Abstract
The objective of the current work was to establish a viable manufacturing technology for 1-3 piezoelectric ceramic/polymer composites, capable of meeting the high volume production requirements. To this end, an extensive development and fabrication programme has been successfully completed, which demonstrates the feasibility of the injection moulding process as a potential fabrication route. The technology will make available to the MOD a new generation of low-cost transducers tailored to perform in the frequency range suitable for underwater applications. The piezoelectric ceramic used throughout this study was PZT-5H. During the course of the investigation two types of feedstock formulation were evaluated. The first employed a binder system developed in-house based on polypropylene, microcrystalline wax and stearic acid. The second formulation employed a commercial binder system designed specifically for ceramic/metal injection moulding. In the case of the 'in-house' binder, two formulations were prepared with approximately 62% by volume PZT-5H but with differing ratios of polypropylene to microcrystalline wax. Three formulations were prepared with the 'commercial' binder system containing approximately 55%, 58% and 60% by volume PZT-5H respectively. Compounding by twin screw extrusion produced a feedstock consisting of discrete particles of PZT-5H, approximately 0.5 ?m in diameter, homogeneously distributed within the organic binder. All formulations displayed shear thinning behaviour when evaluated by capillary rheometry and had shear viscosities below 1000 Pa s at a shear rate of 100 s-1 Further, all formulations displayed tension thinning behaviour-and were characterised as viscoelastic. Attempts to produce an acceptable 1-3 preform moulding from feedstock based on the in-house binder system were unsuccessful. It was necessary to reduce the ceramic loading to 30% by volume before a complete 1-3 preform could be moulded. The failure to mould preforms from the in-house formulation was attributed to its high elongational viscosity. It was therefore concluded that an elongational viscosity of 30 kPa s at a tensile strain rate of 58 S-1 is undesirable for moulding this particular component. Satisfactory conditions were established for the production of 1-3 preform mouldings from the commercial feedstock formulation containing 58% by volume PZT-5H. Optimisation of the injection rate and hold pressure was possible by investigating the variation in pillar-density across the 1-3 preform mouldings. Satisfactory heating schedules for both the binder removal and sintering stages were established. Microstructural characterisation of sintered pillars revealed grain sizes ranging from 2 to 7 ?.m in diameter. Orientation effects in the sintered microstructure were not apparent in samples moulded at different injection rates. The addition of cover plates to poled composites containing epoxy resin was found to increase the hydrophone figure of merit (HFOM) by an order of magnitude. HFOM values measured during the present work for injection moulded 1-3 composites are comparable with similar composites fabricated by the dice-and-fill technique.