Generation Rate Of Point Defects Research Articles

Effect of dose rate on the characteristics of dislocation loops in palladium: In-situ TEM analysis during 30 keV H2+ irradiation

Ion irradiation is usually used to simulate neutron irradiation to accelerate the evaluation of the irradiation behavior of reactor materials. However, the validity of using a high damage rate of ion irradiation to simulate a low damage rate of neutron irradiation has always been a controversial topic. Here, the effect of two dose rates (2.94 × 10–6 and 7.35 × 10–5 dpa s–1) on the characteristics and evolution of dislocation loops in palladium was studied in situ during 30 keV H2+ irradiation using transmission electron microscopy. The dose rate obviously affected the nucleation rate and growth rate of dislocation loops, the types (Frank loops or perfect loops) of dislocation loops, and the irradiation hardening and total damage obtained from the product of average loop size and loop density. At the same irradiation dose, a high dose rate would lead to high loop density, small average loop size, low loop growth rate, and low irradiation hardening and damage induced by loops in pure Pd. Meanwhile, it was found for the first time that a high dose rate was beneficial to the generation of perfect dislocation loops. The effect of dose rate was attributed to the different dynamic equilibrium results between the effective generation rate of point defects and their absorption rate by existing sinks. The present results show that the effect of dose rate should be considered when using ion irradiation to simulate neutron irradiation to evaluate the irradiation damage to materials.

Patterning SiC nanoprecipitate in Si single crystals by simultaneous dual- beam ion implantation

b-SiC nanoprecipitates can be patterned in crystalline silicon with an almost monomodal size distri- bution by simultaneous-dual-beam of C ? and Si ? ion implantations at 550 C. Their shape appears as spherical (average diameter *4-5 nm) ,and they are in epitaxial relationship with the crystalline silicon matrix. The narrow size distribution follows the left wing of the carbon dis- tribution where the nuclear ion stopping, and thus the point defect generation rate is largest. This observation allows us to conclude that the induced damage act as sinks for C atoms leading to the SiC nanoprecipitates formation cen- tered at the maximum of the simulated damage distribu- tion. The nuclear reaction analysis, X-ray diffraction, Raman spectroscopy, and transmission electron micros- copy techniques were used to characterize the samples.

Modelling accelerated solid-state diffusion under the action of intensive plastic deformation

A mathematical model of accelerated solid-state diffusion during mechanical alloying (MA) in a binary substitutional system A-B is developed. An individual lamellar particle formed due to fracturing/cold welding during a preliminary stage of MA is considered. Interdiffusion occurs via the vacancy mechanism. During the plastic deformation, jog dragging by moving screw dislocation generates non-equilibrium point defects (vacancies and interstitial atoms), which can diffuse, interact with edge dislocations and recombinate. To evaluate the point defect generation rate, a simple Hirsch-Mott theory is employed. Numerical simulation has been performed for a repeated “deformation-rest” cycle at 100°C using realistic parameter values. The influence of non-equilibrium vacancies on the atomic diffusion is evaluated. It reveals itself via the increase of partial diffusivities of substitutional atoms and through the cross-link terms in the matrix of interdiffusion coefficients. The incoherent phase boundary between parent phases (pure elements) is considered as a sink for non-equilibrium vacancies. Due to interplay of these factors, substantial alloying by solid-state diffusion is observed after a reasonable time of MA (4000 s).

Influence of boron on radiation enhanced diffusion of antimony in delta-doped silicon

The silicon samples used in this work contain a sequence of alternating boron and antimony spikes grown by molecular beam epitaxy. These samples were irradiated with 2.5 MeV protons at elevated temperatures ranging from 580 °C to 830 °C and characterized by secondary-ion mass spectrometry. The energy of the proton beam was chosen such that the generation rate of point defects can be considered as uniform throughout the delta-doped layers. For each sample the boron and the antimony diffusion coefficient are increased under irradiation as compared to their diffusivity in unirradiated areas. A measurable diffusion of antimony is observed in samples containing both boron and antimony spikes even at temperatures as low as 580 °C while a reference sample containing only an antimony spike do not exhibit any radiation enhanced diffusion, even at 830 °C. The boron diffusion coefficient increases as the irradiation temperature increases but the antimony diffusion coefficient decreases for the highest irradiation temperature investigated.

Molecular Dynamic Simulation of High Speed Deformation Effect on Microstructure Change in Thermal Heated Metals

In the present paper computer simulation of high-speed deformation (shock wave propagation) by molecular dynamic method is performed in thin copper sample, having the form of rectangular parallelepiped (10 a ‐ 10 a ‐ 20 a , where a is the lattice constant) with 8000 atoms. On the surfaces Z 0 =0 and Z max =20 a the mirror boundary conditions with rigid walls and the periodic boundary conditions along X and Y directions corresponding to short sides of deformed crystal are used, which allows to investigate the reflection of shock wave from the surfaces in Z direction. The changes of microstructure have been investigated up to 12 ps. The numerical calculations of microstructure changes have been performed here taking into account the effect of thermal heating of crystal lattice before shock wave front. The numerical results show that comparing with the propagation of shock waves under room temperature in thermal heated structure additional displaced atoms (vacancies and interstitials) are produced. The obtained results show that the production of point defects during high-speed deformation is determined by the thermal softening of microstructure and generation rate of point defects very strong increases with an increasing of high speed deformation rate. The peculiarities of microstructure changes in deformed copper are analyzed here at the different initial temperatures and various high-speed deformations (average ion velocities behind shock wave).

Irradiation enhanced diffusion of boron in delta-doped silicon

Two kinds of silicon samples have been used in this work: one containing a sequence of boron spikes and one with a sequence of alternating boron and antimony spikes, both grown by molecular beam epitaxy. These samples were irradiated with 2.5 MeV protons at an elevated temperature ranging from 500 to 830 °C and characterized by secondary-ion-mass spectrometry. The energy of the proton beam was chosen such that the generation rate of point defects can be considered as uniform throughout the delta-doped Si samples. The influence of the sample surface and of the boron concentration (ranging from 5×1015 to 3.2×1018 B/cm3 in the different samples) on the diffusion of boron have been studied in detail. The effect of antimony on boron diffusion has also been analyzed. For each sample, the B diffusion coefficient is increased under irradiation as compared to the B diffusion coefficient in unirradiated areas. This enhancement is dependent on the irradiation temperature, on the position of the boron spike and on the boron concentration. Further, the presence of antimony enhances the boron diffusion during low-temperature proton irradiation.

Influence of boron concentration on the enhanced diffusion observed after irradiation of boron delta-doped silicon at 570°C

A sequence of three boron spikes, grown in silicon by molecular beam epitaxy (MBE) and characterized by secondary-ion mass spectrometry, has been employed to investigate the mechanisms of boron diffusion during irradiation with 2.5 MeV protons at an elevated sample temperature of 570°C. The energy of the proton beam has been chosen so that the generation rate of point defects can be considered as uniform throughout the boron delta-doped silicon. The influence of the B concentration (ranging from 5×10 15 to 3×10 18 cm −3 in different samples) on the enhanced diffusion of boron have been studied in detail. For each concentration the B diffusion coefficient is increased by a factor greater than 5×10 4 under irradiation as compared to the B diffusion coefficient at 570°C in unirradiated areas. In the first approximation this enhancement is independent on the depth position of the boron spike but depends on the boron concentration. The diffusion of boron under irradiation exhibits an exponential behavior. This kind of diffusion has already been reported in the literature for boron diffusion in silicon during oxidation experiments at 600°C. The direct comparison with oxidation experiments where Si self-interstitials are injected in a non-uniform way provides further information on the basics of the boron diffusion mechanisms in silicon.

Self-interstitial migration during ion irradiation of boron delta-doped silicon

Boron delta layers in silicon, grown by molecular beam epitaxy and characterized by the secondary ion mass spectrometry, have been employed to investigate the migration of silicon self-interstitials during irradiation with MeV protons in the 500–850°C temperature range. After growth, the samples were thinned from the backside to a thickness that made them transparent for the proton energies used. As a result, the generation rate of point defects can be considered as essentially uniform throughout the samples. However, the evolution of the boron profiles is almost identical to that observed after injection of self-interstitials caused by thermal oxidation of the samples at elevated temperature. This strongly indicates that the surface acts as a reflective boundary for the migrating self-interstitials or/and an efficient sink for mobile vacancies. Furthermore, higher value of interstitial supersaturation in the near-surface region in proton-irradiated samples is consistent with experimentally detected depth dependence for immobile fraction in boron clusters. Then, activation energy of boron mobilization, (0.9±0.4) eV, was attributed to the dissociation of boron clusters.

The influence of radiation-induced segregation on precipitate stability in austenitic steels

The influence of radiation-induced segregation of major components and silicon in austenitic steels on the stability of M 23C 6, γ' and G phases under irradiation is theoretically studied. The low temperature boundary T L of M 23C 6 carbide stability and the high temperature boundary T H of γ' and G silcides stability are estimated by comparing the steady-state chromium and silicon concentrations at point defect sinks with corresponding solubility limits for these precipitates. It is shown that both T L and T H are decreased with increasing sink density and decreasing point defect generation rate. The value of T L drops with increasing carbon content while the value of T H does with decreasing silicon concentration.

Generation rate of point defects in silicon irradiated by MeV ions

N-type Si samples have been implanted at room temperature wit low doses of 11B, 12C, 16O, 28Si and 74Ge ions. Energies in the MeV range were used, and deep level transient spectroscopy (DLTS) was applied for sample analysis. Irrespective of ion and material type the DLTS spectra are dominated by three levels at ∼ 0.18, ∼ 0.23 and ∼ 0.43 eV below the conduction band edge ( E c ). The concentration of these levels has been determined as a function of ion dose and sample depth. The generation rate of the E c −0.43 eV level exhibits a close proportionality with the elastic energy deposition rate for the different ions while strong deviations from such a dependence are found for the E c −0.23 eV level. Furthermore, evidence is obtained for a substantial surface-enhanced annihilation of migrating monovacancies; for “shallow” defect distributions (peak position ∼ 1 μM) the measured depth profiles profnarrower in shape towards the surface than simulated vacancy profiles while the opposite trend holds for deep distributions (peak position ∼ μM).

Possible Mechanism Of The Stress Evolution And Point Defects Generation During The Solid Phase Epitaxial Silicide Growth.

ABSTRACTIn this paper we present a model of nonequilibrium point defect generation in the silicon substrate during solid state reactions of the surface suicides formation, resulting from the interaction between the substrate and the thin metal film. The model is based on the following principles. The local stress, which is appearing during each act of the suicide molecule (MexSiy) creation at the suicide - silicon interface, relaxes by the generation of ηv point defects. The point defect generation rate (m is the suicide growth rate) has been defined by the mininimization of the system free energy AG, which includes the enthalpy of chemical reaction ΔG the value of the relaxed elastic energyμ is the silicon shear modulus, Ω = (xΩMe+yΩSi) is the combined volume of metal and silicon atoms with stoichiometric coefficients, ΔΩ = (ΩMexSiy - Ω), ΩV is the vacancy volume in the matrix; and the term ΔGd = ηVkTln(C/C0) which takes into account the energy of the solid solution of noninteracting point defects, where C° is an equilibrium vacancy concentration and C is the real vacancy concentration. The estimations show that there is not any essential thermodynamic force which may prevent stress relaxation for any reasonable point defect supersaturation. For this case point defect generation rate may be written as jV = m(ΔΩ/ΩV). For the reactions of the initial phase formation in the Me-Si structures the values of ΔG*el, ηV and jV have been calculated and it has been shown that vacancy concentration can reach the values of 1015 - 1016 cm-3 at the regions nearest to the interface even during initial low temperature stages of the Ni, Pt, Cr suicide formation with the metal atoms are predominant moving species.

On the Amorphization of Silicon by Light Ion Implantation: Ion Flux and Substrate Temperature Effects

AbstractIn this work, we have observed by cross-sectional electron microscopy (XTEM) and high resolution electron microscopy (HREM) the kinetics of silicon amorphization during nitrogen and helium bombardments for various dose rates and substrate temperatures. It is shown that the progression of the cla interfaces can be accurately described by the “Critical Damage Energy Density” (CDED) model for both ions at 100°K. At this temperature however, dose rate effects are unimportant. When increasing the substrate temperature up to 300°K, the amorphization efficiency is lowered. At 300°K dose rate effects are important and we show that for higher dose rates, the efficiency of the amorphization process is improved. In this case, the amorphous layers created by helium implantation are situated in the near surface region in apparent contradiction with damage calculations. The present experimental study demonstrates the competition which exists between the generation rate of point defects and the anihilation rate of these defects. Therefore, it is shown that the position of the a-layers created by light ion implantation at room temperature can be predicted by using a three-dimensional damage energy calculation.

Kinetics of silicon amorphization by N + implantation: Dose rate and substrate temperature effects

We have observed by cross-sectional electron microscopy the kinetics of silicon amorphization by nitrogen implantation as functions of the dose rate and substrate temperature for 40 keV and 20 keV beam energies. It is shown that amorphization occurs through the accumulation of point defects or complexes. The progression of the c-a interface for increasing dose can be accurately described by the critical damage energy density model (CDED) assuming a CDED value of 30 eV atom −1 for a 40 keV implantation at room temperature using a beam current density of 1.6 μA cm −2. When increasing the dose rate, the efficiency of the amorphization process is improved, suggesting that the generation rate of point defects is a crucial parameter in the case of light ion-induced amorphization. It is finally shown that higher dose rates and low substrate temperature are to be used to promote the complete amorphization of the substrate by light ion implantation.

In-reactor creep of Zr-2.5 at % Nb

Tensile specimens of heat treated Zr-2.5 at % Nb rod have been tested under constant creep loads both unirradiated and during neutron irradiation. No significant differences were found in creep rates for temperatures of 350, 375 and 400 °C. The in-reactor creep rates were faster at 300 °C and stresses of 25.6, 31.6, and 38.7 kg/mm 2 and at 320 °C and 38.7 kg/mm 2 than the corresponding unirradiated control creep rates. The total creep rate during irradiation under these stresses and temperatures can be expressed by the equation: ε = ε u + ε i , where ε is the measured in-reactor creep rate, ε u is the unirradiated control creep rate, and ε i is the increase in creep rate due to neutron irradiation. The values of ε i show little if any dependence upon stress or temperature. The activation energy for creep Q c , determined for a stress of 38.7 kg/mm 2 is approximately 60 000 cal/ mole. Due to the contribution of ε i there is an apparent decrease in the activation energy at the lower strain rates. There is also a decrease in the activation energy of the control data at lower stresses and temperatures which may be due to measurement of faster primary creep rates instead of minimum creep rates, or a different creep process which has a lower activation energy. The mechanism for the increased creep rate during irradiation observed at 300 and 320 °C is tentatively proposed to be due to growth of stress oriented dislocation loops limited by the generation rate of point defects. The possibility of the increased creep rate resulting from a non-conservative dislocation mechanism limited by point defects produced by irradiation is not excluded. The mechanism of the creep represented by an activation energy of approximately 60 000 cal/mole is most likely a diffusion controlled process such as dislocation climb or non-conservative motion of jogs in screw dislocations.