2. Methods
The present study includes a system for aligning graphite nanofibers to
enhance thermal interface material performance. The system includes the
graphite nanofibers configured in a herringbone configuration and a
means for dispersing the graphite nanofibers in the herringbone
configuration into the thermal interface material. The system further
includes a means for applying a magnetic field of sufficient intensity
to align the graphite nanofibers in the thermal interface material.
Nanofibers can be produced by interfacial polymerization and
electrospinning. Carbon nanofibers are graphitized fibers produced by
catalytic synthesis around a catalytic core. The catalytic core around
which graphite platelets are formed is called a metal seed or a
catalytic metal seed, wherein the catalytic metal seed is a material
having magnetic properties such as iron, cobalt, or nickel. Metal-core
graphite nanofibers can be grown in numerous shapes around a catalytic
metal seed. The metal-core graphite nanofibers comprise of graphite
platelets arranged in various orientations with respect to the long axis
of the fiber, giving rise to assorted conformations. A magnetic field is
applied to the metal catalyst prior to deposition of the graphite
nanofibers on the metal-core. With the application of a magnetic field,
the magnetic poles of the seed are aligned with the magnetic field and
will subsequently carry the attached graphite nanofibers along with them
as they rotate in the applied field following deposition.
Conventional manufacture of expanded graphite requires thermal and
chemical treatment in order to expand the interlayers without completely
breaking or separating the layers between the basal planes. Typically,
the expansion is conducted by treating particles of graphite, such as
natural graphite flakes, with an intercalant, for example, a solution of
sulfuric and nitric acid, such that the crystal structure of the
graphite reacts with the acid to form a compound of graphite and the
intercalant. Upon exposure to elevated temperatures the particles of
intercalated graphite expand in an accordion-like fashion in the
direction perpendicular to the crystalline planes of the graphite
flakes. The resulting expanded, or exfoliated, graphite particles are
vermiform in appearance and are commonly referred to as worms. This
expansion of the particles into worms, rather than separation into
separate platelets, occurs as a result of the Van der Waals forces
securing together the basal planes of the graphite structure. The Van
der Waals forces between the basal planes of the graphite prevent
complete separation of the leaflets. Expanded graphite structures may be
used as particulate in composite materials, such as polymers, to provide
reinforcement and add stiffness, strength and other properties.
Attractive benefits of the use of such particulate-filled polymers in
materials-intensive industries include low cost, weight reduction,
styling potential, superior acoustic characteristics, reduced
maintenance and corrosion resistance.
With a diamond shaped catalytic metal seed, the majority of the graphite
platelets will align along the fiber axis as dictated by an external
magnetic field, so that the catalytic metal seed may have its poles
aligned perpendicular to or parallel to the external magnetic field. The
seed particles are not limited to elongated diamonds, so that the
deposited metal-core graphite nanofiber forms the chevrons. The graphite
platelets can assume any of a myriad of shapes. If the catalytic metal
seeds are rectangular plates, then the graphite platelets are deposited
as plates. If the catalytic metal seeds are cylindrical, then the
graphite platelets are deposited as cylindrical plates. If the catalytic
metal seeds are little bars, then the graphite platelets are deposited
as rectangular solids along the long axis of the rectangular bar. The
graphite platelets assume the geometry of the catalytic metal seed
surface. A magnetic field of sufficient strength to cause the domains
within the catalytic metal seeds to align along the external field is
applied. Application of an external magnetic field pre-aligns the
magnetic poles of the catalytic metal seed by applying a magnetic field
to the catalytic metal seed prior to deposition of the graphite
platelets on the metal-core. The chamber is charged with the reactive
gas mixture. By judicious choice of the catalytic metal seeds catalyst,
the ratio of the hydrocarbon-hydrogen reactant mixture, and reaction
conditions, it is possible to tailor the morphological characteristics,
the degree of crystallinity, and the orientation of the precipitated
graphite crystallites with regard to the fiber axis. The gas mixture
thermally decomposes onto the catalytic metal seed to generate the
metal-core graphite nanofibers. The metal-core graphite nanofibers can
be dispersed in silicone-based gels or pastes that are used as thermal
interface materials that are eventually cured into pads.
The thermal interface material having a thickness between a first
surface and a second surface opposite the first surface. The thermal
interface material further includes a plurality of carbon nanofibers,
wherein a majority of the carbon nanofibers are oriented orthogonal to a
plane of the first surface and wherein the carbon nanofibers comprise a
magnetic catalytic seed. Graphite nanofibers have received considerable
attention in the electronics field due to their remarkable thermal
conductivity [29, 30]. Moreover, the thermal conductivity of
graphite nanofibers is anisotropic [31, 32]. Anisotropy is the
property of being directionally dependent, as opposed to isotropy, which
implies homogeneity in all directions. Therefore, the present study
takes advantage of the anisotropic nature of the graphite nanofibers by
effectively aligning them along the conductive axis, thereby generating
a thermal interface material with exceptional thermal conductivity at
comparatively low loading levels. A thermal interface material is used
to fill the gaps between thermal transfer surfaces, such as between
microprocessors and heatsinks, in order to increase thermal transfer
efficiency [33, 34]. These gaps are normally filled with air, which
is a very poor conductor [35, 36]. A thermal interface material may
take on many forms [37, 38]. The most common is the white-colored
paste or thermal grease [39, 40]. Some brands of thermal interface
materials use micronized or pulverized silver [41, 42]. Another type
of thermal interface materials are the phase-change materials [43,
44]. The phase change materials are solid at room temperature, but
liquefy and behave like grease at operating temperatures.
A phase change material is a substance with a high heat of fusion which,
melting and solidifying at a certain temperature, is capable of storing
and releasing large amounts of energy. Heat is absorbed or released when
the material changes from solid to liquid and vice versa; consequently,
phase change materials are classified as latent heat storage units.
Phase change materials latent heat storage can be achieved through
solid-solid, solid-liquid, solid-gas and liquid-gas phase change.
However, the only phase change used for phase change materials is the
solid-liquid change. Liquid-gas phase changes are not practical for use
as thermal storage due to the large volumes or high pressures required
to store the materials when in their gas phase. Liquid-gas transitions
do have a higher heat of transformation than solid-liquid transitions.
Solid-solid phase changes are typically very slow and have a rather low
heat of transformation. Initially, the solid-liquid phase change
materials behave like sensible heat storage materials; their temperature
rises as they absorb heat. Unlike conventional sensible heat storage,
however, when phase change materials reach the temperature at which they
change phase they absorb large amounts of heat at an almost constant
temperature. The phase change material continues to absorb heat without
a significant rise in temperature until all the material is transformed
to the liquid phase. When the ambient temperature around a
liquid-material falls, the phase change material solidifies, releasing
its stored latent heat. A large number of phase change materials are
available in any required temperature range from -8 up to 200 °C. Within
the temperature range of 20 to 60 °C, some phase change materials are
very effective.
Graphite starting materials for the flexible sheets suitable for use in
the present study include highly graphitic carbonaceous materials
capable of intercalating organic and inorganic acids as well as halogens
and then expanding when exposed to heat. Examples of highly graphitic
carbonaceous materials include natural graphite from various sources, as
well as other carbonaceous materials such as carbons prepared by
chemical vapor deposition and the like. Natural graphite is most
preferred. The graphite starting materials for the flexible sheets used
in the present study may contain non-carbon components so long as the
crystal structure of the starting materials maintains the required
degree of graphitization and they are capable of exfoliation. Generally,
any carbon-containing material, the crystal structure of which possesses
the required degree of graphitization and which can be exfoliated, is
suitable for use with the present study. Natural graphite flakes are
intercalated by dispersing the flakes in a solution containing, for
example, a mixture of nitric and sulfuric acid, advantageously at a
level of about 20 to about 300 parts by weight of intercalant solution
per 100 parts by weight of graphite flakes. The intercalation solution
contains oxidizing and other intercalating agents. Examples include
those containing oxidizing agents and oxidizing mixtures. Alternatively,
an electric potential can be used to bring about oxidation of the
graphite. Chemical species that can be introduced into the graphite
crystal using electrolytic oxidation include sulfuric acid as well as
other acids. The quantity of intercalation solution may range from about
20 to about 200 parts per hundred. After the flakes are intercalated,
any excess solution is drained from the flakes and the flakes are
water-washed.
The particles of graphite flake treated with intercalation solution can
optionally be contacted by blending, with a reducing organic agent
selected from alcohols, aldehydes, and esters, which are reactive with
the surface film of oxidizing intercalating solution at temperatures in
the range of 25 °C and 125 °C. The organic reducing agent increases the
expanded volume upon exfoliation and is referred to as an expansion aid.
The amount of organic reducing agent is suitably from about 0.8 to 8
percent by weight of the particles of graphite flake. Another class of
expansion aids that can be added to the intercalating solution, or to
the graphite flake prior to intercalation, and work synergistically with
the above-described organic reducing agents are carboxylic acids. An
expansion aid in this context will advantageously be sufficiently
soluble in the intercalation solution to achieve an improvement in
expansion. More narrowly, organic materials of this type that contain
carbon, hydrogen and oxygen, preferably exclusively, may be employed.
Suitable organic solvents can be employed to improve solubility of an
organic expansion aid in the intercalation solution. Sulfuric acid,
nitric acid and other known aqueous intercalants have the ability to
decompose formic acid, ultimately to water and carbon dioxide. Because
of this, formic acid and other sensitive expansion aids are
advantageously contacted with the graphite flake prior to immersion of
the flake in aqueous intercalant. The intercalation solution will be
aqueous and will preferably contain an amount of carboxylic acid
expansion aid of from about 0.2 percent to about 8 percent, the amount
being effective to enhance exfoliation. Formic acid is contacted with
the graphite flake prior to immersing in the aqueous intercalation
solution, it can be admixed with the graphite by suitable means,
typically in an amount of from about 0.2 percent to about 8 percent by
weight of the graphite flake.