Fraction Of Vacancies Research Articles

Natural ageing clustering under different quenching conditions in an Al-Mg-Si alloy

During quenching of aluminium alloys from the solutionising temperature vacancies are partially conserved as excess vacancies, partially lost to vacancy sinks, the exact fractions depending on the cooling rate. Positron lifetime measurements in samples from interrupted quenching experiments reveal that vacancies are lost during cooling down to 200 °C, after which solute atoms start to form clusters down to 20 °C. Slow cooling leads to 1 to 2 orders of magnitude lower excess vacancies than fast cooling. Since quenched-in vacancies are crucial for natural ageing (NA) in Al-Mg-Si alloys it is surprising to find just small differences between the NA hardening kinetics after different quenches. Specifically, hardening rates differ only in the initial stage (<100 min), after which they are almost identical for NA up to ~1 year. This suggests that interactions between vacancies and early-stage solute clusters help equalising the free vacancy fractions in differently quenched samples.

Exotic closure domains induced by oxygen vacancies in compressed BaTiO3 nanofilm

The molecular dynamics method based on the shell model is used to investigate the polarization configuration evolution in compressed BaTiO3 nanofilms with oxygen vacancy lattices of different volume fractions and positions. A clockwise closure domain surrounding a head-to-head domain is observed for a single oxygen vacancy nanofilm. With an increasing oxygen vacancy fraction, the closure domain around the vacancy becomes larger and gradually transforms its rotation direction from clockwise to counterclockwise. Inside the vacancy, except for the head-to-head domain remaining unchanged, the domain changes from a random configuration to a clockwise vortex, and finally to a segmented strip polydomain with 90°- and 180°-domain walls. For a single oxygen vacancy, its location region where the closure domain can occur is obtained, and the model center is a favored site for clockwise vortex nucleation. The closure domain partly changes its orientation if the oxygen vacancy shifts off the model center. In addition, homogeneous closure domains can occur when the vacancy changes its location within a small area. This study demonstrates the feasibility of tuning this kind of closure domain in ferroelectric nanofilms through external loadings and oxygen vacancies. This could be instructive to develop novel nanoscale memories and logic devices.

Structure, hydration, and proton conductivity in 50% La and Nd doped CeO2 – La2Ce2O7 and Nd2Ce2O7 – and their solid solutions

We have measured water uptake and hydration enthalpy in 50% La and Nd doped CeO2, also to be taken as compositions in the series La2−xNdxCe2O7 (x = 0.0, 0.5, 1.0 and 2.0) using combined thermogravimetry (TG) and differential scanning calorimetry (DSC), TG-DSC. The TG-DSC data unambiguously yield standard molar hydration enthalpies of ~−74 kJ/mol independent of water uptake. The interpretation of the TG results, however, does not fit a classical model of hydration of all oxygen vacancies. Instead, the hydration appears to be limited to a small fraction of the free vacancies. Hydration further decreases as the Nd content (x) and long-range order increases and regions of disorder decrease. We propose a new model explaining why hydration occurs only in a small fraction of the nominally free vacancies: The higher basicity of La/Nd compared to Ce promotes protonation at oxide ion sites with high coordination to La/Nd, and the observed water uptake and modelling suggests that mainly oxide ions fully coordinated to 4 La/Nd neighbours become protonated. The statistical variation of coordination around oxygen sites in a disordered fluorite oxide creates a limited number of such oxide ions sites which results in limited hydration. The model matches well the experimental results and DFT calculations of proton trapping at the fully La-coordinated sites for 50% La-doped CeO2, and also rationalizes conductivity data.

Statistics of primary radiation defects in pure nickel

Molecular dynamics simulations were applied to study radiation damage formation in collision cascades initiated by primary knock-on atoms (PKA) with energy EPKA = 5, 10, 15 and 20 keV in pure nickel at temperature T = 100, 300, 600, 900 and 1200 K. A series of 24 collision cascades per EPKA,T set was simulated to assure statistical reliability of the results. The number of Frenkel pairs, NFP, the fraction of vacancies, σvac, and self-interstitials (SIA), σSIA, in point defect clusters, the average size of vacancy and SIA clusters, Nvac and NSIA, respectively, and the average vacancy and SIA cluster per cascade yield, Yvac and YSIA, respectively, were evaluated as functions of EPKA,T. It is found that NFP∝EPKA under all simulated conditions. Both σvac and σSIA demonstrate similar dependence on EPKA. σvac matches Yvac, while σSIA correlates with NSIA and is affected by SIA diffusivity. Nvac is determined by the irradiation temperature and thermal stability of vacancy clusters. They are stable at T⩽ 300 K and Nvac∝EPKA, whereas at 600 K ⩽T⩽ 900 K, Nvac≈6 and 10, which corresponds to the sizes of regular stacking fault tetrahedra. SIA cluster per cascade yield YSIA∝NFP and therefore ∝EPKA under all simulated irradiation conditions.

Novel black bismuth oxide (Bi2O3) with enhanced photocurrent generation, produced by high-pressure torsion straining

Bismuth oxide (Bi2O3), a yellow semiconductor ceramic, is considered as an advanced photoactive energy material, but its activity still needs further improvement for practical applications. In this study, black Bi2O3 with nanocrystalline structure and large fraction of oxygen vacancies is synthesized by mechanical straining via the high-pressure torsion (HPT) method. The black oxide exhibits enhanced light absorbance under both UV and visible lights. It shows a bipolar photocurrent behavior with up to six times higher photocurrent density compared with the unprocessed yellow Bi2O3. This study shows the high potential of severely-strained black Bi2O3 for photovoltaic, photoconductivity and photocatalytic applications.

Vacancy effect of antimony on shear deformation mechanisms of CoSb3 thermoelectric material

Skutterudite CoSb3 is one of the most extensively studied thermoelectric materials in the moderate temperature range, but the easily volatilization of element Sb leads to the vacancy defect as well as the poor mechanical properties of CoSb3. To examine the effect of Sb vacancy on its failure mechanisms, we applied shear loads on defective CoSb3 along (0 0 1)/〈1 0 0〉 slip system by using both density functional theory (DFT) and molecular dynamics (MD) simulations. Both DFT and MD simulation results showed that Sb vacancies can remarkably reduce the mechanical strength of CoSb3. When the Sb vacancy fraction is 4%, the mechanical strength decreases 36% compared with that (7.17 GPa) in flawless CoSb3. Sb vacancies lead to incomplete Sb4-rings and Co-Sb frameworks. This would weaken the local rigidity of the structure, accelerating the breakage of Sb4-rings as well as the slippage of the Co-Sb frameworks. The results provide a perspective for better understanding the vacancy effect on the failure mechanisms of skutterudite CoSb3.

Mie scattering of phonons by point defects in IV-VI semiconductors PbTe and GeTe

Point defects in solids such as vacancy and dopants often cause large thermal resistance. Because the lattice site occupied by a point defect has a much smaller size than phonon wavelengths, the scattering of thermal acoustic phonons by point defects in solids has been widely assumed to be the Rayleigh scattering type. In contrast to this conventional perception, using an ab initio Green's function approach, we show that the scattering by point defects in PbTe and GeTe exhibits Mie scattering characterized by a weaker frequency dependence of the scattering rates and highly asymmetric scattering phase functions. These unusual behaviors occur because the strain field induced by a point defect can extend for a long distance much larger than the lattice spacing. Because of the asymmetric scattering phase functions, the widely used relaxation time approximation fails with an error of ~20% at 300 K in predicting lattice thermal conductivity when the vacancy fraction is 1%. Our results show that the phonon scattering by point defects in IV-VI semiconductors cannot be described by the simple kinetic theory combined with Rayleigh scattering.

Filling vacancies in a Prussian blue analogue using mechanochemical post-synthetic modification.

Mechanochemical grinding of polycrystalline powders of the Prussian blue analogue (PBA) Mn[Co(CN)6]2/3□1/3·xH2O and K3Co(CN)6 consumes the latter and chemically modifies the former. A combination of inductively-coupled plasma and X-ray powder diffraction measurements suggests the hexacyanometallate vacancy fraction in this modified PBA is reduced by approximately one third under the specific conditions we explore. We infer the mechanochemically-driven incorporation of [Co(CN)6]3- ions onto the initially-vacant sites, coupled with intercalation of charge-balancing K+ ions within the PBA framework cavities. Our results offer a new methodology for the synthesis of low-vacancy PBAs.

Mechanical alloying of Si and Fe: Quantum-mechanical calculations applied to Mössbauer and X-ray diffraction studies

Hyperfine parameters of iron nuclei such as isomer shift, quadrupole splitting, and asymmetry parameter are calculated for β-FeSi2 with and without vacancies, using density functional theory. They are applied, in combination with parameters of α-FeSi2 obtained earlier, for analyzing Mössbauer and X-ray diffraction experiments on the mechanical alloying of silicon with iron in the range of 1–33 at. % of Fe. Such an approach allows more detailed and precise information to be obtained on the structure of Fe–Si samples. In particular, the fraction of vacancies in α- and β-FeSi2 is estimated, and the mechanism of formation of these phases and their transformation into each other is discussed. A new model is developed for analyzing experiments on hyperfine interaction, such as Mössbauer spectroscopy, time-differential perturbed-angular correlation, and the like, for Fe–Si systems with a high Si content.

MD simulations of primary damage formation in L12 Ni3Al intermetallics

Computer modelling by molecular dynamics (MD) was employed to study radiation damage created in collision cascades in L12 Ni3Al intermetallic compound. Either Al or Ni primary knock-on atoms (PKA) with PKA energy 5 keV ≤EPKA≤ 20 keV were initiated in the intermetallic crystals at ambient temperatures 100 K ≤T≤ 1200 K. At least 24 different cascades for each (EPKA, T, PKA type) set were simulated in order to mimic an isotropic spatial and random temporal distribution of PKAs. The total yield of nearly 1000 simulated cascades provides the largest yet reported sampling to get statistically reliable quantitative results. The obtained MD simulation data were analysed to get the number of Frenkel pairs, the fraction of point defects in point defect clusters, the fraction of Al and Ni vacancies, self-interstitial atoms (SIA) and anti-sites created in collision cascades as a function of (EPKA, T, PKA type). It was found that under identical simulation conditions, collision cascades in L12 Ni3Al initiated by Al PKAs generate more radiation damage, whereas collision cascades initiated by Ni PKAs produce more chemical disorder. Preferred formation of Ni SIAs in collision cascades in Ni3Al was observed under all ambient temperatures and PKA energies.

The influence of oxygen vacancy concentration in nanodispersed non-stoichiometric CeO2-δ oxides on the physico-chemical properties of conducting polyaniline/CeO2 composites

Cerium oxide (CeO2-δ) ultrafine nanoparticles, with the lower (CeO2-δ-HT) and higher (CeO2-δ-SS) fraction of oxygen vacancies, were used as anchoring sites for the polymerization of aniline in acidic medium. As a result, polyaniline-emeraldine salt (PANI-ES)-based composites (PANI-ES@CeO2-δ-HT and PANI-ES@CeO2-δ-SS) were obtained. The interaction between CeO2-δ and PANI was examined by FTIR and Raman spectroscopy. The PANI polymerization is initiated via electrostatic interaction of anilinium cation and Cl− ions (adsorbed at the protonated hydroxyl groups of CeO2-δ), and proceeds with hydrogen and nitrogen interaction with oxide nanoparticles. Tailoring the oxygen vacancy population of oxide offers the possibility to control the type of PANI-cerium oxide interaction, and consequently structural, electrical, thermal, electronic and charge storage properties of composite. A high capacitance of synthesized materials, reaching ∼294 F g−1 (PANI-ES), ∼299 F g−1 (PANI-ES@CeO2-δ-HT) and ∼314 F g−1 (PANI-ES@CeO2-δ-SS), was measured in 1 M HCl, at a common scan rate of 20 mV s−1. The high adhesion of PANI with cerium oxide prevents the oxide from its slow dissolution in 1MHCl thus providing the stability of this composite in an acidic solution. The rate of electrochemical oxidation of emeraldine salt into pernigraniline was also found to depend on CeO2-δ characteristics.

Radiation Defects in Aluminum: MD Simulations of Collision Cascades in the Bulk of Material

The molecular dynamics method has been used to simulate displacement cascades created by primary knock-on atoms (PKAs) with energies EPKA= 5, 10, 15, and 20 keV in aluminum at temperatures T = 100, 300, and 600 K. A series of 24 cascades was simulated for each pair of the parameters (EPKA, T) to ensure a representative sampling. The number of Frenkel pairs, the fraction of vacancies (evac) and self-interstitial atoms (SIA) (eSIA) in point defect clusters, the average yield of vacancy (Yvac) and SIA (YSIA) clusters per cascade, and the average relaxation time τc have been determined as a function of (EPKA, T). It has been shown that displacement cascades of in aluminum decompose into several sub-cascades along PKA trajectory. Such a spatial structure of cascades is responsible for the absence of the dependence of the values of 〈evac〉, 〈eSIA〉, 〈Nvac〉, 〈NSIA〉, and τc on the energy of PKAs.

Investigating the effects of vacancies on self-diffusion in silicon clusters using classical molecular dynamics.

Due to recent advances in the field of microelectronics, the growth in microelectronics applications, and the exponentially increasing demand for microelectronic devices in the power sector, it is important to study the behavior of silicon at the nanoscale, given that nanoclusters of silicon could be used to design a new kind of lithium-ion batteries with strongly enhanced performance. Here, molecular dynamics was employed to calculate the self-diffusion coefficients of silicon clusters at room temperature and at a temperature approaching the melting point of silicon, complementing experimental efforts in this field. Silicon clusters of the same spherical geometry and size but with different vacancy fractions were studied using molecular dynamics using the Tersoff potential in order to estimate phase changes and self-diffusion coefficients. At 300K, the self-diffusion coefficient was found to vary non-monotonically: the self-diffusion coefficient at a vacancy fraction of 7.5% is half than the vacancy at a fraction of 0%, while the self-diffusion coefficient at a vacancy fraction of 20% is two orders of magnitude larger than that at a vacancy fraction of 0%. However, there is only a marginal monotonic increase in the self-diffusion coefficient values with vacancy fraction at 2000K. The results of this investigation of vacancy-mediated self-diffusion could aid attempts to improve diffusion control, which is crucial to nanocluster applications in various devices, and the results also provide insight into how the temperature, energy, pressure, and phase changes of the silicon clusters depend on vacancy fraction. This may ultimately allow the design and selection of materials for thermoelectric and optoelectronic devices and thermal transducers to be optimized. Our results also indicated that the findings we obtained for the clusters are independent of the particular random vacancy distribution considered and the heating rate applied to the clusters. Graphical Abstract Silicon nanoparticles (SNP) are among the best options to choose from for the design of devices for renewable energies; SNP based material performance can be effectively tailored by controling the vacancy, temperature and other properties of the SNP.

Critical vacancy density for melting in two-dimensions: the case of high density Bi on Cu(111)

The two-dimensional melting/solidification transition of the high density [2012] phase of Bi on Cu(111) has been studied by means of low energy electron microscopy (LEEM). This well defined phase has an ideal concentration of one Bi atom per two Cu surface atoms (θBi = 0.500). The Bi density is determined accurately in situ and the highest melting temperature of 538 K occurs at exactly θBi = 0.500. A significantly reduced melting temperature is observed for lower Bi densities (θBi < 0.500) and, surprisingly, also for θBi > 0.500. At ∣Δ θBi∣ = 0.015 the melting temperature is reduced by about 20 K. This lowering of the melting temperature is attributed to a critical vacancy density at melting and we propose that this quantity triggers the 2D solid–liquid phase transition. For this particular system, the critical vacancy fraction for melting amounts to 5%–6%. Above θBi = 0.500 and near melting a homogeneous, unilaterally compressed phase, ‘[2012]’ is observed, with a density that increases continuously with coverage. It is commensurate along and incommensurate along The ability to distinguish between Bi accommodated within the ‘[2012]’ phase and Bi residing on top as a lattice gas by applying LEEM is of crucial importance for the analysis.

Reversion of natural ageing in Al-Mg-Si alloys

Al-0.59Mg-0.79Si (wt.%) alloy was naturally aged for 2 weeks after solutionising and quenching to produce a high number density of atomic clusters. This caused an increase of both hardness and electrical resistivity. Samples were then subjected to reversion ageing (RA) by applying temperature treatments at 250 °C of 1 s–5 min duration. The associated changes of hardness, resistivity, positron lifetime and the DSC dissolution signal were measured. Subsequent secondary ageing at 20 °C was studied in an analogous way. Ideas are developed how solute supersaturation and vacancy fraction vary during RA. It is found that the most reverted state still contains enough clusters to trap positrons efficiently and to contribute to hardness. Longer ageing at 250 °C leads to the formation of coherent and later incoherent precipitates. The differences between the evolution of resistivity and hardness indicate that besides the level of solute supersaturation the Mg:Si ratio in the matrix changes during reversion.

Evaluation of palladium diffusion in Ti-Rich TiNi alloys

In order to study the diffusion kinetics of Pd into Ti-rich TiNi alloys, diffusion annealing treatments of palladium-coated Ti53Ni47 and Ti50.2Ni49.8 alloys were carried out at temperatures ranging from 700 to 1000 °C. Linear Arrhenius temperature dependencies with the pre-exponential factors of 2 ± 1.1 × 10−10 and 1.05 ± 1.5 × 10−8 m2 s−1 and the activation energies of −128 ± 4 and −153 ± 10 kJ mol−1 were obtained for Pd diffusivity into Ti53Ni47 and Ti50.2Ni49.8 alloys, respectively. The suggested mechanism for Pd diffusion in Ti53Ni47 was anti-site bridge mechanism. While the rapid diffusion of Pd in Ti50.2Ni49.8 was attributed to the high volume fraction of vacancies in Ni sub-lattice of TiNi lattice and Next Nearest jumps of Pd atoms into abundant Ni vacancies. The lower diffusivity of Pd in Ti53Ni47 was related to the dominant diffusion mechanisms in two systems and low solubility of Pd in Ti2Ni precipitates which located in grain boundaries, as well.The other part of the study was denoted to inter-diffusion studies in Ti53Ni47/Pd and Ti50.2Ni49.8/Pd diffusion couples, which confirmed the higher diffusivity of Pd in Ti50.2Ni49.8.

Free energy and concentration of crystalline vacancies by molecular simulation

ABSTRACTWe present an approach for evaluating the concentration of vacancy defects in crystalline materials by molecular simulation. The proposed method circumvents the problem of equilibration of the number of ‘lattice sites’ M, which characterises the trade-off between more, smaller lattice cells (with some vacant), versus fewer, larger cells. Working in a grand-canonical framework, we instead fix M and solve for the chemical potential μ that ensures thermodynamic consistency of the ensemble-averaged pressure and the grand potential. Having determined μ this way for the given M, the equilibrium vacancy concentration and free energy are easily determined. Methods are demonstrated for the classical Lennard–Jones fcc crystal, examining all states where the crystal is stable. We find for this system that the effect of equilibrating M is negligible at all conditions. Also, although the vacancy fraction varies by many orders of magnitude with temperature and density, we find that the value at melting is nearly independent of density, equal to about . Results further show that a lattice-energy approximation (ignoring entropic effects) underestimates the correct concentration by four orders of magnitude at almost all conditions; ignoring only anharmonic contributions underestimates the vacancy concentration at melting by nearly one order of magnitude.

Multi-resonance frequency spin dependent charge pumping and spin dependent recombination - applied to the 4H-SiC/SiO2 interface

We report on a new electrically detected magnetic resonance (EDMR) approach involving spin dependent charge pumping (SDCP) and spin dependent recombination (SDR) at high (K band, about 16 GHz) and ultra-low (360 and 85 MHz) magnetic resonance frequencies to investigate the dielectric/semiconductor interface in 4H-SiC metal-oxide-semiconductor field-effect transistors (MOSFETs). A comparison of SDCP and SDR allows for a comparison of deep level defects and defects with energy levels throughout most of the bandgap. Additionally, a comparison of high frequency and ultra-low frequency measurements allows for (1) the partial separation of spin-orbit coupling and hyperfine effects on magnetic resonance spectra, (2) the observation of otherwise forbidden half-field effects, which make EDMR, at least, in principle, quantitative, and (3) the observation of Breit-Rabi shifts in superhyperfine measurements. (Observation of the Breit-Rabi shift helps in both the assignment and the measurement of superhyperfine parameters.) We find that, as earlier work also indicates, the SiC silicon vacancy is the dominating defect in n-MOSFETs with as-grown oxides and that post-oxidation NO anneals significantly reduce their population. In addition, we provide strong evidence that NO anneals result in the presence of nitrogen very close to a large fraction of the silicon vacancies. The results indicate that the presence of nearby nitrogen significantly shifts the silicon vacancy energy levels. Our results also show that the introduction of nitrogen introduces a disorder at the interface. This nitrogen induced disorder may provide at least a partial explanation for the relatively modest improvement in mobility after the NO anneals. Finally, we compare the charge pumping and SDCP response as a function of gate amplitude and charge pumping frequency.

Short-range order clustering in BCC Fe–Mn alloys induced by severe plastic deformation

The effect of severe plastic deformation, namely, high-pressure torsion (HPT) at different temperatures and ball milling (BM) at different time intervals, has been investigated by means of Mössbauer spectroscopy in Fe100–xMnx (x = 4.1, 6.8, 9) alloys. Deformation affects the short-range clustering (SRC) in BCC lattice. Two processes occur: destruction of SRC by moving dislocations and enhancement of the SRC by migration of non-equilibrium defects. Destruction of SRC prevails during HPT at 80–293 K; whereas enhancement of SRC dominates at 473–573 K. BM starts enhancing the SRC formation at as low as 293 K due to local heating at impacts. The efficiency of HPT in terms of enhancing SRC increases with increasing temperature. The authors suppose that at low temperatures, a significant fraction of vacancies are excluded from enhancing SRC because of formation of mobile bi- and tri-vacancies having low efficiency of enhancing SRC as compared to that of mono vacancies. Milling of BCC Fe100–xMnx alloys stabilises the BCC phase with respect to α → γ transition at subsequent isothermal annealing because of a high degree of work hardening and formation of composition inhomogeneity.

Molecular dynamics simulations of thermal expansion properties of single layer graphene sheets

The area coefficients of thermal expansion (CTEs) of perfect single layer graphene sheet (SLGS) and SLGS with vacancy defects of different distributions were calculated in this work through molecular dynamics (MD) simulations. The effects of some parameters such as temperature, SLGS size, sample area size, vacancy fraction and vacancy distribution on CTE were investigated extensively. Numerical results clearly revealed that for both perfect and defective SLGSs, the area CTEs are negative and nonlinear with the temperature variation within a wide temperature range. Moreover, the area CTEs tend to be more insensitive to the temperature when temperature is higher than 600 K. The area CTE of a perfect SLGS converges only when the SLGS size and the ratio of the sample size to the SLGS size is above a critical value. When the SLGS size or the sample size is small, the area CTE shows distinct size-dependence. In addition, a set of empirical formulations is proposed for evaluating the area CTEs of perfect SLGSs within a wide temperature range. For the SLGS with vacancy defects, the area CTE decreases with the increase of vacancy fraction within the temperature range considered. Furthermore, compared with a decentralised distribution of vacancy defects, a concentrated distribution leads to a smaller value of area CTE of SLGS, especially for the case of high vacancy fraction.