Diffusion Of Point Defects Research Articles

Origin of ultrafast annihilation effect of martensite aging: Atomistic simulations

Martensite aging effects, e.g., martensitic stabilization and rubberlike behavior, have attracted considerable attention in the past decades. It is known that martensite aging effects can be quickly eliminated once the aged martensite is brought into the parent phase even for a very short time, i.e., the annihilating effect of martensite aging (AEMA). However, the underlying mechanism of AEMA remains unclear due to the lack of an effective tool to probe the atomic processes during AEMA. It is unclear (1) whether AEMA is caused by a mere diffusionless transformation into the parent phase or by a diffusional process in the parent phase and (2) why long-time aging in martensite can be so easily eliminated in the parent phase. In this paper we use combined molecular-dynamics and Monte Carlo simulations to show that the origin of AEMA is related to atomic diffusion in the parent phase, not merely the reverse phase transformation. The open structure in the B2 parent phase and high-temperature result in a significantly higher diffusivity of point defects in the parent phase compared with that in the martensite; this explains the ultrafast annihilation of martensite aging. We attribute the driving force of AEMA to the symmetry-conforming short-range ordering tendency of point defects.

Evolution of an iron passive film in a borate buffer solution (pH 8.4)

The evolution under open-circuit conditions of iron passive films formed at 0.8 VSCE in a borate buffer solution at pH 8.4 was investigated with electrochemical impedance spectroscopy (EIS) and cyclic voltammetry. The composition of the freshly formed passive film as determined by X-ray photoelectron spectroscopy (XPS) was found to be in agreement with a bilayer model, where the inner layer is composed mainly of iron oxide and the outer layer consists of a hydrated material. Results of XPS measurements also showed that the open-circuit breakdown of passive films was consistent with a reductive dissolution mechanism. When the iron electrode reached an intermediate stage in the open-circuit potential decay (approximately −0.3 VSCE), the oxide film, containing both Fe(II) and Fe(III), was still protective. The impedance response in this stage exhibited a mixed control by charge transfer at the metal/film and film/solution interfaces and diffusion of point defects through the film. At the final stage of the open-circuit potential decay (approximately −0.7 VSCE), the oxide film was very thin, and the ratio of Fe3+/Fe2+ and O2−/OH− had decreased significantly. The impedance response also exhibited a mixed charge-transfer–diffusion control, but the diffusion process was related to transport of species in the electrolyte solution resulting from dissolution of the oxide film.

Lattice Strain and Strain Evolution in Hydrogen-Implanted Materials: The Roles of Mechanical Properties and Hydrogen Diffusion

The strain introduced by the implantation of hydrogen into a material for subsequent exfoliation and layer transfer provides a useful monitor for the kinetics of the subsequent exfoliation process. We demonstrate that the level of strain induced by the implantation process, for a given dose, and at low temperatures, is determined by the energy loss per implanted ion by nuclear collisions, which, in turn, correlates strongly with the mechanical parameters such as the Young's modulus and bulk modulus. The magnitude of the dose linearly changes the maximum strain in the layer. At higher temperature implants, the maximum strain is reduced and is associated with the diffusion of point defects during implantation. These observations provide a means to predict the implant profile for any combination of implant dose, energy, and target material for parameters used for exfoliation processes at low temperatures. These results are also consistent with the literature that shows such a relationship for heavier ions at lower doses and for hydrogen implanted into semiconductors with different crystallographic orientation.

Simulation of the diffusion of point defects in structures with local elastic stresses

The stress-mediated diffusion of nonequilibrium point defects into the bulk of a semiconductor is investigated by computer simulation. It is assumed that the point defects are generated on the surface of a semiconductor and that in the course of diffusion they pass through the local region of elastic stresses because the average length of defect migration is greater than the thickness and depth of the strained layer. Within the strained layer, point defect segregation or heavy defect depletion occurs if defect drift under stresses is directed in or out of the layer, respectively. The calculations also show that, in contrast to the case of local defect sink, the local region of elastic stresses practically does not change the distribution of defects beyond this region if there is no generation/absorption of point defects within the strained layer.

Clarification of high density electronic excitation effects on the microstructural evolution in UO 2

In order to understand the properties of ion tracks and the microstructural evolution under accumulation of ion tracks in UO 2, 100 MeV Zr 10+ and 210 MeV Xe 14+ ions irradiation examinations have been done at a tandem accelerator facility of JAEA-Tokai, and it has been observed the microstructure by means of a transmission electron microscope (TEM) and a scanning electron microscope (SEM) in CRIEPI. Comparison of the diameter of ion tracks between UO 2 and CeO 2 under irradiation with 100 MeV Zr 10+ and 210 MeV Xe 14+ ions at room temperature clarify that the sensitivity on high density electronic excitation of UO 2 is much less than that of CeO 2. By the cross-sectional observation of UO 2 under irradiation with 210 MeV Xe 14+ ions at 300 °C, elliptical changes of fabricated pores that exist till ∼6 μm depth and the formation of dislocations have been observed in the ion fluence over 5 × 10 14 ions/cm 2. The drastic changes of surface morphology and inner structure in UO 2 indicate that the overlapping of ion tracks will cause the point defects, enhance the diffusion of point defects and dislocations, and form the sub-grains at relatively low temperature.

Newtonian viscous creep in metals

All materials exhibit Newtonian viscous creep behavior at low stresses and high temperatures. We review here such creep behaviors in metals comprising of pure metals and alloys. The underlying creep mechanism(s) depends mainly on the grain size and test temperature while other factors such as the initial dislocation density might also be a factor. Coble creep due to diffusion of point defects through grain boundaries is known to be the dominant creep mechanism in metals with very small grain sizes and relatively low temperatures while Nabarro-Herring creep becomes important for intermediate grain sizes and/or high temperatures. Large grain size and bulk single crystalline metals exhibit Harper-Dorn creep due to dislocation motion rather than point defect diffusion-dominated mechanisms albeit the underlying mechanism is still unclear. Microstructural studies of the specimens deformed in the Harper-Dorn regime have provided some insights. Recent studies suggest that microstructural characterization of deformed specimens is necessary for accurate determination of the rate controlling mechanism. The aim of this paper is two fold namely, to first review the viscous creep mechanisms and to present recent results on Ti3Al2.5V alloy emphasizing the importance of post creep microstructural characterization in establishing the rate controlling mechanism(s).

Determination of the Diffusivity of Point Defects in Passive Films on NiTi and NiTiAl Alloys

A point defect model based on the movement of cation and anion defects in an electrostatic field was carried out to explain the growth and dissolution behavior of a passivation layer on NiTi and NiTiAl thin films. The calculated value of diffusivity was in range of 10−16 to 10−17 cm2/s. The defect of oxygen vacancy revealed that the passive film was an n-type semiconductor. Mott-Schottky analysis showed that the doping level within a passive film was rather large and in the order of 1020 to 1021 cm−3 film, which was considered to be a highly doped structure. The high-resolution transmission electron microscopy (HRTEM) images showed that the highly doped structure consisted of amorphous and crystalline structures of TiO2 and Al2O3, respectively. A thermodynamic evaluation for the difference between crystalline and the fully amorphous oxides was calculated to be 57.57 and 96.87 kJ/mol, respectively. In the amorphous region, the electronic level arises from the presence of an energy band gap in the ideal crystalline structure. Therefore, the smaller donor density and the lower diffusion coefficient retarded the defect movement in the passivation layer, and improved the stability of the passive film during corrosion.

Low-symmetry T d-distorted Co 2+ centres in ceramic ZnO:Co

The reflectivity and photoluminescence of cobalt-doped ZnO ceramics samples have been studied by optical spectroscopy. The result reveals, in addition to the emission bands associated with intrinsic defects, weak narrow emission peaks which are ascribed to a mixed 4T 1(P), 2T 1(G), 2E(G) → 4A 2(F) transition between Co 2+ d-levels located in regular Zn 2+ sites, [Co 2+] Zn and a broad near infrared emission band centred at 710 nm which has been interpreted as belonging to Co 2+ ions located in a perturbed Zn 2+ site, [ Co 2 + ] Zn ∗ . This study shows that the annealing treatment at 800 °C activates the diffusion of point defects thus creating more [Co 2+] Zn centres.

Programmed strengthening of crystalline materials using copper and aluminum as an example

Fundamental propositions of the well-known method of programmed strengthening of crystalline solids are described. It is shown that when mechanical (deformation) and thermal (tempering, annealing, and aging) actions are combined, the loading rate is determined by the rate of diffusion and microshear processes of relaxation of internal stresses in regions of defect accumulation and by the formation of stable complexes as a result of directional diffusion of point defects.

Inductively Coupled Argon Plasma-Enhanced Quantum-Well Intermixing: Cap Layer Effect and Plasma Process Influence

Comprehensive investigation on inductively coupled argon plasma-enhanced quantum-well intermixing is done on an InGaAs-InP quantum-well structure with p-/n-doped InP and InGaAs caps using polarized edge-emitting photoluminescence analysis technique. The derived diffusion lengths on group V and III sublattices show that the cap material plays an important role as it influences both the accumulation and diffusion of point defects during plasma process and annealing process, respectively. The large blue and red shift can be realized with p-InP and n-InGaAs caps, respectively, by controlling the diffusion length ratio for interdiffusion on two sublattices.

On intrinsic point defect cluster formation during Czochralski crystal growth

AbstractProgress in characterizing, understanding and controlling intrinsic point defect generation and clustering during Cz growth of single crystals is discussed. It is shown that intrinsic point defects in Si and Ge behave quite similar and that the observed clustering behaviour can be understood taking into account the impact of extrinsic point defects, doping and strain on the formation, recombination and diffusivity of the intrinsic point defects in both materials. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Diffusion of Point Defects from Ion Implanted 4H-SiC: Cathodoluminescence Observation

Depth profiles of ion-implantation induced defect centers have been investigated by cross-sectional CL measurements in the energy range from visible to near infrared. CL observation has shown that point defects diffused out from implanted region to ~10 µm depth during activation annealing. Annealing temperature dependence of the depth distribution of CL intensity of these defects has suggested that structural transformation of point defects proceeds as “silicon vacancy (VSi) → carbon vacancy-antisite pair (VC-CSi ; UD2) → antisite pair (CSi-SiC ; DI)”.

Ab-Initio Modelling of Point Defect-Impurity Interaction in Tungsten and other BCC Transition Metals

Ab-initio calculations have been performed to investigate systematically defect-impurity interaction in Tungsten and other bcc transition metals. It is found that the most stable configuration of C and N atoms is the octahedral interstitial site whereas O and H atoms are located in the tetrahedral configuration. For the particular case of bcc-W, the binding energies formed by the carbon and nitrogen atoms located at octahedral sites, and mono-vacancy on a nearest neighbor site are very large, 1.39 eV and 1.91 eV, respectively. Implication of these results of diffusion of point defects in tungsten is discussed and compared with the case of bcc-Fe.

The use of extended-defect dissolution as a probe for stress-induced interstitial diffusion anisotropy

In this paper, the influence of biaxial strain-induced diffusion anisotropy on the evolution of extended defects in silicon has been analyzed. Point-defect diffusion anisotropy has been modeled and implemented within an atomistic kinetic Monte Carlo framework. The anneal of {3 1 1}-defects has been simulated for self-interstitial diffusion anisotropies varying within the plausible ranges. From these simulations, it is observed that diffusion anisotropy has a significant effect on the competition between defect ripening and dissolution. In particular, it is shown that the plot of {3 1 1} density versus {3 1 1} mean size could be used to check for the existence of self-interstitial diffusion anisotropy.

DYNAMICS OF RADIATIVE POINT DEFECTS IN GALLIUM ARSENIDE DURING RELAXATION OF LOCAL HEATING

In this paper we have analyzed the dynamics of redistribution of the radiative point defects in gallium arsenide during relaxation of the local heating in a semiconductor sample, generated by a radiative particle in the initial stage of stabilization of clusters of defects. We calculated: (i) the maximum heating temperature of a semiconductor sample, and (ii) the point defect diffusion coefficient corresponding to the maximum temperature.

Anisotropic diffusion of point defects in a two-dimensional crystal of streptavidin observedby high-speed atomic force microscopy

The diffusion of individual point defects in a two-dimensional streptavidin crystal formedon biotin-containing supported lipid bilayers was observed by high-speed atomicforce microscopy. The two-dimensional diffusion of monovacancy defects exhibitedanisotropy correlated with the two crystallographic axes in the orthorhombicC 222 crystal; in the 2Dplane, one axis (the a-axis) is comprised of contiguous biotin-bound subunit pairs whereas the other axis (theb-axis) is comprised of contiguous biotin-unbound subunit pairs. The diffusivity along theb-axis is approximately 2.4 times larger than that along thea-axis. This anisotropy is ascribed to the difference in the association free energybetween the biotin-bound subunit–subunit interaction and the biotin-unboundsubunit–subunit interaction. The preferred intermolecular contact occurs betweenthe biotin-unbound subunits. The difference in the intermolecular bindingenergy between the two types of subunit pair is estimated to be approximately0.52 kcal mol−1. Another observed dynamic behavior of point defects was fusion of two pointdefects into a larger defect, which occurred much more frequently than the fissionof a point defect into smaller defects. The diffusivity of point defects increasedwith increasing defect size. The fusion and the higher diffusivity of larger defectsare suggested to be involved in the mechanism for the formation of defect-freecrystals.

Correlation between self-diffusion in Si and the migration mechanisms of vacancies and self-interstitials: An atomistic study

The migration of point defects in silicon and the corresponding atomic mobility are investigated by comprehensive classical molecular-dynamics simulations using the Stillinger-Weber potential and the Tersoff potential. In contrast to most of the previous studies both the point defect diffusivity and the self-diffusion coefficient per defect are calculated separately so that the diffusion-correlation factor can be determined. Simulations with both the Stillinger-Weber and the Tersoff potential show that vacancy migration is characterized by the transformation of the tetrahedral vacancy to the split vacancy and vice versa and the diffusion-correlation factor ${f}_{V}$ is about 0.5. This value was also derived by the statistical diffusion theory under the assumption of the same migration mechanism. The mechanisms of self-interstitial migration are more complex. The detailed study, including a visual analysis and investigations with the nudged elastic band method, reveals a variety of transformations between different self-interstitial configurations. Molecular-dynamics simulations using the Stillinger-Weber potential show that the self-interstitial migration is dominated by a dumbbell mechanism, whereas in the case of the Tersoff potential the interstitialcy mechanism prevails. The corresponding values of the correlation factor ${f}_{I}$ are different, namely, 0.59 and 0.69 for the dumbbell and the interstitialcy mechanisms, respectively. The latter value is nearly equal to that obtained by the statistical theory which assumes the interstitialcy mechanism. Recent analysis of experimental results demonstrated that in the framework of state-of-the-art diffusion and reaction models the best interpretation of point defect data can be given by assuming ${f}_{I}\ensuremath{\approx}0.6$. The comparison with the present atomistic study leads to the conclusion that the self-interstitial migration in Si should be governed by a dumbbell mechanism.

Anisotropic diffusion of point defects in metals under a biaxial stress field simulation and theory

We study the diffusion anisotropy (DA) of point defects in fcc and bcc metals in the presence of an applied biaxial stress. The DA depends sensitively on both the crystalline structure and the crystallographic direction in which the stress is applied. For example, interstitials in fcc metals diffuse faster in the plane of the compressive stress than normal to it when the stress is applied to the (001) plane, but they diffuse slower in-plane than out-of-plane when the stress is applied in the (111) plane. In contrast, an applied biaxial stress in the (001) plane of a bcc metal does not cause DA. These results can be explained by considering the interaction of the defects at their saddle point configurations with the external field, together with the constraints imposed by the crystal structure on the defect jump directions. Our calculations show that the DA can be large and lead to surprising results in a number of practical situations.

A mechanism of nucleation during thermal decomposition of solids

A microscopic model based on the appearance, diffusion, and aggregation of point defects allows to calculate the time of appearance of the first nucleus on a surface during a reaction between a solid and a gas. Calculated distributions of these times of appearance of the first nucleus are qualitatively compared to experimental ones, previously determined. The appearance of the point defects seems to be the most influential step on the time of appearance of the first nucleus. Moreover the comparison between experimental and calculated distributions allows to conclude that the frequency of appearance of the defects is higher on the edges than on the faces of the single crystal.

Investigation on Aging Behavior in Ferroelectric Materials with Perovskite Structure

The aging effect in ferroelectric materials plays a central role in the reliability of the related devices. These materials exhibit various kinds of aging behavior. Potassium doped barium titanate was aged below its Curie temperature for different periods of time. The transformation temperature was measured precisely by differential scanning calorimetry. It was observed that the transformation temperature increased with the increase of aging time. The dielectric performance in lanthanum doped ferroelectric lead titanate ceramics after aging was measured. A change of the dielectric spectrum was revealed. Furthermore, the fourier transformation infrared spectroscopy (FTIR) in the (Ba,Mg)TiO 3 system during aging was used and it showed unstable FTIR spectra as well. These phenomena are discussed in terms of diffusion of point defects during aging.