1. Piezoelectricity GLY 4200 – Lecture 3 –Fall, 2017
Some crystals, when squeezed in certain directions, will develop a momentary electric current. This effect is known as piezoelectricity.

2. Pierre and Jacques Curie
Pierre and Jacques Curie discovered piezoelectricity in 1881
Image: curie.jpg
Piezoelectricity was discovered in 1880 by Pierre and Jacques Curie, in the mineral quartz.
They were brothers (Jacques was three years older). Pierre married Maria Skłodowska. Pierre and Marie Curie, together with Henri Becquerel won the Nobel Prize for Physics "in recognition of the extraordinary services they have rendered by their joint researches on the radiation phenomena discovered by Professor Henri Becquerel".
They were the first to demonstrate the generation of electricity (surface charges) on well prepared crystals of quartz as a result of mechanical pressure.
When a crystal is subject to stress it may deform elastically. Ions will move.

3. Piezoelectric Effect Diagram
Image: piezo.jpg
If opposite charges move away from or toward each other, a current is developed.

4. Illustration of Piezoelectric Effect
Generation of current by compression
File piezo.avi
If both charges move in the same direction no net current will develop.

5. Lighting a Neon Bulb Demonstration of piezoelectricity File: pbulb.wmv
The current lasts only as long as the ions are in motion. In order for a crystal to show the piezoelectric property it must have a polar axis. In order to have a polar axis the crystal must belong to a non-centrosymmetric crystal class. Non-centrosymmetric means the class does not have a center of symmetry. A polar axis is an axis that possesses different forms at opposite ends of the axis. Twenty-one crystal classes are non-centrosymmetric. One class, the gyroidal class 432, has no polar axis. The remaining twenty classes do have at least one polar axis and therefore are capable of showing the piezoelectric effect.
When a piezoelectric crystal is subjected to an electric current the crystal will physically deform. This effect is known as the converse piezoelectric effect or the electrostriction effect. Discovered in 1881 by the Curie brothers, the converse piezoelectric effect was first used in 1921 to control radio frequencies. When a plate of quartz is subjected to an alternating electric current the plate vibrates. The thinner the slice the greater the frequency of vibration. If the quartz plate is in a radio circuit, the frequency of the circuit is stabilized at the resonant frequency of the plate. Quartz oscillators greatly improved radio transmission and reception. The same phenomenon has been used in recent years in quartz watches which use a circuit to count the vibrations of the plate and use the number of counts to determine time.
Demonstration of piezoelectricity

6. Ion Generation
Cellophane is wrapped around nylon strings and pulled down
The strings repel each other
A gun, containing a piezoelectric crystal, is aimed at the string
Pulling the trigger causes the gun to emit positive ions
Negative ions are emitted when the trigger is released
When the trigger is released, the strings relax
Video: 5E6025.wmv
The beginning of the twentieth century gave birth to most of the classic applications of piezoelectricity. Significant progress in applications was made possible after the discovery of lead zirconate titanate - Pb(Zr,Ti)O3 or PZT - with a very strong piezoelectric response.
Quartz crystals have also been used in piezometers, which detect pressure waves. Such devices can sense the sound waves produced by submarines. Tourmaline piezometers are less sensitive than quartz and can be used to detect large pressure changes such as those produced in atomic bomb blasts. Other common minerals which show a measurable piezoelectric effect are sphalerite, topaz, and low boracite.

7. Polarization Pi = dijkσjk where Pi is the polarization vector,
dijk is the piezoelectric modulus, and
σjk is the stress tensor
Piezoelectricity is a directional property. It is described mathematically as a third-order tensor. Scalers are numbers. Vectors, or first-order tensors, given direction and magnitude. A vector has three components. A second-order tensor, such as the stress tensor, has nine components. Third-order tensors have twenty-seven components, while fourth-order tensors, such as the electrostriction effect, have eighty-one components.