Aqueous processing of SiC green sheets I: Dispersant
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Poly(ethylene imine) (PEI) has been used as a dispersant for tape casting of SiC powders in aqueous media. The stability of SiC suspensions was studied and characterized in terms of zeta potential, sedimentation, adsorption, and rheology measurements. Zeta potential studies showed that the particle surface was negatively charged in the absence of PEI in the pH 2.5–13 ranges. Adsorptions of PEI increased the zeta potential and led to the shift of isoelectric point from pH 2.4 to pH 10.5. Sedimentation study showed that, in the absence of PEI, SiC slurries were stable around pH 6, while, in the presence of PEI, stabilization could be achieved at a condition of saturated adsorption (1.07 mg/m2) and was related to the high-affinity adsorption in the pH ⳱ 10.5–11.5 range. The rheological measurements showed that SiC slurries (50 vol%) were well stability with slight thixotropical behavior. Finally, the best conditions to obtain a homogeneous stable slurry with high powder loading suitable for tap casting were determined.
I. INTRODUCTION
Tape casting is a prominent process used to produce thin and flat green sheets for the fabrication of ceramic substrates and multilayered structures.1,2 Basically, it consists in the preparation of a well-stabilized suspension composed of ceramic powders dispersed in a solvent, with the addition of dispersants, binders, and plasticizers. Organic solvents, such as alcohols, ketones, or hydrocarbons, are commonly used as the liquid vehicle, but the volatility and toxicity of these solvents lead to increasing interest in research on the aqueous tape casting process. In recent years, slurry formulations with aqueous solvent have appeared in the literature.3–7 A comparison between aqueous and nonaqueous slurries for tape casting has been reported by Nahass et al.8 Compared with solvent-based slurries, aqueous ones have the following drawbacks: (i) lower particle settling rate issues; (ii) greater difficulty to achieve higher solids loading;9–11 (iii) higher crack sensitivity. However, aqueous processing was favored because of safety, environmental, and cost considerations.12–14 Furthermore, a lot of extensive experience from other colloidal processing techniques with the use of water as solvent such as slip casting could also be beneficial for the aqueous tape casting process. As in other wet-forming processes, a homogeneous mixture of the solid, liquid, and additive components is a prerequisite for the manufacture of a
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2012
http://journals.cambridge.org
J. Mater. Res., Vol. 17, No. 8, Aug 2002
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defect-free microstructure in the green tapes and the final ceramic materials. This requires a careful selection and an accurate control of the processing additives to prepare well-dispersed and stabilized slips. In the literature, several studies have been carried out concerning slip casting of SiC powders.15–19 Surface modification of SiC powder was also studied and applied to multicomponent systems.20,21 SiC membranes pre
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