mm Processing of Thick-

103 Figure 4-18: Photographs of free-standing structures: a original designed model, with length, l and width, w ; b samples of fabrication outcome. Figure 4-19: Sample A1 with no additional PZT covers on the upper electrode. Polarisation is the final fabrication step, which is needed to induce remnant polarisation in piezoceramic materials before they are piezoelectric. An electric field up to several MVm typically 2-5 MVm is applied to the upper and lower electrodes of the sandwich structure, at an elevated temperature typically 80-150 C. The electric field is applied for around 30 minutes, and the sample is allowed to cool down to room temperature for another 30 minutes before the electric field is removed. This is to prevent the sample from depolarising below the Curie temperature. The influence of poling conditions such as poling temperature, poling electric field and poling duration on piezoelectric properties of thick-film PZT had been studied by Dargie et al [104]. Top View Side View Potential Free-Standing Structure Length, l b Width, w b Solder Pad Sacrificial Layer a b 104 The experimental results showed that a higher piezoelectric charge coefficient, d 33 was measured at a higher polarisation temperature of 150 °C. Combining with polarisation field strength of 2.5 MVm, a maximum value of coefficient was obtained at about 200 pCN. Further increments of the electric field strength did not show any improvement in the piezoelectric activity. Some of the plated and IDE fabricated samples were polarised to further investigate their mechanical and electrical properties in the following chapters. A set-up of the polarisation is shown in Figure 4-20. The samples were polarised with different field strengths at a constant temperature of 200 °C on a hot-plate. It was found that, the piezoelectric layer suffered electrical short circuits when it was polarised with an electric field strength greater than 5 MVm. Therefore, all the samples were polarised at slightly lower field strengths to prevent the high voltage from damaging the device. All the successful polarised samples are listed in Table 4-2. Figure 4-20: Polarisation set-up. Sample Cooling Fans Hot Plate Poling Rigs 105 Table 4-2: Summary of Polarised Samples. Sample Geometry Poling Process Length mm Width mm Total Thickness um VV Temp C Time min A1 13.5 9 114 180 200 30 2PZT + 2SP + 4PZT + 2SP B1 11.25 9 114 180 200 30 2PZT + 2SP + 4PZT + 2SP C1 6.75 9 135 200 200 30 2PZT + 2SP + 4PZT + 2SP + 2PZT C2 9 9 135 200 200 30 C3 13.5 9 135 200 200 30 C4 18 9 135 200 200 30 D1 4.5 9 124 220 200 30 2PZT + 2SP + 4PZT + 2SP + 1PZT D2 6.75 9 124 220 200 30 D3 9 9 154 220 200 30 D4 11.25 9 124 220 200 30 D5 13.5 9 124 220 200 30 D6 18 9 124 220 200 30 IDa1 9 9 208 300 200 30 2PZT + 2SP + 4PZT + 2SP + 2PZT Note: 2PZT + 2SP denotes a process of printing and drying two layers of PZT followed by print and dry two layers of silverpalladium. All the samples are co-fired with Profile 850. 106

4.8 Conclusion

A one-step air co-firing technique was used to fabricate free-standing structures, where the different thermal expansion coefficient between the conductors and cermets was exploited. The method was based on a combination of conventional thick-film technology and a sacrificial layer technique. A carbon sacrificial layer was printed on a substrate followed by a series of prints of PZT pastes and AgPd pastes one after another and co-fired together at a peak temperature of 850 C in a multi-zone furnace with a constant air flow. At the same time that the carbon sacrificial layer was burnt out, the composite films were sintered, resulting in solid and firm free-standing structures which were released at the end of the process, as a result of pre-stress effect. The resultant PZT cermets were found to be brittle and fragile, and were not able to establish on their own. From the experiments, the AgPd conductors were found to be able to support the cermets structures, besides acting as the electrodes. Higher thermal expansion coefficient compared to the PZT piezoceramic is the major weakness of AgPd conductor. As the rate of expansion and contraction of AgPd conductors are faster than the cermet, the films suffer from warping effect after a co-firing process. This problem was solved by printing an additional layer of PZT cermet on each of the exposed areas at the lower and upper electrode layers. The fabricated free-standing structures were in the form of an elevated free-standing structure with a gap height of 2 mm and a flat beam extended from the S -beam. The overall structures were shrunk by about 10 from the original design. Finally the samples were polarised to increase the piezoelectric activity in the PZT layers. 107

Chapter 5 Piezoelectric Materials

Characterisations

5.1 Introduction

Five series of samples as described were characterised; A, B, C, D and IDa. Samples A, B, C and D are multilayer cantilever structures with plated electrodes, while sample IDa is a cantilever with interdigitated IDT electrodes. Each series of samples was fabricated in the same way, but with slight differences in the printing process as summarised in Table 4-. Firstly, the thickness of the samples was measured with a Solder Paste Inspection Data Analyst SPIDA system. SPIDA is a non-contact, optical inspection and measurement system designed for measuring wet or dry solder paste deposits, which is suitable to measure the thickness of thick-films deposited on rigid substrates. In order to investigate the structural and electrical properties of the piezoceramic samples, the free-standing part of the samples were detached from the base. In this condition, the samples are flexible and easier to handle. The surface and structural properties of the free-standing piezoceramic samples were inspected using a Scanning Electron Microscope SEM at magnifications of 300, 800 and 4000. Two series of samples similar to sample D but co-fired at different profiles with peak temperatures of 850 °C and 950 °C were also inspected. These samples were also measured for their piezoelectric charge constant, d 33 using the Berlincourt method. A dynamic measurement method, however, requires an external excitation voltage to produce the