Preparation of concentrated aqueous alumina suspensions for tape casting

• Published in: Journal of the European Ceramic Society 1997, Vol.17 (12), p.1393-1401
• Main Authors: Greenwood, R., Roncari, E., Galassi, C.
• Format: Article
• Language: English
• Subjects: Applied sciences; Building materials. Ceramics. Glasses; Ceramic industries; Chemical industry and chemicals; Exact sciences and technology; Miscellaneous; Technical ceramics
• ISSN: 0955-2219; 1873-619X
• DOI: 10.1016/S0955-2219(97)00034-4

Aqueous suspensions suitable for the tape casting of alumina were produced, such that ceramic bodies with very high final sintered densities were obtained (100% of theoretical). This enables high purity, smooth, substrates to be produced without using environmentally damaging solvents. To prevent competitive adsorption of the binder with the dispersant, which leads to an increase in the viscosity and in the elastic modulus of the suspension, the binder was mixed very slowly for a short time in a ball mill. In the presence of the binder the optimum amount of dispersant was 0.3 mg m −2 (dry weight basis). These suspensions were not thixotropic. The minimum amount of binder required to produce a flexible, robust tape was established from tape casting experiments, as was the minimum amount of binder required to produce a tape that did not crack on drying. All suspensions were cast with both blades set at a gap of 0.4 mm and a Mylar tape speed of 20 cm min −1; the removal of air bubbles was achieved by exposing the slip to a vacuum for a few minutes. The final sintered density could be correlated directly with the volume fraction of powder (at a fixed binder content) and directly with the binder content (at fixed volume fraction). Sintering the discs revealed a greater shrinkage in the thickness direction than laterally owing to the non-spherical nature of the particles. However no correlation could be found between the thickness of the green body after casting/drying and the suspension viscosity.