28.6: Fabrication of a KNO_3 Ferroelectric Capacitor
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A capacitor can be made from potassium nitrate (KNO3), which is ferroelectric below 120°C. (The temperature dependence of ferroelectrics will be explained later.) The following video clip shows the construction of a KNO3 capacitor, and the hysteresis loop it displays. The circuit used is the standard Sawyer-Tower circuit.
The result is a hysteresis loop. This arises from the fact that a system does not respond immediately to a given set of external conditions. Rather, there is a history dependence and this is the basis for memory (two states are possible in E=0).
The final hysteresis loop appears like this:
When the field is removed, the polarisation does not disappear like a dielectric. (a→ c). The polarisation which remains after a material has been fully polarised and then had the field removed is called the remanent polarisation (Pr).
Only after a field is applied in the opposite direction to the original polarising field does the polarisation diminish significantly. There is a specific field which results in zero net polarisation (d). This is called the coercive field (EC).
Finally, if a sufficiently strong electric field is applied in the reverse direction, the polarisation will reach its maximum value in the opposite direction (e).
To understand how the polarisation switches we must consider domains more fully.