handbook of modern sensors

3.7 Pyroelectric Effect 77
circuit to measure the charge. Contrary to thermoelectrics (thermocouples) which
produce a steady voltage when two dissimilar metal junctions are held at steady but
different temperatures (see Section 3.9), pyroelectrics generate charge in response
to a change in temperature. Because a change in temperature essentially requires
propagation of heat, a pyroelectric device is a heat-flow detector rather than heat
detector. Sometimes it is called a dynamic sensor, which reflects the nature of its
response. When the pyroelectric crystal is exposed to a heat flow (e.g., from an
infrared radiation source), its temperature elevates and it becomes a source of heat,
in turn. Hence, there is an outflow of heat from the opposite side of the crystal, as is
shown in Fig. 3.26.
A crystal is considered to be pyroelectric if it exhibits a spontaneous temperaturedependent
polarization. Of the 32 crystal classes, 21 are noncentrosymmetric and 10
of these exhibit pyroelectric properties. In addition to pyroelectric properties, all of
these materials exhibit some degree of piezoelectric properties as well: They generate
an electrical charge in response to mechanical stress.
Pyroelectricity was observed for the first time in tourmaline crystals in the eighteenth
century (some claim that the Greeks noticed it 23 centuries ago). Later, in the
nineteenth century, Rochelle salt was used to make pyroelectric sensors. A large variety
of materials became available after 1915: KDP (KH 2 PO 4 ), ADP (NH 4 H 2 PO 4 ),
BaTiO 3 , and a composite of PbTiO 3 and PbZrO 3 known as PZT. Presently, more than
1000 materials with reversible polarization are known. They are called ferroelectric
crystals. The most important among them are triglycine sulfate (TGS) and lithium
tantalate (LiTaO 3 ). In 1969, H. Kawai discovered strong piezoelectricity in the plastic
materials, polyvinyl fluoride (PVF) and polyvinylidene fluoride (PVDF) [20]. These
materials also possess substantial pyroelectric properties.
A pyroelectric material can be considered as a composition of a large number of
minute crystallities, each of which behaves as a small electric dipole. All of these
dipoles are randomly oriented (Fig. 3.23A). Above a certain temperature, known as
the Curie point, the crystallities have no dipole moment. Manufacturing (poling) of
pyroelectric materials is analogous to that of piezoelectrics (see Section 3.6).
There are several mechanisms by which changes in temperature will result in
pyroelectricity. Temperature changes may cause a shortening or elongation of individual
dipoles. It may also affect the randomness of the dipole orientations due to
thermal agitation. These phenomena are called primary pyroelectricity. There is also
secondary pyroelectricity, which, in a simplified way, may be described as a result
of the piezoelectric effect, (i.e., a development of strain in the material due to thermal
expansion). Figure 3.26 shows a pyroelectric sensor whose temperature, T 0 ,is
homogeneous over its volume. Being electrically polarized, the dipoles are oriented
(poled) in such a manner as to make one side of the material positive and the opposite
side negative. However, under steady-state conditions, free-charge carriers (electrons
and holes) neutralize the polarized charge and the capacitance between the electrodes
appears not to be charged (see Fig. 3.23C); that is, the sensor generates zero charge.
Now, let us assume that heat is applied to the bottom side of the sensor. Heat may
enter the sensor in a form of thermal radiation, which is absorbed by the bottom electrode
and propagates toward the pyroelectric material via the mechanism of thermal