Electronic applications of flexible graphite

Journal of Electronic Materials, May 2002 by Luo, Xiangcheng, Chugh, Randy, Biller, Brian C, Hoi, Yie Meng, Chung, D D L

Flexible graphite is effective for electronic applications, specifically electromagnetic interference (EMI) gasketing, resistive heating, thermoelectric-energy generation, and heat dissipation. It is comparable to or better than conductive-filled silicone materials for EMI gasketing. The shielding effectiveness reaches 125 dB. Flexible graphite as a heating element provides temperatures up to 980 deg C, response half-time down to 4 sec, and heat output at 60 sec up to 5600 J. The through-thickness, absolute thermoelectric power of flexible graphite is -2.6 (mu)V/deg C. Flexible graphite is effective as a thermal-interface material if the thickness is low (0.13 mm), the density is low (1.1 g/cm^sup 3^), and the contact pressure is high (11.1 MPa). These applications make use of the flexibility and compliance of flexible graphite, in addition to its electronic and thermal behavior. Compliance is particularly important for the use of flexible graphite as interface materials, whether the interface is electromagnetic, thermoelectric, or thermal.

Key words: Flexible graphite, electronic, EMI, thermoelectric heating, heat dissipation

INTRODUCTION

Flexible graphite is a flexible sheet made by compressing a collection of exfoliated graphite flakes (called worms) without a binder.' During exfoliation, an intercalated-graphite flake (graphite compound with foreign species, called the intercalate, between some of the graphite layers) expands typically by over 100 times along the c axis. Compression of the resulting worms (like accordions) causes the worms to be mechanically interlocked to one another so that a sheet is formed without a binder.

Due to the exfoliation, flexible graphite has a relatively large, specific surface area (e.g., 15 m^sup 2^/g^sup 2^). As a result, flexible graphite is used as an adsorption substrate.3'4 Due to the absence of a binder, flexible graphite is essentially entirely graphite, other than the ash and the residual amount of decomposed intercalate (such as sulfur in the case of sulfuric acid being the intercalate). As a result, flexible graphite is chemically and thermally resistant and low in coefficient of thermal expansion (CTE). Due to its microstructure involving graphite layers that are preferentially parallel to the surface of the sheet, flexible graphite is high in electrical and thermal conductivities in the plane of the sheet.5,6 Due to the graphite layers being somewhat connected perpendicular to the sheet (i.e., the honeycomb microstructure of exfoliated graphite), flexible graphite is electrically and thermally conductive in the direction perpendicular to the sheet (although not as conductive as the plane of the sheet).5,6 These in-plane and out-of-plane microstructures result in resilience and impermeability to fluids perpendicular to the sheet. The combination of resilience, impermeability, and chemical and thermal resistance makes flexible graphite attractive for use as a gasket material for high-temperature or chemically harsh environments.

Gasketing (i.e., packaging and sealing)7-11 is by far the main application of flexible graphite, which can replace asbestos. Other than gasketing, a number of applications have emerged recently, including adsorption,3,4 electromagnetic interference (EMI) shielding,2 vibration damping,12 electrochemical applications,13 and stress sensing. 14

This paper is focused on the electronic applications of flexible graphite, namely, EMI gasketing, resistive heating, thermoelectric-energy conversion, and heat dissipation.

ELECTROMAGNETIC INTERFERENCE GASKETING

The EMI shielding is increasingly needed due to the increasing abundance and sensitivity of electronics, particularly radio-frequency devices, which tend to interfere with digital devices. A shielding material needs to be an electrical conductor, although the electrical conductivity does not have to be very high. Due to the skin effect (i.e., the phenomenon that high-frequency electromagnetic radiation only interacts with the surface region of a conductor), a high-surface area of the conductor is desirable. As the electrical conductivity (especially that in the plane of the sheet) and specific surface area are both quite high in flexible graphite, the effectiveness of this material for shielding is exceptionally high (up to 130 dB).2

In addition to conventional shielding applications, flexible graphite can serve as a shielding-gasket material because of its resilience. As the resilience of a polymer-matrix composite decreases rapidly with increasing filler content, the attainment of a shielding gasket using a polymer-matrix composite has been a challenge.15,16

The high-thermal conductivity, low CTE, hightemperature resistance, and excellent chemical resistance of flexible graphite add to the attraction of this material for use in EMI shielding.

The EMI shielding is achieved by using electrical conductors, such as metals and conductive-filled polymers.17-19 The EMI shielding gaskets20-31 are resilient conductors. They are needed to electromagnetically seal an enclosure. The resilient conductors are most commonly elastomers that are filled with a conductive filler or elastomers that are coated with a metallized layer. Metallized elastomers suffer from poor durability due to the tendency of the metal layer to debond from the elastomer. Conductive-filled elastomers do not have this problem, but they require the use of a highly conductive filler, such as silver particles, in order to attain a high shielding effectiveness while maintaining resilience. The highly conductive filler tends to be expensive, making the composite expensive. The use of a less conducting filler results in the need for a large volume fraction of the filler in order to attain a high shielding effectiveness; the consequence is diminished resilience or even loss of resilience. Moreover, these composites suffer from degradation of the shielding effectiveness in the presence of moisture or solvents. In addition, the polymer matrix in the composites limits the temperature resistance, and the thermal expansion mismatch between filler and matrix limits the thermal-cycling resistance. A new class of EMI gasket material is flexible graphite, which is resilient. Although it is not as ductile as silicone, it does not suffer from stress relaxation. Moreover, it is conductive, in addition to being chemically inert and thermally resistant.