Page 266 - Electrical Properties of Materials

248 Dielectric materials

(a) declines. If the distribution is more random then there is less resultant sur-
face charge and the voltmeter would show a smaller voltage. And of course
Crystal
conversely: less random distribution would result in a higher voltage.
V But what happens while the temperature changes? To monitor that let us
replace our voltmeter by an ammeter (Fig. 10.21(b)) and measure current. The
change of temperature causes a change in the randomness of the dipole mo-
(b) ments, the change of randomness causes a change in surface charge, a change
in surface charge causes a current to flow, in one direction when the crystal is
heated, in the other direction when the crystal is cooled.
A As the temperature rises, randomness increases until a temperature is
reached at which there is a transition to full randomness. This happens at the
Curie point and the transition is called a phase transition. The pyroelectric
Fig. 10.21 coefficients are higher in the vicinity of the Curie point but are strongly de-
(a) Voltmeter and (b) ammeter being pendent on ambient temperature. Therefore, typically, they are used well below
used to measure the condition of a the Curie point.
crystal. There is a variety of crystal structures that give rise to the pyroelectric
effect. The most common one is the perovskite structure, based on crystals
Perovskites with the general chemical formula ABO 3 . One example is BaTiO 3 , which is
ferroelectric as well. The positions of the A, B, and O atoms are shown in
Fig. 10.22.
We need to mention here that pyroelectrics, being piezoelectrics as well,
undergo a secondary effect when heated. Heat may cause thermal expansion
and the subsequent change in dimension, due to the piezoelectric effect, will
B
also cause an electric field to appear. This changes the effective pyroelectric
O coefficient (see Table 10.4) and might be responsible for electrical ‘noise’.
The application that comes immediately to mind is use of the heat sens-
A
itivity of pyroelectrics for infrared detection and imaging. They have many
advantages over detectors based on excitation of electrons across a bandgap.
In particular:
• sensitivity over a wide spectral band
Fig. 10.22 • sensitivity over a wider range of temperatures
ABO 3 perovskite crystal structure.
• low power requirement
• fast response
• low cost.
Figure 10.23 shows a thermal image taken by a thin film array of pyroelectric
detectors. The image is clearly of a road. In the distance we can see people,
and on the left a house. All are brighter because they are warmer.

10.13.3 Ferroelectrics
As mentioned before all ferroelectrics are pyroelectrics; they both have spon-
taneous polarization. The difference between them is that the polarization of
Fig. 10.23 ferroelectrics can be reversed by applying an electric field but that of pyro-
Thermal image from a pyroelectric electrics cannot be reversed. They would break down before the reversal could
camera from Lang S.B. occur.
‘Pyroelectricity: from ancient Why were these new materials, that were discovered to have interesting
curiosity to modern imaging tool’
properties, called ferroelectrics? Just a historical accident. Magnets preceded
(Physics Today 58 (8), 31 (2005)).
these new materials by a couple of thousand years. So when the new materials