Rheological and Related Colloidal Aspects of Aqueous Processing that affect the Development of Microstructure
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RHEOLOGICAL AND RELATED COLLOIDAL ASPECTS OF AQUEOUS PROCESSING THAT AFFECT THE DEVELOPMENT OF MICROSTRUCTURE Alan Bleier and C. Gary Westmoreland Oak Ridge National Laboratory, Metals and Ceramics Division, P. Oak Ridge, TN 37831-6068
0. Box 2008,
ABSTRACT Shear flow in a-A120 3 suspensions having a volume fraction of solids () in the range between 0.17 and 0.50 was investigated between pH 4 and 12. It is Newtonian if the magnitude of the zeta potential exceeds a critical value which depends on 4); its value is 39 mV if = 0.40 and 74 mV if 0.50. If this potential is less than the critical value, shear flow is pseudoplastic; its yield value markedly changes (e.g., 0 to >100 Pa) in a slightly ()-dependent, narrow pH range (O). These researchers extended the study to the effects of PAA on the processing of a-Al 2 0 3 :m-ZrO2 composites [3] and found that the stabilizing force when PAA is present is electrostatic. The presence of this polymer requires pH conditions differing from those for the PAA-free case if the association of m-ZrO2 and a-A12 0 3 is to occur. Colloidal forces that effect a net attraction exceeding -8 kTwhen Rt> 1 underpin the association process. [4] The present study also examines the processing of a-Al 2 0 3 :ZrO2 composites but focuses on the use of tetragonal ZrO2 containing ceria (Ce0 2 ) as the crystallographic stabilizer and the rheological behavior of binary suspensions used to prepare pellets by colloidal filtration. Other data include sedimentation and electrokinetic properties; density measurements and scanning electron microscopy are used to evaluate sintered pellets.
Mat. Res. Soc. Symp. Proc. Vol. 180. @1990 Materials Research Society
186
EXPERIMENTAL MATERIALS AND PROCEDURES Materials Alumina and Zirconia Table I summarizes the physical properties of these Concentration is expressed herein powders, after fractionation by size [1]. as volume fraction (4)and ýT for unary and binary systems, respectively). Other Chemicals, HCl and NaOH were used to adjust pH and NaCl was used 3 to maintain ionic strength at 0.01 mol dm- ; these chemicals were of analytical grade (Fisher Scientific). Lastly, distilled deionized water was used.
Procedures Powder Characterization, Electrophoretic mobility, ILE, was measured (4)8 x 10-5) at 200 C using an automated analyzer (Pen Kem, Model 3000); samples Zeta potential (t were subjected to ultrasonication prior to measurement. was calculated from mobility following Henry [6] and the guidelines given by Smith [6b]; thus, experimentally derived t-values are denoted as tH. t,(inmV;20 •)-21.12PE/f1(Xa)
.
(1)
The term, f,, depends [6] on ionic strength, via the Debye-Hickel reciprocal length parameter (x), and particle radius (a). Its values were calculated [6b] to be 1.437 for a-A1 2 0 3 and 1.422 for t-ZrO2 in 0.01 mol dm-3 NaCl. Slurry Preparation and Evaluation. The concentration of a-A1 2 0 3 was varied, such that 0.17]5ý 5O.50 in unary suspensions; the volume-based ratio of concentrations (t-ZrO 2 :a-AlO 3 ) was either 1:9 or 1:4 in binary system
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