Tailoring Suspension Flow for the Gelcasting of Oxide and Nonoxide Ceramics
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Table I: Suppliers and properties of the powders used. POWDER MEAN DIAMETER (Ltm) TEP (pHienl Silicon Nitride SN-E 10, Ube Industries, Japz 0.56 5.8 0.39 Zirconia TZ-3YS, Tosoh Corp., Atlanta, GA 7.5 Alumina RCHP-DBM, Malakoff Inc, Malakc 0.55 8.7 SiA1ON AA, Vesuvius Research, Pittsburgh, 0.44 7.2 the sources of the powders. Colloidal stability was assessed from sedimentation behavior. The slurries were prepared by mixing the powder either in demineralized water or in a 15 wt % aqueous solution of the gelcasting monomer, acrylamide. The mixing was accomplished by hand stirring and then agitating the slurry with a mechanical shaker for two hours. The pH of the slurries were varied by using nitric acid, tetramethylammonium hydroxide or by the addition of the basic dispersant, Darvan C, ammonium polymethacrylate (R. T. Vanderbilt Co. Inc., Norwalk, CT). A few samples were ball-milled. The rheological measurements were obtained (RFS 8400, Rheometrics Inc., Piscataway, NJ) in a thixotropic loop mode. The shear rate was varied from 0 -
100 s-1 in 60 s, held at 100 s- 1 for 60 s, reduced to 0 s-I in 60 s, and increased to 100 s-1 in 60 s. The data from the last step of the thixotropic loop were used in the study. These measurements were carried out for A12 0 3 at low and high solids loading in Region I (Regions are shown in Fig. 1) and at high solids loading only, in Region III with the aid of the dispersant, Darvan C. Similar measurements were made for ZrO 2. For Si 3N 4 , measurements were made in all the three stability regions; furthermore, the effects of the addition of a dispersant to the slurry and of milling the slurry, were examined. Measurements were made for SiAlON at high pH (Region III) in water, in the acrylamide solution, and in the acrylamide solution plus dispersant. The non-Newtonian flow properties of the slurries were best represented by the Bingham plastic model, with a yield stress and an apparent viscosity for each slurry [6]. At a shear rate of 38.5 s- 1 and above, the data points were adequately approximated by a linear relationship. RESULTS AND DISCUSSIONS Figure 1 shows the variation of the electrophoretic mobility with pH for each powder. The pHiep obtained from the figure is tabulated in Table I, and these are acidic, neutral to basic.
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Mobility vs pH for A12 0 3 , ZrO2 , SiAION, & Si 3 N 4 ; Regions I, II: Stability, Region III: Instability.
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For each powder in Figure 1, there are three regions of colloidal stability corresponding to (a) the low pH values far below the pHiep (Region I), (b) the high pH values far above the pHiep (Region III), and (c) the pH values around the pHiep (Region II). These regions, based on the absolute electro
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