The effect of microstructure on the middle and short waveband emissivity of CuO-doped CuxCo1-xFe2O4 spinel

Abstract Ferrites with spinel structure (FSS), especially FSS with doping, show excellent thermal stability and high infrared emissivity performance and can thus be used for improving radiation heat transfer in industrial flame furnaces. In this study, CuxCo1-xFe2O4 samples with varying CuO doping contents (0%, 12.5%, 25%, 37.5%, 50%, 62.5% and 75%) were prepared by a sintering process to study the mechanism of doping on the emissivity of FSS in the 8–14 μm and 3–5 μm wavebands to improve the preparation of FSS with excellent performance. The emissivity in the 8–14 μm and 3–5 μm bands was measured by an IR-2 dual-band emissivity measurement instrument. The results show that in the 8–14 μm band, the samples have an obvious increase from 0.869 to 0.925 with CuO addition to 12.5% and a slight increase from 0.93 to 0.94 with further doping from 12.5% to 75%. For all the samples, the force constant for the octahedral position (KO) was calculated based on the data obtained from Fourier transform infrared (FT-IR) spectroscopy and XRD refinement. The result shows that the sample with smaller KO has higher emissivity in the 8–14 μm band. While for the 3–5 μm band, there is a minimum value (0.62) in the emissivity at a CuO doping content of 12.5%, with an increase in emissivity observed from 0.62 to 0.77 upon further doping from 12.5% to 75%. The band gap for the as-prepared FSS samples was measured by UV–visible absorption spectra, and it was found that the smaller the band gap is, the higher is the emissivity in the 3–5 μm band, which indicates that a decreased band gap can increase the energy of electron absorption or emission during the electron transition process for FSS. Thus, the effect of KO and band gap on the emissivity in the middle and short waveband was identified in this study, which is of significance for the design and preparation of FSS materials with high emissivity in the middle and short wavebands.