Medium infrared transparency of MgO-MgAl2O4 directionally solidified eutectics

Abstract MgAl2O4-MgO eutectic ceramics have been fabricated by the laser floating zone method. Increasing growth rates from 10 to 50 mm/h, the microstructure transformed from irregular MgO rod-to-lamellae phase and it approached to almost homogeneous rod morphology. At the highest used velocity of 750 mm/h, the cell structure was completely dominant and the samples were free from transversal cracks. Although the highest flexure strength was found at 750 mm/h growth rate, the maximum optical transmittance in the medium-infrared range was obtained for 50 mm/h growth rate and for 1 mm thick samples reached values higher than 75% in the wavelengths between 4 and 5.3 µm. The enhanced transmittance for the sample with 50 mm/h growth rate can be explained in terms of the close refraction indexes of the component phases and the characteristic lengths of the resulting microstructure showing fully dense ceramics with the finest and almost homogeneous microstructure.