Papers by Keyword: Oxygen Diffusion

Abstract: Elemental semiconductors play an important role in high-technology equipment used in industry and everyday life. The first transistors were made in the 1950ies of germanium. Later silicon took over because its electronic band-gap is larger. Nowadays, germanium is the base material mainly for γ-radiation detectors. Silicon is the most important semiconductor for the fabrication of solid-state electronic devices (memory chips, processors chips, ...) in computers, cellphones, smartphones. Silicon is also important for photovoltaic devices of energy production.Diffusion is a key process in the fabrication of semiconductor devices. This chapter deals with diffusion and point defects in silicon and germanium. It aims at making the reader familiar with the present understanding rather than painstakingly presenting all diffusion data available a good deal of which may be found in a data collection by Stolwijk and Bracht [1], in the author’s textbook [2], and in recent review papers by Bracht [3, 4]. We mainly review self-diffusion, diffusion of doping elements, oxygen diffusion, and diffusion modes of hybrid foreign elements in elemental semiconductors.Self-diffusion in elemental semiconductors is a very slow process compared to metals. One of the reasons is that the equilibrium concentrations of vacancies and self-interstitials are low. In contrast to metals, point defects in semiconductors exist in neutral and in charged states. The concentrations of charged point defects are therefore affected by doping [2 - 4].

Abstract: This paper summarizes the diffusion mechanism and relevant research method of oxygen which is diffused from courtyard greenbelt with the action of wind. This paper dynamically simulates the oxygen condition of courtyard space by the method of CFD. According to the simulation results, researchers make triaxial quantitative analysis for greenbelt distribution, oxygen diffusion and concentration distribution of courtyard space. Finally, this paper summarizes the basic law of oxygen diffusion influenced by greenbelt distribution of courtyard and puts forward some design strategies of greenbelt which is in favor of oxygen diffusion and oxygen environment of courtyard.

Abstract: The a-axis oriented ZnO thin films deposited on sapphire substrates by pulsed laser deposition were studied to investigate the effects of pre-annealing on oxygen diffusion. The effect was as follows: the oxygen diffusion coefficient decreased, and the oxygen concentration in the tailing regions of the profiles reduced. Ion images of an oxygen tracer revealed the high-diffusivity paths for oxygen tracer diffusion. The temperature dependence of oxygen tracer diffusion coefficients (D_b) in as-deposited and pre-annealed thin films were determined to be D_b [cm²/ = 9.2x10² exp (- 405 [kJ/mo / RT) and D_b [cm²/ = 1.8x10³ exp (- 418 [kJ/mo / RT), respectively. On basis of these results, the crystal orientation on D_b and the mechanism for oxygen diffusion were discussed.

Abstract: Oxygen ionic transport in mixed-conducting Ca₂Fe₂O_5-δ brownmillerite was analyzed in light of potential applications in the composite materials for oxygen separation membranes and solid oxide fuel cell cathodes. The lattice defect formation and oxygen diffusion mechanisms were assessed by the computer simulations employing molecular dynamics and static lattice modeling. The most energetically favorable oxygen-vacancy location is in the octahedral layers of the brownmillerite structure, which provide a maximum contribution to the ionic migration in comparison with the structural blocks comprising iron-oxygen tetrahedra. The activation energies for the vacancy and interstitial diffusion in the tetrahedral layers, and also between the octahedral and tetrahedral sheets, are several times higher. The calculated values were found comparable to the experimental activation energy for ionic conduction in Ca₂Fe₂O_5-δ, 147 kJ/mol, determined by the steady-state oxygen permeation measurements. The dense membranes of model composite Ca₂Fe₂O_5-δ - Ce_0.9Gd_0.1O_2-δ with equal weight fractions of the components (CGCF5) were sintered and characterized. No critical interdiffusion of the composite constituents, leading to their decomposition, was found by X-ray diffraction and electron microscopic analyses. The electrical conductivity of this composite, with an activation energy of 37 kJ/mol, is intermediate between two parent compounds and is dominantly p-type electronic as for Ca₂Fe₂O_5-δ. Since the ion- and electron-conducting phases are well percolated in the composite ceramics, the oxygen permeation fluxes through CGCF5 are considerably higher than those of both constituents.

Abstract: Oxygen diffusion in MgO stabilized zirconia has been studied by molecular dynamics simulation method with MOLDY software. The results illustrate that oxygen diffusion in MgO stabilized zirconia can be decided by temperature and amount of MgO. Both increase of amount of MgO and temperature in the system may promote the oxygen diffusion. Further increase in temperature is helpful to the oxide ions diffusion. There is always a maximum value of oxygen diffusion coefficient, which is represented by the slope of MSD curves, corresponding to a certain amount of MgO. In MgO stabilized zirconia, the most suitable doped amount of MgO exists in a range , which is decided by the working temperature of MgO stabilized zirconia.

Abstract: On a metal surface covered with a moisture layer of variable thickness and shape, the dissolved oxygen may induce a spatial separation of the anodic and cathodic reactions on space-time scales characteristic of the roughness, droplet size and the local kinetics of the system. This leads to a spatio-temporal variations in species concentrations, current and potential over the metal surface and thus atmospheric corrosion. Here a fully three-dimensional transient model is developed that addresses the corrosion of a metal under an aerosol droplet. The effects of various parameters, such as exchange current densities, initial concentrations, shape and size of the droplet, and diffusivity of oxygen on ionic, potential and current distributions are investigated.

Abstract: Oxygen self-diffusion and oxygen surface exchange investigations in the yttria-stabilized zirconia (3 and 8 mol% of Y2O3) and in the gadolinia-doped ceria (10 and 20 mol% of Gd2O3) ceramics in the temperature range from 250 to 450°C were performed by means of the 18O isotopic labeling and the SIMS profiling methods. The activation energies of these processes were determined. The results were discussed in comparison with the literature data extrapolated from higher temperatures.

Abstract: Oxygen diffusion coefficients were measured in polycrystalline ZnO by means of the gas-solid exchange method using the isotope 18O as the oxygen tracer. The diffusion annealings were performed at 892oC and 992oC, in an Ar+18O2 atmosphere under oxygen partial pressures from 0.1 to 1atm. After the diffusion annealings, the 18O diffusion profiles were established by secondary ion mass spectrometry (SIMS). Increasing the oxygen pressure leads to an increase of the oxygen diffusion in ZnO. The bulk diffusion coefficients depends on oxygen pressure according to , at 882oC, or , at 992oC, which indicates that the oxygen bulk diffusion mechanism should preferentially take place by means of interstitial oxygen having a null effective charge. The grain boundary diffusion coefficients show little dependence on oxygen pressure at 882oC, given by , which should correspond to a diffusion mechanism by means of interstitial oxygen, with a double negative charge, but at 992oC this dependence is corresponding to a diffusion mechanism by interstitial oxygen having a null effective charge. The results also show that the grain boundary is a fast path for the oxygen diffusion in polycrystalline ZnO.

Abstract: Oxygen uptake and oxygen diffusion in Mayenite (Ca12Al14O33) were investigated using the stable tracer 18O2. Mayenite contains one intrinsic, highly mobile oxygen anion. It was shown that for high temperatures (above 700 °C), the diffusion goes through a interstitialcy mechanism, where the interstitial oxygen anion knocks out a lattice oxygen anion. The activation enthalpy for this process is around 0.9 eV, suggesting that the ionic migration is very fast.

Abstract: Oxygen diffusion coefficients were determined in a commercial ZnO-based varistor by means of the gas-solid exchange method using the isotope 18O as the oxygen tracer. The diffusion annealings were performed at 892, 942, 992 and 1092oC, in an Ar + 18O2 atmosphere under an oxygen partial pressure of 0.2 atm. After the diffusion annealings, the 18O diffusion profiles were established by secondary ion mass spectrometry (SIMS). The results show an increase of the oxygen diffusion in the varistor, both in bulk and in grain boundaries, when compared to the oxygen diffusion in undoped ZnO. The increase of the oxygen bulk diffusion in the varistor agrees with an interstitial mechanism for the oxygen diffusion. The results also show that the grain boundary is a fast path for the oxygen diffusion in the varistor. However, the oxygen diffusion in the grain boundaries of the varistor seems to depend on several chemical and microstructural parameters and does not allow a simple explanation.