Study of irradiation temperature effect on change of structural, optical, and strength properties of BeO ceramics when irradiated with Ar8+ and Xe22 heavy ions

This paper presents the results of the study of the effect of irradiation temperature on structural and optical distortions and deformations, as well as the strength properties of BeO ceramics as a result of irradiation with Ar8+ and Xe22+ ions at a radiation dose of 5 × 1013 cm-2. The choice of radiation dose is due to the effect of overlapping defective areas arising along the trajectories of ions in ceramics, which makes it possible to model radiation damage caused by the effect of accumulation as a result of cascade collisions and overlapping damaged areas. The temperature range of 300–1000 K was chosen to simulate different operating conditions, as well as the possibility of simulating partial annealing of defects during irradiation at high temperatures. During the research, it was established that high-temperature radiation reduces influence of size of electronic and nuclear power losses of ions of Ar8+ and Xe22+ with energy of 70 MeV and 231 MeV, respectively, on extent of radiation damage of ceramics of BeO. Irradiation at a temperature of 1000 K results in an equal 14% change in dislocation density for these particles, a comparable decrease in the yield intensity of optically stimulated luminescence by 5% and 15%, as well as microhardness by 25% and 30%, respectively.


Introduction
Today, in modern materials science, the most promising materials used in various branches of science and technology are oxide nanomaterials or ceramics, perovskite structures, and ferroelectric and dielectric ceramics [1][2][3][4][5][6][7][8][9][10]. Interest in them is due to their unique physicochemical properties and high resistance to external influences, including radiation damage, thermal heating, and aggressive action of acidic or alkaline environments. In most cases, the use of these types of materials is limited by their service life, which is determined by the preservation of indicators of resistance to external influences for a long time [11][12][13][14]. Moreover, in case of application of these oxide nanostructured ceramics or perovskites as structural materials, their resistance to radiation embrittlement and degradation of physical and mechanical properties is the most important applicability condition [15,16].
One of the main requirements for structural materials for nuclear reactors is to ensure the stability of operating characteristics such as thermal conductivity, electrical resistance, high radiation, and corrosion resistance to external influences such as radiation, thermal heating, etc. [17][18][19]. As is known, the impact of ionizing radiation leads to the accumulation of defects and changes in structural characteristics, the formation of areas of disorder, and amorphous inclusions in the near-surface layer, which, in turn, leads to a decrease in thermal conductivity and further deterioration of the performance characteristics of structural materials [20,21]. Therefore, the research of defect formation processes and the study of radiation resistance of new types of structural materials are of scientific and practical interest. Structural materials based on beryllium or its oxide form have found their application not only as components for optical devices, but also as reflectors and moderators for nuclear reactors [22][23][24]. Moreover, the use of ceramics of this class can significantly increase the temperature of the working area, which is due to the high values of the melting temperature, thermal conductivity, strength, and also resistance to external influences. However, the use of ceramics based on oxides, carbides, or nitrides, which have a similar nature of physicochemical and strength properties, as structural materials for nuclear power in the XXI century requires studying the resistance of these materials to the effects of ionizing radiation, as well as subsequent processes of defect formation [25][26][27][28][29][30]. At the same time, the service life of these structural materials, approved during the construction of nuclear power plants, must be determined with high accuracy, which is due not only to the strict requirement to minimize the risk of deformation of the vessel structure or the main components of nuclear reactors exposed to radiation, as well as the impossibility of quickly replacing deformed components from the accumulated background radiation during operation. The main sources that create defects in structural materials of nuclear reactors during operation are neutron radiation and fragments of fission of uranium nuclei with an energy of 50-250 MeV, which can lead to partial disordering and deformation of the structure as a result of the accumulation effect [31][32][33][34][35][36]. In this case, the greater the energy of the incident particles, the more defects are created along the trajectory of the ions in the material. Most defects are annihilated during migration, which leads to relaxation of deformations and microstresses. However, at high irradiation densities, defect overlap regions are formed, which form regions with a non-equilibrium defect concentration, which leads to disordering and further amorphization of the structure and 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 affecting the reflection ability of ceramics BeO [37,38]. However, the question still remains open related to the behavior of the structural and strength properties of BeO ceramics to high-temperature irradiation. As is known, when materials are heated, partial annealing and relaxation of defects occurs due to changes in the vibrational modes of atoms and subsequent annihilation of point defects with each other [39][40][41]. In this case, during the passage of heavy ions in the material, according to the theory of thermal peaks, regions with an increased temperature are formed along the trajectory of the ions, in which defects are formed [42][43][44][45][46]. Moreover, these regions have short lifetimes, but very high temperatures, which leads to the formation of a large number of mobile defects [44][45][46]. In the case of high-temperature irradiation, the process of defect formation and their annealing will be competing processes, while the question of which of the two processes will dominate in the case when the ion trajectories and, therefore, the defect regions arising from irradiation will overlap at high radiation doses is not clear. In this regard, the study of the processes of radiation damage to ceramics BeO during hightemperature irradiation with high-energy heavy ions is an urgent and demanded direction of scientific research in the field of nuclear technologies.
The main purpose of this work is to conduct research on the effect of the irradiation temperature on the degree of structural distortions and changes in the optical and strength properties of ceramics based on beryllium oxide under irradiation with heavy Ar 8? and Xe 22? ions. The relevance of this work is due to the acquisition of new experimental data on the processes of radiation damage during irradiation with heavy ions of structural materials based on oxide ceramics, which have broad prospects for use in thermonuclear and nuclear power engineering when creating reactors of the GenIV generation [47][48][49][50][51][52]. The proposed hypothesis is based on the assumption that, under high-temperature irradiation, some of the point defects will annihilate among themselves due to accelerated vibrations of the atomic lattice not only in the area of damage as a result of local heating during the passage of ions through the material, but also as a result of subsequent cascade collisions. In this case, the supply of heat for heating the sample during irradiation will significantly increase the amplitude of the vibrational (thermal) motion of the lattice atoms, which will lead to a higher mobility of vacancies and interstitial atoms, with a subsequent decrease in the concentration of point defects during irradiation due to their annihilation.

Experimental part
The objects of study were samples of ceramics based on BeO with a thickness of 15 lm and an area of 5 9 5 mm. According to the technical data sheet, the chemical purity of beryllium oxide samples was 99.9%. The samples under study were polycrystalline structures with a hexagonal lattice of the space group of symmetry P63mc(186) and crystal lattice parameters a = 2.671 Å and c = 4.332 Å (PDF-01-077-9751). The choice of the type of ceramics as the object of research is due to their huge potential for use as structural materials for new generation nuclear energy. Test samples of materials were used as samples, which are later planned to be used as the basis for structural materials of high-temperature nuclear reactors.
Irradiation was carried out on DC-60 heavy ion accelerator of the Astana branch of the Institute of Nuclear Physics (Nur-Sultan, Kazakhstan) by Ar 8? and Xe 22? ions with energies of 70 MeV and 231 MeV, respectively, at irradiation temperatures of 300, 800, and 1000 K and a particle fluence of 5 9 10 13 cm -2 . The samples under study were irradiated in a vacuum, and the samples were placed on special holders combined with a heating element. Temperature control was carried out using thermocouples. The irradiation dose was controlled using a Faraday cup. The choice of irradiation conditions and temperature range made it possible to model with high accuracy the processes of radiation damage resulting from the fission of uranium nuclei in a nuclear reactor. Table 1 shows the data on the energy losses of incident ions as a result of interaction with the BeO target, obtained using the simulation method in the SRIM Pro 2013 program code.
The Monte Carlo simulation in the SRIM-2013 program of BeO ceramics irradiation at a temperature of 300 K by Ar 8? and Xe 22? ions with energies of 70 MeV and 231 MeV, respectively, indicates a significant predominance of radiation damage degree during xenon ion irradiation in comparison with argon ion irradiation, which is associated with higher energy losses of xenon ions by approximately three times when interacting with the electronic and nuclear subsystems of the target (See Fig.1).
According to the calculated data in Table 1, irradiation with Xe 22? ions with an energy of 231 MeV is characterized by a greater spatial development of the damage cascade created in the structure of beryllium oxide in comparison with irradiation with Ar 8? ions with an energy of 70 MeV, which is caused by about three times higher values of energy losses xenon ions when interacting with the electronic and nuclear subsystems of the material. This is additionally confirmed by the results of calculating the number of vacancies created in the structure, which in the case of irradiation with Xe 22? ions is more than six times higher than that for irradiation with Ar 8? ions.
The study of changes in structural parameters, including distortions of the crystal lattice and dislocation density, was carried out on the basis of the obtained X-ray diffraction data performed on a D8 ADVANCE ECO powder diffractometer (Bruker, Germany) using CuKa radiation. Diffraction patterns were recorded in the Bragg-Brentano geometry, in the angular range 2h=30-75°, with a step of 0.02°, and the spectrum acquisition time was 3 s at a point. The crystallographic parameters were determined using the DiffracEVA V.4.2 program code.
Optically stimulated luminescence (OSL) measurements were performed on an automated TL-OSL setup (RNL Denmark). To initiate luminescence, a LED with a wavelength of 470 nm and a power of 35 mW/cm 2 was used. Linear Modulated OSL (LM-OSL) was measured at room temperature after preheating the samples to 160°C at a rate of 5°C/s and holding for 10 s. This procedure is necessary to remove unstable signals.
Determination of microhardness and changes in strength characteristics as a result of irradiation was carried out using the indentation method under variable load, on a digital microhardness meter. The Vickers pyramid was used as an indenter. 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 2h°. 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 Ar 8? 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.

Results and discussion
For samples irradiated with Xe 22? ions at a temperature of 300 K, a similar situation is observed with    (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 3a 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 Ar 8? and Xe 22? ions with energies of 70 MeV and 231 MeV, respectively, at a particle fluence of 5 9 10 13 cm 2 and temperatures of 300, 800, and 1000 K. Structural distortions were evaluated by measuring changes in crystal lattice parameters before and after irradiation and comparing them with reference values.It can be seen from the presented data that, upon irradiation with Ar 8? and Xe 22? 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 annihilation 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 3b 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. The dislocation density was calculated on the basis of X-ray data on changes in grain sizes as a result of irradiation and temperature exposure, which were estimated from the width and shape of diffraction reflexes.
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 non-equilibrium defect concentrations, which lead to disordering and further amorphization of the structure, 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 4 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 nearsurface 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 5 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 Ar 8? ions and more than 60% for the samples irradiated with Xe 22? 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. The obtained data are in good agreement with the data of changes in structural distortions and dislocation density, which change as a result of different exposure modes. As is known, the optical properties are greatly influenced by the change in the concentration of point defects in the structure. At the same time, in the case of irradiation, the end result of radiation damage is the formation of disordered areas containing a large number of point defects and dislocations. The presence of such areas leads to a change in the electronic density of the material, as well as a change in optical properties, both by changing the forbidden zone value and by creating additional obstacles and absorbing centers. In the case of irradiation at elevated temperatures, where some of the resulting defects from irradiation recombine with each other or anneal, the degree of influence on the change in optical characteristics is significantly less than in the case of irradiation at room temperatures. Figure 6 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. According to the data obtained, in the case of irradiation with Ar 8? ions, the maximum depth of radiation damage is 10-12 lm, while for samples irradiated with Xe 22? ions, this value is more than 15 lm. This difference is primarily due to the various energy losses of ions in the material depending on their energy, as well as in the case of irradiation with Xe 22? ions, inelastic collisions of ions with the electron subsystem dominate most of the distance, which gives rise to a large number of primary knocked-out electrons in the structure of ceramics that can create cascades of secondary electrons. In this case, irradiation results in the formation of a large number of localized defect regions, leading to the appearance of deformation distortions of the crystal structure, as well as partial destruction of crystal and chemical bonds, which leads to a decrease in strength properties. However, in the case of irradiation at elevated temperatures, the formation of radiation defects competes with their recombination and thermal annealing of defects. In this case, an increase in the irradiation temperature leads to the fact that most of  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 7. 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 8 a b 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 Ar 8? 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 hillock-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 Xe 22? 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 Ar 8? 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.

Conclusion
The paper presents study results on high-temperature irradiation of BeO ceramics with swift heavy ions. Distortion of diffraction peaks and their shift are associated with deformation processes arising from changes in concentration of point defects, as well as their migration in the structure as a result of irradiation. Irradiation of BeO ceramics with Ar 8? and Xe 22? ions at a temperature of 1000 K leads to comparable for given particles changes in dislocation density, optically stimulated luminescence intensity, and microhardness. The results obtained indicate the process of defect annealing and microstress relaxation of the crystal lattice, which arise during irradiation as a result of increase in thermal vibrations of atoms in the lattice at elevated temperatures. Thus, it has been established that high-temperature irradiation of BeO ceramics reduces effect of electron and nuclear energy losses of Ar 8? and Xe 22? ions with energies of 70 MeV and 231 MeV, respectively, on defect formation efficiency. This is associated with an increase in magnitude of thermal vibrations of lattice atoms, leading to an increase in their mobility, as well as accelerated annihilation of generated defects with partial recovery of the optical properties and hardness of the ceramics.
In conclusion, the data obtained on the effect of high-temperature irradiation of beryllium oxide on the decrease in the contribution of the electronic and nuclear energy losses of high-energy heavy ions to the degree of radiation damage make it possible to determine the thermal stability and the range of applicability of BeO ceramics in high-temperature fields.