15 Thick-
film PZT materials can be classified as ‘hard’ and ‘soft’, according to their coercive field during field-induced-strain actuation and Curie temperature [30]
. A ‘hard’ piezoceramic has larger coercive field greater than 1 kVmm and higher Curie point
T
C
250 C compared to ‘soft’ piezoceramic, which has moderate coercive field
between 0.1 and 1 kVmm and moderate Curie point 150 C
T
C
250 C .
Examples of ‘hard’ PZTs are Pz26 from Ferroperm Piezoceramics [22] and PZT-401 from Morgan Electroceramics [31]. Their typical applications are high power
ultrasonics for cleaning, welding and drilling devices. Their distinctive characteristics include high mechanical factor, high coercive field, and low dielectric constant, which
make them capable to be used in underwater applications and high voltage generators. Compared to its counterpart, ‘Soft’ PZTs have lower mechanical
Q
-factor, higher electromechanical coupling coefficient, and higher dielectric constant, which are useful
to fabricate sensitive receivers and applications requiring fine movement control, for instant in hydrophones and ink jet printers. Other applications ranging from combined
resonant transducers for medical and flow measurements to accelerometer and pressure sensors [32]. Examples of soft PZTs are Pz27 and Pz29 from Ferroperm
Piezoceramics. Pz27 and Pz29 have similar properties as PZT-5A and PZT-5H respectively from Morgan Electroceramics [31] Appendix A.
2.3 Piezoelectric Applications
The applications of piezoelectric materials can be categorised into sensors, actuators, transducers and generators depending on the type of piezoelectric effect. Sensors make
use of the direct piezoelectric effect, transforming mechanical energy into measurable voltage signal. If the output power from this conversion is large enough to power
microelectronic devices, it can therefore be used as a microgenerator. Actuators transform electrical into mechanical energy by means of the inverse piezoelectric effect.
Finally, transducers use both effects to operate as single devices.
One of the earliest applications of piezoelectric devices was in the area of sonar. They were developed during World War 1 in 1917 in France by Paul Langevin
et al
[33]. It was used as an ultrasonic submarine detector which consisted of a transducer made of
16 thin quartz crystals glued between two steel plates, and a hydrophone to detect the
returned echo.
The successful practical use of piezoelectricity in sonar created intense development interest in piezoelectric devices. Over the next few decades, new piezoelectric materials
and new applications for those materials were explored and developed, for instance, in 1927 Morrison and Horton demonstrated the Quartz crystal clock [34], which had been
developed into various modern day applications such as computers, calculators, digital watches and mobile phones.
With the rapid development in micro-fabrication technology, microscopic devices based on piezoelectric materials were able to be fabricated. One of the earliest examples is the
piezoelectric cantilever developed by Blom
et al
[35]. They used ZnO as the piezoelectric material to sputter on CVD Chemical Vapour Deposition SiO
2
. Later, Lee
et al
[36] used the method to develop a piezoelectric acoustic transducer for the application of highly sensitive micro-phone and micro-speaker.
Due to piezoelectric direct energy conversion between the electrical domain and the mechanical domain and thus prompt response ~ns, the application of piezoelectric
materials has expanded into the detection of atomic masses. Itoh
et al
[37] had developed the first self-excited force-sensing micro-cantilever for dynamic scanning
force microscopy SFM. The devices have two piezoelectric ZnO layers on a SiO
2
film. One of the layers was utilised for excitation and detection of the lever and the other for
its static deflection. Yi
et al
[38] reported both experiment and theoretical investigations of the resonance frequency change of a piezoelectric unimorph cantilever due to the
mass loaded at the tip of the cantilever, which is possible for bio-sensing applications.
As the piezoelectric activity in some materials has greatly improved over time, the electrical energy significantly increased and the idea of energy harvesting became
popular. One of the earliest piezoelectric energy harvesting systems was developed by Umeda
et al
[39] based on mechanical impact using a piezoelectric transducer. However, the details of the materials used to fabricate the transducer were not
discussed. From their initial experiment, they dropped a 5.5 g steel ball bearing from 20 mm onto a piezoelectric transducer which consisted of a 19 mm diameter, 0.25 mm
