Study of silver diffusion in silicon carbide

Diffusion of silver in 6H-SiC and polycrystalline CVD-SiC was investigated using α-particle channeling spectroscopy and electron microscopy. Fluences of 2 × 10¹⁶ cm⁻² of ¹⁰⁹Ag⁺ were implanted with an energy of 360 keV at room temperature, at 350 °C and 600 °C, producing an atomic density of approximately 2% at the projected range of about 110 nm. The broadening of the implantation profile and the loss of silver through the front surface during vacuum annealing at temperatures up to 1600 °C was determined. Fairly strong silver diffusion was observed after an initial 10 h annealing period at 1300 °C in both polycrystalline and single crystalline SiC, which is mainly due to implant induced radiation damage. After further annealing at this temperature no additional diffusion took place in the 6H-SiC samples, while it was considerably reduced in the CVD-SiC. The latter was obviously due to grain boundary diffusion and could be described by the Fick diffusion equation. Isochronal annealing of CVD-SiC up to 1400 °C exhibited an Arrhenius type temperature dependence, from which a frequency factor D_o ∼ 4 × 10⁻¹² m² s⁻¹ and an activation energy E_a ∼ 4 × 10⁻¹⁹ J could be extracted. Annealing of 6H-SiC above 1400 °C shifted the silver profile without any broadening towards the surface, where most of the silver was released at 1600 °C. Electron microscopy revealed that this process was accompanied by significant re-structuring of the surface region. An upper limit of D < 10⁻²¹ m² s⁻¹ was estimated for 6H-SiC at 1300 °C.     

Atom probe tomography characterization of radiation-sensitive KS-01 weld

The microstructure of a radiation-sensitive KS-01 test weld has been characterized by atom probe tomography. The levels of copper, manganese, nickel and chromium in this weld were amongst the highest of all the steels used in Western reactor pressure vessels. After neutron irradiation to a fluence of 0.8 × 10²³ n m⁻² (E>1 MeV) at a temperature of 288 °C, this weld exhibited a large Charpy T_41J shift of 169 K, a large shift of the fracture toughness transition temperature of 160 K, a decrease in upper shelf energy from 118 to ∼78 J, and an increase in the yield strength from 600 to 826 MPa. However, the mechanical properties data conformed to the master curve. Atom probe tomography revealed a high number density (∼3 × 10²⁴ m⁻³) of Cu-, Mn-, Ni-, Si- and P-enriched precipitates and a lower number density (∼1  × 10²³ m⁻³) of P clusters.     

Characterization of defect accumulation in neutron-irradiated Mo by positron annihilation spectroscopy

Positron annihilation lifetime spectroscopy measurements were performed on neutron-irradiated low carbon arc cast Mo. Irradiation took place in the high flux isotope reactor, Oak Ridge National Laboratory, at a temperature of 80 ± 10 °C. Neutron fluences ranged from 2 × 10²¹ to 8 × 10²⁴ n/m² (E > 0.1 MeV), corresponding to displacement damage levels in the range from 7.2 × 10⁻⁵ to 2.8 × 10⁻¹ displacements per atom (dpa). A high density of submicroscopic cavities was observed in the neutron-irradiated Mo and their size distributions were estimated. Cavities were detected even at a very low-dose of ∼10⁻⁴ dpa. The average size of the cavities did not change significantly with dose, in contrast to neutron-irradiated bcc Fe where cavity sizes increased with increasing dose. It is suggested that the in-cascade vacancy clustering may be significant in neutron-irradiated Mo, as predicted by molecular dynamics simulations.     

Evolution of the nanostructure of VVER-1000 RPV materials under neutron irradiation and post irradiation annealing

A high nickel VVER-1000 (15Kh2NMFAA) base metal (1.34 wt% Ni, 0.47% Mn, 0.29% Si and 0.05% Cu), and a high nickel (12Kh2N2MAA) weld metal (1.77 wt% Ni, 0.74% Mn, 0.26% Si and 0.07% Cu) have been characterized by atom probe tomography to determine the changes in the microstructure during neutron irradiation to high fluences. The base metal was studied in the unirradiated condition and after neutron irradiation to fluences between 2.4 and 14.9 × 10²³ m⁻² (E > 0.5 MeV), and the weld metal was studied in the unirradiated condition and after neutron irradiation to fluences between 2.4 and 11.5 × 10²³ m⁻² (E > 0.5 MeV). High number densities of ∼2-nm-diameter Ni-, Si- and Mn-enriched nanoclusters were found in the neutron irradiated base and weld metals. No significant copper enrichment was associated with these nanoclusters and no copper-enriched precipitates were observed. The number densities of these nanoclusters correlate with the shifts in the ΔT_{41 J} ductile-to-brittle transition temperature. These nanoclusters were present after a post irradiation anneal of 2 h at 450 °C, but had dissolved into the matrix after 24 h at 450 °C. Phosphorus, nickel, silicon and to a lesser extent manganese were found to be segregated to the dislocations.     

Volume conservation during irradiation growth of Zr-2.5Nb

Dimensional changes are reported in three dimensions for cold-worked Zr-2.5 Nb pressure tube material irradiated to a fast fluence of 174 × 10²⁴ nm⁻², E > 1 MeV at a nominal temperature of 250 °C. The observed dimensional changes in the longitudinal and transverse directions (up to ∼1.2% and ∼−0.5%, respectively) are consistent with earlier data at 280 °C and 310 °C, and the previously reported negative temperature dependence. The observed growth in the radial direction is negative (up to ∼0.7%). Initially, there is a small volume increase (0.05-0.1%) but this gradually decays to < 0.05% and the long term rate of volume change is negligible, within the accuracy of the measurement, demonstrating that the phenomenon observed is, indeed, irradiation growth.     

Permeability of hydrogen and deuterium of Hastelloy XR

Permeation of hydrogen isotope through a high-temperature alloy used as heat exchanger and steam reformer pipes is an important problem in the hydrogen production system connected to be a high-temperature engineering test reactor (HTTR). An experiment of hydrogen (H₂) and deuterium (D₂) permeation was performed to obtain permeability of H₂ and D₂ of Hastelloy XR, which is adopted as heat transfer pipe of an intermediate heat exchanger of the HTTR. Permeability of H₂ and D₂ of Hastelloy XR were obtained as follows. The activation energy E₀ and pre-exponential factor F₀ of the permeability of H₂ were E₀=67.2±1.2 kJ mol⁻¹ and F₀=(1.0±0.2)×10⁻⁸ m³(STP) m⁻¹ s⁻¹ Pa^−0.5, respectively, in the pipe temperature ranging from 843 K (570 °C) to 1093 K (820 °C). E₀ and F₀ of the permeability of D₂ were respectively E₀=76.6±0.5 kJ mol⁻¹ and F₀=(2.5±0.3)×10⁻⁸ m³(STP) m⁻¹ s⁻¹ Pa^−0.5 in the pipe temperature ranging from 943 K (670 °C) to 1093 K (820 °C).     

Formation of silicon hydride using hyperthermal negative hydrogen ions (H) extracted from an argon-seeded hydrogen sheet plasma source

An E × B probe (a modified Wien filter) is constructed to function both as a mass spectrometer and ion implanter. The device, given the acronym EXBII selects negative hydrogen ions (H⁻) from a premixed 10% argon-seeded hydrogen sheet plasma. With a vacuum background of 1.0 × 10⁻⁶ Torr, H⁻ extraction ensues at a total gas feed of 1.8 mTorr, 0.5 A plasma discharge. The EXBII is positioned 3 cm distance from the sheet core as this is the region densely populated by cold electrons (T_e ∼ 2 eV, N_e ∼ 3.4 × 10¹¹ cm⁻³) best suited for H⁻ formation. The extracted H⁻ ions of flux density ∼0.26 A/m² are segregated, accelerated to hyperthermal range (<100 eV) and subsequently deposited into a palladium-coated 1.1 × 1.1 cm², n-type Si (1 0 0) substrate held at the rear end of the EXBII, placed in lieu of its Faraday cup. The palladium membrane plays the role of a catalyst initiating the reaction between Si atoms and H⁻ ions simultaneously capping the sample from oxidation and other undesirable adsorbents. AFM and FTIR characterization tests confirm the formation of SiH₂. Absorbance peaks between 900-970 cm⁻¹ (bending modes) and 2050-2260 cm⁻¹ (stretching modes) are observed in the FTIR spectra of the processed samples. It is found that varying hydrogen exposure time results in the shifting of wavenumbers which may be interpreted as changes in the frequencies of vibration for SiH₂. These are manifestations of chemical changes accompanying alterations in the force constant of the molecule. The sample with longer exposure time exhibits an additional peak at 2036 cm⁻¹ which are hydrides of nano-crystalline silicon.     

Neutron irradiation-induced dimensional changes in MEMS glass substrates

A study is made of radiation-induced expansion/compaction in Pyrex^® (Corning 7740) and Hoya SD-2^® glasses, which are used as substrates for MEMS devices. Glass samples were irradiated with a neutron fluence composed primarily of thermal neutrons, and a flotation technique was employed to measure the resulting density changes in the glass. Transport of Ions in Matter (TRIM) calculations were performed to relate fast (∼1 MeV) neutron atomic displacement damage to that of boron thermal neutron capture events, and measured density changes in the glass samples were thus proportionally attributed to thermal and fast neutron fluences. Pyrex was shown to compact at a rate of (in Δρ/ρ per n/cm²) 8.14 × 10⁻²⁰ (thermal) and 1.79 × 10⁻²⁰ (fast). The corresponding results for Hoya SD-2 were 2.21 × 10⁻²¹ and 1.71 × 10⁻²¹, respectively. On a displacement per atom (dpa) basis, the compaction of the Pyrex was an order of magnitude greater than that of the Hoya SD-2. Our results are the first reported measurement of irridiation-induced densification in Hoya SD-2. The compaction of Pyrex agreed with a previous study. Hoya SD-2 is of considerable importance to MEMS, owing to its close thermal expansivity match to silicon from 25 to 500°C.     

Enthalpy measurements of La2Te3O9 and La2Te4O11

Enthalpy increment measurements on La₂Te₃O₉(s) and La₂Te₄O₁₁(s) were carried out using a Calvet micro-calorimeter. The enthalpy values were analyzed using the non-linear curve fitting method. The dependence of enthalpy increments with temperature was given as: H°(T) − H°(298.15 K) (J mol⁻¹) = 360.70T + 0.00409T² + 133.568 × 10⁵/T − 149 923 (373 ? T (K) ? 936) for La₂Te₃O₉ and H°(T) − H°(298.15 K) (J mol⁻¹) = 331.927T + 0.0549T² + 29.3623 × 10⁵/T − 114 587 (373 ? T (K) ? 936) for La₂Te₄O₁₁.     

The interdiffusion and solid-state reaction of low-energy copper ions implanted in silicon

At room temperature, single-crystal silicon was implanted with Cu⁺ ions at an energy of 80 keV using two doses of 5 × 10¹⁵ and 1 × 10¹⁷ Cu⁺ cm⁻². The samples were heat treated by conventional thermal annealing at different temperatures: 200 °C, 230 °C, 350 °C, 450 °C and 500 °C. The interdiffusion and solid-state reactions between the as-implanted samples and the as-annealed samples were investigated by means of Rutherford backscattering spectrometry (RBS) and X-ray diffraction (XRD). After annealing at 230 °C, the XRD results of the samples (subject to two different doses) showed formation of Cu₃Si. According to RBS, the interdiffusion between Cu and Si atoms after annealing was very insignificant. The reason may be that the formation of Cu₃Si after annealing at 230 °C suppressed further interdiffusion between Si and Cu atoms.     

Structural studies of silicon oxynitride layers formed by low energy ion implantation

Silicon oxynitride (Si_xO_yN_z) layers were synthesized by implanting ¹⁶O₂⁺ and ¹⁴N₂⁺ 30 keV ions in 1:1 ratio with fluences ranging from 5 × 10¹⁶ to 1 × 10¹⁸ ions cm⁻² into single crystal silicon at room temperature. Rapid thermal annealing (RTA) of the samples was carried out at different temperatures in nitrogen ambient for 5 min. The FTIR studies show that the structures of ion-beam synthesized oxynitride layers are strongly dependent on total ion-fluence and annealing temperature. It is found that the structures formed at lower ion fluences (∼1 × 10¹⁷ ions cm⁻²) are homogenous oxygen-rich silicon oxynitride. However, at higher fluence levels (∼1 × 10¹⁸ ions cm⁻²) formation of homogenous nitrogen rich silicon oxynitride is observed due to ion-beam induced surface sputtering effects. The Micro-Raman studies on 1173 K annealed samples show formation of partially amorphous oxygen and nitrogen rich silicon oxynitride structures with crystalline silicon beneath it for lower and higher ion fluences, respectively. The Ellipsometry studies on 1173 K annealed samples show an increase in the thickness of silicon oxynitride layer with increasing ion fluence. The refractive index of the ion-beam synthesized layers is found to be in the range 1.54-1.96.     

Natural and anthropogenic U in environmental samples

The interaction of thermal neutrons with ²³⁵U results in fission with a probability of ∼85% and in the formation of ²³⁶U (t_1/2 = 2.3 × 10⁷ yr) with a probability of ∼15%. While anthropogenic ²³⁶U is, therefore, present in spent nuclear fuel at levels of ²³⁶U/U up to 10⁻², the expected natural ratios in the pre-anthropogenic environment range from 10⁻¹⁴ to 10⁻¹⁰. At VERA, systematic investigations suggest a detection limit below ²³⁶U/U = 5 × 10⁻¹² for samples of 0.5 mg U, while chemistry blanks of ∼2 × 10⁷ atoms ²³⁶U per sample limit the sensitivity for smaller samples. We have found natural isotopic ratios in uranium reagents separated before the onset of human nuclear activities, in uranium ores from various origins and in water from a subsurface well in Bad Gastein, Austria. Anthropogenic contamination was clearly visible in soil and rivulet samples from Salzburg, Austria, whereas river sediments from Garigliano river (Southern Italy) were close to the detection limit. Finally, our natural in-house standard Vienna-KkU was calibrated against a certified reference material (IRMM REIMEP-18 A).     

The materials test station: A fast-spectrum irradiation facility

The United States Department of Energy is developing technologies needed to reduce the quantity of high-level nuclear waste bound for deep geologic disposal. Central to this mission is the development of high burn-up fuel with significant inclusion of plutonium and minor actinides. Different fuel forms (e.g., nitrides, oxides, and metal matrix) and composition are under study. The success of these cannot be judged until they have been irradiated and tested in a prototypic fast neutron spectrum environment. In 2005, the US Congress authorized funding for the design of the materials test station (MTS) to perform candidate fuels and materials irradiations in a neutron spectrum similar to a fast reactor spectrum. The MTS will use a 1-MW proton beam to generate neutrons through spallation reactions. The peak neutron flux in the irradiation region will exceed 1.2 × 10¹⁹ n m⁻² s⁻¹ and the fast neutron fluence will reach 2 × 10²⁶ n m⁻² per year of operation. Site preparation and test station fabrication are expected to take four years.     

Hydrogen diffusive transport parameters in W coating for fusion applications

Combined Magnetron Sputtering and Ion Implantation (CMSII) technology was used for W coating of carbon based materials (Carbon Fibre Composite - CFC and fine grain graphite) for the first wall in fusion devices. The coating thickness was 10-15 μm or 20-25 μm depending on the position of the tile at the wall. While such coatings successfully passed the demanding thermo-mechanical tests, not much is known about its hydrogen interaction. The latter is particularly important for the assessment of tritium retention. Due to the low hydrogen diffusivity and very small volume of W in the coated layer, the gaseous hydrogen permeation measurement at 400 °C was selected for the experimental technique, where increasing & decreasing transient and steady state permeation flux was monitored. Problems that could arise with the CFC membrane sealing were overcome by deposition of the identical W layer on the 0.5 mm Eurofer substrate. Two such membranes were investigated. Obtained hydrogen permeability in tungsten layer (∼10⁻¹³ mol H₂/m s Pa^0.5) is comparable to the upper range of published data. Measured diffusivity (∼10⁻¹⁴ m²/s) is several orders of magnitude lower compared to the average of published data for tungsten, while the measured solubility (∼1 mol H₂/m³ Pa^0.5) is several orders of magnitude higher. The explanation is given in terms of hydrogen trapping that has significant impact on hydrogen migration.     

Behaviour of chromium steels in liquid Pb-55.5Bi with changing oxygen content and temperature

The behaviour of protective oxide layers on P122 steel and its welds and of ODS steel in liquid Pb_44.5Bi_55.5 (LBE) is examined under conditions of changing temperatures and oxygen concentrations. P122 (12Cr) and its welded joints are exposed to LBE at 550 °C for 4000 h with oxygen concentrations of 10⁻⁶ and 10⁻⁸ wt% (p(O₂) = 8.1 × 10⁻²³ bar and 5.2 × 10⁻²⁷ bar) which change every 800 h. It is found that like in case of constant oxygen concentration of 10⁻⁶ wt% a protective spinel layer (Fe(Fe_1−xCr_x)₂O₄) was maintained on P122 and also on its welded joint. Two experiments with exposure times of 4800 h are conducted on ODS steel, both with temperatures changing from 550 to 650 °C and back every 800 h, one experiment with 10⁻⁶ the other with 10⁻⁸ wt% oxygen in LBE. Both experiments show strong local dissolution attack after 4800 h which is in agreement with the behaviour of ODS in LBE at a constant temperature of 650 °C. However, dissolution attack is less in LBE with 10⁻⁸ wt% oxygen (p(O₂) = 3.0 × 10⁻²⁵ bar).     

Online time-differential perturbed angular correlation study with an O beam - Residence sites of oxygen atoms in highly oriented pyrolytic graphite

The online time-differential perturbed angular correlation (TDPAC) method was applied to a study of the physical states of a probe ¹⁹F, the β⁻ decay product of ¹⁹O (t_1/2 = 26.9 s), implanted in highly oriented pyrolytic graphite. The observed magnitude of the electric field gradient at the probe nucleus, ∣V_zz∣ = 2.91(17) × 10²² V m⁻², suggests that the incident ¹⁹O atoms are stabilized at an interlayer position with point group C_3v. Exhibiting observed TDPAC spectra having a clear sample-to-detector configuration dependence, we demonstrate the applicability of the present online method with a short-lived radioactive ¹⁹O beam.     

Formation of InAs nanocrystals in Si by high-fluence ion implantation

We have studied the formation of InAs precipitates with dimensions of several nanometers in silicon by means of As (245 keV, 5 × 10¹⁶ cm⁻²) and In (350 keV, 4.5 × 10¹⁶ cm⁻²) implantation at 500 °C and subsequent annealing at 900 °C for 45 min. RBS, SIMS, TEM/TED, RS and PL techniques were used to characterize the implanted layers. The surface density of the precipitates has been found to be about 1.2 × 10¹¹ cm⁻². Most of the crystallites are from 3 nm to 6 nm large. A band at 1.3 μm has been registered in the low-temperature PL spectra of (As + In) implanted and annealed silicon crystals. The PL band position follows the quantum confinement model for InAs.     

Tungsten behavior under proton flux and high temperature

An experimental study of the physico-chemical behavior of tungsten under severe conditions is presented. High temperatures (1300 ? T ? 2500 K) generated by concentrated solar energy, high vacuum (∼10⁻⁶ hPa) and proton flux (1 keV, ∼10¹⁷ ions m⁻² s⁻¹) have been applied on polycrystalline W samples to simulate expected and also unexpected high heat loads that can occur on the ITER divertor (nominal and accidental conditions). During experiment, in situ measurements are performed and the material degradation, the mass loss kinetics, the characterization of the different species coming from the materials under coupled proton flux and high temperatures and the optical properties (reflectivity) are followed. Material characterization using SEM and XRD was investigated before and after treatment to understand the observed behavior. Bidirectional reflectivity measurements were carried out on the tested samples to explain the surface modifications, between the reference sample, the heated sample and the heated and ion irradiated one that can act on the thermo-radiative properties of tungsten.     

Incidence-angle dependent Cs incorporation in Si during low-energy bombardment: A dynamic computer simulation study

The implantation of Cs atoms in silicon was investigated by dynamic computer simulations using the Monte-Carlo code T-DYN that takes into account the gradual change of the target composition due to the Cs irradiation. The incorporation of Cs atoms was studied for incidence angles ranging from 0° to 85° and for four impact energies (0.2, 0.5, 1 and 3 keV). The total implantation fluences were (1-2) × 10¹⁷ Cs/cm², well above the values required to reach a stationary state. The steady-state Cs surface concentrations exhibit a pronounced dependence on impact angle and energy. At normal incidence, they vary between ∼0.57 (at 0.2 keV) and ∼0.18 (3 keV), but decrease with increasing incidence angle. Under equilibrium, the partial sputtering yield of Si exhibits the typical dependence on incidence angle, first increasing up to a maximum value (at ∼70°-75°) and declining sharply for larger angles. For all irradiation conditions a strongly preferential sputtering of Cs as compared to Si atoms is found, increasing with decreasing irradiation energy (from 4.6 at 3 keV to 7.2 at 0.2 keV) and for nearer-normal incidence.     

Angular dependence of electronic sputtering from HOPG

We have studied the angular distribution of 120 MeV Au ion beam induced sputtering yield for three cases: from crystalline highly oriented pyrolytic graphite (HOPG) for (A) normal and (B) 70° incidence and from (C) amorphous carbon sample for normal incidence. An anisotropic distribution of sputtering is observed for HOPG samples studied with a distribution Y = Acosⁿθ + Bexp[−(θ − μ)²σ²]. Though the over-cosine function dependence is observed for all the cases, the anomalous peak observed at 53° for normal incidence for HOPG sample is found to shift to 73° when the sample is tilted by 20°. No peak is observed in the amorphous carbon sample which further confirms that the anisotropy observed is due to the crystal structure and formation of a pressure pulse. The high exponent of over-cosine distribution of sputtering yield (n = 3.2-3.8) signifies formation of intense pressure pulse induced jet like sputtering.