Diffusion Coefficient Of Iron Research Articles

Enhanced iron self-diffusion in the near-surface region investigated by nuclear resonant scattering

The access to X-rays of third generation synchrotron radiation sources enables studies of dynamics in metallic systems in grazing incidence geometry. Combining grazing incidence reflection of X-rays with nuclear resonant scattering of synchrotron radiation allows depth-selective investigations of hyperfine parameters and diffusion phenomena of iron and iron compounds. The unique feature of this method is its sensitivity to near-surface motions of atoms and not exclusively to the atoms on the surface. The depth sensitivity can be varied between about two and more than 10 nm. A 300 nm thick 57 Fe sample grown by molecular beam epitaxy on a cleaved MgO(0 0 1) substrate was investigated. The diffusion coefficient of iron in the near-surface layer (thickness about 2 nm) is almost two orders of magnitude larger than in bulk bcc iron at the same temperature.

Thermodynamics of the redox equilibrium Fe 2+/Fe 3+ and the diffusivity of iron in 5Na 2O·15MgO· xAl 2O 3·(80− x)SiO 2 ( x=0–20) melts

Melts with the basic compositions 5 Na 2 O·15 MgO·x Al 2 O 3·(80−x) SiO 2 (x=0,5,10,15,20) doped with 0.5 mol% Fe 2O 3 were studied by means of square-wave voltammetry at temperatures in the range from 1300 to 1600 °C. Increasing temperatures resulted in less negative standard potentials, i.e. in a shift of the redox equilibrium to the reduced state. The redox equilibria were notably affected by the alumina concentration. Introducing 5 mol% Al 2O 3 to a soda–magnesia–silicate melt resulted in less negative standard potentials, while further increasing the alumina concentration led to more negative standard potentials. Diffusion coefficients of iron could be fitted to Arrhenius equation. They increased with increasing alumina concentration. A structural model is proposed which explains the effect of the alumina concentration on the thermodynamics of the Fe 2+/Fe 3+-equilibrium and on the diffusivity of iron.

Volume and dislocation diffusion of iron, chromium and cobalt in CVD ß-SiC

Impurity tracer diffusion of 59Fe, 51Cr and 57Co in CVD b-SiC has been studied in the temperature range between 973 and 1873 K. The temperature dependence of the volume diffusion coefficients of iron and chromium can be expressed by linear Arrhenius equations. The preexponential factor and the activation energy are estimated to be 8.7 × 1015 m2 s-1 and 111 kJ mol-1 for iron, respectively, and 9.5 × 1015 m2 s-1 and 81 kJ mol1 for chromium, respectively. The diffusion coefficients of iron and chromium are much higher than those of the self-diffusion in ß-SiC. Furthermore, the activation energies for the diffusion of iron and chromium are about one-tenth of those for carbon and silicon in b-SiC. Therefore, it seems that an interstitial mechanism is predominant for the diffusion of iron and chromium in b-SiC. On the other hand, the diffusion coefficient of cobalt above 1673 K is higher than that of iron, while at lower temperatures it is much lower than that of iron. The difference in the diffusion coefficients at 1173 K is more than three orders of magnitude. Thus, the temperature dependence of the diffusion coefficients of cobalt shows a strongly curved Arrhenius relation. This suggests that cobalt atoms diffuse by an interstitial mechanism at higher temperatures and by a substitutional mechanism at lower temperatures. From the deeper regions of the penetration profiles of iron, chromium and cobalt the dislocation diffusion coefficients of them have been estimated.

Preparation and characterisation of La 1− xSr xGa 1− yMg yO 3−( x+ y)/2 for the investigation of cation diffusion processes

Polycrystalline, dense samples of lanthanum gallate doped with strontium on the A and magnesium on the B site, La 1− x Sr x Ga 1− y Mg y O 3−( x+ y)/2 (LSGM), were prepared using the Pechini method. Characterisation by X-ray diffraction (XRD) showed that the resulting samples annealed at 1400°C in air are single phase. Using in situ XRD measurements the dissociation and crystallisation of the amorphous precursor powders of the Pechini preparation was followed up to 1100°C. First results for the cation diffusion coefficients of gallium, iron and manganese were obtained by the radiotracer sectioning method. Two different mechanisms (grain boundary and bulk diffusion) were found and ratios of grain boundary to bulk diffusion coefficients were obtained.

Rolling Contact Testing of Vapor Phase Lubricants—Part IV: Diffusion Mechanisms©

Auger depth profile data, obtained from vapor-lubricated T15 bearing steel, were modeled using the solution for diffusion in a semi-infinite pair. A tertiary-butyl phenyl phosphate was used as the vapor lubricant. The primary species undergoing diffusion are iron and carbon. The objective of the experiment was to determine the order of magnitude of the diffusion coefficient and to qualitatively assess what types of diffusion mechanisms are involved. The experimental results indicate that the diffusion profile travels at a velocity equal to the bearing wear rate under dynamic conditions. This is possible if iron diffusions at a faster rate than carbon, i.e., the Kirkendall effect. Analyses of the data were performed using Darken's equations. The results indicate that the diffusion coefficient of iron is of the order of 1 × 10−14 cm2/s at test temperatures of 370° and 430°C. Diffusion is thought to occur via the migration of iron cations through an anionic lattice of polyphosphate and phosphite, i.e., cation diffusion. The diffusion mechanism provides a satisfactory explanation of how several hundred monolayers containing iron can exist in vapor deposition film grown from organophosphorus lubricants. The chemical gradient, ionic attraction, and the extreme stress gradients of the Hertzian contact are thought to contribute to diffusion under dynamic conditions. Presented at the 53rd Annual Meeting in Detroit, Michigan May 17–21, 1998

Formation of intermetallic compound in iron-aluminum alloys

The mechanism of iron and aluminum intermetallics formation in the reactive sintering of iron and aluminum mixing powders has been studied by investigating iron-aluminum diffusion couples. The couples were treated at 600°C, 700°C, 800°C, 900°C and 1000°C respectively. It was found that an Al-rich intermetallics FeAl3 has formed in iron adjacent to the interface of iron and aluminum by aluminum diffusion into iron at 600°C (below the eutectic temperature), and that in the case above 700°C (above the eutectic temperature) there was a liquid, an intermetallics Fe2Al5 has formed in both side of the interface. The diffusion of iron and aluminum atoms is companied with the Fe-Al reaction during the treatment under the both conditions. The diffusion coefficients of iron and aluminum and the activation energy were determined. The mechanism of the intermetallics formation in the couples is also discussed.

Self-diffusivity of iron in alkali–lime–alumosilicate glass melts

Self-diffusion coefficients of iron were measured in glass melts with the basic mol% composition of 16 R 2O, 10 CaO, x Al 2O 3 and (74 − x) SiO 2 with x=0, 5, 10, 15 and R=Li, Na, K and Cs in the temperature range of 900–1300°C using square-wave voltammetry. All diffusion coefficients had an Arrhenian dependence which depended on the type of alkali present and the Al 2O 3-concentration. Larger alkali cations, e.g. Cs +, as well as an increase in the Al 2O 3-content led to a decrease in the diffusion coefficients and also to an increase in viscosity. Within one glass composition, the Stokes–Einstein equation is fulfilled with respect to the dependence of the diffusivity upon viscosity. At constant viscosity, however, increasing size of the alkali cation and increasing Al 2O 3-content led to larger iron diffusion coefficients.

Determination of The Diffusion Mechanism by A Method with New Possibilities: Nuclear Scattering of Synchrotron Radiation

ABSTRACTThe elementary diffusion jump in crystalline solids can be determined by methods derived from nuclear physics. With these methods not only diffusion rate(s) but also diffusion vector(s), i.e. the complete diffusion mechanism can be deduced. We report on a new method for probing the elementary diffusion jumps in crystalline lattices on an atomistic scale and demonstrate its potential by a study of 57Fe diffusion in different intermetallic alloys. Compared to the results of conventional tracer (macroscopic) technique, the new method provides clear and doubtless statements concerning the direction and distance of elementary jumps. One can also determine (though less precisely than with tracer diffusion), iron diffusion coefficients.

High-temperature wall-tube cell Design, characterization and results of mass transport phenomena

A high-temperature wall-tube electrode cell (WTE) for electrochemical studies in hydrothermal systems is described. The cell contains a jet impinging on a wall (electrode) where the hydrodynamics in the stagnation region is well defined and the WTE has uniform accessibility and is equivalent to a rotating disc electrode (RDE). The construction and testing of the high-temperature WTE is described and its performance tested using different redox couples such as hexacyanoferrate(II/III) and quinone/hydroquinone. Diffusion coefficients of iron(II)/iron(III) are reported at temperatures up to 142 °C in sulfate solutions and are analysed taking into account the speciation in the solution. The application of the Stokes–Einstein equation to describe the temperature dependence of the diffusion coefficients in systems with strong ionic association is discussed for iron(II)/iron(III) and other species.

The effect of Fe diffusion on structural and superconducting properties of YBaCuO

The Fe diffusion in superconducting YBa 2Cu 3O 7 (YBaCuO) ceramic has been studied over the temperature range 615–880 °C by the energy dispersive X-ray fluorescence (EDXRF) technique. The temperature dependences of iron diffusion coefficient in grains ( D 1) and over the grain boundaries ( D 2) are described by the relations D 1 = 1.5 × 10 −4exp(−1.25/ kT) and D 2 = 1.8 × 10 −5exp(−0.90/ kT). Effects of iron diffusion and cooling rate on the crystal structure and superconducting properties of YBaCuO have been investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM) and resistivity measurements. It is found that iron diffusion is accompanied by the formation of iron oxides in YBaCuO. The fast cooling of iron-doped samples stimulates the formation of predominantly orthorhombic phase, while the undoped sample, employing the same cooling rate, remained tetragonal.

Determination of the diffusion coefficients of iron and chromium in pb17li

A method with controlled forced convection using a cylindrical rotating sample has been devised to determine the diffusion coefficients of iron and chromium dissolved in Pbl7Li liquid metal at 500°C. The following values have been obtained: D(Fe/Pb17Li) 500°C = 4±2 × 10 −14 m 2 s −1 fD(Cr/Pb17Li) 500°C = 8±2.5 × 10 −11 m 2 s −1. A value of chromium solubility in Pb17Li at 500°C has also been deduced, C s = 90.9 g m −3 (10 wppm), which is compatible with the existing data at lower temperatures.

The ratio of diffusion coefficients in growing and non-growing compound layers

The experimental data available in the literature show considerable, if not huge, differences between the values of diffusion coefficients of the elements A and B in a compound layer growing at the A-B interface and the values of diffusion coefficients of those elements in a piece of the same binary compound material but taken along. For example, the diffusion coefficients of iron and aluminium in the Fe2A15 phase material at 520 °C were reported by Larikov et al. [1] and Larikov [2] to have values of 1.9 x 10 -16 mZs -1 and 8.7 x 10 -15 m2s -1, respectively. On the other hand, when investigating the growth kinetics of the Fe2A15 layer at the interface between iron and aluminium, Heumann and Dittrich [3] found that the growth process was due mainly to the diffusion of aluminium atoms across the layer bulk. They established the following temperature dependence of the diffusion coefficient of aluminium in the growing Fe2A15 layer: DA1 = 3.3 X 10 -6 exp (--13 IO0/RT) m 2 s -1 (R in cal mole -1 K -1)

Interaction of iron-nickel alloys with liquid aluminium

The dissolution of Fe-Ni alloys containing 90 to 5 mass % Fe in liquid aluminium at 700‡C was found by the rotating-disc technique to be non-selective and diffusion controlled. Experimentally determined values of the saturation concentration (solubility),Cs, of iron and nickel from Fe-Ni alloys in the aluminium melt are presented. A dependence of the total iron and nickel concentration,Ctotal, in the saturated melt upon the iron contentCFe, in initial Fe-Ni alloys could well be described by the linear equations:Ctotal = 7.51−0.054CFe at 90% <CFe < 28% andCtotal = 11.0−0.19CFe at 28% <CFe < 5%. Considerable deviations of the concentration-time relationships from the Nernst-Shchukarev equation were revealed. The value of the dissolution rate constant was found to decrease by about 20 to 30% in the 0 toCs/2 iron or nickel concentration range. Accordingly, the decrease in value of the iron and nickel diffusion coefficient from an Fe-Ni alloy into liquid aluminium was 25 to 40%. For a 90% Fe-10% Ni alloy the value of the diffusion coefficient tends, with increasing iron and nickel concentration in the melt, to the value of the diffusion coefficient of pure iron in liquid aluminium. For other Fe-Ni alloys investigated, these values are less than or close to the diffusion coefficient of pure nickel in liquid aluminium.

Diffusion of iron in β-iron telluride (Fe 1.12Te) by Mössbauer spectroscopy and tracer method

The diffusion coefficient of iron in a β-iron telluride (Fe 1.12Te) polycrystalline sample was measured by Mössbauer diffusional line broadening method which relates to the collapse of coherence in gamma-ray photon by the atomic jump at local sites. The diffusion coefficient of iron along the c- axis in nearly single crystal of β-iron telluride was also measured by tracer technique which shows the results of an atom transport in long distance. The activation energies for the diffusion of iron in Fe 1.12Te obtained by the Mössbauer spectroscopy and the tracer method were 91.5±5.4 and 106±23 kJ/mol , respectively. The diffusion coefficients of iron in β-iron telluride obtained by Mössbauer line broadening are in fair agreement with the values averaged from that along c- axis obtained by tracer method and that along a- and b- axes obtained from reaction rate constant between iron and tellurium by the previous study of the present authors.

Diffusion of iron in δ-iron telluride (Fe 0.743Te) by Mössbauer spectroscopy

The diffusional line broadening of iron in δ-iron telluride (Fe 0.743Te) polycrystal was measured by means of Mössbauer spectroscopy in the temperature range from 851 to 1000 K. The diffusional jump frequency and the activation energy for the diffusion of iron in δ-iron telluride obtained in this study are compared with those in β-, δ'-and ϵ-iron tellurides reported by the present authors previously. By assuming random walk, the diffusion coefficients of iron in δ-iron telluride are estimated from the diffusional jump frequencies for three different cases, and their results ar compared with eath other.

How far does the charge state affect the iron behavior in silicon?

Low-temperature precipitation of neutral interstitial iron in n-type silicon is investigated by means of photocapacitance measurements. Isothermal kinetics as well as iron depth profiles are observed which agree very well with an outdiffusion mechanism of iron to the sample surface. From these data the diffusion coefficient of neutral iron in silicon could be determined. Comparison with published results on positively charged iron in p-type silicon reveals a higher stability of neutral iron as well as a charge state dependence of its diffusion mechanism.

Diffusion of Iron into GaAs from a Spin‐on Source

Iron was diffused from a spin‐on glass film into n‐type wafers at temperatures of 700°–900°C. The observed diffusion depths can be explained by a model with exhaustible diffusion sources. The diffusion coefficient of iron in has been estimated to be in this temperature range. The resistivity of the diffused layer was in the order of 104 Ω cm. The activation energy of the introduced levels was 0.53 eV for the diffusion at 700° and 800°C, which agrees well with the acceptor level of iron. The current‐voltage characteristics of mesa diodes made of the diffused wafers showed no excess leakage current, which ensures that high‐resistivity, p‐type, iron‐diffused regions can be used for junction isolation in integrated circuits.

On bainite formation

Driving force calculations for Fe-C, Fe-X-C, and Cu-Zn alloys show that the formation of bainite by a shear mechanism is thermodynamically impossible. There exist superledges on the broad faces of bainite in steels, revealing that the thickening of bainite probably proceeds by a ledge mechanism. In some Fe-Ni-C alloys and commercial steels, no simple relationship was found between the strength of austenite and theBs temperature; however, there is a linear relationship betweenBs and the diffusion coefficient of carbon and iron in austenite, as well as between the incubation period and the function containing DFeγ. The bainite reaction seems to be controlled by diffusional processes. In a low-carbon Ni-Cr steel, the morphology of the bainite/matrix interface boundary is different from that of the martensite, and the habit plane of the bainite (1 7 11)α deviates 13.3 deg from that of the martensite (1 1 0)α, indicating that the mechanism of the bainite reaction is not necessarily analogous to that of the martensitic transformation. At temperatures nearMs, as the driving force will be large enough, the growth of bainite by shear may be able to occur, and evidence is given by the morphology of bainite showing shear characteristic. X-ray diffraction study in Ag-Cd and internal friction measurements in Ag-Cd and Cu-Zn-Al alloys, 18CrNiWA steel, and its decarburized specimen reveal that the nucleation process occurs within the incubation period of bainite formation.

Iron diffusivity measurement with deep-level transient spectroscopy at room temperature

Time dependence of iron interstitial concentration changes in p-type silicon wafers was measured with deep-level transient spectroscopy. A slab diffusion model was applied to the iron-to-FeB transformation in order to calculate the room temperature diffusion coefficient of iron. It was very close to the extrapolated value obtained from high temperature iron diffusivity and some room temperature diffusion data obtained by the electron paramagnetic resonance or by resistivity change measurements. The effects of oxygen and its precipitation on iron diffusion was considered, by comparing floating-zone wafers with Czochralski wafers of various oxygen contents. It was observed that these variables do not affect iron diffusivity at room temperature. It was also noted that there was no dependence of the substrate resistivity on the iron diffusivity at room temperature. From these results it is concluded that the apparent iron diffusion is mostly caused by the random walk of iron atoms.

Diffusion Coefficient of Interstitial Iron in Silicon

The in-depth profiles of iron in silicon diffused with iron at 800, 900, 1000 and 1070°C were investigated by DLTS measurements. The diffusion coefficient of interstitial iron in silicon was represented by the expression DFe=9.5×10-4 exp (-0.65/kT) cm2s-1 in the temperature range of 800–1070°C.