Production of porous ceramic materialusing different sources of alumina and calcia

Abstract

Numerous papers and publications report the use of microporous calcium hexaluminate (CaO.6Al 2 O 3 ; CA 6 ) 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 CA 6 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 o C and results in a low thermal conductivity. Given the advantages presented by this material, it is necessary to understand the formation mechanism of CA 6 grains in order to better develop the potential applications of this material. CA 6 can be fabricated using organic binders to consolidate the Al 2 O 3 -CaCO 3 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 (CaCO 3 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.

REFRACTORY CERAMICS SYNTHESIS BY SOLID-STATE REACTION BETWEEN CaCO₃ (MOLLUSK SHELL) AND Al₂O₃ POWDERS

“…As the sintering temperature increases, the bulk density increases, and the porosity decreases (5.4 and 3.0 % respectively); this fact explains why the volumetric shrinkage value shown in Table 2 is higher for the material sintered at 1500°C. In another work [28], a porous ceramic material was processed from commercial Al 2 O 3 and commercial CaCO 3 by mechanical milling, uniaxial compaction, and sintering. The authors reported an apparent poro- sity of more than 20 %, with an average pore size of 1 µm, while the apparent porosity in the materials of the present work, sintered at 1450°C and 1500°C is 5.4 and 3.0 % respectively, with pore sizes between 0.5 and 20 micrometers.…”

Calcium aluminate cement in castable alumina: From hydrate bonding to the in situ formation of calcium hexaluminate

“…The porous structures produced typically show a small average pore size and higher strength, which are features particularly attractive in the production of thermal insulators [14,27]. The process requires a careful preparation of stable low-viscosity codispersions of particles of Al 2 O 3 and CaO sources [28,29], the definition of an adequate heating schedule for the drying, dehydroxylation, and decarbonation steps [6,14], and use of hydraulic binders prior to sintering. However, little attention has been devoted to the role of such binders in the evolution of the physical properties of porous CA 6 .…”

Porogênese em hexaluminato de cálcio (CaAl>sub<12>/sub<O>sub<19>/sub<): processamento, microestrutura e propriedades termomecânicas

“…Apesar disso, eles abrem a possibilidade para sua utilização como um material poroso para temperaturas elevadas, como no caso dos isolamentos térmicos, onde a resistência à densificação e a preservação dos altos níveis de porosidade a alta temperatura são mais importantes do que a resistência mecânica [18,19]. Nestas aplicações, a heterogeneidade do grão e a expansão volumétrica resultante das reações in situ conferem uma resistência intrínseca à densificação que ajuda a preservar os poros até altas temperaturas e por períodos prolongados [20].…”

“…Eles serão comparados com as amostras de referência AF-Mold, AF-Pren [18], AG-Mold e AG-Pren. [18,19,23,[89][90][91][92][93] e, portanto, modificar as propriedades mecânicas.…”