1. Introduction

A porous medium can be defined as a material composed of a solid matrix consisting of interconnected voids. This solid matrix is usually assumed to be rigid, but sometimes it may undergo limited deformations. The interconnected pores allow for a single type of fluid or more to flow through the material. There are many examples of these permeable materials available in nature such as sand beds, limestone, sponges, wood, and so on.

Porous media have become one of the most important materials used in insulating, transferring, storing, and dissipating thermal energy. The benefits these characteristics confer have led to porous materials being widely used in practical applications such as thermal insulation, geothermal applications, cooling systems, recuperative/regenerative heat exchangers, and solar energy collection systems, in addition to chemical and nuclear engineering. Thus, convective flows in porous materials have been investigated widely over recent decades and various aspects have been considered for different applications so far.

© The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons © 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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High-porosity metal foams are usually porous media with low density and novel structural and thermal properties. This sort of media is mainly formed from multi-struts interconnected to each other at joint nodes to shape pores and cells (see the SEM image in Figure 1) (Liu et al. [1]). They offer very high porosity (ε ≥ 0.89), light weight, high rigidity and strength, and a large surface area, which make them able to recycle energy efficiently. This capacity to transport a large amount of heat is attributed to their superior thermal conductivity compared to ordinary fluids and high surface-area density (surface area per a given volume of metal foam) as well as enhanced convective transport (flow mixing) due to the tortuous flow paths existing within them, as shown in Figure 1 (Zhao [2]). Also, their open-cell structure makes them even less resistant to the fluids flowing through them, and hence, the pressure drop across them is much less than it is in the case of fluid flow via packed beds or granular porous materials.

Open-cell metal foams were first invented by the ERG Materials and Aerospace Corp. in 1967, and since then, they have been continuously developed. This invention was patented to Walz [3], where the manufacturing processes were based on an organic preformation cast. However, this invention was originally intended for only classified military and aerospace applications. Accordingly, nonclassified applications had not made use of this technology until the mid of 1990s, when it has become generally available for industrial applications. Since then, other manufacturers have joined the global competition in this industry. To name a few, M-Pore GmbH in Germany, the French company Alveotec, and Constellium from Netherlands are currently making open-cell metal foams on a large scale for a wide range of applications.

or sand plus a polymer bonding agent. After metal solidification, the spheres are dissolved through washing them simply with water. Using this casting technique results in foams having more uniformly spherical-shaped cells unlike those formed by the investment casting, as

Figure 2. Open-cell metal foams formed by: Investment casting (left); (b) leachable bed casting (right).

Open-cell metal foams possess unique characteristics, making them a promising candidate for plenty of practical and engineering applications. Among these potentials are the following:

2. The open-core structure makes them attractive for applications where lightweight is a

3. The open-cell nature makes them even less resistant to the fluids flowing through them,

transfer area per a given volume of metal foam is offered. The surface area density can be further increased through compressing them in a particular direction, where the specific

6. The tortuous flow paths existing within them considerably enhance the convective trans-

/m<sup>3</sup>

High-Porosity Metal Foams: Potentials, Applications, and Formulations

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. Therefore, exceptional heat

/m<sup>3</sup> [2].

shown in Figure 2 (De Schampheleire et al. [6]).

4. Very high effective thermal conductivity.

1. Very high porosity, ranges usually from 80% and up to 97.5%.

resulting in a significant saving in the pressure drop resulted.

surface area of such a compressed metal foam can reach up to 8000 m2

5. High surface-area density, roughly from 1000 to 3000 m2

7. They are efficient sound-absorption materials [8].

2. Potentials

crucial requirement.

port (flow mixing).

The traditional way of casting open-cell metal foams is still adopted by ERG Materials and Aerospace [4] as well as M-Pore GmbH [5], where the foams are cast with an investment process based on polyurethane preformation. As the fabrication process is affected by gravity, the foams resulted will be shaped from oval rather than spherical cells, as illustrated in Figure 2 (De Schampheleire et al. [6]). Alveotec [7] and Constellium, on the other hand, use a different way called leachable bed casting, in which metal is cast over a stack of soluble spheres to shape out the interconnecting open-cells desired. The spheres used are usually made out of either salt

Figure 1. Open-cell metal foams: SEM image of the structure (left); mechanism of flow mixing (right).

Figure 2. Open-cell metal foams formed by: Investment casting (left); (b) leachable bed casting (right).

or sand plus a polymer bonding agent. After metal solidification, the spheres are dissolved through washing them simply with water. Using this casting technique results in foams having more uniformly spherical-shaped cells unlike those formed by the investment casting, as shown in Figure 2 (De Schampheleire et al. [6]).
