Basic research on ionic-covalent materials for nuclear applications

Resume : Radiation-induced structural defects strongly affect the functionality of wide gap materials, incl. those considered as promising materials for optical (diagnostics) windows in future fusion reactors. A comparative study of radiation damage caused by ~MeV fission neutrons, ~GeV swift heavy ions and 100-keV protons in MgAl2O4, Lu3Al5O12 and Lu/Gd mixed oxyorthosilicates has been performed. The damage was analyzed using the methods of induced optical absorption (region of 1.4-9 eV), cathodo- and photo-luminescence and thermoactivation spectroscopy. The thermal annealing (up to 1000 K) of F-type center absorption, measured in a stepwise regime, was compared to that of the EPR signal for new paramagnetic hole centers in a neutron-irradiated spinel crystals (3e18 n/cm2). The possible inversion of spinel lattice is considered to be responsible for high tolerance of MgAl2O4 to heavy irradiation. Optical characteristics of antisite defects, revealed earlier in YAG and LuAG crystals, were tentatively determined for spinel ceramics in optical absorption and luminescence excitation spectra. The accumulation of proton-induced radiation damage (a fluence varies from 1e15 to 5e17 p/cm2) was analyzed for spinel ceramics by means of optical absorption and cathodoluminescence. The joint contribution of universal knock-on mechanism and several nonimpact mechanisms (related to the decay of electronic excitations) to radiation damage of wide gap materials is considered.

Resume : Electron and hole localization in solids is fundamental for our understanding of the mechanisms of radiation damage. However, strong localization in complex 3D systems is material specific and difficult to elucidate. We will discuss the results of recent theoretical and experimental studies of intrinsic electron and hole trapping in crystalline and amorphous SiO2, HfO2 and Al2O3. These materials in a bulk crystalline phase exhibit polarons with trapping energies of about 0.2 eV. Recent experimental evidence suggests that both negative and positive charging can occur in amorphous oxides in much deeper states. To investigate whether such charging can be caused by intrinsic electron and hole trapping, we modelled the behaviour of extra electrons and holes in stoichiometric amorphous SiO2, HfO2 and Al2O3 structures using classical simulations and Density Functional Theory. The results demonstrate that single- and bi-polaron electron and hole states can form in a-SiO2 and a-HfO2, where the effect of local disorder is amplified by polaronic relaxation of amorphous network. Only hole trapping is found in a-Al2O3. Both electron and hole bi-polarons can facilitate the formation of Frenkel pairs of oxygen vacancies and interstitial oxygen ions. These results broaden the concept of intrinsic polaron trapping to disordered oxides. We will discuss challenges in predicting polaron trapping and comparing the predicted properties with experiment.

Resume : In solid materials, micro-structural patterns resulting from self-organization are responsible for phase transitions and properties changes. Understanding the origin and the selection of possible spatio-temporal patterns far from thermal equilibrium is a major theme of research opening doors to many technological applications ranging from plasmonics to meta-materials. Almost forty years after Turing’s celebrate paper on patterning, progress on modeling instabilities leading to pattern formation has been achieved. The initial concept of dissipative structure is now clearly understood within the Phase-Field framework. Features of spatio-temporal pattern formation are now described in this framework. So far, such an approach obtained promising results in various aspects of materials research from pattern formation during solidification to defect dynamics. In this presentation, we will try to develop an unified presentation of tools, concepts and methods in view to discussing experimental results observed during aging of solids under irradiation. The approach followed in this presentation is comprehensive and not specialized in specific aspects of the Phase-Field modelling (mechanics, mathematics, or numerical methods) at the expense of a holistic picture. Although specialized treatments of this topic are available, this presentation is focused on applications of this appoach to materials submitted to radiation damage.

Resume : Refractory metals are widely used in nuclear engineering and aerospace industry, due to their high resistance to extreme conditions, such as high temperatures, and wear. When used in this context, these materials are subjected to a flux of high-energy radiations that change their microscopic structure by means of the generation of a high number of defects. The production of defects can reach a dramatic amount, thus affecting the macroscopic properties of the material. It is crucial to understand the microscopic changes that occur in these materials, in order to enhance their performances and endurance. Damage production and displacement cascade occur rapidly are impossible to be studied directly. Molecular Dynamics, operating in the nanometer scale and covering a timescale that spans from tenths of femtoseconds up to nanoseconds, can provide a deep insight into the microstructure of the materials in the first moments after the impact. Our study focuses on the analysis of the response of different refractory metals towards radiation damage, by means of studying their behavior after the impact with a high-energy debris. The goal of this analysis is to compute some target properties of the materials, classify them, and state their efficiency as shielding components in aerospace and nuclear industry. Our aim is to compare niobium, for which we already have satisfactory results, to tungsten and tantalum, and to study the same structures with and without a grain boundary interface.

Resume : Oxide ceramics, such as ceria, yttria-stabilized zirconia, magnesium-aluminate spinel, are of importance for nuclear applications because of their superior resistance to amorphization. Radiation damage induced by fission fragments (FFs) is the most crucial issue for fuel and target materials, in which high-density electronic excitation is induced to form continuous ion tracks. This paper reports the structure of ion tracks in those oxides throurgh a variety of transmission electron microscopy (TEM) and scanning transmission electron microscopy (STEM) techniques, to discuss the atomic structure of ion tracks and microstructure evolution at high dose. Atomic resolution STEM observations showed that core damage regions of ion tracks for oxides, CeO2, yttria-stabilized ZrO2 and MgAl2O4, maintain their own crystal structure. The atomic density in core region of ion tracks is, however, depressed and oxygen sublattice is preferentially disordered in fluorite oxides (CeO2, yttria-stabilized ZrO2). Modelling of the accumulation of ion tracks against ion fluence succeeded in evaluation of ion tracks size, size of recovery region, and track formation efficiency. Those values were discussed as a function of electronic stopping power among the oxides. References [1] S. Takaki et al., Nucl. Instrum. Meth. B, 326 (2014) 140. [2] K. Yasuda et al., Int. J. Mater. Res., 102 (2011) 9. [3] K. Yasuda et al., Nucl. Instrum. Meth. B, 314 (2013) 185. [4] S. Takaki et al. Prog. Nucl. Energy 92 (2016) 306. [5] AKM S.I. Bhuian et al. Trans Mat. Res. Soc.-J. 41 (2016) 319.

Resume : The “velocity effect” (VE) in latent track formation has been observed for a variety of materials irradiated with swift heavy ions. The effect manifests in the observation that at the same level of ionization energy losses, high-velocity ions produce less damage than low-velocity ions. Data on track radii in Y3Fe5O12 used to formulate the VE conjecture was obtained mostly through (RBS/C) measurements which is an indirect and highly averaged method for obtaining such data. These track radii and thus the VE effect itself should ideally be confirmed by more direct track imaging methods such as TEM. Recent studies based on direct and indirect measurements of swift heavy ion induced damage in GeS, HOPG and graphene have demonstrated that VE manifests itself differently for these materials. In this work, the results of a TEM study of the morphology of latent tracks in Y3Al5O12 and Y3Fe5O12 formed by Kr and Xe ion irradiation in high- and low-velocity regimes will be presented. Additionally, a possible effect of near surface stress induced by high energy ion irradiation on the results of RBS/C measurements, which may affect the reliability of deduced track sizes from damage cross-sections, is discussed.

Resume : Cubic ZrO2 single crystals have been reported to exhibit excellent radiation resistance both in the elastic and inelastic collision regimes. However, the use of Yttria Fully Stabilized Zirconia (FSZ) in realistic scenarios requires to study its radiation resistance at the nanoscale. FSZ thin films 600 nm nominally thick were grown on (100) Si by UV pulsed laser ablation. FSZ films are polymorphous, as shown by X-ray diffraction (XRD). They were irradiated with 2.6 GeV Uranium ions at different fluences in the range 5-20×1011 ions cm−2. XRD and μ-Raman spectroscopy evidenced a change in FSZ crystalline structure depending on the irradiation fluence. While low fluences do not modify film structure, at progressively higher fluences the relative fraction of monoclinic to cubic/tetragonal phases evolves towards the latter phase. Across the mid-range fluences of 8-15×1011 ions cm-2, partial amorphization is also observed. Further, the irradiation modifies FSZ crystalline structure as a function of film depth. XRD at synchrotron at different incidence angles (0.2-1°) shows that the contribution of the monoclinic phase is enhanced in the sub-surface region while the cubic/tetragonal phase dominates in depth. These observations evidence that FSZ radiation tolerance is stressed at particularly high fluences of swift heavy ions, especially in the region of few hundreds nm in the proximity of the surface

Resume : At irradiation of swift heavy ions, the defects formation frequently takes place in crystals. High energy transfer into the electronic subsystem and relaxations processes lead to the formation of structural defects and cause specific effects, such as the track formation. There is a large interest to understanding of the mechanisms of defects/tracks formation due to the heating of the electron subsystem. In this work, the atomistic simulation of defects formation and structure transitions in UO2 and U-Mo alloys at irradiation of swift heavy ions has been carried out. We use the two-temperature atomistic model with explicit account of electron pressure and electron thermal conductivity. This two-temperature model describes ionic subsystem by means of molecular dynamics while the electron subsystem is considered in the continuum approach. The various mechanisms of structure changes at irradiation are examined. In particular, the simulation results indicate that the defects formation may be produced without melting and subsequent crystallization. Also it is shown that the mechanism of surface track formation differs from the mechanism of track formation in the bulk. The threshold values of the stopping power for track formation are estimated.

Resume : Al2O3 (sapphire, corundum) is a promising material for fusion reactors, mainly for diagnostics as a general insulator or optical components. Anion-deficient sapphire doped with carbon is also used in personal dosimeters. Thus, it is very important to predict and simulate the kinetics of defect accumulation in sapphire under neutron irradiation as well as long-time defect structure evolution. There were numerous experimental measurements of the primary defect accumulation kinetics (first of all, F color centers—oxygen vacancy with two trapped electrons) as a function of radiation dose rate and temperature with subsequent post-irradiation annealing. As is well known, the F center mobility is much smaller than that of the complementary Frenkel defects – interstitial oxygen ions. Thus, at moderate radiation doses and temperatures, the kinetics of the F center annealing is controlled by their diffusion-controlled recombination with mobile oxygen interstitials. Despite numerous experimental data, very few theoretical efforts were devoted so far to the quantitative analysis of available kinetics, in order to extract main kinetic parameters- interstitial migration energy Ea and diffusion pre-exponent Do, necessary for further prediction of the secondary defect kinetics and radiation stability of sapphire and related materials. Such theory was developed and applied to irradiated insulators in our recent studies [1,2]. In this paper, we analyzed carefully the annealing kinetics for neutron irradiated sapphire available from literature. The extracted migration energies in different experiments vary from 0.8 eV (close to theoretical prediction [3]) down to 0.3 eV, as radiation dose increases. The pre-exponent Do is also much smaller than the estimate for a regular diffusion in crystalline solids. This could be attributed to a strong spatial correlation of radiation-induced defects along neutron trajectories. Moreover, we have observed a strong correlation between the activation energy Ea and pre-exponent Do (Ea) which is no longer constant but fits very well to the exponential function of Ea. In fact, as radiation dose increases, both the migration energy Ea and Do decrease, as the defect diffusion rate effectively grows. Moreover, we demonstrate this correlation for two other neutron irradiated ionic materials- MgO and MgF2. These results are analysed in terms of the Meyer-Neldel rule [4] observed earlier in glasses and disordered materials. [1] V.N. Kuzovkov, E.A. Kotomin, A.I. Popov, R.Villa, Nucl. Inst. Meth. B 374, 107 (2016). [2] V.N. Kuzovkov, A.I. Popov, E.A. Kotomin et al, Low Temp. Phys., 42, 748 (2016). [3] A. Platonenko, D.Gryaznov, S.Piskunov, E.A.Kotomin , Phys. Stat. Sol. C 13, 932 (2016) [4] W. Meyer, H. Neldel, Phys. Zeitschrift, 38, 1014 (1937).

Resume : The radiation-resistant oxide insulators (MgO, Al2O3, MgAl2O4, BeO etc) are important materials for applications in fusion reactors. It is very important to predict/simulate not only the kinetics of diffusion-controlled defect accumulation under neutron irradiation, but also a long-time defect structure evolution including thermal defect annealing after irradiation. In thus work, the kinetics of the F-type center annealing after electron, heavy ions or neutron irradiation was treated as the bimolecular process with equal concentrations of the complementary F and Oi defects. It is controlled by the interstitial oxygen ion mobility, which is much higher than that of the F centers. The F center annealing begins in binary oxides at temperatures 500-700 K, when both F and F centers are practically immobile, due to the recombination with mobile Oi defects. It is demonstrated how the shape of the F-annealing curves is determined by two control parameters: Ea and effective pre-exponential factor and strongly depends on irradiation conditions. The appropriate migration energies were obtained from available from literature annealing kinetics for electron, neutron and ion irradiated MgO, Al2O3, MgAl2O4, BeO, ZnO, PLZT etc. The results obtained are compared with recent ab initio calculations of interstitial oxygen migration. Special attention is paid to: (1) dose effects on F center annealing in neutron and fast electron irradiated MgO and MgF2; (2) a detailed comparison of diffusion-controlled F center thermal annealing in neutron, electron and heavy-ion irradiated MgO, MgF2, Al2O3, ZnO and MgAl2O4; (3) the F center annealing and metal colloid formation in thermochemically reduced MgO, Al2O3 and BeO.

Resume : ZnO is a wide and direct band gap semiconductor with potential applications in electronic devices able to work in harsh environments including fusion/fission reactors. However, despite that ZnO exhibits efficient dynamic annealing even at cryogenic temperatures, radiation-induced defects of different types accumulate and can affect device performance. In the present contribution, we study the role of buried pre-existing extended defects on damage formation and annealing in ZnO single crystals implanted with N ions at RT and 15 K. A crystal c-axis aligned 500 keV Ag implantation was used to create a dense band of extended defects in the samples [1]. After that 80 keV N implantations were performed in the region between the sample surface and the Ag peak. Damage evolution was analyzed by RBS/C. The results show that thermally activated migration of point defects from the implanted volume and subsequent trapping/annihilation by the pre-existing defects determine the defect accumulation in the pre-damaged samples at RT. In particular, this leads to the suppression of the defect formation around the projected range of N ions and enhanced radiation tolerance of the material in this region. In its turn, annealing leads to an apparent increase of the damage in all the N implanted samples i.e. a reverse annealing takes place and the role of the pre-existing defects on reverse annealing is discussed. [1] A. Azarov et al., Appl. Phys. Lett. 110, 022103 (2017). [2] A. Azarov et al., Appl. Phys. Lett. 110, 172103 (2017).

Resume : Radiation-induced degradation processes in solid methane have aroused particular interest in connection with its use as a moderator of hot neutrons in advanced neutron sources [1]. Another aspect of the interest is related to planetary and interstellar astrophysics [2]. Dramatic changes of solid CH4 under ionizing radiation were discovered in 80’s [3,4]. Here we present new trends in the study of radiation damage of CH4 solids with an accent on relaxation processes in the films pre-irradiated with a 1 keV electron beam. Because there is no thermoluminescence from pure solid CH4 we focused on measurements of relaxation currents to discriminate charge recombination reactions. Thermally stimulated exoelectron emission TSEE was measured at T from 5 to 120 K. Two strong peaks of TSEE at about 18 and 43 K were detected from solid CH4 for the first time and activation energies were estimated. Measured in a correlation manner (similar to used in [5]) thermally stimulated particle ejection (post-desorption) from pre-irradiated solid CH4 was found. Strong peaks of anomalous low-temperature post-desorption ALTpD were observed starting at temperatures much lower than the characteristic sublimation temperature. The ALTpD and TSEE yields were compared with the temperature behavior of “burps” in solid CH4 moderator. Basic mechanisms behind of these phenomena are discussed. [1] E. Kulagin, S. Kulicov, V. Melikhov, E. Shabalin. NIM B 215 (2004) 181. [2] R. Miles Icarus 272, (2016) 387. [3] J.M. Carpenter Nature 330, (1987) 358. [4] G. Foti, L. Calcagno, K. Sheng, G. Strazzulla, Nature, 310 (1984) 126. [5] E.V. Savchenko, I.V. Khyzhniy, S.A. Uyutnov, A.N. Ponomaryov, G.B. Gumenchuk, V.E. Bondybey, Low Temp. Phys. 39 (2013) 446.

Resume : Yttrium orthoaluminate YAlO3 (YAP) crystals doped by rare earth ions have long-term interest for thermoluminescent (TL) dosimetry of ionizing radiation as well as fast scintillator detection. Recently, application potential of Mn-doped YAP for TL dosimetry of ionizing radiation has been clearly demonstrated. In this presentation, after introducing some basics on the radiation point defects and impurities in YAlO3, we will summarize the current status of luminescence studies (PL, OSL, TSL, TR-OSL, and dose dependence of OSL) of YAlO3:Mn single crystals and ceramics. Then, we will report the recent results of Raman and EPR measurements of the set of undoped YAlO3 and YAlO3:Mn samples, prepared and characterized in [1, 2]. Finally, detailed comparison with earlier published data will be presented. [1] Ya.Zhydachevskii, et al., Radiation Measurements 45 (2010) 516. [2] Ya. Zhydachevskii, et al., Radiation Measurements 94 (2016) 18. The financial support of Latvian-Ukrainian Joint Research Project No LV-UA/2016/1 is greatly acknowledged.

Resume : Uranium dioxide (UO2) is an important industrial material which is employed as a fuel in most nuclear reactors world-wide. The doping of UO2 with small amounts of chromium sesquioxide (Cr2O3) is technically applied to obtain a larger average grain size after the fuel sintering process. In this study the local environment of cromium in UO2 was investigated using X-ray absorption spectroscopy. An interpretation of the X-ray absorption spectra is a non-trivial task, especially for so complicated systems like doped fuel. Here we will address this challenging problem using two approaches, in which the ab initio EXAFS theory is combined with classical molecular dynamics (MD) or ab initio molecular dynamics (AIMD) as well as electronic structure simulations based on density functional theory (DFT). The atomic scale MD and DFT simulations were carried out using the CP2K code. Alternatively, DFT simulations were performed to determine the influence of the chromium doping on the fuel matrix. The atomic structure around the chromium impurity was relaxed, and the resulting structures were used to calculate the Cr K-edge EXAFS spectra. The comparison of the simulated EXAFS spectra derived from DFT results with the experimental one allows the identification of valid atomic configurations. The limitations of this approach are discussed.

Resume : Cathodoluminescence (CL) spectra of the aluminum nitride nanotubes and nanoparticles, synthesized by using a highly nonequilibrium dc-arc plasma method have been measured at 80 K and room temperature (RT) under electron irradition with 10 keV energy. Low-temperature CL spectra of nanostructured AlN have been compared with those of the commercially available AlN powder. The significant difference between emission spectra of the three investigated samples has been established. Commercial AlN has been found to emit a band peaked at 3.47 eV which is commonly ascribed to oxygen impurities. Emission of the AlN nanoparticles is centered around 3.66 eV while CL spectrum of AlN nanotubes show complex character with at least three peaks at 2.2, 3.0 and 3.5 eV in the photon energy range of 1.8 – 3.8 eV. CL intensity of the nanostructured samples has been found to decrease significantly at RT, most probably due to a combination of non-irradiative relaxations at the surface, electron-phonon interactions and the reabsorption of the emitted light. CL of AlN-nanotube/CsI-scintillator composites has been also studied. Energy transfer via luminescence emission from CsI scintillator to AlN nanotube is demonstrated. Luminescence properties AlN nanotube/polymer composites were also studied and compared with those obtained for AlN nanotubes and nanoparticles.

Resume : High Temperature Oxidation Behavior of Silicon Carbide Ceramics for Enhanced Performance Fuel in Hydrothermal Exposure Conditions Seung-Jae Lee1, Kwang-Young Lim1, Young-Wook Kim2 1Materials Development Section, KEPCO Nuclear Fuel, Daejeon 305-353, Republic of Korea 2Functional Ceramics Laboratory, Department of Materials Science and Engineering, The University of Seoul, Seoul 130-743, Republic of Korea Keywords : SiC, Nuclear Fuel, PBAT, TRISO, ATF The recent investigations focus on the nuclear ceramic fuels with enhanced accident tolerance under severe accidents. In this sense, the use of particle-based accident tolerant (PBAT) fuels is an alternative strategy for overcoming this issue by multiple silicon carbide (SiC) barriers such as SiC layer in TRISO particle and the dense SiC matrix of the ceramic capsules. The performance on high temperature oxidation of PBAT fuel is one of important factors for plant safety of nuclear reactors supposing the accidents. In this work, safety performance evaluation of SiC ceramics for accident tolerance fuel (ATF) in hydrothermal exposure condition was performed at 1200 and1700 oC for ~25hr. PBAT fuel and monolithic SiC ceramics was fabricated by hot-pressing and normal pressureless sintering, respectively. After 25hr hydrothermal oxidation, some TRISO particles in PBAT fuel were exposed and inner surface portion of PBAT fuel shows pore channel by permeated liquid phase through the grain boundary and junction. The high temperature oxidation resistance of SiC ceramics was dependent on the chemistry inside the grain structure, the additive compositions and the additive contents. More importantly, suppression of liquid phase formation by reaction between formed SiO2 and junction area is one of the essential factors for oxidation resistance performance of SiC ceramics. Detail data for oxidation behavior of SiC ceramic under hydrothermal exposure conditions will be dealed.

Resume : To study the effect of uniaxial deformation by <100> and <110> directions on the radiative annihilation of self-trapped excitons (STE), experiments were carried out with KI, KCl and KBr crystals. At deformation by <110> direction, in KBr spectra the emission line with the maximum at 2.95 eV was recorded. In KI and KCl under deformation by <110> and in KBr by <100> direction in the X-ray luminescence spectrum, in addition to intrinsic  -,  - luminescence, additional emission bands were not recorded. This effect was explained by the appearance of excitons with a strongly asymmetric configuration in KBr crystal. Since the ratio between the radii of the anion and the cation for KI, KBr and KCl are equal to 1.5, 1.9 and 2.2, respectively, then it is obvious that the size of the cavity containing STE will be the largest for KCl and the smallest for KI. Therefore, in KI, there are no configurational transformations of STE, since its molecular core cannot move. For KCl, the conversion effect is possible, but in this crystal the luminescence of STE is extinguished at 40K. In KBr crystal, at deformation by <110>, the core of the STE is "pushed out" into the interstitial void. Thus, STE of a less symmetrical configuration is formed than the "strong-off", which corresponds to the emission band at 2.95 eV. This effect is also explained on the basis of the difference in the energies of the exciton states by the surface of adiabatic potential [2]. Calculated energy difference for KI, KBr and KCl crystals was 0.0045 eV, 0.0016 eV, 0.0232 eV, respectively. It follows from these calculations that for the configuration transformations for KBr, the lowest energy is required, which explains the possibility of creating of STE with a less symmetrical configuration than "strong-off".

Resume : The modern development of optical materials is impossible without the development of optical materials of a new generation. Therefore, the creation of new optical materials is an important, relevant and strategic objective in an optical instrument and optical material science. Great interest for the modern element base of photonics, represent transparent ceramic materials. This is explained by a better technological performance of transparent ceramics compared with single crystals and transparent glass. Spinel oxides find many applications as magnetic, electric, ceramic, catalysis and optical materials. MgAl2O4 has received a great deal of attention as a technologically important material because of its mechanical strength, chemical inertness, wide band gap, relatively low density, high melting point and thermal shock resistance, low thermal expansion coefficient, resistance to neutron irradiation. In this work, we investigated the optical and cathodoluminescent properties of MgAl2O4 ceramics. The polycrystalline MgAl2O4 spinel from nanopowder was consolidated by the spark plasma sintering (SPS) technique using the installation SPS-515S with different temperature and pressure. The integrated cathodoluminescence spectra and decay kinetics were investigated with the pulsed optical spectrometer based on a high-current electron accelerator GIN-600 (Еex=0.25 MeV, FWHM=15 ns, W=3 mJ/cm2) and AvaSpec-2048 fiber spectrometer. The dependences of luminescent properties on the conditions of sintering of ceramics are established. Nature of luminescent centers and mechanisms of luminescence are discussed. This work is financially supported by the project of Russian Science Foundation No 17-13-01233.

Resume : MX2 (where M is a metal and X typically denotes Iodine or Bromide) halogenides with pronounced layered structure are interesting materials due to their capability of transforming ionizing radiation into visible light. Specific structure of these compounds provides a number of ways for doping with different impurities. Localization of impurities at interlayer voids and formation of complex aggregates involving intrinsic defects stimulate grow of nanostructures in these materials. Since micron and submicron sized clusters are quantum systems, their discrete energy spectra need to be taken into account when one deals with optical absorption or luminescence of MX2. Unlike thoroughly studied semiconductors, for example, GaSe or InSe, formation and properties of nanostructures in wide band gap halogenides are rather unexplored, while definitely represents a subject of interest. This interest is not only fundamental, as the morphology of clusters in layered MX2 change luminescence color, intensity and kinetics opening several possibilities for designing adjustable scintillators or efficient UV detectors. This report is focused on luminescence properties of a representative crystal from MX2 family – cadmium iodide. Results of spectroscopic experiments at different temperatures are discussed and mechanisms of non-radiative and radiative recombination centers formation are analyzed.

Resume : Usually the correlation between the optical and paramagnetic properties of point defects has been studied by the optically detected magnetic resonance (ODMR) in crystalline materials, where the angular dependencies of the magnetic resonances are well pronounced [1]. Nevertheless, the correlation of optical and paramagnetic properties of activator centres is an actual task also in glass-ceramics and ceramic samples [2]. In this work, we present the results of the magnetic circular dichroism (MCD) and the MCD-detected EPR measurements of the Er3+, Gd3+[3] and Mn2+ doped oxyfluoride glass-ceramics. Through the MCD-EPR, the direct correlation between the optical absorption and the local structure of the activator centres in these glass-ceramics has been established. More complicated task with the estimation of the correlation between the optical spectra and paramagnetic centres is for the ceramics, e.g. Mn-doped YAP ceramics [4]. The MCD is not available due to the opacity of the ceramics. In this case the only possibility is to establish the correlation by the MCD-EPR first in the crystalline analogous and then to verify it for the corresponding ceramics. [1] J.-M. Spaeth, H. Overhof, Springer-Verlag, Berlin, Heidelberg (2003). [2] U. Rogulis, Low temperature physics, 2016, 42, 689-693. [3] A. Fedotovs, et al, J. of Non-Crystalline solids, 2015, 429, 118-121. [4] Ya. Zhydachevskii, et al, Optical Materials, 2014, 37, 125?131.

Resume : The GeS2–Ga2S3 and GeSe2–Ga2Se3 chalcogenide glasses (ChG) have shown many advantages for potential applications of optical modulator, fiber-optical amplifier in the IR region, etc. Atomic arrangement in such ChG can be studied with numerous techniques, while number of probes available to study atomic-deficient distribution is rather limited, especially at a sub-nanometer scale. One of the best techniques capable to probe such fine free volumes is the positron annihilation lifetime spectroscopy (PALS). In the present paper, we imply the PAL method to study of free-volume structure of GeS2-Ga2S3 ChG. The PALS spectra were recorded with conventional ORTEC system. The PALS spectra of ChG were processed with standard LT 9.0 computer program, the obtained curve being fitted by two components with tau1, tau2 lifetimes and I1, I2 intensities. The positron trapping modes in the studied ChG were calculated using a formalism of two-states trapping model. Since (1, I1) component has no strong physical meaning, we focused our analysis on second component, where tau2 lifetime is directly related to a size of defect free-volume entity and the intensity I2 is proportional to the number of such “defects”. At values of PALS fitting parameters for Ge-Ga-S/Se system, bulk positron lifetime is near 0.28 ns and average lifetime is 0.32 ns. It is shown that the size of free-volume trapping centers in the ChG is higher than in the 80GeSe2-20Ga2Se3 glasses, weak increases of I2 intensities testify growing behaviour in the number of defect-related entities. Other positron trapping parameters for Ge-Ga-S system are similar to Ge-Ga-Se glasses.

Resume : Nowadays, one of the most important materials are elastomeric materials and their products. These materials are playing a serious role in almost types of machines. Once, there is no common material could be used for manufacturing details for any working conditions. Science researches are doing their best to promote technical properties. It is known that elastomeric materials products which ensure the capacity of many aggregates of rocket and space equipment that can be operated in extreme conditions. Definitely it involves high heat resistance materials for solid propellant rocket motors that must withstand high temperatures. Nowadays, a special rubber produced by various rubber fillers which have different activities is using as internal thermal insulation material. At the same time, their heat-shielding properties need to be improved. One of methods for improving protective properties of rubber composite materials is using carbon nanotubes (CNTs) as filler. CNTs structure makes ability for creating more effective heat resistance coating of rubber composite. According to our estimation vulcanizates with CNTs as an active filler will improve heat-shielding properties such strength and elastomer properties, abrasion and tear resistance. Moreover, the thermal insulation coating volume in solid rockets is substantial and density of CNTs does not exceed 1400 kg/m3 . These properties of CNTs make them auspicious to be used as filler. As well it can be used for reducing the structure net weight of solid propellant rock

Resume : With the steady increase in the availability of supercomputing power and capabilities of computational software the computer-aided modeling techniques become popular research tools in many research fields. They are also frequently used in the research on nuclear materials, including the ones relevant to nuclear waste management [1]. In our research we aim at joint computational and experimental investigation of nuclear materials which allows for a superior characterization of properties of nuclear waste. In this presentation we will show the results of joint investigation of selected ceramic waste forms (monazite, xenotime, pyrochlore) and potential secondary phases that may form in the spent nuclear fuel under disposal conditions (studtite/metastudtite & coffinite). We will discuss the structural incorporation of actinides into these materials and the derivation of related thermodynamic [2] and thermochemical parameters [3]. We will also show the results on the structural and disordering effects [4], on the stability of selected solid solutions and on the derivation of phase diagrams of actinide-bearing compounds [5]. The discussed materials properties are important for assessment of long-term durability and stability of disposed nuclear waste. [1] Chroneos et al., J. Nucl. Mater. 441, 29 (2013). [2] Kowalski et al., J. Nucl. Mater. 464, 147 (2015). [3] Kowalski et al., J. Eur. Ceram. Soc. 36, 2093 (2016). [4] Finkeldei et al., Acta Materialia 25, 166 (2017). [5] Xiao et al., Chem. Eur. J. 22, 946 (2016).

Resume : The YAlO3 single crystals and ceramics have a long interest for scintillator detectors, laser and optical recording media as well as a substrate for the high-temperature superconductor thin films growth. Among them Mn-doped YAlO3 is a promising material for thermoluminescent dosimetry of ionizing radiation. In this presentation we report the results of the first principles calculations obtained within the formalism of hybrid Density Functional Theory and Hartree-Fock method (using the HSE0 Hamiltonian as implemented in total energy LCAO-CO CRYSTAL14 computer code [1]) properly adapted and carefully verified for atomic, electronic and phonon structure of pristine YAlO3 bulk. The YAlO3 crystal is described by the Pbnm space symmetry group. Equilibrium lattice constants obtained in our study a = 5.189 Å, b = 5.317 Å, c = 7.388 Å are in good agreement with those experimentally observed: a = 5.167 Å, b = 5.315 Å, and c = 7.354 Å. Band gap calculated for the YAlO3 bulk is equal 7.26 eV vs. 7.1 eV known from the experiment. Doping of YAlO3 with Mn ions results in the formation of mid band gap states that alter its electronic properties. In this study we discuss electronic structure of YAlO3 having host Al substituted with both divalent and trivalent Mn ions. In the later case neutral F-center has been inserted to keep the unit cell neutral. Careful comparison with available experimental data is provided. [1] http://www.crystal.unito.it/

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