## 3 Results  
### Microstructure  
Eutectic rods of Co-doped MgO-MgAl\({}_{2}\)O\({}_{4}\) were obtained by directional solidification using two different solidification rates in order to achieve different microstructures. Large transverse cracks were observed in the eutectic rods grown at 50 mm/h whereas the rods grown at 750 mm/h were free of cracks. This cracking in samples grown at low rates has been previously reported in undoped MgO-MgAl\({}_{2}\)O\({}_{4}\) eutectic ceramics [21; 22] and is related to the thermal expansion mismatch and the high aspect ratio of microstructure at these rates.  
Figure 1 shows transverse cross sections of Co-0.12-50 and Co-0.15-750 eutectic rods grown at 50 mm/h and 750 mm/h, respectively. Two different phases were observed corresponding to the eutectic phases, MgO (dark contrast) and MgAl\({}_{2}\)O\({}_{4}\) (light contrast). Table 1 shows EDS estimation of the Co content in both eutectic phases. We can see that cobalt ions enter mostly in the MgO eutectic phase; the Co content in MgAl\({}_{2}\)O\({}_{4}\) phase is in some samples near the EDS detection level. No primary phases were found in any of the samples. Microstructure features did not show significant dependence on the cobalt content.  
Formation of colonies were observed for both processing rates. The colonies presented a boundary formed by coarsened eutectic phases. Colonies in eutectic ceramics processed at 50 mm/h, with a 70-90 um diameter, showed a boundary of 15-25 um in thickness whereas the samples solidified at 750 mm/h presented colonies with a diameter about 30-40 um and a boundary of \(\sim\)10 um in thickness. Inside the colony, the eutectic microstructure consisted of MgO fibers embedded in a spinel matrix. The observed fibrillar microstructure is consistent with the volume fractions of the eutectic phases calculated from the eutectic composition (23.4%vol), as fibrous geometries are favored when the volume fraction of the minority phase is lower than 28% [11]. The presence of colonies has been previously observed in undoped MgO-MgAl\({}_{2}\)O\({}_{4}\) eutectic ceramics when using high solidification rates [21; 22]. However, homogeneous eutectic microstructures have been achieved in MgO-MgAl\({}_{2}\)O\({}_{4}\) eutectic composites solidified using the LFZ technique at rates of 50 mm/h [22], contrary to the observed in the present research. This may be related to the slight deviation of the ceramic composition from the true eutectic composition due to the cobalt addition, which leads to a non-planar growth and the formation of colonies.  
A refinement of the microstructure was observed when the solidification rate increases, in agreement with Hunt-Jackson law, which predicts a decrease of the eutectic interphase spacing, \(\lambda\), when the processing rate increases [23]. The interphase spacing was calculated by linear interception methods using the transverse cross section SEM images for both processing rates. \(\lambda\)-values of 1.4 \(\pm\) 0.2 um and 0.6 \(\pm\) 0.1 um were obtained for Co0.12-50 and Co0.15-750 eutectic rods, respectively. The interphase spacings measured for Co-doped eutectic were slightly larger than those obtained for undoped eutectic samples (1.06 um and 0.53 um for samples processed at 50 mm/h and 750 mm/h, respectively) [22], and a bit smaller than the one measured by [PERSON] et al. in eutectic rods prepared by the micro-pulling down method at 50 mm/h [24].