Aqueous processing and mechanical properties of PLZT green tapes
Introduction
Tape casting [1], [2] is widely used in manufacturing thin sheets of ceramic materials for various electronic applications, such as transducers based on lanthanum-modified lead zirconate titanate (PLZT) piezoceramics. In the past, organic solvents were favored to prepare ceramic slurries for tape casting due to their low latent heat of evaporation and low surface tension. Recently, more efforts have been put on the development of aqueous processing because of safety, environmental, and cost considerations [3]. Hydrogen bonding linkages in water easily lead to flocculation of fine ceramic particles, which makes it more difficult to achieve uniform microstructures in ceramic bodies. Wetting of aqueous slurries is another crucial factor in fabricating thin tapes due to the high surface tension of water. In this respect, the differences between non-aqueous and aqueous tape casting have been reported by Nahass et al. [4].
Tape casting slurries contain organic additives that control the stability and rheological behavior of slurries as well as the strength and flexibility of green tapes [5], [6]. Various binders for aqueous tape casting have been studied, such as acrylic emulsion [7], [8], polyacrylic acid [9], and cellulose binders [10]. Poly(vinyl alcohol) (PVA) is another common binder for ceramic processing, which is extensively used for spray drying and dry pressing of powders [11], [12], [13]. Commercial PVA is usually made from polyvinyl acetate and characterized by its hydrolysis level and molecular weight. The percentage of acetate groups converted to alcohol groups determines the hydrolysis level of PVA, which affects the degree of polymer crystallinity [14]. For high hydrolysis PVAs, the hydroxyl groups on one polymer chain can form hydrogen bonding with hydroxyl groups of another chain as illustrated in Fig. 1(a). Consequently, the polymers will line up and orient with each other. The acetyl groups in a partial hydrolysis PVA act as spacers, which limit the crystallinity by preventing binder chains from close approach as illustrated in Fig. 1(b).
Poly(ethylene glycol) (PEG) and glycerol are two common plasticizers used with PVA binder [11], [12], [14], [15]. Plasticization may depend on the molecular size of the plasticizer. Rambaldini et al. [16] measured the bending strength of green bodies containing 1.2–2 wt.% of PVA and PEG. It was found that PEG 300 has an improved plasticizing effect in comparison with PEG 1000 and PEG 10 000. It is also common to add two or more different plasticizers into a tape casting slurry in order to occupy different size spaces between binder chains for optimal plasticization [15].
Several reports have addressed the effects of binder quantity [17], [18], [19] and binder/plasticizer ratio [10], [20] on mechanical strength and density of ceramic green tapes as well as the rheological properties of slurries. The present work aims to study the effects of molecular size and chemical structures of plasticizers and PVA binders on tensile properties of PLZT green tapes. Since strongly flocculated particles may introduce defects in green tapes and affect their mechanical properties, it is important to achieve a well-dispersed suspension prior to the addition of binder and plasticizer. Colloidal behavior of PLZT suspensions dispersed by polyelectrolyte dispersant was also investigated.
Section snippets
Materials
PLZT powders (Praxair Specialty Ceramics, Woodinville, WA) with an average particle size of 0.49 μm and specific surface area of 7.4 m2 g−1 were used. A polyacrylate ammonium salt (Duramax 3021, Mw=2500, Rohm and Haas, Spring House, PA) was used as a dispersant. This polyelectrolyte dispersant has ionizable groups [-COO(NH4)], which can dissociate in water. The binders utilized for tape casting slurries were four different grades of PVAs (Alfa Aesar, Ward Hill, MA) with varying molecular
Colloidal properties of PLZT suspensions
Fig. 2 shows the zeta potential of a 1 vol.% PLZT suspension as a function of pH, where the isoelectric point is ≈9.5. After adding dispersant, the pH of PLZT suspensions was measured between 9.7 and 10. Therefore, one can expect that the particle surfaces are slightly negative charged. Dissociation of polyacrylate or polymethacrylate type polyelectrolytes in water also strongly depends on pH [24], [29]. At low pH (<4), the polyelectrolyte is almost undissociated, which leads to low charge on
Conclusions
Relationships between organic additives and colloidal properties of tape casting slurries as well as mechanical properties of green tapes were studied using polyelectrolyte dispersant, commercial PVA binders, and different plasticizers. PLZT suspensions were stabilized electrosterically by adsorption of dissociated dispersant molecules on particles. An optimum amount of dispersant was determined by viscosity measurements to be about 1.1 mg per square meter of particle surface area.
Plasticizers
Acknowledgements
This work was supported by the Washington Technology Center under Contract No. 97-A6, Praxair Specialty Ceramics, Inc., and ATL Ultrasound.
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