FAQ – Piezo Ceramics
The most Frequently Asked Questions (FAQ) we get about Piezo Ceramics. Let us know if there is any question not found below which you would like to have the answer to.
In 1880, the French physicists brothers of Jacques & Pierre Curie discovered that pressure generates electrical charges in a number of ceramics and crystals; they called this phenomenon the “piezoelectric effect”. Later they noticed that electrical fields can deform piezoelectric materials. This effect is called the “inverse piezoelectric effect”. Piezo Ceramics is the material in which one can take advantage of the piezoelectric effect.
Pressure generates charges on the surface of piezoelectric materials. This direct piezoelectric effect, also called generator or sensor effect, converts mechanical energy into electrical energy. Vice versa, the inverse piezoelectric effect causes a change in length in this type of materials when an electrical voltage is applied. This actuator effect converts electrical energy into mechanical energy.
The piezoelectric effect of natural monocrystalline materials such as quartz, tourmaline and Rochelle salt is relatively small. Polycrystalline ferroelectric ceramics such as lead zirconate titanate (PZT) exhibit larger displacements or induce larger electric voltages. PZT piezo ceramic materials are available in many variations and are most widely used for actuator or sensor applications.
At temperatures below the Curie temperature TC , the lattice structure of the PZT crystallites becomes distorted and asymmetric. This brings about the formation of dipoles and the rhombohedral and tetragonal crystallite phases, which are of interest for piezo technology. The ceramic exhibits spontaneous polarization. Above the Curie temperature the piezoceramic material loses its piezoelectric properties.
- Soft piezo ceramic materials can be polarized fairly easily even at relatively low field strengths. The advantages of soft PZT materials are their large piezoelectric charge coefficient, moderate permittivities and high coupling factors. Important fields of application for soft piezo ceramics are: Actuators for micropositioning and nanopositioning, sensors, such as conventional vibration detectors, ultrasonic transmitters and receivers, e.g., for flow or level measurement, object identification or monitoring, as well as for electro-acoustic applications as sound transducers and microphones, and also as sound pickups on musical instruments.
- Hard PZT materials can be subjected to high electrical and mechanical stresses. Their properties hardly change under these conditions. The advantages of these materials are their moderate permittivity, large piezoelectric coupling factors, high mechanical qualities, and very good stability under high mechanical loads and operating field strengths. Especially high-power acoustic applications benefit from the properties of hard piezo materials. Examples of their fields of application include ultrasonic cleaning (typically in the kHz frequency range), the machining of materials (ultrasonic welding, bonding, drilling, etc.), ultrasonic processors (e.g., to disperse liquid media), the medical sector (ultrasonic tartar removal, surgical instruments, etc.) and sonar technology.
A strong electric field of several kV/mm is applied to create an asymmetry in the previously unorganized ceramic compound. The electric field causes a reorientation of the spontaneous polarization. At the same time, domains with a favorable orientation to the polarity field direction grow and those with an unfavorable orientation shrink. After polarization, most of the reorientations are preserved even without the application of an electric field. However, a small number of the domain walls are shifted back to their original position, e.g., due to internal mechanical stresses.
The coupling factor k is a measure of the magnitude of the piezoelectric effect (not an efficiency factor!). It describes the ability of a piezoelectric material to convert electrical energy into mechanical energy and vice versa. The coupling factor is determined by the square root of the ratio of stored mechanical energy to the total energy absorbed. At resonance, k is a function of the corresponding form of oscillation of the piezoelectric body.
As it is hard to define the maximum velocity of a piezo actuator, which is mostly dependant of the application, more common is to define the time it takes for an actuator to expand from resting point to maximum travel range. This is referred as rise time (T) which depends on the is the resonance frequency (Rf) of the piezo actuator defined as
T = 1 / (3 * Rf)
As the tensile strength is low for piezoceramics it is recommeneded to apply a preload onto the piezo actuator. It is important to optimize the preload on the piezo actuator to accommodate the load capacity and dynamic forces of the application.
Oscillation mode and the deformation are defined by the geometry of the ceramic body, orientations of the electric field and polarization as well as the mechano-elastic properties