CARBON FOAMS AND APPLICATIONS
RICHARD L. SHAO, IRWIN C. LEWIS AND DOUGLAS J. MILLER
GrafTech International Ltd.
Parma, OH 44130, USA
SUMMARY
GrafTech International has developed a new high-strength carbon foam
(GRAFOAM™) product line. The carbon foam is currently produced in densities
ranging from 0.03 g/cm3 to 0.6 g/cm3 with nominal block size of 180 cm × 45 cm × 15
cm. The foam has a novel cell structure, which leads to high compressive strength
and makes it amenable to sealing to achieve vacuum tightness. The carbon foam
properties of low density, low thermal conductivity, high compressive strength, low
gas permeability and outstanding fire resistance make it suitable for use in tooling for
carbon fiber composites, sandwich structures for building, construction and
transportation, and for high temperature thermal insulation.
1. INTRODUCTION
Carbon foams have attracted considerable recent activity because of their properties
of low density coupled with high thermal stability and either very high or very low
thermal conductivity. Carbon foams have been produced from a number of different
precursors including resins [1], coal [2], hydrotreated coal extracts [3], isotropic
pitches [4], and mesophase pitches [5-6].
The properties of these foams differ based on their ability to transform to graphitic
carbon. Carbon foams, which are poorly or non-graphitizing, exhibit low conductivities
suitable for their use as insulating materials. Carbon foams, which are highly
graphitizable, develop high thermal and electrical conductivities [7]. A variety of
applications have been proposed for carbon foams including: thermal management,
composite reinforcement, electrochemical applications and the use in composite
tooling [8].
The structural features vary for the different types of carbon foam. In general, foams
derived from pitches and coal-based precursors have an open cell configuration
consisting of irregular shaped cells with a range of sizes up to greater than 1000 m.
The structure of these foams consists of interconnected cells and solid carbon
ligaments. Mechanical properties such as strength increase with density for carbon
foams. However, they are substantially weaker than conventional bulk graphites due
to their highly porous nature. The foams derived from mesophase or liquid crystalline
pitch develop a crystalline graphitic structure and tend to be lower in strength than the
less crystalline carbon foams.
In this paper, we describe a unique carbon foam product that is suitable for a number
of applications. The foam has a largely closed cell structure composed of a uniform
bimodal distribution of small-sized cells. This carbon foam has a high strength to
density ratio and can be manufactured in large block sizes ranging up to 180 cm in
length.
2. CARBON FOAM PROPERTIES
This new carbon foam is produced at carbonization temperatures above 500ºC. The
properties of the foam change with final carbonization temperature up to about
3000ºC. The foam produces a non-graphitizing carbon as determined by X-ray
diffraction (XRD). Carbon foam blocks are presently produced with densities ranging
from 0.03 g/cm3 to 0.6 g/cm3. Typical properties for foams of selected densities are
shown in Table I.
GRAFOAMTM Grades
Property
Bulk Density
Specific Resistance
Young's Modulus
CTE (30-100°C)
Thermal Conductivity
Flex Strength (4pt)
Compressive Strength
Gas Permeability
FPA-05 FPA-10 FPA-20
units
g/cm3
m
GPa
x10-6/°C
W/mK
MPa
MPa
Darcy
0.08
6800
0.1
2.3
0.1
0.5
1
nm
0.16
2500
0.5
2.3
0.15
1.5
6
nm
0.32
650
2
2.3
0.3
5.5
30
0.2
FPA-35
0.56
400
3.5
2.3
0.3
8
60
0.02
Table I: Carbon Foam Properties
3. CARBON FOAM STRUCTURE
The cell structure of this new carbon foam has been characterized using optical
microscopy, scanning electron microscopy (SEM) and image analysis. Figure 1
shows an optical photomicrograph of the FPA-20 foam with a density of 0.32g/cm 3.
The foam has a unique structure with a bimodal distribution of larger and very fine
cells, all of which have a circular shape. Figure 2 presents an SEM photomicrograph
of the fine porosity of the FPA-35 foam taken at a magnification of 2000X. These
small circular cells have a relatively uniform size range of about 1-3 m. The total
porosity of this foam was measured by an image analysis technique as 79.5%.
Additional quantification results for the foam porosity are summarized in Table II. The
circular nature of the cells is confirmed by the calculated cell aspect ratio close to 1.0.
Figure 1: Optical Micrograph of Carbon
Foam FPA-20
Cell Size (μm)
35  4.7
1.73  0.35
Cell Aspect Ratio
1.16
1.10
Figure 2: SEM Micrograph of Fine
Porosity of Carbon Foam FPA-35
Distribution
96%
4%
Table II: Quantitative Image Analysis of Carbon Foam
FPA-20 with 79.5% Total Porosity
4. APPLICATIONS
The unique combinations of properties of these carbon foams make them excellent
candidates in numerous applications such as composite tooling, sandwich structures,
thermal insulation, etc.
4.1 TOOLING
A variety of materials, including metals, carbon or glass fiber composites, ceramics,
and bulk graphite, are used as tooling in the manufacture of carbon fiber composite
parts. The low bulk density of carbon foams results in a substantial weight savings
over other currently used materials. The new carbon foam offers some unique
features for its use in tooling. The very low CTE of the carbon foam provides a good
match for carbon fiber composite part production. Carbon foam machines far more
easily than metals, resulting in both cost and flow time reduction. Carbon foams have
similar machining characteristics to conventional bulk graphite and are easily
machined to intricate shapes. Additionally, expensive master models can be
eliminated. The fine cell structure of this new carbon foam leads to an excellent
machined surface finish. The low gas permeability allows effective sealing to be
rendered vacuum tight. A sealed carbon foam surface thus can be used directly as a
working tool surface.
Alternatively, face sheets, including carbon fiber/epoxy and carbon fiber/bismaleimide
(BMI) laminates can be attached directly to the carbon foam base structure for use as
a working tool surface. A laminated carbon foam tool could have a substantially
longer service life than a sealed carbon foam tool without a face sheet. We have
successfully demonstrated that after sealing, wet lay-up of prepregs on the sealed
carbon foam surface followed by curing, creates a well-bonded laminate/carbon foam
tool. The laminate is then machined to the final specifications. Such tools are being
evaluated in commercial composite manufacture.
Studies have shown that carbon foam tooling can provide cost saving of 35% to 60%
over other conventional tools [8].
4.2 SANDWICH PANELS
A potentially significant application for the carbon foam is to provide the core material
for composite sandwich structures for use in buildings, construction, transportation,
etc. Carbon foams exhibit outstanding fire resistance as illustrated by the cone
calorimeter results in Figure 3, which show the carbon foam does not exert an
exothermic heat flux due to burning. The properties of fire resistance, thermal
insulation and sound damping in addition to lightweight, are major benefits for carbon
foam cores. Unlike most conventional polymeric core materials, the carbon foam
does not emit toxic decomposition by-products. We have successfully prepared
sandwich structures by attaching the carbon foam to polymer laminates, metals and
other construction materials.
Heat Flux at 25 kW/m2
160
140
kW/m2
120
100
80
60
40
Wood Cores
20
FPA-05 & FPA-10 Carbon Foams
0
0
200
400
600
800
1000 1200 1400 1600 1800 2000
Time (sec)
Figure 3: Cone Calorimeter Test Results on Carbon Foams and Other
Wood Cores
4.3 HIGH-TEMPERATURE THERMAL INSULATION
Carbon foams have outstanding thermal and dimensional stability to over 2500C in
inert atmosphere or vacuum. Their low thermal conductivities and high strength
make the carbon foam ideally suited for load-bearing high temperature furnace
fixtures. Some high-temperature properties of the FPH-20 carbon foam are shown in
Table III.
PROPERTIES
Density (g/cm3)
CTE ( 10-6/C)
1500C
2500C
Specific Heat (cal/gC)
1500C
2500C
Thermal Conductivity in Argon (W/mK)
1500C
2500C
Thermal Conductivity in Vacuum (W/mK)
1500C
2000C
Compressive Strength (Mpa)
0.32
4.3
5.4
0.49
0.52
1.6
2.1
1.6
1.7
9.6
Table III: High-temperature Properties of FPH-20 Carbon Foam
6. CONCLUSIONS
GRAFOAM™ carbon foam is a new commercial product. Carbon foam’s unique
properties makes it suitable for a variety of applications including use in tooling for
carbon fiber composites, sandwich structures for building, construction and
transportation, and for high temperature thermal insulation.
7. REFERENCES
[1]
U.S. Patent 3,302,999 (Feb. 7, 1967) C.V. Mitchell (to Union Carbide
Corporation).
[2]
D.K. Rogers, J. Plucinski, P.G. Stansberry, A.H. Stiller, and J.W. Zondlo,
Procedings of the 45th SAMPE Conference, Long Beach, CA, May 21-25 (2000),
pp.295-305.
[3]
U.S. Patent 5,888,469 (Mar. 30, 1999) A.H. Stiller, P.G. Stansberry, and J.W.
Zondlo (to West Virginia University).
[4]
U.S. Patent 5,961,814 (Oct. 5, 1999) K.M. Kearns.
[5]
U.S. Patent 6,033,506 (Mar. 7, 2000) J.R. Klett (to Lockheed Martin Energy
Research Corporation).
[6]
M.Brow, R. Watts, A. Alam, R. Koch and K. Lafdi, Procedings of the 35th
SAMPE Technical Conference, Dayton, Ohio, September 28 - October 2,
(2003), paper 1.
[7]
J.R. Klett, R. Hardy, E. Romine, C. Walls and T. Burchell, Carbon 38 953
(2000).
[8]
M.M. Rowe, R.A. Guth, and D.J. Merriman, Proceedings of the SAMPE ‘05
Conference, Long Beach, CA, May 1-5, (2005).
____________________________
GRAFOAM™ is a trademark of the UCAR Carbon Company Inc., a subsidiary of
GrafTech International Ltd.