Aluminum self-diffusivity

Henderson J., YangL., andDerge G. (1961) Self-diffusion of aluminum in Ca0-Si02-Al203 melts. Trans. Met. Soc. AIME 221, 56-60. 

Y. Oishi and W. D. Kingery, Self-Diffusion of Oxygen in Single Crystal and Polycrystalline Aluminum Oxide, J. Chem. Phys., 33, 480-486 (1960). 

In addition, a model has been formulated to predict the self-diffusion coefficient of diffusing species, which are hydrogen and aluminum ions. The interdiffusion coefficient value for the coupled transport of these two ions is also determined for the two types of membranes. 

A numerical integration technique is applied to calculate the aluminum ion concentration at intermediate times. The input parameters are (1) initial concentration of the ions in the feed and the sweep solutions, CAi,feed> Cai,sweep Cn.feed CH,sweep (2) volume of the two solutions, Vfeed and Vsweep (3) area of exchange A, (4) self-diffusion coefficient Dai and Dai/Dh determined from Step 4, and (5) wet membrane thickness, L. Two sets of equations are generated from the isotherm plot for... 

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. 

Even for lonac MC 3470, it was the H" " ion that was the trace species. A comparison of the diffusion coefficient values for the two membranes is presented in Table 34.3. From this table, it can be observed that the self-diffusion coefficient values for aluminum and hydrogen ions are similarly lowered as the experimental interdiffusion coefficient value. This lowering is about one order for both the cases. Explanation for low Dai,h values in the heterogeneous membrane is desired, as it explains the slow transport kinetics in heterogeneous membrane. 

Oishi Y, Ando K (1984) Oxygen diffusion in MgO and AI2O3. Adv Ceram Structural Prop 10 379-395 Oishi Y, Kingeiy WD (1960) Self-diffusion of oxygen in single crystal and poly cry stalhne aluminum oxide. J Chem Phys 33 480-486... 

A novel one-sided NMR magnet has been developed for self-diffusion measurements in thin samples [7]. Using this technique, researchers have demonstrated measurements in bulk [BMIM][TFSI] and [BMIM][TFSI] confined in nanoporous anodized aluminum oxide membranes, as shown in Fig. 3. This operates at a proton Larmor frequency of 14.08 MFlz, and the expected appHcation is toward in situ measurements in portable energy devices. This method utilizes the fringe field to measure diffusion coefficients instead of conventional PFG-NMR. 

The self-diffusion coefficient of A1 was determined in polycrystalline aluminum oxide at 1670 to 1905C. Diffusion couples were employed which used in oxide form as a radioactive tracer. Within the above temperature range, the experimental data were represented by ... 

Kingery et al. [10a] have collected the experimental results for cation and anion self-diffusion coefficients in various oxides. For instance, the rate of diffusion of copper in CU2O, a modifier, is six orders of magnitude larger than that of aluminum in AI2O3, a network former when the aluminum is four coordinate. This result is for a temperature of approximately 1500°C, but the relative relationship is expected to hold at room temperature. This finding also supports the postulate that network modifiers can recrystallize more easily than network formers. 

Fig. 9. Apparent diffusion coefficients calculated from the volume kinetics portions of the shrinkage isotherms compared with the reported self-diffusion coefficients for aluminum and oxygen.

It follows from this equation that in the case of molybdenum sulfidation one half of A//f participates in the activation energy of sulfur self-diffusion, because the concentration of both ionic and electronic defects increases with increasing temperamre. In aluminum doped M0S2, on the other hand, the concentration of electronic defects is fixed on the constant level (Eq. (20.12)) and consequently the whole value of A//f participates in E. Eliminating the equilibrium constant K from Eq. (20.13) and considering Eqs (20.6) and (20.15) yields ... 

Monoliths made of metal foils can also be used as substrates in combustion catalysts [19, 20]. The metal is generally an iron- or nickel-based steel containing small amounts of aluminum. The aluminum diffuses to the surface on heating and oxidizes to form an adherent alumina layer. This alumina layer gives the alloy high oxidation resistance and is essentially self-healing as it arises from diffusion from the bulk material. It also provides good adhesion for the alumina washcoat. 

The meehanism for creep reduction is not understood although the relationship between dopant concentration and creep resistance has been confirmed to be a true solid-solution effect. One school of thought is that creep is controlled by grain boundary diffusion and that large segregated ions simply hinder diffusion in the core sites at the boundary. This theory is supported by kinetic measurements of self-difEiision in undoped and yttria-doped alumina and by studies of the oxidation of aluminum alloys (Le Gall et al., 1995). There is... 

Metals such as titanium, aluminum, nickel, and stainless steels have been pursued for bipolar plate applications [5,8,10-12], However, these research efforts met limited success because of the chemical instability of the metals in the fuel ceU environment, especially when in contact with the acidic electrolytic membrane. Corrosion of the metal bipolar plate leads to a release of cations, which can both lead to an increase in membrane resistance and poisoning of the electrode catalysts [12]. The oxide film formed on the surface of the self-passivating metals also results in high voltage losses across the plate/macro-diffuser interface [8,11]. 

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