Atomistic simulation of point defects behavior in ceria

The cerium and oxygen threshold displacement energies and the Frenkel pair recombination in CeO₂ have been investigated with empirical potential molecular dynamics simulations. The oxygen and cerium threshold displacement energies have been evaluated to be 27 and 56 eV, respectively, in agreement with experimental results. The short-range cerium and oxygen Frenkel pair recombination processes are found to be strongly dependent on the local topology and the pathways. The oxygen Frenkel pair recombinations are either spontaneous or thermally activated unlike the standard recombination usually defined in rate equations. The lifetime of Frenkel pair recombination is in general found to be much shorter than the characteristic time for single defect diffusion.     

Effect of 200 MeV Ag ion irradiation on structural and electrical transport properties of Fe3O4 thin films

Thin films of Fe₃O₄ have been deposited on single crystal MgO(1 0 0) and Si(1 0 0) substrates using pulsed laser deposition. Films grown on MgO substrate are epitaxial with c-axis orientation whereas, films on Si substrate are highly 〈1 1 1〉 oriented. Film thicknesses are 150 nm. These films have been irradiated with 200 MeV Ag ions. We study the effect of the irradiation on structural and electrical transport properties of these films. The fluence value of irradiation has been varied in the range of 5 × 10¹⁰ ions/cm² to 1 × 10¹² ions/cm². We compare the irradiation induced modifications on various physical properties between the c-axis oriented epitaxial film and non epitaxial but 〈1 1 1〉 oriented film. The pristine film on Si substrate shows Verwey transition (T_V) close to 125 K, which is higher than generally observed in single crystals (121 K). After the irradiation with the 5 × 10¹⁰ ions/cm² fluence value, T_V shifts to 122 K, closer to the single crystal value. However, with the higher fluence (1 × 10¹² ions/cm²) irradiation, T_V again shifts to 125 K.     

Structure and electrophysical properties of liquid Pb83Mg17 and Pb83Li17 eutectics

Short range order structure of Pb₈₃Mg₁₇ and Pb₈₃Li₁₇ liquid alloys has been studied by means of X-ray diffraction method. The structure factors and pair correlation functions are analyzed. Experimental structure data were used to calculate the partial structure characteristics by means of Reverse Monte Carlo method. It is shown also that Li₄Pb significantly affects the structure of Pb₈₃Li₁₇ eutectic melt. For Pb₈₃Mg₁₇ eutectic melt the electrical resistivity and thermo-e.m.f. were measured in the temperature range of 550-1300 K. Their analysis confirms the diffraction data concluding the heterocoordinated atomic distribution Pb and Mg atoms.     

Synthesis of nanodimensional TiO2 thin films using energetic ion beam

Nanophases of TiO₂ are achieved by irradiating polycrystalline thin films of TiO₂ by 100 MeV Au ion beam at varying fluence. The surface morphology of pristine and irradiated films is studied by atomic force microscopy (AFM). Phase of the film before and after irradiation is identified by glancing angle X-ray diffraction (GAXRD). The blue shift observed in UV-vis absorption edge of the irradiated films indicates nanostructure formation. Electron spin resonance (ESR) studies are carried out to identify defects created by the irradiation. The nanocrystallisation induced by SHI irradiation in polycrystalline thin films is studied.     

Solar parameters of induced WO3-coated glass

WO₃ films were prepared by sol-gel deposition process on Corning 2947, microscope slide substrates. The effects of irradiation on the solar parameters of WO₃ films were investigated between ∼1 and 21 kGy absorbed dose by Co-60 radioisotope. Three characteristic optical density bands explained the causes of color due to the absorption of sunlight at induced color centers of the transition elements such as W, Fe and Zr after the gamma irradiation. These bands lead to variations on the solar control in terms of shading coefficient. The absorbed dose plays a key role in the improvement of the shading coefficient dramatically, hence the solar parameters changed considerably depending on the induced color centers of the transition elements and the variations of the grains and the void sizes of the film. The results of the solar parameters of irradiated WO₃ films were compared with the unirradiated WO₃ films and uncoated corning in the solar range.     

Evaluation of melting point of UO2 by molecular dynamics simulation

The melting point of UO₂ has been evaluated by molecular dynamics simulation (MD) in terms of interatomic potential, pressure and Schottky defect concentration. The Born-Mayer-Huggins potentials with or without a Morse potential were explored in the present study. Two-phase simulation whose supercell at the initial state consisted of solid and liquid phases gave the melting point comparable to the experimental data using the potential proposed by Yakub. The heat of fusion was determined by the difference in enthalpy at the melting point. In addition, MD calculations showed that the melting point increased with pressure applied to the system. Thus, the Clausius-Clapeyron equation was verified. Furthermore, MD calculations clarified that an addition of Schottky defects, which generated the local disorder in the UO₂ crystal, lowered the melting point.     

Characteristics of the electron-emission defects introduced in Si-SiO2 structures by MeV electron irradiation

Defects induced by high-energy electrons in Si-SiO₂ structure have been studied by the optically stimulated electron emission (OSEE) method. Si-SiO₂ structures with oxide thickness of 100 nm are irradiated with 23 MeV electrons for different durations. It is shown that most of the defects created by electron irradiation at the interface and in the oxide bulk are vacancies like E′-centers. Most of the photoemission activity changes are observed during low doses electron irradiation. Some uncharged defects like diamagnetic oxygen-deficient centers are also observed, together with E′-centers.     

Characterization of ThO2-UO2 pellets made by co-precipitation process

The co-precipitation technique renders an excellent route to obtain a homogeneous mixture of ThO₂ and UO₂ powders. In this process, after the nitrate solutions of Th and U are mixed in the intended ratio, oxalic acid is added for co-precipitation. The precipitate is then dried and calcined to get a solid solution of ThO₂ and UO₂. In this study, ThO₂-30%UO₂ and ThO₂-50%UO₂ (% in weight) powders were characterized in terms of particle size, particle shape, surface area, phase content, O/M ratio etc. The pellets obtained by sintering these powders were characterized with the help of optical microscopy, scanning electron microscopy (SEM) and electron probe microanalysis (EPMA). The XRD data for ThO₂-30%UO₂ and ThO₂-50%UO₂ pellets showed the presence of a small amount of U₃O₈ phase besides fluorite phase. The grain size of ThO₂-30%UO₂ and ThO₂-50%UO₂ was found to be 5.7 and 4.5 μm, respectively.     

Irradiation effect on dielectric properties and electrical conductivity of Au/SiO2/n-Si (MOS) structures

The Au/SiO₂/n-Si (MOS) structures were exposed to beta-ray irradiation to a total dose of 30 kGy at room temperature. Irradiation effect on dielectric properties of MOS structures were investigated using capacitance−voltage (C−V) and conductance−voltage (G/ω−V) characteristics. The C−V and G/ω−V measurements carried out in the frequency range from 1 kHz to 10 MHz and at various radiation doses, while the dc voltage was swept from positive bias to negative bias for MOS structures. The dielectric constant (ε′), dielectric loss (ε″), loss factor (tan δ) and ac electrical conductivity (σ_ac) were calculated from the C−V and G/ω−V measurements and plotted as a function of frequency at various radiation doses. A decrease in the ε′ and ε″ were observed when the irradiation dose increased. The decrease in the ε′ and ε″ of irradiated MOS structures in magnitude is explained on the basis of Maxwell−Wagner interfacial polarization. Also, the σ_ac is found to decrease with increasing radiation dose. In addition, the values of the tan δ decrease with increasing radiation dose and give a peak. From the experimental results, it is confirmed that the peak of loss tangent is due to the interaction between majority carriers and interface states which induced by radiation.     

Theory of He trapping, diffusion, and clustering in UO2

We have performed ab initio total energy calculations to investigate the behavior of helium and its diffusion properties in uranium dioxide (UO₂). Our investigations are based on the density functional theory within the generalized gradient approximation (GGA). The trapping behavior of He in UO₂ has been modeled with a supercell containing 96-atoms as well as uranium and oxygen vacancy trapping sites. The calculated incorporation energies show that for He a uranium vacancy is more stable than an oxygen vacancy or an octahedral interstitial site (OIS). Interstitial site hopping is found to be the rate-determining mechanism of the He diffusion process and the corresponding migration energy is computed as 2.79 eV at 0 K (with the spin-orbit coupling (SOC) included), and as 2.09 eV by using the thermally expanded lattice parameter of UO₂ at 1200 K, which is relatively close to the experimental value of 2.0 eV. The lattice expansion coefficient of He-induced swelling of UO₂ is calculated as 9 × 10⁻². For two He atoms, we have found that they form a dumbbell configuration if they are close enough to each other, and that the lattice expansion induced by a dumbbell is larger than by two distant interstitial He atoms. The clustering tendency of He has been studied for small clusters of up to six He atoms. We find that He strongly tends to cluster in the vicinity of an OIS, and that the collective action of the He atoms is sufficient to spontaneously create additional point defects around the He cluster in the UO₂ lattice.     

Modification of a shell model for the study of the radiation effects in BaTiO3

In order to study the radiation effects in BaTiO₃ ferroelectric crystal, a previously developed shell model is modified. The modifications include adding the ZBL universal potentials at short distances and distance-dependent spring constants for core-shell interactions. The phase transition sequences in BaTiO₃ were correctly reproduced using molecular dynamics simulations with this modified shell model. Also, the calculated Frenkel pair formation energies agree well with results obtained by first principles calculations, which suggests that this model is suitable for the simulation of the radiation effects in BaTiO₃. The dependence of polarization on the number of oxygen vacancies was also studied.     

Temperature accelerated dynamics study of migration process of oxygen defects in UO2

We studied the migration dynamics of oxygen point defects in UO₂ which is the primary ceramic fuel for light-water reactors. Temperature accelerated dynamics simulations are performed for several initial conditions. Though the migration of the single interstitial is much slower than that of the vacancy, clustered interstitial shows faster migration than those. This observation gives us important insight on the formation mechanism of high-burnup restructuring, including planar defects and grain sub-division (the rim structure), found in UO₂.     

Photoluminescence and XEL in Y2O3:Eu phosphor

Eu-activated Y₂O₃ phosphors were prepared by combustion synthesis and also by precipitation techniques. Photoluminescence and X-ray excited luminescence of prepared Y₂O₃:Eu phosphor, under two different techniques were compared and reported in this paper. Y₂O₃:Eu³⁺ phosphor were prepared by precipitation technique followed by annealing at 900 °C. It gives cubic nature of the particle that may be more favourable for high lumen output. X-ray excited luminescence of Y₂O₃:Eu³⁺ phosphors also reported in this paper.     

Strain-free Al0.08In0.018Ga0.902N/GaN heterostructure: Combined Rutherford backscattering/channeling and high resolution X-ray diffraction

A 160 nm Al_0.08In_0.018Ga_0.902N layer was grown by metal-organic chemical vapour deposition (MOCVD) on sapphire (0 0 0 1) with thick (>1 μm) GaN intermediate layer. The chemical compositions can be determined by Rutherford backscattering (RBS). The perpendicular and parallel strain of Al_0.08In_0.018Ga_0.902N layer was derived to be zero by using a combination of high resolution X-ray diffraction (HRXRD) and RBS/channeling. The conclusion is further evidenced by transmission electron microscopy (TEM).     

Improvement on thermoelectric properties of multilayered Si1−xGex/Si by ion beam bombardment

We made n-type nano-scale thin film thermoelectric (TE) devices that consist of multiple periodic layers of Si_1−xGe_x/Si. The period is about 10 nm. The structure was modified by 5 MeV Si ion bombardment that formed a nano-scale cluster structure. In addition to the effect of confinement of the phonon transmission, formation of nanoclusters by the ionization energy of incident MeV Si ions further increases the scattering of phonons, increasing the chance of inelastic interaction of phonons, resulting in more annihilation of phonons. This limits phonon mean free path. Phonons are absorbed and dissipated along the layers rather than in the direction perpendicular to the layer interfaces, therefore cross plane thermal conductivity is reduced. The increase of the density of electronic states due to the formation of nanocluster minibands increases the cross plane Seebeck coefficient and increases the cross plane electric conductivity of the film. Eventually, the thermoelectric figure of merit of the TE film increases.     

Existence states of deuterium irradiated into LiAlO2

The existence states of deuterium in LiAlO₂ were analyzed by in situ IR absorption spectroscopy during irradiation with 3 keV at room temperature. Multiple IR absorption peaks that were related to O-D stretching vibrations were observed, mainly at 2650 cm⁻¹ (O-D_α), 2600 cm⁻¹ (O-D_β), and 2500 cm⁻¹ (O-D_γ). The O-D_α was assigned to the surface O-D. The O-D_β and O-D_γ were interpreted as two distinct O-D states for three candidates: O-D of substitutional D⁺ for Li⁺; O-D of substitutional D⁺ for Al³⁺; and O-D of interstitial D⁺. O-D_β was the dominant O-D state for deuterium irradiated into LiAlO₂, and had higher stability than O-D_γ. Heating after ion irradiation led to the desorption of D₂ and an increase in the intensity of O-D_β, which implies that some of the deuterium irradiated into LiAlO₂ exists in non-O-D states, such as D⁻ captured by F centers.     

Formation of metastable conductive nanowire in a thiospinel compound CuIr2S4 induced by ion irradiation

We observed an increase in the conductivity of a thiospinel compound, CuIr₂S₄, induced by H⁺ and He⁺ irradiation with energies of 1-2 MeV. It was indicated that the metastable conductive phase was produced by electronic excitation due to the ion beam and this phase was similar to the X-ray-induced phase. Conductivity as a function of ion fluence was analyzed by a simple model where the ion-induced change occurred in a cylindrical region around an ion trajectory. The cross-sectional area of the cylinder was obtained by analyzing the conductivity as a function of ion fluence for each ion, and it was found that an impinging ion produced a nanowire in the conductive phase. In addition, the yield of the Ir dimer displacement, which was related to the increase in conductivity, was considerably high. The ion irradiation effect reported in this paper is unique with regard to the high yield and low linear energy transfer (LET) in the formation of the conductive-phase nanowire. Both these unique aspects could be ascribed to the low band-gap energy and strong electron-lattice interaction of this compound.     

Z2 structure and gas-solid effect in the stopping of slow ions

A recent claim by Paul of a systematic gas-solid difference in stopping cross sections for ions such as nitrogen and oxygen in the velocity range v ? v₀ is studied on the basis of existing experimental data. We find that all existing data support the commonly known Z₂ structure which, by and large, follows the valence structure of the target material. Existing experimental evidence is not found to support a specific gas-solid difference in the velocity range under consideration. The possibility of such an effect due to a gas-solid difference in charge state is rejected on theoretical grounds. Data for compound gases and solids are found to be well described by the Bragg additivity rule.We have also studied nitrogen/helium and oxygen/helium stopping ratios which determine the so-called effective-charge ratio. Taking into account the scatter of experimental data, we do not find clear evidence against Northcliffe’s assumption of a stopping ratio independent of Z₂ and common for gases and solids in the considered velocity range, although the absolute value appears too high.     

Interface strain study of thin Lu2O3/Si using HRBS

The interface of thin Lu₂O₃ on silicon has been studied using high-resolution RBS (HRBS) for samples annealed at different temperatures. Thin rare earth metal oxides are of interest as candidates for next generation transistor gate dielectrics, due to their high-k values allowing for equivalent oxide thickness (EOT) of less than 1 nm. Among them, Lu₂O₃ has been found to have the highest lattice energy and largest band gap, making it a good candidate for an alternative high-k gate dielectric. HRBS depth profiling results have shown the existence of a thin (∼2 nm) transitional silicate layer beneath the Lu₂O₃ films. The thicknesses of the Lu₂O₃ films were found to be ∼8 nm and the films were determined to be non-crystalline. Angular scans were performed across the [1 1 0] and [1 1 1] axis along planar channels, and clear shifts in the channeling minimum indicate the presence of Si lattice strain at the silicate/Si interface.     

Hyperfine interactions in the itinerant system UFeGa5

We report ⁶⁹Ga NMR measurements on the itinerant compound UFeGa₅ using a single crystal sample. Hyperfine coupling constants at both the Ga(1) and Ga(2) sites are determined for H∥a and c-axes based on Knight shift versus susceptibility plots. The T-dependence of the spin-lattice relaxation time (T₁) has been determined at both sites for H∥a and c-axes. From these quantities, the magnetic fluctuations are estimated for both axes. The complex nature of the hyperfine interactions is discussed.