39 dielectric material is printed on top of the filler and parts of the substrate, where the part
that printed on the substrate will form a rigid base to support the free-standing structure.
The dried paste is then co-fired in a nitrogen atmosphere. The nitrogen must be used because the filler must not be burnt out before the glass-ceramic has sintered. The
process is repeated to form a multilayer composite film. Finally, the composite film is co-fired in an air environment, where the carbon filler acting as a sacrificial layer is
burnt out without residues, releasing a composite thick-film free-standing structure. The fabrication steps are shown in Figure 2-18.
Figure 2-18: Fabrication steps for thick-film sacrificial layer technique. Alumina
substrate
Carbon-like
filler
Step 1: Carbon-like filler printing
Step 2: Electrode and PZT layers print on top of the filler and
fired in nitrogen environment Electrode
PZT
Step 3: Fire in air environment to burn carbon-like filler
Free-Standing Structure
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2.7 Conclusion
Free-standing structures in the form of a cantilever are interesting features which find application in sensing and actuating. Incorporated with high piezoelectric activity
materials like PZT, the structures can be operated as micro-generators for powering low power microelectronic devices. The micro-generators can be modelled as a single-
degree-of-freedom mass-spring-damper system, where the electrical output power can be estimated and improved with optimised designs. Conventionally, free-standing
structures were fabricated with thin-film and silicon micro-engineering technologies. Thick-film technology, however, has not received significant attention compared with
its competitor technologies, for fabricating free-standing structures. One of the main reasons for this is because piezoceramics are considered too fragile to form free-
standing structures. In this work, studies on the free-standing structures fabricated by thick-film technology will be presented.
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Chapter 3 Free-Standing
Cantilever Structure Designs
3.1 Introduction
Making the reality of ambient vibration energy harvesting using thick-film free-standing structure is very challenging. Some of the challenges include, fabricating a robust
piezoceramic structure, ensuring the structure resonates with the vibration sources, solving the problem of unpredictable ambient vibrations and meeting the minimum
electrical energy requirement.
First and foremost the characteristics of potential vibration sources from the environment have to be investigated before any energy harvester device can be
designed. Once the vibration sources are identified, energy harvesters can be tailored to suit that specific environment. Besides that, the design of the energy harvesters has to be
based on the limitation of the fabrication technology in this case, thick-film technology and the physical constrains of the real device e.g. the maximum allowed
displacement and stress before the device fails to respond accordingly or is broken in order to fabricate a robust piezoceramic structure.
The output voltage and electrical power are the crucial factors in making the device useful. For this reason, the multimorph structure was developed to enhance the electrical
performance of the device. Besides improving the electrical energy output, the multimorph can be deployed as either current source or voltage source depending on the
