Determination of the standard free energies of formation for tungsten silicides by EMF measurements using lithium silicate liquid electrolyte

EMF measurements on silicon concentration cell were carried out using lithium silicate electrolyte in the temperature range 1300-1500 K. Using the EMF-temperature relations obtained, the standard free energies of formation for W5Si3 and WSi2 have been calculated as functions of temperature: ΔG 0(W5Si3) (kJ mol-1) = -128.6 - 0.0132 (T (K)) ± 0.42 and ΔG 0(WSi2) (kJ mol-1) = -98.5 + 0.0095 (T (K)) ± 0.15, respectively. Combining the results with thermodynamic data, the standard enthalpies of formation for silicides at 298 K have been obtained as ΔH 0(W5Si3, 298) (kJ mol-1) = -133 ± 25 and ΔH 0(WSi2, 298) (kJ mol-1) = -82 ± 6, respectively. A new eutectoid reaction W5Si3 => W + WSi2 occurring around 700 K is suggested in W-Si binary system