SURFACE EXCHANGE OF SODIUM, ANISOTROPY OF DIFFUSION AND DIFFUSIONAL CREEP IN CARBON CATHODE MATERIALS

摘要

Experimental data for the expansion of a semigraphitic cylinder due to sodium diffusion in the axial and radial directions are explained by anisotropic diffusion coefficients and surface exchange coefficients. Compressive creep deformation due to sodium diffusion into the binder phase (short-term creep) produces reduction of the sodium expansion when an external pressure is applied to the material. A constitutive model describes the diffusional short-term creep of an anthracitic material. The long-term creep deformation is considered related to sodium diffusion into grains. Creep measurements have been made for a semigraphitic material under axial loading applied after the maximum value of free sodium expansion. A new constitutive model considering the surface boundary conditions with the exchange rate of sodium describes the carbon swelling and short-term creep. Modeling of the diffusional long-term creep in grains under loading after binder phase sodium saturation is also described. Validation of the proposed model through correlation of theoretical results and experimental data for the sodium concentration, sodium expansion and creep strain is performed. Practical recommendations on how to extend the model to full-scale reduction cells are formulated.