Medium infrared transparency of MgO-MgAl2O4 directionally solidified eutectics

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 1mm thick samples reached values higher than 75% in the wavelengths between 4 and 5.3μm.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.


Introduction
Ceramics composed of several phases are studied in the frame of many different applications. Frequently it is for structural purposes (better mechanical properties), but this comes usually combined with other functional properties rendered to the material by the appropriate properties combination of the components, as thermal or electrical conductivity, optical activity, electronic or magnetic properties, etc. In particular, examples of ceramics for optical applications in which it is required or take advantage of multiple phases or gradient in composition are light guiding materials [1], hosts for selective doping or multiple active ions in lasers [2,3], phosphors or scintillators [4], selective emitters with low emissivity in the IR [5,6], or metamaterials [7][8][9] etc .An approach-for creating multiphase composites, exhibiting useful microstructure/texture patterns, is the directional solidification of eutectics (DSE) [10,11]. One of the techniques most used -for DSE work, especially in the case of systems from which transparency may be expected-is the laser floating zone (LFZ) method [10 ,11]. 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. Actually, a CO 2 laser is focused on the molten zone and a precursor is brought into the focused laser beam. Growth starts by moving the precursor and mass conservation dictates that the diameter can be reduced as the square route of the feed rate-pull rate ratio. Moreover, the control of the crystal microstructure is possible by means of growth rates. Let us note that the LFZ can generate pores free parts-a feature critical in the case of transparent ceramics [10,11].
Advances in the manufacture of both materials have been made, and results can be found in the literature [15,19,20,25], optimizing powder characteristics with impurity level to ppm range and particle size of less than 100 nm, rigorous compaction and sintering schemes by HIP and/or SPS to obtain fully dense ceramics with nanograin retention, or post annealing treatment to avoid color-center formation. Therefore, large pieces of thickness in the mm range with in-line transmittance up to 80 % in the visible range can be obtained.
Some preliminary studies-regarding the fabrication, by LFZ, of bulk pieces, from the spinel-MgO eutectic-have been already effectuated. It was observed that specimens strength is larger when high pulling rates are used [28][29][30][31][32];the studies also showed that solidification , at pull rates lower than 50 mm/h causes -owing mostly to the thermal expansion mismatch of the two phases-cracking, thus severely reducing strength [29,30,32].
Further work is needed in order to fully master the LFZ fabrication of parts,from the spinel-MgO eutectic. In this context, the purpose of this workis to study the relative in-line

Experimental procedure
The starting materials were commercially available Al 2 O 3 powder (Sigma-Aldrich, 99.99%) and MgO powder (Sigma-Aldrich,>99%). MgO powder was dried in a furnaceat 1200 ºC for 12 h to remove the possible moisture absorption from outside air [33].The eutectic point of MgO and MgAl 2 O 4 appears at 55wt% Al 2 O 3 and 45 wt% MgO and its melting temperature is around 2000 ºC [34]. Two general approaches using lasermelting techniques were studied for the possible fabrication of this eutectic ceramic with dense and fine microstructure retention.
The first was solidification of the eutectic rods by directional solidification from the melt using the laser-heated floating zone (LFZ) method with a CO 2 laser [10]. Precursor rods of ~ 3 mm in diameter and up to 5 cm in length were prepared by cold isostatic pressing for 5 min at 200 MPa followed by pre-sintering in a furnace at 1500 ºC for 12 h. The pre-sintered rods were then grown by LFZ in air, in all cases using two steps of diameter reduction at a pulling rate of 300 mm/h. These two first steps were performed in counter-rotation of the crystal and precursor with 50 rpm which provided fully dense rods grown from the melt without residual porosity.
The last (third)step, however, was performed without rotation and the solidified rod being pulled downwards using growth rates between10 and 750 mm/h to evaluate its effect on the microstructure, average grain size, relative in-line transmittanceand strength of the resulting In-line transmission spectra were measured in the VIS-NIR range with a Cary 5000 UV-Vis-NIR spectrometer from Agilent and in the MIR range with a Spectrum 100 FTIR spectrophotometer form Perkin-Elmer. The instruments overlap in the 2.5 to 3.0 microns wavelength range, but the transmittance was not coincident on all samples, most probably due to different optical arrangement and apertures. Therefore, the short-wavelength (VIS-NIR) spectra were scaled to match the measurements in the MIR range. Calculation of transmission spectra of Mie-scattering [35] of spheres was made using Scott Prahl´s software [36].
Finally the strength of rods in longitudinal direction for the highest grown velocity was measured by flexural tests carried out in a three-point bending test fixture with 10 mm loading span in Instron testing machine (Instron 5565). Eight tests were performed at constant crossheads speed of 0.05 mm/min.

Results and discussion
Directional solidification by the laser assisted floating zone method (LFZ) is frequently performed counter-rotating feed rod and pulling crystal as this better homogenizes the heating flux around the melt. Often, this also adds instabilities in the melt that lead to the formation of bands associated to the rotation period, that is, periodic changes in the microstructural size of the eutectic preferential solidification of one of the phases of the eutectic. Perturbation of the microstructure in composites leads to perturbation in the diffusion of light in the material.
Submitted to Journal of the European Ceramics Society on 23 th July 2019/ Revised 18 th Oct/ Accepted 29 th Oct. 6 Therefore we have solidified the samples without rotation and compared when necessary with the composite solidified with counter-rotation [29].  Table 1 the microstructural properties obtained from the analysis of the SEM images are summarized.By EDS analysis, the bright matrix was detected to be spinel and dark embedded phases were MgO. It is worth knowing that transverse cracks were present for those samples solidified at a pulling rate lower than 100mm/h. In the areas with coarser microstructure, microcracks tend to appear, which lead to poor mechanical resistance [29]. At the lowest growth rates of 10 (Fig. 1(A), inset) and 3.2 µm ( Fig. 1 (B), inset), respectively. Increasing the growth rate to 50 mm/h, a mixed microstructure was observed that indicates transition from rod-to-lamellae to rod morphology ( Fig. 1(C)). At this velocity,MgO rods with triangular section with size of 0.8 µm( Fig. 1(C), inset)and rod-to-lamellae MgO structure with size of 1.9 µm was distinguished. Approaching 100 mm/h growth rate, therod-tolamellae microstructure was almost transformed and the solidification suffered a transition to the formation of cells especially in the center (core of 380 m diameter) of the solidified ceramic ( Fig. 1(D)) which was surrounded by MgO fibrous microstructure in the approximately 250 m thick outer shell of the sample. At this velocity, cells with a diameter of ~35µm and a boundary of ~13 µm in thickness were found. MgO fiber size was estimated about 0.9 µm (Fig. 1(D), inset) boundary of ~7 µm in thickness( Fig. 1(E)). MgO fibers were found to size0.34 µm and 2.1 µm and with almost the same volume fraction inside and in the boundaries of the cells, respectively (Fig. 1E, inset). In the core (center) region of the samples solidified at 750 mm/h, two more features appear. A third bright phase was observed at the cell boundaries in the sample solidified at this rate ( Fig. 1(E)). EDS microanalysis showed it contains O, Ca, Al, Si and Mg, suggesting it is consequence of the segregation of impurities towards the melt upon solidification.

Microstructure of LFZ processed eutectic MgAl 2 O 4 -MgO
Occasionally some dendrites of MgO were observed, too.  shows signs of departure from coupled growth and incipient cell formation, but still not well developed. An average transverse particle size can be used to describe the transverse size of the MgO dispersed phase (see Table 1). At even larger pulling rates, the formation of cells is evident in part (100 mm/h) or all the sample cross-section (750 mm/h). This leads to transverse size of the MgO particles which is finer inside the cells and coarser in the inter-cell areas. Light scattering is expected to be smaller when the microstructure is finer and more homogeneous, and this happens in the samples solidified at 50 mm/h, mostly when banding is avoided.

In-line transmittance
We have not attempted to calculate the theoretical transmittance in this material, which at minimum would require modelling the anisotropic sample microstructure in a way that is simultaneously representative of the samples microstructure and tractable computationally. a volume percentage and equivalent size was assigned to each microstructure. To estimate an extinction coefficient, the number of particles per unit volume was calculated using this filling ratio of MgO, that was taken as f = 0.245 [29]. The volume fraction is large enough so that multiple scattering and particle interaction could be contributing to the scattering. We did not take this into account. Although the density of particles is large, we expect trends in this simpler calculation to mimic the trends in the overall behavior of the material transmission. The reason is that the refractive index contrast is small, so that the scattering strength is also small in the NIR.
Submitted to Journal of the European Ceramics Society on 23 th July 2019/ Revised 18 th Oct/ Accepted 29 th Oct.

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For example, at= 4.5 µm, the predicted extinction length for 2 µm diameter spheres is 1.6 mm (corresponding to particle extinction efficiency 0.00335 with 0.0585 particles per µm 3 ). Figure 4 shows the result of the calculations. Reflection at the surfaces of the slices has also been taken into account. Transmission is moderate with MgO spheres of 4.3 m diameter (black curve), and increases as the diameter of the MgO particle decreases (red, brown and orange curves). The simulation to the 100 mm/h sample calculates the transmittance as a weighted average of the transmittance estimated for different sized spheres, as suggested by the microstructure. Therefore, although the microstructural size inside the cells is smaller, the transmittance decreases because of the populations with larger microstructural features, as is also observed experimentally. Overall, the microstructural sizes of the sample solidified at 750 mm/h is smaller, and thus the calculated transmittance increases again, contrary to what is observed experimentally. Most probably, the presence of other phases at the inter-cell areas should be blamed for the discrepancy. Segregation of impurities can be avoided with starting products with higher purity but the occasional formation of MgO dendrites is growth instability of much uncertain avoidance.

Flexural strength
Three-point bending tests were just performed forthe grown rods with the rate of 750 mm/h which were free from transverse cracks to go into their flexural strength behaviour. No plasticity was observed. A strength value of ~445 MPa was obtained. This is around twofold greater than those (150-200 MPa) of conventional MgAl 2 O 4 ceramics [13,14] and nearer to the values reported for fine-grained SPS´ed Spinel [40]. Furthermore, mild improvement of strength was found compared with its counterpart grown with counter-rotation under identical Submitted to Journal of the European Ceramics Society on 23 th July 2019/ Revised 18 th Oct/ Accepted 29 th Oct.

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LFZconditions (i.e., 345 MPa) [29], which is because better homogeneity of microstructure by minimizing possible instabilities in the melt produced by periodic perturbations. Regarding mechanical properties, LFZ rods of around 1mm in diameter and solidified at 750 mm/h without using counter-rotation of the feedstock and growing crystal found to be the optimal condition to get highest flexure strength. This value amounts to 445 MPa and presents slightly better flexure strength than its counterpart solidified with counter-rotation. The formation of cracks at low solidification rates could not be avoided.    Black squares (4.3 m), red circles (3.2 m), orange up-triangles (0.8 m), brown downtriangles (1.9 m), blue diamonds (0.9 m (32%), 7.1 m (32%) and 2.25 m (36%)), magenta stars (0.34 m (50%) and 2.08 m (50%)).