Initial stages of nucleation of molybdenum and tungsten carbide phases in tungstate–molybdate–carbonate melts

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 was studied by the electrodeposition method. The influence of electrocrystallization conditions (temperature, deposition time, initial current pulse, and current density) was investigated. 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, ηmax, which includes crystallization and electrolysis overvoltages, is proportional to the electrode surface area. Under these conditions, surface diffusion limits the electrode process rate. An increase in carbide deposition rate leads to an increase in the number of crystallization centers. This reduces the duration of surface diffusion. A rise in the melt temperature complicates the crystallization process by alloy-formation phenomena. Overvoltage maximum height for metals, which do not form alloys, is proportional to the reciprocal of their formation time. Melt temperature increase promotes interdiffusion of refractory metal, carbon, and substrate, and also intensifies their chemical interaction. Structural mismatch is observed during molybdenum and tungsten carbide electrodeposition onto different single-crystal substrates.