5 Chapter 5 presents the experimental results of piezoelectric materials characterisation
using Berlincourt direct and resonant measurement method for determining the properties of the PZT materials. Comparison is made between a clamped and
unclamped sample to verify the analytical model developed by other researchers.
Chapter 6 discusses the results of testing a unimorph free-standing structure under harmonic base excitation. The mechanical and electrical properties of the piezoelectric
cantilever having different lengths are characterised with and without a proof mass attached to the end of the structure. The efficiency of energy conversion is compared
between cantilevers with different lengths and proof masses.
Chapter 7 describes multimorph cantilever structures. These structures are an extension of the unimorph structure arranged in a multi-layer fashion. Experimental results reveal
an improved performance compared to the unimorph structure. Two polarisation modes are studied; series and parallel. The electrical outputs from both of these configurations
are measured and discussed in the chapter.
Chapter 8 considers an alternative approach for wide-bandwidth operations. An array of multi-cantilevers is designed to operate in multi-frequency environments, with the
intention to harvest energy in a broader frequency spectrum. Experiment results of multi-frequency response are presented and discussed in this chapter.
6
Chapter 2 Literature Review
2.1 Introduction
The research in designing and fabricating free-standing thick-film piezoelectric devices for the application of energy harvesting involves the understanding of the mechanics of
vibrating cantilever structures, the mechanic-to-electric conversion mechanism, thick- film materials and fabrication processes. This chapter will give an overview of all the
fundamental knowledge which makes up the back-bone of this book.
This chapter is divided into four main topics: piezoelectricity, vibration energy harvesting, thick-film technology and free-standing structures. In order to understand
the interesting phenomena of mechanical to electrical energy conversion, piezoelectricity is first reviewed. This is followed by a few examples of its applications,
with the main focus on ambient energy harvesting. The relevant progress in energy harvesting technology will be discussed in detail.
The piezoelectric materials are usually fabricated in the form of a cantilever structure. Electrical energy is produced when the cantilever operates in bending mode at resonant
frequency. The cantilever can be fabricated into micro-scale by thin-film and micro- machining technology but as the physical size decreases, the natural frequency of the
structure increases, which is not desirable for ambient energy harvesting. An alternative for fabricating cantilever-type energy harvester is by using thick-film technology, where
the piezoelectric materials are usually printed on a substrate such as stainless steel and need to be manually clamped at one end to form a cantilever. In most cases, these
devices are attached with a proof mass in order to operate at lower vibration levels,
7 which make the whole device bulky in a range of millimetres and with thickness
around 50 µm.
Cantilevers in the form of free-standing structures are one solution for the above mentioned issues. In free-standing form, the piezoelectric materials are more flexible to
move and there are other advantages which will be discussed in this chapter.
2.2 Piezoelectricity
Piezoelectricity is the ability of certain crystals to generate a voltage when a corresponding mechanical stress is applied. The piezoelectric effect is reversible, where
the shape of the piezoelectric crystals will deform proportional to externally applied voltage.
Piezoelectricity was first discovered by the brothers Pierre Curie and Jacques Curie in 1880. They predicted and demonstrated that crystalline materials like tourmaline,
quartz, topaz, cane sugar, and Rochelle salt sodium potassium tartrate tetrahydrate can generate electrical polarization from mechanical stress. Inverse piezoelectricity was
mathematically deduced from fundamental thermodynamic principles by Lippmann in 1881. Later the Curies confirmed the existence of the inverse piezoelectric effect [17].
Figure 2-1: Schematic diagram of the electrical domain: a before polarisation, b during polarisation and c after polarisation.
Poling axis
a b
c
+
