Diffusion of nickel in single- and polycrystalline Cr2O3

Chromia protective layers are formed on many industrial alloys to prevent corrosion by oxidation. Their role is to limit the inward diffusion of oxygen and the outward diffusion of cations. A number of chromia-forming alloys contain nickel as a component, such as steels, FeNiCr and NiCr alloys. To ascertain if chromia is a barrier to outward diffusion, nickel diffusion in chromia was studied in both single crystals and polycrystals in the temperature range 900–1100°C at an oxygen pressure of 10^?4 atm (argon + 100 ppm O₂). A nickel film of ～35 nm thick was deposited on the chromia surface and, after diffusing treatment, nickel penetration profiles were established by secondary ion mass spectrometry (SIMS). Two diffusion domains appear in polycrystals, the first domain is assigned to bulk diffusion and the second is due to diffusion along grain boundaries. For the bulk diffusion domain and diffusion in single crystals, using a solution of Fick's second law for diffusion from a thick film, bulk diffusion coefficients were determined at 900 and 1000°C. At the higher temperature, a solution of Fick's second law for diffusion from a thin film could be used. For the second domain in polycrystals, Le Claire's model allowed the grain boundary diffusion parameter (αD _gb δ) to be established. Nickel bulk diffusion does not vary significantly according to the microstructure of chromia. The activation energy of grain boundary diffusion is slightly greater than the activation energy of bulk diffusion, probably on account of segregation phenomena. Nickel diffusion was compared with cationic self-diffusion and with literature data on Fe and Mn heterodiffusion in the bulk and along grain boundaries. All results were analyzed in relation to the oxidation process of stainless steel.     

Effect of Soret diffusion on lean hydrogen/air flames at normal and elevated pressure and temperature

The influence of Soret diffusion on lean premixed flames propagating in hydrogen/air mixtures is numerically investigated with a detailed chemical and transport models at normal and elevated pressure and temperature. The Soret diffusion influence on the one-dimensional (1D) flame mass burning rate and two-dimensional (2D) flame propagating characteristics is analysed, revealing a strong dependency on flame stretch rate, pressure and temperature. For 1D flames, at normal pressure and temperature, with an increase of Karlovitz number from 0 to 0.4, the mass burning rate is first reduced and then enhanced by Soret diffusion of H₂ while it is reduced by Soret diffusion of H. The influence of Soret diffusion of H₂ is enhanced by pressure and reduced by temperature. On the contrary, the influence of Soret diffusion of H is reduced by pressure and enhanced by temperature. For 2D flames, at normal pressure and temperature, during the early phase of flame evolution, flames with Soret diffusion display more curved flame cells. Pressure enhances this effect, while temperature reduces it. The influence of Soret diffusion of H₂ on the global consumption speed is enhanced at elevated pressure. The influence of Soret diffusion of H on the global consumption speed is enhanced at elevated temperature. The flame evolution is more affected by Soret diffusion in the early phase of propagation than in the long run due to the local enrichment of H₂ caused by flame curvature effects. The present study provides new insights into the Soret diffusion effect on the characteristics of lean hydrogen/air flames at conditions that are relevant to practical applications, e.g. gas engines and turbines.     

Effects of Soret diffusion on the laminar flame speed and Markstein length of syngas/air mixtures

The effects of Soret diffusion on premixed syngas/air flames at normal and elevated temperatures and pressures are investigated numerically including detailed chemistry and transport. The emphasis is placed on assessing and interpreting the influence of Soret diffusion on the unstretched and stretched laminar flame speed and Markstein length of syngas/air mixtures. The laminar flame speed and Markstein length are obtained by simulating the unstretched planar flame and positively-stretched spherical flame, respectively. The results indicate that at atmospheric pressure the laminar flame speed of syngas/air is mainly reduced by Soret diffusion of H radical while the influence of H₂ Soret diffusion is negligible. This is due to the facts that the main reaction zone and the Soret diffusion for H radical (H₂) are strongly (weakly) coupled, and that Soret diffusion reduces the H concentration in the reaction zone. Because of the enhancement in the Soret diffusion flux of H radical, the influence of Soret diffusion on the laminar burning flux increases with the initial temperature and pressure. Unlike the results at atmospheric pressure, at elevated pressures the laminar flame speed is shown to be affected by the Soret diffusion of H₂ as well as H radical. For stretched spherical flame, it is shown that the Soret diffusion of both H and H₂ should be included so that the stretched flame speed can be accurately predicted. Similar to the laminar flame speed, the Markstein length is also reduced by Soret diffusion. However, the reduction is found to be mainly caused by Soret diffusion of H₂ rather than that of H radical. Moreover, the influence of Soret diffusion on the Markstein length is demonstrated to decrease with the initial temperature and pressure.     

X-ray topography study of monocrystalline silicon wafers diffused with phosphorus by different methods

To reduce the cost of the emitter diffusion process, there has been increasing interest to substitute the standard process of batch POCl₃ emitter diffusion used in the silicon solar-cell manufacturing industry with in-line diffusion processes such as the spray-on and screen-printing process. For this reason, it is essential to study and compare the processes of different diffusion methods from the point of view of the crystalline quality of the final wafers. X-ray transmission topography was employed to characterize the possible precipitates and other microdefects generated in Czochralski-grown silicon (Cz Si) during the emitter diffusion process carried out by screen-printing, spray-on and the standard process, in which the emitter was provided by a liquid (POCl₃) source. The results indicate that the phosphorus diffusion process influences the crystalline quality of the wafers and the efficiency of the external gettering process that takes place during phosphorus diffusion depends on the diffusion method employed.     

Structure and diffusion processes in laminated composites of a Cu–Ti system

Characteristics of diffusion transformations in a multi-layer composite of a Cu–Ti system, which was produced by high-temperature diffusion welding followed by rolling at room temperature, are reported. It is shown that phase-formation at the layer boundaries in this system at the temperature below 823 K is determined by diffusion along the inner surfaces of these boundaries; while above this temperature it is controlled by bulk diffusion.     

Diffusion Imaging with HyperpolarizedHe Gas

We used MRI of hyperpolarized³He to demonstrate some novel aspects of gas diffusion. Two different techniques were used. First, a slice was burned into a one-dimensional image by inverting the spins in the slice and diffusion was studied by measuring the magnetization as it filled the depleted slice. A diffusion coefficient was determined by the fit of these data. Second, one-dimensional diffusion images were made using a Stejskal–Tanner PGSE method. This was done with and without a temperature gradient present, showing that the effect of temperature can be dynamically monitored by such diffusion images.     

Gold and platinum diffusion: The key to the understanding of intrinsic point defect behavior in silicon

The study of gold and platinum diffusion is found to allow the separate observation of the intrinsic point defects, i.e., of silicon self-interstitials and of vacancies. The diffusion of gold in float zone (FZ) silicon is found to be dominated by the kick-out mechanism for temperatures of 800° C and higher. The diffusion of platinum in FZ silicon is described by the kick-out mechanism for temperatures above approximately 900° C, whereas for temperatures below approximately 850° C the dissociative mechanism governs platinum diffusion. As a result of numerical simulations, we suggest a complete and consistent set of parameters which describes the diffusion of platinum in silicon in the temperature range from 700° C to 950° C and the diffusion of gold in the temperature range from 800° C to 1100° C. The generation or recombination of self-interstitials and vacancies is found to be ineffective at least below 850° C. The concentration of substitutional platinum is determined by the initial concentration of vacancies at diffusion temperatures below 850° C. Platinum diffusion below 850°C can be used to measure vacancy distributions in silicon quantitatively.     

Diffusion Measurement in Phantoms and Tissues Using SLIM Localization

A new approach to efficient localized diffusion measurements has been developed and evaluated on phantoms and isolated tissues. The combination of a diffusion-sensitive pulse sequence with SLIM (spectral localization by imaging) makes efficient and accurate localized water and metabolite diffusion measurements possible with a substantial improvement in spatial or time resolution compared to standard methods. Phantom experiments showed that diffusion of substances present in relatively low concentration within small compartments can be measured accurately by this method, suggesting potential applications for diffusion measurements of metabolitesin vivo.Experiments on excised rat uterine horns demonstrated the ability of this method to measure localized diffusion of water within irregularly shaped regions of biological samples. Accurate diffusion measurements were achieved in the localized regions with acquisition times less than would have been required by standard diffusion imaging methods.     

Oxygen diffusion in uranium dioxide in the temperature range of phase transitions

The structure of and oxygen diffusion in UO₂ are studied by the molecular dynamics method in the range of transition to the superionic state (melting of the oxygen sublattice) and near the melting point of UO₂. The temperature dependence of the diffusion coefficient of a doubly charged oxygen ion in UO₂ is constructed. In the crystalline state at temperatures between 1800 and 2600 K, this dependence is described by an exponential dependence with a diffusion activation energy of 2.6±0.2 eV. In the superionic state (2600–3100 K), the activation energy of diffusion of an oxygen anion decreases to 1.88±0.13 eV. In melt (3100–3600 K), the exponential dependence of the diffusion coefficient of O^2- persists but the activation energy of diffusion decreases still further, to 0.8±0.2 eV. Our experimental results agree (within the limits of experimental error) with data on oxygen diffusion in the crystalline phase obtained by other researchers.     

Application of the Method of Secondary-Ion Mass Spectroscopy to the Study of Heterodiffusion in Alkali-Halide Crystals

The present paper gives the results of investigations of the high-temperature diffusion of Na and Mg ions into single crystals of potassium bromide by the method of secondary-ion mass spectrometry (SIMS). It is demonstrated that the diffusion profiles are nonelementary in character, and the diffusion in the near-surface layer is decelerated. To determine the volume diffusion coefficient, the diffusion profiles must be approximated for depths 1 m. The efficiency of the procedure for determining the diffusion coefficient is confirmed by a comparison of the diffusion coefficients for Na ions determined in the present work with their values obtained by other methods having high reliability and measurement accuracy. The diffusion coefficients of the magnesium impurity are determined for various initial chemical states.     

First study of manganese diffusion in Cr2O3 polycrystals and thin films by SIMS

Chromia layers are formed on many industrial alloys and act as a protective barrier against the corrosion of the materials by limiting the diffusion of oxygen and cations. Most of these alloys contain manganese as an impurity, and manganese oxides are often found at the outer surface of the oxide films. In order to clarify the oxidation mechanism and to check if chromia acts as a barrier, manganese diffusion in chromia was studied in both polycrystals and oxide films formed by oxidation of Ni–30Cr alloy in the temperature range 700–1100°C at an oxygen pressure of 10^?4?atm. After deposition of Mn on the chromia surface and a diffusing treatment, the manganese penetration profiles were established by secondary ion mass spectrometry. In all cases, the diffusion profiles showed two domains. For the first domain, using the solution of Fick's law for diffusion from a thick film into a semi-infinite medium, bulk diffusion coefficients were determined. For the second domain, the Le Claire model allowed the grain boundary diffusion parameter (αD _gbδ) to be obtained. Manganese diffusion does not vary significantly according to the nature and microstructure of chromia. The activation energy of grain boundary diffusion is not far from that obtained for bulk diffusion, probably on account of segregation phenomena. Manganese diffusion was compared to cationic self-diffusion and iron diffusion, and related to the protective character of chromia.     

磷通过热生长二氧化硅层在硅中的扩散

用四探针测量薄层电导方法及阳极氧化去层技术,测定了磷在硅中扩散的具体分布,在恒表面浓度下,它们偏离余误差函数分布。如认为这是由于扩散系数是杂质浓度的函数,实验得到了当杂质浓度大于10¹⁹原子/厘米³时,扩散系数随杂质浓度增加而增大的强烈依赖关系。用同样方法测定了磷通过二氧化硅层后在硅中扩散的具体分布,研究了这些杂质分布的特性,实验表明,不同厚度的氧化层在1300℃高温下仍具有掩蔽效应,在完全掩蔽失效时间附近,杂质分布的共同特点是表面浓度较低(～10¹⁷原子/厘米³)、结较浅(～1微米)。对不同厚度的氧化层,经过足够的时间后,硅中表面浓度不受氧化层厚度的影响,而只由扩散源的蒸气压决定。磷通过氧化层后扩散的具体分布情况还与扩散源的性质、条件等密切相关。扩散过程中观察到的氧化层厚度增长有可能影响表面附近杂质的具体分布情况。     

Photothermal deflection measurement of effective gas diffusion coefficient of a porous medium

In this work, an effective gas diffusion coefficient of a porous medium was measured using photothermal deflection (PD) technique. An in-house made Loschmidt diffusion cell with a photothermal-deflection probe were employed to measure the effective gas diffusion coefficient of a gas diffusion layer (GDL) with a porosity ε ≈ 0.7. The concentration evolutions of CO₂ in O₂ with and without the GDL were measured, respectively, using a transverse normal PD technique. The concentration variations were used to deduce the gas diffusion coefficients in the presence and absence of the GDL by solving mass diffusion equations. The effective gas diffusion coefficient of the GDL was calculated from the diffusion coefficients using a model of an equivalent resistance to diffusion and found to be 4.39 × 10^-6 m²s^-1, demonstrating that PD technique can be employed to determine the effective gas diffusion coefficient of a porous medium.     

Diffusion and aggregation of adatoms on faceted Pd/Mo(111) surface

Diffusion of Pd adatoms on faceted Pd/Mo(111) surface with hill and valley structure is studied by kinetic Monte Carlo method. The surface consists of {211} channeled facets separated by concave and convex edges. It is shown that diffusion on the faceted surface is significantly reduced with comparison to diffusion on (112) surface by the presence of concave edges. A simple formula for the diffusion coefficient is proposed. The growth of linear structures, studied at low coverage θ = 0.1, depends on the cross-channel diffusion, although the cross-channel diffusion coefficient is by 6–8 orders of magnitude smaller than the in-channel diffusion coefficient. We observed that most of atomic structures grow in valleys and the average size of the structure increases with increase of facet size.     

Variational estimates of the diffusion coefficient of cosmic rays in a random magnetic field

A variational method is used to obtain estimates of the effective particle transport coefficients in a random static magnetic field. The particle propagation is described by an anisotropic diffusion equation. The diffusion coefficient parallel to the local magnetic field is much greater than the transverse diffusion coefficient. For large-scale magnetic-field variations the diffusion is described by effective coefficients. The variational approach can be used to find the effective parallel and perpendicular diffusion coefficients. It was shown that the instability growth rate of the magnetic field lines determines the upper estimate of the effective transverse diffusion coefficient. Zh. éksp. Teor. Fiz. 114, 398–405 (August 1998)     

On the analysis of coordinate-dependent diffusion

Coordinate-dependent diffusion in solids, in which case the probability of presence of a vacancy, φ, and the probability of migration, γ, depend on coordinate x is analyzed. The coordinate dependence of parameters φ and γ is shown to cause the drift component of the diffusion flux. In the presence of this component, the direction of atomic motion may coincide (enhanced diffusion) or not coincide (retarded diffusion and up-diffusion) with the direction of the diffusion component. The criteria of enhanced diffusion, retarded diffusion, and up-diffusion are established. These criteria are specified by the behavior of function F(x)=φ(x)/γ(x).     

Calculation of radon diffusion coefficient and diffusion length for different building construction materials

The diffusion of radon in dwellings is a process determined by the radon concentration gradient across the building material structure between the radon source and the surrounding air, and can be a significant contributor to indoor radon inflow. Radon can originate from the deeply buried deposit beneath homes and can migrate to the surface of earth. Radon emanates to the surfaces mainly by diffusion processes from the point of origin following α-decay of ²²⁶Ra in underground soil and building materials used, in the construction of floors, walls, and ceilings. In the present study radon diffusion through some building materials viz. coarse sand and stone dust of different grain size has been carried out using LR-115 type II solid-state nuclear track detectors (SSNTDs). The radon diffusion coefficients and diffusion lengths through these building construction materials have been calculated. The effect of grain size on radon diffusion through these building materials shows the decrease in radon diffusion with decrease in grain size.     

Molecular dynamics simulation on boron diffusion in diamond

The atomic-scale diffusion mechanism of boron in diamond is investigated by molecular dynamics simulation. A substitutional boron atom diffuses to the self-interstitial site when there exists a self-interstitial carbon atom in its nearest tetrahedral center and the system temperature is high. More important, the bond between boron and the self-interstitial carbon atom is never broken during the diffusion process, indicating that B_s-C_i pairs diffuse in the lattice by the interstitial mechanism. The results suggest that boron diffusion is mediated by carbon self-interstitial and not by the vacancy mechanism. In addition, the estimated activation energy and the diffusion exponential prefactor of boron diffusion in diamond are found to be 0.23 eV and 1.123×10⁻⁶cm²/s, respectively.     

Photoelectrical characteristics of as-grown highresistance GaAs single crystals

The electrophysical and photoelectrical properties of high-resistance GaAs produced by single crystal growth are studied and analyzed. The electron (τ _n ) and hole (τ _p ) lifetimes are estimated. The charge-carrier lifetimes are compared in as-grown and diffusion high-resistance GaAs. The conclusion is made that in high-resistance GaAs produced by chromium diffusion, the charge-carrier recombination mechanism qualitatively differs from that in as-grown GaAs. The charge-carrier recombination in diffusion GaAs is determined by the occurrence of recombination barriers due to chromium diffusion.     

Simulations of anomalous ion diffusion in experimentally measured turbulent potential

The diffusion of plasma impurities in tokamak-edge-plasma turbulence is investigated numerically. The time-dependent potential governing particle motion was measured by 2D array of 8×8 Langmuir probes in edge region of CASTOR tokamak. The diffusion of particles is found to be classical in the radial direction, but it can be of an anomalous Lévy-walk type in the poloidal direction. The diffusion is found to be dependent on the ratio of particles’ mass and charge. When this ratio grows, the diffusion coefficient in radial direction grows as well, whereas poloidal diffusion coefficient drops down. Moreover, movement of particles in the time-frozen snapshot of this potential is investigated showing that also the time-independent potential is much more favorable for the particle diffusion in poloidal direction than in radial one. In the case of single ionized carbon ions the poloidal diffusion in time-independent potential transits to the Lévy-walk type for temperatures greater than 25 eV, for radial diffusion Lévy-walk was not observed even for 500 eV.