Effect of low temperature neutron irradiation and 20–400° C annealing on the density and lattice parameters of aluminum oxide and magnesium oxide

Abstract

When irradiated at 77° K aluminum oxide and magnesium oxide show a lattice expansion and density decrease several times larger than when irradiated at “pile ambient” temperature; results are equivalent for single crystal and polycrystalline samples. For Al 2O 3 measured at 91° K, c-axis expansion is 2.69 times larger than a-axis expansion; after annealing at 400° C this ratio has decreased to 1.04. Lattice volume expansion and density decrease are equal for Al 2O 3, indicating the presence of the same numbers of interstitials and vacancies after irradiation and during annealing at temperatures up to 400° C. For MgO, lattice volume expansion and density decrease are equal up to 100° C, but differ at 250° C. This probably results from interstitial precipitation at dislocations. Isothermal annealing of density and lattice parameter follows a relation P- p 0 = − k In t, where P is a property change ( Δc, Δa, Δδ) and t is time. Observations of the temperature, time, and time-rate behavior of annealing are consistent with interstitials having a variety of detailed configurations combining with vacancies with an activation energy for the process distributed over the range 0.2–0.8 eV.