Thick-Film Dimensions Measurement Piezoelectric Materials

109 layers of AgPd are necessary to provide this physical support. The electrodes gold and AgPd do not contribute to the piezoelectric activity in the structure and therefore it is desirable that they are thin, which can be realised by printing using screens with higher mesh densities. Thinner and finer electrodes can produce higher definition of print which is important when fabricating multilayer structures as well as reducing the overall cost of fabrication. Figure 5-1: Photographs show: a A SPIDA system set-up and b a thick-film sample under inspection. Laser pointer Device under measurement White Light source Video camera with magnificent up to ×40 Monitor screen “High point” of the laser light “Low point” of the laser light Thickness readings a b 110 The thickness of the films is an important parameter when predicting the mechanical and electrical performance of the device, especially the PZT and the AgPd electrode layers. Three samples for each device were measured after co-firing at 850 °C, the average thicknesses of the devices are listed in Table 5-1. Table 5-1: Thick-film thickness measured with SPIDA system. Thick-Film Material Process No. of Layer Average Thickness µm Gold ESL 8836 Co-fired at 850 °C 1 12 AgPd ESL 9633B 1 12 2 20 The thicknesses of four samples for each of the device were measured before and after co-firing. The effective thickness to be taken into account when predicting the mechanical and electrical performance of a free-standing structure is the thickness after co-firing, which suffers a reduction of around 10 compared to the thickness before they are co-firing. Figure 5-2 shows the thickness of PZT as a function of the number of layer for two conditions; printed-dried and printed-dried-co-fired. The plot shows that the thickness of the printed-dried films increases linearly with the number of layers. However, when the films were co-fired, the overall thickness decreases which becomes significant as the number of layers increases. For instance, eight layers of printed-and- dried PZT produced a thickness of about 125 µm, which reduced to 112 µm after co- firing at 850 °C as shown in Figure 5-2. 111 Figure 5-2: PZT thickness before and after co-firing.

5.3 Thick-Film Free-Standing Samples under SEM

Inspection Multilayer composite structures with five sections of laminar PZT and four layers of AgPd, printed in a sequence of PZT-AgPd-PZT-AgPd-PZT-AgPd-PZT-AgPd-PZT were examined. Figure 5-3 shows SEM images of two multilayer samples co-fired with temperature profiles with peaks at 850 °C and 950 °C. The fabrication process has a very important influence over the formation of microstructure and hence the mechanical and electrical properties of the piezoceramic materials. From the SEM micrographs at a magnification of 300 it can be seen that, sample co-fired at 850 °C profile produced AgPd layers which are in a relatively uniform shape and have a definite separation between the electrode and PZT layers, compared to sample co-fired with 950 °C profile. One of the risks of co-firing at high temperature is the electron migration from electrode to PZT layers, which may reduce the capacity of polarisation of the PZT material. The population of pores and void spaces for both samples are rather similar when inspected under a magnification of 800. The voids are formed by intercrystalline boundaries between the piezoceramic and electrode layers, and spaces between the grains, which range from several to several tens of micrometers. The presence of the 112 voids is largely due to the nature of the screen-printing process and the size of the granules used in the thick-film paste formulation. They are responsible for the reduction of fracture strength [105] and capacity of polarisation, which explains why some piezoelectric material samples experienced internal short circuits when polarised above an electrical field strength of 3 MVm, as discussed in the previous chapter. Figure 5-3: SEM micrographs of samples co-fired at 850 °C and 950 °C under magnification of ×300, ×800 and ×4000. a d b e c f 850 Profile × 300 950 Profile × 300 850 Profile × 800 950 Profile × 800 850 Profile × 4000 950 Profile × 4000 PZT PZT PZT AgPd AgPd AgPd AgPd PZT AgPd AgPd PZT AgPd