The alumina for making a suspension according to this concept must be in the form of a highly pure (preferably >99.99 percent) powder with a suitable particle-size distribution — preferably monodisperse with a mean particle size of a fraction of a micron, as illustrated in the figure. The alumina powder is mixed with an ethanol solution, the pH of which has been lowered to a predetermined value (typically between 1 and 5) as explained below. The amounts of alumina powder and ethanol solution are chosen to obtain the desired concentration of solid material in the suspension. Optionally, the suspension can be made initially in a high concentration (e.g., 20 weight percent alumina), then diluted. During mixing or dilution, the liquid and solid contents are agitated with an ultrasound to break up agglomerations of particles.
The lowering of the pH of the ethanol solution to a predetermined value is effected by addition of a suitable amount of 1N HCl. The predetermination is based on the following principle: In a polar aqueous solution or a polar nonaqueous liquid like ethanol, the surfaces of the particles become electrically charged by amounts that depend on the pH, among other factors. The stability of the suspension depends on the sign and magnitude of the total energy of interaction among the particles. One can choose a value of pH for which the amount of surface charge is large enough that the resulting electrostatic repulsion among particles is sufficient to overcome the attractive London-van der Waals forces that cause agglomeration. Thus, the chosen pH should be far from that value of pH (called "pHpzc" for which the surface charge is zero). In general, different grades of alumina powder and alumina powders from different manufacturers exhibit different values of pHpzc. The pHpzc of the powder to be used in a specific application must be determined precisely by titration or approximately via sedimentation tests.
This work was done by Mark P. Wernet of Lewis Research Center,Gary J. Skoch of the Vehicle Technology Center of the U. S. Army Research Laboratory, and Judith H. Wernet of Case Western Reserve University. For further information, access the Technical Support Package (TSP) free on-line at www.techbriefs.com under the Materials category, or circle no. 143 on the TSPOrder Card in this issue to receive a copy by mail ($5 charge).
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