Metallic Foams

Ashby has taken his approach a stage further with the introduction of physically based estimates of material properties where these have not been measured (Ashby 1998. Bassett et al. 1998), where an independent check on values is thought desirable or where property ranges of categories of materials would be useful. Figure 5.5(c) is one example of the kind of estimates which his approach makes possible. A still more recent development of Ashby s approach to materials selection is an analysis in depth of the total financial cost of using alternative materials (for different number of identical items manufactured). Thus, an expanded metallic foam beam offers the... 

Other types that have been used but are not currently commercially available in the United States are compressed wire wool and metal foam. However, compressed wire wool flame arresters are available in the United Kingdom. 

Foam vs. Sheet Metal Foam vs. Die Casting Compound Fiberglass Molding... 

A completely different way involves use of metal foams, or metal made porous otherwise (North and Cho 2003 Hetsroni et al. 2006a). 

Aluminum foam can be used as a porous medium in the model of a heat sink with inner heat generation (Hetsroni et al. 2006a). Open-cell metal foam has a good effective thermal conductivity and a high specific solid-fluid interfacial surface area. 

Depending on the metal foam configuration, its specific surface area varies from 500 for original foam to 10,000 m /m for compressed foam. Aluminum foam of 40 pores per inch (ppi) was studied. The structure of the porous material is presented in Fig. 2.77. 

A gas suspended in a solid is also called a foam. This form of colloid is relatively rare in nature, unless we stretch our definition of solid to include rock, in which case pumice stone is a colloidal foam. Synthetic foams are essential for making cushions and pillows. There is also presently much research into forming metal foams, which have an amazingly low density. 

Microscope image of a Ni metal foam used as a DL in direct methanol fuel cells (reference bar indicates 1 mm). (Reprinted from S. Arisetty et al. Journal of Power Sources 165 (2007) 49-57. With permission from Elsevier.)... 

Metal foams have been used in the past in the development of FF plates. However, Gamburzev and Appleby [53] used Ni foams as both a DL and a flow field plate with an MPL layer on one of its surfaces. They observed that such a design had high contact resistance between the nickel foam and the MPL and also increased gas diffusion resistance due to the required MPL thickness. Arisetty, Prasad, and Advani [54] were able to demonstrate that these materials can also be used as potential anode diffusion layers in DMFCs (see Figure 4.10). In fact, the nickel foam used in this study performed better than a carbon cloth (Avcarb 1071HCB) and a stainless steel mesh. However, it was recognized that a major drawback for these foams is their susceptibility to corrosion. 

In another study, Chen and Zhao [55] demonstrated that by using a Ni-Cr alloy metal foam as the cathode DL (and current collector), instead of a CFP or CC, the performance of a DMFC can be enhanced significantly due to the improvement of the mass transfer of oxygen and overall water removal on the cathode side. Fly and Brady [56] designed a fuel cell stack in which the distribution layers were made out of metal foams (open cell foams). In addition, more than one foam (with different porosity) could be sandwiched together in order to form a DL with variable porosity. 

The bipolar plates are usually fabricated with non-porous machined graphite or corrosion-resistant metal plates. Distribution channels are engraved in these plates. Metallic foams can also be used for distributing the reactants. One key point is to ensure a low ohmic resistance inside the bipolar plate and at the contact with the M EA. Another point is to use materials with high corrosion resistance in the oxidative environment of the oxygen cathode. 

Foams have a large variety of applications. Solid foams are widely used as insulating materials. Due to the presence of air bubbles they have a low thermal conductivity. Polyurethane foams and Styrofoam are examples. Styrofoam is also used as a packing material. The light weight of polymer foams makes them attractive as filling materials to stabilize otherwise hollow structures. A natural solid foam is pumice stone. Metal foams are used in the automotive and aerospace industry as light and stable materials [567], Ceramic foams are developed for electronic applications as piezoelectric transducers and low dielectric constant substrates [568],... 

These problems can be avoided by measuring capillary pressure curves of GDMs directly with water as the working fluid. A number of techniques have been recently proposed. Of the different types of GDM (carbon paper, carbon cloth, metal foam/mesh, etc.), only carbon paper GDMS have been studied to any appreciable extent using these methods. An ideal capillary pressure measurement technique must be able to vary water saturation in directions of increasing saturation from Sw = 0 to Sw -> 1 and decreasing saturation from Sw = 1 to Sw -> 0. This requires the application... 

On testing of the metal foams at a catalyst loading of 200 mg, 72% conversion of the gasoline was achieved. Further increases in catalyst loading led to coke formation, which was attributed to lower reaction temperatures towards the reactor outlet. The better dilution of the initial hot-spot was assumed to lead to the lower reaction temperatures observed. Additionally, suspicion arose that some of the feed was channelling through the metal foam. 

The BP are usually fabricated with nonporous machined graphite or corrosion-resistant metal plates. Distribution channels are engraved in these plates. Metallic foams can also be used for distributing the reactants. 

In the last decade several novel types of gas-filled polymers appeared which belong to the second generation integral (structural) plastic foams syntactic foams reinforced polymer foams multilayer foams (foamed laminates), metallized plastic foams mineral and metallic foamed materials obtained on the basis of foamed polymers laminated constructions on the basis of foamed polymers and monolithic (unfoamed) plastics, metals, paper, leather, etc. 

It is well known that, depending on the field of application, preference is given either to closed-cell or open-cell foams. The latter are widely used for sound and vibration insulation, in chromatography, as absorbents and filters of liquids and gases, for the preparation of metallized foamed materials, etc. 

Gokon, N., Osawa, Y., Nakazawa, D., and Kodama, T. Kinetics of C02 reforming of methane by catalytically activated metallic foam absorber for solar receiver-reactors. International Journal of Hydrogen Energy, 2009, 34 (2), 1787. 

Chin, P., Sun, X., Roberts, G.W., and Spivey, JJ. Preferential oxidation of carbon monoxide with iron-promoted platinum catalysts supported on metal foams. Applied Catalysis. A, General, 2006, 302, 22. 

T. Dillard, F. NGuyen, S. Forest, Y. Bienvenu, J. Bartout, L. Salvo, R. Dendievel, E. Maire, P. Cloetens, and C. Lantuejoul, in Cellular metals and metal foaming technology, 2003, pp. 1-6. Verlag MIT Publishing,. 

Fig. 14.10 Microphotographs of porous HSVRMs [53-55], (a) Metallic foam, (b) mesh structure, and (c) woven mesh structure...

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