Cellular Ceramics / p2

2.6 Carbon Foams 147

mechanical and thermal properties. As in film blowing (e.g., plastic bags), the biaxial stresses appear to impart better orientation in the liquid crystal than normal shear in fiber spinning. This may result in better orientation of the graphitic planes along the struts in mesophase pitch-derived foam than that seen in round fibers. This alignment is observable in Figure 7, as evidenced by the striations in the strut and node surfaces.

There are several techniques for producing mesophase pich-derived graphitic foams. The first, by Hagar et al. [1, 2, 17–20] and later by Kearns [21, 22], was a traditional blowing technique in which the mesophase is softened, saturated with a blowing agent (gas), and then flashed (blown) into a foam. The process requires oxidative stabilization of the foam to prevent remelting during carbonization and graphitization.

This process produces a foam with a microcellular structure and uniform pore size (Fig. 8). First, a quantity of a pitch is pressed in a mold to provide a pressed article. Then the article is placed in a pressure vessel and covered with an inert gas and pressurized to about 200–500 psi. The furnace is heated about 10–40 C above the melting temperature of the pitch. Following a short hold time to allow equilibrium to be reached, additional inert gas is added to obtain a final pressure in the pressure vessel of about 1000–1500 psi. The furnace is held constant for about 10–40 min and then quickly vented to atmospheric pressure, thereby resulting in a thermodynamic flash of the dissolved gases in the precursor. The resulting expansion provides a porous foam due to the high viscosity of the mesophase precursor at these temperatures. After removal from the furnace, the foams are stabilized with oxygen at an elevated temperature near the softening point of the precursor. The maximum temperature at which the stabilization is carried out should not markedly exceed the temperature at which the pitch foam softens. In general, stabilization can be carried out by subjecting the porous foam to an oxygen or air atmosphere for about 8–24 h at about 150–260 C, preferably about 150–220 C, or until a weight gain of about 5–10 % is achieved. Slow cooling (ca. 0.1–5 C min–1) of the stabilized porous pitch foam to ambient temperature is necessary. This is a critical step because a fast cooling rate may result in thermal stresses and cracking of the sample (dependent on billet size). The porous pitch foam can then be converted to a porous carbon foam by heating to 900–1100 C. The porous carbon foam can then be converted to a porous graphitic foam by heating to graphitization temperatures (> 2400 C). This process has been licensed by MER Corporation, Tucson, Arizona (www.mercorp.com).

An advantage of this process is that resulting graphitic foam has a very low density of about 0.1 g cm–3 and very high specific strength. However, thermal and electrical transport are limited by the processing conditions. First, the foaming step is performed at a temperature not much higher than the melting point of the pitch. During biaxial extension of the mesophase domains, the very high viscosity limits the mobility of the mesophase domains and prevents formation of a highly oriented structure parallel to the cell walls. In the fabrication of mesophase fibers, the precursor first travels through a narrow capillary designed to induce very high shear rates which can reorient the domains in a very viscous material. However, in the normal

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Figure 8 Microcellular carbon foams produced by Kearns (photo courtesy of Dave Anderson, University of Dayton Research Institute).

blowing process, it is expected that the stresses imparted to the mesophase domains are significantly lower than those occurring in carbon fiber production. Hence, the mesophase domains do not reorient as well as desired. Next, stabilization of the porous foam is intended to render the surface layer infusible so that the porous structure of the foam is maintained during subsequent treatments. Additionally, the stabilization locks in the domain size of the mesophase regions, thus limiting the crystal size of the resultant graphitic structure [56]. Thus, during post-stabilization heat treatments, any reorganization of the mesogens into a pre-graphitic and eventually a graphitic structure is limited to the original domain size. Hence, domain size of the original mesophase becomes the maximum crystallite size after graphitization [56]. This helps the mechanical strength, as small crystallite sizes (or grain boundaries) in flaw-limited ceramics, such as foams, tend to increase strength. However, it severely limits the thermal and electrical transport properties of foams produced with this method.

Another process for producing graphitic foams was developed at Oak Ridge National Laboratory (ORNL) by Klett et al. [21, 22, 29–31, 57–61]. This process requires fewer steps and overcomes some of the deficiencies of the Hagar–Kearns processes. It results in a foam that is denser, significantly more thermally and electrically conductive, and structurally weaker. In fact, they were the first to produce carbon foams sufficiently conductive to be considered for heat sinks and other thermal-manage- ment applications, rather than thermally insulating or structural applications. The ORNL process is relatively simple and is free of the oxidative stabilization traditionally required for processing of pitches and mesophases.

First, a mesophase pitch precursor is heated in an oxygen-free environment to about 50 C above its softening point. Once the pitch has melted, the pressure is elevated and the temperature is raised at a controlled rate. While the pitch is molten, low molecular weight species begin to evolve. These volatile gases form bubbles at nucleation sites on the bottom and sides of the crucible and rise to the top, beginning to orient the mesophase crystals in the vertical direction. With time, a significant amount of the mesophase crystals are oriented vertically.

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At high temperatures the mesophase begins to pyrolyze (polymerize) and create additional volatile species. This pyrolysis, which can be very rapid and is dependent on the precursor, is accompanied by an increase in the molecular weight of the precursor which, in turn, increases the melt viscosity of the liquid mesophase. As the rapid evolution of gases progresses, the increase in viscosity tends to capture the bubbles in place, forcing the material to foam in the unrestrained z direction. As the temperature is further increased, the foamed mesophase continues to pyrolyze, further increasing the viscosity of the material until it has sufficiently cross-linked and is rendered infusible.

While the foam synthesis process is rather simple, the morphological changes occurring during processing are complex. There is a delicate relationship between the viscosity–temperature behavior, melting temperature, and pyrolysis temperature of the mesophase pitch. Initially, the pyrolysis gases develop at a temperature such that the viscosity is sufficient to result in a stable foam. Premature evolution of pyrolysis gas causes the pitch to froth and results in foam with a significant density gradient (which may be desirable in some applications). If the gases are evolved too late, when the pitch viscosity is high, the bubbles may not be uniform, and cracking can occur due to thermal stresses. If the pyrolysis gases are evolved very slowly, as for certain high-melting Conoco pitches, the pores tend to be smaller [60].

Bubble formation is closely related to the autoclave operating pressure and temperature. Typically, the higher the autoclave gas pressure, the higher the temperature at which gas evolution occurs and the smaller the resulting pores. However, depending on the unique rheological properties of the starting pitch, the cell walls have different thicknesses, the bubble sizes can be dramatically different, and the mechanical and thermal properties can be affected. Unlike some other foaming techniques, such as slurry-derived metallic foams, the resultant properties of the graphitic foam (e.g., bubble size, ligament size, relative density, thermal and mechanical properties) are not independent properties. They are all dependent on the precursor’s melt viscosity, pyrolysis temperature, and other pitch rheological properties.

The foamed mesophase is carbonized by heating to between 600 and 1000 C to yield a relatively pure carbon foam. In this state, the foam is an excellent thermal insulator with a bulk thermal conductivity of about 1.2 W m–1 K–1 at a density of 0.5 g cm–3. Because the carbonized foam was formed with a mesophase that was not oxidatively stabilized during the pyrolysis/carbonization stages, the mesophase crystals are not inhibited and can grow to very large sizes. Consequently, when the carbon foam is converted to a highly graphitic foam by heat treatment above 2800 C under an argon purge, the resultant graphite crystals are highly aligned and significantly larger than those found in mesophase pitch-derived carbon fibers (similar to needle coke). Hence, the ligaments of the graphite foam produced with this method are more thermally conductive than even the best mesophase pitch-based graphite fibers.

A typical resultant mesophase foam is illustrated in Fig. 9. The foam typically exhibits uniformly shaped bubbles with a normal distribution. The average pore size, orientation, and distribution are determined primarily by the pitch viscosity and pro-

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cessing pressure during foaming. Additionally, the mesophase-derived foam has a preferred orientation of crystals in the z direction with accompanying anisotropy of properties in the xy plane even though bubble shape may not appear anisotropic.

Figure 9 SEM images of mesophase pitch-derived foams.

This material differs from the carbon and graphite foams produced previously. It has a predominately spherical porosity with smaller openings between the cells (Fig. 10). Highly oriented sheets of graphite parallel to the cell walls yield thermal conductivities in the cell walls as high as 1640 W m–1 K–1 [59]. At a relative density of approximately 0.3, this results in a bulk apparent thermal conductivity of more than 180 W m–1 K–1. Hence, a graphite foam can be produced with a bulk thermal conductivity almost equivalent to those of dense aluminum alloys, but at one-fifth the weight. This process has been licensed by Poco Graphite, Inc. of Decatur, TX (www.poco.com).

To examine the unique graphitic structure, the ORNL foams were examined by TEM. Figure 11 shows a typical TEM image of a cell wall. Unlike many synthetic graphitic materials, the graphene planes here are highly aligned and relatively defect

Figure 10 Spherical pore structure of mesophase pitch-based graphite foams.

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stabilization would have minimized the domain size of the mesophase during formation and thus limited the size of the crystals in the final heat treatment. Hence, while the crystals are not developed in the foaming stage, the ability to form large crystals in later heat treatments is regulated in this stage.

Figure 12 X-Ray diffraction pattern of highly graphitic foams.

Figure 13 Development of crystalline structure with heat treatment temperature.

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Figure 14 Images of thermally conductive graphitic foams made from synthetic and petroleumand coal-derived mesophases.

The ORNL process can be used with different mesophase precursors to form different foams without changing process parameters. Figure 14 shows scanning electron micrographs of foams made with the same operating conditions and a synthetic mesophase (AR mesophase), a petroleum mesophase (Conoco Dry Mesophase), and a coal-derived mesophase (Koppers mesophase). The strikingly different morphologies of the resultant foams are due to the viscosities of the precursor at the temperature when the pyrolysis gases evolve. It is feasible to blend the three pitches, even with isotropic pitches, to produce a hybrid foam with a tailored pore structure.

2.6.5

Properties of Carbon and Graphite Foam

Because of the wide range of structures developed and specific precursors used to fabricate carbon and graphite foams, there is an equally wide range in mechanical and thermal properties. The advantage is that virtually any desired material property within the range can be fabricated. The disadvantage is that customers usually want the product customized to their individual needs, which increases costs. Unfortunately, overall engineering is as important to heat transfer with graphite foams as is material tailorability. First, there are foams with very low pressure drops (reticulated carbon foams) and ones with very high pressure drops (ORNL foams, see Fig. 15). However, selecting a specific foam for heat- (or mass-) transfer applications where fluids flow through the foams is more complicated than selecting a pore size to minimize pumping power. In the case of heat transfer, the pumping power must be balanced with heat transfer. In addition, there are engineering solutions to the design of the system that can be utilized to reduce pressure drop without seriously affecting heat transfer. Such a design is used frequently with low permeability filters for HEPA vacuums by corrugating the structure. Corrugation, or pleating, can reduce the pumping power by a factor of more than 10, but not significantly decrease heat transfer.

Table 1 lists several properties of various types of carbon and graphite foams. This is not all-inclusive, just representative of what is available commercially. Many specific properties can be obtained by tailoring various processing conditions.

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Figure 15 Pressure drops as a function of air and water velocity through

Pocofoam , as measured at ORNL.

An interesting aspect of carbon is its electrical conductivity. The thermal insulating foams exhibit high electrical resistivity, and some are even used with ohmic heating for furnace elements. However, as the structure becomes more graphitic, the foams become electrically conductive. The mechanisms that scatter phonons and interfere with thermal transport also scatter electrons and interefere with electrical transport [62]. Hence, there is a relationship between electrical resistivity and thermal conductivity in the thermally conductive foams, similar to that found in mesophase derived carbon fibers. Figure 16 illustrates that foams with high thermal conductivity (k > 100 W m–1 K–1), exhibit an electrical resistivity of less than 10 lX m, which is nearing that of semiconductors.

Figure 16 Thermal conductivity of graphitic foams as a function of electrical resistivity.

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Table 1 General properties of various experimental and commercial graphite foams (data from manufacturer’s website unless otherwise specified).

* Measured at ORNL. ** Measured at WPAFB.

2.6.6

Summary

Carbon foams are a very broad class of materials. In addition, they can exhibit some of the most widely varied properties and can be made with a multitude of processes. They can be everything from thermally insulating to thermally conductive, structural or nonstructural, or reticulated or spherical. Their uses include catalyst supports, filters, gas diffusers, and templates for other metallic and ceramic foams. Recently, the low-density, high-strength foams developed by the US Air Force offer the opportunity for replacing very expensive fiber weaves in 3D structural applications. Furthermore, the development of foams with high thermal conductivity opens even more applications for these unique materials. Now, a material which can be thermally insulating and used as a heat shield can simply be heat-treated to very high temperatures and converted to a thermally conductive material which can readily transfer heat. These applications range from computer cooling to engine cooling to residential coolers. The ability to transfer heat at a rate equivalent to aluminium but at one-fifth its weight has significant applications if the systems utilizing this technology can be engineered properly to capture the full potential of the foams.

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References