Preparation of γ-alumina ceramic foams employing hydrophilated polyester polyurethane sponges

Abstract

Single phase c-Al2O3 ceramic foams with high surface area (179 m/g) and porosity (67%) were prepared by the polyurethane sponge replica method from slurries containing c-Al2O3. Ceramic foams are highly porous brittle materials with large voids (cells) [1]. These cellular ceramics are special classes of porous materials comprised of cells with size ranging from a few microns to a few millimetres, where the cells can be surrounded by ceramic walls or contain solid material at only cell edges (struts), thus creating an interconnected structure (open cell foam) [1–3]. Ceramics foams are finding increasing applications as catalytic supports, ceramic membranes, sensors, filters, thermal and acoustic insulators, due to their properties such as low density, low thermal conductivity, high temperature stability, and high resistance to chemical attack [4]. Due to the high calcination temperatures employed in the conventional preparation of alumina foams, a-Al2O3 is always obtained [5]. For catalytic applications, post-treatments such as washcoating of the a-Al2O3 monoliths with suspensions of c-Al2O3 or sol–gel deposition procedures can be employed in order to obtain materials with higher surface areas [6]. However, when the foam is coated with c-alumina or any hydrated form of alumina, the maximum useful temperature is limited to approximately 700 C to avoid loss of surface area as the result of the transformation to more crystalline forms of alumina [7]. To our best knowledge, there has been only one, very recent report on the preparation of single phase c-Al2O3 ceramic foam [8]. In the present work, the preparation of single phase c-alumina foams of high surface area employing the polyurethane sponge replica method is reported, which consists of dipping the polymeric foam into a slurry containing the ceramic precursors, followed by a thermal treatment that leads to the burning out of the sponge and to the pressureless sintering at elevated temperatures of the ceramic skeleton [9]. It is expected that the meso/ macroporous characteristics of the ceramic foams will parallel those of the templating organic material. A commercial polymeric sponge (ACE Hardware Corp.), corresponding to hydrophilated polyester polyurethane with pore density of 122 ppi, was tested as template for the preparation of ceramic foams. c-Alumina which has 283 m/g BET area, 0.53 cm/g pore volume, and mean pore radius of 7.6 nm, was prepared in this laboratory following the method reported by Xu et al. [10], and was used in the present study for preparing the slurries employed in the impregnation process of polyurethane sponge. Slurries were prepared using the following procedure: 21.8 g of the synthesized alumina was mixed with 24 mL of deionized water and 5 mL of concentrated HCl. The viscosity of the slurry prepared with c-alumina at pH 3.5 was 103 9 10 Pa s. The alumina coating was applied by repeated immersiondrying cycles of the substrate (PU sponge) in the alumina slurries. A minimum of four immersion-drying cycles were Electronic supplementary material The online version of this article (doi:10.1007/s10853-009-3693-3) contains supplementary material, which is available to authorized users.