**4.4 Monolithic finned foams**

To maximize heat transport between components, monolithic finned copper foams with different geometries of pores were fabricated by a new manufacturing process presented in [33]. 3D printed polymeric or wax patterns were used as sacrificial materials in an investment casting process. This process eliminates the need to restrict design geometries to shapes that can be easily separated from a reusable mold. Their structure, hence, allow these materials to be classified as a combination of monolithic materials with a continuous matrix (Section 3.2.4).

### **4.5 Composite finned foams**

### *4.5.1 Aluminum/graphite flake composite finned foams*

These multiphase materials were also inspired by those presented in [40, 41]. Preforms were prepared by uniaxial pressure packaging of alternating layers of graphite flakes and NaCl particles. Preforms were infiltrated by the gas pressure technique with liquid aluminum and later leached away by water dissolution.

In this type of preforms, there are no restrictions concerning percolation, as the structure could ideally be understood as composed of alternating porous NaCl and Gf monoliths. Even in the extreme case where the percentage of NaCl monoliths is negligible compared to that of Gf monoliths, the NaCl particles in the monolith still have enough coordination to be effectively removed by dissolution. Nevertheless, a compaction limit is detected for preforms prepared without external pressure as a result of the natural tendency of graphite flakes to lie on top of one another (**Figure 14a**). The resulting multiphase open-pore foams present microstructures with alternating layers of oriented graphite flakes and metal foam, as it is shown in **Figure 14b**.

For alternating layers of Al foam and Gf monoliths, the longitudinal thermal conductivity of the composite finned foams *Kc<sup>L</sup>* can be estimated by the well-known Maxwell approach [13, 41]:

$$K\_C^L = V'f.K\_f^L + (1 - V'f).K\_{foam} \tag{5}$$

where the symbols have the same meaning as in Eq. (4) and *Kfoam* is again calculated with Eq. (3).

Analytical values obtained from Eq. (5) are correlated with the experimental results in **Figure 15a**. As it can be seen, the model represented by Eq. (5) can reasonably predict the longitudinal thermal conductivities for the Al/Gf composite finned foams, which reach experimental values up to 290 W/mK. The power dissipation density results obtained under working conditions with the setup described in [8] are represented in **Figure 15b**. The experimental results show increments in power dissipation density up to 325% compared with conventional aluminum foams.

### **Figure 14.**

*(a) Preform composition ternary phase diagram as a function of compaction pressure (in MPa) and (b) photograph of an Al/Gf foam with alternating layers of oriented graphite flakes and metal foam. Reproduced with permission from [13].*

**17**

*Open-Pore Foams Modified by Incorporation of New Phases: Multiphase Foams for Thermal…*

This chapter reviews recent developments in the manufacture and characterization of multiphase foams developed by incorporation of new phases into open-pore foam materials. The new incorporated phases can significantly alter the macro-/ microstructure of the starting materials or modify the pore surfaces to achieve new

The incorporation of new phases into open-pore foams opens up a new range of properties in foam materials since improvements can be obtained in the mechanical, thermal, catalytic, or adsorptive properties, among others. The design and conception of multiphase open-pore foams seem to be a very suitable way to overcome the growing demands for very specific properties in some modern applications in sectors such as electronics, catalysis, or medical

*(a) Calculated vs. experimental thermal conductivities for different Al foams and Al/Gf composite finned foams and (b) a comparison of power dissipation density as a function of airflow between some selected Al/ Gf composite finned foams and a conventional aluminum foam. Graphite flake dimensions: 1000 μm average* 

The authors acknowledge partial financial support from the Spanish Agencia Estatal de Investigación (AEI) and European Union (FEDER funds) through grant

*h* matrix/inclusion interface thermal conductance (W/m<sup>2</sup>

*KC* thermal conductivity of composite (W/mK)

K)

*DOI: http://dx.doi.org/10.5772/intechopen.88977*

*diameter and 20–45 μm thickness. Partially reproduced from [13].*

**5. Conclusions**

**Figure 15.**

functionalities.

implantology.

**Acknowledgements**

MAT2016-77742-C2-2-P.

**Conflict of interest**

**Nomenclature**

The authors declare no conflict of interest.

*D* graphite flakes diameter (m)

Gf graphite flakes

*Open-Pore Foams Modified by Incorporation of New Phases: Multiphase Foams for Thermal… DOI: http://dx.doi.org/10.5772/intechopen.88977*

**Figure 15.**

*Foams - Emerging Technologies*

**4.5 Composite finned foams**

foam, as it is shown in **Figure 14b**.

Maxwell approach [13, 41]:

lated with Eq. (3).

conductivity of the composite finned foams *Kc<sup>L</sup>*

*KC*

*<sup>L</sup>* =*V*´ *f*. *Kf*

*4.5.1 Aluminum/graphite flake composite finned foams*

These multiphase materials were also inspired by those presented in [40, 41]. Preforms were prepared by uniaxial pressure packaging of alternating layers of graphite flakes and NaCl particles. Preforms were infiltrated by the gas pressure technique with liquid aluminum and later leached away by water dissolution. In this type of preforms, there are no restrictions concerning percolation, as the structure could ideally be understood as composed of alternating porous NaCl and Gf monoliths. Even in the extreme case where the percentage of NaCl monoliths is negligible compared to that of Gf monoliths, the NaCl particles in the monolith still have enough coordination to be effectively removed by dissolution. Nevertheless, a compaction limit is detected for preforms prepared without external pressure as a result of the natural tendency of graphite flakes to lie on top of one another (**Figure 14a**). The resulting multiphase open-pore foams present microstructures with alternating layers of oriented graphite flakes and metal

For alternating layers of Al foam and Gf monoliths, the longitudinal thermal

where the symbols have the same meaning as in Eq. (4) and *Kfoam* is again calcu-

Analytical values obtained from Eq. (5) are correlated with the experimental results in **Figure 15a**. As it can be seen, the model represented by Eq. (5) can reasonably predict the longitudinal thermal conductivities for the Al/Gf composite finned foams, which reach experimental values up to 290 W/mK. The power dissipation density results obtained under working conditions with the setup described in [8] are represented in **Figure 15b**. The experimental results show increments in power dissipation density up to 325% compared with conventional aluminum foams.

*(a) Preform composition ternary phase diagram as a function of compaction pressure (in MPa) and (b) photograph of an Al/Gf foam with alternating layers of oriented graphite flakes and metal foam. Reproduced* 

can be estimated by the well-known

*<sup>L</sup>* + (1 − *V*´ *f*). *Kfoam* (5)

**16**

**Figure 14.**

*with permission from [13].*

*(a) Calculated vs. experimental thermal conductivities for different Al foams and Al/Gf composite finned foams and (b) a comparison of power dissipation density as a function of airflow between some selected Al/ Gf composite finned foams and a conventional aluminum foam. Graphite flake dimensions: 1000 μm average diameter and 20–45 μm thickness. Partially reproduced from [13].*
