Piezoelectric Effect in Rochelle Salt, Quartz Crystal and Tourmaline

Piezoelectric Effect: Some crystalline materials like Rochelle Salt, Quartz and Tourmaline exhibit the Piezoelectric Effect. When we apply an AC voltage across them they vibrate at the frequency of the applied voltage. Conversely, if we mechanically force them to vibrate, they generate an AC voltage of same frequency.

Rochelle Salt: It has a Greatest piezoelectric effect. It is Mechanically Weakest, so break easily. It is used in Microphones, Phonograph pickups, Headsets and Loud-Speakers.

Quartz: It has Moderate characteristics. It is Inexpensive and readily available. It is used in RF Oscillator and Filters.

Tourmaline: It has Lowest Piezoelectric activity. It is Mechanically Strongest. It is Most Expensive so occasionally used at Very High Frequency.

Quartz Crystal: The natural shape of a quartz crystal is a hexagonal prism with pyramids at the ends.
  • We use its slices in Rectangular Slab of thickness t. This slab is mounted between two metal plates. Each crystal has a natural frequency like pendulum.

    • fundamental frequency: f = k/t Where: k = constant and t = thickness of the crystal.

  • Crystal vibrate best at one of its resonant frequencies, usually fundamental frequency. Higher resonant frequencies are called Overtones (multiples of f).

    • If f = 1 MHz then First overtones = 1 MHz, Second overtones = 2 MHz, and Third overtones = 3 MHz.

  • Quartz crystal work well up-to 10 MHz on fundamental frequency. By vibrating them on overtone frequencies, we can reach frequencies up-to 100 MHz.

  • For Higher frequencies, we use Tourmaline occasionally.

  • When crystal is Not Vibrating, it is equivalent to a capacitance Cm (Mounting Capacitance) because it has two metal plates separated by dielectric.

  • When crystal is Vibrating, it is equivalent to Tuned Circuit.

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