_applied sciences_  
_Article_  
Directionally Solidified Cobalt-Doped MgO-MgAl2O4 Eutectic Composites for Selective Emitters  
[PERSON] 1, [PERSON] 1,*, [PERSON] 1,2\({}^{\copyright}\), [PERSON] 3,4 and [PERSON] 1  
Footnote 1: [[https://www.mdpi.com/journal/applsci](https://www.mdpi.com/journal/applsci)]([https://www.mdpi.com/journal/applsci](https://www.mdpi.com/journal/applsci))  
Footnote 2: [[https://www.mdpi.com/journal/applsci](https://www.mdpi.com/journal/applsci)]([https://www.mdpi.com/journal/applsci](https://www.mdpi.com/journal/applsci))  
Footnote 3: [[https://www.mdpi.com/journal/applsci](https://www.mdpi.com/journal/applsci)]([https://www.mdpi.com/journal/applsci](https://www.mdpi.com/journal/applsci))  
Footnote 4: [[https://www.mdpi.com/journal/applsci](https://www.mdpi.com/journal/applsci)]([https://www.mdpi.com/journal/applsci](https://www.mdpi.com/journal/applsci))  
Footnote 5  
However, the narrowness of the 4f band emission of the rare earth ions maintains the converted energy at low levels which have moved the research to the study of ceramics doped with transition metal ions. The broader selective emission required for the TPV conversion can be obtained by the use of 3d ions, with intense absorption bands in the near infrared range. Thermal emission of Ni\({}^{2+}\) and Co\({}^{2+}\) ions in some ceramic oxides have been previously studied [12]. Intense selective emission in the 1-2 um spectral range was reported in MgO:Ni and MgO:Co ceramics [13,14]. Recently, thermal emission of Ni\({}^{2+}\)-doped MgO-ZrO\({}_{2}\) eutectic has been reported [15]. An intense broad emission band centred in 1.5 um matching with the sensitive region of the GaSb photovoltaic cell was found. The emission was attributed to the radiative de-excitation of the nickel ions in the MgO lattice. However, the selectivity in the emission was lower than in the case of the rare earth-doped DSEC due to a non-negligible matrix emission, ascribed to the emission of the zirconia phase, and to the increase of the thermal emission at non-convertible wavelengths when the nickel doping content was raised. The aim of the present work is to study the thermal emission of directionally solidified Co-doped MgO-MgAl\({}_{2}\)O\({}_{4}\) ceramic composites. The choice of Co\({}^{2+}\) as the selective emitter ion for this eutectic is relevant as, considering that cobalt ions can enter substituting Mg\({}^{2+}\) ions in both MgO and MgAl\({}_{2}\)O\({}_{4}\) eutectic phases, the content of the dopant ion in the composite could be higher than in the case of nickel, increasing the thermal emission of the composite. In addition, Co\({}^{2+}\) ions show a high effective absorption cross section in MgAl\({}_{2}\)O\({}_{4}\)[16,17], which would lead to a higher emission. Finally, recent results in directionally solidified MgO-MgAl\({}_{2}\)O\({}_{4}\) eutectic ceramics show that this eutectic composite is translucent in the medium infrared range under some processing parameters [18], which would make it a suitable matrix for selective emission by reducing the emission out of the TPV convertible wavelength range. Nonetheless, both phases, MgO and MgAl\({}_{2}\)O\({}_{4}\), have transparent windows extending deep into the infrared range (6.8 and 5.3 um, respectively) [19]. In addition, materials based on MgO and MgAl\({}_{2}\)O\({}_{4}\) have also been proposed for elevated temperature nuclear applications due to their radiation resistance [20]. Different directional solidification procedures can be used to process ceramic oxides from the melt. In this study, eutectic ceramics were obtained by means of the Laser Floating Zone (LFZ) technique, which achieves very large gradients at the liquid/solid interface allowing for the use of high growth rates and the crystal microstructure control in a wide variety of processing parameters [11]. In this work, two growth rates (50 and 750 mm/h) were used in order to develop different microstructures.