Medium infrared transparency of MgO-MgAl2O4 directionally solidified eutectics

Bibi Malmal Moshtaghioun,Jose I Peña,Rosa I Merino

doi:10.1016/j.jeurceramsoc.2019.10.053

Bibi Malmal Moshtaghioun, Jose I Peña + Show 1 more

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https://doi.org/10.1016/j.jeurceramsoc.2019.10.053

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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.

Highlights

Ceramics composed of several phases are studied in the frame of many different applications
laser floating zone (LFZ) is an excellent method among the different directional solidification procedures to grow ceramic oxides from the melt, as the large thermal gradients at the liquid/solid interface achieved with this method allow high growth rates to be used
Microstructure of LFZ processed eutectic MgAl2O4-MgO Fig. 1 compares representative transverse cross section images of the MgAl2O4-MgO eutectic ceramics grown at different velocities without rotation

Summary

Introduction

Ceramics composed of several phases are studied in the frame of many different applications. That may be of interest for DSE processing (LFZ variant), is the spinel-MgO eutectic From both components monolithic bulk parts of high transparency were already obtained by powder- sintering [12,13,14,15,16,17,18,19,20,21,22,23,24,25]. They are high melting point materials, have relatively large thermal conductivity [26] with transparency window extending deep into the IR (5.3 m for MgAl2O4 and 6.8 m for MgO) [27].In both cases, fully dense ceramics can only be fabricated at high sintering temperatures coupled with high pressure. High isostatic pressure (HIP) and spark plasma sintering (SPS) are the most efficient techniques, so far [12,13,14,15,16,17,18,19,20,21,22,23,24,25]

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