Thermodynamic Studies on the Mg-B System Using Solid State Electrochemical Cells

Abstract

This study provides a reasonable evaluation of the Mg-B binary system. Magnesium and Boron (Mg-B) alloys were investigated to determine their thermodynamic properties employing solid state electrochemical cells based on CaF₂ electrolyte represented as $(-) {\text{Pt}},{\text{Ar}}/\left\{ {{\text{Mg}} + {\text{CaMgF}}_{4} } \right\}\left\| {{\text{CaF}}_{2} } \right\|\{ [{\text{Mg}} - {\text{B}}]_{\text{alloy }} + {\text{CaMgF}}_{4} \} /{\text{Ar}},{\text{Pt}}(+)$ Investigations were performed over the temperature range of 773–873 K to measure the electromotive force (EMF), which was used to derive the partial Gibbs Free energies of the alloys in the composition range of X_B = 0.07 to 0.95. The activities of Mg were also calculated from partial Gibbs Free energies. The activities of Mg in MgB₂ , MgB₄ , and MgB₇ were expressed as a function of temperature respectively, ln a_Mg = 4.27–9.32 × 10³/T, ln a_Mg = −7.54 + 6.67×10³/T, and ln a_Mg = 4.93–19.57 × 10³/T, where T is the temperature in K. From this expression, the activity of Mg for these intermediate phases can be extrapolated at a higher temperature to get the accurate phase boundaries for the Mg-B system.