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4.6 Experiment Results and Discussion
A few experiments have been carried out to investigate the structure of thick-film free- standing cantilever as a result of different co-firing profiles and fabrication sequence of
PZT-AgPd. Another experiment where AgPd was printed in an IDT pattern exploiting the piezoelectric effect of
d
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, was used as a mean to investigate the role of the AgPd material in supporting free-standing structure. Finally, multilayer composite structures
of PZT-AgPd were fabricated with improvements to produce robust and flat cantilevers.
4.6.1 Effect of PZT-AgPd Fabrication Sequence
There are a few problems faced by piezoceramic free-standing structures. One of which is thermal shock, which may result in structures cracking as an effect of rapid
temperature change during the co-firing process. For a thick-film printed directly on a substrate, the thermal shock can be reduced as the expansion and contraction of the film
is prohibited as it is rigidly clamped to the substrate. PZT films are not able to be free- standing by themselves as shown in Figure 4-4 a, where the films broke off from the
base after the carbon sacrificial layer burnt out.
Figure 4-4 b shows that AgPd films were able cope with rapid temperature change in holding together the film as part of a free-standing structure but the rates of expansion
and contraction of the materials are relatively fast therefore they collapse and adhere to the alumina substrate after the carbon film burnt out forming a wave-like structure.
These experiments conclude that none of the materials is able to be free-standing by itself.
Composite free-standing structures consisted of sandwich layers of piezoceramics and AgPd conductors were investigated. Because the structure consists of two different
materials with two different coefficients of thermal expansion, increasing or decreasing the processing temperature will produce a surface stress on the structure and thus create
a pronounced bending. The direction of bending depends on the arrangement of the layers between ceramics and conductors.
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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
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