## 3 Results
### Thermal Emission  
Figure 2 shows the thermal emission spectra of Co0.12-50 and Co0.15-750 for several temperatures in the infrared range. For comparison, the thermal emission measured at 1000 \({}^{\circ}\)C in the undoped MgO-MgAl\({}_{2}\)O\({}_{4}\) eutectic is also presented. We can observe an intense band centered at 1675 nm in the spectra of the cobalt-doped composites. The strong emission was attributed to the radiative deexcitation of the thermally excited Co\({}^{2+}\) ions, with an almost negligible contribution of the matrix. When comparing spectra from eutectics processed at 50 mm/h (solid lines) and at 750 mm/h (dotted lines) we can observe that the effect of the microstructure was negligible in the maximum position and half-width of the band, although slightly more intense spectra were detected for the samples grown at higher rate. The thermal emission increased when the temperature raised, as expected from the greater number of thermally excited ions. The effect of the composition of the eutectic rods in the thermal emission was also investigated. Figure 3a shows the emission spectra at 1300 \({}^{\circ}\)C for samples solidified at 50 mm/h (with the same microstructure) for different Co content. In order to analyze the spectral features of the band in more detail, we have to take into account that the thermal emission is modulated by the Plank profile. Figure 3b shows the thermal emission divided by the blackbody radiance (BBR), which, in fact, corresponds to the emissivity of the sample. An increase of the emission at wavelengths lower than 1000 nm can be observed in these spectra which was attributed to the Co\({}^{2+}\) very intense electronic transitions peaking in the visible range. Other transitions to and from excited state levels might also contribute, as explained below. Figure 1: SEM micrographs of the transverse cross section of (**a**) Co0.12-50 and (**b**) Co0.15-750 eutectic rods. Insets: amplification of the microstructure inside the colony. The features of the spectra strongly depended on the cobalt percentage of the composite, thermal emission becoming more intense when the cobalt content increased. However, this increase is not linear with the dopant level, the thermal emission clearly saturating for Co0.22-50. In addition, an enhancement of the emission at low energies (at wavelength \(>\)2000 nm) was observed when raising the cobalt content, which lowered the  
Figure 3: (**a**) Thermal emission of Co0.04-50, Co0.12-50 and Co0.22-50 eutectic rods measured at 1300 \({}^{\circ}\)C; (**b**) spectra obtained by dividing the thermal emission spectra by the Planck profile at 1300 \({}^{\circ}\)C.  
Figure 2: Thermal emission spectra of Co0.12-50 (solid lines) and Co0.15-750 (dotted lines) eutectic rods measured at 1000, 1200 and 1400 \({}^{\circ}\)C. Black solid line shows the emission measured in the undoped MgO-MgSZ eutectic rod at 1000 \({}^{\circ}\)C.  
selectivity of the emission. This behavior is more clearly noted when comparing the normalized spectra of the thermal emission for all the eutectic samples investigated in this work (Figure 4). It is found that a high contribution to the emission is present in the spectral range of 2000-2400 nm for samples with a cobalt content >0.2% at Co.  
Cobalt ions can enter both MgO and MgAl\({}_{2}\)O\({}_{4}\) eutectic phases. In order to elucidate in what lattice the cobalt ions responsible of the thermal emission enter, thermal emission in MgO:Co and MgAl\({}_{2}\)O\({}_{4}\):Co single crystals was studied. Figure 5 shows the spectra measured at 1000 \({}^{\circ}\)C for MgO:Co and Spinel0.06-50. From the comparison of these spectra with those obtained for the Co-doped eutectic samples investigated in this work, we can conclude that the measured emission is mainly due to the radiative emission of the cobalt ions in the spinel matrix.