Physical, fluid dynamic and mechanical properties of alumina gel-cast foams manufactured using agarose or ovalbumin as g
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rek Potoczek, Anna Chmielarz, and Elwira Kocyło Faculty of Chemistry, Rzeszow University of Technology, Rzeszow 35-959, Poland (Received 27 February 2017; accepted 19 June 2017)
Alumina gel-cast foams were manufactured in a broad total porosity range (43–86%) by using agarose or ovalbumin as gelling agents. The foams were examined in terms of microstructural, permeability, and mechanical properties. For the achieved open porosity level (19–85%), the mean cell size (19–375 lm), and mean window size (8–77 lm) of the alumina foams manufactured using ovalbumin were slightly wider than those obtained using agarose (34–262 lm and 18–33 lm, respectively). By using different contents of agarose (0.3–1 wt%) or albumin (5 wt%) and solids (30–45.9 wt%), it was possible to vary the foaming yield from 1.6 to 4.4 and produce bodies with a very wide permeability level that included several classes of porous ceramics. Darcian (k1) and non-Darcian (k2) permeability coefficients displayed values in the range 3.2 1018 to 4.3 109 m2 and 1.8 1018 to 6.5 105 m respectively. Compressive strength of bodies was dependent upon the porosity level, with a variation of 8.5–149.7 MPa.
I. INTRODUCTION
The high interest in ceramic foams has been associated with their unique physical and chemical properties such as low mass, low thermal conductivity, high surface area, high permeability, and corrosion resistance etc.1–3 To produce a ceramic foam, many different approaches can be followed: (i) replication of a sacrificial foam template,4 (ii) direct foaming of a liquid slurry,5 (iii) burn-out of fugitive pore formers,6 (iv) foams from preceramic polymers that cross-link during foaming,7 (v) foaming of ceramic precursors that undergo a sol-to-gel transition,8 (vi) sintering of hollow spheres.9 More information one can find in a published book1 and references therein. One of the most common methods for fabricating the ceramic foams is the replication of polymeric sponges. One of the problems of the replica template method is that the ceramic struts are hollow, due to volatilization of the organic filament used as substrate.10,11 This internal void and the flaws present on outer surface due to the difficulty in homogeneously coating the polyurethane foam template leads to the low mechanical strength levels that characterize this type of structure. On the other hand, direct foaming
Contributing Editor: Paolo Colombo a) Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2017.263
followed by consolidation of the wet foam by gelcasting leads to higher strength than replica foams at the same porosity level, although typically the components possess a somewhat lower permeability.10 Gelcasting of porous materials was developed as a result of the combination of the gel-casting process and the aeration of ceramic suspensions.5 Wet foams stabilized by surfactants require a gelling agent to consolidate the foam microstructure before extensive coalescence and disproportionation take place. Historically, the
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