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The Development of a Natural Graphite Heat-Spreader
Julian Norley*, Jim J.-W. Tzeng, George Getz, Jeremy Klug and Brian Fedor
Graftech Inc.
12900 Snow Road, Parma, OH 44130, USA
*
Corresponding author: julian.norley@ucar.com, (216) 676-2434
Copyright 2001 IEEE
Abstract
The ongoing need for miniaturization and speed in the
electronics industry has brought about a requirement for better
performing thermal management systems. Thermal
management technology remains a vital part of electronics
innovations for notebook computers, high-performing CPU
chipsets, mobile electronic appliances and power
conversion [1]. Typical thermal management systems consist
of external cooling mechanisms, heat dissipaters, and thermal
interfaces. The primary function of the heat dissipaters, e.g.
heat sinks, is to create the maximum effective surface area
where heat is transferred into and carried away by the external
cooling medium. Performance of a heat dissipater is
conventionally characterized by its intrinsic thermal
conductivity, physical surface area, and pressure drop (or
drag) coefficient [2]. An additional variable, namely heat
spreading coefficient (α), has been introduced by Tzeng [3].
The heat spreading coefficient has to be considered when the
heat dissipater is a thermally anisotropic material. A high
degree of thermal anisotropy reduces the temperature gradient
in the plane of the part and increases the effective heat transfer
area, characteristics that are most desirable for electronics
with high heat-intensity components. The ability to direct heat
in a preferred direction is an additional advantage of an
anisotropic heat-spreader material. Carbon and graphite-
based materials are attracting interest as anisotropic heat-
spreaders, with an additional advantage being their low
density. Most carbon and graphite-based materials used to
date are based around carbon fibers. These are high cost by
virtue of the need to conduct high temperature graphitization
processes to develop the required thermal properties in the
fiber. A new form of graphite heat-spreader material is
described in this paper, based around naturally occurring
graphite. Because this material has been graphitized "by
nature", anisotropic heat-spreaders with high thermal
conductivity can be manufactured without using traditional
carbon fiber-based additives.
Manufacture of Natural Graphite Heat Sinks
Natural graphite, the mineral form of graphitic carbon,
occurs worldwide [4]. The ore usually contains silicate
materials which have to be removed. In its final form it is
classified according to its purity, with high grade containing
95-96% carbon and low grade 90-94% carbon. It is possible
to chemically purify the material to achieve a carbon content
greater than 99.5%.
Graphite is a crystalline form of carbon wherein the atoms
are bonded in flat layers (basal planes) with weaker van der
Waals bonds between the layers. Each of the basal planes is
comprised of hexagonal arrays or networks of carbon atoms.
These basal planes are substantially flat and are oriented or
ordered parallel to and equidistant from one another to form
crystallites. Highly-ordered graphite consists of crystallites of
considerable size, with the crystallites being highly aligned or
oriented with respect to one another and having well-ordered
basal planes. The graphite structure is typically described as
having two axes -- the "a" and "c" axes or directions. In this
context, the "a" axis is in a direction parallel to basal planes
and the "c" axis is in a direction perpendicular to the basal
planes and the "a" axis. Because of the markedly different
nature of the bonding in the "a" and "c" directions, graphite
exhibits structural anisotropy and possesses many properties
that are highly directional, e.g., thermal and electrical
conductivity. Natural graphites possess a very high degree of
structural anisotropy which make them ideal starting materials
for heat-spreader components.
Natural graphite is commonly obtained in the form of a
flake. There are many ways of manufacturing products from
this natural graphite material, depending on the finished shape
required. In the manufacture of flexible graphite sheet for
example [5], natural graphite flake is treated with an oxidizing
agent such as a solution of nitric acid and sulfuric acid to form
an intercalated compound with graphite. Upon heating at high
temperature, the intercalants in the graphite crystal form a gas
that causes the layers of the graphite to separate and the
graphite flakes to expand or exfoliate in an accordion-like
fashion in the c-direction, i.e., the direction perpendicular to
the crystalline planes of the graphite. The result is the
production of particles having a vermicular or worm-like
structure. The expanded flakes are then compressed into
sheets which are flexible and can be formed and cut into
various shapes. The sheets retain the structural anisotropy of
the raw flake. The material has found wide use as a thermal
interface material [3] because of its combination of high
thermal conductivity and excellent surface conformability.
For heat-spreader applications, the same high thermal
conductivity properties are important, together with the low
density afforded by a graphite-based material. Described
below are two forms of natural graphite heat-spreader
developed to date, one of which uses a laminating process to
produce thick plates of graphite, and the other which uses
natural graphite in the form of a molding compound to allow
manufacture of more complex net shapes by compression
molding.
Laminate Material
This material is formed by chemically bonding individual
sheets of graphite together to produce a laminate up to
1.5 meters wide and several centimeters thick. The material
can be heat treated after bonding to prevent outgassing during
operation. If required in other than plate form, the material
can be subsequently machined into components; materials at
various stages of processing being shown in Figure 1. A
typical cross-section through a laminate material is shown in
Figure 2, with the bond lines between the individual graphite
sheets being apparent.
Figure 1 - Examples of Laminated Plate Material.
Figure 2 - Polished Cross-Section Through
Graphite Laminate.
As discussed above, the material properties retain the
characteristics of the graphite flake and are highly anisotropic.
Typical properties of the material are shown in Table 1. All
measurements were performed at room temperature unless
otherwise specified. Thermal conductivity values were
obtained using a thermal diffusivity technique (ASTM
designation C 714-85). Electrical resistivity was measured
according to ASTM C-611 with flexural strength (3 point,
span/thickness = 8:1) and interlaminar shear strength
(span/thickness = 4:1) being measured according to ASTM
standards C1161-94 and D2344-84, respectively. Thermal
expansion coefficient was measured using the Parma
Automated Thermal Extensometer (PATE) method.
Table 1
Properties of Typical Graphite Laminate Materials
Property Units Direction Value
Density g/cm
3
1.33
Thermal
Conductivity
W/m
∙
K In-Plane 233
Thermal
Conductivity
W/m
∙
K Thickness 4.5
Thermal
Anisotropy
52
CTE
(30 °C-100 °C)
10
-6
m/m/°C
In-Plane -0.77
Resistivity
µohm
∙
m
In-Plane 4.9
Young's
Modulus
GPa In-Plane 13
Flexural
Strength
MPa In-Plane 11
Shear Strength MPa In-Plane 0.18
The main feature of the material is its thermal anisotropy.
In-plane thermal conductivity (in-plane is parallel to the sheet
direction) is ~230 Wm
-1
K
-1
, compared to ~4.5 Wm
-1
K
-1
through the thickness of the laminate. This yields a thermal
anisotropy ratio of ~52 and allows for directional control of
heat flow for applications in which this is desired. The low
density of the material (~1.3 g/cm
3
) results in specific in-plane
thermal conductivity values 2.6 x that of aluminum 6061 and
3.8 x that of copper (Table 2). There is, therefore, the
potential of significant weight savings using these graphite
materials. Obviously, no directional control of heat flow is
possible with isotropic metals. The low in-plane resistivity
and negative thermal expansion values are typical of highly
oriented graphitic materials. The Young's modulus and
strength values are typical for a coarse grained graphite. The
material is significantly lower in strength than carbon fiber-
based polymer composites and cannot be considered a
structural material, although this is not an issue for a number
of passive heat management applications. In addition, in cases
where it is required to directly retrofit a metal part with a
natural graphite heat sink, the graphite can be cast within a
metal casing.
The processing of the laminate allows some control of the
density of the material, with densities varying in the range of
1.1 - 2.0 g/cm
3
. Experience with graphite sheet [6] has shown
that increasing density results in an increase in strength and
thermal conductivity and a decrease in electrical resistivity. It
seems reasonable that the thermal conductivity will approach
that of copper as the density of the laminate approaches
2.0 g/cm
3
. The density of the material needs to be optimized
for the particular application.
1 mm
Table 2
Thermal Conductivity Comparison of Graphite Laminates
with 6061 and Copper
Graphite
Laminate
Aluminum
6061
Pure
Copper
Density (g/cm
3
) 1.33 2.7 8.96
Thermal
Conductivity
(In-Plane)
(W/m
∙
K)
233 180 400
Specific Thermal
Conductivity
(W
∙
m
2
/kg
∙
K)
0.175
0.067
0.045
Thermal
Anisotropy
52 1 1
The material contains no binder phase and so it can
withstand temperatures typical for graphite materials (up to
~400 °C in air, up to 3000 °C in a non-oxidizing
environment).
Some interesting composite derivatives of this material can
be produced by laminating graphite with other metals, this
process being well established in traditional markets for
natural graphite sheet in fluid sealing and automotive
applications. As an example, Figure 3 shows an optical
micrograph of a laminate prepared using a perforated copper
sheet. Perforation or tanging of the copper results in
mechanical interlocking of the graphite sheets, increasing the
strength, stiffness and toughness of the material. The thermal
expansion coefficient, resistivity and thermal conductivity of
the material is altered significantly by the addition of copper,
allowing the ability to tailor properties according to the
application.
Figure 3 - Polished Cross-Section of Graphite/Copper
Laminate.
Compression Molded Materials
In the manufacture of natural graphite sheet, various
techniques have been developed for impregnating the
expanded graphite with resin (as an example [7]) so that a
composite material can be manufactured comprising natural
graphite in a resin matrix. It is possible to produce molding
compounds from these materials which can then be
subsequently pressed into shapes using various methods (die-
pressing, isomolding). Depending on the complexity of the
heat-spreader design, it is possible to manufacture near-net
shape components or to machine these blocks into the final
shape. Examples of both net shape and machined products
produced from epoxy-impregnated natural graphite are shown
in Figure 4.
Figure 4 - Examples of Products Manufactured from
Epoxy-Impregnated Natural Graphite.
A typical micrograph of an epoxy/graphite compression-
molded part is shown in Figure 5. An intimate mixture of
natural graphite and epoxy resin is apparent with very little
porosity.
Figure 5 - Polished Cross-Section Through Epoxy/Graphite
Compression-Molded Part.
The properties of the material can be varied over a broad
range according to the proportions of resin and graphite and
the molding technique used. Typical properties of the material
are shown in Table 3. As with the laminate materials it is
possible to produce highly anisotropic materials, with values
of in-plane thermal conductivity as high as 200 Wm
-1
K
-1
, with
corresponding through thickness values being ~ 8 Wm
-1
K
-1
.
The ratio of the in-plane to through thickness thermal
conductivity can be varied through processing changes, and
nearly isotropic properties can be achieved. It is therefore
possible to produce a material in which the relative amounts of
1mm
heat flowing in different directions can be controlled, this not
being possible with aluminum and copper.
Table 3
Typical Properties of Compression Molded Materials
Property
Units
Direc-
tion
Typical
Value
Range
Volume
Fraction
Graphite
% 68 55-100
Volume
Fraction
Resin
% 32 0-45
Density g/cm
3
1.59 1.5-1.9
Specific Heat
Capacity
(25 °C)
J/g°C
0.94 0.71-
1.05
Thermal
Conductivity
W/m
∙
K In-Plane 201 57-202
Thermal
Conductivity
W/m
∙
K Thick-
ness
7.7 7-71
Thermal
Anisotropy
26 1-27
Resistivity
µohm
∙
m
In-Plane 9 9-18
Resistivity
µohm
∙
m
Thick-
ness
98 52-98
CTE
(30-100 °C)
10
-6
m/m°C
In-Plane 20 3-24
CTE
(30-100 °C)
10
-6
m/m°C
Thick-
ness
28 9-41
Flexural
Strength
MPa In-Plane 37 31-40
Young's
Modulus
GPa In-Plane 20 19-24
The density of the materials produced via this method is in
the range of 1.5-1.9 g/cm
3
, so significant weight benefits can
still be achieved compared with aluminum and copper.
The electrical resistivity also varies with direction, and the
degree of anisotropy can again be modified. Typical values
are ~9 µohm•m in-plane and ~100 µohm•m through thickness.
The thermal expansion coefficient is largely dominated by
the epoxy resin, usually resulting in much higher values than
those reported for the laminate material. Reducing the mass
fraction of resin reduces both the in-plane and through
thickness values of thermal expansion coefficient.
The strength and modulus of these materials are
significantly higher than those of the laminate materials (by a
factor of three for strength and two for modulus), by virtue of
the epoxy matrix. Machinability of the compression-molded
materials is also superior.
The upper use temperature of these materials is limited by
the epoxy resin system. Some resin systems (e.g., phenolics)
can be used which can be converted to a carbon matrix during
inert atmosphere pyrolysis to produce a graphite/carbon
composite material. This will extend the temperature
capability of this type of material.
Conclusions
New natural graphite-based heat-spreader materials have
been developed, being available in the form of both laminates
and compression-molded products. The materials are
anisotropic, with the ratio of in-plane to through-plane thermal
conductivity typically ranging from 5 to 50. The ability to
control the anisotropy of these materials offers thermal
engineers flexibility to channel heat in a preferred direction,
and offers new design options that can reduce thermal failures
in electronic devices. In addition, the low density offers the
potential for replacing aluminum and copper in weight-
sensitive applications.
References
1. Edwards, M. R. , “Electronic Technology Drivers and
Their Implications for Thermal Management,” 7th
AIAA/ASME Joint Thermophysics and Heat Transfer
Conference, 1998.
2. Kraus, A. D. and A. Bar-Cohen, Design and Analysis of
Heat Sinks, 1995.
3. Tzeng, J. W. et al., “Anisotropic Graphite Heat Spreaders
for Electronics Thermal Management”, PCIM 2000.
4. Kirk-Othmer Encyclopedia of Chemical Technology 4
th
Ed.
, Volume No. 4, Carbon (Natural Graphite) Pgs. 1097-
1116.
5. Shane, J. H., R. J. Russell, and R. Bochman, “Flexible
Graphite Material of Expanded Particles Compressed
Together”, U. S. Patent 3,404,061.
6. GRAFOIL
®
Engineering Design Manual, Volume One,
Union Carbide.
7. Mercuri, R. A., J. P. Capp, and J. J. Gough, “Flexible
Graphite Composite”, U. S. Patent 6,074,585.
