Stabilization of aqueous BaTiO 3 suspensions with ammonium salt of poly(acrylic acid) at various pH values
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Stabilization of aqueous BaTiO3 suspensions with ammonium salt of poly(acrylic acid) at various pH values Jau-Ho Jean and Hong-Ren Wang Department of Materials Science and Engineering, National Tsing Hua University, Hsinchu, Taiwan, Republic of China (Received 18 March 1997; accepted 24 October 1997)
Ammonium salt of poly(acrylic acid) (PAA-NH4 ) has been used to stabilize aqueous BaTiO3 suspensions at various pH values. Adsorption of PAA-NH4 causes the zeta potential to become more negative, although this effect becomes less dramatic as the pH increases. The concentration of PAA-NH4 required to stabilize aqueous BaTiO3 suspensions decreases with increasing pH. The critical amount of PAA-NH4 as a function of pH is plotted in a stability map, as determined by adsorption, rheology, and sedimentation studies.
I. INTRODUCTION 1
In our previous paper, colloidal stability of aqueous BaTiO3 powder suspensions with various amounts of ammonium salt of poly(methacrylic acid) (PMAANH4 ) at different pH values was investigated. Several critical factors such as pH, dissolution of powders, surface chemistry of powders, degree of polyelectrolyte dissociation, and quantity of polyelectrolyte added were identified. At pH , 7, the dissolution of Ba12 from BaTiO3 , which complicated the preparation process of slurry, was observed. At pH . 7, the negatively charged PMAA-NH4 was adsorbed onto the similarly charged BaTiO3 powder through the minority of positive surface sites of Ba12 . The adsorption of PMAA-NH4 decreased with increasing pH, because the PMAA-NH4 was more negatively dissociated, resulting in a larger energy barrier for adsorption and a greater gyration size at a higher pH. The required amount of PMAA-NH4 to obtain stable aqueous BaTiO3 suspensions decreased with increasing pH. A combination of electrostatic and steric stabilization, referred to as electrosteric stabilization, was believed to be operative at pH 7–12 with various concentrations of PMAA-NH4 investigated. A stability map based upon the results of adsorption, rheology, and sedimentation describing the critical amount of PMAANH4 needed to reach colloidal stability of aqueous BaTiO3 powder suspensions as a function of pH was presented. The above results were consistent with those reported2,3 on the system of Al2 O3 with sodium salt of poly(acrylic acid) (PAA-Na), poly(methacrylic acid) (PMAA), and polyacrylic acid (PAA). Cesarano and Aksay2,3 showed that the required polyelectrolyte concentration to stabilize Al2 O3 aqueous suspensions corresponded to the adsorption saturation limit of the powers by the PMAA. As the pH decreased, the adsorption saturation limit increased until charge J. Mater. Res., Vol. 13, No. 8, Aug 1998
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neutralization of PMAA was reached. At high solid contents, moreover, the presence of excess dissociated polyelectrolytes degraded the stability of suspensions, which became more significant as the molecular weight increased. A similar s
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