Elastic and electronic properties and stability of SrThO3, SrZrO3 and ThO2 from first principles

First-principle calculations in the framework of the full-potential linearized-augmented-plane-wave method (FLAPW, as implemented into the WIEN-2k code) have been performed to understand the structural, elastic, cohesive and electronic properties of the meta-stable cubic strontium thorate SrThO₃. The optimized lattice parameters, elastic parameters, formation energies, densities of states, band structures and charge density distributions are obtained and discussed in comparison with those of cubic SrZrO₃ and ThO₂.     

Ion beam induced desorption from thin films: SiO2 single layers and SiO2/Si multilayers

The occurrence of O₂ molecular loss from the bulk of SiO₂ single layers and SiO₂/Si multilayers as a result of 50 MeV Cu⁹⁺ irradiation has been investigated. This process did not take place with a significant rate, if it occurs at all. Instead both Si and O are removed from the SiO₂ surface region, releasing molecular O₂. If an elemental Si layer is on top in a multilayer, removal of Si and O with an appreciable rate is not observed. The irradiation creates bubbles in the SiO₂/Si multilayers, which contain O₂. The distinct SiO₂ sublayers remain chemically intact. The bubbles deteriorate the depth resolution in elastic recoil detection.     

Effects of MeV Si ions bombardment on the thermoelectric generator from SiO2/SiO2 + Cu and SiO2/SiO2 + Au nanolayered multilayer films

The defects and disorder in the thin films caused by MeV ions bombardment and the grain boundaries of these nanoscale clusters increase phonon scattering and increase the chance of an inelastic interaction and phonon annihilation. We prepared the thermoelectric generator devices from 100 alternating layers of SiO₂/SiO₂ + Cu multi-nano layered superlattice films at the total thickness of 382 nm and 50 alternating layers of SiO₂/SiO₂ + Au multi-nano layered superlattice films at the total thickness of 147 nm using the physical vapor deposition (PVD). Rutherford Backscattering Spectrometry (RBS) and RUMP simulation have been used to determine the stoichiometry of the elements of SiO₂, Cu and Au in the multilayer films and the thickness of the grown multi-layer films. The 5 MeV Si ions bombardments have been performed using the AAMU-Center for Irradiation of Materials (CIM) Pelletron ion beam accelerator to make quantum (nano) dots and/or quantum (quantum) clusters in the multilayered superlattice thin films to decrease the cross plane thermal conductivity, increase the cross plane Seebeck coefficient and cross plane electrical conductivity. To characterize the thermoelectric generator devices before and after Si ion bombardments we have measured Seebeck coefficient, cross-plane electrical conductivity, and thermal conductivity in the cross-plane geometry for different fluences.     

CeO_2/SiO_2吸附剂的制备及其对碘酸根的吸附

以多孔二氧化硅为载体、六水合硝酸铈为原料,采用真空灌注法制备了CeO_2/SiO_2吸附剂,并利用扫描电镜(SEM)、X射线衍射(XRD)、红外光谱(IR)等方法对其进行表征。同时,通过静态和动态吸附实验探究了CeO_2/SiO_2对碘酸根的吸附和脱附行为,考察了pH、反应时间、碘酸根浓度以及共存阴离子等因素对CeO_2/SiO_2吸附性能的影响。研究结果表明:CeO_2/SiO_2在较广的pH值范围内对碘酸根有较高的吸附率,吸附过程符合准二级动力学模型以及Langmuir和Redlich-Peterson模型;共存阴离子对吸附的影响大小为NO_3~-SO_4~(2-)HCO_3~-H_2PO_4~-≈PO_4~(3-);动态吸附实验的柱利用率高达90.96%,且穿透曲线符合Thomas模型;通过IR谱图分析推测CeO_2/SiO_2的吸附机理为阴离子交换。     

Trapping and diffusion of fission products in ThO2 and CeO2

The trapping and diffusion of Br, Rb, Cs and Xe in ThO₂ and CeO₂ have been studied using an Ab Initio total energy method in the local-density approximation of density functional theory. Fission products incorporated in cation mono-vacancy, cation-anion di-vacancy and Schottky defect sites are found to be stable, with the cation mono-vacancy being the preferred site in most cases. In both oxides, Rb and Cs are the most likely to be trapped, and Xe is more difficult to incorporate than other fission products. The energy barriers for migration of each species in ThO₂ and CeO₂ are also calculated. Alkali metals are relatively more mobile than other fission products, and bromine is the least mobile.     

添加Al2O3和SiO2的大晶粒UO2芯块制备研究

研究了Al2O3和SiO2添加剂对UO2芯块晶粒尺寸的影响.结果表明加入少量的Al2O3和SiO2,可有效促进烧结过程中UO2芯块的晶粒度长大,过量加入则会阻碍烧结过程中UO2芯块的致密化;在添加量一定的情况下,添加不同比例的Al2O3和SiO2,对芯块晶粒尺寸有较大影响,只添加SiO2,对芯块晶粒尺寸影响不大,Al2O3添加量增加,芯块晶粒尺寸随之增加;添加Al2O3和SiO2促进UO2芯块晶粒长大的机制是在烧结期间发生了液相烧结.     

Monte Carlo study of ion-induced secondary electron emission from SiO2

Here we describe a recently developed direct Monte Carlo program to study kinetic electron emission from SiO₂ target. The program includes excitation of the target electrons (by projectile ions, recoiling target atoms and fast primary electrons), subsequent transport and escape of these electrons from the target surface. The program can be used to calculate the electron yields, distribution of electron excitation points in the target and other physical parameters of the emitted electrons. In order to demonstrate the capabilities of this program, we report a study on the kinetic electron emission from SiO₂ induced by fast (1-10 keV) rare gas ions. The calculated kinetic electron yield for various ion energies and masses is in good agreement with the predictions of most frequently applied theoretical model. In addition, the effects of projectile energy, mass and impact angle on the depth distribution of electron excitation points and average escape depth of the outgoing electrons were investigated. It is important to mention that the existing experimental techniques are not capable to measure these parameters.     

Defect- and strain-enhanced cavity formation and Au precipitation at nano-crystalline ZrO2/SiO2/Si interfaces

Defect- and strain-enhanced cavity formation and Au precipitation at the interfaces of a nano-crystalline ZrO₂/SiO₂/Si multilayer structure resulting from 2 MeV Au⁺ irradiation at temperatures of 160 and 400 K have been studied. Under irradiation, loss of oxygen is observed, and the nano-crystalline grains in the ZrO₂ layer increase in size. In addition, small cavities are observed at the ZrO₂/SiO₂ interface with the morphology of the cavities being dependent on the damage state of the underlying Si lattice. Elongated cavities are formed when crystallinity is still retained in the heavily-damaged Si substrate; however, the morphology of the cavities becomes spherical when the substrate is amorphized. With further irradiation, the cavities appear to become stabilized and begin to act as gettering sites for the Au. As the cavities become fully saturated with Au, the ZrO₂/SiO₂ interface then acts as a gettering site for the Au. Analysis of the results suggests that oxygen diffusion along the grain boundaries contributes to the growth of cavities and that oxygen within the cavities may affect the gettering of Au. Mechanisms of defect- and strain-enhanced cavity formation and Au precipitation at the interfaces will be discussed with focus on oxygen diffusion and vacancy accumulation, the role of the lattice strain on the morphology of the cavities, and the effect of the binding free energy of the cavities on the Au precipitation.     

Helium diffusion behavior and its retention in LaNiAl alloy from molecular dynamic simulations

Molecular dynamic method has been used to investigate the diffusion behavior of helium atom in the LaNiAl alloy system. The results have shown that diffusion coefficient of helium atom increases as the temperature increases from 300 to 1500 K, which indicates that helium atom gets more active via thermal desorption. The diffusion coefficient in LaNi₅ and LaNi₄Al are about 1.5 × 10⁻⁵ and 1.1 × 10⁻⁶ cm²/s at 1300 K, respectively, and the diffusion barrier of He are 1.45 and 0.52 eV, respectively. The helium diffusion is shown to be enhanced with temperature increasing. Compared with metallic La, Ni, Al and Pd, the simulation result implies that helium atom is more stable and difficult to diffuse in LaNi₅ and LaNi₄Al alloys, which are the most promising materials for helium retention in experiments. Our results indicate the correlation between diffusion behavior of helium atom and capacity of materials for helium retention. Therefore, it can be considered as a feasible method to evaluate the helium retention capacity of materials via determining the diffusion properties of interstitial helium atom.     

Surface desorption and bulk diffusion models of tritium release from Li2TiO3 and Li2ZrO3 pebbles

The release of tritium from Li₂TiO₃ and Li₂ZrO₃ pebbles, in batch experiments, is studied by means of temperature programmed desorption. Data reduction focuses on the analysis of the non-oxidized and oxidized tritium components in terms of release limited by diffusion from the bulk of ceramic grains, or by first or second order surface desorption. By analytical and numerical methods the in-furnace tritium release is deconvoluted from the ionization chamber transfer functions, for which a semi-empirical form is established. The release from Li₂TiO₃ follows second order desorption kinetics, requiring a temperature for a residence time of 1 day (T_1dRes) of 620 K, and 603 K, of the non-oxidized, and the oxidized components, respectively. The release from Li₂ZrO₃ appears as limited by either diffusion from the bulk of the ceramic grains, or by first order surface desorption, the first possibility being the more probable. The respective values of T_1dRes for the non-oxidized component are 661 K, according to the first order surface desorption model, and 735 K within the bulk diffusion limited model.     

Structure and morphology of ion irradiated Au nanocrystals in SiO2

We have investigated the effect of ion irradiation on the structure and morphology of Au nanocrystals (NCs) fabricated by ion beam synthesis in a thin SiO₂ layer on a Si substrate. Extended X-ray absorption fine structure (EXAFS) spectroscopy measurements show a significant drop in the average Au–Au coordination, as well as a loss of medium and long range order with increasing irradiation dose. Small angle X-ray scattering (SAXS) measurements reveal a concomitant reduction in average NC size. These observations are a consequence of structural disorder and collisional mixing induced by the irradiation. The observed reduction in average Au–Au coordination by EXAFS differs significantly from that estimated from the average NC sizes evaluated using SAXS. This behavior can be explained by the dissolution of Au NCs into the SiO₂ matrix. A significant bond-length contraction indicates that part of this material forms small Au clusters (dimers, trimers, etc.) during irradiation that cannot be detected by SAXS. Combining the results from SAXS and EXAFS measurements, we estimate the volume fraction of such clusters.     

Structural studies of Ge nanocrystals embedded in SiO2 matrix

Germanium nanoparticles embedded in SiO₂ matrix were prepared by atom beam sputtering on a p-type Si substrate. The as-deposited films were annealed at temperatures of 973 and 1073 K under Ar + H₂ atmosphere. The as-deposited and annealed films were characterized by Raman, X-ray diffraction and Fourier transform infrared spectroscopy (FTIR). Rutherford backscattering spectrometry was used to quantify the concentration of Ge in the SiO₂ matrix of the composite thin films. The formation of Ge nanoparticles were observed from the enhanced intensity of the Ge mode in the Raman spectra as a function of annealing, the appearance of Ge(3 1 1) peaks in the X-ray diffraction data and the Ge vibrational mode in the FTIR spectra. We have irradiated the films using 100 MeV Au⁸⁺ ions with a fluence of 1 × 10¹³ ions/cm² and subsequently studied them by Raman and FTIR. The results are compared with the ones obtained by annealing.     

Heavy-ion-induced luminescence of amorphous SiO2 during nanoparticle formation

Silica glass was implanted with negative 60 keV Cu ions at an ion flux from 5 to 75 μA/cm² up to a fluence of 1 × 10¹⁷ ions/cm² at initial sample temperatures of 300, 573 and 773 K. Spectra of ion-induced photon emission (IIPE) were collected in situ in the range from 250 to 850 nm. Optical absorption spectra of implanted specimens were ex situ measured in the range from 190 to 2500 nm.

IIPE spectra showed a broad band centered around 560 nm (2.2 eV) that was assigned to Cu⁺ solutes. The band appeared at the onset of irradiation, increased in intensity up to a fluence of about 5 × 10¹⁵ ions/cm² and then gradually decreased indicating three stage of the ion beam synthesis of nanoclusters: accumulation of implants, nucleation and growth nanoclusters. The IIPE intensity normalized on the ion flux is independent on the ion flux below 20 μA/cm²at higher fluences. The intensity of the band increased with increasing samples temperature, when optical absorption spectra reveal the increase of Cu nanoparticles size.     

Evolution of ion-induced ripple patterns on SiO2 surfaces

The evolution of nanoscale ripple patterns during sub-keV ion sputtering of thermally grown, fused and single crystalline SiO₂ surfaces has been investigated by means of atomic force microscopy. For all three materials, different dependencies of the ripple wavelength and the surface roughness on the ion fluence have been found. Within the Bradley-Harper model of pattern formation, the observed differences are consistent with different amounts of surface and near-surface mass transport by ion-enhanced viscous flow which might result from different surface energies of the SiO₂ specimens.     

Three-dimensional depth profiles of boron ions implanted in SiO2

Silicon dioxide films were implanted at room temperature with boron ions at 7° and 35°. Implantation energies ranged from 20 to 250 keV and the dose was 5×10¹⁴ ions/cm². The depth profiles of ion-implanted boron in SiO₂ were measured using secondary ion mass spectrometry and least-squares fitted to a Pearson distribution. The results demonstrated that the measured depth profiles are well approximated by Pearson distributions, while the experimentally determined range parameters correspond fairly well to the theoretical predictions yielded by the SRIM (stopping and range of ions in matter) Monte Carlo simulation code. The overall differences between the measured and calculated values are 4%, 7%, 15%, 22% and 10%, for projected range, longitudinal range straggling, skewness, kurtosis and transversal range straggling, respectively. Rapid thermal annealing (1050°C for 30 s) of the as-implanted specimens revealed that radiation-enhanced diffusion tends to increase projected range, longitudinal range straggling and transversal range straggling, but decrease (in absolute values) skewness and kurtosis. Notably, the increase in transversal range straggling is smaller than that of longitudinal range straggling.     

Visible photoluminescence of SiO2 implanted with carbon and silicon

The structures formed after sequential implantation of silicon plus carbon in amorphous SiO₂ and annealing presented strong photoluminescence bands in the deep red (1.4–1.6 eV) and green (2.0–2.2 eV) regions of the visible spectrum. The energy and intensity of the bands depended strongly on the temperature and duration of annealing. Different behaviours with post-processing were encountered for the red and green bands, including deexcitation kinetics and structural origin. The FTIR, Raman and HRTEM measurements showed that silicon crystallites were reponsible for the red emitting band while carbon aggregates were probably the origin of the green one.     

Hydrogen release in SiO2: Source sites and release mechanisms

We investigate molecular scale mechanisms for radiation-induced release of hydrogen from precursor sites using density functional theory applied to a fully periodic model of SiO₂. We focus on proton release from H-decorated oxygen vacancies in the bulk oxide. After hole-capture at the vacancy, a proton can hop to an energetically favorable bound state at a neighboring oxygen atom. In -quartz, this release mechanism has an activation energy of about 1.2 eV. In amorphous silica, this hop has a range of low barriers, from 0.1 to 0.5 eV. Furthermore, another proton release mechanism involves cracking of H₂ molecules by a reaction with an isolated, positively charged Si-dangling bond.