EMF Measurements of the Na-Si System

The electromotive force (EMF) of galvanic cells with β-alumina solid electrolyte was measured to determine the phase relations and thermodynamic properties of the Na-Si binary system below 800 °C. Na was added by coulometric titration to cells containing known amounts of silicon. The galvanic cells sealed with Corning 1720 Kovar glass had unpredictable lifetimes, lasting from several days to less than one reading. A new ceramic seal consisting of Na2O-Al2O3 eutectic was developed which allowed high temperature EMF measurements up to 800 °C. The solubility of Si in molten Na was determined to be very limited. Properties of the NaSi and Na4Si23 compounds were derived from the EMF measurements. Differential thermal analysis (DTA) experiments were carried out to determine the onset temperature of phase changes.