Nickel self-diffusivity

Nickel self-diffusion in single-crystals was measured as a function of temperature and O2 pressure at 1182 to 1762C and from 1 to 5 x 10" atm. In pure O2 and in pure... 

Figure 4.38 Schematic illustration of a self-diffusion experiment in which (a) a thin layer of radioactive nickel is deposited on one surface of a nonradio active nickel specimen. After heating and time (b), the radioactive nickel has diffused into the sample, as monitored with a radioactive detector. From K. M. Ralls, T. H. Courtney, and J. Wulff, Introduction to Materials Science and Engineering. Copyright 1976 by John Wiley Sons, Inc. This material is used by permission John Wiley Sons, Inc.

Current collectors in Ni/MH batteries are usually made of nickel foam, passivated by a NiOOH layer at the positive side. Separators are usually microporous plastic films impregnated with liquid electrolyte. It is, therefore, necessary that the active masses be insoluble, or at least sparingly soluble, in order to avoid mixing of components via diffusion through the separator, thus leading to self-discharge. 

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. 

In spite of their low solubility ( 5 x 10 M litre), HFeOJ ions diffuse to the positive electrode and are oxidized to solid FeOOH causing further dissolution of iron and its continous transfer to the positive electrode. The process is irreversible, the potential of the nickel electrode being too positive, even during discharge, for the reduction of trivalent iron. Further decrease of capacity is caused by the lowering of oxygen overpotential on the nickel oxide in the presence of FeOOH. The self-discharge and iron transfer processes are somewhat inhibited by additives to the electrode (sulfur) or electrolyte (e.g., lithium and sulfide ions, or hydrazine sulfate). 

The formation of (II) provides a quite selective spot test for palladium. Gold must be removed prior to the test because it will cause the development of a deep ruby red in the spot plate test and a diffused violet spot on the paper, apparently due to the reduction of the gold ions to the colloidal metal. Interference may also arise from 0s04 , Os+, Ru+, and RuCle ions because they have distinct self-colors. Mercurous ion causes partial interference by the reduction of part of the palladium to the elementary state, but a positive response can still be seen. It is possible to detect I part of palladium in the presence of 200 parts of platinum or 100 parts of rhodium. Less favorable ratios should be avoided because of the color of these salts. No interference is caused by mercuric and iridic chloride, but free ammonia, ammonium ions, stannous, cyanide, thiocyanate, fluoride, oxalate, and tetraborate ions do interfere. Lead, silver, ferrous, ferric, stannic, cobaltous, nickel, cupric, nitrite, sulfate, chloride, and bromide ions do not interfere. 

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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