JEC COMPOSITES MAGAZINE - Issue #46 - January/February 2009 - 63

A major breakthrough was the development of injection moulding compounds incorporating thermally-conductive discontinuous carbon fibres, resulting in composites with reported thermal conductivities as high as 20 W/m.K. These materials open up countless commercial applications, which are discussed further on in the article. Thermal conductivities as high as 180 W/m.K – in the range of aluminium alloys – have been reported with compression-moulded parts. Improved properties result from the use of longer fibres and higher fibre volume fractions. In addition, greater in-plane isotropy is possible compared to injection-moulded parts. As always, cost is a key issue. The price of discontinuous, thermally-conductive carbon fibres is competitive with that of boron nitride particles, which are widely used as thermallyconductive reinforcements. Because of this, there is reason to be optimistic about carbon fibres, since they can produce materials with much higher thermal conductivities. Injection moulding compounds reinforced with thermallyconductive fibres have other significant advantages. Their higher thermal conductivities result in faster cooling, reducing cycle time and part distortion. In addition, they are electrically conductive, and their low coefficients of thermal expansion (CTEs) facilitate the fabrication of precision parts. Furthermore, the unique properties of these materials allow for greater design freedom and part consolidation. A negative aspect of injection moulding is that the process tends to orient fibres, producing parts with anisotropic properties that may be undesirable in some applications. Use of compression moulding compounds can overcome this problem. mechanical equipment, lighting, office machines, and industrial equipment used in corrosive environments. Microelectronic and optoelectronic applications are discussed further on. Examples of current and potential mechanical system applications include: heat exchangers, a diesel fuel pump which uses a thermally-conductive plastic to help keep fuel flowing in subfreezing temperatures, all-plastic car radiators that fit around bumper contours instead of the traditional square box, and tubing to convey paints and adhesives that must be kept at a constant temperature. Microelectronic and optoelectronic thermal management Heat dissipation and thermal stress are critical issues in many microelectronic and optoelectronic (photonic) packaging applications. The former include microprocessors, power semiconductors, application-specific integrated circuits (ASICs), and RF devices. Photonic applications include laser diodes, light-emitting diodes (LEDs), liquid crystal and plasma displays, photoelectric (i.e. solar) devices, and detectors. The criticality of thermal management in microprocessors is highlighted by Intel’s acknowledgement that it has hit a “thermal wall” [4]. The use of multicore chips at least temporarily relieves this situation. Further evidence of the severity of the thermal management problem is the fact that liquid cooling of computers is under serious study throughout the industry. The Apple Computer Power Mac G5 used a pumped liquid cooling system similar to those found in automobiles. The introduction of liquid cooling requires a completely new manufacturing and servicing infrastructure. It also raises significant reliability and cost concerns. Note that regardless of the type of cooling used, thermal stress arising from CTE differences is still an issue. Weight is another important concern for portable and aerospace systems and applications subjected to shock and vibration during shipping or in service. For example, commercial electronic equipment is commonly required to meet 50 g shock loads. Key requirements for electronic and photonic thermal management materials are: 1) high thermal conductivities, 2) to minimize thermal stress affecting reliability, CTEs that are close to those of ceramics (which are widely-used electrical insulators), and semiconductors like silicon, gallium arsenide and indium phosphide [5,6,7]. The CTEs of ceramics and semiconductors range from 2 to 7 ppm/K. Copper has a high thermal conductivity (400 W/m-K), but its CTE is 17 ppm/K, which causes thermal stress-related problems, such as solder joint failure and warping. Aluminium alloys have roughly half the thermal conductivity of copper and their CTEs are even higher (about 23 ppm/K). Traditional low-CTE materials, which date from the mid 20th century, have high densities, and thermal conductivities that are little or no better than that of aluminium. The table shows one widely-used example, copper/tungsten, Fig. 2: Injection-moulded, thermally-conductive carbon/thermoplastic motor cover (source: Cool Polymers). Figure 2 shows an injection-moulded stepper motor part made of carbon/thermoplastic that combines a housing and a heat sink [3]. This component is reportedly half the cost of the aluminium casting it replaces, while being much lighter and requiring no painting. There is a virtually limitless number of potential commercial and industrial applications for these materials, such as heat exchangers, No46 January-February 2009 / jec composites magazine 63

JEC COMPOSITES MAGAZINE - Issue #46 - January/February 2009

Table of Contents for the Digital Edition of JEC COMPOSITES MAGAZINE - Issue #46 - January/February 2009