Piezoelectric Phenomena

Historical Background - The history of piezoelectricity dates back to 1880 when Pierre and Jacques Curie first discovered the piezoelectric effect in various substances includin Rochelle Salt and quartz. Piezoelectric materials can generate an electric charge with the application of pressure; conversely, they can change physical dimensions with the application of an electric field (called converse piezoelectricity). In material having piezoelectric properties, ions can be moved more easily along some crystal axes than others. Pressure in certain directions results in a displacement of ions such that opposite faces of the crystal assume opposite charges. When pressure is released, the ions return to original positions.

The piezoelectricity phenomena was developed and applied in sonar and quartz oscillation crystals. In 1940 the first synthetic piezoelectric substance was demonstrated. Barium titanate has piezoelectric activity nearly comparable to Rochelle salt - it is not water soluable and can withstand high operating temperatures. This ceramic material was soon followed by others, including lead metaniobate and lead titanate zirconate. In 1958, synthetic quartz material became available.

If a piece of piezoelectric material is heated above a certain temperature, called a Curie temperature, it will lose piezoelectric properties. After cooling below the Curie temperature, the piezoelectric material will not regain its piezoelectric properties.

Well known applications are phonograph pickups, microphones, accelerometers and roughness indicators.

Piezoelectric Effect - By putting piezolectric material under mechanical stress, a shifting of the positive and negative charge centers of the elementary units in the material takes place, which then shows up in an external electrical field. This effect is proportional to the deformation, which depends on the state of tension found in the corresponding material. Reversed, an outer electrical field either stretches or compresses such material.

The piezoelectric effect is dependent on the direction. When the piezoelectric actuators stretch in one direction, then they contract in another direction, twist and bend themselves in a rather complicated way. For example, the bending indicates an hysteresis which rotates around an axis. Therefore, in order to obtain precise movements, the parasitic movements must be eliminated. This desired effect is achieved when the piezoelectric element propels a mechanism.

Quartz is the most popularly used substance of this group of materials. That also includes SiO2, LiNbO3, LiTaO3, and ZuO. For technical use, the most significant substances are the ceramic lead and zircon titanite (PZT). These PZTs are marked by high piezoelectric efficiencies. The displacement is in the order of magnitude of 10 micrometer. Among the plastic, for example polyvinylidene fluoride (C2H2F2), can be brought to a state in which it works piezoelectrically by means of electrical polarization at higher temperatures.

The possibility of reacting to deformation and electrical fields in proportion to their strength, the piezoelectric materials offer not only the opportunity to propel structures, but to control them simultaneously.

Though a solid is electrically neutral, it may consist of electrically charged particles, such as cations and anions that make up salts. Also, polar bonds between atoms of different electronegativities may lead to a large number of dipole moments. However, no net electric dipole moment exists in many such materials because the arrangements of the atoms or ions cause a cancellation of individual dipole moments or charges.

A material like quartz is considered piezoelectric because it has the ability to develop a net dipole moment if it is mechanically deformed in certain directions with respect to the atomic arrangement, and it is mechanically deformable by an electric field with an appropriate direction with respect to the atoms in the solid.

When quartz is in its natural state, the positive and negative ions are uniformly distributed and the geometric centers of positive and negative charge correspond. When the crystal is compressed, the ions are displaced and the centers of charge no longer coincide. A net dipole moment has been created, but it will disappear when the pressure is released. The displacement of electron density leads to a voltage across opposite sides of the crystal, and to a current if a wire is connected to the opposite sides. The net movement of negative positive charge in one direction is enhanced by the movement of positive charge in the opposite direction.

A giant piezoelectric effect

A GIANT PIEZOELECTRIC EFFECT has been observed in strontium titanate at low temperatures. The piezoelectric process, by which mechanical energy in a crystal is converted into electricity (and vice versa), generally gets worse below 50 K, but in the case of SrTiO3, it gets better. At 1.6 K, in fact, STO competes with the best room-temperature piezoelectrics. The ultralow-temperature manifestation of this effect might result in new forms of microscopy or thermometry. (Science, 18 April 1997.)

These delicate, sensitive wafers of quartz are hermetically sealed in a dry nitrogen atmosphere in "cans" with pins for electrical contact.

 A Quartz "Crystal"

Once a wafer has been cut, the next stage of the production process involves the reduction in thickness of the quartz to get it to the correct size - this skilled process is called 'lapping'. During this stage the sliver of quartz is 'lapped' to the correct size so it will resonate at the correct frequency.

Electrodes of silver or gold are added by vacuum deposition to allow wires to be connected. The final frequency of the crystal is adjusted by adding an extra layer of silver to one side of the quartz sliver. Tolerances are extremely fine (measured in parts per million) and define how close the resonant frequency is to the required frequency - the smaller the tolerance the more expensive it will be. Crystal frequency is usually specified at 25ºC since crystal accuracy is very temperature dependent - some crystals are made to operate in temperature controlled ovens.

The Piezoelectric effect is an effect in which energy is converted between mechanical and electrical forms. It was discovered in the 1880's by the Curie brothers. Specifically, when a pressure (piezo means pressure in Greek) is applied to a polarized crystal, the resulting mechanical deformation results in an electrical charge. Piezoelectric microphones serve as a good example of this phenomenon. Microphones turn an acoustical pressure into a voltage. Alternatively, when an electrical charge is applied to a polarized crystal, the crystal undergoes a mechanical deformation which can in turn create an acoustical pressure. An example of this can be seen in piezoelectric speakers. (These are the cause of those annoying system beeps that are all too common in today's computers).

Figure 1: Internal Structure of an electret

Electrets are solids which have a permanent electrical polarization. (These are basically the electrical analogs of magnets, which exhibit a permanent magnetic polarization). Figure1 shows a diagram of the internal structure of a electret. In general, the alignment of the internal electric dipoles would result in a charge which would be observable on the surface of the solid. In practice, this small charge is quickly dissipated by free charges from the surrounding atmosphere which are attracted by the surface charges. Electrets are commonly used in microphones.

Permanent polarization as in the case of the electrets is also observed in crystals. In these structures, each cell of the crystal has an electric dipole, and the cells are oriented such that the electric dipoles are aligned. Again, this results in excess surface charge which attracts free charges from the surrounding atmosphere making the crystal electrically neutral. If a sufficient force is applied to the piezoelectric crystal, a deformation will take place. This deformation disrupts the orientation of the electrical dipoles and creates a situation in which the charge is not completely canceled. This results in a temporary excess of surface charge, which subsequently is manifested as a voltage which is developed across the crystal.

Figure 2: A sensor based on the piezoelectric effect

In order to utilize this physical principle to make a sensor to measure force, we must be able to measure the surface charge on the crystal. Figure 2 shows a common method of using a piezoelectric crystal to make a force sensor. Two metal plates are used to sandwich the crystal making a capacitor. As mentioned previously, an external force cause a deformation of the crystal results in a charge which is a function of the applied force. In its operating region, a greater force will result in more surface charge. This charge results in a voltage , where is the charge resulting from a force f, and C is the capacitance of the device.

In the manner described above, piezoelectric crystals act as transducers which turn force, or mechanical stress into electrical charge which in turn can be converted into a voltage. Alternatively, if one was to apply a voltage to the plates of the system described above, the resultant electric field would cause the internal electric dipoles to re-align which would cause a deformation of the material. An example of this is the fact that piezoelectric transducers find use both as speakers (voltage to mechanical) and microphones (mechanical to electrical).

Since the piezo effect exhibited by natural materials such as quartz, tourmaline, Rochelle salt, etc. is very small, polycrystalline ferroelectric ceramic materials such as BaTiO3 and Lead Zirconate Titanate (PZT) have been developed with improved properties. Ferroelectric ceramics become piezoelectric when poled. PZT ceramics are available in many variations and are still the most widely used materials for actuator or sensor applications today. PZT crystallites are centro-symmetric cubic (isotropic) before poling and after poling exhibit tetragonal symmetry (anisotropic structure) below the Curie temperature. Above this temperature they lose the piezoelectric properties.

Charge seperation between the positive and negative ions is the reason for electric dipole behavior. Groups of dipoles with parallel orientation are called Weiss domains. The Weiss domains are randomly oriented in the raw PZT material, before the poling treatment has been finished. For this purpose an electric field (> 2000 V/mm) is applied to the (heated) piezo ceramics. With the field applied, the material expands along the axis of the field and contracts perpendicular to that axis. The electric dipoles align and roughly stay in alignment upon cooling. The material now has a remanent polarization (which can be degraded by exceeding the mechanical, thermal and electrical limits of the material). As a result, there is a distortion that causes growth in the Dimensions aligned with the field and a contraction along the axes normal to the electric field.

When an electric voltage is applied to a poled piezoelectric material, the Weiss domains increase their alignment proportional to the voltage. The result is a change of the Dimensionss (expansion, contraction) of the PZT material.