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Abstract
Alumina replica foams were manufactured by the Schwartzwalder sponge replication technique and were provided with an additional strut porosity by a freeze-drying/ice-templating step prior to thermal processing. A variety of thickeners in combination with different alumina solid loads in the dispersion used for polyurethane foam template coating were studied. An additional strut porosity as generated by freeze-drying was found to be in the order of ~20%, and the spacings between the strut pores generated by ice-templating were in the range between 20 µm and 32 µm. In spite of the lamellar strut pore structure and a total porosity exceeding 90%, the compressive strength was found to be up to 1.3 MPa. Combining the replica process with freeze-drying proves to be a suitable method to enhance foams with respect to their surface area accessible for active coatings while preserving the advantageous flow properties of the cellular structure. A two-to-threefold object surface-to-object volume ratio of 55 to 77 mm−1 was achieved for samples with 30 vol% solid load compared to 26 mm−1 for non-freeze-dried samples. The freeze-drying technique allows the control of the proportion and properties of the introduced pores in an uncomplicated and predictable way by adjusting the process parameters. Nevertheless, the present article demonstrates that a suitable thickener in the dispersion used for the Schwartzwalder process is inevitable to obtain ceramic foams with sufficient mechanical strength due to the necessarily increased water content of the ceramic dispersion used for foam manufacturing.
Highlights
Ceramic foams with an open pore structure are commonly used, e.g., as filters, supports for heterogeneous catalysts, heat exchangers and silencers [1,2,3,4,5]
The specific surface area has been increased in various studies, for instance, by coating them with a high surface area washcoat [8] or using a porous alumina powder [9]
For those applications where active coatings are a component of the foam strut surface, a higher surface area may be of advantage
Summary
Ceramic foams with an open pore structure are commonly used, e.g., as filters, supports for heterogeneous catalysts, heat exchangers and silencers [1,2,3,4,5]. The manufacturing is typically carried out by a sponge replication technique, known as the Schwartzwalder process [6], and the resulting ceramic foams possess hollow struts. Those cellular materials own a specific surface area of 1–2 m2 g−1 , which is comparably low [7]. The specific surface area has been increased in various studies, for instance, by coating them with a high surface area washcoat [8] or using a porous alumina powder [9] For those applications where active coatings are a component of the foam strut surface, a higher surface area may be of advantage. A potential processing route to access the inner strut surface and to increase the strut porosity is the combination of the Schwartzwalder process with the freeze-drying technique, as shown in literature for hydroxyapatite-ceramics (with coating of 75 ppi Polyurethane (PU)
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