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Multi-principal component transition metal (TM) diborides represent a class of high-entropy ceramics (HECs) that have received considerable interest in recent years owing to their promising properties for extreme environment applications that include thermal/ environmental barriers, hypersonic vehicles, turbine engines, and next-generation nuclear reactors. While the addition of chemical disorder through the random distribution of TM elements on the cation sublattice has offered opportunities to tailor elastic stiffness and hardness, the effects of irradiation-induced structural damage on the physical properties of these complex materials have remained largely unexplored. To this end, changes in the hardness and elastic moduli of a high-entropy TM diboride (Hf0.2Nb0.2Ta0.2Ti0.2Zr0.2)B2 and three of its quaternary subsets following irradiation with 10 MeV gold (Au) ions to fluences of up to 6 × 1015 Au cm−2 are investigated at the micrometer and sub-micrometer length-scales via the dispersion of laser-generated surface acoustic waves (SAW) and nanoindentation, respectively. The nanoindentation measurements show that the TM diborides exhibit an initial increase in hardness following irradiation with energetic Au ions, with a subsequent decrease in hardness following further irradiation. One quaternary composition, (Hf1/3Ta1/3Ti1/3)B2, exhibits a notable exception to the trend and continues to exhibit an increase in hardness with ion irradiation fluence. Although differences in the absolute values of the effective elastic moduli obtained from the measured SAW dispersion and nanoindentation are observed (and attributed to microstructural variations at the measurement length-scale), both techniques yield similar trends in the form of an initial reduction and subsequent saturation in the elastic modulus with increasing ion irradiation fluence. The quaternary TM diboride (Hf1/3Ta1/3Ti1/3)B2 again exhibits a departure from this trend. The high-entropy TM diboride (Hf0.2Nb0.2Ta0.2Ti0.2Zr0.2)B2 exhibits the greatest recovery in hardness and modulus when irradiated to high ion fluences following initial changes at low fluence, indicating superior resistance to radiation-induced damage over its quaternary counterparts. Opportunities for designing HECs with superior hardness and modulus for enhanced radiation resistance (compared to their single constituent counterparts) by tailoring chemical disorder and bond character in the lattice are discussed. [ABSTRACT FROM AUTHOR]

The high-entropy concept was applied to synthesize a set of rare-earth perovskites REBO3 (RE = La, Pr, Nd, Sm, Eu, Gd) with the B-site occupied by Sc, Al, Cr, Ni, and Fe in equimolar ratios. All samples crystallize in the orthorhombic Pnma space group. Using an extended set of characterization measurements, the effects of multi-component material design and rare-earth selection on the electronic properties are explored. Transport measurements show semiconducting behavior. PrBO3, SmBO3, and LaBO3 show low-temperature magnetic ordering, with the ordering temperature shifting with the moment on the A-site. [ABSTRACT FROM AUTHOR]

The selective amorphization of SiGe in Si/SiGe nanostructures via a 1 MeV Si+ implant was investigated, resulting in single-crystal Si nanowires (NWs) and quantum dots (QDs) encapsulated in amorphous SiGe fins and pillars, respectively. The Si NWs and QDs are formed during high-temperature dry oxidation of single-crystal Si/SiGe heterostructure fins and pillars, during which Ge diffuses along the nanostructure sidewalls and encapsulates the Si layers. The fins and pillars were then subjected to a 3 × 1015 ions/cm2 1 MeV Si+ implant, resulting in the amorphization of SiGe, while leaving the encapsulated Si crystalline for larger, 65-nm wide NWs and QDs. Interestingly, the 26-nm diameter Si QDs amorphize, while the 28-nm wide NWs remain crystalline during the same high energy ion implant. This result suggests that the Si/SiGe pillars have a lower threshold for Si-induced amorphization compared to their Si/SiGe fin counterparts. However, Monte Carlo simulations of ion implantation into the Si/SiGe nanostructures reveal similar predicted levels of displacements per cm3. Molecular dynamics simulations suggest that the total stress magnitude in Si QDs encapsulated in crystalline SiGe is higher than the total stress magnitude in Si NWs, which may lead to greater crystalline instability in the QDs during ion implant. The potential lower amorphization threshold of QDs compared to NWs is of special importance to applications that require robust QD devices in a variety of radiation environments. [ABSTRACT FROM AUTHOR]

Micro-Raman spectroscopy, X-band electron paramagnetic resonance (EPR) spectroscopy, and UV-visible optical absorption spectroscopy were used to study the damage production in cerium dioxide (CeO2) single crystals by electron irradiation for three energies (1.0, 1.4, and 2.5 MeV). The Raman-active T2g peak was left unchanged after 2.5-MeV electron irradiation at a high fluence. This shows that no structural modifications occurred for the cubic fluorite structure. UV-visible optical absorption spectra exhibited a characteristic sub band-gap tail for 1.4-MeV and 2.5-MeV energies, but not for 1.0 MeV. Narrow EPR lines were recorded near liquid-helium temperature after 2.5-MeV electron irradiation; whereas no such signal was found for the virgin unirradiated crystal or after 1.0-MeV irradiation for the same fluence. The angular variation of these lines in the {111} plane revealed a weak g-factor anisotropy assigned to Ce3+ ions (with the 4f¹ configuration) in a high-symmetry local environment. It is concluded that Ce3+ ions may be produced by a reduction resulting from the displacement damage process. However, no evidence of F+ or F0 center or hole center formation due to irradiation was found from the present EPR and optical absorption spectra. [ABSTRACT FROM AUTHOR]

Defect energetics in structural materials has long been recognized to be affected by specific alloy compositions. Local structural distortion greatly affects the physical properties and performance of alloys. To reveal the atomic-level lattice distortion, the local structures of Ni and Fe in Ni1-xFex (x=0.10, 0.20, 0.35 and 0.50) solid solution alloys were measured with extended X-ray absorption fine structure (EXAFS) technique. The EXAFS measurements have revealed that the bond length of Fe with surrounding atoms is 0.01-0.02Å larger than that of Ni with its neighbors in the alloys. Both the lattice constant and the interatomic distance of the nearest neighbors increase with the addition of Fe content in the solid solutions. The local bonding environments in Ni1-xFex alloys were also calculated from ab initio and compared with the experimental results. [ABSTRACT FROM AUTHOR]

Understanding scintillation physics and nonproportionality is essential to accelerate materials discovery that has been restricted due to the difficulties inherent to large crystal growth and complex nature of gamma-solid interaction. Taking advantage of less restrictive growth and deposition techniques for smaller crystal sizes or thin films and better fundamental understanding of ion-solid interactions, a unique ion approach is demonstrated to effectively screen candidate scintillators with relatively small size and evaluate their nonlinear scintillation response. Response of CaF2:Eu and YAlO3:Ce scintillators to single ions of H+, He+, and O3+ are measured by the corresponding pulse height over a continuous energy range using a time-of-flight-scintillator-photoelectric multiplier tube apparatus. Nonlinear response of the scintillators under ionizing ion irradiation is quantitatively evaluated by considering the energy partitioning process. In a differential energy deposition region with negligible displacement damage, the low, medium and high excitation energy deposition density (Dexci) can be produced by energetic H+, He+ and O3+ ions, respectively, and significantly different impacts on the response characteristics of these two benchmark scintillators are observed. For CaF2:Eu, the scintillation efficiency under ion irradiation monotonically decreases with increasing excitation-energy density. In contrast, the response efficiency of YAlO3:Ce scintillation initially increases with excitation-energy density at low excitation-energy densities, goes through a maximum, and then decreases with further increasing excitation-energy density. The fundamental mechanism causing these different response behaviours in the scintillators is based on the competition between the scintillation response and the nonradiative quenching process under different excitation densities, which is also the main origin of the nonlinear response of the scintillators to irradiation. [ABSTRACT FROM AUTHOR]

Density functional theory (DFT) with a tailored Hartree-Fock hybrid functional, which can overcome the band gap problem arising in conventional DFT and gives a valence band width comparable with experiment, is applied to determine formation energies and electronic structures of intrinsic defects in cubic silicon carbide (3C-SiC). Systematic comparison of defect formation energies obtained with the tailored hybrid functional and a conventional DFT functional clearly demonstrates that conventional DFT results are not satisfactory. The understanding on intrinsic defects, which were previously investigated mainly with conventional DFT functionals, is largely revised with regard to formation energies, electronic structures and transition levels. It is found that conventional DFT functionals basically lead to (i) underestimation of the formation energy when the defect charge is more negative and (ii) overestimation when the defect charge is more positive. The underestimation is mainly attributed to the well-known band gap problem. The overestimation is attributed to shrinkage of the valence bands, although in some cases such band shrinkage may lead to underestimation depending on how the defect alters the valence band structure. Both the band gap problem and the valence band shrinkage are often observed in semiconductors, including SiC, with conventional DFT functionals, and thus need to be carefully dealt with to achieve reliable computational results. [ABSTRACT FROM AUTHOR]

Understanding material responses to energy deposition from energetic charged particles is important for defect engineering, ion-beam processing, ion-beam analysis and modification, geologic aging, space exploration, and nuclear applications. As an incident ion penetrates a solid, its energy is transferred to electrons and to atomic nuclei of the solid. Much of this electronic energy deposition is subsequently transferred to the atomic structure via electron–phonon (e–ph) coupling, leading to local inelastic thermal spikes in which energy dissipation is influenced by the local environment. In addition, intense ionization can lead to high densities of localized electronic excitations in wide-bandgap materials and ceramics that can affect defect dynamics and atomic mobility. Specifically, energy exchange between electrons and atomic nuclei, along with localized electronic excitations, can lead to substantial competitive (ionization-induced annealing), additive (both electronic and nuclear energy depositions contributing to damage production), and synergistic (more damage than the sums of separate damage processes) effects. Although nonmonotonic effects of the e–ph coupling strength and athermal processes are demonstrated for pre-existing defects and residual damage during ion–solid interactions, there is limited understanding of when such electronic effects must be considered in atomic-scale models of damage production and evolution in a broad variety of materials. Complex ceramics and chemically disordered solid solution alloys with different constituent elements allow a systematic evaluation of defect dynamics and irradiation performance with increasing complexity. Current knowledge regarding tuning of bonding characteristics and chemical disorder to control atomic-level dynamics is reviewed. Although a lack of fundamental understanding obstructs the advancement of reliable predictions for ion beam material modification, it highlights challenges and opens research opportunities. Insights into the complex electronic and atomic correlations with extreme energy deposition will strengthen our ability to design materials and predict ion-irradiation-induced damage in a radiation environment, and they may pave the way to better control fundamental processes and design new material functionalities for advanced technologies. [ABSTRACT FROM AUTHOR]

We have studied the effect of the yttria content on the paramagnetic centers in electron-irradiated yttria-stabilized zirconia (ZrO2: Y3+) or YSZ. Single crystals with 9.5 mol % or 18 mol % Y2O3 were irradiated with electrons of 1.0, 1.5, 2.0, and 2.5 MeV. The paramagnetic center production was studied by X-band electron paramagnetic resonance (EPR) spectroscopy. The same paramagnetic centers were identified for both chemical compositions, namely two electron centers, i.e., (i) F+-type centers (involving singly ionized oxygen vacancies), and (ii) so-called T centers (Zr3+ in a trigonal symmetry site), as well as hole-centers. A strong effect is observed on the production of hole-centers that is strongly enhanced when doubling the yttria content. However, no striking effect is found on the electron centers (except the enhancement of an extra line associated with the F+-type centers). It is concluded that hole-centers are produced by inelastic interactions, whereas F+-type centers are produced by elastic collisions with no effect of the yttria content on the defect production rate. In the latter case, the threshold displacement energy (Ed) of oxygen is estimated from the electron-energy dependence of the F+-type center production rate, with no significant effect of the yttria content on Ed. An Ed value larger than 120 eV is found. This is supported by classical molecular dynamics (MD) simulations with a Buckingham-type potential that show Ed values for Y and O are likely to be in excess of 200 eV. Due to the difficulty in displacing O or Y atoms, the radiation-induced defects may alternatively be a result of Zr atom displacements for Ed = 80 ± 1 eV with subsequent defect rearrangement. [ABSTRACT FROM AUTHOR]

Although GaN is an important semiconductor material, its amorphous structures are not well understood. Currently, theoretical atomistic structural models which contradict each other, are proposed for the chemical short-range order of amorphous GaN: one characterizes amorphous GaN networks as highly chemically ordered, consisting of heteronuclear Ga-N atomic bonds; and the other predicts the existence of a large number of homonuclear bonds within the first coordination shell. In the present study, we examine amorphous structures of GaN via radial distribution functions obtained by electron diffraction techniques. The experimental results demonstrate that amorphous GaN networks consist of heterononuclear Ga-N bonds, as well as homonuclear Ga-Ga and N-N bonds. [ABSTRACT FROM AUTHOR]

Atomic configurations and formation energies of native defects in an unsaturated GaN nanowire grown along the [001] direction and with (100) lateral facets are studied using large-scale ab initio calculation. Cation and anion vacancies, antisites, and interstitials in the neutral charge state are all considered. The configurations of these defects in the core region and outermost surface region of the nanowire are different. The atomic configurations of the defects in the core region are same as those in the bulk GaN, and the formation energy is large. The defects at the surface show different atomic configurations with low formation energy. Starting from a Ga vacancy at the edge of the side plane of the nanowire, a N–N split interstitial is formed after relaxation. As a N site is replaced by a Ga atom in the suboutermost layer, the Ga atom will be expelled out of the outermost layers and leaves a vacancy at the original N site. The Ga interstitial at the outmost surface will diffuse out by interstitialcy mechanism. For all the tested cases N–N split interstitials are easily formed with low formation energy in the nanowires, indicating N2 molecular will appear in the GaN nanowire, which agrees well with experimental findings. [ABSTRACT FROM AUTHOR]

The mechanical behavior of twinned silicon carbide (SiC) nanowires under combined tension-torsion and compression-torsion is investigated using molecular dynamics simulations with an empirical potential. The simulation results show that both the tensile failure stress and buckling stress decrease under combined tension-torsional and combined compression-torsional strain, and they decrease with increasing torsional rate under combined loading. The torsion rate has no effect on the elastic properties of the twinned SiC nanowires. The collapse of the twinned nanowires takes place in a twin stacking fault of the nanowires. [ABSTRACT FROM AUTHOR]

Depth profiles of local volume expansions are precisely measured in 6H-SiC after irradiation at 150 K with 2 MeV Pt ions and following annealing at 770 K using transmission electron microscopy equipped with electron energy loss spectroscopy. It is found that the depth profile of local volume expansion from the as-implanted sample matches well with the depth profile of irradiation-induced local disorder measured by Rutherford backscattering spectrometry. Further, the local volume expansion increases linearly with local dose up to ∼10%. By systematically comparing the depth profiles of local volume expansion and local relative disorder, it is revealed that the atomic volume of amorphous SiC continues to increase until it saturates at ∼14% due to the increased chemical short-range disorder. This is believed to be one of the reasons for significant scatter in values of volume expansion previously reported for the irradiation-induced amorphous state of SiC. [ABSTRACT FROM AUTHOR]

The defect creation at low energy events was studied using density functional theory molecular dynamics simulations in silicon carbide nanotubes, and the displacement threshold energies determined exhibit a dependence on sizes, which decrease with decreasing diameter of the nanotubes. The Stone–Wales (SW) defect, which is a common defect configurations induced through irradiation in nanotubes, has also been investigated, and the formation energies of the SW defects increase with increasing diameter of the nanotubes. The mean threshold energies were found to be 23 and 18 eV for Si and C in armchair (5,5) nanotubes. [ABSTRACT FROM AUTHOR]

Atomistic structures of high-energy ion irradiated GaN were examined using transmission electron microscopy (TEM). Single crystalline GaN substrates were irradiated at cryogenic temperatures with 2 MeV Au2+ ions to a fluence of 7.35×1015 Au/cm2. Cross-sectional TEM observations revealed that damaged layers consisting of amorphous and nanocrystalline phases are formed at the surface and buried depth of the as-irradiated GaN substrate. Atomic radial distribution functions of the amorphous/polynanocrystalline regions showed that not only heteronuclear Ga–N bonds but also homonuclear Ga–Ga bonds exist within the first coordination shell. It was found that the ratio of heteronuclear-to-homonuclear bonds, i.e., the degree of chemical disorder, is different between the surface and buried damaged layers. The alternation of chemical disorder was attributed to the difference in the defect formation processes between these layers. [ABSTRACT FROM AUTHOR]

Slow heavy ions inevitably produce a significant concentration of defects and lattice disorder in solids during their slowing-down process via ion-solid interactions. For irradiation effects research and many industrial applications, atomic defect production, ion range, and doping concentration are commonly estimated by the stopping and range of ions in matter (SRIM) code. In this study, ion-induced damage and projectile ranges of low energy Au ions in SiC are determined using complementary ion beam and microscopy techniques. Considerable errors in both disorder profile and ion range predicted by the SRIM code indicate an overestimation of the electronic stopping power, by a factor of 2 in most cases, in the energy region up to 25 keV/nucleon. Such large discrepancies are also observed for slow heavy ions, including Pt, Au, and Pb ions, in other compound materials, such as GaN, AlN, and SrTiO3. Due to the importance of these materials for advanced device and nuclear applications, better electronic stopping cross section predictions, based on a reciprocity principle developed by Sigmund, is suggested with fitting parameters for possible improvement. [ABSTRACT FROM AUTHOR]

This report presents the results of classical molecular dynamics simulations of the diffuse premelting transition, melting, and defect production by 1 keV U recoils in UO2 using five different rigid ion potentials. The experimentally observed premelting transition occurred for all five cases. For all the potentials studied, dynamic defect annealing is highly effective and is accompanied by replacement events on the anion sublattice. The primary damage state after ∼15 ps consists of isolated Frenkel pairs and interstitial and vacancy clusters of various sizes. The average displacement energy varies from ∼28 to ∼83 eV and the number of Frenkel pairs is different by a factor of 3 depending on the choice of potential. The size and spatial distribution of vacancy and interstitial clusters is drastically different for the potentials studied. The results provide statistics of defect production. They point to a pressing need to determine defect formation, migration, and binding energies in UO2 from first principles and to develop reliable potentials based on this data for simulating microstructural evolution in nuclear fuel under operating conditions. [ABSTRACT FROM AUTHOR]

The thermal evolution of the microstructure created by irradiation of a GaN single crystal with 2 MeV Au2+ ions at 150 K is characterized following annealing at 973 K using transmission electron microscopy. In the as-irradiated sample characterized at 300 K, Ga nanocrystals with the diamond structure, which is an unstable configuration for Ga, are directly observed together with nitrogen bubbles in the irradiation-induced amorphous layer. A simple model is proposed to explain Ga nanocrystal formation. Upon thermal annealing, the thickness of the amorphous layer decreases by ∼13.1% and nanobeam electron diffraction analysis indicates no evidence for residual Ga nanocrystals, but instead reveals a mixture of hexagonal and cubic GaN phases in the annealed sample. Nitrogen molecules, captured in the as-irradiated bubbles, appear to disassociate and react with Ga nanocrystals during the thermal annealing to form crystalline GaN. In addition, electron energy loss spectroscopy measurements reveal an volume change of 18.9% for the as-irradiated amorphous layer relative to the virgin single crystal GaN. This relative swelling of the damaged layer reduces to 7.7% after thermal annealing. Partial recrystallization and structural relaxation of the GaN amorphous state are believed responsible for the volume change. [ABSTRACT FROM AUTHOR]

Molecular dynamics simulations with Tersoff potentials were used to study the response of single crystalline SiC nanotubes under tensile, compressive, torsional, combined tension-torsional, and combined compression-torsional strains. The simulation results reveal that the nanotubes deform through bond-stretching and breaking and exhibit brittle properties under uniaxial tensile strain, except for the thinnest nanotube at high temperatures, which fails in a ductile manner. Under uniaxial compressive strain, the SiC nanotubes buckle with two modes, i.e., shell buckling and column buckling, depending on the length of the nanotubes. Under torsional strain, the nanotubes buckle either collapse in the middle region into a dumbbell-like structure for thinner wall thicknesses or fail by bond breakage for the largest wall thickness. Both the tensile failure stress and buckling stress decrease under combined tension-torsional and combined compression-torsional strain, and they decrease with increasing torsional rate under combined loading. [ABSTRACT FROM AUTHOR]

Thermally induced structural relaxation in amorphous silicon carbide (SiC) has been examined by means of in situ transmission electron microscopy (TEM). The amorphous SiC was prepared by high-energy ion beam irradiation into a single crystalline 4H-SiC substrate. Cross-sectional TEM observations and electron energy-loss spectroscopy measurements revealed that thermal annealing induces a remarkable volume reduction, so-called densification, of amorphous SiC. From radial distribution function analyses using electron diffraction, notable changes associated with structural relaxation were observed in chemical short-range order. It was confirmed that the structural changes observed by the in situ TEM study agree qualitatively with those of the bulk material. On the basis of the alteration of chemical short-range order, we discuss the origin of thermally induced densification in amorphous SiC. [ABSTRACT FROM AUTHOR]

A kinetic lattice Monte Carlo (KLMC) model is developed to investigate the recovery and clustering of defects during annealing of a single 10 keV cascade in cubic silicon carbide. The 10 keV Si cascade is produced by molecular dynamics (MD), and a method of transferring the defects created by MD simulations to the KLMC model is developed. The KLMC model parameters are obtained from MD simulations and ab initio calculations of defect migration, recombination, and annihilation. The defects are annealed isothermally from 100 K to 1000 K in the KLMC model. Two distinct recovery stages for close Frenkel pairs are observed at about 200 and 550 K, and the growth of complex clusters is observed above 400 K. These simulation results are in good agreement with available experimental results. [ABSTRACT FROM AUTHOR]

Molecular dynamics simulations have been used to study the melting of GaN nanowires with triangular cross sections. The variation in potential energy as a function of the cross-sectional area of GaN nanowires, along with the atomic configuration, is used to monitor the phase transition. The thermal stability of GaN nanowires is strongly size dependent. The melting temperature of the GaN nanowires increases with increasing cross-sectional area to a saturation value. Melting of the nanowires is initiated at the surface edges formed by the triangular shape and then spreads across the nanowire surface. As temperature increases, the melting expands into the inner regions of the nanowires. [ABSTRACT FROM AUTHOR]

SiC thermal conductivity is known to decrease under irradiation. To understand this effect, we study the variation of the thermal conductivity of cubic SiC with defect accumulation induced by displacement cascades. We use an empirical potential of the Tersoff type in the framework of nonequilibrium molecular dynamics. The conductivity of SiC is found to decrease with dose, in very good quantitative agreement with low temperature irradiation experiments. The results are analyzed in view of the amorphization states that are created by the cascade accumulation simulations. The calculated conductivity values at lower doses are close to the smallest measured values after high temperature irradiation, indicating that the decrease of the conductivity observed at lower doses is related to the creation of point defects. A subsequent decrease takes place upon further cascade accumulation. It is characteristic of the amorphization of the material and is experimentally observed for low temperature irradiation only. [ABSTRACT FROM AUTHOR]

Molecular dynamics simulations with a Stillinger-Weber potential have been used to investigate the melting behavior of wurtzite-type single-crystalline GaN nanotubes. The simulations show that the melting temperature of the GaN nanotubes increases with the thickness of the nanotubes to a saturation value, which is close to the melting temperature of a GaN slab. The results reveal that the nanotubes begin to melt at the surface, and then the melting rapidly extends to the interior of the nanotubes as the temperature increases. The melting temperature of a single-crystalline GaN nanotube with [100]-oriented lateral facets is higher than that with [110]-oriented lateral facets for the same thickness. [ABSTRACT FROM AUTHOR]

During the past half-century, the nuclear fuel cycle has generated approximately 1400 metric tons of plutonium and substantial quantities of the “minor” actinides, such as Np, Am, and Cm. The successful disposition of these actinides has an important impact on the strategy for developing advanced nuclear fuel cycles, weapons proliferation, and the geologic disposal of high-level radioactive waste. During the last decade, there has been substantial interest in the use of the isometric pyrochlore structure-type, A2B2O7, for the immobilization of actinides. Most of the interest has focused on titanate-pyrochlore because of its chemical durability; however, these compositions experience a radiation-induced transition from the crystalline-to-aperiodic state due to radiation damage from the alpha-decay of actinides. Depending on the actinide concentration, the titanate pyrochlore will become amorphous in less than 1000 years of storage. Recently, systematic ion beam irradiations of a variety of pyrochlore compositions has revealed that many zirconate pyrochlores do not become amorphous, but remain crystalline as a defect fluorite structure-type due to disordering of the A- and B-site cations. The zirconate pyrochlores will remain crystalline even to very high doses, greater than 100 displacements per atom. Systematic experimental studies of actinide-doped and ion beam-irradiated pyrochlore, analyses of natural U- and Th-bearing pyrochlore, and simulations of the energetics of the disordering process now provide a rather detailed understanding of the structural and chemical controls on the response of pyrochlore to radiation. These results provide a solid basis for predicting the behavior and durability of pyrochlore used to immobilize plutonium. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We investigate the energy dissipation and track formation due to ion irradiation in SrTiO3 and KTaO3. We use molecular dynamics simulations combined with the inelastic thermal spike model to simulate 21 MeV Ni ion irradiation in pristine and predamaged samples. The results are validated against experimental findings, showing that the level of initial disorder affects the electron-phonon interactions and the energy dissipation and deposition to the atoms. It is predicted that the ion track size increases linearly for low disorder levels, while its size saturates for high levels of disorder. [ABSTRACT FROM AUTHOR]

The tensile mechanical behavior of single crystalline gallium nitride (GaN) nanotubes under combined tension-torsion is investigated using molecular dynamics simulations with an empirical potential. The simulation results show that a small torsion rate (<0.010 deg ps-1) does not affect the tensile behavior of GaN nanotube, i.e., the nanotubes show brittle properties at low temperatures; whereas at high temperatures, they behave as ductile materials. However, the failure stress decreases with increasing rate of torsion above 0.010 deg ps-1, and the nanotube fails in a different manner. The torsion rate has no effect on the elastic properties of GaN nanotubes. [ABSTRACT FROM AUTHOR]

We use the inelastic thermal spike model for insulators and molecular dynamic simulations to investigate the effects of pre-existing damage on the energy dissipation and structural alterations in KTaO3 under irradiation with 21 MeV Ni ions. Our results reveal a synergy between the pre-existing defects and the electronic energy loss, indicating that the defects play an important role on the energy deposition in the system. Our findings highlight the need for better understanding on the role of defects in electronic energy dissipation and the coupling of the electronic and atomic subsystems. [ABSTRACT FROM AUTHOR]

We used a combination of ion cascades and the unified thermal spike model to study the electronic effects from 800 keV Kr and Xe ion irradiation in zircon. We compared the damage production for four cases: (a) due to ion cascades alone, (b) due to ion cascades with the electronic energy loss activated as a friction term, (c) due to the thermal spike from the combined electronic and nuclear energy losses, and (d) due to ion cascades with electronic stopping and the electron-phonon interactions superimposed. We found that taking the electronic energy loss out as a friction term results in reduced damage, while the electronic electron-phonon interactions have additive impact on the final damage created per ion. [ABSTRACT FROM AUTHOR]

Ab initio molecular dynamics simulations of low energy recoil events in wurtzite AlN have been performed to determine threshold displacement energies, defect production and evolution mechanisms, role of partial charge transfer during the process, and the influence of irradiation-induced defects on the properties of AlN. The results show that the threshold displacement energies, Ed, along the direction parallel to the basal planes are smaller than those perpendicular to the basal planes. The minimum Ed values are determined to be 19 eV and 55 eV for N and Al atom, respectively, which occur along the [ 1 ¯ 1 ¯ 20 ] direction. In general, the threshold displacement energies for N are smaller than those for Al atom, indicating the N defects would be dominant under irradiation. The defect production mechanisms have been analyzed. It is found that charge transfer and redistribution for both the primary knock-on atom and the subsequent recoil atoms play a significant role in defect production and evolution. Similar to the trend in oxide materials, there is a nearly linear relationship between Ed and the total amount of charge transfer at the potential energy peak in AlN, which provides guidance on the development of charge-transfer interatomic potentials for classic molecular dynamics simulations. Finally, the response behavior of AlN to low energy irradiation is qualitatively investigated. The existence of irradiation-induced defects significantly modifies the electronic structure, and thus affects the magnetic, electronic and optical properties of AlN. These findings further enrich the understanding of defects in the wide bandgap semiconductor of AlN. [ABSTRACT FROM AUTHOR]

A triple-containment magic-angle spinning rotor insert system has been developed and a sample handling procedure formulated for safely analyzing highly radioactive solids by high-resolution solid-state NMR. The protocol and containment system have been demonstrated for magic-angle spinning (MAS) experiments on ceramic samples containing 5–10 wt % 239Pu and 238Pu at rotation speeds of 3500 Hz. The technique has been used to demonstrate that MAS NMR experiments can be used to measure amorphous atomic number fractions produced by accelerated internal radiation damage. This will allow incorporated α-emitters with short half-lives to be used to model the long-term radiation tolerance of potential ceramic radioactive waste forms. This is an example of MAS NMR spectroscopy on samples containing fissionable isotopes. [ABSTRACT FROM AUTHOR]

Codoping of p-type GaN nanowires with Mg and oxygen was investigated using first-principles calculations. The Mg becomes a deep acceptor in GaN nanowires with high ionization energy due to the quantum confinement. The ionization energy of Mg doped GaN nanowires containing passivated Mg–O complex decreases with increasing the diameter, and reduces to 300 meV as the diameter of the GaN nanowire is larger than 2.01 nm, which indicates that Mg–O codoping is suitable for achieving p-type GaN nanowires with larger diameters. The codoping method to reduce the ionization energy can be effectively used in other semiconductor nanostructures. [ABSTRACT FROM AUTHOR]

Well-aligned ripple structures on the surface of a single crystal of 3C-SiC were created by focused ion beam bombardment, and the resulting morphology and topography were characterized using in situ focused ion beam/scanning electron microscopy, as well as ex situ atomic force microscopy. The ripple structure formed as a result of ion sputtering beyond a critical incident angle (∼50°), and its characteristic wavelength varied from 158 to 296 nm with changes in the incident angle and ion beam flux. The geometry, ordering, and homogeneity of the ripples can be well controlled by varying the ion beam incident angle and beam current, as required for the fabrication of nanostructures that use SiC for optical and electronic applications. [ABSTRACT FROM AUTHOR]

Electronic band structures of single-walled silicon carbide nanotubes are studied under uniaxial strain using first principles calculations. The band structure can be tuned by mechanical strain in a wide energy range. The band gap decreases with uniaxial tensile strain, but initially increases with uniaxial compressive strain and then decreases with further increases in compressive strain. These results may provide a way to tune the electronic structures of silicon carbide nanotubes, which may have promising applications in building nanodevices. [ABSTRACT FROM AUTHOR]

An ion approach is demonstrated to determine energy resolution in both semiconductor detectors and scintillators over a continuous energy range. For semiconductors, the energy resolution of a silicon detector was measured as a function of helium ion energy, and the values from extrapolation to high energies are in good agreement with the literature data from alpha measurements. For scintillators, benchmark crystals subject to He+ irradiation were investigated, and the agreement of energy resolution between the ion and gamma measurements indicates that the ion approach can be used to predict the energy resolution of the candidate materials in thin-film form or small crystals when large crystals necessary for gamma-ray measurements are unavailable. [ABSTRACT FROM AUTHOR]

The thermal conductivity of GaN nanowires has been determined computationally by applying nonequilibrium atomistic simulation methods using the Stillinger-Weber [Phys. Rev. B 31, 5262 (1985)] potentials. The simulation results show that the thermal conductivity of the GaN nanowires is smaller than that of a bulk crystal and increases with increasing diameter. Surface scattering of phonons and the high surface to volume ratios of the nanowires are primarily responsible for the reduced thermal conductivity and its size dependence behavior. The thermal conductivity is also found to decrease with increasing temperature and exhibits a dependence on axial orientation of the nanowires. [ABSTRACT FROM AUTHOR]

The authors have investigated the effects of 200 and 300 keV electron-beam irradiations on amorphization in 6H-SiC at 100 and 295 K. Amorphization is induced by the accumulation of defects produced by direct atomic displacements. Dynamic recovery of these defects during irradiation, due to temperature increases and ionization effects, results in increases in the amorphization dose. By comparing with previous data for 2 MeV electrons and 1.5 MeV Xe ions, the results demonstrate that ionization-enhanced recovery in 6H-SiC dramatically increases above an ionization rate threshold. [ABSTRACT FROM AUTHOR]

Electron-beam induced recrystallization of irradiation-induced amorphous Sr2Nd8(SiO4)6O2 is investigated in situ using transmission electron microscopy with 200 keV electrons at room temperature. Epitaxial recrystallization is observed from both the amorphous/crystalline interface and the surface, and the recrystallization is more pronounced with increasing electron-beam flux. Since the temperature increase induced by electron-beam irradiation is estimated to be less than 7 K and maximum energies transferred to target atoms are below the displacement energies, ionization-induced processes are considered to be the primary mechanisms for the solid-phase epitaxial recrystallization observed in the present study. [ABSTRACT FROM AUTHOR]

Molecular dynamics methods with a Stillinger-Weber [Phys. Rev. B 31, 5262 (1985)] potential have been used to investigate the mechanical properties of wurtzite-type single-crystalline GaN nanotubes under applied tensile strains. At low temperatures, the nanotubes show brittle properties, whereas at high temperatures, they behave as ductile materials. The brittle to ductile transition (BDT) is systematically investigated and the corresponding transition temperatures have been determined in GaN nanotubes. The BDT temperature generally increases with increasing thickness of nanotubes and strain rate. [ABSTRACT FROM AUTHOR]

The kinetics of amorphization in La2Zr2O7 pyrochlore is investigated using molecular dynamics simulations. Irradiation damage is simulated by continuous accumulation of cation Frenkel pairs at various temperatures. As observed experimentally, La2Zr2O7 first transitions to the fluorite structure, independent of the temperature, and amorphization occurs at low temperatures. A model fit of the simulated dose-temperature curve reproduces experimental results in the literature, with a low temperature amorphization dose D0=1.1 displacement per cation and an activation energy Eact=0.036 eV. Present simulations indicate that point defect recombination can control the temperature dependence of amorphization driven by point defect accumulation. [ABSTRACT FROM AUTHOR]

We use the two-temperature model in molecular dynamic simulations of 150 keV Ni ion cascades in nickel and nickel-based alloys to investigate the effect of the energy exchange between the atomic and the electronic systems during the primary stages of radiation damage. We find that the electron-phonon interactions result in a smaller amount of defects and affect the cluster formation, resulting in smaller clusters. These results indicate that ignoring the local heating due to the electrons results in the overestimation of the amount of damage and the size of the defect clusters. A comparison of the average defect production to the Norgett-Robinson-Torrens (NRT) prediction over a range of energies is provided. [ABSTRACT FROM AUTHOR]

We have recently found that the radiation tolerance of SiC is highly enhanced by introducing nanolayers of stacking faults and twins [Y. Zhang et al., Phys. Chem. Chem. Phys. 14, 13429 (2012)]. To reveal the origin of this radiation resistance, we used in situ transmission electron microscopy to examine structural changes induced by electron beam irradiation in 3C-SiC containing nanolayers of (111) planar defects. We found that preferential amorphization, when it does occur, takes place at grain boundaries and at [formula] and [formula] planar defects. Radiation-induced point defects, such as interstitials and vacancies, migrate two-dimensionally between the (111) planar defects, which probably enhances the damage recovery. [ABSTRACT FROM AUTHOR]

This report presents the results of classical molecular dynamics simulations of the diffuse premelting transition, melting, and defect production by 1 keV U recoils in UO(2) using five different rigid ion potentials. The experimentally observed premelting transition occurred for all five cases. For all the potentials studied, dynamic defect annealing is highly effective and is accompanied by replacement events on the anion sublattice. The primary damage state after approximately 15 ps consists of isolated Frenkel pairs and interstitial and vacancy clusters of various sizes. The average displacement energy varies from approximately 28 to approximately 83 eV and the number of Frenkel pairs is different by a factor of 3 depending on the choice of potential. The size and spatial distribution of vacancy and interstitial clusters is drastically different for the potentials studied. The results provide statistics of defect production. They point to a pressing need to determine defect formation, migration, and binding energies in UO(2) from first principles and to develop reliable potentials based on this data for simulating microstructural evolution in nuclear fuel under operating conditions.