Modeling of the coalescence of gas pores during annealing

The kinetics of the coalescence of gas bubbles under the conditions of high-temperature annealing is modeled. The case where the growth of gas pores is controlled by thermal dissolution of gas atoms from bubbles is examined. The modeling is performed by solving the basic kinetic equation numerically. The numerical calculations are compared with the results of an analytical model which describes the asymptotic behavior of the gas pores. The time characteristics of the process which are obtained in this work are at variance with the results of the analytical model. The discrepancies found are discussed.     

Void nucleation, growth, and coalescence in irradiated metals

A novel computational treatment of dense, stiff, coupled reaction rate equations is introduced to study the nucleation, growth, and possible coalescence of cavities during neutron irradiation of metals. Radiation damage is modeled by the creation of Frenkel pair defects and helium impurity atoms. A multi-dimensional cluster size distribution function allows independent evolution of the vacancy and helium content of cavities, distinguishing voids and bubbles. A model with sessile cavities and no cluster–cluster coalescence can result in a bimodal final cavity size distribution with coexistence of small, high-pressure bubbles and large, low-pressure voids. A model that includes unhindered cavity diffusion and coalescence ultimately removes the small helium bubbles from the system, leaving only large voids. The terminal void density is also reduced and the incubation period and terminal swelling rate can be greatly altered by cavity coalescence. Temperature-dependent trapping of voids/bubbles by precipitates and alterations in void surface diffusion from adsorbed impurities and internal gas pressure may give rise to intermediate swelling behavior through their effects on cavity mobility and coalescence.     

Interpretation of inert gas release from irradiated solids

This paper considers the interpretation of inert gas release experiments in cases where surface effects are of minor importance. The present study is based on numerical solutions to the general problem of a composite solid with reversible trapping, including the possibility of the implanted ions residing in traps initially. Gas release data reported by Ong and Elleman for Xe implanted CaF₂ are considered. A two-region model consisting of the implanted volume and the sub-implant volume is utilized to show that more than one model can fit the experimental data. For example, using a homogenous trapping model, Ong and Elleman fit one set of release data with trapping and detrapping constants of 9.1 × 10^?4s^?1 and 6.0 × 10^?5s^?1, respectively. Using a composite model with 38% of the implanted gas initially trapped, the corresponding trapping constants are determined to be zero and 10^?6 s^?1.     

The emission process of secondary ions from solids bombarded with large gas cluster ions

We investigated the effects of size and energy of large incident Ar cluster ions on the secondary ion emission of Si. The secondary ions were measured using a double deflection method and a time-of-flight (TOF) technique. The size of the incident Ar cluster ions was between a few hundreds and several tens of thousands of atoms, and the energy up to 60 keV. Under the incidence of keV energy atomic Ar ions, mainly atomic Si ions were detected, whereas Si cluster ions were rarely observed. On the other hand, under the incidence of large Ar cluster ions, the dominant secondary ions were  (2 ? n ? 11). It has become clear that the yield ratio of secondary Si cluster ions was determined by the velocity of the incident cluster ions, and this strong dependence of the yield ratio on incident velocity should be related to the mechanisms of secondary ion emission under large Ar cluster ion bombardment.     

The relaxation of strong electron excitations in solids induced by multicharged ion bombardment

The relaxation of electron excitations arising from irradiation by multicharged ions of metals and insulators is considered. The excitation processes in solids are described. In metals, the relaxation is determined by the electron thermal conductivity, and electron energy transfer to the lattice is small. In insulators, the electron excitation relaxation is dependent on the low temperature ionization wave, the electron heat is localized, and lattice atoms receive enough energy for melting. The electron-phonon interaction at high electron temperatures and lattice relaxation is considered.     

The effects of dilution gas on nanoparticle growth in atmospheric-pressure acetylene microdischarges

A two-dimensional multi-fluid model is developed to investigate the effects of dilution gas on microplasma properties and nanoparticle behavior in atmospheric-pressure radio-frequency acetylene discharges. The percentage of dilution gases (argon and helium) percentage varied from 0% to 90%, with the pressure kept constant. Simulation results show that the dilution gas percentage has a significant influence on the spatial distributions of the electron density and temperature, as well as on the formation of nanoparticles in acetylene microplasmas. With increasing dilution gas percentage, the electron density profile changes continuously from being high at the edge to high in the center. A mode transition from a mixed discharge mode with both α regime and drift-ambipolar regime into α regime occurs, which is associated with a sudden decrease in the electron density of the presheaths and an increase in the electron temperature of the bulk plasma. The mode transition point corresponds to the lowest number density ratio of hydrocarbon ions to acetylene. The highest number density ratio is observed at a dilution percentage of 90%, and causes more effective nucleation and coagulation of nanoparticles. Furthermore, owing to the high ionization potential of helium, the transition point moves to a larger dilution gas percentage in ${{rm{C}}}_{2}{{rm{H}}}_{2}$/He microplasmas. Finally, the growth of nanoparticles via coagulation is studied.     

Grain boundaries as vacancy sources during the growth of creep voids and gas bubbles

Irradiation-induced creep embrittlement of metals and alloys may be a consequence of the diffusional growth, and eventual impingement, of intergranular fission gas bubbles. It is proposed that a suitable array of intergranular precipitates will prevent the grain boundaries acting as vacancy sources and thus inhibit the growth of the gas bubbles and prevent them evolving into creep voids. A model is developed enabling an estimate to be made of the size and distribution of precipitates necessary to prevent this form of embrittlement. It is also suggested that a similar mechanism is responsible for the inhibition, by precipitates, of fission gas swelling in irradiated uranium.     

The effects of gas flow pattern on the generation of ozone in surface dielectric barrier discharge

Ozone production utilizing surface dielectric barrier discharge (SDBD) was experimental studied for different flow patterns considering the influences of transversal flow, lateral flow and different lateral flow positions. Results show that the flow patterns have a remarkable impact on the ozone yield by affecting the uniformity and turbulence of gas flow. Meanwhile, distributing the O₂ flow rate according to the intensity of the plasma reaction would also increase the generation efficiency of SDBD for ozone production. By improving the uniformity and introducing the lateral flow to the transversal flow, the highest ozone yield was obtained in flow pattern ‘F’. In this case, the ozone yield increased by 28.4% to 131 gkWh ⁻¹ from 102.8 gkWh⁻¹ in flow pattern ‘A’.     

The possible role of anisotropy in kinetic electronic excitation of solids by particle bombardment

The kinetic excitation of a solid surface by impact of energetic particles is investigated by means of internal electron emission across a buried metal-insulator-metal (MIM) tunnel junction. By bombarding the top metal surface of such a device with keV noble gas ions, internal emission yields were determined as a function of projectile impact energy and angle of incidence with respect to the surface normal. In order to understand the observed impact angle dependence, we apply a modified formalism originally published to describe external electron emission. As a result, we find that the measured data can be explained by assuming the spatial distribution of excited electrons propagating towards the buried oxide interface to be strongly influenced by the projectile impact angle. A simple ballistic model assuming excited electrons generated by direct collisions with the projectile to preferably propagate along the direction of the original projectile motion, while electrons excited by scattering from moving recoils propagate isotropically, appears to describe the observed experimental data quite well.     

The effects of irradiation on solids

A short survey is given of the reports on the effects of radiation on solids that were given at the Second International Conference on the Peaceful Use of Atomic Energy (Geneva, 1958). Consideration is given to the experimental work devoted to quantitative estimates of radiation damage, and also to questions of the action of neutron radiation on fissionable materials (uranium, plutonium, and certain alloys of these metals). New data are presented on the effect of very high burnups (up to 2 atom per cent) on the size and shape of units made of uranium and its alloys, and also on the increase of volume (swelling) of uranium units under the action of radiation. Detailed attention is also given to data on studies of uranium alloy with 9 per cent of molybdenum by weight, and also of pure uranium, which give evidence of high mobility of the atoms in uranium and its alloys under irradiation. Data are presented on the effects of temperature and of radiation dosage, and also on a number of other factors, on the mechanical properties of steels and other construction materials. Results are presented from studies of the effects of irradiation on nonmetallic materials: BeO, UO₂-BeO and UO₂ThO₂ mixtures, graphite, and so on.     

The generation of shallow dopant layers by low-angle implantation

A simple arrangement for the generation of shallow dopant profiled by low-angle ion implantation is described. High resolution Rutherford backscattering has been employed to measure profiles of arsenic and antimony which have been obtained by implantation into (100) silicon at angles of incidence as low as 4° with respect to the wafer surface. These profiles have been compared with Monte Carlo calculations using the TRIM II code and found to be in good agreement. Electrical activity > 5 × 10¹⁹ cm^?3 has been achieved for dopant profiles with peak concentrations within 30 Å of the surface. The technique is amenable to the generation of tailored (e.g. uniform) implantation profiles at constant energy by variation of implant angle.     

The release of fission gas from nuclear fuels during temperature transients

The fractional release of rare gas atoms from a spherical grain of uranium dioxide containing a uniform concentration of gas atoms and intragranular bubbles is calculated for short-duration temperature transients and post-irradiation annealing. The released fraction is shown to reach a small maximum value which is dependent only on the grain size, gas atom concentration and intragranular bubble distribution. The analysis therefore may be applied to any fuel material providing these parameters are known. The conclusion therefore is that during a brief temperature transient, only a very small fraction of gas will be released from grain interiors to grain boundaries. Consequently the majority of gas released into the free volume of a pin will come from gas already residing on grain boundaries and released by mechanical cracking; the fractional gas release is small and should not cause undue concern. The calculations do not cover the case of fuel melting.     

Dissolution of uranium metal without hydride formation or hydrogen gas generation

This study shows that metallic uranium will cleanly dissolve in carbonate-peroxide solution without generation of hydrogen gas or uranium hydride. Metallic uranium shot, 0.5–1 mm diameter, was reacted with ammonium carbonate–hydrogen peroxide solutions ranging in concentration from 0.13 M to 1.0 M carbonate and 0.50 M to 2.0 M peroxide. The dissolution rate was calculated from the reduction in bead mass, and independently by uranium analysis of the solution. The calculated dissolution rate ranged from about 4 × 10⁻³ to 8 × 10⁻³ mm/h, dependent primarily on the peroxide concentration. Hydrogen analysis of the etched beads showed that no detectable hydrogen was introduced into the uranium metal by the etching process.     

Ion beam induced defects in solids studied by optical techniques

Optical methods can provide important insights into the mechanisms and consequences of ion beam interactions with solids. This is illustrated by four distinctly different systems.X- and Y-cut LiNbO₃ crystals implanted with 8 MeV Au³⁺ ions with a fluence of 1 × 10¹⁷ ions/cm² result in gold nanoparticle formation during high temperature annealing. Optical extinction curves simulated by the Mie theory provide the average nanoparticle sizes. TEM studies are in reasonable agreement and confirm a near-spherical nanoparticle shape but with surface facets. Large temperature differences in the nanoparticle creation in the X- and Y-cut crystals are explained by recrystallisation of the initially amorphised regions so as to recreate the prior crystal structure and to result in anisotropic diffusion of the implanted gold.Defect formation in alkali halides using ion beam irradiation has provided new information. Radiation-hard CsI crystals bombarded with 1 MeV protons at 300 K successfully produce F-type centres and V-centres having the structure as identified by optical absorption and Raman studies. The results are discussed in relation to the formation of interstitial iodine aggregates of various types in alkali iodides. Depth profiling of and aggregates created in RbI bombarded with 13.6 MeV/A argon ions at 300 K is discussed.The recrystallisation of an amorphous silicon layer created in crystalline silicon bombarded with 100 keV carbon ions with a fluence of 5 × 10¹⁷ ions/cm² during subsequent high temperature annealing is studied by Raman and Brillouin light scattering.Irradiation of tin-doped indium oxide (ITO) films with 1 MeV protons with fluences from 1 × 10¹⁵ to 250 × 10¹⁵ ions/cm⁻² induces visible darkening over a broad spectral region that shows three stages of development. This is attributed to the formation of defect clusters by a model of defect growth and also high fluence optical absorption studies. X-ray diffraction studies show evidence of a strained lattice after the proton bombardment and recovery after long period storage. The effects are attributed to the annealing of the defects produced.