Effect of PZT-AgPd Fabrication Sequence

93 Figure 4-4: Photographs of failed free-standing structure comprising only a PZT and b Agpd materials. Composite films with AgPd printed as the bottom layer and PZT as the top layer were co-fired at 850 °C, produced a free-standing structure which bend inward to the substrate as shown in Figure 4-5 a. This is because the thermal expansion coefficient for the conductor is greater than for the ceramic, therefore expansion of the conductor is faster than the ceramic at high temperature in the furnace. However, once the composite films were cooled to room temperature at the end of the process, the conductors contract faster than the ceramic and cause the structure to bend inward. Composite films with the arrangement the other way round produced a free-standing structure which bends outward from the substrate as shown in Figure 4-6 a. A sequence of composite films with AgPd conductor as the bottom layer was co-fired together with the carbon sacrificial layer to release the structure. The resultant of the arrangement of AgPd-PZT-AgPd A-P-A collapsed inward to the substrate but with a higher rising angle. An extension series of composite layers of A-P-A-P produced side- way curving structures as shown in Figure 4-5 c. Composite layers of A-P-A-P-A seem to be able to pull the films away from the substrate due to the complex combination of expansion and contraction of the composite films. a b 94 Figure 4-5: Composite structures of AgPd conductors and PZT ceramics printed in sequence and co-fired together: a conductor-ceramic A-P, b conductor-ceramic-conductor A-P-A, c conductor-ceramic- conductor-ceramic A-P-A-P and d conductor-ceramic-conductor- ceramic-conductor A-P-A-P-A. In another experiment, a sequence of film printed with PZT layers as the bottom layer and followed by a layer of AgPd results in an upward bending structure as shown in Figure 4-6. This is because the thermal expansion coefficient of the conductor is greater than the ceramic layer, therefore the upper layer of conductor contracts faster than the lower layer of ceramic when cooled down to room temperature at the end of the co- firing process. This effect caused the structures to be pulled away from the substrate. There is also a sign of curl effect at both sides of the free-standing structure. A smoother surface for the free-standing structures was obtained when more layers of film were printed and co-fired together. Figure 4-6 a shows the result of fabrication a b c d 95 with a series of films of PZT-AgPd-PZT-AgPd P-A-P-A. Therefore, it can be concluded that free-standing structures with PZT as the bottom layer act as an important factor to raise the structure away from the substrate. Figure 4-6: a Composite structures of PZT ceramics as the lower layer followed by printed AgPd conductors and co-fired together, b Composite of ceramic-conductor-ceramic- conductor.

4.6.2 Effect of Air-Flow and Co-Firing Profile

In another experiment, free-standing structures with longer cantilever beams were designed and fabricated in a multilayer manner. PZT layers were designed 1 mm longer in perimeter compared to electrode layers. A series of composite samples printed in the sequence of PZT-AgPd-PZT-AgPd-PZT PAPAP were co-fired with 850 Profile. Figure 4-7 shows a sign of rising from the base but fail to maintain the height at the end of the structure and fall back onto the substrate. This maybe because the rate of the contraction and expansion of the bi- material are slow in an arrangement with very little air passage, therefore at the end of the co-firing process the gravity force becomes more dominant than the residual stress of the bi-material structure and hence bends downward to the substrate. This experiment concludes that air flow plays an important role in fabrication of free- standing structures. a b 96 Figure 4-7: A photograph of failure free-standing structure fabricated with reduced air flow. In another experiment with lower co-firing temperature at 550 °C Figure 4-3, the samples were found to be free-standing before the carbon sacrificial layer completely burnt off. This resulted in a free-standing structure as shown in Figure 4-8. The films were released from the substrate to form free-standing structures because the polymer binder of the sacrificial layer was burnt out at 550 °C but the thick-films were not properly cured, therefore the structures were fragile and easily broken. Figure 4-8: Thick-film co-fired with 550 Profile. Samples co-fired with 850 Profile were found to be more robust as shown in Figure 4-9 a. As the temperature of the co-firing was increased to 950 °C, a sign of electro- migration from AgPd to PZT layer can be observed as the structures turned to a darker colour as shown in Figure 4-9 b. It also shows signs of cracks especially near to the base of the structure which was a result of stress caused by the surface tension after the fabrication process. Carbon Residual Adhered to substrate