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Piezoelectricity

Piezoelectricity. Medical Physics Notes: Ultrasound. The Piezoelectric Effect. The piezoelectric effect describes an interchange between mechanical and electrical energy which occurs in certain crystals known as piezoelectric crystals.

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Piezoelectricity

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  1. Piezoelectricity Medical Physics Notes: Ultrasound

  2. The Piezoelectric Effect • The piezoelectric effect describes an interchange between mechanical and electrical energy which occurs in certain crystals known as piezoelectric crystals. • Examples - quartz or the synthetic ceramic, lead zirconate titanate.

  3. Unstressed crystal • In an unstressed state, the centres of symmetry of both the positive and negative ions of such a crystal lattice coincide. • Therefore no effective charge appears on electrodes attached to either side of the crystal. • The charge is balanced.

  4. Stressed Crystal • When the crystal is compressed or extended and, the centres of symmetry of the charges move apart. • They no longer coincide, and give rise to net charge on the electrodes. • Therefore there is a voltage across them.

  5. Applying a voltage to the crystal • If a voltage is applied across an unstressed piezoelectric crystal, the centres of symmetry move. • This deforms the crystal. • An applied alternating voltage gives rise to mechanical vibrations in the crystal. • A maximum response (or resonance) occurs when the applied frequency matches a natural frequency of vibration of the crystal • If these frequencies of vibration are higher than 20,000 Hz they are in the ultrasonic range

  6. The Piezoelectric Transducer • A piezoelectric transducer can operate as: • (a)an ultrasound generator, by applying a ‘stimulating voltage’ of suitable frequency, thereby causing crystal vibrations andthe emission of ultrasonic waves, • (b) an ultrasound detector, by monitoring the piezoelectric voltage developed across the crystal when it is forced to vibrate by incoming ultrasonic waves.

  7. A Typical Transducer for Medical Applications

  8. A Typical Transducer for Medical Applications • The electrodes must be light and they usually consist of thin layers of silver. • The one nearest the patient is connected to the earthed metal case for safety. • The other is called the 'active' or 'live' electrode and is connected via a coaxial cable to either a source of power (when used as a transmitter) or an amplifier and cathode ray tube (when used as a receiver).

  9. Clarity of Signal • Clear distinct echoes are obtained using short pulses. • To generate such pulses, the vibrations must be damped out as quickly as possible. • This is achieved by bonding a damping material (araldite or other epoxy resin) to the back face of the crystal.

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