Diffusion Of Cation Vacancies Research Articles

Effect of the temperature on the passivity of the modified martensitic stainless steels

ABSTRACT The limits and nature of the passive behavior exhibited by modified martensitic stainless steels (MMSS) is studied in brine solutions at different temperatures using electrochemical methods and the point defect model (PDM), in an environment at pH is 3 and temperatures up to 150°C, using autoclaves with CO2 gas up to 2.8 MPa. The results show that passivity on the MMSS is characterized by the metal cation vacancy diffusivity (D) which indicates that the passivity up to temperatures of 75°C is more stable than at higher temperatures up to 150°C. The value of D is used to describe the transition between the passive layers at the threshold temperature of 75°C; at which exhibits the largest stability. MMSS specimens passivated at 75°C exhibit higher passivity stability in experiments performed at 25°C, when compared the pre passivation, suggesting that the change experienced at 75°C is at least partially irreversible.

Extremely enhanced thermal stability of Y2/3Cu3Ti4O12 ceramics by a Pechini method

AbstractY2/3Cu3Ti4O12 ceramic is a novel semiconductor with potential applications in electronic devices. However, its resistance drift and surface structure defects limit the application for negative temperature coefficient thermistor devices. A comparative study of solid‐phase and a Pechini processing of Y2/3Cu3Ti4O12 is presented in this paper, in the particular comparative study of stability. The Pechini method is found to be effective in lowering sintering temperature and improving thermal stability (ΔR/R0 < 0.3％) in comparison with solid‐phase method. The origin of the stability deterioration is related to the oxidation state of Cu ions and the migration of sublattice. The Pechini method overcomes stoichiometric ratio deviations and cation vacancy diffusion, thus reducing metal ion segregation in a continuous high‐temperature environment and thereby showing unique insensitivity to oxygen. The important role of the Pechini method in improving thermal stability is revealed, which is expected to be used in the preparation of high stability electron devices.

Influence of crystallization temperature and atmosphere on the phase composition, microstructure and electrical properties of Ni–Mn–O thin films

Nickel manganite thin films were prepared by chemical solution deposition from nitrate and acetate precursors on polished alumina substrates. By variation of crystallization temperature between 650 ​°C and 900 ​°C and atmosphere during cooling of the films, the morphology, phase formation and electrical properties of the films were investigated. The electrically conductive spinel phase was formed as a main phase independent of processing conditions exhibiting NTCR characteristics. Cooling down in nitrogen atmosphere favored the decomposition of the spinel phase into nickel oxide and a spinel phase with higher manganese content, but it also reduced the resistance drift of the samples. From this finding it can be deduced that a reduced oxygen partial pressure influences defect equilibria and by that decreases cation vacancy concentration. This supports the assumption that cation redistribution by a cation vacancy diffusion mechanism is a relevant mechanism of resistance drift in such thin films.

Theoretical and experimental studies of passivity breakdown of Aermet 100 ultra‐high stainless steel in chloride ion medium

AbstractPassivity breakdown and pitting behavior in the presence of aggressive Cl− on ultra‐high strength steel Aermet 100, commonly used for landing gear, has been studied, the data are explained in view of the point defect model (PDM). Cyclic potentiodynamic polarization (CPP), electrochemical impedance spectroscopy (EIS), and X‐ray photoelectron spectroscopy (XPS) were examined to observe corrosion progress and corrosion products in the salt spray tests at 4, 8, 12, 16, and 20 days intervals. The breakdown behavior was studied using CPP, whereas scanning electron microscopy (SEM) and digital confocal microscopy were used to explore the micromorphology and depth distribution of pits. The cumulative distribution basically satisfies a normal probabilistic distribution at the different concentrations of chloride and pH, which is consistent with PDM assuming a near‐normal distribution of potential breakdown sites regarding to the cation vacancy diffusivity.

Experimental constraints on hydrogen diffusion in garnet

The incorporation mechanisms and diffusional loss of hydrogen in garnet have been experimentally investigated. A suite of gem-quality hydrous spessartine- and grossular-rich garnets were analysed by Fourier transform infrared spectroscopy (FTIR) and by ion microprobe (SHRIMP-SI) to determine the calibration coefficients for quantification of FTIR data. The excellent agreement between measured absorption and OH/O indicates that the same molar extinction coefficient can be used for spessartine and grossular. The coefficient of 14400 l mol− 1 cm− 2 proposed by Maldener et al. (Phys Chem Miner 30:337–344, 2003) seems the most appropriate for both minerals. A grossular with 6.4% andradite and 1.6% almandine containing 834 ppm H2O, and an almost pure spessartine with 282 ppm H2O, were selected for diffusion experiments. 1.5-mm cubes of garnets were heated between 12 h and 10 days at 1 atm under various temperature (750–1050 °C) and oxygen fugacity ( $${f_{{{\text{O}}_2}}}$$ ) conditions, (ΔQFM + 15.2 to − 3.0). Diffusion profiles were acquired from sections through the cubes using FTIR, with a deconvolution algorithm developed to assess peak-specific behaviour. Different families of peaks have been identified based on their diffusive behaviour, representing hydrogen incorporated in different H-bearing defects. A dominant, fast, strongly $${f_{{{\text{O}}_2}}}$$ -dependent oxidation-related diffusion mechanism is proposed $$\left( {\{ {{\text{M}}^{2+}}+{{\text{H}}^+}\} +\frac{1}{4}{{\text{O}}_2}={{\text{M}}^{3+}}+\frac{1}{2}{{\text{H}}_2}{\text{O}}} \right)$$ (M=Fe, Mn) with a relatively low activation energy (158 ± 19 kJ mol− 1). This diffusion mechanism is likely restricted by availability of ferrous iron in grossular. At low oxygen fugacity, this diffusion mechanism is shut off and the diffusivity decreased by more than three orders of magnitude. A second, slower hydrogen diffusion mechanism has been observed in minor bands, where charge balance might be maintained by diffusion of cation vacancies, with much higher activation energy (≈ 200–270 kJ mol− 1). Spessartine shows clear differences in peak retentivity suggesting that up to four different H sites might exist. This opens exciting opportunities to use hydrogen diffusion in garnet as speedometer. However, it is essential to constrain the main diffusion mechanisms and the oxygen fugacity in the rocks investigated to obtain timescales for metamorphic or igneous processes.

Dielectric response and thermistor behavior of lead-free x NaNbO3 - (1-x) BiFeO3 electroceramics

The electroceramics of the x NaNbO3 - (1-x) BiFeO3 (x = 0.15, 0.25, 0.50, 0.75, 0.85, 1) system (NBNFO) have been prepared at moderate temperatures (600 °C). Microstructural analysis from high resolution electron microscopy reveals the existence of small crystals (10–150 nm), which agglomerate leading to micro-sized crystals. All the NBNFO compounds show perovskite-type structure and a transition from rhombohedral (S.G. R3c) to orthorhombic (S. G. P21ma) mean macroscopic symmetry takes place for x = 0.50, as analyzed by refinement of XRD data. Nevertheless, at the microstructural scale, the system can be described by a compositional-segregation picture in which structural heterogeneity is present. The materials show a positive temperature coefficient of resistivity (PTCR)-thermistor behavior above room temperature for x ≥ 0.50, up to maxima temperature values of 410, 393, 405 and 463 K for x values of 0.50, 0.75, 0.85 and 1, respectively. Above these temperatures, in the NTCR temperatures-region, conduction and polarization phenomena are analyzed from impedance and modulus formalisms in terms of two mechanisms implying different charge carriers: polaron hopping and/or diffusion of cation and oxygen vacancies. The activation energy values, Ea, corresponding to these processes are of about 0.50 and 1 eV, respectively. Finally, in the dielectric response displayed by the NBNFO ceramics, the development of a FE contribution as x increases is detected, which can be interpreted considering microstructural features.

Passivity breakdown on 300 M and S280 ultra-high strength steels in borate buffer solutions containing chloride ion

Passivity breakdown and the pitting behavior of ultra-high strength 300M and S280 steels, which are commonly employed for aircraft landing gear, in the presence of chloride has been explored and compared, and the data were interpreted using the point defect model (PDM). The breakdown behavior was studied using cyclic potentiodynamic polarization while SEM and digital confocal microscopy were employed to explore the micro morphology and depth distribution of pits. The cumulative distributions in the breakdown voltage are near-normal, which are consistent with the PDM, assuming the distributions of potential breakdown sites in terms of the cation vacancy diffusivity for different concentrations of chloride are normal. Compared to 300M, S280 steel exhibits shallower pitting and is more resistant to breakdown. From the potential scan rate dependence of the breakdown potential, the critical areal cation vacancy concentration, ξ, resulting in passivity breakdown, is 8.6×1013 No.cm−2 for 300M and 2.4×1013 No.cm−2 for S280. These values are structurally in close agreement with the calculated results, considering the vacancy condensation is either on the cation sublattice of the barrier layer or on the steels lattice.

Chemical diffusion and defect chemistry of grain boundaries in n-type barium titanate ceramics

Conductivity relaxation experiments have been performed on n-type barium titanate ceramics, co-doped with Y (donor) and Mn (acceptor), as a function of temperature (700–900°C) and oxygen partial pressure (0.001–1.0bar) by employing the van der Pauw technique. The chemical diffusion process owing to a change of the oxygen partial pressure in the surrounding atmosphere can be described by fast transport of oxygen vacancies along the grain boundaries (mainly core regions) and slow diffusion of cation vacancies into adjacent bulk regions (including space charge layers). A careful analysis of the experimental conductivity relaxation experiments allows the determination of the chemical diffusion coefficient for oxygen along grain boundary core regions with typical values in the range of 10−8 to 10−9cm2s−1 and an activation energy of 0.9eV (900>T/°C>700). Furthermore, the partial pressure dependence of the electronic conductivity can be interpreted by means of a defect chemical model involving grain boundary core regions where Mn is segregated and oxygen vacancies are the decisive mobile ionic charge carriers.

Passivity Breakdown on Copper: Influence of Temperature

Passivity breakdown and pitting on pure copper has been studied in simulated repository ground water (x M NaCl + 2 × 10−4 M Na2S, pH = 9.0 ± 0.2 at 40°C, as established by 0.2 M boric acid/sodium hydroxide buffer, with x = 0.05 to 5), at elevated temperatures (22°C to 82°C) by means of potentiodynamic polarization, cyclic voltammetry, and potentiostatic polarization, as well as by using SEM/ EDS to explore the elemental composition of the film. By assuming that the potential breakdown sites are normally distributed with respect to the cation vacancy diffusivity, a near normal distribution in breakdown potential is predicted by the Point Defect Model (PDM) and is found experimentally at different temperatures (42 - 82°C). From the potential sweep-rate dependence of the breakdown voltage, the critical, areal cation vacancy concentration, ξ, leading to passivity breakdown on copper is in the range of 9.15 × 1014 - 1.01 × 1015 cm−2. These values are in excellent agreement with that estimated structurally (1014 - 1015 cm−2), assuming vacancy condensation on either the copper substrate or upon the cation sublattice of the barrier layer (Cu2S).

Conductivity relaxation experiments on donor doped barium titanate ceramics: Effect of microstructure

The re-equilibration kinetics of n-conducting BaTiO3 ceramics upon a sudden change of the oxygen partial pressure of the surrounding gas phase has been investigated by application of conductivity relaxation experiments at 900°C using the van der Pauw method. The relaxation curves show two distinct equilibration processes. The first (fast) relaxation refers to diffusion of oxygen most probably via oxygen vacancies along the core regions of grain boundaries. The second (sluggish) relaxation process is related to slow diffusion of cation vacancies from the grain boundaries into the bulk. Even after 1000h, disk-shaped samples with an average grain size of 4μm and thicknesses between 0.5 and 1.0mm are not fully equilibrated with the gas phase. A modified Schottky barrier model, taking account of diffusion profiles of cation vacancies at the grain boundaries, enables a phenomenological interpretation of the conductivity relaxation curves. In addition, the defect chemistry of core regions of grain boundaries in n-type BaTiO3 ceramics has been studied by conductivity measurements as a function of oxygen partial pressure [0.5>p(O2)/bar>0.001].

Why a steady state void size distribution in irradiated UO2? A modeling approach

In UO2 pellets irradiated in standard water reactor, Xe nano-bubbles nucleate, grow, coarsen and finally reach a quasi steady state size distribution: transmission electron microscope (TEM) observations typically report a concentration around 10−4nm−3 and a radius around 0.5nm. This phenomenon is often considered as a consequence of radiation enhanced diffusion, precipitation of gas atoms and ballistic mixing. However, in UO2 thin foils irradiated with energetic ions at room temperature, a nano-void population whose size distribution reaches a similar steady state can be observed, although quasi no foreign atoms are implanted nor significant cation vacancy diffusion expected in conditions. Atomistic simulations performed at low temperature only address the first stage of the process, supporting the assumption of void heterogeneous nucleation: 25keV sub-cascades directly produce defect aggregates (loops and voids) even in the absence of gas atoms and thermal diffusion. In this work a semi-empirical stochastic model is proposed to enlarge the time scale covered by simulation up to damage levels where every point in the material undergoes the superposition of a large number of sub-cascade impacts. To account for the accumulation of these impacts, simple rules inferred from the atomistic simulation results are used. The model satisfactorily reproduces the TEM observations of nano-voids size and concentration, which paves the way for the introduction of a more realistic damage term in rate theory models.

Passive films growth on different Ni-Cr alloys from the migration of cation vacancies

One of the most popular models taking into account the semiconductive behavior of passive film is the Point Defect Model (PDM) which assumes that growth and breakdown kinetics are mainly governed by the migration of point defects under the high electric field within the film. Based on these PDM approaches and employing electrochemical technics such as a multi-frequency Mott–Schottky analysis, the role of Cr addition in synthetic Nickel-based alloys is investigated in this work. Particular attention is paid to cation vacancy diffusivity and flux. The accuracy of the transport coefficient of point defects as a good parameter for assessing the kinetics of film growth is then debated. Moreover, a beneficial synergic alloying effect on cation vacancy diffusivity for Nickel alloy with 24 wt% Cr is also observed and discussed.

(Olin Palladium Award) Some Critical Issues in the Breakdown of Passive Films

Passivity breakdown is the precursor to most forms of localized corrosion, including pitting, stress corrosion cracking, corrosion fatigue, and crevice corrosion, and also results in the phenomenon of depassivation, including electrochemical polishing and trans-passive dissolution, corresponding to passivity breakdown over macroscopic dimensions. In this OLIN Palladium Medal address, I will review my work in this area in terms of developing the Point Defect Model (PDM) for passivity and passivity breakdown and will identify principal challenges, as I see them. Starting with the rate equation for barrier layer growth, as predicted by the PDM, and by using phase-space analysis, I derive the condition that must be met for passivity to exist; the rate of growth of the barrier layer at the metal/barrier layer interface, due to the generation of oxygen vacancies at zero barrier layer thickness, must exceed the dissolution rate of the barrier layer. If this condition is met, the barrier layer may exist as a meta-stable phase and passivate the surface. In this regard, it is important to note that passivity is a kinetic state, not a equilibrium thermodynamic phenomenon, as has been so often assumed in the past. This fact has been used to resolve Faraday’s famous paradox arising from his iron-in-nitric acid experiments of 1833. Violation of this condition implies that the barrier layer cannot exist on the surface, even as a meta-stable phase and, hence, that the surface depassivates (loss of the barrier layer). If depassivation occurs over a macroscopic area, this phenomenon accounts for acid depassivation, transpassive dissolution, resistive depassivation, and electrochemical polishing. However, if depassivation occurs on a microscopic area, localized corrosion phenomena, including pitting, stress corrosion cracking, corrosion fatigue, and crevice corrosion occur, resulting in rapid penetration of a cavity into the metal. This unified theory of corrosion (UTC) accounts for all known forms of corrosion, except for hydrogen embrittlement, which lies outside of the realm of the PDM. After having established the basis of the UTC, I will then proceed to discuss the mechanism by which both general and localized depassivation occurs, which has been identified as being due to cation vacancy condensation at the m/bl interface, such that the barrier layer separates from the metal and film growth into the metal via oxygen vacancy generation cannot occur. This condition assures the depassivation inequality stated above, because the rate of growth of the barrier layer into the metal is reduced to zero and because all oxides exhibit a finite dissolution rate. The lecture will then proceed to show how the UTC predicts the critical breakdown voltage and induction time for a single breakdown site, as a function of the chloride activity, pH, and potential. It will then be shown that, by postulating that the breakdown sites are normally distributed with respect to the cation vacancy diffusivity, it is possible to derive the distribution functions for the breakdown voltage and the induction time for a large population of potential breakdown sites on the surface and to identify the critical characteristic of a breakdown site (high cation vacancy diffusivity). From there, I will demonstrate a variety of phenomena, including photo-inhibition of passivity breakdown, the impact of other halides (F-, Br-, I-), anion inhibition via competitive absorption into surface oxygen vacancies, the role of the precipitated outer layer, and the predicted impact of alloying elements on passivity breakdown. Where possible and appropriate, these predictions will be compared with experiment. As expected the PDM contains many model parameters that are poorly-known. However, values for all of the parameters may be obtained by optimizing the expressions for the impedance of the barrier layer as a function of frequency and film formation potential, as derived from the PDM, on experimental electrochemical impedance spectroscopic (EIS) data and optimization of the expressions for passivity breakdown (potential and induction time) on the corresponding experimental data. Finally, I will demonstrate how the PDM has led to the development of a deterministic protocol, Damage Function Analysis (DFA), for predicting the evolution of localized corrosion damage on metal and alloy surfaces, as a system evolves along a pre-determined corrosion evolutionary path, which is defined as the path taken by the system in terms of those properties of the system (temperature, potential, pH, [Cl-], etc) that have a significant impact of the damage accumulation rate. Time permitting, I will demonstrate how DFA may be used to predict the evolution of general and localized corrosion damage in practical systems.

Kinetics of photostimulated transformations in nanosized bismuth films

Optical spectroscopy, gravimetric analysis, and microscopy studies show that irradiating bismuth films (d = 3–55 nm) with light (λ = 360 nm and I = 1.8 × 1015–7.0 × 1015 quantum cm−2 s−1) leads to major changes in their absorption and reflectance spectra and in film mass. The kinetic curves of the degree of photochemical transformation of bismuth films are shown to obey linear, inverse logarithmic, cubic, and logarithmic laws. The contact potential difference (CPD) of Bi and Bi2O3 films and the photo-electromotive force (emf) of Bi-Bi2O3 systems are measured. An energy band diagram of Bi-Bi2O3 systems is constructed. A model that includes the stages of generation, recombination, and redistribution of nonequilibrium charge carriers in the contact field of a Bi-Bi2O3 system, oxygen adsorption, diffusion of cation vacancies, and Bi2O3 formation is proposed.

Chemical diffusion in non-stoichiometric cuprous oxide

The kinetics and thermodynamics of point defect diffusion in non-stoichiometric cuprous oxide, Cu 2− y O, has been studied as a function of temperature (1173–1373 K) and oxygen pressure (10 2–7×10 4 Pa) using microthermogravimetric reequilibration technique. It has been shown that the defect diffusion coefficient of cation vacancies, constituting the predominant point defects in this oxide does not change with their concentration, clearly indicating that these defects do not interact and are randomly distributed in the crystal lattice. Consequently, cation vacancy diffusion coefficient, D V, being the direct measure of defect mobility, can be described by the following empirical equation: D V = 3.0 × 1 0 - 3 exp ( - 52.4 kJ mol - 1 RT ) cm 2 s - 1 .

Effects of rapid thermal annealing on the optical properties of In0.53Ga0.47As/In0.52Al0.48As multiple quantum wells with InGaAs and dielectric capping layers

We investigated the effects of InGaAs and dielectric (SiO2 and Si3N4) capping layers on the intermixing behavior of InGaAs/InAlAs multiple quantum wells (MQWs) after rapid thermal annealing (RTA). With a fixed RTA time of 45 s, no appreciable shift of photoluminescence (PL) peak energy was observed in the samples annealed up to 800 and 850 °C for the SiO2- and Si3N4-capped MQWs, respectively. But they exhibited a significant improvement of PL intensity without a broadening of the PL spectra after the RTA. After an RTA at 900 °C for 45 s, a blueshift of 17 meV and a redshift of 25 meV were observed for the SiO2- and Si3N4-capped MQWs, respectively. Microstructures of the InGaAs/InAlAs MQWs observed by cross-sectional transmission electron microscopy before and after RTA reveal that the above improvement of PL intensity is associated with the curing of nonradiative recombination centers in the course of the relaxation of vertical composition modulation along the growth direction in the InGaAs well and the InAlAs barrier layers. The influence of dielectric and InGaAs cap layers on the improvement of PL intensity is explained by the ease of the cation and anion vacancy diffusion from the dielectric layers and through the InGaAs cap layer.

The mechanism of oxidation in semiconducting a(=Sr, Ca,…)TiO3 ceramic materials

Ceramic samples of Nb doped (Sr, Ca)TiO3, with a 1% TiO2 excess with respect to the stoichiometry, have been prepared by using liquid phase sintering at high temperature in a reducing atmosphere. These semiconducting materials are n-type, the electrons being provided by the presence of niobium dopants and oxygen vacancies. The latter are eliminated through an oxidation treatment at moderate temperature (1000–1200°C). The oxidation kinetics of ceramic materials and single crystals are very different. The presence of a TiO2-rich liquid phase at grain boundaries during the sintering process modifies, on cooling, the stoichiometry of thin adjacent layers, which show an excess of strontium vacancies. These defects are so slow that transport through grain boundaries is the limiting step in such a ceramic material. The cation vacancy diffusion coefficient has been determined. The particular microstructure leads also to a complicated band structure of the grain boundaries which are resistive while the grains are semiconducting. Its time evolution is followed by using resistivity and capacitance measurements.

Cation vacancy diffusion on the NiO(001) surface by molecular dynamics simulations

We present results concerning the cation vacancy diffusion mechanisms occurring on the NiO(001) surface along with their corresponding migration energies. Using a rigid ion potential and molecular dynamics simulations we found that the cation vacancy diffusion is the main diffusion process in this surface employing the simple and two different double hopping mechanisms. Although the hopping rates of the latter are about two times lower than that of simple jumps, at high temperatures their contribution to the total vacancy diffusion coefficient is comparable with that of simple jumps. In addition, for all mechanisms saturation is found in the Arrhenius diagrams above a temperature T s=2850 K. We attributed this behavior to collective and correlated movements of the surface atoms that are activated from multi-phonon processes taking place above T s. Similar behavior was found recently in the case of metals.

Thermally stimulated depolarization currents in nickel doped NaCI crystals

Depolarization currents have been investigated for thermally and mechanically pretreated NaCI crystals doped with divalent nickel ions. The models of Debye and Maxwell-Wagner-Sillars polarizations have been exploited to describe the behavior of ITC bands detected below (dipolar), near and above (interfacial) room temperature, respectively. The NaCI:Ni2+ system was especially interesting for kinetic studies because the dopant ions are distinctly smaller than the matrix cations. It has been found that the precipitation phenomena can be understood in terms of an Avramiapproach of the process mainly occurring in the stress fields of dislocations. The migration of impurities is controlled by the diffusion of cation vacancies.

Semi-empirical simulations of F-center diffusion in KCl crystals

The semi-empirical method and 224 atom quantum clusters were used for calculating the activation energy for diffusion of cation and anion vacancies and F-centers in KCl crystals. The relevant activation energies of 1.19 eV, 1.44 eV and 1.64 eV, respectively agree well with the experimental data.