有机溶剂保护对Al/Ni扩散连接的影响规律研究

采用有机溶剂保护实现了纯铝和纯镍的扩散连接。利用扫描电子显微镜、能谱分析以及X射线衍射等分析手段,确定了Al/Ni扩散连接接头典型的界面结构为Al/Al_3Ni_2/Ni。在扩散连接过程中利用有机溶剂防止铝表面发生二次氧化,相比直接扩散连接可得到更好的焊接质量。研究了连接温度对Al/Ni接头界面结构的影响规律,随着连接温度的升高各反应层厚度逐渐增加。当连接温度为490℃,连接时间为60 min,连接压力为2 MPa时,接头抗剪强度达到最大值,为17.83 MPa,比该工艺下直接扩散连接得到的焊接接头强度提高了约55%。     

二次离子质谱仪测量扩散曲线时“零曲线”的分离

二次离子质谱仪测量的元素深度分布曲线受离子注入引发的原子混合、坑壁、晶体取向以及表面粗糙度等因素的影响,使得实测曲线偏离真实元素分布曲线.通过测量扩散退火前溶质原子在样品中的分布曲线,即"零曲线",可以表征各种因素的综合效果.本文提出一种使用Fourier级数解卷积的方法有效地从实测曲线中分离"零曲线"的影响,得到元素的真实分布曲线的方法.并利用此法分析了Zn在粗晶Cu及用动态塑性变形方法制备的纳米结构纯Cu中扩散的浓度-深度分布曲线.     

Calculation of diffusion coefficients by the Matano-Boltzmann method

Some aspects of Boltzmann’s solution for one-dimensional diffusion in an infinite medium when the initial distribution is a step function are clarified. Thus, the fact that the concentration is a function of the single variable x/t^1/2 is established by a dimensional argument. The basis of the Matano-Boltzmann method for determining diffusion coefficients from concentration curves is discussed, and a new way of applying the method is derived and illustrated. As in the work of Hall, emphasis is placed on the curve obtained when (in effect) concentration is plotted against position on probability paper. This curve is approximated by a polynomial, and the diffusion coefficient is calculated by using an equation in which the coefficients of the polynomial appear as parameters. The new procedure makes the judgment of the effect of errors in concentration measurements easier than do the graphical methods frequently used.     

Bulk diffusion of iron in copper

The temperature dependence of bulk diffusion coefficient of iron in copper has been determined by electron microprobe analysis (EMA) in the temperature range from 923 to 1273 K to be $D_{Fe} = 0.03 \times 10^{ - 4} \exp \left( { - \frac{{187 kJ/mol}} {{RT}}} \right)$ m²/s. The results obtained differ from the parameters of bulk diffusion determined by the tracer method: the activation energy is less by 30 kJ/mol and the preexponential factor is less by approximately a factor of 50. The deviations of the solutions from the ideality does not explain the discrepancies obtained.     

Mechanisms of formation of diffusion aluminide coatings on nickel-base superalloys

Studies on microstructures and compositions of model diffusion aluminide coatings coupled with existing information on diffusion in nickel-aluminum intermetallic compounds indicate that only two basic types of these coatings can be formed on nickel-base superalloys. The first type is formed by inward diffusion of aluminum from coating media of sufficiently high aluminum activity to cause formation of the (Ni₂Al₃) phase. Subsequent stabilizing heat treatments required for practical use of this type of coating cause some outward diffusion of nickel to form a three-zone (NiAl) coating. The second type is formed by outward diffusion of nickel from the substrate alloy to react with aluminum of suitable activity to stabilize that (NiAl) phase of composition through which only nickel motion can occur. This process produces a two-zone (NiAl) coating. It follows that the structural type of coating to be expected from any given coating process can, in principle, be predicted from knowledge of the aluminum activity present during formation of the coating. The initial stages of diffusional degradation of these coatings involve continued outward diffusion of nickel from the substrate through the (NiAl) coating with the resultant dilution of aluminum content of the coating; little inward diffusion of aluminum occurs.     

Self-Diffusion of Iron in Molten Fe-C Alloys

Self-diffusion coefficients of iron in molten Fe-C alloys have been measured by using the capillary method. In addition, the samples have been autoradiographed and sectioned to insure that no significant convection has occurred during the diffusion. The results can be represented by the equation D = 4.3×10^?3 exp (—12.200/RT) for carbon = 4.6 pct and T = 1513° to 1633°K; and D = 1.0×10^?2 exp (—15,700/RT) for carbon = 2.5 pct and T = 1613° to 1673°K. The D values are higher and the heat of activation for diffusion lower in alloys containing more carbon. Calculation based on the Einstein-Stokes equation indicates that the diffusing species is iron ion.     

Mathematical modeling of the formation of diffusion coatings with periodic structure

To obtain coatings of thickness to 0.7 mm or more with controllable porosity, having in contrast with the traditional coatings a more uniform chemical composition and properties that are uniform through the thickness, we propose to create on the surface of the part that is being coated a periodic relief of a definite geometry, consisting of alternating protuberances (ribs) and depressions, after which diffusional saturation is performed. The inter-rib channels provide for accelerated diffusion of the saturating element through the entire thickness of the coating, equal to the rib height. Thus, the structure of the diffusion coating will be determined not only by the technological factors of diffusional saturation (temperature, duration, chemical composition of the medium and of the material being coated) but also by the geometry of the relief, which makes it possible to broaden the capabilities for varying the structural state of the diffusion coatings. The multifactor nature of the proposed method leads to certain difficulties in developing the regimes for obtaining coatings with the desired periodic structure. In this connection it is advisable to formulate a mathematical model that will make it possible to predict the structure of the obtained coatings. The mathematical model consists of a system of differential equations that can be solved numerically and describe the changes in time of the concentration of the coating elements and the location of the interphase boundaries. The adequacy of the proposed mathematical model is confirmed herein.Translated from Metallovedenie i Termicheskaya Obrabotka Metallov, No. 6, pp. 6–9, June, 1994.     

Oxygen-18 tracer study of the passive thermal oxidation of silicon

This work focuses on the thermal oxidation of silicon near 1273 K using the double-tracer oxidation method. The results confirm that oxidation occurs by the transport of electrically neutral non-network oxygen through the interstitial space of the vitreous silica (-SiO₂) scale. Simultaneously, self- (or isotopic-) diffusion occurs in the network, resulting in characteristic isotopic fraction distributions near the gas-scale interface. The self-diffusion coefficients calculated from these profiles agree with those reported for tracer diffusion in -SiO₂, and the diffusion coefficient calculated from the scale growth is consistent with reported O₂ permeation data. An important parameter that describes the double-oxidation behavior is the ratio of the value of /(D _nt),where is the scale thickness grown during the second oxidation, D_n is the network self-diffusion coefficient for oxygen, and t is the time of the second oxidation.     

Comparison between the apparent chemical diffusion coefficient evaluated from relaxation experiments by means of electrical conductivity changes and the corresponding theoretical value for a pure and doped p-type oxide

The chemical diffusion coefficient for a p-type oxide either pure or doped with an aliovalent impurity as evaluated from electrical conductivity changes in simulated relaxation experiments is compared with the corresponding theoretical values obtained on the basis of Fick's first law using a convenient model to represent the defect structure of the oxide. It is found that if the relaxation process is purely diffusion controlled, the experimental value of obtained ( ) is in rather good agreement with the theoretical value calculated by considering the diffusion of lattice defects rather than with that obtained by considering the diffusion of the prevailing electronic defects , even when the latter two values differ. This is shown to be the result of relatively small departures from a proportionality (for the pure oxide) or from a linear dependence (for a doped oxide) in the relationship between the deviation from stoichiometry and the concentration of the electron holes in restricted range of oxygen activity as used in relaxation experiments.     

Application of tight-binding and path probability methods to the junction relaxation of semiconductor heterostructures

The atomistic and thermodynamic properties of semiconductor heterostructures are investigated by using the tight-binding (TB) electronic theory and path probability method (PPM). The atomic diffusion in the semiconductor interface is studied via the vacancy mechanism of diffusion using the nonequilibrium irreversible statistical mechanical approach, PPM. The effective pair interaction energies between the constituent atoms are derived by using the zeros-poles method, taking into account the misfit strains at the interface. We study the junction relaxation processes of semiconductor heterostructures such as SiGe/Si(001), GaAs/Si(001), and ZnSe/GaAs(001) systems. It has been found that the junction relaxation exhibits characteristic features, e.g., overshooting and uphill diffusion along the chemical potential gradient depending on the temperature and relative magnitude of effective pair interaction energies. It is also shown that, even for the very early stage of the junction relaxation, the interface electronic properties are strongly influenced by the interface disorder.     

Dislocation structure of diffusion layers of boron in iron

Conclusion The dislocation structure and the microstructure of diffusion layers, which at the temperature of diffusion annealing represent a single-phase solid solution of boron in -iron, are determined by the local boron concentration and by the cooling rate from the austenitic state. The high dislocation density in the layer, which is found independently of the cooling rate, testifies to the fact that diffusion of boron in iron is accompanied by the formation of a large number of dislocations. The presence of boron in the solution slows down their redistribution and annihilation in the porcess of high-temperature annealing.The distribution of microdistortions across the thickness of the layer is characterized by the existence of a maximum near the diffusion line; this is due to the high density of the randomly distributed dislocations on the boundary between the diffusion zone and iron.Tula Polytechnic Institute. Translated from Metallovedenie i Termicheskaya obrabotka Metallov, No. 1, pp. 20–23, January, 1988.     

Determination of H^＋ diffusion coefficient in the course of H^＋ response of a W/WO3 pH electrode

A WAVO3 pH electrode was prepared by a method of sol-gel. In order to study the H^＋ response dynamic mechanism, the electrochemical impedance spectroscopy （EIS） experiment was conducted. It was found that the H^＋ response course is controlled by the H^＋ diffusion from the solution to the WO3 film, based on the analysis of EIS spectra. The EIS and potential step method were used to determinate the H＋ diffusion coefficient （D） in the course of H^＋ response of this WAVO3 electrode, and the values of D calculated by these two method correspond very well, which all are about 10^-19cm^2/s The imposed different potential steps make little effect on the calculation of H^＋ diffusion coefficient, and it was found that the limiting Cottrell equation of short elapsed time fits well to the current transient caused by a potential step, based on the analysis of the time constant.     

Multi-scale simulation study for growth dynamics of irreversible islands during submonolayer epitaxy

Using empirical embedded atom method (EAM) potential, activation barriers of adatom diffusion on the surface, diffusion along the step edge, and island corner diffusion are calculated for metal (111) surface (Pt, Ag) and the behavior of those barriers is investigated with respect to lattice strain. Based on the calculated barriers, we perform the kinetic Monte Carlo simulation for monolayer island growth and for how the variation of the activation barriers with lattice strain changes the island growth. Results from this work show that the lattice strain transforms the island growth shape from dendritic through random fractal to denditic at 100 K and from triangular through hexagonal to inverse triangular at 400 K. This article is based on a presentation made in the 2002 Korea-US symposium on the “Phase Transformations of Nano-Materials,” organized as a special program of the 2002 Annual Meeting of the Korean Institute of Metals and Materials, held at Yonsei University, Seoul, Korea on October 25–26, 2002.     

INFLUENCE OF ALLOYING ELEMENTS ON HYDROGEN DIFFUSION USING SCM-DV-Xα

The influence of alloy composition (Ti, Mn, TiN) on hydrogen diffusion in Fe was studied in detail using SCM-DV-Xα method. The voltage barriers were obtained via calculation on Fe clusters containing the alloy elements such as Ti, Mn as well as the chemical compound TiN respectively. The results showed that Ti element produced deep trap in Fe, decreasing the diffusion coefficient of hydrogen elements, Mn element did not produce deep trap in Fe, decreasing the diffusion coefficient slightly and TiN in Fe produced very deep "trap" decreasing the diffusion coefficient obviously. The calculation results were in agreement with experiment results.     

Application of tight-binding and path probability methods to the junction relaxation of semiconductor heterostructures

The atomistic and thermodynamic properties of semiconductor heterostructures are investigated by using the tight-binding (TB) electronic theory and path probability method (PPM). The atomic diffusion in the semiconductor interface is studied via the vacancy mechanism of diffusion using the nonequilibrium irreversible statistical mechanical approach, PPM. The effective pair interaction energies between the constituent atoms are derived by using the zeros-poles method, taking into account the misfit strains at the interface. We study the junction relaxation processes of semiconductor heterostructures such as SiGe/Si(001), GaAs/Si(001), and ZnSe/GaAs(001) systems. It has been found that the junction relaxation exhibits characteristic features, e.g., overshooting and uphill diffusion along the chemical potential gradient depending on the temperature and relative magnitude of effective pair interaction energies. It is also shown that, even for the very early stage of the junction relaxation, the interface electronic properties are strongly influenced by the interface disorder.     

Protective behavior of alumina scales in sulfidizing atmospheres

Fe-17Cr steel was aluminized (three different pack-cementation treatments were used) and then oxidized before exposure to a sulfidizing (H ₂/10%H₂S) atmosphere. When the oxidation time is sufficiently high (30 min), the corrosion remains nil for a period termed the immunity time. The corrosion initiates by sulfur diffusion through the alumina scale, which had been formed previously. A diffusion model was established: the sulfur diffusion coefficient in the oxide depends on the oxidation duration and on the aliminizing treatment. Its activation energy was calculated to be 353 kJ and was constant regardless of the prior cementation treatment.     

Kinetic study on Li2.8(V0.9Ge0.1)2(PO4)3 by EIS measurement

Kinetics for lithium ion transfers in the fast ionic conductor Li_2.8(V_0.9Ge_0.1)₂(PO₄)₃ prepared by solid-state reaction method has been studied by electrochemical impedance spectroscopy (EIS) at various temperatures and the results were correlated with observed cathodic behavior. The specific conductivities of Li_x(V_0.9Ge_0.1)₂(PO₄)₃ (x = 0.9–2.8) versus temperatures were analyzed from blocking-electrodes by Wagner's polarization method and the activation energy was calculated. It was observed that electronic conductivities of Li_x(V_0.9Ge_0.1)₂(PO₄)₃ increased with lithium contents in the materials. The compounds show a reversible capacity of 131 mAh g⁻¹ at low current density (13 mA g⁻¹). Modeling the EIS data with equivalent circuit approach enabled the determination of charge transfer and surface film resistances. The Li ion diffusion coefficient (D_Li⁺) versus voltage plot shows three valleys during the first charge cycle coinciding with the irreversible plateau of the voltage versus lithium content profiles reflecting the irreversible phase change in the compound. The obtained D_Li⁺ from EIS varies within 10⁻⁸ to 10⁻⁷ cm² s⁻¹, so Li_2.8(V_0.9Ge_0.1)₂(PO₄)₃ shows excellent chemical diffusion performance.     

Transport processes in the oxidation of Ni studied using tracers in growing NiO scales

Experimental techniques have been developed for determining Ni⁶³ and O¹⁸ tracer distributions in NiO scales ranging in thickness from 0.1 to 100 . These have been used to investigate Ni and O transport in scales on {100} Ni crystals and polycrystalline Ni in the temperature range 500–1300° C. NiO grown on {100} Ni crystals at 1000°C was uniform and compact and grew by the bulk diffusion of Ni in NiO by a vacancy mechanism. At temperatures below 800°C the principal transport mechanism was short-circuit diffusion of Ni in NiO. At all temperatures short-circuit diffusion of oxygen contributed to scale growth on polycrystalline Ni and was responsible for growth of the inner layer of duplex scales. The oxygen diffusion paths are believed to be micro-cracks induced by growth stresses.     

Kinetics and mechanism of the accelerated oxidation of low carbon scrap steels

Especially with accelerated oxidation, the kinetic curves of steel strips in CO₂-CO gas mixtures at 900° C do not adhere to either the parabolic or the linear growth law. Linear kinetics is essentially obeyed in the oxidation of untreated steel, whereas mixed parabolic or linparabolic laws determine the oxidation of steels treated with oxidation promoters. The observed kinetics are best explained with the Zener diffusion model, which assumes that the activation energy for diffusion is spent in deforming the crystal lattice elastically in the vicinity of the saddle point. The model results in a strong dependence of diffusivity on the melting point and elastic moduli of the diffusion medium (oxide scale). A diffusion coefficient that decreases exponentially with melting point and the Tammann differential relation for oxide growth account adequately for the observed kinetics.     

Chronoammetry and Chronopotentiometry on Electrodes with a Microrough Surface: Theoretical Consideration

The problem of transient two-dimensional diffusion to a microrough electrode under galvano- and potentiostatic polarization conditions is solved. The effect of the scale and morphology of roughness, as well as the diffusivity on the time dependences of I current and overpotential is analyzed. Both of them are found to be practically independent of the type of the roughness, while its degree (the microroughness factor f _r ) decisively affects the shape of the current- and overpotential-decay curves. At a relatively short polarization when the diffusion front profile repeats the electrode surface relief, the parameters of transient electrochemical methods (the I current in the potentiostatic measurements and the ^1/2 rooted transient time in the galvanostatic ones) linearly depend on the microroughness factor. Upon a relatively long polarization when the diffusion front is noticeably distant from the surface and almost planar, the current and overpotential values fit in with classical equations of chronoammetry and chronopotentiometry, being independent of f _r . At a thickness of the diffusion layer comparable to the height of surface irregularities, the current (as well as the rooted transient time) ratio of the microrough to perfectly smooth electrode surface looses its linear dependence on the surface parameters. The time location of this range is chiefly determined by the diffusivity and the mean distance between the microscopic surface irregularities.__________Translated from Zashchita Metallov, Vol. 41, No. 3, 2005, pp. 234–243.Original Russian Text Copyright © 2005 by Kozaderov, Vvedenskii.