18.4: Curie Point
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On warming towards the Curie point, above which the spontaneous polarisation of a pyroelectric disappears, the pyroelectric coefficient typically increases as the temperature dependence of the polarization becomes stronger.
The polarisation also depends on the order of the phase transition, covered in more detail here.
The diagram below demonstrates how the polarisation changes near the Curie point.
For second order transitions, the pyroelectric coefficient is observed to be large.
Materials which undergo first order transitions cannot be used for applications as they undergo hysteresis, such that the transition occurs at different temperatures depending on whether the material is being heated or cooled. This makes the Curie point unreliable.
These factors mean that the pyroelectric is typically used at temperatures much lower than the Curie point. This results in pyroelectric coefficients being lower (as they are directly related to the temperature), but less variable with ambient temperature.