Hafnium Sponge Sintering Kinetics

Abstract:

Hafnium metal is produced industrially by electrolysis of tetrachloride dissolved in molten equimolar mixture of sodium and potassium chlorides, to which some sodium fluoride was added, or by the ingenious process developed by Kroll for producing zirconium and titanium. In the latter case separation of hafnium formed from the excess magnesium and by-product magnesium chloride is achieved by vacuum distillation resulting in the formation of hafnium sponge. To obtain compact hafnium metal the sponge produced must undergo sintering at elevated temperatures under vacuum. Until now there was no information concerning sintering conditions and a mechanism of the process.In the present study we attempted to investigate mechanism of the mass transport through studying the kinetics of sintering of hafnium sponge. Two different techniques were employed to determine the mechanisms of sintering: activation energy measurements using dilatometry and subsequent analysis of sintered hafnium structure by scanning electron microscopy. The experiments on hafnium sponge sintering were conducted between 1000 and 1500°C in high vacuum (residual pressure less than 0.1 Pa). It was found, that hafnium sponge sintering begins at 1100°C. Increasing temperature to 1300°C leads to enlargement of grains through their agglomeration. Samples obtained at 1400 and 1500°C represent a compact metal with small pores in the volume.During sintering the samples shrink equally in axial and radial directions. Depending on the process temperature the samples of different densities were obtained. Hafnium sponge sintering rates were calculated for different temperatures on the basis of the results of dilatometry measurements. The Arrhenius plot of sintering rate represents a superposition of two straight lines. Using calculated apparent activation energies (350±70 kJ/mol for 1100-1400°C and 47±2 kJ/mol for 1400-1500°C) and data concerning β-titanium sintering we conclude, that the diffusion of metal atoms in the crystal lattice (or lattice self-diffusion) is the liming stage of the process at 1100-1400°C, while at higher temperatures, 1400-1500°C, the sintering process is controlled by grain boundary self-diffusion.

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