Flexible graphite sheet material can be obtained by first intercalating graphite with an intercalating agent to form a graphite intercalation compound that is then exposed to a thermal shock, for example, at a temperature of 700° C.-1,050° C. for a short duration of time (20-60 seconds) to expand or exfoliate the graphite. The exfoliated graphite particles are vermiform in appearance, and are commonly referred to as “worms”. The worm is essentially a network of interconnected, thin graphite flakes, with pores present between flakes that make the worms compressible. The worms can be re-compressed together into flexible sheets (foils or films), referred to as “flexible graphite” or “exfoliated graphite sheet” or “graphite sheet” that can be wound up on a drum to form a roll. U.S. Pat. No. 3,404,061 describes the preparation of flexible graphite from expanded or exfoliated graphite particles.
Most of the graphite flakes in flexible graphite are oriented parallel to the two opposed major exterior surfaces. Although flexible graphite is typically highly electrically conductive (typically around 1,300 S/cm) in the in-plane directions, flexible graphite's through-plane electrical conductivity is significantly less (often only about 15 S/cm). The anisotropy ratio, the ratio of highest electrical conductivity to lowest conductivity values, is typically as high as 86:1 (and often higher than this value). The thermal properties of flexible conventional flexible graphite are similarly highly anisotropic with the in-plane thermal conductivity being many times greater than the through-plane conductivity.
The properties of flexible graphite (such as its density, flexibility and its electrical and thermal conductivity) can be adjusted by incorporating a resin during forming of the material or impregnating it with a resin or another suitable impregnation medium after it is formed. The impregnation medium at least partially fills the pores between the graphite flakes. Resins suitable for impregnation of flexible graphite include phenolic, furan, epoxy and acrylic resins.
During compression or embossing processes, air can become trapped within the flexible graphite as it is compressed. This can cause problems including blistering and/or delamination of the embossed material. This is particularly undesirable for some end-use applications. For example, blistering or delamination in flexible graphite materials can weaken the material and make it more permeable to fluids. The material is also rendered less homogeneous as a result and can exhibit undesirable localized differences in thermal and electrical conductivity. The foregoing problems can be difficult to detect during fabrication and may only surface at a later date. Finally, in applications where the material is subsequently impregnated with a resin, delamination and/or blistering can result in voids in the plate material that become filled with resin. Where the resin employed is electrically nonconductive, this can result in undesirable nonconductive regions within the material.
For thermal management applications, such as heat sinks, heat spreaders and thermal interfaces, flexible graphite offers many advantages over other materials that are commonly used in these applications such as copper, steel and aluminum. For example, relative to these metals, flexible graphite is often lighter, less susceptible to corrosion, has lower thermal expansion and has higher thermal conductivity in the in-plane direction.
The present application relates to methods and apparatuses for processing of flexible graphite that can be used to modify or enhance its properties, in particular for use in thermal management applications.