A.A. Turkin

On the dynamic roughening transition in nanocomposite film growth

Surface roughness and dynamic growth behavior of TiC/a-C nanocomposite films deposited by nonreactive pulsed-dc (p-dc) magnetron sputtering were studied using atomic force microscopy, cross-sectional scanning, and transmission electron microscopy. From detailed analyses of surface morphology and growth conditions, it is concluded that a transition in growth mechanisms occurs, i.e., a mechanism dominated by geometric shadowing at a p-dc frequency of 100 kHz evolving to a surface diffusion mechanism driven by impact-induced atomistic downhill flow process by Ar+ ions at a p-dc frequency of 350 kHz. It is shown that rapid smoothening of initially rough surfaces with rms roughness from ∼6 to <1 nm can be effectively achieved with p-dc sputtering at 350 kHz pulse frequency, leading to a transition from a strong columnar to a columnar-free microstructure.

Theory of the late stage of radiolysis of alkali halides

Abstract Recent results on heavily irradiated natural and synthetic NaCl crystals give evidence for the formation of large vacancy voids, which were not addressed by the conventional Jain–Lidiard model of radiation damage in alkali halides. This model was constructed to describe metal colloids and dislocation loops formed in alkali halides during earlier stages of irradiation. We present a theory based on a new mechanism of dislocation climb, which involves the production of VF centers (self-trapped hole neighboring a cation vacancy) as a result of the absorption of excess H centers. Voids are shown to arise due to the reaction between F and VF centers at the surface of halogen bubbles. Critical parameters associated with the bubble-to-void transition are evaluated. Voids can grow to sizes exceeding the mean distance between colloids and bubbles, eventually absorbing them, and, hence, igniting a back reaction between the halogen gas and metal. The amount of radiation damage in alkali halides should be evaluated with account of void formation, which strongly affects the radiation stability of material.

A new mechanism for radiation damage processes in alkali halides

We present a theory of radiation damage formation in alkali halides based on a new mechanism of dislocation climb, which involves the production of VF centers (self-trapped hole neighboring a cation vacancy) as a result of the absorption of H centers of dislocation lines. We consider the evolution of all experimentally observed extended defects: metal colloids, gas bubbles, and vacancy voids. Voids are shown to arise and grow large due to the reaction between F and VF centers at the surface of halogen bubbles. Voids can ignite a back reaction between the radiolytic products resulting in decomposition of the irradiated material.

Surface roughness evolution of nanocomposite thin films

An analysis of dynamic roughening and smoothening mechanisms of thin films grown with pulsed-dc magnetron sputtering is presented. The roughness evolution has been described by a linear stochastic equation, which contains the second- and fourth-order gradient terms. Dynamic smoothening of the growing interface is explained by ballistic effects resulting from impingements of ions to the growing thin film. These ballistic effects are sensitive to the flux and energy of impinging ions. The predictions of the model are compared with experimental data, and it is concluded that the thin film roughness can be further controlled by adjusting waveform, frequency, and width of dc pulses.

A kinetic model of zircon thermoluminescence

Abstract A kinetic model of zircon thermoluminescence (TL) has been constructed to simulate the processes and stages relevant to thermoluminescent dating such as: filling of electron and hole traps during the excitation stage both for natural and laboratory irradiation; the time dependence of fading after laboratory irradiation; TL experiments both after laboratory and natural irradiation. The goal is to inspect qualitative behavior of the system and to unravel the processes and determine the parameters controlling TL phenomena of zircon. The input parameters of the model, such as types and concentrations of the TL centers and energy distributions of the hole and electron traps, were obtained by analyzing the experimental data on fading of the TL-emission spectra of samples from different locations. EPR data were used to establish the nature of the TL centers. Glow curves and 3D TL emission spectra are simulated and compared with the experimental data on time-dependent TL fading. Theoretical dating curves for combined natural plus laboratory irradiation have been calculated for as-irradiated, faded and preheated samples.

Thermoluminescence of zircon: a kinetic model

The mineral zircon, ZrSiO4, belongs to a class of promising materials for geochronometry by means of thermoluminescence (TL) dating. The development of a reliable and reproducible method for TL dating with zircon requires detailed knowledge of the processes taking place during exposure to ionizing radiation, long-term storage, annealing at moderate temperatures and heating at a constant rate (TL measurements). To understand these processes one needs a kinetic model of TL. This paper is devoted to the construction of such a model. The goal is to study the qualitative behaviour of the system and to determine the parameters and processes controlling TL phenomena of zircon. The model considers the following processes: (i) Filling of electron and hole traps at the excitation stage as a function of the dose rate and the dose for both (low dose rate) natural and (high dose rate) laboratory irradiation. (ii) Time dependence of TL fading in samples irradiated under laboratory conditions. (iii) Short time annealing at a given temperature. (iv) Heating of the irradiated sample to simulate TL experiments both after laboratory and natural irradiation. The input parameters of the model, such as the types and concentrations of the TL centres and the energy distributions of the hole and electron traps, were obtained by analysing the experimental data on fading of the TL-emission spectra of samples from different geological locations. Electron paramagnetic resonance (EPR) data were used to establish the nature of the TL centres. Glow curves and 3D TL emission spectra are simulated and compared with the experimental data on time-dependent TL fading. The saturation and annealing behaviour of filled trap concentrations has been considered in the framework of the proposed kinetic model and compared with the EPR data associated with the rare-earth ions Tb3+ and Dy3+, which play a crucial role as hole traps and recombination centres. In addition, the behaviour of some of the SiOmn- centres M has been compared with simulation results.

Recombination mechanism of point defect loss to coherent precipitates in alloys under irradiation

Abstract A new mechanism of defect loss by enhanced recombination inside coherent precipitates in alloys under irradiation is described. The mechanism is examined quantitatively to find the microstructural parameters responsible for resistance to dimensional instability. The proposed model explains why radiation properties of Zr–Nb alloys depend on density of fine-grained precipitates of βNb-phase.

New mechanism for radiation defect production and aggregation in crystalline ceramics

In many ceramic solids, the number of primary displaced ions is different for different sublattice components, either because the ion masses and displacement energies differ in simple binary collisions (like in alumina) or because radiolytic displacements occur on a single sublattice (like in halides). However, irradiation produces not only metal colloids or gas precipitates, but stoichiometric dislocation loops, and voids as well. We propose a secondary displacement mechanism of vacancy production at a dislocation as a result of its interaction with a primary interstitial ion in another sublattice which explains the observed phenomena.

On the effect of radiation-induced segregation on void shape and growth rate

Abstract Microstructural changes have been studied in Cr18Ni10Ti steel irradiated in BOR-60 up to 40 dpa at 580–600 °C. It was found that in the vicinity of the voids associated with G-phase particles the composition of steel is practically the same as the average matrix one, whereas the isolated voids are surrounded by Ni and Si-enriched zones. The shape and size of a void depend on local composition near it — the mean size of octahedral voids (associated with G-phase particles) is greater than that of isolated cubic voids. A theoretical model is developed to explain the observed difference between growth rates of free and precipitate-attached voids.

Monte Carlo simulation of cascade-induced mixing in two-phase systems

Abstract A model of low temperature transport due to atomic mixing in displacement cascades is formulated. In this model details of the structure of a cascade, as known from recent molecular dynamics (MD) calculations, are taken explicitly into account. Using this information in the lattice Monte Carlo (LMC) simulation of overlapping cascades, the long range transport of tracer atoms as well as the destruction of precipitates by high fluence irradiations at low temperature is modelled.