Discontinuous metal matrix composites (i.e. short fibre and particle reinforced materials) have attained a significant degree of scientific and technological maturity as advanced structural materials. Initial commercialisation has been achieved, with the unique combinations of mechanical and physical properties afforded by metal-ceramic systems proving appropriate for a variety of structural and semistructural applications. In recent years there has been important consolidation in the understanding of basic structural properties in such composites, which are addressed in the present review. The outstanding requirement for an improved understanding of damage tolerance characteristics in these materials is particularly noted. ‘Mesoscopic’ materials architectures (e.g. laminated and functionally graded materials) are also discussed, and the associated potential for development offracture resistant discontinuous metal composite materials highlighted.

Structural performance of discontinuous metal matrix composites

The paper focuses on thermal and electrical properties of copper–graphite composites. Copper–graphite composites in the range of 0–50 vol% of graphite were prepared from the mixture of copper and graphite powder by the powder metallurgy method. Such composites combine high thermal and electrical conductivities provided by copper matrix and low thermal expansion coefficient and lubricating properties due to the graphite phase. We model thermal and electrical conductivities and thermal expansion coefficient using methods of micromechanics in connection with the material microstructure and measure these quantities experimentally to compare with the results of modeling. Cross-property connections between thermal and electrical properties of the composites are established and experimentally verified.

Copper–graphite composites: thermal expansion, thermal and electrical conductivities, and cross-property connections

Cross property connection between the electric and the thermal conductivities of copper graphite composites

Copper-graphite composite: Shear modulus, electrical resistivity, and cross-property connections

High thermal conductivity carbon fibers are orthotropic, having significantly different properties in the longitudinal(650-1 100 W/mK) and radial directions (15-50 W/mK). These fibers can function like thermal "pipes" to directionallytransport heat from one location to another. The potential of such high thermal conductivity materials to facilitate enhancedthermal management in miniaturized electronics systems used in space applications is considered. Visualization of heat flowusing time-resolved infrared imaging and analysis of the capabilities of different fabricated structures for directing heat flowwill be described.Keywords: thermal management, thermal properties measurement, carbon fibers; graphite, ultra-high thermal conductivitygraphite fibers, electronic packaging 1. INTRODUCTION The management of excess heat in electronics applications is becoming an even more significant problem with new developments in the microelectronics industry. The drive towards pushing microelectronics components down to smallerand smaller sizes, packed into closer and closer proximity on circuit boards greatly increases the demands on thermalmanagement systems. Decreasing the temperature at the integrated circuits has a marked role in increasing the reliability andlifetime ofthese devices1. There are a wide variety ofapproaches currently in use as well as another whole suite of conceptsunder development. These include both active and passive means of cooling. One example of an active approach is the useof spray cooling. Here a fluid is sprayed over the components requiring cooling to remove heat by boiling of the fluid. Thefluid is then collected, circulated past a heat exchanger and then used again. Convective cooling using forced air blown overthe part is another example of active cooling which is very widely practiced.Passive approaches include the use ofheat pipes2 which are closed tubes containing a material such as ammonia. There is aphase change in this material which allows heat to be extracted from the part of concern. Another passive method is toimprove the conduction from the high power dissipating component to the backplane of the circuit board or to a dedicatedheat sink. Different materials systems have been used in these heat spreading efforts ranging from the aluminum and copperplates which have had a long history of application to the more recent developments in diamond films3. Another recentdevelopment has been the use of composite plates incorporating graphite fibers with very high thermal conductivity, ic, in thelay-up46. This approach is very attractive for space applications where minimizing weight is a key design goal.The specific thermal conductivity ofthese high ic graphite fibers is particularly impressive as shown in Table I .

D.L. Vrable

Papers

Applications of advanced composites for satellite packaging for improved electronic component thermal management