Physical property changes of neutron-irradiated aluminum nitride and their recovery behavior by annealing using a step-heating dilatometer

Aluminum nitride (AlN) is a candidate tritium permeation, electric insulation and corrosion barriers for several kinds of blankets such as molten salt-cooled (FLiBe) or liquid metal (Li-Pb or Li)-cooled blankets because of its desirably low dielectric constant and tritium diffusion. Commercially available AlN ceramic specimens were neutron-irradiated at two different fluences but the same irradiation temperature in the Japan Materials Testing Reactor. Specimen swelling was found to be slightly different for both conditions, with higher dose causing greater swelling. All irradiated specimens consisted from hexagonal AlN phase with alpha-Al2O3 phase occurring on the surface after long-time post-irradiation annealing in He atmosphere. The a- and c-axis experienced isotropic increase and degree of unit-cell volume change was almost the same with the macroscopic volume increase obtained from the length change. This result indicates uniform distribution of Frankel pairs. After step-wise thermal annealing by using a dilatometer up to 1673 K for 6 h at each step, the maximum recovery was found at 1573 K. Based on the recovery rates at each step by first-order analysis, macroscopic length recovery during annealing can be divided into three regions with different activation energies, low temperature (373-523 K) with similar to 4.5 eV, intermediate temperature (523-873 K) with 0.5-1.0 eV, and high temperature (873-1273 K) with 2.0-2.5 eV. Over 1273 K, a slight increase of length was observed. It is thought that the expansion is due to oxidation.