Processing, structure and properties of an alumina fiber based high temperature filter membrane

Alumina fiber filter membranes were made by wet forming and heat treatment, with pressure optionally applied during wet forming to control density. Properties of the alumina fiber filter membranes made using commercially available binders were compared to membranes made using a non-commercial acid phosphate binder which had not been used previously for filter membranes.; The filter membranes containing the acid phosphate binder exhibited the highest flexural strength, compressive strength, work of fracture and elastic modulus in comparison to those containing the other binders (silica, alumina and monoaluminum phosphate) at equivalent binder contents. The fracture toughness of the membrane containing the acid phosphate binder increased with increasing density as shown under three-point bending. A micromechanical model that accounted for the presence of a binder phase in a ceramic fiber based porous membrane was developed to predict the fracture toughness.; The acid phosphate bonded alumina fiber filter membrane demonstrated superior thermal and thermomechanical properties in comparison to the silica bonded membrane. The acid phosphate bonded membranes exhibited lower thermal conductivity, which also was calculated by using existing models, though the calculated and measured values agreed to a limited degree. In comparison with silica bonded membranes, acid phosphate bonded membranes demonstrate higher dynamic storage modulus and damping capacity up to 500°C., higher flexural strength up to 800°C, higher room-temperature flexural strength after thermal shock up to 1400°C, and significantly superior creep performance up to 800°C. The superior thermomechanical properties of the acid phosphate bonded membranes are ascribed to the formation of type-A Al(PO₃) ₃ in the binder phase and its presence through a wide temperature range.; Microscopy showed that the acid phosphate caused the fibers to bond at their junctions only, whereas colloidal alumina and the colloidal silica binders caused free binder particles within the fiber network. The filter membranes containing acid phosphate exhibited the lowest pressure drop in comparison to membranes with other binders having equivalent flexural and compressive strengths. The pore size distribution is narrow in comparison to silica bonded membranes. The filter membrane containing the acid phosphate binder had a mean flow pore size of 2.6 ± 0.1 μm in comparison to the silica bonded filter membrane which had a mean flow pore size of 4.8 ± 0.1 μm The permeability of the acid phosphate bonded membrane is approximately four times that of the silica bonded membrane.