Increase In Defect Concentration Research Articles

Positron annihilation study of defects in copper irradiated with swift Xe26+ ions

Variable energy positron beam and conventional positron lifetime spectroscopy were used to study pure copper exposed to irradiation with 167 MeV Xe26+ heavy ions with different doses of 1012, 1013, 5 × 1013, 1014 ions/cm2. The presence of vacancy-type defects induced by implantation was confirmed in Doppler spectroscopy characteristics. Decreasing of the positron diffusion length from 135 nm for unirradiated sample, to 82 nm for the lowest and to 46 nm for the highest applied dose was noted. It points out increasing defect concentration with the increase of the fluence. Additionally, the existence of one kind of defects in all irradiated samples was noticed. The defected zone covered with the implanted range and the so called “long range effect” was not confirmed.

Physical and optical properties of bulk and thin films of a–Ge–Sb–Te lone-pair semiconductors

ABSTRACTPhysical parameters and their correlation with optical band gap and refractive index for amorphous Ge20Te80-xSbx (x = 0, 2, 4, 6, 8, 10) lone-pair semiconductors were discussed. Using average coordination number lone-pair electrons (L) were calculated and found to decrease with an increase in the Sb content. However, L value greater than 3 indicated that Ge20Te80-xSbx alloys can retain their vitreous nature. The density of alloys was measured both experimentally and theoretically. Increased density values with the increase of Sb content further accounts for the concurrent increase in the refractive index values. The results were explained on the basis of increased polarizability and larger Bohr radius of Sb. Compactness of the structure was found to decrease with the alterations of the atomic arrangements. Molar volume (calculated from the measured density values) found to decrease with Sb content. The number of atoms per unit volume (N) was calculated, using molar volume and average coordination number and were found to increase. This increase in N accounts for the decrease in the optical band gap. Other optical parameters viz. optical density, penetration depth and Urbach energy were also calculated. An increase in Urbach energy showed the reduction in band tail width with increased defect concentration. Positions of the conduction bands and valence bands were found to shift towards the Fermi edge, which accounts for the reduction in the optical band gap. Chemical bond approach (CBA) model was applied to estimate the cohesive energy of the system and found to decrease with the Sb content. A linear relation is found between the behaviour of cohesive energy and band gap (calculated both experimentally and theoretically). Deviation of stoichiometry confirmed chalcogen-rich region for all the compositions. The mean bond energy was found to be proportional to the glass transition temperature and showed maxima at the chemical threshold. The obtained results were discussed in terms of average coordination number or equivalently structure of the glassy matrix, decrease in average stabilization energy, electronegativity and average heat of atomization of the system. In amorphous materials, maximum optical non-linearity has been predicted. To open the prospects of Ge–Te–Sb vitreous system for non-linearity, Sheikh and Bahae relationship was applied, using experimentally obtained parameters, at a telecommunication wavelength. Obtained results showed a decent agreement with values available in the literature at 0.8 eV or 1550 nm. Non-linear refractive indices, almost three orders higher than silica glass, were obtained. These results may lead to yielding more sensitive optical limiting devices, and these glasses may be used as an optical material for a high-speed communication fibre.

RECYCLING OF CARBONE OXIDES (CO, CO2) CONVERSION INTO METHANOL AT ATMOSPHERIC PRESSURE OVER MECHANOCHEMICAL ACHTIVATED CUO-ZNO-AL2O3 CATALYST

The catalytic process for methanol production by synthesis gas conversion under the conditions of mechanochemical activation (MCA) of copper-zinc-aluminum oxide catalyst in the temperature range 160–280 °C at a pressure of 0.1 MPa are investigated. The use of mechanical action force is one of the promising ways to improve the activity of heterogeneous catalysts designed to simplify the manufacturing process lines, improving the efficiency of catalytic processes and reduce the cost of the target product. Given the importance of technology for methanol production on copper-zinc-aluminum oxide catalysts and high demand for methanol in the world [1–3], clarification of the peculiarities of the process of methanol production by synthesis gas conversion in terms of mechanical load on the catalyst is important in scientific and applied ways. It is established that specific catalytic activity, performance of methanol synthesis catalyst and the conversion of initial reagents are increased in the conditions of mechanochemical activation, because of the increasing concentration of defects and formation of additional active centers. It is revealed that mechanochemical treatment of copper-zinc-aluminum oxide catalyst can reduce reaction initiation temperature and optimum temperature synthesis by 20–30 °C, and increase the maximum performance of the catalytic system. Increase of the catalyst activity under mechanical stress is explored by increase of defect concentration of crystal lattice of the catalyst, as confirmed by the tests of catalyst surface structure by scanning electron microscopy, Raman spectroscopy and X-ray analysis. A new effective method for synthesis gas conversion into the methanol under conditions of mechanochemical activation of the catalyst can be used in industry as an alternative to methanol production at high pressures.

Influence of annealing on microstructure and properties of Cr-doped ZnO thin films deposited on glass surface

Cr-doped ZnO thin films were deposited onto glass substrates by radio frequency magnetron sputtering technique. The effects of annealing temperature and annealing time on the structural and optical properties of the Cr-doped ZnO films were comprehensively investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM), ultraviolet–visible (UV) and photoluminescence (PL) measurements. XRD studies reveal that the Cr-doped ZnO films are highly oriented along (002) direction. The intensity of ZnO (002) peak firstly increases and then decreases with increasing annealing temperature and annealing time. SEM analysis show that the grain size increases and the surface of films becomes rougher as the annealing temperature increases to 700 °C. The optical transmittance spectra indicate that the average transmittance in the range of the visible region is approximately 80%, and the band gaps of the films decrease with increasing annealing temperature. Two peaks of UV emission and green emission are observed in the PL spectra. The UV emission increases first and then decreases with increasing annealing temperature, which show that the increasing defect concentration in the films leads to the poor crystallinity. The green emission greatly increases with increasing annealing temperature, which could be attributed to the increase of oxygen vacancies and increased nonradiative process.

Accelerating ferroic ageing dynamics upon cooling

Once a structural glass is formed, its relaxation time will increase exponentially with decreasing temperature. Thus, the glass has little chance of transforming into a crystal upon further cooling to zero Kelvin. However, a spontaneous transition upon cooling from amorphous to long-range ordered ferroic states has been observed experimentally in ferroelastic, ferroelectric and ferromagnetic materials. The origin for this obvious discrepancy is discussed here conceptually. We present a combined theoretical and numerical study of this phenomenon and show that the diffusive and displacive atomic processes that take place in structural glass and amorphous ferroics, respectively, lead to markedly different temperature-dependent relaxation behaviors, one being ‘colder is slower’ and the other being ‘colder is faster’. Once a structural glass (e.g., window glass) is formed, it will not turn into a crystal upon further cooling to zero Kelvin. This is, however, not true for ferroic glasses — disordered states of magnetization, polarization or strain. Junyan Zhang from Xi'an Jiao Tong University in China and collaborators developed a theoretical framework to shed light on how the relaxation processes in these two types of glasses are fundamentally different, one being dominated by atomic diffusion and activation enthalpy, and the other being dominated by collective displacements and activation volume/entropy; one being ‘colder is slower’ as expected, and the other being ‘colder is faster’. Numerical simulations unveil the change from ‘colder is faster’ to ‘colder is slower’ with an increasing defect concentration in a generic ferroelastic system. Microstructure evolutions upon cooling in three ferroelastic systems having different defect concentrations, c=0.02, 0.14 and 0.40, respectively. Green color in the figures represents the parent phase; blue and red colors represent the two different martensitic variants. As the defect concentration increases, the behavior of the ferroelastic system shifts from ‘the cooler the faster’ (c=0.02 and 0.14) to ‘the cooler the slower’ (c=0.40).

Recent progress of hydrogen isotope behavior studies for neutron or heavy ion damaged W

This paper reviews recent results pertaining to hydrogen isotope behavior in neutron and heavy ion damaged W. Accumulation of damage in W creates stable trapping sites for hydrogen isotopes, thereby changing the observed desorption behavior. In particular, the desorption temperature shifts higher as the defect concentration increases. In addition, the distribution of defects throughout the sample also changes the shape of TDS spectrum. Even if low energy traps were distributed in the bulk region, the D diffusion toward the surface requires additional time for trapping/detrapping during surface-to-bulk transport, contributing to a shift of desorption peaks toward higher temperatures. It can be said that both of distribution of damage (e.g. hydrogen isotope trapping sites) and their stabilities would have a large impact on desorption. In addition, transmutation effects should be also considered for an actual fusion environment. Experimental results show that production of Re by nuclear reaction of W with neutrons reduces the density of trapping sites, though no remarkable retention enhancement is observed.

Molecular dynamics study of the diffusion properties of H in Fe with point defects

Molecular dynamics (MD) simulations have been performed to investigate the diffusion properties of hydrogen (H) atoms in body-center-cubic (bcc) iron (Fe) with point defects, i.e., vacancies (Vs) and self-interstitial atoms (SIAs). Firstly, the binding energies of a H atom to a H/SIA-H cluster have been calculated by molecular statics simulations. The results show that single-H diffusion should be the major diffusion mechanism in perfect bcc Fe and bcc Fe with SIAs, but for bcc Fe with Vs, results from literatures show that multi-H diffusion should be taken into account. Further, the mean squared displacements method has been employed to determine the diffusivities and diffusion energy barriers of single-H at different temperatures in perfect bcc Fe. The diffusion energy barrier varies with the temperature. Furthermore, the H diffusion properties with different point defect concentrations have also been demonstrated, which showed that diffusivities reduce as the point defect concentration increases and the influence of point defects reduces as the temperature increases. Especially, SIAs exhibit negligible effect when the temperature is higher than 600K.

Elastic moduli of pure and gadolinium doped ceria revisited: Sound velocity measurements

The Young's, shear and bulk moduli of dense (>95%) Gd-doped ceria (Ce1-xGdxO2-x/2) ceramics were determined as a function of Gd content using the ultrasonic pulse-echo method and corrected for porosity according to both static and dynamic models. With increasing defect concentration (0–20mol% Gd), the Young's and shear moduli decrease linearly by 10% while for 29mol%, the values lie above the linear fit; the bulk modulus decreases linearly within the complete range. We conclude that structural changes, induced by oxygen vacancy ordering for x>0.2, may influence the uniaxial and shear moduli, leaving the bulk modulus unaffected.

Operando X-ray Investigation of Electrode/Electrolyte Interfaces in Model Solid Oxide Fuel Cells.

We employed operando anomalous surface X-ray diffraction to investigate the buried interface between the cathode and the electrolyte of a model solid oxide fuel cell with atomic resolution. The cell was studied under different oxygen pressures at elevated temperatures and polarizations by external potential control. Making use of anomalous X-ray diffraction effects at the Y and Zr K-edges allowed us to resolve the interfacial structure and chemical composition of a (100)-oriented, 9.5 mol % yttria-stabilized zirconia (YSZ) single crystal electrolyte below a La0.6Sr0.4CoO3−δ (LSC) electrode. We observe yttrium segregation toward the YSZ/LSC electrolyte/electrode interface under reducing conditions. Under oxidizing conditions, the interface becomes Y depleted. The yttrium segregation is corroborated by an enhanced outward relaxation of the YSZ interfacial metal ion layer. At the same time, an increase in point defect concentration in the electrolyte at the interface was observed, as evidenced by reduced YSZ crystallographic site occupancies for the cations as well as the oxygen ions. Such changes in composition are expected to strongly influence the oxygen ion transport through this interface which plays an important role for the performance of solid oxide fuel cells. The structure of the interface is compared to the bare YSZ(100) surface structure near the microelectrode under identical conditions and to the structure of the YSZ(100) surface prepared under ultrahigh vacuum conditions.

Effect of structure, size and copper doping on the luminescence properties of ZnS

Luminescence properties of wurtzite and cubic forms of bulk ZnS have been investigated in detail and compared with that of ZnS nanoparticles. Blue emission observed in both hexagonal and cubic forms of undoped bulk ZnS is explained based on electron–hole recombination involving electron in conduction band and hole trapped in Zn2+ vacancies where as green emission arises due to electron hole recombination from Zn2+ and S2− vacancies. Conversion of wurtzite form to cubic form is associated with relative increase in intensity of green emission due to increased defect concentration brought about by high temperature heat treatment. Copper doping in ZnS, initially leads to formation of both CuZn and Cui (interstitial copper) centers, and latter to mainly CuZn centers as revealed by variation in relative intensities of blue and green emission from the samples.

Radiothermoluminescence of n-eicosane mixtures with n-tetracosane: Effect of mixture composition

Radiothermoluminescence (RTL) curves of mixtures of C20 and C24 n-alkanes have been deconvoluted into the curves of crystalline and disordered regions, and their change depending on the mixture composition has been analyzed. The shape of the total RTL curve of the mixture regardless of the its composition is fundamentally different from that shape of the sum of the curves of the individual components taken in the relevant ratios. The difference becomes noticeable at as low a C20 amount in the mixture as 2% and is manifested in the disappearance of the intense RTL peaks of the individual components and appearance of new intense peaks in temperature intervals other than those of the RTL peaks of the components. It has been shown that these changes correlate with changes of the crystalline structure of the mixture: a new crystalline structure other than the structure of any of the individual components emerges and the mixture becomes amorphous. High selectivity of charge trapping in disordered regions of the mixture has been revealed: the contribution of RTL of the disordered regions to the total curve is many times higher than their volume fraction in the mixture. This selectivity might be due to an increase in concentration of defects in the unordered regions of the mixture in comparison with their concentration in the pure alkanes. These defects are traps for electrons that are accessible to acceptors introduced into the mixture.

Variable energy positron beam studies of defects in heavy ion implanted palladium

Abstract The paper reports experimental studies of Palladium (Pd) exposed to 107 MeV Kr 17 + ion irradiation. Heavy ion implantation of samples with fluences of 10 12 , 10 13 , 5 × 10 13 and 10 14 was performed at IC-100 cyclotron at Flerov Laboratory of Nuclear Reactions at Joint Institute for Nuclear Research (JINR) in Dubna, Russia. Radiation damages were investigated with variable energy positron beam (VEP). Doppler broadening spectroscopy (DB) was applied and the line shape S parameter of annihilation line was extracted. The analysis of S parameter profiles gives information about the presence of open volume defects in irradiated samples. The positron diffusion length extracted from the profile decreases with the dose increasing from 112 nm for a non-irradiated sample to 26 nm for the biggest fluence. It points out the increase of defect concentration with the increase of the fluence.

Atomic Transport in Nano-Сrystalline Thin Films

Ge and B diffusion was studied in nanocrystalline Si, and Pd and Si self-diffusion was studied in nanocrystalline Pd2Si during and after Pd/Si reactive diffusion. These experiments showed that grain boundary (GB) diffusion kinetic is the same in micro-and nanoGBs, whereas triple junction (TJ) diffusion is several orders of magnitude faster than GB diffusion. In addition, GB segregation and GB migration can significantly modify atomic diffusion profiles in nanocrystalline materials, and atomic transport kinetics can be largely increased in nanograins compared to micro-grains, as well as during reactive diffusion, probably due to an increase of point defect concentration. These observations show that atomic transport in nanometric layers during reactive diffusion is complex, since GBs and TJs are moving and the proportion of GBs and TJs is changing during the layer growth.

Nanoscaled Martensitic Domains in Ferroelastic Systems: Strain Glass

Strain glass, a frozen disordered ferroelastic state characterized by randomly distributed nanoscaled, non-ergodic martensitic domains, was recently found in ferroelastic TiNi-based alloys. Point defects like anti-site defects or dopants are found essential for the formation of the strain glass, but their role has remained obscure. In this paper, phase field method is employed to simulate how point defects change a normal ferroelastic system into a strain glass and as the result to clarify the role of point defects. We found that the local symmetry-breaking caused by the anisotropic local strain field of point defects plays a central role in the formation of strain glass. Such point-defect-induced local symmetry-breaking hinders the formation of long-range strain-ordering (twinned martensite) and results in a decrease in transition temperature Tc and a reduction of ferroelastic domain size. Above a critical defect concentration, the system transforms to a frozen state of nano-sized ferroelastic domains, i.e., strain glass. In addition, our simulation shows a gradual vanishing of heat capacity peak at the transition temperature with increasing defect concentration, and it also reproduces a characteristic non-ergodic ZFC/FC behavior of the glass state, which is in good agreement with recent experimental observations. Our results also indicate that the concentration fluctuation effect of point defects cannot stabilize a strain glass to very low temperature; thus the direct interaction between the random anisotropic strain field caused by point defects and the strain order parameter of the system is crucial for the formation of strain glass. Keywords: Martensitic phase transition, nanoscaled martensite, phase field, point defect, strain glass.

A circuit model for defective bilayer graphene transistors

This paper investigates the behaviour of a defective single-gate bilayer graphene transistor. Point defects were introduced into pristine graphene crystal structure using a tightly focused helium ion beam. The transfer characteristics of the exposed transistors were measured ex-situ for different defect concentrations. The channel peak resistance increased with increasing defect concentration whilst the on–off ratio showed a decreasing trend for both electrons and holes. To understand the electrical behaviour of the transistors, a circuit model for bilayer graphene is developed which shows a very good agreement when validated against experimental data. The model allowed parameter extraction of bilayer transistor and can be implemented in circuit level simulators.

Influences of material defects and temperature on current-driven domain wall mobility

Current-induced domain wall motion, which has potential application in the next-generation data storage and logic device, has attracted much interest in recent years. However, how the material defect and its joule heat influence current-driven domain wall motion in magnetic nanostripe is still unclear. This paper is to deal with these issues by using the Landau-Lifshitz-Gilbert spin dynamics. The results show that the material defect can pin domain wall motion and this pinning effect strongly depends on the defect concentration, location and shape. The pinning effect induced by the defect on domain wall motion results in the increase of threshold current, and the domain wall moves steadily and continuously. Specifically, the probability for domain wall motion induced by pinning effect is nonlinearly increasing with the increase of defect concentration. Namely, the increasing of the pinning ability with the increase of the defect concentration becomes fades away. Initially, when the defect is near to domain wall, the pinning ability is obvious. However, the pinning ability is not linearly increasing with the decrease of the initial distance between the defect and the domain wall. The results also show that the single defect is larger, the probability for domain wall motion induced by defect pining is bigger. Moreover, the material defect can suppress the domain wall trending toward breakdown and make domain wall move faster, but the suppressing ability is not obviously increasing with the increase of the defect concentration. On the other hand, the temperature field can remove the pinning phenomenon, which will result in the threshold current decrease. The decrease of the threshold current is of benefit to the working of the data storage and logic device. Also the temperature field can suppress the domain wall trending toward breakdown, but the suppressing ability is less than that of the defect. In addition, the Joule heat around defects can obviously eliminate the pinning effect of the defects, so the pinning effect for a few defects on current-induced domain wall motion can be ignored. Further analysis indicates that these effects are due to the change of the out-of-plane magnetization of the domain wall induced by the material defects and the temperature field, because the velocity of the domain wall motion induced by the applied current greatly depends on the out-of-plane magnetization of the domain wall.

Sintering and grain growth in SrTiO<sub>3</sub>: impact of defects on kinetics

The microstructural evolution of undoped and iron doped SrTiO3 is analyzed during sintering at 1280°C in air and reducing atmosphere. The focus is on densification and grain growth during different holding times investigated by dilatometric studies and microstructural analysis. The sintering equations developed by Coble are used to characterize sintering. The influence of point defects on diffusion, densification and grain growth is evaluated using basic defect chemistry equations. A space charge concept at the grain boundaries is added to bulk defect chemistry concepts to understand sintering of perovskites, since the major part of mass transport during sintering occurs in this region. The extension of the defect chemistry concept allows to explain the change in diffusion mechanism during sintering (grain boundary diffusion or bulk diffusion) as well as grain growth stagnation observed in iron doped SrTiO3. The results are used to separate the complex interplay of densification and grain growth. While grain growth decreases with increasing defect concentration, no clear trend is observed for the densification kinetics, since both grain growth and diffusion are relevant. The results show that grain growth during sintering provides comparable results to grain growth experiments in dense SrTiO3 and, thus, pore drag seems not to be important. The calculated diffusion coefficients are in good agreement with literature data and show a strong dependency on the concentration of strontium vacancies.

Radiation-accelerated precipitation in Fe–Cr alloys

The kinetics of phase separation in Fe–Cr alloys under irradiation is modeled by Atomistic kinetic Monte Carlo simulations that include the formation, migration and elimination of vacancies and self-interstitials at point defects sinks. The evolution of the sink density is modeled by cluster dynamics, and taken into account in the Monte Carlo simulations by a rescaling of the time. The results are in good agreement with available experimental observations of neutron irradiation at 290 °C. The irradiation is found to accelerate the kinetics of phase separation by orders of magnitude, with an acceleration factor given by the increase in point defect concentrations. The microstructure evolution is qualitatively the same as during isothermal annealing, except in the vicinity of point defect sinks. The effects of equilibrium and radiation induced segregations at grain boundaries are considered. The ballistic mixing occurring in displacement cascades is modeled, and is found to be insufficient to produce the dissolution of chromium rich precipitates at 290 °C, even at high dose rates. Therefore, it cannot explain the absence of precipitation observed during ion irradiations.

Building a Better Battery: Ionic Conductivity of Granular Hydrate Crystals

Although solid state batteries are an alternative to conventional batteries, they generally produce low power due to their poor ionic conductivity. This study tested the ionic conductivities of powdered potassium sodium tartrate and potassium aluminum sulfate, two double salts that have garnered attention as potential solid state electrolytes. The substances were hypothesized on having abnormally high conductivities. Conductivity was measured using electrical impedance spectroscopy on an Arduino UNO. The results indicated that the double salts had indeed high conductivities for an ionic crystal at 5.6e-5±6e-6 S/m for PST and 7.8e-6±9e-7 S/m for PAS. However, the other 3 materials tested all produced similar values. Furthermore, one of the materials tested, sodium chloride, has a documented conductivity of 10-12 S/m. The high conductivities as well as the discrepancy between the measured and documented conductivities of sodium chloride were attributed to the powder form of the materials. Caking may have occurred and increased conductivity by increasing defect concentration as well as by creating moisture channels through which free ions could flow. These results have vast implications if a powdered electrolyte material may be successfully applied to existing fast ion conductors instead of insulators such as sodium chloride.

Effect of high-temperature aging on microstructure and mechanical properties of Ni–Mo–Cr based superalloy subjected to simulated heat-affected zone thermal cycle

In the work, the effect of high-temperature (800–1100 °C) aging treatment on microstructural evolution and mechanical properties of the Ni–17Mo–7Cr based superalloy subjected to heat-affected zone (HAZ) thermal cycle. When the aging temperature is in excess of 800 °C, massive tiny MoC particles and a few chromium carbides are precipitated in the alloy. The lamellar-like structures grow in the alloy and tiny MoC are precipitated around them. In addition, some nano-sized chromium carbides with needle-like morphology are formed in the MoC. With increasing the aging temperature, the tensile and yielding strength decrease slightly due to increased precipitates in the alloy. In comparison with the alloy that does not go through the HAZ simulation, both of their mechanical properties share the close value at each temperature. The retention interval above 1300 °C during the HAZ simulation also influences the alloy's microstructure and mechanical properties. More lamellar-like structures are produced when elevating the retention interval over 5 s. Due to the increase of solidification defect and stress concentration, the tensile and yielding strength decrease sharply when the interval is increased from 5 s to 10 s. Further improving the interval has a limited influence on the alloy's mechanical properties.