Effects of Surface Charge on the Processing of Nonaqueous Silicon Slurries
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EFFECTS OF SURFACE CHARGE ON THE PROCESSING OF NONAQUEOUS SILICON SLURRIES
Alan Bleier Oak Ridge National Laboratory,
P. 0. Box 2008,
Oak Ridge TN
37831-6068
ABSTRACT The influence of surface charge on the dispersion aspects of the nonaqueous processing of silicon powder is explained by the relationship between the liquid medium's dielectric constant and the maximum particle size that is stabilized by colloidal forces. The effects of surface charge, derived from the chemistry of the solid-liquid pair or the adsorption of ionic processing aids, are either great, significantly increasing the maximum particle size that is stable against agglomeration from the nanometer range to the micrometer range, or virtually undetectable, contributing nothing to enhanced stability. Guidelines for the processing of silicon powder when surface charge exists are discussed.
INTRODUCTION Quantitative evaluation of the colloidal stability of silicon slurries needs to be developed in order to identify and to predict the relevant processing behavior for routes such as filtration and casting. This need seems greatest for the fabrication of green compacts from nonaqueous slurries. In these systems, London-van der Waals forces [1], surface charges [2], adsorbed species at the silicon surface [3], and solvation or structured solvent [4] combine to regulate the colloidal stability against particle agglomeration. These factors are assessed here using the interactions between two model silicon particles, stressing the effects of surface charge on stability. Whereas the behavior of a concentrated slurry also depends on higher-order interactions, the two-particle interactions introduce the roles of particle size and the dielectric constant. FACTORS AFFECTING STABILITY OF SILICON SLURRIES The stability of a silicon slurry can be assessed by summing the potential energy of interaction from each source just cited. The total energy , VT, is VT=VA÷VDL+VS+VL,
(1)
where VA, VDL, VS, and VL denote the respective binary interaction energies due to London-van der Waals forces, surface charge via the electrical double layer, steric forces, and solvation or structured liquid. Since VL is often negligible, (2)
VT VA +VDL
in
the absence of processing additives,
and
V• vA+ VDL+ vS,
(3)
when a deflocculant or other adsorbing additive is present. If neither surface charge nor adsorbing additives are present, this equation simplifies to Mat. Res. Soc. Symp. Proc. Vol. 249. 01992 Materials Research Society
248
VTVA .
London-van der Waals Forces These forces are silicon powder. [5a]
the primary ones that affect slurry stability for ideal They account for the energy, VA, given by
(5)
VA=-A'd/24H 0 ,
where A%, d, and H. are, in turn, the effective Hamaker constant, the particle diameter, and the interparticle distance, which is taken to be much less than the particle radius, i.e., Ho2ladB, especially when surface charge exits and lads-l/i and VDL is small compared to VA, even if the layer consists of polymer molecules. Clearly, an ionic adsorbed
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