March 6-11, 2016
The study is focussed on the rapid generation of whisker-like magnesia-aluminate spinel (MgAl2O4) crystals. Two recently developed carbon-bonded refractory ceramics, pyrolyzed MgO-C and Al2O3-C , were used as the starting materials. The heating to 1600 °C was performed in a SPS/FAST device to facilitate carbothermic reactions, which resulted in the reduction of the specific oxide and the subsequent nucleation and growth of fibre-like crystals.
The research was based on a recently published study  that had established the possibility of the MgAl2O4 whisker formation in a composite structure consisting of MgO-C/Al2O3-C with iron, which acts as a catalyst in a vapour-liquid-solid (VLS) growth process. This process had been performed using SPS/FAST at 1600 °C under vacuum atmosphere and relied on the reduction of both oxides under given conditions. Resulting CO gas and vaporised metals (Mg, Al) had dissolved in the catalyst leading to the formation of the typical shape of VLS-grown whiskers .
The aim of the present study was to transfer these known conditions to a system without a catalyst and to clarify the influence of atmospheric pressure. For this reason, MgO-C was placed on Al2O3-C, which served as substrate for the whisker growth. The heating was performed indirectly with 200 K∙min‑1; the heat was produced by the surrounding graphite die. No mechanical pressure was applied during the SPS/FAST process. Vacuum as well as argon gas with the pressure of 9 ´ 104 Pa were used as sintering atmospheres.
It was shown that with Fe/steel as the catalyst, MgAl2O4 whiskers are generated independently from the atmosphere by the VLS mechanism. In the absence of the liquid metal catalyst, argon atmosphere at near-ambient pressure supports the formation of the fibre-like crystals by a vapour-solid (VS) growth process. Without the catalyst, no whiskers are obtained under vacuum conditions.
In the presentation, results of XRD, SEM/EDX and EBSD will be presented and the growth mechanisms will be discussed in details. The rapid and easy generation of the MgAl2O4 whiskers in a SPS/FAST apparatus makes them a candidate for reinforcing carbon bonded structures, while the method of production can generally be transferred to many different applications.
 M. Emmel, C. Aneziris, F. Sponza, S. Dudczig and P. Colombo, "In situ spinel formation in Al2O3-MgO-C filter materials for steel melt filtration," Ceramics International, no. 40, pp. 13507-13513, 2014.  A. Salomon, M. Dopita, M. Emmel, S. Dudczig, C. G. Aneziris and D. Rafaja, "Reaction mechanism between the carbon bonded magnesia coatings deposited on carbon bonded alumina and a steel melt," Journal of the European Ceramic Society, no. 35, pp. 795 - 802, 2015.