Authors:
Prof. Dr. Rafael Salomão | São Carlos School of Engineering, University of São Paulo | Brazil
Dr. Leandro Fernandes | São Carlos School of Engineering, University of São Paulo | Brazil
Dr. Luiz Carlos Simão | Fibertechnic Ltda. | Brazil
Microporous refractory ceramics are effective thermal barriers for high-temperature insulation due to a useful combination of features. Their microstructure contains pores with a careful balance between volumetric fraction (50-80 %), diameter distribution (0.5-5 microns), and geometry and degree of interconnection (ideally isolated spheres). The solid ceramic phase surrounding such functional pores presents significant corrosion and densification resistance to avoid microstructural changes, extend materials’ service life, and prevent maintenance operations. In comparison to ceramic fibers-based insulators, microporous refractories show straightforward manufacturing and installation and do not produce hazardous particulate debris. They also present lower thermal conductivity and diffusivity above the temperature range at which heat loss by infrared radiation becomes significant (600-800ºC). This study addressed the development of a novel MgO-based thermal insulator whose temperature-resistant microporous structure was engineered to introduce controlled packing flaws amongst their highly asymmetric particles during the compacting. Hard-burnt magnesia (MgO) and dolomite ((Mg.Ca)CO3) particles were dry-mixed with exfoliated expanded aluminum phyllosilicate ((Mg,Fe2+,Fe3+)3(Al,Si)4O10(OH)2.4H2O), polymeric fibers, before being sprayed with anionic colloidal silica. The mixture was uniaxially pressed as boards (400x100x20 mm) and dried overnight (110ºC). Samples were thermally treated at different temperatures (120-1200ºC) for the evaluation of their physical properties (compression strength, rigidity, permanent dimensional thermal variation, solid density, total porosity, thermal conductivity), crystalline phases, and microstructure. The microporous microstructure generated during pressing was not significantly affected by thermal treatment below 1200ºC. Samples’ thermal conductivity, total porosity, and compression strength varied between 0.20-0.14 W.(m.K)-1, 54-56 %, and 29-15 MPa in the 200-1000ºC temperature range, respectively.