Effect of gamma irradiation and thermal annealing on defect formation in ZrB2 nanocrystals

Abstract

The study investigated the microstructural changes and defect formation for ZrB₂ nanocrystalline sample (grain size: 43 nm) under the effect of irradiation and combined effects of irradiation and thermal annealing. The sample was irradiated using a ⁶⁰Co gamma-ray source at a dose rate of 6.05 Gy/h, with total absorbed dose of 1500 and 3000 kGy. Raman spectroscopic analysis showed that the degree of amorphization increases up to 1.2 % for the sample irradiated with absorption dose 3000 kGy, while a 0.2 % decrease was observed after thermal treatment at 1173 K. moreover the broadening of Raman peaks and the decrease in intensity after thermal treatment confirm the persistence of defect states in the crystalline structure. Doppler Broadening Positron Annihilation Spectroscopy (DBPAS) results revealed that the S parameter increased within the range of 3.1–3.6 %, while the W parameter decreased within the range of 2.8–2.4 %. The variation of the S and W components indicates the formation of free-volume defects, as well as Zr and B vacancies in the crystal structure ZrB₂ nanocrystalline sample. Ab initio calculations based on the two-component density functional theory (TCDFT) showed the formation of 1V_Zr, 2V_Zr+1V_B, and 3V_Zr+2V_B vacancy complexes. In the electron momentum density (EMD) spectra, characteristic intensity variations were detected in the range of 0.0–4.0 × 10⁻³ m₀c.

After combined effect of irradiation and thermal treatment, the highly defective EMD features demonstrate that the recombination of defect centers significantly decreases. The research results show that in ZrB₂ material irradiated at a high absorbed dose, complex vacancy-type defects are formed, and depending on the selected temperature interval, effective recombination of defects occurs. All these findings suggest that nanocrystalline ZrB₂ is a promising material for future applications as a radiation-resistant ceramic.