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.