Pyroelectrics are the bridge between ferroelectrics and piezoelectrics. They possess a spontaneous polarisation which is not necessarily switchable by an electric field. If their polarisation is switchable, i.e. they are ferroelectric, then they are mainly used in situations in which ferroelectric properties are required. However, if they are not ferroelectric, then their properties as pyroelectrics are more useful. Pyroelectrics fill an entirely separate niche.
In pyroelectrics, the material has a spontaneous dipole moment, as a result of the ionic positions. This dipole moment induces a polarisation. Depending on the formation of the pyroelectric, the pyroelectric may possess a net polarisation. This polarisation will increase when a stress is applied to the material, as it is piezoelectric. But it will not reverse under the application of an electric field.
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However, the polarisation will also increase when its temperature is raised. Consider, these hysteresis loops of barium titanate at different temperatures.
As you can see, the polarisation increases at 90°C, as a result of a phase transition. Between this temperature and room temperature, the polarisation increases steadily, as a direct relation with temperature, such that:
where p = pyroelectric coefficient (C m-2 T-1).
We have to measure changes in polarisation, as mobile charges will mask the true polarisation.
The main use of pyroelectrics is in a temperature sensitive burglar alarm. This is possible because of the way a polarisation results in a charge on the surface of a pyroelectric. If a burglar enters a house, they emit heat. This increases the temperature of the pyroelectric. As its temperature increases, the polarisation of the pyroelectric increases. This leads to the development of charge on the surface of the pyroelectric, as described by the equation:
This charge will produce a current in a closed circuit, and this can be detected, activating an alarm.
As seen, the pyroelectric has rather limited use. However, it is a rather important type of material, due to the position it holds between piezoelectrics and ferroelectrics, and as such, should be understood.