Basic research on ionic-covalent materials for nuclear applications

Resume : Reliable simulations of actinide materials are important as a complementary alternative to experimental investigations, because experiments with such usually hazardous materials are difficult and costly. During the recent decades density functional theory (DFT) has proven to be such a method for condensed phase actinide systems. The accurate description of the strong correlation of the 5f electrons, however, still poses a challenge. Standard DFT functionals are usually augmented with a Hubbard-type U term (DFT+U) or with a contribution of exact exchange nowadays to improve the description of the strongly correlated 5f electrons. Unfortunately, this creates also a manifold of possible localization patterns for the 5f electrons at each uranium site which complicates the detection of the correct ground state due to the occurrence of metastable states. Fully unconstrained cell optimizations of uranium dioxide bulk model systems using the Gaussian plane waves method are presented. Different U values are employed for all possible uranium 5f orbital occupation patterns in the framework of a spin polarized DFT+U approach. The detected low-lying states are presented and their properties are analysed. A new lowest energy state has been detected different from the proposed ones so far which shows also favourable properties. The presented results are relevant for future investigations using DFT+U or hybrid functional methods for the study of actinide systems, especially defective ones.

Resume : Models of thermodynamic properties of nuclear fuel are necessary for analysis and numerical simulations of processes in nuclear reactors under normal and abnormal mode conditions. In this work, a semiempirical equation-of-state model for uranium dioxide is presented with taking into account polymorphic transformation and melting effects. A new form of thermodynamic potential Helmholtz free energy as a function of volume and temperature is proposed. This function provides for a more correct thermal contribution of heavy particles (atoms, ions, nuclei) in the solid and liquid phases as well as the phase changes. Obtained results are critically analyzed in comparison with available experimental data at high temperatures and pressures.

Resume : The behavior of Pu and U oxides is made complex by the broad variety of point defects they exhibit, both on the anion and cation sub-lattices. In these non-stoichiometric actinide oxides, changes in temperature and deviation from stoichiometry can lead to modifications of the charge state of defects or even clustering, both of which are expected to substantially modify defect mobility, hence atomic transport properties. This has very practical implications upon how materials are manufactured, handled and stored. In this talk we discuss properties relating to both anion and cation sub-lattices. Several examples are given of changes in bulk material properties which may be interpreted by assuming modifications in the overall charge of the dominant defect species. With look at oxygen self- and chemical diffusion and discuss methods for measuring these properties. From the application of the Darken equation, we show that under certain specific conditions, it is possible to derive single oxygen defect diffusion coefficients from chemical diffusion measurements. Cation vacancy characteristics and in particular the change in their charge state as deviation from stoichiometry increases are also discussed based on known uranium self-diffusion coefficient measurements or trace diffusion coefficients of certain important fission products.

Resume : Neutron scattering was used to investigate the structure of stoichiometric uranium dioxide. A statistical physics approach was used to model these experimental results and to relate them to the presence of polarons in the fluorite lattice.

Resume : It is well known fact that in UO2 pellets irradiated in reactor, Xe nano-bubbles nucleate, grow, coarsen and finally reach a steady state distribution (TEM observation typically report 1023-24 bubbles per m3). This phenomenon is usually explained by radiation enhanced diffusion and precipitation of gas atoms. Nevertheless [Sabathier NIMB 326(2014)247], 4 MeV Au ion irradiation of UO2 thin foils at room temperature yields a nano-void population whose size distribution reaches a similar steady state, although no foreign atoms are implanted nor significant cation vacancy diffusion expected. Molecular dynamics (MD) simulations performed at low temperature support the assumption of heterogeneous nucleation: 10 keV sub-cascades produce defect aggregates and in particular voids that grow up to a nanometer scale through sub-cascade overlapping. In this work a semi-empirical model is proposed to extend these results to the simulation of the size distribution evolution of a representative void population in a fraction of a material grain under a cascade overlap regime. This model accounts for the main features of MD simulations and rely on simple rules for damage accumulation when cascade overlaps occur. It satisfactorily reproduces the TEM observations of nano-voids size and concentration, which paves the way for the introduction of a more realistic damage term in rate theory models.

Resume : We review our recent results dealing with bulk and (001) surface F center calculations in SrZrO3 as well as SrTiO3/BaTiO3 and SrZrO3/PbZrO3 (001) interfaces [1,2]. Combining B3PW hybrid exchange-correlation functional within the density functional theory (DFT) and a supercell model, we calculated from the first principles the electronic structure of both F center in the bulk and F center located on the ZrO2-terminated srZrO3 (001) surface. Thermodynamical analysis of pure surfaces indicates that ZrO2-termination is energetically more favorable than SrO-termination. The F center on the ZrO2-terminated SrZrO3 (001) surface attracts ~0.8e (~1.2e in the SrZrO3 bulk). The calculated defect formation energy on ZrO2-terminated (001) surface is smaller than in the bulk which should lead to the vacancy segregation to the surface. We present a first-principles results of our study of SrTiO3/BaTiO3 and SrZrO3/PbZrO3 (001) interfaces. The optical band gap of BaTiO3/SrTiO3 (001) interface [2] depends mostly from BaO or TiO2 termination of the upper layer. The optical band gap for BaO-terminated BaTiO3/SrTiO3 (001) interfaces varies from 3.47 eV (single BaTiO3 (001) surface monolayer) till 3.22 eV (9 monolayers). References: 1. S. Piskunov and R. I. Eglitis, Phys. Rev. B, 2015, will be submitted. 2. S. Piskunov and R. I. Eglitis, Solid State Ionics, 2015, submitted.

Resume : Magnesium oxide (MgO) is considered as a candidate material for neutron reflector in sodium fast reactors, matrix of cercer nuclear fuel for minor actinides transmutation or electrical insulator for diagnostics components in ITER. Therefore, a deep understanding of the behaviour of this material under ion irradiation environment is required. Very recently [1], we demonstrated, through the use of RBS/C, XRD and TEM experiments that the damage build-up in MgO irradiated with 1.2 MeV Au+ ions at RT occurs in three steps, and transitions from one step to the next one reveal microstructural transformations. Each step is characterized by specific features such as damage and strain levels and nature of defects. The present study aims at investigating the effect of the irradiation temperature on the response of MgO. For this purpose, additional 1.2 MeV Au irradiations have been performed in a broad fluence range at 300°C, 500°C and 800°C. Results show that disorder accumulation is still a multi-step process at high temperature, but occurrence of the microstructural transformations is modified; in addition, disorder and strain levels also change with temperature. These findings reveal a competition between defect creation and recovery processes. A comparison with cubic zirconia suggests, at first sight, that MgO is less radiation resistant. However, electronic excitation effects can lead to damage healing in MgO. [1] S.Moll, Y. Zhang, A. Debelle et al, Acta Mater, in press

Resume : It is well known that beryllium hydride and beryllium deuteride are important components for thermonuclear fusion. But it is turned out that those compounds are very useful initial materials for some problems solution of X-ray optics and spectrometry. Thermal decomposition of those compounds allows to get the nanoporous metallic Be with small pollutions concentrations (DL for Fe smaller 100 ppm). This material is called for X-ray flux filtering in X-ray microscopy owing to its imitation of a turbid medium for the radiation. Be nanoporous compaction leads to the polycrystalline material formation with submicrone structure. This material is the ideal substance for preparation of the vacuum tight foil with thickness 5-8 mkm. It can be used with success for fabrication of X-ray refractive optics elements. Similar elements can be prepared with using of BeH2 and BeD2 with beautiful results, too. Work presents experimental data about properties of BeH2, BeD2 and metallic Be produced from hydride and deuteride of beryllium in result of its pyrolysis. There is described X-ray optics details fabricated from those materials.

Resume : In nuclear materials studies, ion beams are used to reproduce, in a controlled way, the different sources of irradiation to which these materials are submitted to in actual conditions. The collision of the incident ions with the nuclei of the target atoms leads to displacements of these atoms and, ultimately, to the creation of crystalline defects all along the path of the incident ions. X-ray scattering techniques are extremely sensitive to atomic displacements and are in principle well suited to study radiation damage. However, the very complex microstructure of these materials, containing inhomogeneously distributed defects, of different nature and different sizes, substantially complicate the analysis and often limit the interpretations to a phenomenological level [1]. In this work we show that the analysis of the full X-ray scattering signal (coherent and diffuse) opens the path towards a complete and non-destructive characterization of radiation damage in crystals. In particular we show that well established phenomenological quantities such as 'lattice swelling' and 'lattice damage' can be rationalized in terms of defect properties such as the defect size, concentration and spatial distribution [2]. The potential of the approach will be illustrated with the example of irradiated SiC and ZrO2 single crystals. [1] A. Boulle, A. Debelle, J. Appl. Cryst. 43, 1046 (2010) [2] J. Channagiri, A. Boulle, A. Debelle, to be published in J. Appl. Cryst. (2015)

Resume : In this paper we suggest an approach for the theoretical determination of the threshold displacement energy (Ed). The components of the proposed expression are defined on the basis of analysis of processes accompanying the subthreshold movement of the atom ejected from the lattice site. The expression for the Ed consists of the atomic binding energy, work of electrostatic and elastic forces of interaction between a vacancy and moving interstitial atom, as well as its dissipative energy loss. The theoretical calculations have been made for a number of metals (Al, Ti, V, Cr, Fe, Co, Ni, Cu, Zr, Mo, Cd, Ta, W, Au, Bi) and semiconductors (Si, C, Ge). The obtained results were compared with the experimental data. It has been found that the partial contribution of the binding energy to the Ed value does not exceed 30 %, and the dissipative loss is about 10 %. The main part of the Ed is determined by the work of electrostatic (in the case of semiconductors and insulators) and elastic (for metals) forces of interaction between vacancies and interstitial atoms. It was shown that the threshold displacement energy is related to the physical properties of solids (Young's modulus, sublimation energy, Debye temperature, binding energy, thermal expansion coefficient, lattice constant).

Resume : In the past decades, there has been considerable amount of study for manufacturing of nanoporous layers due to their potential application in micro- and nano-electronics. Here, we have formed nanoporous SiO2 layers on silicon wafers using swift heavy ion irradiation followed by chemical etching of latent track zones in dielectric matrix. Thermal spike model was used in order to predict the possibility of etched tracks formation in SiO2. We have calculated radius of the molten region formed along the swift ion trajectory and used this parameter as a criterion for track "etchability". SiO2/Si templates were irradiated with Xe (133, 200 MeV), Kr (54 MeV) and Ar (38 MeV) ions using an ion cyclotron accelerator DC-60 (Astana, Kazakhstan). Pore density and size in SiO2 as well as dependence of these parameters on treatment duration in hydrofluoric acid solution were investigated. It was found that not every "light" Ar ion could create latent track since the estimated radius of the molten region was less than established threshold value (3 nm). In contrary, "heavy" ions of Kr and Xe were able to create regularly shaped nanochannels.

Resume : Reduced activation ferritic-martensitic (RAFM) steels strengthened by Y2O3 precipitates are promising structural materials for advanced fission and future fusion reactors. Tough oxide particles in oxide dispersion strengthened (ODS) steels efficiently hinder dislocation motion thus increasing the strength and improving the high-temperature creep resistance of ODS steels, as compared to conventional RAFM steels. The degree of the radiation resistance improvement of ferritic-martensitic steels is highly sensitive to the parameters of ODS particle ensembles, including the size, shape, and spatial distribution of ODS nanoparticles. Therefore, a deep understanding of the process of the ODS particle formation at the atomistic level that allows to create nanoparticle ensembles with highly optimized parameters for the best in-reactor steel performance is essential for the successful design and production of ODS steels [1]. The report summarizes the results of first-principles calculations for yttrium, oxygen and vacancy clusters embedded into bcc-Fe lattice. An ample database of the energetics of multiple vacancies, Y and O solute atoms in various combinations and spatial arrangement has been collected. Both stable cluster configrations and transition states were studied, suggesting the most probable diffusion pathways. The accumulated information provides reliable basis for the analysis of the early stages of oxide particle precipitations, in particular in the framework of kinetic Monte Carlo simulations. [1] A. Gopejenko, Yu.F. Zhukovskii, P.V

Resume : We have studied the formation of defects in GaN irradiated by molecular and atomistic ions. We found experimentally that the measured photoluminescence (PL) decay time decreased faster during the molecular ion irradiation compared to light ion irradiation. We performed atomistic simulation of different ions (P, F, Pf2 and PF4), in all cases the energy of projectile was 0.6 keV/amu. To investigate the mass effect, we have also included in our simulations the case of Ag ions, whose mass is close to the mass of the PF4 molecule. The PL decay time change is connected with the types of defect produced by different projectiles. Simulation results show that the light ions mainly produce isolated point defects while molecular and heavy ions produce clusters of point defects. This and the similar depth profiles of damage produced by molecular and light ion irradiations suggest that the defect clusters are one of the important reasons for fast PL decay. We also studied the location and type of defects produced in GaN nanowires in the Er ion irradiation. We saw that Er implantation leads to a net positive (expansion) strain in the nanowire and especially at the top region a clear expansion has been observed in the lateral and axial directions. The lattice expansion is due to the hydrostatic strain imposed by a large number of radiation induced defects at the top of the NW. Due to the large surface-to-volume ratio, most of the defects were concentrated at the surface region, which suggests that the experimentally observed yellow luminescence (YL) in ion implanted GaN NWs arises from surface defects. We observed big clusters of point defects and vacancy clusters which are correlated with stable lattice strain and the yellow luminescence band.

Resume : The caracterization of materials under irradiation, or after irradiation, has traditionally been performed with a variety of techniques. One can probe macroscopic properties, like mechanical response, thermal or electrical conductivities, optical or magnetic responses, or one can try to observe directly features at the nanoscale, like, e.g., dislocation loops, with local probes like STM or hrTEM. In the former case the observed signals depend on the type of defects induced by irradiation, their concentration, their spatial distribution and their time evolution. Raman spectroscopy has been increasingly used in the last few years for irradiated materials, including silicon carbide [1]; however, the interpretation of spectra is not always an easy task. We have considered silicon carbide not only because of recent experimental works, but also because it is a material for which a lot of knowledge on point defects has been gathered in the last two decades; we have calculated the Raman intensities of defect containing supercells in the framework of linear response theory [2] with the goal of providing tools for the interpretations of experimental spectra. We present a few examples and discuss the limitations and opportunities of our approach, also in connection with the prediction of the time evolution of defect concentrations under irradiation. [1] S. Miro et al., J. Raman Spectroscopy 45, 481 (2014). [2] M. Lazzeri and F.Mauri, Phys. Rev. Lett. 90, 036401 (2003).

Resume : The majority of studies about the effect of radiation on the structure and properties of materials have been performed on single crystals or sintered pellets having large grain sizes. Thin films used as protective coatings for nuclear fuel encapsulation are polycrystalline or even nanocrystalline. The effect of radiation on such films has only recently been studied, due to the lack of good samples. The Pulsed Laser Deposition (PLD) technique could grow high quality thin films from inexpensive targets. By changing the deposition parameters, films possessing different chemical compositions and/or structures could be synthesized. PLD grown ZrC, ZrN, and SiC thin films were ion-irradiated at room temperature by 800 keV Ar ions. X-ray reflectivity and diffuse scattering investigations were used to estimate changes induced after irradiation on the mass density and surface and interface roughness, while glancing incidence X-ray diffraction investigations were used to assess changes induced on the structure of the films. X-ray photoelectron spectroscopy investigations using hard X-ray allowed for sampling a deeper region than that usually investigated by using the Al or Mg Kalpha radiation. Also, changes in the chemical bonding were investigated by Raman scattering investigations. Nanoindentation and optical reflectance measurements were employed to assess the modification of the mechanical and optical properties of the films after irradiation.

Resume : Influence of impurities on the efficiency of radiation defects (RD) creation in the bulk and at the surface of wide-gap metal oxides (WGO, Eg> 5 eV) is studied using optical and thermoactivation spectroscopy methods. Besides knock-out mechanism, the recombination of hot electron-hole pairs or the decay of long-lived core electronic excitations contributes to radiation damage of WGO with the formation energy of a Frenkel pair Ed > Eg. The decrease of the efficiency of RD creation is detected in lightly doped WGO (BaMgAl10O17:Eu, Lu3Al5O12:Ce) via "luminescent protection" - competition between a luminescent energy dissipation channel and RD creation. However, in WGO with Ed>3Eg, a high impurity concentration and super-dense excitation (e.g., within cylindrical tracks of ~GeV Bi or Au ions), the situation is more complicated. In parallel with F-type defects the creation of antisite defects (an ion in a "wrong" cation position) is efficient in complex WGO (LuAG, Gd2SiO5, Lu2SiO5). Radiation damage in WGO predisposed to anharmonic interactions is also caused by a collapse of intrinsic discrete breathers creating irreversible 3D defects - rearrangement of many host ions - that facilitate a cracking and brittle destruction. The contribution of complex impurity centres (Ce-Ce pairs in LuAG, Cr-Cr in MgO) to such processes is analysed. Particular attention is also given to a role of calcium (effective complexing agent)- or hydrogen (proton)-containing impurities to the creation of RD.

Resume : Structural and chemical modifications in complex oxides (e.g., Dy2Ti2O7, Dy2TiO5, Dy2Sn2O7) induced by high energy ion irradiation were characterized using pair distribution function analysis (PDF) from total neutron scattering experiments at the Nanoscale Ordered Materials Diffractometer (NOMAD) beamline at the Spallation Neutron Source at Oak Ridge National Laboratory. Key to the experimental procedure is the use of GeV ions to produce sufficient sample material irradiated with a near uniform electronic energy loss. Such experiments required a new strategy to facilitate the preparation of ~150 mg homogeneously irradiated powder sample required to conduct neutron characterization experiments. Samples were irradiated at the GSI Helmholtz Center in Darmstadt, Germany at room temperature with swift heavy ions (e.g., Au ions, 2.2 GeV) to fluences up to 8×1012 ions/cm2. Ion-beam induced structural modifications including defect formation, amorphization and crystalline-to-crystalline phase transformations were studied for the first time in terms of both the average and local sample structure. Unlike X-rays, neutrons scatter strongly from low-Z elements, permitting a detailed analysis on both the cation and oxygen-anion sublattices. This information is crucial to better understand sublattice-specific disordering which swift heavy ions induce in many complex oxides. PDF analysis provides data on the local defect structure, including changes in site occupation, coordination, bond distance and bond angle. These results are compared to complimentary synchrotron powder XRD performed at the Advanced Photon Source at Argonne National Laboratory.

Resume : The interaction of ions with solids results in energy loss to atomic nuclei and electrons, and we have integrated computational and experimental approaches to investigate the separate and combined effects of nuclear and electronic energy loss on the response of ceramics to ion irradiation. The loss of energy to electrons results in significant ionization and formation of hot electrons that create a local thermal spike via electron-phonon coupling, and we have discovered that a significant synergy occurs between electronic energy loss by ions and pre-existing atomic defects created by elastic energy loss in single-crystal complex oxides. This synergy results in the formation of nanometer-sized amorphous tracks, but only in the region with pre-existing defects. These defects decrease the electronic and atomic thermal conductivities and increase the electron-lattice coupling, which highly localizes the thermal spike from each ion, resulting in the formation of an amorphous track. Molecular dynamics simulations confirm that the introduction of small concentrations of defects sensitize these materials to amorphous track formation at relative low values of electronic energy loss. This work identifies a major gap in fundamental understanding on the role of defects in electronic energy dissipation and electron-lattice coupling. This work was support by U.S. DOE, BES, MSED.

Resume : Pyrochlores (A2B2O7) are widely investigated as potential nuclear waste disposal forms because of high resistance to the radiation damage shown by some of these materials. The radiation resistance is associated with the radiation-induced disordering of the cation and anion sublattices and subsequent formation of defective fluorite, but the mechanisms governing this process are poorly understood. We performed the ab initio calculations of the formation energies of the two main point defects occurring in pyrochlore, the cation antisite and the anion Frenkel pair. The resulting contour maps of the defect formation energies are substantially different from those obtained previously with the force-field atomistic calculations. The new defect formation energy diagrams correlate well with the stability diagram of pyrochlore. We found that in pyrochlores that form the defective fluorite the anion Frenkel pair formation energy becomes negative. This compensates for the energy required to disorder the cation lattice through the formation of cation antisite defects. Thus our ab initio studies show that the low energy cost of defect accumulation is the driving force behind the selective order-dissorder transition in pyrochlores.

Resume : Experimental data on the size and distribution of strain around SHI tracks is sought as input to and verification of mathematical models of the ion-solid interaction. In this paper we present new results on estimation of these parameters through TEM imaging and subsequent image processing. Through focal image series were obtained from which the complex electron wave at the specimen exit surface was computationally restored. This method mitigates some of the difficulties in previous attempts where unprocessed HRTEM micrographs were used as input. Particular attention will be given to the structure of the strain field. Single crystal c-oriented Al2O3 crystals were irradiated with swift heavy Xe and Bi ions. Plan view TEM specimens were prepared and imaged using a Cs corrected JEOL ARM200F TEM. Geometrical phase analysis was carried out on the phase image of the reconstructed complex electron wave at the specimen exit surface.

Resume : Radiation stability of dielectric nanoparticles embedded into metallic matrix is of considerable practical value due to growing interest to oxide dispersion strengthened (ODS) steels as promising nuclear reactor materials. In this report we present a TEM and the electron diffraction analysis data on structural changes in Cr23C6 and Y-Ti-O nanoparticles in Cr16 ODS alloys irradiated with 1.2 MeV/amu Kr and Xe and 3.4 MeV/amu Bi ions. It was found that swift heavy ion irradiation leads to formation of amorphous latent tracks in both materials. The electronic stopping power threshold for track formation in pyrochlore nanoparticles lies in the range 7.4– 9.7 keV/nm and the mean track diameter varies linearly with the electronic energy loss. Multiple ion track overlapping leads to complete amorphization of carbide and Y-Ti oxide nanoparticles.

Resume : Al2O3 (corundum) is a promising material for fusion reactors, e.g. for components such as diagnostic systems, Heating and Current Drive devices and even possibly in the breeder blanket zone. Thus, this is very important to predict/simulate the kinetics of defect accumulation under neutron irradiation as well as long-time defect structure evolution. There are numerous experimental measurements of the primary defect kinetics (first of all, F color centers—oxygen vacancy Vo with two trapped electrons) as a function of dose rate and temperature and references therein). It is known that at high irradiation doses and/or high temperatures the primary defects aggregate, giving rise to metallic colloids. Another way to produce colloids is additive coloration of corundum crystals by heating at 2000 C under strongly reducing conditions. A study of metallic colloids in corundum is also important since they are related to material degradation. We developed and applied here new approach based on the formalism of the correlation functions describing spatial distribution of similar (F-F centers) and dissimilar (Frenkel pair of defects: F center – interstitial O ions) which is much better suited for the study of defect kinetics and aggregation. Based on our calculations, we estimate the migration energy of the F centers, interstitial O defects and their interaction energy and estimate colloid size. For a comparison we performed also colloid formation in well studied NaCl crystals.

Resume : Recent studies have shown that swift heavy ion irradiation may significantly modulate hydrogen and helium behavior in certain materials. This phenomenon is of considerable practical interest for various ceramics and semiconductors. ZrN is a candidate-inert-matrix-fuel-host (IM) and is prone to blister formation following He irradiation and thermal annealing. However blister formation is suppressed in nanocrystalline ZrN by subsequent swift heavy ion (SHI) irradiation. Due to the complex microstructure of nc-ZrN the exact mechanism involved is difficult to deduce by transmission electron microscopy (TEM). Blister/Bubble suppression has however also been observed in He and H irradiated Si. The microstructural damage resulting from low energy He radiation is very obvious in Si when observed in the TEM even before annealing. After irradiation with SHIs however the peak damage region in Si broadens which suggest that the implanted He is dispersed by these ions. Blister/Bubble formation is therefore most likely impeded by the dispersion of He by SHIs in both ZrN and Si. In the current investigation the microstructural response of both Si and nc-ZrN to He and subsequent SHI irradiation is compared.