Aluminum diffusion rate

Current-potential curves depend on the stirring conditions in the investigated potential range. It was thus assmned that the aluminum deposition rate is determined by convective diffusion as well as by discharging. 

Experiments were carried out with Ionac MC 3470 to determine the self-diffusion coefficient values for H+ and Al + in the coupled transport. Data points were used from the experiment involving 2N acid sweep solution in Figure 34.24b, presented later. These values formed the basis for aluminum transport rate or flux (7ai) calculation at different time intervals. The equilibrium data generated in Figure 34.20b were used in conjunction with Equation 34.25 to determine the interdiffusion coefficient values. Local equilibrium was assumed at the membrane-water interface. Eigure 34.24a shows computed Dai,h values for this membrane. When compared with Dai,h values for Nafion 117, it was noticed that the drop in interdiffusion coefficient values was not so steep, indicative of slow kinetics. The model discussed earlier was applied to determine the self-diffusion coefficient values of aluminum and hydrogen ions in Ionac MC 3470 membrane. A notable point was that the osmosis effect was not taken into account in this case, as no significant osmosis was observed in a separate experiment. 

Chromium and aluminum are known to form extremely stable oxides and are included in many alloys. Generally, 10 at.% is sufficient to provide the alloy with a continuous oxide layer. In some metals, like Co, the diffusion rate of Cr or A1 is lower, and a greater concentration of Cr (25 at.%) is needed to achieve a continuous layer. If temperatures above 850 °C are anticipated, it is generally preferable to use A1 in place of Cr as chromium oxides will oxidize to C1O3, a volatile compound at elevated temperatures [17]. 

The rate of aluminum diffusion from the Al-doped epitaxial layer is lower than from the vapor phase. It is assumed that Al diffuses via silicon vacancies. 

In some cases, diffusion barriers are used at the interface to reduce diffusion. For example, W Ti or electrically conductive nitrides such as TiN are used as dififiasion barriers in aluminum metallization of silicon to inhibit aluminum diffusion into the silicon during subsequent high temperature processing. Barrier layers, such as tantalum, nickel, and nickel-chromium, are used to prevent diffusion and reaction in metallic systems. The presence of compound-forming species in the depositing material reduces the diffusion rate. Alternatively, materials may be alloyed with film material to reduce diffusion rates.t ... 

The Beckstead-Derr-Price model (Fig. 1) considers both the gas-phase and condensed-phase reactions. It assumes heat release from the condensed phase, an oxidizer flame, a primary diffusion flame between the fuel and oxidizer decomposition products, and a final diffusion flame between the fuel decomposition products and the products of the oxidizer flame. Examination of the physical phenomena reveals an irregular surface on top of the unheated bulk of the propellant that consists of the binder undergoing pyrolysis, decomposing oxidizer particles, and an agglomeration of metallic particles. The oxidizer and fuel decomposition products mix and react exothermically in the three-dimensional zone above the surface for a distance that depends on the propellant composition, its microstmcture, and the ambient pressure and gas velocity. If aluminum is present, additional heat is subsequently produced at a comparatively large distance from the surface. Only small aluminum particles ignite and bum close enough to the surface to influence the propellant bum rate. The temperature of the surface is ca 500 to 1000°C compared to ca 300°C for double-base propellants. 

Permeability of an FML is evaluated using the Water Vapor Transmission test.28 A sample of the membrane is placed on top of a small aluminum cup containing a small amount of water. The cup is then placed in a controlled humidity and temperature chamber. The humidity in the chamber is typically 20% relative humidity, while the humidity in the cup is 100%. Thus, a concentration gradient is set up across the membrane. Moisture diffuses through the membrane, and with time the liquid level in the cup is reduced. The rate at which moisture is moving through the membrane is measured. From that rate, the permeability of the membrane is calculated with the simple diffusion equation (Fick s first law). It is important to remember that even if a liner is installed correctly with no holes, penetrations, punctures, or defects, liquid will still diffuse through the membrane. 

As mentioned by Mathias et al. [9], reliable methods to measure the thermal conductivity of diffusion layers as a function of compression pressures are very scarce in the open literature. Khandelwal and Mench [112] designed an ex situ method to measure accurately the thermal conductivities of different components used in a fuel cell. In their apparatus, the sample materials were placed between two cylindrical rods made out of aluminum bronze (see Figure 4.28). Three thermocouples were located equidistantly in each of the upper and lower cylinders to monitor the temperatures along these components. Two plates located at each end compressed both cylinders together. The temperatures of each plate were maintained by flowing coolant fluids at a high flow rate through channels located inside each of the plates. A load cell was located between two plates at one end so that the compression pressure could be measured. 

Rates of sorption and desorption of phosphate. Eur. J. Soil Sd. 48 101-114 Strens, R.G.S. Wood, B.J. (1979) Diffuse reflectance spectra and optical properties of some iron and titanium oxides and oxyhydr-oxides. Min. Mag. 43 347—354 Stumm, W. Eurrer, G. (1987) The dissolution of oxides and aluminum silicates Examples of surface-coordination-controlled kinetics. 

The major results of this study are consistent with a simple picture of mordenite catalysts. An increase in effective pore diameter, whether by extraction or exchange, will increase the rate of transport of reactant and product molecules to and from the active sites. However, aluminum ions are necessary for catalytic activity as aluminum is progressively removed by acid extraction, the number of active sites and the initial activity decrease. Coke deposition is harmful in two ways coke formation as the reaction proceeds will cause a decrease in effective pore diameter and effective diffusivity, and coke deposited on active sites will result in a chemical deactivation as well. 

One can distinguish the following steps in the agglomeration of silica dispersions with hydrolyzed aluminum (1) hydrolysis and multimerization of Al(III) to isopolycations (2) diffusion of these aluminum hydroxo complexes to the colloid surface and adsorption and (3) transport of suspended particles and collision resulting in certain instances in attachment of colloids. The transport of colloids to each other has been observed to proceed more slowly than all other steps under the given circumstances (3). It follows that the rate of agglomeration, — dN/dt, is obtained from the collision frequency, as determined solely by the trans-... 

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