Nucleation Of Carbide Phases Crystals During Tungstate-Molybdate-Carbonate Melts Electrolysis

Abstract:

The electrochemical nucleation of Mo2C and W2C crystals for tungstate-molybdate-carbonate melts of specific compositions on Ag, Au, Cu, Pt and Ni substrates has been studied by the electrodeposition method. The influence of the electrocrystallization conditions - temperature, depo¬sition time, initial current pulse and current density was characterized. Experimental measurements indicate that crystallization overvoltage is associated with three-dimensional nucleation. Carbide deposition onto the substrates prepared from the same solid materials was not associated with crystallization overvoltage. The maximum value of the initial overvoltage max which includes crystallization and electrolysis overvoltages is propor¬tional to the electrode surface area. Under these conditions, the stage of surface diffusion does limit the rate of the process at the electrode. Increase in the rate of carbide deposition leads to the number of crystallization centres increase which reduces deceleration related to surface diffusion. At high overvoltages, change the limiting stage is observed so that the process rate is determined either by the rate of electron transfer or by the rate of diffusion from the melt volume. An increase in melt temperature leads to complication of the crystallization process by alloy-formation phenomena. The height of the overvoltage maximum for metals which do not form alloys is proportional to the reciprocal time of their formation. An increase in the melt temperature promotes the interdiffusion rate of the refractory metal, carbon and the substrate, and also intensifies their chemical interaction. Structural mismatch is observed during molybdenum and tungsten carbide electrodeposition onto different single-crystal substrates.

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