Abstract
Numerous papers and publications report the use of microporous calcium hexaluminate (CaO.6Al2O3; CA6) as a key raw material for high temperature insulating materials. This material has unique properties with respect to chemical purity and mineral composition. Another important property of CA6 is its structure, which consists of platelet-shaped crystals that interlock. The free distance between the crystals defines the microporous structure. The low density in combination with the micropores hampers heat transfer by radiation at temperatures exceeding 1000 oC and results in a low thermal conductivity. Given the advantages presented by this material, it is necessary to understand the formation mechanism of CA6 grains in order to better develop the potential applications of this material. CA6 can be fabricated using organic binders to consolidate the Al2O3-CaCO3 powder mixture and to provide green strength so that a green body can be formed and retains the desired shape before heating. However, these organic binders must be completely thermally decomposed so that they do not remain in the sintered body as carbon or ash. Moreover, the use of organic binders releases large volumes of gases such as carbon dioxide from the green body during heating. Therefore, an eco-friendly ceramic fabrication process has been developed that employs an inorganic binder (hydraulic alumina). The aim of the present work was to study the synthesis of porous calcium-hexaluminate ceramics using calcined alumina or hydraulic alumina combined with different sources of calcia (CaCO3 and Ca(OH)2) at different temperatures. The materials produced were characterized by X-ray diffraction, scanning electron microscopy, apparent porosity and mercury intrusion porosimetry. The materials produced by hydraulic alumina presented higher porosity and larger pores compared to those produced from calcined alumina
Highlights
Ceramic fiber-based materials are widely employed as thermal insulators for high temperature applications due to their low thermal conductivity and thermal shock resistance
Numerous papers and publications report the use of microporous calcium hexaluminate (CaO.6Al2O3; CA6) as a key raw material for high temperature insulating materials
The aim of the present work was to study the synthesis of porous calcium-hexaluminate ceramics using calcined alumina or hydraulic alumina combined with different sources of calcia (CaCO3 and Ca(OH)2) at different temperatures
Summary
Ceramic fiber-based materials are widely employed as thermal insulators for high temperature applications (above 1200°C) due to their low thermal conductivity and thermal shock resistance. Due to their potential for causing lung diseases and allergies, efforts have been made to replace these materials by nonfiber containing alternatives [1-4]. From the fiber-based boards, in porous monolithics, the ceramics matrixes comprise sintered particles surrounding micro-to-macro porous. Due to this, this class of materials presents low thermal conductivity and reduces the generation of hazardous dust during installation and use. Recent studies report porous calcium hexaluminate structures as suitable candidates for this application [1-5]
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