Thermodynamic Evaluation of Reaction Abilities of Structural Units in Fe-O Binary Melts Based on the Atom–Molecule Coexistence Theory

A thermodynamic model for calculating the mass action concentrations \( N_{i} \) of structural units in Fe-O binary melts based on the atom–molecule coexistence theory, i.e., AMCT-\( N_{i} \) model, has been developed and verified to be valid through comparing with the calculated activities \( a_{{{\text{R,}}i}} \) of both O and Fe over a temperature range from 1833 K to 1973 K (1560 °C to 1700 °C). Moreover, activity coefficients \( \gamma_{\text{O}}^{{}} \) or \( f_{{{\%,{\text O}}}} \) or \( f_{\text{H,O}} \) of O coupled with activity \( a_{\text{R,O}} \) or \( a_{{{\% , \text{O}}}} \) or \( a_{\text{H,O}} \) of O and the corresponding first-order activity interaction coefficient \( \varepsilon_{\text{O}}^{\text{O}} \) or \( e_{\text{O}}^{\text{O}} \) or \( h_{\text{O}}^{\text{O}} \) of O to O have also been determined by the developed AMCT-\( N_{i} \) model and verified to be credible. In addition, the molar mixing thermodynamic properties of Fe-O binary melts have been determined to be accurate. Values of the calculated mass action concentration \( N_{\text{Fe}} \) of free Fe are in good agreement with results of the calculated activity \( a_{\text{R,Fe}} \) of Fe relative to pure liquid Fe(l) as standard state in Fe-O binary melts. The calculated mass action concentration \( N_{\text{O}} \) of free O has a closely corresponding relationship with the calculated activity \( a_{\text{R,O}} \) of O relative to ideal O₂ at 101,325 Pa as standard state in Fe-O binary melts. However, values of the calculated mass action concentration \( N_{\text{O}} \) of free O are much greater than results of the calculated activity \( a_{\text{R,O}} \) of O in Fe-O binary melts. The converted mass action concentration \( N_{\text{O}}^{\prime} \) of total O relative to ideal O₂ at 101,325 Pa as standard state can be obtained through transferring standard state of the calculated mass action concentration \( N_{\text{O}} \) of free O. The converted mass action concentration \( N_{\text{O}}^{\prime} \) of total O or the converted activity \( a_{\text{R,O}}^{\text{AMCT}} \) of O can well be matched with the calculated activity \( a_{\text{R,O}} \) of O in Fe-O binary melts. Although the obtained expression of first-order activity interaction coefficient \( \varepsilon_{\text{O}}^{\text{O}} \) or \( e_{\text{O}}^{\text{O}} \) or \( h_{\text{O}}^{\text{O}} \) by the developed AMCT-\( N_{i} \) model for Fe-O binary melts is different with that based on the calculated activity \( a_{\text{R,O}} \) or \( a_{{{{\%,{ \text O}}}}} \) or \( a_{\text{H,O}} \) of O, they can be applied to accurately predict activity \( a_{\text{R,O}} \) or \( a_{{{{\%, {\text O}}}}} \) or \( a_{\text{H,O}} \) of O in Fe-O binary melts. The molar mixing thermodynamic properties such as molar mixing enthalpy change/entropy change/Gibbs energy change of Fe-O binary melts can reliably be determined from the converted mass action concentration \( N_{\text{O}}^{\prime} \) of O or the converted activity \( a_{\text{R,O}}^{\text{AMCT}} \) of O as well as the calculated mass action concentration \( N_{\text{Fe}} \) of [Fe] by the developed AMCT-\( N_{i} \) model for Fe-O binary melts.