What is the piezo material made of?



The effect of Piezoelectricity (also piezoelectric effect or short: Piezo effect, out of date: Piëzo-) describes the interaction of mechanical pressure (Greek. πιέζειν - pressing, pressing) and electrical voltage in solids. It is based on the phenomenon that when certain materials are deformed, electrical charges occur on the surface (direct piezo effect). Conversely, these (mostly crystals) deform when an electrical voltage is applied (inverse piezo effect, see also electrostriction). In physics, the piezo effect is the link between electrostatics and mechanics.

However, the piezoelectric effect is relatively small with all known materials: the deformation when a voltage is applied can hardly exceed one per thousand of the body's dimensions.

history

The direct piezo effect was discovered in 1880 by the brothers Jacques and Pierre Curie. In experiments with tourmaline crystals, they found out that when mechanical stress occurs, electrical charges are generated on the crystal surface, the amount of which is proportional to the stress.

The first applications were piezoelectric ultrasonic transducers and, soon after, quartz oscillators for frequency stabilization. With the patent on the charge amplifier granted to Walter P. Kistler in 1950, piezoelectric measurement technology achieved its breakthrough in broad industrial application.

principle

The directional deformation of a piezoelectric material creates microscopic dipoles within the unit cells (shifting the center of gravity of the charges). The summation over all unit cells of the crystal leads to a macroscopically measurable electrical voltage. Directed Deformation means that the applied pressure does not act on the sample from all sides, but only from opposite sides, for example.

Conversely, the crystal (or the piezoceramic component) can be deformed by applying an electrical voltage.

Like any other solid body, piezoelectric bodies can perform mechanical vibrations. In the case of piezoelectrics, these vibrations can on the one hand be electrically excited, on the other hand they also cause an electrical voltage. The frequency of the oscillation depends only on the speed of sound (a material constant) and the dimensions of the piezoelectric body. This means that piezoelectric components are also suitable for oscillators (e.g. quartz crystals, see Applications).

Piezoelectric materials

Basics

The piezo effect can only occur in non-conductive materials. Furthermore, all non-conductive ferroelectric materials or materials with a permanent electric dipole are also piezoelectric, for example barium titanate and lead zirconate titanate (PZT). However, only some of the piezoelectrics behave ferroelectrically.

In the case of crystals, the crystal symmetry is another criterion for the occurrence of piezoelectricity. The piezoelectric polarization does not occur if the crystal has a center of inversion. Piezoelectricity can occur in all 21 non-centrosymmetric point groups, with the exception of the cubic point group 432. In other words, a unit cell must not have a center of symmetry (= a point at which a point reflection converts the crystal into itself).

The best-known material with piezo properties is quartz (SiO2). Quartz crystals have the non-centrosymmetric point group 32. Each Si atom sits in the middle of a tetrahedron made up of four oxygen atoms. A force acting in the direction of the base tip (crystallographic direction: [111]) now deforms these tetrahedra in such a way that the compressed tetrahedra are electrically polarized and a net voltage occurs on the surfaces of the crystal (in the [111] direction).

Technically used materials that show a stronger piezo effect than quartz are often derived from the perovskite structure, e.g. E.g .: barium titanate (BaTiO3). The cubic perovskite modification itself has the centrosymmetric point group m3m and is thus non-piezoelectric, but the material can be used below a critical temperature - the piezoelectric Curie temperature TC. - change into a non-centrosymmetrical perovskite structure (rhombohedral / tetragonal, see lead-zirconate-titanate). It then shows a spontaneous polarization and is ferroelectric.

Piezoelectric Crystals

  • The most important piezoelectric crystal is quartz, more precisely the trigonal crystal structure α-quartz, which is stable up to 573 ° C. The most important application are quartz crystals.
  • Lithium niobate has higher piezoelectric constants than quartz and is used for piezoelectric filters and SAW components (surface acoustic wave Acoustic surface wave) is used.
  • Gallium orthophosphate has only been available as a piezoelectric material since the 1990s. This material is similar to quartz, but has higher piezoelectric constants and better temperature stability. It is stable up to over 900 ° C.

Other piezoelectric crystals are berlinite, minerals of the tourmaline group, seignette salt, and all ferroelectrics such as barium titanate (BTO) or lead zirconate titanate (PZT). However, BTO and PZT are normally not used as single crystals, but in polycrystalline form (ceramics).

Compared to piezoelectric ceramics, the piezoelectric crystals quartz, gallium orthophosphate and lithium niobate have lower piezoelectric coefficients, higher temperature stability, lower losses, significantly lower hysteresis and hardly show any creep after changing the applied voltage.

Piezoelectric ceramics

Industrially produced piezo elements are mostly ceramics. These ceramics are made from synthetic, inorganic, ferroelectric and polycrystalline ceramic materials. Typical base materials for high-voltage actuators are modified lead-zirconate-titanate (PZT) and lead-magnesium-niobate (PMN) for low-voltage actuators.

The composite of the PZT ceramics (Pb, O, Ti / Zr) crystallizes in the perovskite crystal structure; Below the piezoelectric Curie temperature, distortions of the ideal perovskite structure create a dipole moment. In ceramic piezo elements, the internal dipoles are still disordered after the sintering process, which is why no piezoelectric properties are found. The Weiss districts or domains have an arbitrary spatial orientation and compensate each other. A clearly measurable piezoelectric property can only be impressed by an external constant electric field (a few 106 V / m), while the material is heated to just below the Curie temperature and then cooled again. The imprinted orientation is then largely retained (retentivePolarization) and is called Polarization direction designated.

The rotation of the Weiss domains by the polarization leads to a slight distortion of the material as well as a macroscopic increase in length in the direction of polarization.

Other piezoelectric materials

  • Piezoelectric thin films are also increasingly being used as active sensor materials. With the help of semiconductor technologies it is possible to deposit these active piezoelectric thin films on silicon. This is mostly zinc oxide (ZnO) or aluminum nitride (AlN).
  • The plastic polyvinylidene fluoride (PVDF) can be polarized - similar to piezoelectric ceramics - and is then piezoelectric. Application are e.g. B. Hydrophones.

calculation

geometry

A coordinate system is selected to describe the spatially different properties. An x, y, z coordinate system is usually used for indexing, the axes of which are designated by the numbers 1, 2, 3 (axis 3 corresponds to the polarization axis). The shearings on these axes have the numbers 4, 5, 6. Based on these axes, the piezoelectric properties are expressed in formulas with tensors.

Equations

The simplest equations for the piezo effect involve polarizationP.pz (Unit [C / m²]) and the deformationS.pz (Dimensionless size):

where d, e are the piezoelectric coefficients, E is the electric field strength (V / m) and T is the mechanical stress (N / m²). The first formula describes the direct, the second the inverse piezo effect.

The piezoelectric coefficients are:

  • piezoelectric distortion coefficients (reaction of the distortion to the electric field)
  • piezoelectric stress coefficients (reaction of the mechanical stress to the electric field)

The two coefficients are related to the elastic constants:

Second order effects (inverse piezo effect) are described by the electrostrictive coefficients.

The tensors given above are usually rewritten in matrix form (Voigt notation). This gives matrices with hexavalent components that correspond to the axis definition shown above. The piezoelectric effects are then by means of two coupled equations in which the dielectric shift D is used instead of the polarization.
Permittivity at constant mechanical stress
sE. Constant of elasticity with constant electric field strength

It is common practice to summarize the elements of these equations in the coupling matrix.

The most important material parameter for the inverse piezo effect and thus for actuators is the piezoelectric charge constant d. It describes the functional relationship between the applied electric field strength and the strain generated by it. The characteristic sizes of a piezoelectric transducer are different for the different directions of action.

 

In the area of ​​actuators, two main effects are relevant. For these two effects, the equation for the expansion is simplified as follows:

piezoelectric transverse or transverse effect (d31-Effect). The mechanical force acts across the applied field.
piezoelectric longitudinal or longitudinal effect (d33-Effect). The mechanical force acts parallel to the applied field.

Applications

In general, the applications can be divided into three areas:

Sensors

The occurrence of the piezoelectric charge in the event of mechanical deformation can be used for force, pressure and acceleration sensors, in which an acting mechanical force causes a deformation. The resulting charge can be converted into a measurable electrical voltage with the aid of a charge amplifier.

Piezo elements are primarily suitable for recording dynamic processes. In static applications, the charges are usually too low to be accurately detected.

  • Piezo elements are used in music as pickups for acoustic instruments. They are mainly used on stringed instruments such as guitar, violin or mandolin. The dynamic deformation of the instrument (vibration of the sound body) is converted into a low alternating voltage. The voltage of the electrical output signal is parallel to the vibration, i.e. to the sound, and only needs to be amplified electrically.
  • In the case of piezoelectric acceleration sensors or pickups, mechanical deformation (compression or shear) results in charge separation due to the acceleration and thus a charge (or voltage) that can be tapped on the vapor-deposited electrodes.
  • In a piezo lighter, a piezo igniter uses a sudden and large pressure to generate a short-term high electrical voltage. The sparkover then ignites the gas flame.
  • In the case of quartz oscillators, the influence of various variables on the resonance frequency can be used; in the case of surface acoustic wave components, the influence on the delay time can be used. An important application is the measurement of the mass applied to the quartz, e.g. B. in industrial coating processes to determine the layer thickness. The temperature dependence of the oscillation frequency can also be measured; however, such oscillating quartz thermometers are no longer on the market.

Actuators

The piezoelectric transverse and longitudinal effect results in three different basic elements for piezoelectric actuators: the thickness transducer, the transverse expansion element, the bimorph. The bimorph is a combination of two transverse expansion elements. Activating the elements in the opposite direction causes the actuator to bend, which is why it is given a separate name.

The deformation when a voltage is applied can be used for actuators: Piezo positioners allow precise movements by deforming one or more piezo elements, piezo loudspeakers generate sound waves by means of a voltage applied at the appropriate frequency, in inkjet printers (continuous ink jet) can through the high-frequency oscillation a piezo element, the ink can be atomized. Ink printers (drop-on-demand) (e.g. from Epson) also work in part with piezoelectric actuators.

Another important application for piezoelectric actuators are Braille displays for the blind. When the voltage is applied to piezoceramic components, pins that can be felt by the blind are pushed up; This means that the monitor text can be converted into tactile Braille characters on the PC.

Diesel injection valves with stacked piezoelectric actuators are currently revolutionizing common rail technology. Piezo actuators have also been used in the pump-nozzle system since 2005.

Since the piezo effect is always fixed to certain directions of the material, several piezo elements must be combined for two- or three-dimensional movements in such a way that they act in different directions.

Electrical components

In these applications, a mechanical vibration of a piezoelectric solid is electrically excited and electrically detected again. A distinction is made between two types:

  • Volume resonators in which essentially the entire piezoelectric element oscillates. The most important representatives are quartz crystals and ceramic filters.
  • SAW components are based on surface acoustic waves (SAW). Examples are SAW filters and delay lines.

other applications

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The effect is used in:

  • Piezo lighters for generating the ignition spark
  • Piezo microphones (crystal microphone)
  • Headphones (earphones)
  • Sonic heads of ultrasonic devices (ultrasonic diagnostic devices), for the generation and detection of mechanical vibrations, formerly also in remote controls
  • Ultrasonic probes for the generation and detection of mechanical vibrations for non-destructive material testing
  • Ultrasonic sensors for generating and detecting mechanical vibrations for flow and level measurement (process measurement technology)
  • Accelerometers
  • Pressure sensors
  • Force sensors
  • Rotation rate sensors
  • Nanopositioning systems
  • Micropositioning systems
  • Pickups
  • Injection nozzles of cars (series start 2000 for diesel engines)
  • Ink jet printer printheads
  • Ultrasonic motors for z. B. the lens autofocusing or clocks
  • Sensors of measuring devices for traffic monitoring, the so-called Star box
  • Delay lines (e.g. used in PAL or SECAM color televisions and many more)
  • Battery-free radio technology (switch)
  • Optical modulators
  • Michelson interferometers
  • Quartz microbalances
  • Feeding technology
  • Knock sensors in cars
  • Piezo Motors (Squiggler)
  • Proportional pressure regulator

See also

Category: Solid State Physics