Nanotechnology-Enabled Sensors

46 Chapter 2: Sensor Characteristics and Physical Effects
2.3.13 Piezoelectric Effect
Piezoelectricity is the ability of crystals that lack a centre of symmetry
to produce a voltage in response to an applied mechanical force, and vice
versa (Fig. 2.20). It was discovered by the Curie brothers in 1880.
Force
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Fig. 2.20 (a) A piezoelectric material. (b) A voltage response can be measured as a
result of a compression or expansion. (c) An applied voltage expands or compresses
a piezoelectric material.
Out of thirty-two crystal classes, twenty-one do not have a centre of
symmetry (non-centro-symmetric), and of these, twenty directly exhibit
piezoelectricity (except the cubic class 432). The most popular piezoelectric
materials are quartz, lithium niobate, lithium tantalite, PZT and langasite.
Many piezoelectric materials are ferroelectric ceramics, which
become piezoelectric when poled with an external electric field (Fig.
2.21). Piezoelectric crystallites are centro-symmetric cubic (isotropic) before
poling and after poling exhibit tetragonal symmetry (anisotropic structure)
below the Curie temperature. Above this temperature they lose their
piezoelectric properties.
Polymers such as rubber, wool, hair, wood fiber and silk also exhibit
piezoelectricity to some extent. Polyvinylidene fluoride (PVDF) is a thermoplastic
material that when poled exhibits piezoelectricity several times
greater than quartz.
Piezoelectric materials are an extremely popular choice for a broad variety
of sensing applications. In Chap. 3, several transducers exploiting the
piezoelectric effect will be presented and examples of piezoelectric materials
employed in nanotechnology enabled sensing applications will be given
in Chaps. 6 and 7.
V
Voltage
(a) (b) (c)