Phase equilibria and reaction kinetics of borides based high-temperature thermoelectric materials
In this study, the phase equilibria of the binary borides (Mg-B and Si-B) were determined by solid-state electrochemical measurements. This study provides the equilibrium thermodynamic properties of boride based thermoelectric materials. Experiments were performed to measure the electromotive force (EMF) as a function of temperature. The activities of Mg/Si in the boride alloys were determined to obtain the partial and integral thermodynamic properties (∆GM, ∆HM) of boride based TE materials. The tangent rule was used to estimate the Gibbs energies of formation (∆G0f) of SiB3, SiB6, and SiB14-50 from 823 K to 923 K in the Si-B system. The ∆G0f of SiB6 evaluated to be -12.78 ± 0.64 kJ/mole-atoms at 923 K, was found to be the most stable within this temperature range. In the Mg-B system, the integral Gibbs energy of formation (∆G0f ) of MgB2, MgB4, and MgB7 was also estimated using tangent rule and reported for 773 k to 873 k. The Gibbs energy of formation (∆G0f ) of MgB2, MgB4, and MgB7 are -15.48, -22.03, and - 15.89 kJ/mol-atoms at 873 K. Additionally, Thermogravimetric Analysis (TGA) were also done to study the oxidation stability and reaction kinetics of the boride base TE materials. The activation energy for the oxidation process was also calculated from the parabolic rate constant, obtained from the mathematical fitting of the specific weight gain with time. The oxidation activation energy for the SiB6 is 250.72, 235.64, and 232.65 kJ/mol for PO2 = 0.1, 0.23 and 0.33 atm respectively. Again, The thermal decomposition of MgB2 to MgB4 was studied to determine the kinetic barriers associated with the decomposition process. The activation energy of decomposition is 205.81 ± 1.5 kJ/mol and formation is 241.5 ±2.6 kJ/mol, which is in close agreement with the published literature, 238.1±2.6 kJ/-mol.Three different rates (10K/min, 15K/min and 20K/min) were used for an iso-conversional method as model-free kinetics to compare with the model based kinetics. The activation energy is 239.98 kJ/mol for Kissinger-Akahira-Sunrose (KAS) method and 247.8 kJ/mol for Ozawa-Flynn-Wall (OFW) method, which is in close agreement with the published literature. The CALPHAD approach was also exploited using the equilibrium emf data to get a better understanding of the binary boride base TE materials.