TABLE OF CONTENTS Ceramics typically have a low thermal conductivity and a low TCE, whereas metals have a high thermal conductivity and a high TCE. It is a logical step to combine these properties to obtain a material with a high thermal conductivity and a low TCE. The ceramic in the form of particles or continuous fibers is mixed with the metal to combine the desirable properties of both. The resultant structure is referred to as a metal matrix composite (MMC).
The most common metals used in this application are aluminum and copper, with aluminum being more common due to lower cost. Fillers include SiC, AlN, BeO, graphite, and diamond. Compatibility of the materials is a prime consideration. Graphite, for example, has an electrolytic reaction with aluminum but not with copper.11 Two materials will be described here: AlSiC, a composite made up of aluminum and silicon carbide, and Dymalloy , a combination of copper and diamond.
Aluminum silicon carbide (AlSiC) is produced by forcing liquid aluminum into a porous SiC preform. The preform is made by any of the common ceramic processing technologies, including dry pressing, slip molding, and tape casting. The size and shape of the preform is selected to provide the desired volume fraction of SiC. The resulting combination has a thermal conductivity almost as high as that of pure aluminum, with a TCE as low as 6.1 ppm/ C. AlSiC is also electrically conductive, prohibiting its use as a conventional substrate.
The...
