Analysis and development of an aqueous tape casting ceramic process
The laboratory scale process developed by Navarro [Navarro, 2001 ] for the production of pyroelectric ceramics was used as a case study for the design of a high-level methodology for the scale-up of ceramic processes. A twofold approach was adopted as the basis of the methodology to perform the process scale-up. A "process focussed" approach was used that considered the sequence of processing operations, their feasibility on a larger scale and the potential problems foreseeable for a scaled-up process. Secondly, a "product focussed" approach analysed the quality of the product in respect to the customer requirements (the specifications).This approach also concerned the analysis of the process potential to perform within an essential tolerance interval. This aim was pursued through the use of a statistical technique, the Statistical Process Control. With the product focussed approach, the mechanical and electrical characteristics of the wafers obtained with the laboratory scale process were compared with the industrial requirements. The ceramic wafers possessed satisfactory characteristics except for their strength. The processing parameters were investigated in order to optimise the sintered ceramic microstructure and to understand their relative influence on the achievement of adequate characteristics for allowing handling and machining. Sintered density and grain size were studied as functions of green density and firing temperature. Increasing the green density from the original 37% to approximately 65% of the theoretical density by means of warm pressing allowed the wafers to achieve a more homogeneous microstructure and a higher sintered density up to approximately 97% of the theoretical. Thanks to the increase in green density, the sintering temperature could be reduced by 100°C and the total sintering time was shortened by 190 minutes. The samples produced with warm pressing were more resistant as 75% of them survived during the poling and machining test, in contrast with the 5% of the original samples. The suitability of the warm pressing technique to increase the green density up to 5 g.cm 3 (61 % of the theoretical density) was investigated by means of the statistical process control tool. A correct design of the press equipment was judged the most relevant factor for determining the control and the capability of the process. A rheological study was directed to understand how the slurry ingredients influenced the ceramic suspension, the green tape density and morphology. Two preparation routes were compared in order to discriminate among the effects induced by the electrosteric dispersant and the polyvinyl alcohol binder on the PZT particles suspension. It was shown how the dispersant influenced both the suspension characteristics and the comminution efficiency of the milling. The binder was shown to adsorb onto the ceramic particles. A model was proposed describing the interaction among ingredients which suggested that the adsorbing polymer induced particles flocculation by bridging mechanism, especially when added to stabilised slurries. As a result, PVA was judged inappropriate for tape casting of this PZT powder as it yielded low green density tapes.