Ceramic foams made from a wide range of ceramic materials, both oxide and nonoxide, are being considered for a whole range of potential applications. These include hot gas filters, interpenetrating composites and biomedical applications as well as thermal insulation, kiln furniture and catalyst supports amongst others. Three of these are described briefly below as well as a new processing route based on gel casting.
The process for fabricating the ceramic foams, which has now been taken to full commercial status by Hi-Por Ceramics, a new company created specifically for the purpose, is given by the flowchart in figure 1. A stable, well-dispersed, high solids content, aqueous ceramic suspension is prepared which also incorporates an acrylate monomer together with an initiator and catalyst. The latter is used to provide in-situ polymerisation. After the further addition of a foaming agent, a high shear mixer is used to provide simple mechanical agitation that results in the formation of a wet ceramic foam that can be dried and then fired.
Figure 1. Process flow chart for production of ceramic foams.
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One of the advantages of the in-situ polymerisation method is that it is common to observe a period of inactivity between the addition of reagents and the actual beginning of the polymerisation reaction. This is known as the induction period or idle time (ti). The induction period is beneficial since it allows the casting of the fluid foam into a mould prior to polymerisation and control over the pore size.
A wide range of ceramic materials have been produced as foams using the new process. Although the majority of work has focused on the engineering ceramic oxides, materials such as alumina, cordierite, mullite and zirconia, a large number of other ceramics can be foamed. These include the bioceramic hydroxyapatite, the electroceramic lead zirconate titanate (PZT), low thermal expansion sodium zirconium phosphate (NZP) and, more recently, non-oxides such as silicon carbide and aluminium nitride. In general, foams in the range 5% to 40% of theoretical density can be produced; a typical micrograph showing the structure can be seen in figure 2. Note how the pore walls and struts are solid and fully dense; this provides high strength and chemical resistance.
Figure 2. Typical micrograph of a 30% dense alumina foam.
Although the lower the density of the foam the larger the cell or pore size, recent research has allowed a far greater degree of control to be achieved. Foams can now be produced with cells as large as 1 mm and densities as high as 30% of theoretical, whilst 20% dense foams have been produced with cells as small as 20-50 μm.
Foams made of the engineering ceramics such as alumina offer comparatively high strengths, up to 80 MPa crush strength and 25 MPa modulus of rupture. Thermal insulation is almost as good as fibre-based products whilst also offering a totally fibre- and dust-free working environment. With zirconia the service temperature can be as high as °C. A wide range of component shapes is also available. The production route itself is intrinsically a casting process and hence tiles, tubes and a range of other custom shapes can all be produced very easily. In addition, both the green and fired foam may be readily machined, drilled, turned and slit opening up the possibility of producing some very complex shapes indeed. In addition, it is a simple process to apply a dense coating to one or more surfaces, either to eliminate permeability or to increase the mechanical properties of the surface layers. One application that could utilise this is the production of ultra-low mass ceramic crucibles. These can significantly reduce the thermal mass to be heated during processing of their contents, thus improving energy efficiency.
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