Stable, highly concentrated suspensions for electronic and ceramic materials applications
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Highly concentrated solid-in-liquid suspensions find applications in a number of areas such as electronics, ceramics, paints, coatings, etc. Highly loaded, stable suspensions which exhibit desirable rheological characteristics (moderate viscosity, shear thinning behavior, thixotropy, and a small yield stress, for example), and which have high thermal or electrical conductivities are frequently sought after. We describe here some techniques which can be used to obtain such highly concentrated suspensions. These involve employing mixed size grades of particles and effective dispersants. For thermal paste applications, for example, compliant pastes of up to 78 vol. % solids with thermal conductivity values as high as 6 W/mK (hence, a few times greater than the values reported previously by others), low electrical conductivity, and moderate viscosity have been prepared by mixing different particle size grades of materials such as A12O3, SiC, A1N, Al, and diamond. Effective dispersants, both commercial as well as those synthesized in our laboratory as novel variations of previously known molecular architectures, have been used to facilitate the achievement of these very high loading and stable suspensions.
I. INTRODUCTION
Stable, concentrated solid-in-liquid dispersions find applications in a number of areas such as electronics, ceramics, paints, and coatings. In the electronic packaging industries, for example, concentrated suspensions are used in substrate preparation, screen printing of circuits, and in thermal grease preparation. In order to enhance the performance (computer speed, for example) and/or to miniaturize electronic devices, denser packaging of the components is essential. However, such dense packagings are realizable only when the large amount of heat generated is rapidly carried away from the active components. Thermal grease or joint compound is a material which plays a pivotal role in minimizing the contact resistance between the heat source (integrated circuit chips or other hot regions) and heat sink.1 Of course, the use of thermal joint compounds is not restricted to computer and other electrical and electronic device applications; it is also useful in a number of areas where an effective thermal contact is sought, such as between a thermocouple probe and the heat source and between two bolted metal parts which necessarily trap an air gap and across which heat is transferred.2 For effectiveness one prefers the thermal compound to be a highly thermally conducting, electrically insulating (if necessary), Theologically cona
'l. Sushumna is currently associated with IBM, East Fishkill, New York. b) Address correspondence to E. Ruckenstein. 1082 http://journals.cambridge.org
J. Mater. Res., Vol. 6, No. 5, May 1991 Downloaded: 16 Mar 2015
venient (moderate viscosity and yield stress, shear thinning, and in some cases thixotropy), easily applicable and removable, compliant, stable material. A stable, highly concentrated solid-in-liquid dispersion can be tailor-fit to satisfy all these requirements. V
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