Dimensional changes of ceramic materials under proton irradiation

Ceramic materials including Al2O3, SiC, AlN, Si3N4, MgO, MgAl2O4, Si and SiO2 as well as the refractory metal tungsten are irradiated with protons in the 10 MeV energy range at temperatures from 75 C to 630 C under uniaxial tensile stresses from 6 to 330 MPa to dose levels up to about 0.1 dpa. Changes in length and electrical resistivity are monitored after each irradiation or during beam-off periods. TEM and optical absorption technique are applied to extract information on the radiation induced microstructural changes. For materials with crystalline structure a modified rate theory is used to discuss the measurements whereas for SiO2 glass a model is developed to describe the radiation induced deformation (compaction) and viscous flow. Alternatively a recently developed visco-elastic model based on thermal spike formation is also applied to explain the observed viscous flow in SiO2 glass. It is found that crystalline materials irradiated in the intermediate temperature range (0.2 Tm<Tirr<0.4 Tm) deform in four different stages: (1) at very low doses mainly point defects or small defect clusters (loops) are formed, the strain increases linearly with dose. (2) After the nucleation of interstitial-type loops has finished and they become the dominating sinks, the strain follows a 2/3 power law dose dependence. (3) The growth and intersection of loops resulting in the formation of dislocation networks leads to a 1/2 dose dependence of the strain. (4) Finally, the strain saturates when the vacancy concentration becomes so high that spontaneous recombination dominates. Thermal spike formation can account for at most 10% of the observed viscous flow of vitreous SiO2 under 9.2 MeV proton irradiation. It is therefore proposed that energy deposition through electronic processes must also contribute to the viscous flow in glassy materials through processes of bond-breaking and re-bonding during irradiation. (orig.)