Initial Defect Concentration Research Articles

Damage recovery stages revisited: Thermal evolution of non-saturated and saturated displacement damage in heavy-ion irradiated tungsten

Establishing the correlation between damage recovery and temperature in nuclear materials is vital to understanding material property degradation. Among the handful of topics related, one long-standing concern is the differences in damage recovery caused by different initial defect concentrations, with respect to ranges below and beyond the damage saturation limit. In this study, the temperature dependence of damage recovery has been comprehensively elucidated for pure tungsten, irradiated with 6 MeV copper ions at room temperature (RT), up to 0.05 dpa (non-saturated damage, n-SD) and 0.6 dpa (saturated damage, SD), and followed by isochronal annealing. Coupled transmission electron microscopy and Doppler broadening positron annihilation spectroscopy characterizations provided a full-probe-range picture of defect evolution. The varied initial defect concentrations caused retaining differences in damage recovery, but no substantial influence on temperature-dependent defect evolution. Moderate and dramatic damage recovery were identified below 973 K and above 1123 K in n-SD and SD tungsten, respectively. Most strikingly, nano-sized voids were confirmed at 473 K in both samples. Annealing at 1873 K saw the complete removal of defects in n-SD tungsten, but survival of a few dislocations via void pinning in SD tungsten. Full damage recovery in SD tungsten was projected to be ∼2000 K. The current revisit of damage recovery stages demonstrated the significance of encompassing scenarios of non-saturated and saturated defect concentrations. Accordingly, four damage recovery stages were redefined for heavy-ion irradiated tungsten, namely stage I (20 K–RT), stage II (RT–653 K), stage III (653–1123 K), and stage IV (1123–2000 K).

Radiation-induced attenuation of hollow-core photonic bandgap fiber for space applications

Radiation-Induced Attenuation (RIA) is a crucial parameter for the applications of fiber optic gyroscopes in space environments. Under the irradiation of a 60Co source with a radiation rate of 10 rad/s and a total radiation dose of 138 krad, we make real-time measurements of RIA and evaluate the recovery process of homemade 19-cell HC-PBGF and commercial PPMF. Experimental results demonstrate that there is only a 0.45 dB/km loss increment for the RIA of homemade HC-PBGF at the wavelength of 1550 nm, which is only about 1/27 of that of the PPMF. After the radiation source is turned off, the loss of HC-PBGF returns to the initial level before radiation in the tenth hour, but the RIA of PPMF has only a 20% recovery on the seventh day. We first theoretically investigate the kinetics and phenomenological equation of RIA in the Hollow-Core Photonic Band-Gap Fiber (HC-PBGF) and commercial Panda Polarization Maintaining Fiber (PPMF) in the γ-rays environment. Theoretical results proved that the initial defect concentration and RIA increase rate of HC-PBGF are only 1/3 and 1/6 of that of the PPMF. This proves that HC-PBGF can significantly reduce the RIA and exhibits a strong capacity of irradiation resistance, which is of great significance to promote fiber optic gyroscopes for space applications.

Enhancing the thermal stability of n-type Mg3+xSb1.5Bi0.49Te0.01 by defect manipulation

N-type Mg3+x(Sb, Bi)2-based thermoelectrics have quickly attracted considerable interest because of their excellent thermoelectric performance over a wide temperature range. Most studies on these compounds have thus far focused on improving their thermoelectric performance, with little consideration given to the equally essential issue of thermal stability. Mg3+x(Sb, Bi)2 is highly disordered due to having many kinds of defects, resulting in features like low thermal conductivity. However, the lattice distortion introduced by defects and the evolution of non-equilibrium defects both may impair the thermal stability of these thermoelectric materials. Additionally, incorporating Mg as the most prominent element in Mg3+x(Sb, Bi)2 has a significant impact on the compound’s initial defect concentration and its stability performance at high temperatures due to Mg loss resulting from the element’s high vapor pressure. Here we used in situ stability testing to reveal the evolution of intrinsic defects in n-type Mg3+xSb1.5Bi0.49Te0.01. A low-temperature annealing treatment was employed to improve stability by regulating non-equilibrium defects. Results from both experiments and theoretical calculations show that filling vacancy defects with transition metals rather than with additional excess Mg is effective in improving thermal stability due to the resulting enhanced chemical bonding and increased defect formation energy. This study has important implications for understanding and overcoming instability in other similar thermoelectric materials.

Vacancy and interstitial atom evolution with the separation of the nanoscale phase in Fe-Cr alloys: phase-field simulations.

Point defects induced by irradiation affect the phase decomposition and morphology evolution of Fe-Cr alloys, which results in the increase of hardness and embrittlement. The coevolution of point defects and the Cr-enriched nanoscale α' phase was studied by using phase-field simulations, and the interactive influence of point defects and Cr-enriched nanoscale particles on the kinetics evolution was clarified. It is found that the clustering of vacancies and interstitial atoms is earlier than the separation of the α' phase, and the point defects migrate from the initial cluster position to the interface of the α/α' phases, and accumulate into a defect concentration loop around the α' phase with the growth of the α' phase. In addition, with the increase of the initial defect concentration, phase separation is accelerated via the vacancy diffusion mechanism, and also the separation of Cr-enriched particles promotes the clustering of point defects. Point defects show a significant influence on phase separation with elevated temperature.

Influence of defects on the shock Hugoniot of tantalum

Using molecular dynamics simulations, we investigate the effect of vacancies and dislocations on the dynamic response of single crystal tantalum to shock loading along the ⟨110⟩ axis. A Hugoniostat technique is employed, for which a series of states along the Hugoniot are sampled by many individual simulations. We show that defects have a limited effect on the shock/particle velocity relationship and that the shock pressure/volume relationship can be well predicted by taking into account the changes in the initial density and sound speeds of the samples. The principal effect of initial defects is the activation of heterogeneous dislocation nucleation and expedited dislocation multiplication during shock. The heat generated by plastic work, caused by defects moving through the lattice, is substantial. The result is significantly divergent final shock temperatures for different initial defect concentrations and pronounced changes in the resultant shock melting temperatures. The motion of dislocations also leaves behind a noninconsequential concentration of vacancies that is quantified.

Effects of dopant type and concentration on the femtosecond laser ablation threshold and incubation behaviour of silicon

In laser micromachining, the ablation threshold (minimum fluence required to cause ablation) is a key performance parameter and overall indicator of the efficiency of material removal. For pulsed laser micromachining, this important observable depends upon material properties, pulse properties and the number of pulses applied in a complex manner that is not yet well understood. The incubation effect is one example. It manifests as a change in the ablation threshold as a function of number of laser pulses applied and is driven by photoinduced defect accumulation in the material. Here, we study femtosecond (800 nm, 110 fs, 0.1-1 mJ/pulse) micromachining of a material with well-defined initial defect concentrations: doped Si across a range of dopant types and concentrations. The single-pulse ablation threshold (Fth,1) was observed to decrease with increasing dopant concentration, from a maximum of 0.70 J/cm2 (+/-0.02) for undoped Si to 0.51 J/cm2 (+/-0.01) for highly N-type doped Si. The effect was greater for N-type doped Si than for P-type, consistent with the higher carrier mobility of electrons compared to holes. In contrast, the infinite-pulse ablation threshold (Fth,inf) was the same for all doping levels and types. We attribute this asymptotic behaviour to a maximum defect concentration that is independent of the initial defect concentration and type. These results lend insight into the mechanism of multipulse, femtosecond laser ablation.

Formation of Thermally Induced Defects in Silica Optical Material

Characteristics of silica optical material largely depend on its thermal history. In this paper, formation of thermally induced defects in silica optical material is studied. The formation process of defect is analyzed in detail. The results show that there is an obvious difference in defect formation induced by heating treatment when the composition of silica optical material changes. Defect formation mainly displays as the produce process when the initial defects of the silica material are zero. However, defect formation expresses as the produce and annealing process when the initial defects of the silica material are not zero. The initial defect concentration can be decreased significantly when the silica material is heated in high temperature. At the same time, the new defect is also produced. These theoretic results are consistent with the previous experimental ones.

On the Formation of Intrinsic Defects in 4H-SiC by High Temperature Annealing Steps

In this paper the impact of high temperature annealing on the formation of intrinsic defects in 4H-SiC such as Z1/2 and EH6/7 was examined. Therefore, three epitaxial layers with various initial concentrations of the Z1/2- and EH6/7-centers (1011 – 1013 cm-3) were investigated. It turns out that depending on the initial defect concentration the high temperature annealing leads to a monotone increase of the Z1/2- and EH6/7-concentration in a temperature range from 1600 to 1750°C. For a defined temperature above these values, the resulting defect concentration is independent of the sample’s initial values. Beside the growth conditions themselves such as C/Si ratio the thermal post-growth processing has a severe impact on the carrier lifetime which must be taken into account during device fabrication.

On the merits of Raman spectroscopy and thermogravimetric analysis to asses carbon nanotube structural modifications

Raman spectroscopy, thermogravimetric analysis (TGA), Fourier transform infrared spectroscopy, and transmission electron microscopy are used to assess structural changes generated on the surface of multi-walled (MWCNTs) and single-walled (SWCNTs) carbon nanotubes. Different levels of structural modifications are generated by the use of acidic oxidative treatments. It is found that Raman spectroscopy is a very powerful technique to assess structural modification of SWCNTs with initial low defect concentration. For MWCNTs grown by chemical vapor deposition, which already contain a high density of structural defects in their as-produced state, Raman spectroscopy is not a very sensitive tool to detect the generation of further defects or other structural modifications introduced through acidic treatments. For this later case, TGA is a sensitive technique to assess structural modifications on the nanotubes.

High temperature annealing of n-type 4H-SiC: Impact on intrinsic defects and carrier lifetime

In this paper, the impact of high-temperature annealing of 4H silicon carbide (SiC) on the formation of intrinsic defects, such as Z1/2 and EH6/7, and on carrier lifetimes was studied. Four nitrogen-doped epitaxial layers with various initial concentrations of the Z1/2- and EH6/7-centers (1011-1014cm-3) were investigated by means of deep level transient spectroscopy and microwave photoconductance decay. It turned out that the high-temperature annealing leads to a monotone increase of the Z1/2- and EH6/7-concentration starting at temperatures between 1600 °C and 1750 °C, depending on the initial defect concentration. In the case of samples with high initial defect concentration (1014cm-3) a distinct decrease in Z1/2- and EH6/7-concentration in the temperature range from 1600 °C to 1750 °C was observed, being consistent with previous reports. For higher annealing temperatures (Tanneal≥1750 ∘C), the defect concentration is independent of the samples’ initial values. As a consequence, beside the growth conditions, such as C/Si ratio, the thermal post-growth processing has a severe impact on carrier lifetimes, which are strongly reduced for samples annealed at high temperatures.

A Quantitative Model for ELDRS and ${\rm H}_{2}$ Degradation Effects in Irradiated Oxides Based on First Principles Calculations

A physics-based TCAD model for enhanced low-dose-rate sensitivity in linear bipolar devices is developed. Quantitative agreement is found with measured data over a wide range of dose rates and H <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> concentrations. Analysis of the degradation effects of individual defect types, the implementation of which has been informed by first principles calculations, provides insights into the mechanisms behind enhanced low-dose-rate effects in different hydrogen environments. The effects of initial defect concentration and location and the energetics of the defect-related reactions are explored. Conclusions are drawn about the roles of molecular hydrogen and hydrogenated defects in the radiation response of these devices.

Effect of irradiation parameters on defect aggregation during thermal annealing of LiF irradiated with swift ions and electrons

Absorption spectroscopy were performed to study the effects of thermal annealing on the aggregation of color centers in LiF crystals irradiated with different ions between carbon and uranium of megaelectron volt-gigaelectron volt energy. The beam parameters such as energy, energy loss, and fluence have a pronounced influence on the initial defect composition and concentration as well as their evolution upon thermal annealing. A distinct phenomenon was observed, viz., the enhancement of ${\text{F}}_{n}$ centers for annealing temperatures between 500 and 700 K, followed by Li colloid formation above 700 K. The phenomenon requires specific irradiation conditions whereas the formation of Mg colloids from Mg impurities occurs in all irradiated crystals. The mechanisms of annealing and colloid formation are discussed.

The Effects of Aging and Hydrogen on the Radiation Response of Gated Lateral PNP Bipolar Transistors

<para xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> Complex interplay between hydrogen-related defect formation and passivation is observed in irradiated bipolar transistors. Hydrogen soaking experiments are performed to evaluate the dependence of defect buildup and annealing in gated lateral bipolar transistors on hydrogen exposure. Comparisons of the radiation responses of transistors tested in 2009 to identical devices from the same wafer tested in 2003 show that aging has reduced the amount of radiation-induced interface trap and oxide trapped charge formation in most cases. These results demonstrate that the way in which the radiation response of a hydrogen-sensitive device evolves with age depends on whether hydrogen is diffusing into or out of the device, and whether the initial defect concentration favors passivation or depassivation reactions. These results strongly suggest that hydrogen exposure cannot replace low-dose-rate irradiation in ELDRS tests for bipolar devices and ICs without extensive characterization testing. </para>

Overheating effects on thermal stability and mechanical properties of Cu 36Zr 48Ag 8Al 8 bulk metallic glass

A pronounced effect of overheating is observed on the thermal stability and mechanical properties of Cu 36Zr 48Ag 8Al 8 bulk metallic glass. Higher overheated temperature enhances the thermal stability of bulk amorphous alloys, corresponding to higher specific-heat capacity and the smaller initial defect concentration. And a threshold overheating temperature is found for the fully amorphous structure. Bulk amorphous alloys exhibit good compressive plasticity at small overheat levels, whereas the compressive fracture strength and micro-hardness exhibit a significant increase first and then a slightly decrease. Mechanical properties of BMGs can be tailored in certain extent by controlling the overheated level, which is correlated with free volume and residual stresses.

Computer Simulation of Discrete Crack Propagation

Although statistical methods are widely used to study a large amount o f phenomena ranging from random walk to percolation and particle charging, the application o f these methods to mechanics is limited. Nevertheless the use of senses like fractal dimension can be very handy to describe the mass o f a crack or the roughness of a surface. From this viewpoint subcritical crack growth is studied as a cluster growth process with the aid o f discrete computer experiments. A mechanical ballistic aggregation model motivated by a continuum theory of stress-assisted migration o f point defects is formulated and simulated on a square lattice. These defects move towards the crack tip under the action of the high stress gradients existing there. The crack grows by the inf lux of defects in the tip region with the rate of this process, which depends on the externally applied stress, determining the crack velocity and its dependence on the stress intensity factor. The crucial parameter of the model is the initial concentration of defect panicles. It is shown that there is a characteristic relation between critical stress and crack growth for different initial defect concentrations. The crack path is o f a fractal character and the crack velocity dependence on the stress intensity factor follows a power law relationship, in accordance with experimental trends. 1. I N T R O D U C T I O N Subcritical cracking /1, 2/ is a generic term used to indicate slow crack growth for applied loads below those causing dynamic fracture. Subcritical cracking may occur under creep, fatigue, hydrogen embrittlement

Modeling of primary defect aggregation in tracks of swift heavy ions in alkali halides

Dynamic Monte Carlo simulations of primary defect aggregation in tracks of swift heavy ions in alkali halides have been performed. The study was motivated by recent experimental findings on dense F-center clusters observed even at 15 K in LiF irradiated by GeV heavy ions. We relied on a recently suggested model, which assumes local heating and diffusion of single defects in their excited electronic state. The main parameters (migration energy, initial defect concentration, diffusion time) are estimated from available experimental data. Simulation results confirm the formation of a core with large F-aggregates and a broad halo of single and dimer defects around it. The fraction of large (more than 10 defects) aggregates in the core is negligible, making ESR detection impossible. The low sensitivity of the model to changes in parameters can be explained by its universal features, which we first report in this paper. Unexpected power laws in the kinetics of single defect concentrations are also emphasized.

Modeling of primary defect aggregation in tracks of swift heavy ions in LiF

To simulate aggregation of primary F centers created along the path of swift heavy ions in LiF, Monte Carlo simulations were developed. Parameters relevant for defect aggregation as a result of their random hopping, such as the migration energy, temperature in the track, initial defect concentration, and diffusion time, were estimated from available experimental data. It is estimated that in the electronically excited state and under temperature locally increased up to 1200 K F centers are mobile enough to make several tens of hops. Most of the F aggregates formed are extremely small and consist only of two or three F centers. The fraction of larger F clusters (with more than 10 defects) is negligibly small, at least for defect concentrations reasonable for ion tracks. Even at the largest initial defect densities, the aggregates are isolated from each other and do not form a percolating trail of defects. Such a track morphology is in good agreement with various experimental results.

Argon implantation into GaAs and in situ RBS analysis at 21 and 77 K

In order to investigate the primary effects of ion–solid interaction, low-temperature implantation and measurement of the irradiated samples without warming up is necessary. An experimental setup is used, which allows ion implantation at constant temperatures between 20 and 300 K and defect analysis by RBS without changing the target temperature. First results are presented for 200 keV Ar + implantation into GaAs at 21 K and at 77 K. No difference in the damage formation was noticed between implantation at 21 K or at 77 K, indicating that no visible annealing effects occur at these temperatures. During warming up of the samples to room temperature, the relative defect concentration remains unchanged, if its initial value was above 80%, but it reduces remarkably for lower values of the initial defect concentration.

Dsc Evaluations in F.c.c. Solid Solutions of Short-range-order Kinetics As Influenced by Bound Vacancies

A modified first order kinetic law, which describes the roles of bound and unbound vacancies, is proposed in order to predict defect decay and short-range-order kinetics of quenched binary alloys during linear heating experiments. The model has been applied to differential scanning calorimetry (DSC) curves of Cu–5 at%Zn quenched from different temperatures. Activation energy for migration of solute-vacancy complexes was also assessed from the kinetics of short-range-order using DSC traces. A value of 89.5±0.32 kJ mol–1 was obtained. The relative contribution of bound and unbound vacancies to the ordering process as influenced by quenching temperature was determined. In conjunction, a parametric study of the initial total defect concentration and effective energy for defect migration was performed in order to envisage their influence on the calculated DSC profiles.

Ion beam analysis of 6H SiC implanted with erbium and ytterbium ions

Recrystallization of 6H SiC implanted at room temperature with 800 keV Er + and Yb + ions was analysed using Rutherford backscattering and channeling of 1.5 MeV He + ions. Erbium doses were 10 13, 10 14 and 2 × 10 15cm −2. Ytterbium ions were implanted with doses of 2 × 10 14and 2 × 10 15cm −2 into SiC samples which had been preimplanted with a 20 times lower fluence of Er ions. It has been found that annealing within the 1000–1300°C range results in partial damage removal depending on the initial defect concentration. Perfect recrystallization is achieved at 1350°C, however it is accompanied by the segregation of rare earth impurities to the surface.