Materials for energy and environmentG
Basic research on ionic-covalent materials for nuclear applications
Ionic-covalent (non-metallic) solids play important roles in numerous industrial applications, due to their unique thermal, electrical, magnetic and optical properties. Refractory materials find application at very high temperatures, where metallic materials are not physically stable. Among the extreme environments where such materials are used is the nuclear industry where there is an impressive diversity of applications for ionic-covalent solids, ranging from nuclear fuel (UO2) to inert matrix materials for actinide transmutation, to host materials for nuclear waste disposal. For future nuclear applications, especially high-temperature Gen IV fission reactors as well as fusion reactors, research is needed to develop structural materials and nuclear fuels that can withstand these extreme environments associated with these reactors. Given the different temperature and irradiation conditions in these various applications, it is imperative to better understand basic degradation processes and perform material tests employing a science-based approach. Improved knowledge on underlying principles of materials behaviour in extreme environments will help us to develop suitable materials for nuclear waste forms as well as for the next generation of nuclear power stations.
The previous symposia on “Basic Research on Ionic-Covalent Materials for Nuclear Applications” at the E-MRS in 2011 and 2013 successfully inspired a broad community of researchers about the need for addressing the basic mechanisms involved in irradiation damage induced by energetic particles, such as point and extended defect formation, precipitation/dissolution, dislocation formation/dissolution etc., in different kinds of ionic-covalent solids, regardless of the applications (nuclear industry, microelectronics, etc.). Our program is clearly cross-disciplinary on a wide range of materials (from insulators to semiconductors and even some metal/insulator composite systems), and a broad range of irradiation conditions with charged particles (from keV to GeV), experimental techniques and theoretical computations. Evidence of the similarity of phenomena occurring in semiconductors and transition-metal or actinide oxides was highlighted in a number of invited and contributed presentations in previous symposia. However, many important questions were left open showing that this field still needs research development.
The utmost importance of electronic excitation effects in irradiation damage was also recognized. In this respect, the knowledge of phenomena occurring under irradiation with laser beams is instrumental for advancing the understanding of electronic excitation effects induced by charged particles. On the experimental standpoint, the use of laser-based optical spectroscopies (possibly coupled to ion beams) could be used to probe short time scales and obtain time-resolved data with pulsed beams (or particle bursts), which would be useful for confirming calculations with relevant atomic-scale theoretical models.
In all these atomic-scale processes, charged point defects are known to be produced in ionic-covalent solids under these conditions of electronic excitations. The importance of charged point defects was once again highlighted in the last symposium in 2013 for understanding the behavior of these materials under radiation fields. Moreover, there is a need for improved understanding of the electronic excitation regime and the intermediate electronic/nuclear collision regime of radiation damage production.
It is necessary to obtain experimental data and develop models of the stability and migration of excited defects on the basis of time-resolved experiments and related numerical simulations. Such similar phenomena are known to take place in particle or photon detectors (scintillators) used for medical applications, high-energy physics, and nuclear security, as well as in irradiated electronic systems used for space applications that have to be hardened to perform in high radiation environment of space.
It has also been recognized that (non-standard) specific tailored samples could also be instrumental in devising new experiments, as is commonly done in the semiconductor research community. Engineered multi-layered samples (super-lattices) could be prepared by selected deposition techniques (sputtering, MBE, CVD, etc.) and used for test irradiations and round-robin measurements. In this case, the use of specific built-in atomic probes (isotopic markers) could be contemplated, for instance for atomic diffusion or point-defect studies, either with on-line or off-line measurements.
We propose a new symposium at the E-MRS 2015 following these prospects to develop researches and new collaborations in these fields. Some key topics were highlighted as to be addressed in the next symposium such as: i) amorphous and glassy material production, characterization and properties, ii) dislocation and crack propagation in ceramics, iii) mechanical property evaluations by nano-indentation and related modeling, iv) elastic/plastic behavior of nano-objects, v) complex microstructure evolutions of inhomogeneous materials (dissolution of precipitates, effects of surfaces/interfaces, layered systems), and vi) carbonaceous materials.
The balance between basic research and applications, and between experimental results and theoretical modeling, was also considered as an important feature of this symposium that should be maintained.
Hot topics to be covered by the symposium :
The symposium will address solid-state processes induced by irradiating particles (electrons, neutrons, or ions) over wide ranges of energy, fluence, flux, temperature, pressure, etc., including:
- Interaction processes of energetic particles in solids
- Amorphization and phase transformations
- Role of electronic excitations: track effects, enhanced diffusion…
- Formation of point defects, extended defects, and defect clustering
- Mesoscale architectures for improved radiation resistance
- Dynamic defect recovery, flux effects and radiation-enhanced diffusion
- Recrystallization and nanophase formation
- Dissolution of clusters and precipitates, segregation
- Modifications of physical properties (thermal conductivity, mechanical properties etc.)
- Degradation mechanisms (creep, embrittlement, swelling, corrosion, etc.)
- Modeling and simulation of radiation effects
List of invited speakers:
- Alain Claverie (CEMES, Toulouse, France), On the origin of dislocation loops in irradiated materials: a point of view from silicon
- Steven Conradson (Synchrotron SOLEIL, France), Collective and Dynamical Behavior in UO2
- Takeshi Egami (Univ. Tennessee and Oak Ridge NL, USA), How to characterize disorder?
- François Jollet (CEA/DAM/DIF, Bruyères-le-Châtel, France), Electronic structure of uranium dioxide
- Jan Meijer (University of Leipzig, Germany), Defect engineering for determined production of NV-centers in diamond
- Alex Shluger (University College, London, UK), First principle calculations of defects in oxides
- Izabela Szlufarska (Univ. Wisconsin, USA), High-resolution microscopy and simulation studies of defect clusters in irradiated SiC
List of scientific committee members:
- A. Claverie (CEMES, Toulouse, France)
- F. Djurabekova (Helsinki University, Finland)
- R. Ewing (Stanford University, USA)
- P. Garcia (CEA/Cadarache, France)
- A. Iwase (Osaka Prefecture University, Japan)
- K. Yasuda (Kyushu University, Japan)
- S. Zinkle (University of Tennessee, USA)
The proceedings will be published in a special issue of Nuclear Instruments and Methods Section B (Elsevier).
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Point defects in insulators and semiconductors : -
Authors : Al-Moatasem El-Sayed, Alexander Shluger
Affiliations : Department of Physics and Astronomy, University College London, Gower Street, London WC1E 6BT, UK
Resume : We will discuss the role of exciton and electron trapping in the mechanisms of Frenkel defect creation in oxides. Density functional theory simulations are used to predict the structures of self-trapped triplet excitons in HfO2, HfSiO4, and in α-quartz. The character of the hole and electron localization in these excitons strongly depends on the geometrical structure and dielectric properties of the particular material. In HfO2, the electron is localized predominantly on one Hf atom while the hole is localized on one or two oxygen atoms at the nearest or next-nearest-neighbour sites, depending on the crystal phase. We predict two exciton configurations in HfSiO4 with the excited electron localized either on a Hf or on a Si atom and the hole localized on the nearest-neighbour oxygen atoms. However, Frenkel defects are formed as a result of exciton decomposition only in silica. We demonstrate that electron injection can facilitate the formation of Frenkel pairs. Electrons can be trapped in pure crystalline and amorphous SiO2 in deep band gap states facilitating breaking of Si-O bonds. Trapping of extra electrons in polaron states in a perfect HfO2 lattice and near pre-existing O vacancies facilitates the creation of stable pairs of O vacancies and interstitial O ions. Thus population of anti-bonding states by extra electrons may provide an efficient channel for stable defect creation in oxides.
Authors : Yu F Zhukovskii, A Platonenko, S Piskunov, E A Kotomin
Affiliations : Institute of Solid State Physics, Universiy of Latvia, Riga, Latvia
Resume : Corundum crystal, highly radiation-resistant wide band gap (8.8 eV) insulator, is promising for components in breeder blanket and diagnostics. It could be used also in future fusion reactors as coating material to avoid light gas permeation as tritium (into the pipes steel) and to avoid corrosion produced by lithium-based alloys. Thus, it is important to understand radiation damage in this material. It is known that radiation-induced changes in the structural and optical properties of corundum are mainly associated with single interstitial oxygen atoms and oxygen vacancies as well as complementary Frenkel pairs of these defects. We have performed large-scale ab initio calculations on both above-mentioned single point defects and Frenkel pairs in corundum, using periodic conventional supercells 2×2×1 and 3×3×1 (containing 120 and 270 atoms, respectively). The defect formation energies, Mulliken atomic charges and the electron density distributions for both supercells have been compared, in order to estimate defect-defect interactions. It has been found that oxygen interstitial atom, both single and a component of Frenkel pair, spontaneously forms a dumbbell configuration with the adjacent oxygen atom in the regular sublattice of corundum with the interatomic distance 1.404 Å and the dumbbell charge -1.1 e. The latter is close to that for an O ion in pure corundum. Formation energies for neutral single point will be compared with the formation energy for a close Frenkel pair (11.8 eV). The energy barriers for possible paths of oxygen migration have been estimated.
Authors : V. M. Lisitsyn (1), L. A. Lisitsyna (2), E.A. Kotomin (3,4), A.I. Popov (4), J. Maier (3)
Affiliations : (1) National Research Tomsk Polytechnic University, pr. Lenina 30, Tomsk, 634050 Russia; (2) State University of Architecture and Building, pl. Solyanaya 2, Tomsk, 634003 Russia; (3) Max Planck Inst for Solid State Research, Heisenbergstr. 1, 70569 Stuttgart, Germany; (4) Institute of Solid State Phys, University of Latvia, Kengaraga 8, LV-1063, Riga, Latvia
Resume : In contrast to the alkali halide crystals (AHC), the processes of the electronic excitation nonradiative decay with formation of Frenkel pairs (vacancy-interstitial) in rutile-structure MgF2 crystals are not well understood and need more detailed analysis. Typical time scale of the relaxation of the electronic excitation into a transient defect pair is about 1?50 ps. In AHC, under different conditions of irradiation only a small fraction (<10% ) of the transient defect pairs survive, in MgF2 not more than 0.1% whereas in fluorides MeF2 (Me: Ca, Sr, Ba) much less, <0.001%. Therefore, the key factor determining accumulation of stable radiation defects is stabilization of mobile primary defects. In this talk, on the basis of the time-resolved and stationary optical absorption and luminescence studies and also theoretical calculations, we give the comparative overview of the basic processes of the formation and relaxation of the electronic excitations in MgF2 crystals. Special attention will be paid to the thermally activated motion and stabilization of a hole component (H-center) of the created defect pairs. The specific role of the cation (Mg) sub-lattice in the above-mentioned process will be highlighted. Thermal conversion of the primary hole centers into complex defects of the V-type will be also discussed and compared with those observed in AHC and alkaline fluorides. Finally, the general comparative picture of the radiation processes in MgF2 and other ionic solids will be overviewed.
Authors : F. Moisy, M. Sall, C. Grygiel, E. Balanzat, M. Toulemonde, S. Jublot-Leclerc, H. Lebius, B. Ban dEtat, I. Monnet
Affiliations : CIMAP-GANIL Boulevard Henri Becquerel BP 5133 14070 Caen cedex 5, France
Resume : Nitride semiconductors are attractive materials because of their excellent properties for applications in high power and high frequency optoelectronic devices. In order to integrate them in harsh environment (outer space or nuclear plants), it is necessary to characterize their behavior under irradiation. Thus, wurtzite III-N epilayers (InN, AlN, GaN and (Al,Ga)N ternary compounds) grown on c-plane sapphire substrates, have been irradiated with Swift Heavy Ions (SHI) and fullerenes and thereafter studied by Transmission Electron Microscopy (TEM) and UV-visible spectroscopy. TEM characterizations exhibit latent tracks in InN and GaN under SHI irradiation whereas AlN shows a high resistance to ion track formation. Indeed, latent tracks in AlN are observed only under fullerene irradiations. Tracks appear more continuous as the electronic stopping power (Se) of the incident ions increases. In-situ optical absorption spectra at 15K show new energy levels in the gap of the nitride layers due to the creation of point defects. It was demonstrated that these point defects inducing absorption band at 4.7eV in AlN come from N-vacancies and a synergy between electronic excitations and elastic collisions can explain their formation. Concerning irradiated GaN, absorption spectra exhibit an absorption band at 2.8 eV which seems to be linked to Ga-sublattice related defects. In this communication, the influence of Ga molar ratio in (Al,Ga)N on the behavior under irradiation is discussed.
Authors : A. Yu. Azarov, P. Rauwel, A. Yu. Kuznetsov, E. V. Monakhov and B. G. Svensson
Affiliations : Department of Physics, Centre for Materials Science and Nanotechnology, University of Oslo, P.O. Box 1048 Blindern, N-0316 Oslo, Norway
Resume : It has been shown previously that point defect evolution in ion implanted ZnO can be studied by monitoring the behavior of residual group-Ia impurities, such as Li . Here, we study effects of post-implant annealing on the defect evolution and how the defects interact with Li in hydrothermally grown ZnO single crystals implanted with B or Ag as well as co-implanted with B+Ag ions. Structural defects and Li redistribution have been analyzed by a combination of with RBS/C, TEM and SIMS. Dopant-defect reactions dominate the defect formation for these ion species resulting in enhanced concentration of extended defects, such as stacking faults, or defect clusters for Ag and B, respectively. Despite that neither B nor Ag are considered as truly Zn-substituting elements, the Li exhibits an extremely wide depletion for B, while its redistribution is quite modest for Ag. Furthermore, the Li depletion is partly suppressed in the co-doped samples. The results are explained in terms of different kinetics of defect annealing and injection of Zn interstitials from the implanted region as well as strong trapping of Zn interstitials by the extended defects.  A. Yu. Azarov, et al., Phys. Rev. Lett. 110, 175503 (2013).
Authors : E.V. Savchenko1*, I.V. Khyzhniy1, S.A. Uyutnov1, M.A. Bludov1, G.B. Gumenchuk2, V.E. Bondybey2
Affiliations : 1Institute for Low Temperature Physics and Engineering NASU, 61103 Kharkov, Ukraine 2Lehrstuhl für Physikalische Chemie II TUM, 85747 Garching, Germany
Resume : Interest in research of inert matrices formed from noble gases and nitrogen is associated with their applications as scintillators, moderators, systems for energy storage and as components of interstellar and solar systems. Ionization and relaxation processes in these inert matrices subjected to irradiating electrons are studied with the focus on defect formation. Low-energy electrons were used to simulate the effect of secondary electrons generated by energetic particles. The study was performed employing cathodoluminescence CL, optical and current activation spectroscopy methods. Recording the CL spectra sequences on exposure time and fluence dependence of activation spectra enabled us to monitor defect production, fragmentation of molecules and excited particle ejection . The use of current and optical methods allowed to distinguish between reactions of neutral and charged defects and to restore the relaxation scenario. Ionization of these matrices is followed by substantial structural relaxation hole self-trapping with formation of ionic dimers Ne_2ᶺ+ and N_4ᶺ+ . In solid N_2 ionic centers N_3ᶺ+ were also detected by CL. It was found that neutralization reactions proceed by type dissociative recombination. Products of the reactions elucidated spectroscopically are radiation-induced point defects. Conditions of defect accumulation and their stability are discussed.  E. Savchenko , I. Khyzhniy, S. Uyutnov, A. Barabashov, G. Gumenchuk, A. Ponomaryov and V. Bondybey, Phys. Status Solidi (c) 12 (2015) 49.  E.V. Savchenko, I.V. Khyzhniy, S.A. Uyutnov, A.P. Barabashov, G.B. Gumenchuk, M.K. Beyer, A.N. Ponomaryov, and V.E. Bondybey, J. Phys. Chem. http://dx.doi.org/10.1021/jp5087575
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Authors : Matthias Krack
Affiliations : Laboratory for Reactor Physics and Systems Behaviour Paul Scherrer Institute 5232 Villigen PSI Switzerland
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.
Authors : K. V. Khishchenko
Affiliations : Joint Institute for High Temperatures RAS, Moscow, Russia
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.
Authors : P. Garcia*, R. Vauchy*, A.-C. Robisson*, I. Roure*, P. Bienvenu*, B. Dorado**, C. Stanek***, D. Andersson***
Affiliations : *CEA/Cad/DEN/DEC; **CEA/DIF;***LANL/MSTD
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.
Authors : Steven D. Conradson
Affiliations : Synchrotron Soleil
Resume : Reactor performance and fuel processing, the two factors that determine the efficacy of nuclear power, are ultimately governed by the properties of the fuel material, which is still most commonly UO2 or its derivatives. Although a putatively simple binary compound, underlying complexity is historically suggested by the very large number of U-oxide phases, the continuous range of U-O bond lengths, and the resulting structural disorder. This has very recently been expanded by the discovery of a number of additional unusual or unique properties of the atoms, charges, and spins in chemically and photo-doped UO2 that demonstrate that its polarons are dynamical and aggregate to form domains whose coherence and other properties indicate are droplets of Bose-Einstein-Hubbard condensate superfluid. The underlying stability of this novel type of quantum matter is demonstrated by the observation that even undoped UO2 possesses coherent carriers and that ion irradiated UO2 contains urany l-type bonds that would only form by aggregation of the mobilized O ions. All of these observations are consistent with a mechanism that may be unique to the oxides of the light actinides, the transformation between cubic and layered structures promoted by the spherical geometries of ionic An(III/IV) and highly oblate An(V/VI) with its covalent actinyl-type bonds. Synchronization of this disproportionation reaction over the charge enriched domain could then be a novel coherence mechanism.
Authors : Guillaume Brindelle(1,2,3), Gianguido Baldinozzi (1,2), Lionel Desgranges (3)
Affiliations : (1) SPMS, LRC Carmen, CNRS CentraleSupelec, Chatenay-Malabry, France; (2) CEA, DEN, DMN, Gif-sur-Yvette, France; (3) CEA, DEN, DEC, Saint-Paul-lez-Durance, France
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.
Authors : Serge Maillard, Guillaume Martin, Catherine Sabathier
Affiliations : CEA, DEN, DEC, Centre de Cadarache, 13108, Saint-Paul-lez-Durance, France
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.
Authors : S. Piskunov and R. I. Eglitis
Affiliations : Institute of Solid State Physics, University of Latvia, 8 Kengaraga Str., Riga LV1063, Latvia
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  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.
Authors : Diana Bachiller Perea, Lionel Thomé, Aurélien Debelle
Affiliations : CSNSM, Univ. Paris-Sud, CNRS/IN2P3 Bat 108, 91405 Orsay Cedex
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 , 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.  S.Moll, Y. Zhang, A. Debelle et al, Acta Mater, in press
Authors : V.K. Egorov1, A.S. Anikin2, V.V. Gorlevskiy2, A.V. Zabrodin2, O.V. Koletskiy2, A.V. Lizunov2, A.A. Semeonov2, M.S. Sheverdajev2
Affiliations : 1IMT RAS, Chernogolovka, Moscow District, Russia 2JSC VNIINM, Moscow, Russia
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.
Authors : Alexandre Boulle, Jayanth Channagiri, Aurélien Debelle
Affiliations : SPCTS CNRS UMR 7315, Centre Européen de la Céramique, 12 rue atlantis, 87068 Limoges Cedex, France CSNSM, University Paris-Sud, CNRS-IN2P3, 91405 Orsay Cedex, France
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 . 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 . The potential of the approach will be illustrated with the example of irradiated SiC and ZrO2 single crystals.  A. Boulle, A. Debelle, J. Appl. Cryst. 43, 1046 (2010)  J. Channagiri, A. Boulle, A. Debelle, to be published in J. Appl. Cryst. (2015)
Authors : V.V. Uglov, N.T. Kvasov, R.V. Polikarpov
Affiliations : Belarusian State University, Minsk, 220030, Nezavisimosty ave., 4, Belarus
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).
Authors : F. Komarov1, L. Vlasukova1, A. Dauletbekova2, A.Akilbekov2, V. Yuvchenko1, A. Alzhanova2
Affiliations : 1Belarus State University, Minsk, Belarus; 2L.N. Gumilyov Eurasian National University, Astana, Kazakhstan
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.
Authors : Yu. A. Mastrikov1, P. V. Vladimirov2, V.A. Borodin3, A. Gopejenko1,Yu.F. Zhukovskii1, E. A. Kotomin1, A. Möslang2
Affiliations : 1Institute for Solid State Physics, University of Latvia, Kengaraga str. 8, Riga, Latvia 2Institute for Applied Materials Applied Materials Physics, Karlsruhe Institute of Technology, Hermann-von-Helmholtz-Platz 1, Eggenstein-Leopoldshafen, D-76344 Karlsruhe, Germany 3 NRC Kurchatov Institute, Kurchatov pl. 1, Moscow, 123182, Russia
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 . 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.  A. Gopejenko, Yu.F. Zhukovskii, P.V
Authors : A.I. Popov (1), E.A. Kotomin (1,2), J. Maier (2)
Affiliations : (1) Institute of Solid State Physics, University of Latvia, Kengaraga 8, Riga, Latvia; (2) Max Planck Institute for Solid State Research, Heisenbergstr. 1, Stuttgart, Germany
Resume : The self-trapped hole polarons (called also Vk centres) where a hole is shared by two nearest halogens) are very common color centers created in alkali halides and alkaline-earth halides under all kinds of irradiation (UV light, electrons, gamma rays, neutrons, heavy swift ions). Their stability controls the secondary reactions between electron and hole defects and thus radiation stability/sensitivity of materials. The hole polarons start to migrate and recombine above certain critical temperatures. Their thermally induced decay has been observed by different experimental technigues in almost all alkali halides, as well as in some halides and more complex halides, such as perovskite halides, ammonium halides, halide sodalites etc. In this report, we review and analyse the self-trapped hole center migration temperatures for a series of alkali halides as a function of halogen-halogen distance in a regular crystalline lattice as well as of halogen-halogen distance in isolated molecular ions. The critical temperature dependence on the band gap and the optical absorption energy of self-trapped hole is also discussed. The results discussed here are especially important for clarification of the self-trapped holes in, for example, large family of perovskite halides and more complex halide materials.
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Authors : Yanwen Zhang 1, Ritesh Sachan 1, Olli H. Pakarinen 1, Matthew F. Chisholm 1, Peng Liu 2,3, Haizhou Xue 2, and William J. Weber 2,1
Affiliations : 1 Materials Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831, USA; 2 Department of Materials Science and Engineering, University of Tennessee, Knoxville, TN 37996, USA; 3 School of Physics, Key Laboratory of Particle Physics and Particle Irradiation (MOE), Shandong University, Jinan 250100, China
Resume : A long-standing objective in materials research is to find innovative ways to remove pre-existing damage and heal fabrication defects or environmentally induced defects in materials. As one of the most investigated wide-band gap ceramics, SiC is a key functional material in high-power, high-frequency, and high-temperature electronic and space applications. SiC materials and composites are also being considered as key engineering materials in nuclear energy applications for operation in harsh environments. In this presentation, we show that the energy transferred to the electrons in SiC by MeV ions via inelastic ionization processes can effectively anneal pre-existing defects and restore the structural order. This work reveals that this self-healing process is activated at an unexpectedly low threshold value using ionizing ions with energies from a few of MeV to a few tens of MeV, compared with a few tens of keV/nm reported for swift heavy ions with energy from a few hundreds of MeV to GeV. The results conveyed here can have a considerable impact not only on functionalizing SiC-based devices by providing a room-temperature approach to anneal defects and repair damaged crystalline structure, but also for predicting material performance in nuclear environment and space applications. This work was support by U.S. DOE, BES, MSED.
Authors : M. W. Ullah, A. Kuronen, F. Djurabekova, K. Nordlund, P. A. Karaseov, A. I. Titov
Affiliations : University of Helsinki, Finland; S.Petersburg State Technical University, Russia
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.
Authors : Guido ROMA1, Thomas JOURDAN1, Sandrine MIRO1-2, Fabien BRUNEVAL1, Jean-Paul CROCOMBETTE1, Jean-Marc COSTANTINI3
Affiliations : 1 CEA, DEN, Service de Recherches de Métallurgie Physique; F-91191, Gif sur Yvette ;2 Laboratoire JANNUS ; 3 CEA, DEN, Service de Recherches de Métallurgie Appliquée, F-91191 Gif sur Yvette
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 ; 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  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.  S. Miro et al., J. Raman Spectroscopy 45, 481 (2014).  M. Lazzeri and F.Mauri, Phys. Rev. Lett. 90, 036401 (2003).
Authors : D. Craciun1, G. Socol1, N. Stefan1, D. Simeone2, S. Behdad3, B. Boesl3, E. Lambers4, C. Himcinschi5, D. Pantelica6, P. Ionescu6, C. Martin7, B. Vasile8, H. Makino9, and V. Craciun1
Affiliations : 1National Institute for Lasers, Plasma and Radiation Physics, Măgurele, Romania; DMN/SRMA-LA2M, LRC CARMEN CEA Saclay, France; 3Mechanical and Materials Science Engineering, Florida International University, Miami, USA; 4MAIC, University of Florida, Gainesville, USA; 5Institute of Theoretical Physics, TU Bergakademie Freiberg, Freiberg, Germany; 6Horia Hulubei National Institute for Physics and Nuclear Engineering, Măgurele, Romania; 7Ramapo College of New Jersey, NJ, USA; 8Polytechnic University Bucharest, Romania; 9Research Institute, Kochi University of Technology, Kochi, Japan
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.
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Authors : A. Lushchik*, Ch. Lushchik*, A.I. Popov**, K. Schwartz***, E. Shablonin*, O. Sidletskiy****E. Vasil'chenko*
Affiliations : *Institute of Physics, University of Tartu, Ravila 14c, 50411 Tartu, Estonia; **Institute of Solid State Physics, University of Latvia, Kengaraga 8, Riga LV-1063, Latvia; ***Gesellschaft für Schwerionenforschung (GSI), Planckstr. 1, 64291 Darmstadt, Germany; ****Institute for Scintillation Materials, Lenin Avenue 60, 61001 Kharkov, Ukraine
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.
Authors : M. Lang1, J. Shamblin1, C.L. Tracy2, F.X. Zhang3, R.I. Palomares1, B. Perlov1,C. Trautmann4,5, and R.C. Ewing6
Affiliations : 1 Department of Nuclear Engineering, University of Tennessee, TN, 37996, USA 2 Department of Materials Science & Engineering, University of Michigan, MI, 48109, USA 3 Department of Earth and Environmental Sciences, University of Michigan, MI, 48109, USA 4 GSI Helmholtzzentrum für Schwerionenforschung, 64291 Darmstadt, Germany 5 Technische Universität Darmstadt, 64287 Darmstadt, Germany 6 Department of Geological and Environmental Sciences, Stanford University, CA, 94305, USA
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.
Authors : Eva Zarkadoula 1, Ritesh Sachan 1, Yanwen Zhang 1, Haizhou Xue 2, Ke Jin 2, and William J. Weber 2
Affiliations : 1 Materials Science and Technology Division, Oak Ridge National Laboratory, USA; 2 Department of Materials Science and Engineering, University of Tennessee, USA
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.
Authors : Yan Li, George Beridze, Piotr M. Kowalski
Affiliations : Institute of Energy and Climate Research: Nuclear Waste Management and Reactor Safety (IEK-6), Forschungszentrum J?lich GmbH, 52425 J?lich, Germany
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.
Authors : J. H. OConnell, V. A. Skuratov, J.H. Neethling
Affiliations : CHRTEM, Port Elizabeth, South Africa; FLNR, JINR, Dubna, Russia; CHRTEM, Port Elizabeth, South Africa
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.
Nanostructured materials : C. Gallé
Authors : V.A. Skuratov, A.S.Sohatsky, J.H. O'Connel, K. Kornieieva, A.A. Nikitina, V.V.Uglov, J.H. Neethling, V.S. Ageev
Affiliations : FLNR, JINR, Dubna, Russia; CHRTEM, NMMU, Port Elizabeth, South Africa; JSC VNIINM, Moscow, Russia; 4Belarusian State University, Minsk, Belarus
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.
Authors : V N Kuzovkov, E A Kotomin, A I Popov, and R Vila
Affiliations : Institute of Solid State Physics, Riga, Latvia, CIEMAT, Madrid, Spain
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 centersoxygen 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.
Authors : A. Janse Van Vuuren1, A.S.Sohatsky2, J.H.OConnell1,V.A. Skuratov2, M. Milosavljević3, S. Petrović3
Affiliations : 1CHRTEM, NMMU, Port Elizabeth; 2FLNR, JINR, Dubna, Russia; 3VINCA Institute of Nuclear Sciences, BU, Belgrade, Serbia
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.
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