Bulk Diffusion Mechanism Research Articles

Nitrogen diffusion parameters in ion-implanted tungsten single crystals

Diffusion of nitrogen implanted in single-crystal tungsten was studied in the temperature range 700–820° C. Measurements were carried out using a method of nondestructive determination of diffusivities (developed by the authors) from the dynamics of variation in the surface impurity concentration. The initial distribution and diffusion profiles for various annealing times were determined by secondary ion mass spectrometry. The relative surface concentration of nitrogen was measured by Auger electron spectroscopy. Several fluxes of impurity atoms in the surface region of ion-doped tungsten were experimentally detected to exist. Under the assumption that the fluxes interact with each other, the temperature dependences of the nitrogen diffusivities in the flux associated with dislocations generated by ion implantation and in the flux associated with the bulk diffusion mechanism were determined. Nitrogen diffusion is characterized by a rather low activation energy, namely, 0.15 and 0.75 eV for dislocation and bulk mechanisms, respectively.

Non-Destructive Method of Diffusion Parameters Determination in Solids

Diffusion of nitrogen implanted in tungsten and molybdenum single crystals has been investigated at temperature about 0.3 Tm (Tm is the melting point). Existence of several dopant atom fluxes is found in subsurface region of the ion implanted material. The diffusion coefficients of the nitrogen connected with the radiation damages and that with the bulk diffusion mechanism are determined. Change of the nitrogen surface concentration has been measured by Auger electron spectroscopy. Initial distribution of the nitrogen and diffusion profiles for various annealing time have been determined by secondary-ion mass-spectroscopy technique. Transmission electron microscopy and X-ray diffraction investigations were used to study the microstructure and phase state of the implanted samples.

The surface reaction and diffusion of NO2 in lead phthalocyanine thin film

The responses of lead phthalocyanine thin films upon encountering of NO2 gas were studied both experimentally and theoretically. A surface adsorption/desorption and bulk diffusion mechanism was proposed to describe the response. Laplace transform was employed to solve the resulting diffusion equation and an analytical solution was obtained. The analytical results show that the steady state current depends on parameters such as film thickness, gas diffusivity, adsorption and desorption rate constants, etc. Diffusivity, adsorption and desorption rate constants can be extracted from the nonlinear least square fitting of experimental data with analytical results. The obtained gas diffusivity, which depends on film morphology, remains fairly constant as film thickness changes. From the results of this study, it was concluded that the adsorption of NO2 by lead phthalocyanine is controlled by the surface adsorption and desorption with negligible diffusion resistance.

A study of oxygen diffusion in and out of YBa2Cu3O7−δ thin films

The c-axis oriented YBa2Cu3O7−δ (YBCO) films with superconducting transition temperatures up to 89 K were prepared by ion beam sputter deposition on single crystal (100) MgO and (100) SrTiO3 substrates. The variation of the oxygen concentration δ during annealing in vacuum and in oxygen was measured in situ and in real time by means of spectroscopic ellipsometry. The ellipsometric measurables Δ and Ψ were quantitatively correlated with δ at a photon energy of 4.1 eV. Changes of the oxygen concentration below 1% were resolved. Measurements for different film thicknesses in the range from 15 to 100 nm provided information to elucidate the mechanisms for oxygen out- and in-diffusion. The oxygen out-diffusion rate depends on the layer thickness confirming a bulk diffusion mechanism. The oxygen in-diffusion is relatively independent of layer thickness suggesting that a surface/grain boundary diffusion mechanism is dominant in this case. Additionally, the effect of H2O on the oxygen diffusion was studied. It was found that H2O enhances the oxygen out-diffusion while the in-diffusion remains unchanged or even decreases. The present study shows that ellipsometry provides a powerful tool for the in situ, noninvasive, on-line control of the oxygen content of thin YBCO films.

Self-diffusion in α‒Al2O3. III. Oxygen diffusion in single crystals doped with Y2O3

Abstract Oxygen self-diffusion has been measured in single crystals of α-alumina (α[sbnd]Al2O3) doped with yttria (Y2O3) in the temperature range 1110–1630°C, using 18O2 and secondary-ion mass spectrometry. Two diffusion mechanisms are involved a bulk diffusion mechanism and a subboundary mechanism. At high temperatures, both these two mechanisms are observed while, at lower temperatures, only bulk diffusion is observed. Bulk and subboundary diffusion coefficients obey the equations D′o (cm2s−1) = 67exp[-590(kJ mol−1)/RT] and D′o (cm2 s−1) = 1017exp [-980(kJmol−1)/RT] respectively. On comparison with results obtained for ‘undoped’ α[sbnd]Al2O3 single crystals produced from the same powder, it appears that yttrium slightly enhances oxygen bulk and subboundary diffusion. An extrinsic diffusion mechanism by oxygen interstitials or by defect complexes can explain these results in the bulk of ‘undoped’ Al2O3, and the enhancement of D o in α[sbnd]Al2O3: Y2O3 is due to the donor effect of yttrium which slightly ...

Evolution of dislocation loops in silicon in an inert ambient—I

A plan-view TEM study has been made of the distribution, geometry and the time-dependent annealing behavior of type II (end of range) dislocation loops introduced by 1 × 10 15/cm 2 50 keV Si + implantation into silicon. The size and density distributions of the loops have been quantitatively analyzed, and loop growth behavior has been compared with that predicted by a bulk-diffusion mechanism and by a glide and self-climb mechanism. It has been shown that the loop growth rate is approximately constant for each annealing temperature (700–1000°C) and that the growth is governed by the bulk-diffusion mechanism. Regions of growth and shrinkage have been investigated for different annealing temperatures in terms of interstitial supersaturation and the critical loop growth radius. The activation energy for loop growth is determined to be 1.0 ± 0.2 eV from the Arrhenius plot of loop growth rate versus the reciprocal of annealing temperature.

The diffusion of nitrogen into Nb-modified Sm 2Fe 17 powder

Niobium-modified Sm 2Fe 17 powders were nitrided in pure nitrogen for a range of times and investigated using thermomagnetic analysis, scanning electron microscopy, optical microscopy and electron probe microanalysis (EPMA). Thermomagnetic analysis showed that, on nitriding the Curie point of the core increases continually until the whole particle has been uniformly nitrided. Such results could be explained either by the formation of a continuous solid solution of nitrogen or by the presence of just two compositions with the nitrogen-saturated shell expanding the lattice of the Sm 2Fe 17 core. A shell-core diffusion pattern was imaged using both optical microscopy and backscattered electron imaging. Comparing the EPMA profiles with the backscattered electron images, a diffusion layer between the shell and core with composition Sm 2Fe 17N x (0 < x < 2.7) was identified. This is consistent with the diffusion of nitrogen in the form of a continuous solid solution. The thickness of the nitrided shell varied considerably, not just from particle to particle, but also from different edges of the same particle. In some cases the difference was as large as a 1:10 ratio. Assuming an isotropic bulk diffusion mechanism propagating uniformly from the surface of the particle, such large differences cannot be explained reasonably by the non-central sectioning of the particles in question. Near the edges of some partially nitrided powder particles, the Sm 2Fe 17N 3-δ phase decomposed and this was observed as a mottled region consisting of α-Fe and samarium nitride. The soft magnetic α-Fe can be removed by subsequent zinc bonding, which develops a reaction layer ∼ 4 μm deep for the given processing parameters.

Stripe structure CdTe-CdZnTe-CdTe in a bulk single crystal

In this paper we present a study that was aimed at performing a selective diffusion of Zn into CdTe. A single crystal CdTe wafer fabricated into a “tooth-like” structure which was further subjected to high temperature annealing in the presence of Zn vapour. The sample was then cut parallel to the diffusion direction and a Zn concentration analysis, using an electron microprobe, was performed. As expected, the stripe structure CdTe-CdZnTe-CdTe has been confirmed. The Zn decay profiles were fitted to a modified diffusion model, suggesting a bulk diffusion mechanism coupled with a surface reaction. Practical implementation of this stripe structure for an infrared light waveguide is being evaluated.

TEM Moiré Pattern and Scanning Auger Electron Microscope Analysis of Anomalous Si Incorporation into MBE-grown Ge on Si(111)

Ge islands are grown by molecular beam epitaxy (MBE) on Si(111) surfaces with an SPE-grown buffer layer, which was expected to prevent intermixing between the epitaxial Ge layer and the Si substrate. The epilayer composition analyzed from the spacing of the Moiré fringe indicates that the Ge islands should be a Si-Ge alloy rather than pure Ge and that the alloying of Ge with Si is greater near the edges of the island. The alloying phenomenon was verified by the compositional depth profile analysis using the scanning Auger electron microscope (SAM) technique. The Si incorporation into the Ge islands observed here cannot be explained by a bulk diffusion mechanism and is a seriously anomalous phenomenon since Si diffusion into Ge would be negligible at the growth temperature and time used ill this experiment.

SAM investigations of the segregation of sulphur in polycrystalline α‐iron

AbstractScanning Auger microscopy (SAM) and AES investigations of the segregation of non‐metal impurity atoms (sulphur, nitrogen, carbon and phosphorus) in polycrystalline high‐purity (&gt;99.99%) α‐iron samples were performed in the temperature range 500–800°C. At the beginning of the heating, nitrogen segregates homogeneously and quickly but in the equilibrium state the whole surface is covered with nearly half a monolayer of sulphur. During the first measurements, the segregation of sulphur was ruled by a combined bulk and grain boundary diffusion mechanism with a t1/2 law, resulting in a net‐shaped structure of the surface concentration of sulphur. After repeated heating and sputtering, the contribution of grain boundary diffusion becomes smaller and the segregation rate of sulphur decreases as a consequence of sulphur depletion of the bulk. The segregation kinetics of the individual grains differ strongly owing to the differences between the extent of sulphur depletion of the grains and can be described very well by a t3/2 law in the initial stage of the segregation process. In a later phase, surface diffusion of sulphur from the grains, which have almost reached the equilibrium sulphur coverage, leads to an almost homogeneous sulphur coverage of the surface with only small and diffuse lateral variations.

Defects in nonstoichiometric TiC studied by TDS

Implantation and trapping effects of helium and neon are studied by thermal desorption spectrometry (TDS) in single-crystalline Ti 1C 0.924, which has a structural defect concentration on the order of percents on the carbon sublattice. For neon a first-order surface-related release peak and a very broad diffusional bulk-related release peak are found. For helium only the bulk peak has significant magnitude. The bulk diffusion mechanism can be explained by either a hopping or a vacancy assisted mechanism. In addition, neon is capable of displacing titanium atoms, and hence of creating vacancy clusters which have a stronger binding energy for neon than the structural defects; this is clearly visible in the spectra.

OXYGEN SELF-DIFFUSION IN SUBBOUNDARIES OF ALUMINA SINGLE CRYSTALS

Oxygen self-diffusion has been studied in alumina single crystals in the temperature range 1520-1750°C by means of the gas-solid isotope exchange technique. After diffusion annealing, profiles of oxygen-18 were determined by Secondary Ion Mass Spectrometry. Results indicate that two diffusion mechanisms are involved : a bulk diffusion mechanism and a subboundary one. The bulk diffusion mechanism can be described by: D(cm2/s) = 99.6 exp (- 626(kJ/mol)/RT) while subboundary coefficients obey: D' (cm2/s) = 3 x 1013 exp (-877(kJ/mol)/RT). The large value of the activation energy obtained for the subboundary diffusivity (larger than those related to the volume one) is attributed to the segregation of an impurity along the subboundaries.

X-ray photoelectron forward scattering studies of surface segregation in epitaxial Ni–Cu–Ni(100) sandwich structures

The forward scattering of the Cu 2p3/2 core level photoelectrons by overlying Ni lattice atoms which occurs in the photoemission process has been used to follow the segregation of Cu to the surface of epitaxial Ni–Cu–Ni(100) sandwich structures. The results indicate that when 1 monolayer (ML) of Cu is covered by 1–2 ML of Ni, surface diffusion processes constitute the mechanism by which the segregation occurs, whereas if 1 ML of Cu is covered by 10 ML or more of Ni the segregation involves the usual bulk (vacancy) diffusion mechanism. Since these processes are quite general, it appears that the top few atomic layers of a binary alloy can equilibrate with respect to surface segregation at a temperature significantly lower than that which is required for bulk equilibration. This combination of x-ray photoelectron spectroscopy (XPS) forward scattering and epitaxial sandwich structures thus provides both more clear-cut data and a more specific understanding of segregation phenomena than is usually possible by conventional XPS and Auger studies on random alloys.

Grain boundary diffusion of fast diffusing impurities: Investigation in the lead-silver system

Grain boundary diffusivities of a 110Ag radioactive tracer have been measured in polycrystalline Pb specimens over a temperature range of 57–118 °C. Diffusivity parameter values are expressed by the Arrhenius relation: P(cm3 s−1)=Dgbδα=1.58×10−4 ×exp(−41120 J mol−1/RT). These values are five orders of magnitude higher than those relative to self-grain boundary (gb) diffusivity of lead. Thus, the difference between silver and lead grain boundary mass transport is almost the same as that measured for bulk diffusion. The possible explanations for such a similarity are discussed in relation with the particular bulk diffusion mechanism of silver and a possible high grain boundary segregation of this solute.

The influence of phosphorus on the solid state reaction between copper and silicon

The solid state reaction between copper and silicon has been studied in the temperature range 350 to 650°C. The rate-limiting step is the diffusion of copper through the main product Cu 3Si. When pure copper is used, a bulk diffusion mechanism is operative above 470°C, and the activation energy is 175 kJ mol . Below 470°C incubation times occur. The reaction proceeds by grain-boundary diffusion, with an activation energy of 110 kJ mol . When phosphorus-doped copper is used, bulk diffusion of copper through Cu 3Si occurs above 530°C. Below this temperature grain-boundary diffusion occurs with an activation energy of 92 kJ mol . No incubation times were observed. The segregation of phosphorus at the reaction interface removes the reaction barrier. Cu 15Si 4 and Cu 5Si seem to be absent from the diffusion couples because of kinetic factors.

The diffusion of hydrogen in amorphous silicon

By employing the nuclear 15N 1H reaction technique we have measured H concentration profiles in gd-a-Si in the temperature range of about 250–500°C. A consistent description of these results can be obtained on the basis of a single bulk diffusion mechanism with a surface conductance boundary condition. The thermodynamic parameters assume values of Q = 1.4 eV (±10) and D Q = 1E−3±1.2 cm 2/s for the bulk process, and W = 1.7 eV (±10%) and H 0 = 8 E 3 ± 1.5 cm 2/s for the surface condutance.

Diffusion of Tc-99m in Neutron Irradiated Molybdenum Trioxide and Its Application to Separation

The dry distillation method was studied for rapid separation of 99mTc from neutron irradiated MoO3. Upon heating, 99mTc produced in MoO3 crystal by the 98Mo(n,r)Mo99m β→ 99mTc process was found to be released by diffusion. From observations made of the deposition temperature, it was concluded that 99mTc was present in the form of 99mTc(IV) in the MoO3 crystal lattice and that it was released in the form of 99mTcO2, which, in air, further oxidized to 99mTc2O7. The release of 99mTc from neutron irradiated MoO3 powder was studied in the temperature range of 480°700°C, and its behavior was found to be in good agreement with the bulk diffusion mechanism based on Fick's law. The diffusion coefficient varied with temperature in compliance with Arrhenius' relation, except in the region of temperatures above 700°C. The separation of 99mTc from MoO3 was performed with a yield higher than 80% upon heating for 30 min at 800°C in air. The decontamination factor for 99Mo was consistently found higher than 105.

Steam Oxidation Kinetics and Oxygen Diffusion in UOz at High Temperatures

Results of steam oxidation measurements of UO2 cylinders from 885° to 1835°C may be expressed by the equations: for the parabolic rate of oxidation and for the chemical oxygen diffusion coefficient. Kp and Dc, obtained from thermobalance measurements to 1500° C and from pre- and post-test weights for the higher temperatures, were independent of sample weight from 10 to 149 g and of surfaceto-volume ratios from 2.34 to 18.74 cm-1. Dc varied as the 0.65 power of the final average excess oxygen (x in UO2+x). The oxidation was accompanied by considerable grain growth. A bulk diffusion mechanism was verified by analyses of partially oxidized samples using both X-ray diffraction and analytical chemistry; an oxygen gradient was demonstrated by each technique.

Flux Relationships for Diffusion in Microcapillaries

IT is well known that flux ratios in small capillaries, that is in capillaries where the Knudsen number (Nk, defined as the ratio of capillary diameter to the mean free path of the diffusing molecule) is much less than one, vary as the square root of the inverse molecular weight ratio for binary systems. It has been shown1 that for counter-diffusion in an ‘open’ system, corresponding to the physical situation in most transport experiments of the Wicke–Kallenbach type2, flux ratio may vary from dependence on the square root to the first power of the inverse molecular weight ratio for the bulk diffusion mechanism alone. For capillaries of a fixed length, the progression from square root to first power dependence is associated with increasing Knudsen numbers, the latter mechanism corresponding to Nk≫1. It is clear, then, that at Nk≪1 the predominant mechanism of transport is that of Knudsen diffusion, and flux ratios are equal to the square root of inverse molecular weight ratios, and at Nk≫1 the predominant mechanism of transport is that of bulk diffusion. Flux ratios for sufficiently large Nk will then be equal to the first power of the inverse molecular weight ratios (provided capillary length–diameter ratios do not approach the order of unity).