CARBON FOAM FOR INSULATING APPLICATIONS

Guenter Rinn, Schunk Kohlenstofftechnik GmbH, Rodheimer Strasse 59, 35452 Heuchelheim, Germany

Abstract Carbon foams, produced by the carbonization and subsequent high-temperature treatment of foamed polymer precursors, can be used as insulating materials in high temperature furnaces. In contrast to fibre based materials, the thermal and mechanical properties of the foam are fairly isotropic. A combination of low thermal conductivity and good mechanical strength can be achieved. In addition, the energy transport at high temperatures by thermal conductivity and convection of gases is suppressed due to the spherical cells with small interconnecting pores of the foam. The standard product with a density of 0.15 g/cm³ and a flexural strength of 3 N/mm² has a room temperature thermal conductivity of 0.22 W/mK; at 1200 °C und ambient pressure the thermal conductivity rises to 0.45 W/mK. For applications in oxidative or corrosive atmospheres and a reduction of particle release in clean environments, it might be necessary to apply coatings onto the surface of the carbon foam. Different coatings were applied and investigated; the most promising coating in a corrosive, silicon monoxide containing atmosphere was pyrolytic graphite. The penetration of silicon monoxide into the pore structure and the conversion of the carbon into silicon carbide could be suppressed effectively.

Introduction In high temperature furnaces carbon based materials are frequently used for insulating purposes because of the excellent temperature stability of carbon. Especially the semiconductor and photovoltaic industry has an increasing demand for insulating material with uniform properties, low thermal conductivity and very high purity levels. High purity carbon foams produced by the carbonization and subsequent high temperature treatment of foamed thermoset polymers are alternatives to carbon fiber based materials, which are frequently used in these applications up to now. Advantages are the isotropic mechanical and thermal properties; in contrast in carbon fiber felts the fiber orientation dominates the thermal conductivity and the mechanical strength and leads to a significant anisotropy. Some details of the production process and the material were reported previously [1]; basic properties of different versions are summarized in Table 1. Table 1. Properties of carbon foams.

Density (g/cm³)

Flexural Strength (N/mm²)

Compressive Strength (N/mm²)

Thermal Conductivity at Room Temperature

(W/mK) 0.04 0.5 0.6 0.19 0.07 1.2 1.5 0.20 0.15 3 5 0.22 0.20 6 11 0.30 0.33 10 24 0.35 0.44 13 37 0.46 0.55 14 42 0.51

The most promising version in terms of mechanical and thermal properties and producibility in reasonable sizes was a foam with a density of approx. 0.15 g/cm³. It was the subject of further investigations and evaluations; its structure is characterized by almost spherical cells with an average diameter of approx. 120 µm with small interconnecting pores (Figure 1-2). Alternate carbon foams from pitch or coal based precursors develop fairly high thermal conductivities in high temperature purification processes because of structural rearrangement and crystal growth [2, 3]. Thermoset polymer based foams maintain their disordered microstructure up to very high temperatures and as a consequence a low thermal conductivity, too.

Figure 1-2. Microstructure of carbon foam with density 0.15 g/cm³.

High Temperature Thermal Conductivity The high temperature thermal conductivity was determined up to 1200 °C in argon atmosphere and up to 900 °C in vacuum (Figure 3). Surprisingly, the increase of the thermal conductivity with increasing temperature was fairly low. Especially the small difference between the thermal conductivity in vacuum and at ambient gas pressure indicates, that the small interconnecting pores suppress the energy transport by the gas molecules to a significant extent. Compared to properties of standard carbon fiber felt materials [5], the carbon foam combine the thermal conductivity of a low density felt with the mechanical strength of a high density felt. High density felts with similar mechanical strength have a thermal conductivity at 1200 °C in argon of approx. 2.2 W/mK.

Figure 3. High-temperature thermal conductivity in argon and vacuum.

Corrosion in Silicon Monoxide Atmosphere In the semiconductor and photovoltaic industry molting silicon is typically held in silica crucibles because of purity reasons. The direct contact between silicon and silica leads to the formation of silicon monoxide, which is gaseous already at the melting temperature of silicon. Therefore, it can penetrate deep into pores carbon structures and react with carbon to form silicon carbide and carbon monoxide.

0,0

0,1

0,2

0,3

0,4

0,5

0,6

0 100 200 300 400 500 600 700 800 900 1000 1100 1200 1300

Temperature (°C)

Ther

mal

Con

duct

ivity

(W/m

K)

Argon atmosphereVacuum (< 0.5 mbar)

A calculation of mass and density changes indicate that the total volume remains almost constant. As a consequence, the reaction does not stop because of the formation of a dense protective layer. Instead the reaction proceeds and is governed by physical-chemical parameters (diffusion, concentration, temperature). After 9 month service in a silicon furnace the carbon foam insulation was removed and inspected. The structural integrity and the insulating capability were still maintained. Only in the most critical zones the surface was greenish in color and could be scratch off easily. Cross sections indicated a corrosion zone approx. 10 mm deep (Figure 4); scanning electron micrographs showed that the surface was completely converted into small silicon carbide crystals, but the original pore structure of the carbon foam was still visible (Figures 5, 6).

Figure 4. Optical micrograph of cross section through corrosion zone.

Figure 5, 6. SEM micrograph of cross section through corrosion zone (top surface and 5 mm deep).

Protective Coatings on Carbon Foam Application of Protective Coatings Silicon carbide was applied by chemical vapor deposition (CVD) in a low pressure process. Similar coatings on graphite materials result in excellent oxidation resistance, indicating a very dense coating without any pores through which oxygen could penetrate into and oxidize the graphite material. However, this technology requires a perfect match of the coefficient of thermal expansion between the base material and the layer, otherwise the protective properties will fail because of the formation of micro cracks. Figure 7 shows a SEM micrograph of the siliconcarbide layer. Pyrolytic graphite was coated in a similar CVD process (Figure 8). Even though the graphite layer would be still reactive in the silicon monoxide atmosphere, it could seal the pores in the carbon foam and could react to a silicon carbide layer in situ.

Figure 7 - 8. SEM micrograph of silicon carbide (left) and pyrolytic graphite (right) coating on carbon foam.

Corrosion Testing A source for silicon monoxide was produced by mixing very fine grained graphite and silica powders in stoichiometric ratio according the equation C + SiO2 → SiO + CO. Graphite crucibles with a diameter of approx. 50 mm were filled with 10 g of the reaction powder, covered with foam samples (with and without coating) and heated in vacuum to a max. temperature of 1800 °C. The test conditions were much more severe than in typical silicon furnaces, but should show differences between the materials after a day instead of months. After these corrosion tests the foam samples were inspected with the naked eye as well as optical and SEM techniques of the surface and cross-sections. Results The foam without coating showed a penetration and reaction depth of approx. 10 mm. The surface and the cross section was rather similar to the corrosion zone of the furnace insulation after 9 month service in normal operation. Different colors in the cross section indicate different reaction mechanisms according to the temperature and concentration profiles. The Figures 9 – 11 show optical and SEM micrographs of the corrosion zone. From the side of the contact zone a crack is traveling into the material; it can be expected that the entire corrosion zone would fall off after repeated cycles. The corrosion process leads to a widening of the interconnecting pores and to the growth of some larger silicon carbide crystals.

Figure 9. Optical micrograph of a cross section through a foam sample without coating.

Figure 10 - 11. SEM micrograph of the corrosion zone of a sample without coating.

On the surface of the silicon carbide coating several cracks were visible by the naked eye already. The cross section showed a reduced penetration depth of the silicon monoxide, but the cracks traveled from the surface up to the interface between converted and unconverted foam. Obviously the mismatch between the mechanical properties and the coefficient of thermal expansion between the base material and the coating make this combination useless for this application. The silicon carbide surface showed almost spherical interconnected particles with a size of approx. 50 µm (Figures 12 – 14).

Figure 12. Optical micrograph of a cross section through a foam sample with a silicon carbide coating.

Figure 13 - 14. SEM micrograph (surface and cross section) of a sample with silicon carbide coating.

The coating with pyrolytic graphite resulted in a substantial improvement: The penetration depth was reduced to < 1 mm and cracks were not observed. The top surface of the coating was completely covered by small silicon carbide crystals (average size < 5 µm), but the graphite layer acts as an interface between carbon foam and silicon carbide and balances the mismatch in the CTEs (Figures 15 – 17).

Figure 15. Optical micrograph of a cross section through a foam sample with a pyrolytic graphite coating.

Figure 16 - 17. SEM micrograph of the top surface and the interface of a sample with pyrolytic graphite coating.

Conclusions and Summary Carbon foams from thermoset polymer precursors can be used as insulating materials in high-temperature furnaces. Especially the combination of low thermal conductivity, good mechanical strength and the isotropy of all thermal and mechanical properties create an ideal material for components which combine insulating and structural tasks. If a protective atmosphere can be maintained by a sufficient flow of inert gases (nitrogen or argon) the material can be used without any protective coatings. Even a minor attack by silicon monoxide in silicon furnaces will give sufficient lifetime, if the material stays untouched in the furnace. Fast corrosion by high concentrations of silicon monoxide in the atmosphere can be prevented by a top coating of pyrolytic graphite on the foam. The dense coating reacts on its surface to silicon carbide, but prevents the penetration of silicon monoxide deep into the material through the interconnecting pores. A direct coating with silicon carbide fails in very high temperature applications because of the mismatch of thermal and mechanical properties. However such a coating may give good oxidation protection in a temperature range from 600 – 1000 °C. Further investigations of the performance of different coatings will be performed.

Acknowledgments Thanks for the measurements of the high-temperature thermal conductivity in argon atmosphere and in vacuum to Dr. Hans-Peter Ebert from the Zentrum für Angewandte Energieforschung, Wuerzburg, Germany.

References [1] Rinn, G., Metz, J., Kehr, D. and Scheibel, T.: Polymer based carbon foams for high temperature insulation applications, Proceedings International Conference on Carbon, Session S 05-09, 159 (2005) [2] Klett, J.W., Hardy, R., Romine, E., Walls, C., Burchell, T.: High-thermal-conductivity, mesophase-pitch-derived carbon foams: effect of precursor on structure and properties, CARBON, 38 (7), 953-973 (2000) [3] Productinformation Pocofoam: www.poco.com/us/Literature/files/Thermal/POCO.Foam.Flyer.pdf [4] Stiller, Alfred H., Plucinski, Janusz and Yocum, Aaron Pat. WO 02/18271 [5] Productinformation Calcarb: www.com/pages/sp-graphs.html and www.calcarb.com/data/cbcfapplicationnotes2.pdf