Impact of atmospheric conditions on the high-temperature thermal conductivity of novel Cr-free refractory materials

In high-temperature applications, the effective thermal conductivity (keff) of refractory ceramics significantly influences energy efficiency. However, formulating refractories with low keff remains challenging, especially when avoiding toxic electrofused magnesia-chromium aggregates (EMCAs). This study investigated Cr-free alternative compositions produced by sintering at 1600 degrees C, evaluating their keff up to 1500 degrees C-thermal diffusivity, heat capacity, and density assessed by laser flash analysis (LFA), thermodynamic calculations and immersion test corrected with dilatometric data, respectively. Results showed that compositions with fewer distinct cations exhibited higher keff at low temperatures, whereas increased compositional complexity enhanced lattice distortions, lowering it. Above 1000 degrees C, the formation of solid solutions in the Cr-free systems induced a rapid decrease in this property, ultimately falling below that of EMCA. This behavior suggests that using these nontoxic compositions in high-temperature processes could enhance energy efficiency. Nevertheless, another aspect observed was the reversible effect of the environment inside the LFA equipment on the microstructure and thermal conductivity of the Cr-free samples. Detailed characterization of one composition before and after LFA analysis revealed the generation of a slightly reducing atmosphere, increasing iron solubility in periclase and even inhibiting spinel precipitation during cooling. Consequently, this led to microstructures with greater lattice disorder and reduced thermal conductivity.