Cellular Ceramics / p2

52 Part 2 Manufacturing

ture. A detailed mathematical argument, based on his foam constant concept, can be found in his paper on the subject [5]. However, if the foam volume, (i.e., foam density), is allowed to change, then the opposite behavior is observed. Characterization of foams produced by the gel casting technique [52] has shown that more open structures are associated with lower density foams and these, in turn, tended to have larger cell sizes. This again can be explained quite simply using the same arguments put forward by Minnear: the larger the foam volume the thinner the walls will be on average and hence the weaker they will be.

When a foam is produced, the two most important factors are the foam cell size and the wet foam density. These were systematically investigated by Gido et al. [53] with respect to machine variables for an Oakes foaming unit, albeit for a liquid surfactant solution rather than a ceramic suspension. The foam was produced by injecting pressurized nitrogen gas into a stream of surfactant solution, which was then mixed in a rotary-head mixer from which it exited as a foam. Characteristic shear rates were between 2000 s–1 for a motor speed of 600 rpm and 4000 s–1 for a speed of 1200 rpm. The variables investigated were the liquid and gas flow rates, the speed of the rotary-head mixer and the viscosity of the liquid phase. From the results obtained it was observed that the average mean bubble diameter decreased as the rotational speed increased, and at comparable foam quantities and rotation speeds, the mean bubble diameter was significantly smaller for a foam generated from a higher viscosity liquid. However, one of the most interesting aspects of the work is the lack of correlation between bubble size and wet foam density, and the range of wet foam densities and bubble sizes achievable.

2.1.4

Other Techniques

Volatile or combustible additives that are lost during firing can be incorporated into the ceramic, and the volume, size, shape, and distribution of the resulting porosity is determined by the amount and nature of the fugitive phase [54]. One particular approach for forming porous ceramics used starch as both binder and pore former [55, 56]. It is well known that starches can act as binders due to their gelling ability in water [57], but during firing they result in residual porosity. While this has always been seen as a disadvantage for dense ceramics, it was used to advantage to generate sintered porous ceramics with porosities in the range 23–70 % [55]. The overall pore structure was dominated by the 10–80 mm spherical pores left by the starch particles, and the average size of the small pores connecting the large pores was controlled by the total solids loading and starch content in the precursor slips to between about 0.5 and 10 mm. Chemically modified starch was found to give better dimensional control and uniformity than native starch with regard to the average size of the connecting pores owing to its more stable properties during water processing.

Another organic material that has been used to promote the creation of foams is egg white; since albumin has amphiphilic properties its solutions are prone to foaming. Hence it is possible to generate foams by the simple process of vigorously mix-

2.1 Ceramic Foams 53

ing or ball milling the ceramic suspension with the addition of albumin [58, 59]. Once again, the organic additive acts as both a binder and pore-forming agent.

While the majority of such applications lead to porous, as distinct from cellular, ceramics, if the additives comprise a large enough fraction of the whole then they can result in the material’s developing a cellular nature. For example, poly(methyl methacrylate), PMMA, microbeads have been added to heat-treated methylsilicone resin powder in a weight ratio of 80:20 prior to uniaxial pressing [60]. The PMMA microbeads were then burnt out and the porous green bodies pyrolyzed in an inert atmosphere to yield silicon oxycarbide (SiOC) ceramic microcellular foams. Further information on these foams can be found in Chapter 5.11.

Inorganic pore formers can also be used; these include alkaline earth metals and their oxides, as well as a range of other oxides, nitrides, and carbides. The mineral perlite is also commonly used [61]. After forming and drying, the green bodies are fired, and the pore formers partially or completely melt and/or react with the ceramic powder. As with the organic additives, the structure and porosity of the final porous ceramic can be adjusted by means of the shape, size, and amount of the pore formers. The final pore size of these structures can be in the range of a few micrometers up to several centimeters, the latter usually involving the addition of hollow additives, for example, glass spheres. The use of inorganic pore formers that do not vaporize can have the advantage that they do not pollute the environment, but since the material is retained in the final body the chemistry of the pore former and its effect on the host material increase in importance. An example is the formation of porous silicon carbide ceramics by the addition of graphite powders, the pore diameter being controlled by the size of the graphite particles [62]. In this work, the SiC particles were bonded together by taking advantage of an oxidation-bonding process in which the powder compacts were heated in air so that SiC particles were bonded to each other by oxidation-derived SiO2 glass.

Closely related in conceptual terms is the chemical leaching of one phase in systems that contain two or more phases to yield connected porosity. For example, in 1996 Japanese researchers investigated the consequences of 1 M hydrochloric acid on cordierite [63]. They found that both magnesium and aluminum ions were leached from the structure in the same molar ratio, leaving a porous structure with a large surface area, proportional to the amount of cations removed. The pore radius distribution ranged from 0.4 to 0.9 nm, which, being comparable to those of typical molecular sieves, encouraged them to try simple adsorption tests. They found that the porous substrate was capable of adsorbing some amine compounds.

A final approach, which is slightly different to either of the above since it does not rely on the removal of a discrete phase, is the sintering together of hollow spheres (or other shapes) to yield closed-cell structures with excellent control of cell size. For example, in recent work by Luyten et al. [7], spheres of both polymeric and biological origin were coated with an alumina slurry, packed into a mold and then slurry-coat- ed a second time. After drying, the spheres were burnt out, and the porous green body sintered, to yield a relatively strong and lightweight material that was built up of hollow alumina spheres; the density ranged from 15 to 20 % of theoretical. A wide diversity of materials could be produced by varying the diameters and shapes

54 Part 2 Manufacturing

of the hollow precursor material. Further information on the concept can be found in Chapter 2.8.

2.1.6

Summary

It is clear that the field of cellular ceramics is currently very active with new process routes constantly being developed and reported in the scientific literature and at conferences. Relatively few of them, however, are inherently novel; rather most are modifications or adaptations of existing processes, so that the concept of having three basic categories of manufacturing routes is still fundamentally true. What has changed over the past decade is that more processes are being commercialized. Whereas the polymer replication process was one of the few routes to be in commercial use as little as 15 years ago, now there are several routes available, each of which yields cellular ceramics with different pore structures and degrees of porosity, and hence different properties and potential applications. This provides much greater choice for the end user and far greater potential for the tailoring of structures to meet specific end-use requirements.

Acknowledgements

The author would like to acknowledge the work of some of his former research students whose work on the processing of ceramic foams has helped to contribute to this chapter. In chronological order: Dr. Rob Smith, Dr. Jutta Reichert, Dr. Pilar Sepulveda, Dr. Yongheng Zhang, and Mr. Lars Monson.

References

60Colombo, P., Bernardo, E. and Biasetto, L.,

J. Am. Ceram. Soc. 2004 87 [1] 152–154.