Cellular Ceramics / 5.11

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micro-/macrostructure development). After calcination, the pure zeolitic replica inherits the micro/macro-architecture of the biotemplate in detail (see Fig. 14b,c), and mechanical stability is maintained. An external scaffold of well-intergrown zeolite crystals supports the entire structure. The struts of this hierarchical porous foam are characterized by a vascular bundle of zeolitic hollow microchannels or microrods, made of crystals tightly packed together, running continuously along the network. Self-supporting structured pure zeolite monoliths with hierarchical porosity (e.g., see Refs. [46–50]) are a promising class of materials that are expected to find applications in adsorption/separation, catalysis, and the development of sensors.

5.11.16

Current Collectors in Solid Oxide Fuel Cells

A key problem in the successful use of solid oxide fuel cells (SOFCs) is the collection of current on the cathode side. Most metallic current collectors developed to date have a coarse design. The sizes of the current contacts are typically on the order of millimeters, and this leads to inhomogeneous current densities. La0.84Sr0.16Co0.02MnO3 perovskite open-cell ceramic foams, with porosities greater than 90 vol % and pore sizes smaller than 700 mm, have been proposed by researchers at ETH Zurich (Switzerland) as air distributors, current collectors, and load-bearing structural parts at the cathode or the anode side of an SOFC [51]. The foams are electrically conductive (100 S cm–1 at 634 C, 60 ppi foam), and the electric current in this design is collected by many more and much smaller contact points. The foams, produced by the replica technique, successfully fulfil three important requirements, namely, high electrical conductivity, sufficient mechanical strength, and good creep resistance at 580–690 C.

5.11.17

Sound Absorbers

The sound absorbing property of cellular structures has been thoroughly exploited with polymer and metal foams. Ceramic cellular materials expand this capability to environments where heat or corrosion resistance is required. Mufflers for hypersonic engines have been tested in Japan [52], and Volkswagen developed aeroplane mufflers based on reticulated alumina or SiC foams [53], while silicate ceramic foams obtained by direct foaming have been proposed by researchers at RWTHIKKM (Aachen, Germany) [54]. Open-cell carbon foams have also been demonstrated by Ultramet as acoustic liners in mufflers for general aviation aircraft, but these must be employed below the temperature at which the carbon is oxidized. Open-cell carbon foam coated with CVD SiC has been used as an effective broadband sound absorber in a high-temperature, oxidizing gas environment [55]. Glass foams developed by Pittsburgh Corning Corp. (Pittsburgh, PA) as cellular insulation for building applications, can provide sound attenuation of up to 56 dB.

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The mechanisms for sound absorption in a porous material are viscous and thermal dissipation (see Chapter 4.5), and to effectively dissipate acoustic power, the characteristic pore dimension should be on the order of the viscous and thermal penetration depths. Furthermore, to obtain good broadband attenuation, similar to that of existing liner materials, a comparatively low flow resistance is required [52].

5.11.18

Bacteria/Cell Immobilization

Kent Marine Inc. (Acworth, GA) and other suppliers sell open-cell ceramic foams or sintered glass products that are used as hosts for bacteria to purify water in fish tanks through wet/dry biofiltration [56]. Since inert ceramic foams combine a very high (geometric) surface area with a large and open pore structure, which allows water flow with limited resistance, they can be used as very compact filters for efficiently converting toxic fish wastes into nontoxic compounds and reducing the level of organics and algae. Decaying organic material (plant and animal waste) in a water tank is transformed to ammonia, and suitable bacteria grown on the surface of the foam metabolize the ammonia by converting it to nitrite. Another kind of bacteria grown on the foam consume nitrite producing nitrate, which in turn is consumed by plants, which give off oxygen.

Bacteria have been also immobilized on ceramic foams (12–55 ppi, reticulated Al2O3) for aerobic wastewater treatment (containing various organics, nontoxic solvents, and various salts), and results show that bacteria degrade organic carbon more efficiently when immobilized in a ceramic foam fixed bed in comparison with nonimmobilized bacteria, resulting in shorter residence times for wastewater and smaller reactors [57].

Immobilizing cell cultures on ceramic foams resulted in a high-density system for the growth of animal cells [58], with a fiveto tenfold increase in volumetric cell density per unit surface area over standard culture systems. The foams used (30–100 ppi, reticulated Al2O3) were able to withstand repeated autoclaving, and this allowed sterilization and reuse. The best results were obtained for cellular substrates of larger cell size (ca. 550 mm). Less than 5 % of the total number of cells detached from the substrate during the experiment, and the productivity of the immobilized cells (BHK, which produce human transferrin) was similar to that observed in suspension cultures. Such compact systems offer the advantages of significant reduction in serum requirement and the potential for scale-up.

5.11.19

Light Diffusers

Ceramic foams have also been tested in applications where the light signature needs to be reduced or emitted light needs to be diffused. Besides these technical applications, ceramic foams can be used for other purposes. Architect Harry Allen (Harry

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Allen and Associates, New York, NY) noticed how the material filters light, and created a line of lamps utilizing the material as the shade (see Fig. 15). After showing the lamps at the International Contemporary Furniture Fair, they were included in the “Mutant Materials in Contemporary Design” show at the Museum of Modern Art, New York, NY, in 1995. The lamps have since been added to the museum’s permanent design collection. All the lamps were designed by Harry Allen in 1994; the diffuser is made of alumina foam (reticulated) and the base is made of painted steel.

Fig. 15 a) The entire family of lamps (from left to right: Tower 1, Twist, Tower 2, and Tower 3). b) Detail of a ceramic

foam lamp. Images courtesy of H. Allen (Harry Allen and Associates).

5.11.20

Summary

Cellular ceramics play a key role in a variety of most diverse innovative applications, besides well-established industrial roles such as filtering liquid metals or particules in gas streams. Novel fabrication processes and developments in established manufacturing technologies allow the production of components with improved properties, a wider range of compositions, and varied morphology, which can all be optimized for a targeted application. Because of this, there are increasing efforts to utilize cellular ceramics in very diverse fields, as engineers are coming to realize more and more that these materials are especially suited for fulfilling some of the unique requirements typical of advanced technological developments.

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References