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Figure 3-34: Layouts of a plated electrode a and an IDE cantilever structure b.
1 2
3 4
5
1 2
3 4
5 a
b
Gold layer
Carbon sacrificial layer
Electrode Layer
Piezoelectric layer
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3.8 Conclusion
There are a few challenges in designing a free-standing structure for energy harvesting. One of which is the natural frequency of the structure, which has to be matched to the
vibration sources in order to harvest maximum energy. Since thick-film materials are brittle, it is very important to know the maximum allowed stress.
If the structure is overstressed, it would fail to operate accordingly and might lead to fracture. Small and compact are desirable features for miniature energy harvester,
however a cantilever structure needs some room to manoeuvre and therefore the maximum displacement of the cantilever has to be determined so that the structure can
move freely in a confined space. A useful energy harvester has to produce up to a certain level of voltage and electrical power suitable for microelectronic devices. The
challenge of fulfilling these requirements is enormous especially for miniature thick- film devices. The first step toward making a thick-film free-standing micro-generator a
reality is by computational simulations and analytical calculations. Overall, the calculation results are in relatively good agreement with the simulation results.
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Chapter 4 Processing of Thick-
Film Free-Standing Devices
4.1 Introduction
Generally, piezoceramic thick-film fabrication steps are in sequence starting from paste formulation, screen-printing deposition, drying and co-firing and finally poling process.
The major difference for this study is to apply sacrificial layer techniques to fabricate thick-films in three dimensional free-standing forms. The fabrication technique involves
a one-step air co-firing technique, where the active members of the structure were co- fired together with the electrodes in an air environment.
One of the disadvantages of thick-film lead zirconate titanate PZT materials is their brittleness which makes them too fragile to form free-standing structures. Therefore
silverpalladium AgPd, which is more ductile, was chosen to form the electrode. It also acts as the support platform for the ceramic layers to form a robust free-standing
structure.
AgPd electrodes were printed as planar plate and interdigitated IDT patterns on the PZT layers for two purposes; one of which was to study the electrical outputs from the
d
31
and
d
33
piezoelectric effects. Another reason was to use them as a tool to investigate the consequences of the composite film fabrication process with two materials of
different thermal expansion coefficient.
