Designing Colloidal Silica-Bonded Porous Structures of In-situ Mullite for Thermal Insulation
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Designing Colloidal Silica-Bonded Porous Structures of In-situ Mullite for Thermal Insulation N.C. de Mendonça Spera1, L. Fernandes1,2, J. Sakihama1,3, I. Santos Martinatti1,4, P. Tiba4, R. Salomão1 Abstract: Colloidal silica (CS) is a promising raw material for refractory castable ceramics. It consists of stable suspensions of synthetic amorphous silica nanoparticles that behave simultaneously as liquid medium and binder for ceramic particles and as a porogenic agent and highly reactive source of silica to promote in-situ reactions. The setting mechanism of CS balances two opposite effects. Adding more CS to a suspension increases the bonding potential for gelling reactions and strengthening; on the other hand, it also introduces more water into the system, enhancing pore content. Such effects can be advantageously employed in the preparation of porous structures from aqueous suspensions and applied as high-temperature thermal insulators. The present study addresses the production of porous structures of in-situ mullite attained from aqueous suspensions of highly porous transition alumina particles bonded with colloidal silica. Different grades of CS and transition aluminas were combined to present suitable workability (flowability and gelling time) and to generate stoichiometric mullite or mullite-alumina porous structures after sintering. Keywords: colloidal silica, transition alumina, coral-like, porous mullite structures, thermal insulation
1. Introduction 1.1 Binders for microporous castable ceramics: general aspects Microporous castable ceramics play an important role as thermal insulators for industrial processes that operate at temperatures above 600 °C, for instance in petrochemical unities and the production of cement, primary and secondary aluminium, foundries and steel [1−3]. Such materials combine the high refractoriness of ceramics, the low thermal diffusivity and conductivity of porous structures, and the straightforward processing and energy saving of castable suspensions [4, 5]. Due to this, several recent studies evaluated porous structures based on α-Al2O3 [1, 4−8], magnesium aluminate spinel (MgAl 2O 4) [9, 10], calcium hexaluminate (CaAl12O19) [11−13] and mullite (Al9Si4O13) [14−18] prepared using aqueous suspensions and in-situ solid-state reactions. The production of such porous materials starts with mixing the particles of the ceramic
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matrix (or those of reactants for the in-situ solid-state reactions) with water and a dispersant to promote their individualization. Following this, a binder is added to consolidate and harden the structure during the curing and drying processes to preserve part of the pores formed by water before sintering [19, 20]. In some cases, binders also behave as a porogenic agent and as raw material for in-situ reactions with other raw materials [21]. There are three main types of binders employed in the production of macroporous ceramics. Polymeric binders are previously dissolved in water and restrain particles’ movement through gelation chemical reactions (so
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