In situ TEM Study of Size Dependence on Radiation-Induced Amorphization and Recrystallization of Hydroxyapatite Nanoparticles

Hydroxyapatite, Ca₁₀(PO₄) ₆(OH)₂, is considered as an important apatite-type material for the incorporation and disposal of actinides and fission products. Hydroxyapatite nanoparticles with different size ranging from 20 nm to 280 nm were synthesized via calcining the bovine bones under different temperatures and durations. The samples are irradiated with 1 MeV Kr²⁺ ions and 200 keV electrons to study the displacive and ionizing effects on the irradiation behaviors. In situ transmission electron microscopy (TEM) observation shows hydroxyapatite nanoparticles can be amorphized by 1 MeV Kr²⁺ ions and the previously amorphized samples experience a rapid ionizing-radiation-induced recrystallization upon 200 keV electrons irradiation. A strong size dependence on the displacive irradiation-induced amorphization and ionizing irradiation-induced recrystallization are observed. Under ion irradiation, a lower critical temperature T _c is observed for a larger sized sample, indicating enhanced amorphization tolerance. This result indicates excess surface energy as a result of larger surface area resulting from the reduced size, may lower the energy barrier between the crystalline phase and amorphous state, degrading the radiation tolerance. Whereas under electron irradiation, the recrystallization fluences decrease with the reduction of particle size since a higher density of dangling bonds exist in a smaller sized hydroxyapatite. These unstable bonds are relatively easy to break and reforming under ionizing electron irradiation, which drives the recrystallization process.