Figures 2-3 show the dynamics of changes in the X-ray diffraction patterns of the studied BeO ceramics irradiated with Ar8+ and Xe22+ ions at a radiation dose of 5х1013 cm-2 and temperatures of 300, 800, and 1000 K.
Using the Rietveld method in the analysis of the shape, position and intensity of diffraction peaks, it was found that the initial samples are highly ordered polycrystalline structures with a hexagonal type of the crystal lattice of beryllium oxide (P63mc(186)) with three pronounced directions of texture orientation (100), (002) and (101). In this case, the analysis of the X-ray diffraction patterns of the samples under study before and after irradiation showed the absence of the appearance of any new diffraction reflections or stratification of diffraction maxima, which indicates the absence of phase transformations or the formation of impurity phases as a result of irradiation. The main changes in the diffraction patterns of irradiated samples are associated with a change in the shape and intensity of the diffraction maxima, as well as their angular position relative to 2θ°. The distortion of the diffraction maxima, as well as their displacement, is associated with deformation processes arising as a result of changes in the concentration of point defects, as well as their migration in the structure as a result of irradiation. In the case of samples irradiated with Ar8+ ions at a temperature of 300 K, the greatest change in the intensities of the diffraction maxima, their broadening and line asymmetry are observed. A decrease in intensities indicates a reorientation of crystallites as a result of irradiation, and broadening of lines indicates grain fragmentation and recrystallization processes.
For samples irradiated with Xe22+ ions at a temperature of 300 K, a similar situation is observed with a change in the intensities, shape, and width of diffraction reflections, as in the case of irradiation with Ar8+ ions, however, the changes are more pronounced. This difference is caused not only by an increase in the depth of damage to the material upon irradiation with Xe22+ ions, but also by a large number of vacancy and point defects arising during irradiation. In this case, the increase in energy losses in collisions with the electronic and nuclear subsystems of the target in the case of irradiation with Xe22+ ions is 4 times greater and 8 times for electronic and nuclear losses, respectively, compared with similar values for irradiation with Ar8+ ions. This leads to an increase in the concentration of defect regions in the structure, as well as to a large number of initially knocked out atoms. Also, in the case of irradiation with Xe22+ ions, a sharp decrease in the intensities of the (100) and (002) reflections is observed, which indicates a strong reorientation of grains in the structure, as well as their fragmentation. However, these changes are more pronounced for irradiation with Xe22+ ions, which is caused not only by an increase in the depth of damage to the material, but also by a 6.5 times larger number of vacancy defects generated by irradiation with Xe22+ ions in comparison with irradiation with Ar8+ ions (according to Monte Carlo calculations in the SRIM- 2013). Also, in the case of irradiation with Xe22+ ions, a significant decrease in the intensities of diffraction peaks with Miller indices (100) and (002) is observed, which indicates a strong reorientation of grains in the structure, as well as their fragmentation, which is confirmed by a large broadening of reflections.
Increasing the FWHM value, i.e. physical broadening, diffraction lines provides information on the appearance of distortions and microstresses in the crystal lattice, as well as changes in interplanar distances. Figure 4 shows the results of crystal lattice distortion, in particular, the ratio of the c/a parameters, which characterizes the total deformation of the crystal lattice before and after irradiation of beryllium oxide with Ar8+ and Xe22+ ions with energies of 70 MeV and 231 MeV, respectively, at a particle fluence of 5×1013 cm2 and temperatures of 300, 800 and 1000 K.
It can be seen from the presented data that, upon irradiation with Ar8+ and Xe22+ ions at a temperature of 300 K, the degree of deformation was 0.6 % and 1.2 %, respectively. In this case, the difference in deformation by almost 2 times, depending on the type of ion, is due to energy losses and subsequent radiation damage arising from collision. However, under thermal irradiation, a decrease in the value of the crystal lattice strain is observed, and in the case of irradiation at 1000 K, this value is from 0.06 to 0.2 %, depending on the type of ion, which indicates a small deformation of the crystal lattice. Such a decrease in deformation under high-temperature irradiation is associated with a change in the magnitude of thermal vibrations of atoms in the lattice, which leads to an increase in their mobility, as well as an accelerated anighilation of arising defects in the structure.
It is well known that deformations of the crystal structure lead to an increase in the dislocation density, which significantly affects the change in both the mechanical characteristics and the optical properties of the material. Figure 5 shows the data on changes in the density of dislocations that arise during irradiation, the dynamics of which is associated primarily with the processes of crushing and recrystallization of grains. As can be seen from the data presented, the main changes in the dislocation density occur at an irradiation temperature of 300 K, when the effect of annealing of defects is minimal, and the change in grain sizes is most pronounced. Moreover, in the case of irradiation at temperatures of 800 K and 1000 K, the dislocation density for both ions is practically the same (within the measurement error), and only 10-15 % exceed the values of the initial dislocation density.
As a result of elastic and inelastic collisions of incident ions with atoms of the crystal lattice, a large number of point defects and dislocations are generated, which cause an increase in microstresses and deformations of the structure. Most defects annihilate during migration, leading to relaxation of deformations and microstresses. However, at high irradiation fluences, regions of overlapping defects are formed, which form regions of nonequilibrium defect concentrations, which lead to disordering and further amorphization of the structure, which leads to a decrease in the ceramic hardness along the entire ion path, as well as to the formation of additional optical traps and an increase in the photoionization cross section, which affect on the reflectivity of BeO ceramics.
Figure 6 shows the changes in the OSL signal bleaching curve obtained for samples irradiated with different ions and at different temperatures. The general view of the curves is characterized by an asymmetric bell-shaped maximum with strong shape distortion. The dynamics of the change in the curve for irradiated samples is characterized by a decrease in the intensity of the maximum, as well as by its shift. A decrease in the intensity of the bleaching signal is associated with the presence of point defects and disordered regions in the structure of ceramics, which serve as optical traps. The shift of the maxima is due to a change in their density, both in the near-surface layer and in the distribution over depth.
For the samples irradiated at different temperatures, a less pronounced decrease in the intensity of the maximum with increasing temperature is observed, which indicates a change in the concentration of optical traps with a decrease in the photoionization cross section.
Figure 7 shows the OSL signal bleaching efficiency data depending on the irradiation conditions. The presented data show that in the case of irradiation at room temperature, the decrease in the bleaching efficiency is more than 40 % for the samples irradiated with Ar8+ ions and more than 60 % for the samples irradiated with Xe22+ ions. The difference in the decrease in the bleaching efficiency of 20 % is associated with a change in the density of defects arising from irradiation and the maximum ion path length, which leads to damage at a greater depth. In the case of an increase in the irradiation temperature to 800 K, a smaller decrease in the bleaching efficiency is observed, and at a temperature of 1000 K, the efficiency loss is no more than 5-15 %, depending on the type of ion. This effect is associated with the partial annealing of point defects and lower structural distortions arising during irradiation as a result of an increase in thermal vibrations of atoms in the lattice at elevated temperatures.
Figure 8 shows the graphs of changes in the microhardness value depending on the type of incident ion, as well as the irradiation temperature. As can be seen from the data presented, for all irradiated samples, a decrease in the value of microhardness is observed along the entire path of incident ions. In this case, an increase in the irradiation temperature leads not only to a decrease in radiation damage, but also in the depth of radiation damage.
On the basis of the obtained data on changes in microhardness, the efficiency of microhardness resistance to radiation damage resulting from irradiation with various ions and irradiation temperature was calculated. The calculation data are presented in Figure 9. According to the data obtained, in the case of irradiation of ceramics at a temperature of 300 K, a sharp decrease in microhardness by 50-60% is observed, depending on the type of irradiation. In this case, an increase in the irradiation temperature leads to a decrease in the degree of radiation damage.
The general trend in the change in the surface morphology of irradiated ceramics is associated with a change in the surface relief as a result of defect formation, deformation of the crystal structure, and partial sputtering of the near-surface layer. Figure 10 shows 3D images of the surface morphology of BeO ceramics before and after irradiation, performed using the method of atomic force microscopy.
The general tendency of changes in the surface morphology of irradiated ceramics is associated with a change in the surface relief as a result of defect formation, deformation of the crystal structure, and partial sputtering of the surface layer. For samples irradiated with Ar8+ ions, the main effects of changes in the surface topography are associated with the formation of crater formations, which are formed as a result of partial peeling of the surface layer as a result of the appearance of overvoltage regions and highly disordered regions. For example, on the surface of ceramics, the formation of chilloc-like inclusions is observed, resulting from the extrusion of a deformed volume onto the surface as a result of radiation damage. In this case, an increase in the irradiation temperature leads to a decrease in the density of these inclusions, as well as to a decrease in their size, which indicates the effect of partial annealing of defects at high irradiation temperatures. For samples irradiated with Xe22+ ions, the main changes in the surface relief are associated with partial exfoliation and crater formation as a result of irradiation. In this case, the tendency for a decrease in the change in the surface relief with an increase in the irradiation temperature is similar, as for the samples irradiated with Ar8+ ions.
Thus, during this study, it was found that thermal irradiation leads to a decrease in the degree of radiation damage and a decrease in their contribution to structural changes. It has been shown that BeO ceramics exposed to high-temperature irradiation are more resistant to degradation and deformation of their structural and strength properties.