## The influence of the coarse grain fraction on the microstructure and effective thermal conductivity of alumina castables

Reference Presenter Authors
(Institution)
Abstract
09-094 Jens Fruhstorfer Fruhstorfer, J.(TU Bergakademie Freiberg); Goetze, P.(TU Bergakademie Freiberg); Gross, U.(TU Bergakademie Freiberg); Fieback, T.M.(TU Bergakademie Freiberg); Aneziris, C.G.(TU Bergakademie Freiberg); A modified Andreasen model was used to design castables with different aggregate size distributions with 3mm maximum grain size while the amount of the fine fraction was held constant.

The porosities and pore size distributions were analyzed. The porosity was measured by the water immersion method and the pore sizes over their whole range by mercury porosimetry, optical microscopy and visually. In addition, the microstructure regarding especially microcracking was investigated by scanning electron microscopy. Besides the structural properties, the effective thermal conductivity (ETC) was measured by the transient plane source method.

The porosity decreased firstly with increasing coarse grain amounts but then reached a plateau at 19-20%. Nevertheless, the pore sizes were smallest for the batch with a low porosity and comparatively low coarse grain amounts (at the beginning of the plateau). With increasing coarse fraction amounts, the sintering activity was enhanced due to less disturbed sintering of the matrix. Additionally, pores $\le$250nm increased the sintering activity. The particles' and pores' contributions to the sintering activity led to intensified microcracking for the densest castable with the most small pores. This batch, thus, presented also a decreased ETC.

Hence, the particle size distribution impacts the porosity, pore sizes and also the sintering activity. The sintering activity determines the extent of microcracking. Ultimately, the ETC results from these properties, all influenced by the particle size distribution.
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