Tape-casted transparent alumina ceramic wafers
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Ceramic wafers of alumina (Al2O3) were produced by tape casting of aqueous slurry followed by vacuum sintering. The binder system used to form the tape casting slurry is a copolymer of isobutylene and maleic anhydride, which is environmentally friendly, marketed under the name ISOBAM. The rheological properties of the slurries were studied by varying solid loading and binder addition level. Through the optimization of plasticizer addition, green tapes were casted with excellent plasticity and a thickness of 240–740 lm. The tapes displayed a post-sintering thickness of 150–660 lm. The morphologies, as well as the fracture surface and the as-sintered surface of the powder were examined using a scanning electron microscope (SEM). The in-line transmittance of the transparent unpolished Al2O3 wafers with a thickness of 660 lm was found to be 72% at a wave length of 5 lm and 26% at a wave length of 600 nm.
I. INTRODUCTION
Since Howatt first used tape casting to prepare ceramics in 1947,1 tape casting has been considered a promising method for producing thin and flat ceramic sheets with numerous applications in the electronic industries (e.g., substrates,2 microelectronics,3 transducer,4 piezoelectric transformers,5 capacitors,6 etc.), solid oxide fuel cells,7 structural materials, multilayer packages,6,8 and rapid prototyping.9 The thin, flat ceramics can also be patterned, stacked, and laminated to form threedimensional structures. In terms of the medium used to form slurries, tape casting has been classified into two categories: nonaqueous and aqueous tape casting.10–12 Nonaqueous media have been used for industrial production, with benefits including low boiling points, easy evaporation, low surface tension, hydration prohibition, and shortened production time. However, precautions concerning toxicity, flammability, explosiveness, and volatility exist for nearly all nonaqueous media. Because of these drawbacks of nonaqueous media, much attention has been recently focused on water-based systems, which are more cost-effective and less hazardous. Aqueous tape casting has several disadvantages, including low tape green strength, slow drying rate, heightened crack sensitivity, possible material reactivity with water, and increased agglomeration due to hydrogen bonding. Although the disadvantages of aqueous systems has limited their
applicability, many efforts have been devoted to improve aqueous tape casting by optimizing solid loading, and adding dispersants together with plasticizers. The development of aqueous tape casting has been the subject of recent research studies with several different binders, including acrylic latex,13 acrylic emulsion, 14 cellulose ethers, 15 polyvinyl alcohols, 16 and acrylic polymers.17 To achieve dense and homogeneous ceramic sheets, tape casting slurries must be uniformly dispersed and well stabilized. The viscosity, solid loading, and shear thinning behavior of the slurry have significant impacts on the shrinkages during the drying and subsequent densification process. Because of these require
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