WEB Thermodynamic assessment of the Al2O3-MnO systemWednesday (01.01.2020) 00:45 - 01:00 Part of:
The Al2O3-MnO system is essential for steel industry. Given the experimental results, manganese is the main reactive alloying element in steel in combination with oxide ceramics. Thereby, the interactions between steel and alumina should be thermodynamically modelled including the Al2O3-MnO system into ceramic materials description.
The Al2O3-MnO system was thoroughly investigated. However, it is still unclear whether the MnAl2O4 spinel phase melts congruently or according to the peritectic reaction L + Al2O3 ↔ MnAl2O4. Moreover, the first thermodynamic description of the Al2O3-MnO system  was then improved  by correcting the Gibbs energy of MnAl2O4 from the original experimental data. In both works, Farina and Beneduce  who modeled the MnAl2O4 spinel phase using the three sublattice model and considering the Mn+2 and Al+3 distribution between tetra- and octahedral sites and Navarro et al.  who considered MnAl2O4 as stoichiometric, the Gibbs energies of MnAl2O4 were not consistent with the experimental values. Recently, the Al-Mn-O system was re-modeled  by taking into account various oxidation states for Mn and describing cubic spinel as (Mn+2,Al+3)T(Mn+2,Mn+3,Mn+4,Al+3,Va)2OO4. Their calculations showed that MnAl2O4 spinel melts with decomposition at reducing atmosphere.
Despite the presence of numerous studies performed to optimize the Al2O3-MnO system, the available data are partially contradictory. The aim of the present work is thus an experimental study of the Al2O3-MnO system, critical evaluation and optimization of all available data to get reliable thermodynamic description of the system.
The MnAl2O4 compound was prepared by co-precipitation routine  and then annealed at 1100°C followed by air-quenching. XRD confirmed the presence of pure MnAl2O4 spinel. Melting behavior of MnAl2O4 was checked by DTA followed by SEM/EDX. Heat capacity measurements of the spinel compound was carried out in the temperature range between -70°C and 600°C using DSC; the data obtained agree with results of Navarro et al. .
New results and available thermodynamic data were used to derive a new thermodynamic description of the Al2O3-MnO system. Using the CALPHAD approach by applying the Thermo-Calc software, liquid was described by the two-sublattice model for ionic liquids while all solid solutions – using CEF. A low inversion degree of spinel previously reported was also accounted for the MnAl2O4 modeling.
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