## 5 Conclusions  
In this work, cobalt-doped MgO-MgAl\({}_{2}\)O\({}_{4}\) 2 mm diameter rods with a eutectic microstructure have been successfully solidified by means of the LFZ technique at 50 mm/h and 750 mm/h. The emittance at RT and the thermal emission between 1000 and 1500 \({}^{\circ}\)C have been studied as a function of the microstructure and the cobalt content. The main conclusions are as follows:  
* Most of the cobalt was introduced in the material sites in the MgO phase. Around 30% of Cobalt located in the MgAl\({}_{2}\)O\({}_{4}\) phase is responsible for the optical properties of the material which are relevant to the thermal emission, due to the much larger optical activity of cobalt in tetrahedrally coordinated sites. * The materials have a high emittance band around 1500-1700 nm, even for low cobalt ion concentration, which would make the material appropriate as selective emitter for thermophotovoltaic systems based on InGaAs detector. Very low cobalt concentration (0.04% at Co; 0.2 wt % CoO) is enough to produce emittances above 0.5 at 1.5 \(\upmu\)m.
* However, the multiple levels of Co\({}^{2+}\) in tetrahedral coordination diminish the selectivity further at high temperatures and higher cobalt content. Low intense electric dipole forbidden transitions show up in emittance and thermal emission spectra at lower energies than the useful emission band as the cobalt concentration increases. Added to that, other radiative transitions involving the first excited state to higher levels may also contribute at high temperatures. * The microstructural size is not relevant to the thermal emission properties. * To conclude, as a selective emitter, the cobalt content in the eutectic composite must be kept to a low level in order to avoid selectivity loss. With this limitation, the peak emissivity a 1700 nm remains at 0.6. Methodology, all authors; validation, all authors; investigation, all authors; writing--original draft preparation, P.B.O. and R.I.M. ; writing--review, all authors; funding acquisition, P.B.O., R.I.M. and J.I.P. All authors have read and agreed to the published version of the manuscript. This research was funded by the Spanish Ministerio de Economia, Industria y Competitividad (MINECO) and the CE (FEDER FUNDS), grants number MAT2013-41045-R-EMITEME and MAT-2016-77769R-FLASCERAMAT. The Departamento de Ciencia, Universidad y Sociedad del Conocimiento del Gobierno de Aragon also contributed through the financial support to the Research  
Figure 11: Ratio of emittances at 2300 nm vs. 1545 nm at room temperature (filled symbols), and at 2400 nm vs. 1700 nm at 1300 \({}^{\circ}\)C (open symbols). With black symbols, eutectic bar-samples solidified at 50 mm/h, with red symbols, samples solidified at 750 mm/h. Emittance at 1300 \({}^{\circ}\)C estimated as the thermal emission intensity divided by the black body profile. Group T02-20R. [PERSON] wants to acknowledge the support of the Spanish MINECO by means of a \"[PERSON]- Incorporacion\" fellowship during her sabbatical stay in Zaragoza. [PERSON] thanks the PIT2 program of the University of Murcia's own research plan. Not applicable. The data presented in this study are available on request from the corresponding author. Authors acknowledge the use of Servicio de Microscopia Electronica (Servicios de Apoyo a la Investigacion), Universidad de Zaragoza. The authors also wish to thank [PERSON] for providing the Co-doped MgO single crystals. The authors declare no conflict of interest. Appendix A Tetrahedrally and Octahedrally Coordinated Co\({}^{2+}\) Ions in Mg1-yCoyAl\({}_{2}\)O\({}_{4}\) Spinel  
It is well known that antisite defects in CoAl\({}_{2}\)O\({}_{4}\) or MgAl\({}_{2}\)O\({}_{4}\) spinels consisting of Co and Al changing crystallographic sites occur. This disorder is normally quantified with an inversion parameter, \(x\), where \(1-x\) is the fraction of Co or Mg ions in the tetrahedral site. It is also well known that the inversion parameter increases with temperature.