Ferroelectricity is the phenomenon in which a spontaneous electric polarization occurs in some materials by the application of an external electric field. Ferroelectricity was observed in Rochelle salt by Valasek in 1920 and the term was introduced in analogy to Ferromagnetism which is the magnetic property of some materials. A very good ferroelectric material is Lead Zirconate Titanate crystals used in piezoelements. The combined properties like piezoelectricity, Pyroelectricity and memory make the ferroelectric materials ideal for making sensors.
Most materials are polarized linearly when subject to an electric field. This phenomenon is called as dielectric polarization. Ferroelectric materials show a spontaneous polarization and are generally referred to as Pyroelectric. In Ferroelectric materials, the direction of polarization can be reversed by applying an electric field. This creates a hysteresis loop.
In the ferroelectric materials, the electric dipoles are coupled to the lattice of the material so that any change in the lattice results in the strength of the dipoles resulting in spontaneous polarization. This causes a change in surface charge leading to a current flow. Two important external forces that cause spontaneous polarization are mechanical force and temperature. The generation of surface charge in ferroelectric materials by stress is called as the piezoelectricity and spontaneous polarization due to change in temperature is called as Pyroelectricity.
Displacive phase and Order-disorder transitions
There are two Ferroelectric phase transitions like Displacive and order-disorder transitions. In Displacive phase transition; polarization catastrophe occurs if an ion is displaced from its equilibrium by an external force .This leads to an asymmetrical shift from the equilibrium of ion positions resulting in permanent ion movement. Barium Titanate is the typical ferroelectric material showing Displacive transition. The ionic displacement in barium Titanate is related to the relative position of titanium ion with the oxygen ion. Lead Titanate is another material showing Displacive shift.
The most important ferroelectric materials used in many applications is the Lead Zirconate Titanate (PZT). It is the combination of ferroelectric Lead Titanate and Anti-ferroelectric Lead Zirconate. To make the Lead Zirconate Titanate material, various compositions of lead Titanate and lead Zirconate are used. In memory applications, composition close to lead Titanate is preferred while in piezoelectric applications, composition close to 50: 50 lead Titanate and lead Zirconate is used.
Ferroelectric property is used to make sensors, capacitors, memory devices etc. Tunable capacitors use the non linear property of ferroelectric material to change the capacitance through tuning. The ferroelectric capacitor has two electrodes sandwiched by a layer of ferroelectric material. The ferroelectric capacitors are Tunable and very small in size compared to non tunable dielectric capacitors.
The hysteresis of the ferroelectric materials during spontaneous polarization is useful to make memory devices ferroelectric RAM and RFID cards.
Ferroelectric material is used as the input devices in ultrasound imaging devices, infrared cameras, fire sensors, vibration sensors etc.
CIRCUITS USING PIEZO SENSOR