Cellular Ceramics / p2

137

2.6

Carbon Foams

James Klett

2.6.1

Introduction

Carbon is a unique element in that in one of its allotropes is extremely thermally conductive and another is insulating. It has the ability to span orders of magnitudes in properties simply by how it is processed into a structure and the degree of heat treatment. One such structure is a foam made from carbon or graphite. Typical process sequences include blowing, carbonizing, and then thermally treating the foam. Autoclave processes are also feasible. Foams can be blown into a mold for netshape composites or machined into intricate parts. Analytical models of graphitic foams have predicted them to have a compression modulus of approximately 2 GPa with a density of about 0.1 g cm–3 [1, 2]. Few, if any, other foams or core materials have a density and compression modulus near these theoretical values. Graphitic foams have been produced with various processes and precursors. The resultant foam properties are dependent on the process path and precursor. Some foams have a very high thermal conductivity, while others exhibit greater structural integrity. The creation of these foams has begun to foster novel thinking on how to design structures and thermal-management systems with these “alternative” carbon materials.

2.6.2

History

The first carbon foam, developed in the late 1960s, was reticulated vitreous carbon foam. Ford [3] reported on carbon foams produced by carbonizing thermosetting organic polymer foams through a simple heat treatment. Then, Googin et al. [4] at the Oak Ridge Atomic Energy Commission Laboratory reported the first process dedicated to controlling the structure and material properties of carbon and graphitic foams by varying the precursor material (partially cured urethane polymer). In the several decades following these initial discoveries, many researchers explored a variety of applications for these materials [5–14] ranging from electrodes to insulating liners for temperatures up to 2500 C. Reticulated carbon foams have been used as the template for many of the metal and ceramic foams currently used in industry.

Cellular Ceramics: Structure, Manufacturing, Properties and Applications.

Michael Scheffler, Paolo Colombo (Eds.)

Copyright 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

ISBN: 3-527-31320-6

138 Part 2 Manufacturing

In the 1970s, research focused primarily on producing carbon foams from alternative precursors. For example, Klett [8] at Sandia National Laboratories produced the first carbon foams from cork, a natural cellular precursor. Others worked on various changes to processing and precursors in an attempt to modify properties and reduce cost. One technique produced a reticulated vitreous carbon foam, commonly called glassy carbon foam [15, 16]. Other researchers also produced carbon foams by carbonization of thermoplastic or thermoset foams, such as phenolic resins and polyurethanes. For example, syntactic foams were obtained by molding a mixture of hollow plastic microspheres with a binder followed by carbonization; such products have a closed porosity of greater than 90 %. A process for the preparation of carbon foam from polyurethanes has been described, as has a process starting from phenolic foams. The majority of these “glassy” carbon foams were used for thermal insulation, although some structural applications were found.

In the early 1990s, researchers at the Wright Patterson Air Force Base (WPAFB) Materials Laboratory pioneered mesophase pitch-derived graphitic foams, specifically for replacing expensive 3D woven fiber performs in polymer composites and as replacements for honeycomb materials [1, 2, 17–22]. Their work was centered on developing a highly structural material that was lightweight. Concurrently, Ultramet Corp. performed research on RVC foam and used chemical vapor deposition (CVD) as a templating technique to place pyrolytic graphite on the glassy carbon ligaments, producing three-dimensional carbon structures with high-modulus ligaments.

With the goal of producing very inexpensive carbon foams, researchers at West Virginia University developed a method that used coal as a precursor for highstrength foams with excellent thermal insulation properties [23–26]. Then, in 1997, Klett [27–37] at the Oak Ridge National Laboratory (ORNL) reported the first graphitic foams with bulk thermal conductivities greater than 40 W m–1 K–1 (recently, conductivities over 180 W m–1 K–1 have been measured [38]). By combining an open cellular structure with a thermal conductivity to density ratio (k/ ) of greater than 345 W m–1K–1 g–1 cm3 (compared to 45 for copper), this material presents a unique opportunity to radically change the approach to solving many heat-transfer problems. This graphite material has been examined for the core of heat-transfer devices such as radiators and heat sinks, evaporative coolers, and phase-change devices.

This chapter discusses the various processing methods for several major types of carbon foams. This is not all-inclusive but is a cross-section of the many fundamental techniques for producing carbon and graphite foams.

2.6.3

Terminology

Before embarking on a review of carbon and graphitic foams, it is necessary to review the terminology used in the carbon community. The first terms that are critical to understanding these materials are carbon, graphite, and graphitization. These have been defined in many circles [39–45], but the most relevant definitions were

2.6 Carbon Foams 139

developed by the carbon and graphite committee of ASTM subcommittee D02.F0 and are listed below.

2.6.3.1

Carbon

Element number 6 of the periodic table, with the electron ground state 1s22s22p2. In carbon and graphite technology, an artifact consisting predominantly of the element carbon and having limited long-range order.

2.6.3.2

Graphite

An allotropic crystalline form of the element carbon, occurring as a mineral, commonly consisting of a hexagonal array of carbon atoms but also known in a rhombohedral form. In carbon and graphite technology, a material consisting predominantly of the element carbon that possesses extensive long-range three-dimensional crystallographic order, as determined by X-ray diffraction studies.

The presence of long-range order is usually accompanied by high electrical and thermal conductivity within the hexagonal plane. This also results in a material having relatively easy machinability when compared to non-graphitic materials. The use of the term graphite without reporting confirmation of long-range crystallographic order should be avoided, as it can be misleading.

2.6.3.3

Graphitization

Graphitization is a solid-state transformation of thermodynamically unstable nongraphitic carbon into graphite by thermal treatment. The degree of graphitization is a measure of the extent of long-range 3D crystallographic order, as determined by diffraction studies alone. The degree of graphitization significantly affects many properties, such as thermal conductivity, electrical conductivity, strength, and stiffness.

A common, but incorrect, use of the term graphitization is to indicate a process of thermal treatment of carbon materials above 2200 C regardless of any resultant crystallinity. The use of the term graphitization without reporting confirmation of long-range 3D crystallographic order determined by diffraction studies should be avoided, as it can be misleading.

The term graphitization is quite often used to indicate a heat treatment to very high temperatures. Many assume that the common heat treatment referred to as graphitization of a carbon part necessarily yields a graphitic structure. To avoid confusion, the term should not be used without confirmation of extensive crystallinity in diffraction studies. Other studies such as TEM are suggestive of, but do not confirm, long-range 3D crystallinity.

2.6 Carbon Foams 141

foams with densities up to 0.8 g cm–3 (r.d. = 0.35), and Stiller et al. [25] reported foams with densities up to 1.0 g cm–3 (r.d. = 0.44). Knippenburg et. al. reported a foam with r.d. greater than 0.3 [6]. As can be seen in Fig. 1, when the relative density of a foam is sufficiently high, the failure mode changes significantly, and the plateau is lacking (here, the graphite foam was densified with mesophase pitch, yielding a structure with interconnecting pores, but very large ligaments; see Fig. 2).

Clearly, most researchers classify their materials as foam when it exhibits a structure in accord with the first definition of Gibson and Ashby, but are not in agreement when it comes to a specific cutoff of relative density. In addition, the polymer industry does not use relative density in its definition of foam. Because of these inconsistencies, ASTM defined a foam without using relative density as a delineation mark. Instead ASTM agreed on the definitions given below based on the 3D cellular structure commonly assumed to be the essence of a foam. This chapter attempts to adhere to the ASTM definitions. However, some of the patents referenced in this section were granted prior to the ASTM and Gibson and Ashby definitions.

Carbon foam is a porous carbon product containing regularly shaped, predominantly concave, homogeneously dispersed cells which interact to form a threedimensional array throughout a continuum material of carbon, predominantly in the non-graphitic state. The final result is either an openor a closed-cell product.

Graphite foam is a porous graphite product containing regularly shaped, predominantly concave, homogeneously dispersed cells which interact to form a threedimensional array throughout a continuum material of carbon, predominantly in the graphitic state. The final result is either an openor a closed-cell product.

2.6.4

Foaming Processes

Foams can be fabricated by many different techniques outlined in the next section. There are three general types of precursors for foams: 1) thermosetting, 2) thermoplastic, and 3) hydrocarbon vapors. However, since the process of chemical vapor deposition (CVD) of carbon requires a foam substrate to begin with, this is included in the discussions of thermosetting and thermoplastic precursors.

2.6.4.1

Thermosetting Precursors

The first carbon foams were prepared by heat-treating existing polymeric foams, such as those from polyurethane, polyester, or phenolics [3]. Then, Googin et al. at the U.S. Atomic Energy Commision’s Oak Ridge plant adjusted the precursor to tailor the resulting foam properties [4]. By mixing furfuryl alcohol with urethaneforming compounds prior to foaming, the resulting glassy carbon foam had drastically different properties than the converted polyurethane foam. In addition, graphite powder was added to the mix to result in the first “graphitic” foam. While the

2.6 Carbon Foams 143

Finally, the coated polyurethane foam is pyrolyzed to convert the structure to a glassy carbon foam typically referred to as reticulated vitreous carbon foam. This type of process (with many variations) is practiced by Ultramet Corp. which markets the foam as Ultrafoam. For specific applications, Ultramet produces foams with a controlled aspect ratio of cells near unity and with billet thicknesses greater than 20 cm (Fig. 3). A similar process is patented by Energy Research and Generation, Inc. (ERG) [48], which markets a reticulated vitreous carbon foam under the tradename RVC (Fig. 4), which is limited to 10 cm in thickness.

By varying the starting foam structure (linear cell count, ligament thickness, etc.) and varying the coating process, foams with the same overall density but drastically different structures (i.e. different cell count, ppi, strengths, thermal conductivities, and permeability factors) can be made.

Another process for producing a glassy carbon foam is described in ref. [49]. This process carbonizes a polystyrene-based low-density foam to yield a carbon foam. First, an inverse emulsion of a styrene monomer in water is prepared and polymerized to produce a styrene foam. The cells of the styrene foam are filled with an aqueous resorcinol–formaldehyde solution, which is cured to form a gel which coats the pores of the styrene foam. Subsequent heating to elevated temperatures converts the gel-filled polystyrene foam to a carbon foam.

Hayward et al. [50] combined expanded graphite with phenolic resins prior to foaming. First, a flexible graphite (e.g., Grafoil gasket material) is chopped into small particles and thermally shocked to expand the graphitic planes into a very low density structure (i.e., exfoliation). A phenolic resin is mixed with the expanded graphite, and application of heat and pressure produces a foamed structure with both graphitic and glassy regions. This product was shown to be an excellent thermal insulator, despite being partially graphitic.

Another process for the production of carbon foam was developed by Simandl et al. at the US DOE Y-12 plant in Oak Ridge [51]. This process variation is unique because a polyacrylonitrile (PAN) precursor was used instead of a glassy carbon precursor (like phenols and urethanes). This process utilizes a phase inversion of polyacrylnitrile in a solution of an alkali metal halide in a solvent such as propylene carbonate, tetramethylene sulfone, or mixtures of both. Typically, gels of polyacrylonitrile produce a structure incapable of self-support when converted to a carbon material. However, the alkali metal, in low concentrations, promotes solubilization and unraveling of the highly crystalline, helical PAN molecules in solution and then maintains these extended molecules for the desired strut formation of the gel during phase inversion. Thus, a stable carbon foam can be produced. This carbon foam, after heat treatment at elevated temperatures (> 2400 C), was found to be an excellent thermal insulator at densities up to 0.3 g cm–3.

Another process for producing foam is the lost-foam technique. While this technique is more widely used for metal foams, it has been adapted for production of carbon foams. In general, the process begins by producing a solid structure with interconnected porosity from a salt. Then the precursor for the foam is infiltrated into the interconnecting porous structure and cured. At the appropriate stage in curing, the salt is dissolved out of the polymer foam. The polymer foam is then carbo-

144 Part 2 Manufacturing

nized to form carbon foam. One such technique [52] describes such a product as a machinable and structurally stable, low-density, microcellular carbon foam. In this process, pulverized sodium chloride is first classified to improve particle size uniformity. The particles are cold pressed into a compact having internal pores and then sintered. The sintered compact is submerged in a phenolic polymer solution to uniformly fill the pores of the compact with phenolic polymer. The compact is then heated to pyrolyze the phenolic polymer to carbon in the form of a foam. Finally, the sodium chloride is leached away with water to give a carbon foam suitable for catalyst supports or high-temperature insulation.

2.6.4.2

Thermoplastic Precursors

Carbon foams from pitches have been under development for several decades [9, 53]. Recently, the processes have been investigated for large-scale manufacture. As material becomes available, data are being generated to support realization of potential applications for theses carbon foams. Traditionally, pitches are cheaper than synthetic polymers but can be difficult to process.

The first process for the manufacture of pitch foams was developed by Bonzom et al. [9, 53]. In this process petroleum-derived pitches are expanded at elevated temperature by a pore-forming agent in a mold. The pitches used by Bonzom et al. were residues from petroleum cracking, subjected to thermal aging to increase the ratio of aromatic to aliphatic componets. Typically, the pitch had a Kraemer–Sarnow (KS) softening point between 70 and 210 C. The initial pressure was selected such that, at the decomposition temperature of the porogenic agent, gases would not evolve (i.e., the initial pressure is higher than the pressure of normal pyrolysis under atmospheric conditions). After the decomposition of the porogenic agent, the pressure was lowered to expand the pitch (thermodynamic flash). The temperature could also be adjusted between the decomposition and decompression stages to control growth rate of the cells by adjusting the viscosity of the pitch. After expansion, the pitch requires oxidative stabilization to render it infusible and prevent melting. Bonzom originally showed that these pitch foams could be used as floor coverings, as insulators, or as surfaces for the collection of solar energy. Further heat treatment to 2400 C was performed to convert the pitch foam to carbon foam. Since the precursor pitch is isotropic in nature, oxidative stabilization rendered the structure nongraphitizable. Therefore, the foam was not converted to a graphitic structure and remained primarily a noncrystalline carbon material. The carbon foams could be used as thermal insulation, catalyst supports, or as filters for corrosive products. In addition, because of its noncrystalline nature, the carbon foam can be activated to produce a highly adsorbent material for separation of liquids and gases.

Another method, developed at West Virginia University in the late 1990s by Stiller et. al., uses coal-derived pitch, an even cheaper precursor than the petroleum-derived pitches used by Bonzom et al. [9]. The process begins by hydrogenating and deashing bituminous coal in a specialized reactor at temperatures between 325 and 450 C at a hydrogen overpressure of about 3.4–17.2 MPa for between 15 and 90 min. Tetra-

2.6 Carbon Foams 145

lin can be employed as a proton-donating agent. After the reactor has cooled, the contents are removed and, if necessary, the tetralin is separated by distillation. Then the product is deashed according to ref. [54]. The resulting deashed hydrogenated coal can be extracted with tetrahydrofuran (THF), and the solution filtered to remove inorganic matter. Under these hydrogenating conditions, more than one-half of the coal mass is rendered soluble in THF. The THF portion contains all of the asphaltenes, or coal-derived pitch precursors, and oils. When extraction is complete, the THF can be evaporated for recycling, and the recovered coal-derived pitch precursor separated by employing a suitable solvent, such as toluene. The toluene-soluble fraction is generally referred to as “oils”, and the remainder as the asphaltene fraction or coal-derived pitch-precursor fraction. After separating the asphaltenes from the oils by decanting, the asphaltenes are foamed by heating at about 325–500 C and pressures up to 100 MPa under inert conditions (argon or nitrogen). The pressure is maintained at a generally constant level by means of a ballast tank and throttle valve. Pyrolysis of the asphaltenes produces volatile components which generate a foam at these high pressures and increased pitch viscosity. The foam is then soaked for up to 8 h to sufficiently coke it and render it infusible. After coking, the foamed material is calcined at a temperature substantially higher than the coking temperature, between 975 and 1025 C, to remove any residual volatile material. The carbonized foam is then subjected to graphitization temperatures of up to 2600 C to further heat-treat the material and increase its strength (see Fig. 5). Despite these temperatures, the material is not generally graphitized into a highly crystalline structure, but is only partially graphitic with small crystalline regions dispersed among more noncrystalline regions. Variations of this process have been commercialized by Touchstone Research Lab.

Figure 5 Foams produced from coal-derived pitches by West

Virginia University (sample courtesy of Al Stiller).

In the late 1980s, research demonstrated that production of fibers from mesophase pitch could result in graphitic fibers with very high moduli. The unique discotic nematic liquid-crystal precursor, mesophase pitch, resulted in highly oriented graphitic structures aligned along the fiber axis. During spinning, the mesophase crys-