, 1180°, 1330° and 1650°C to fast neutron exposures of 1.9, 2.6, 3.1 and 3.3 × 10 21 nvt ( E >; 0.18 MeV), respectively. Changes in linear dimensions, density, apparent crystallite height, layer spacing, and anisotropy were measured. Significant changes in preferred orientation resulting from irradiation at temperatures above 1100°C were measured. The Bacon anisotropy factor for many specimens increased more than 20 per cent due to the irradiation. The kinetics of densification at 600°C were found to be first order with respect to the density defect and independent of the microstructural appearance of the carbon. At irradiation temperatures above 1100°C, the densities of the carbons with initial densities less than about 2.06 g/cm 3 were increased by the irradiation, but the densities of those with initial densities greater than this value were decreased. The crystallite dimensional changes, ΔX c X c and ΔX a X a , produced by irradiation were derived from the measured dimensional changes parallel and perpendicular to the deposit and compared with the crystallite dimensional changes previously published for carbons and graphites with crystallite densities above 2.0 g/cm 3. The data show no dependence on crystallite size below ~500°C, where the mobility of radiation-induced point defects is low. Above ~500°C, where the mean diffusion distance of the displaced atoms is large compared with the crystallite size, dimensional change rates are higher for carbons with smaller crystallite sizes.