Thermal diffusion galvanizing

Similar phenomena such as diffusion potential and thermal diffusion potential in systems where ion transport is involved are also of considerable interest. Coupling of flow of ions relative to solvent is involved in the development of diffusion potential, while in the case of thermal diffusion potential, coupling of flow of ions and energy flow is involved. In such situations, the effective transference number as compared to Hittorf transference number is affected. Interesting experimental results have been reported in the context of galvanic cells (thermo-cells), in which the two electrodes are not at the same temperature where results have been interpreted in terms of thermodynamics of irreversible processes [3]. 

Solid electrolytes can withstand higher temperatures than liquids which is important for the Carnot efficiency of a thermo-galvanic cell. In the case of devices featuring a liquid electrolyte and a redox couple, the electroactive species diffuse from one electrode to the other. To have high steady state current, diffusion gradient should be as steep as possible which means bringing the electrodes close to each other. There results an increase in thermal loss by conduction. On the other hand, with a solid electrolyte such as 3-A 20s the electroactive species migrate in the electrolyte where it is the only possible current carrier. Consequently the current will not be limited by mass diffusion but by heat diffusion in metallic electrodes or by the electrical resistance of the solid electrolyte. 

This is the case, for instance, for K2Sip6 which in an aluminothermic reaction provides in situ the Al-Si clad (molten fiUer metal) normally used for controlled atmosphere brazing (CAB) of aluminium. In the case of zinc, the primary objective is not only to form a clad but to obtain a homogeneous diffusion layer into the aluminium, which is needed for some applications and which would have to be otherwise applied by costly intensive methods like electrochemical galvanization, plasma coating or thermal spray coating. 

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