What are piezoelectricity and ferroelectricity

Piezo and ferroelectric effect

After the development of electronic amplifiers with cathode-heated tubes, it was possible to play records with crystal pickups. They replaced the mechanically stressful, heavy steel needle pick-ups with which the deflections were brought to the diaphragm of the bell with lighter and better quality systems. The first crystal buyers consisted of seignette salt, potassium sodium tartrate, a double salt of tartaric acid. When mechanical forces act, the crystal generates small electrical voltages. There are now many areas in electronics where materials with piezoelectric properties are used.

A crystal can become electrically charged through mechanical pressure (from the Greek piezein = to press). Crystal electrical properties are based on the coupling between mechanics and electrostatics, whereby the material must meet certain requirements. There are electrically non-conductive crystals, polycrystalline ceramics and some polymer plastics. They have at least one polar axis. The unit cell of the crystal lattice has no center of symmetry. There must be no mutual alignment of the unit cells due to the formation of twins. The sum of all resulting dipole moments in the crystal must not be equal to zero.

Instead of the Seignette salt KNaC4H4O6, crystalline gallium orthophosphate GaPO4 and lithium niobate LiNbO3 are used today. Crystal ceramics consist of barium titanate BaTiO3 and lead zirconate titanate (PZT). Natural minerals such as berlinite AlPO4, tourmalines (complex ring silicates) and the most widely used α-quartz have crystal-electrical properties. The piezoceramics generate greater electrical effects and, like the α-quartz, are manufactured under controlled laboratory conditions. The crystal-electrical properties of the piezocrystals are stable over a wide temperature range and of high quality.

Direct piezo effect

If mechanical compressive or tensile forces act on a piezo material along certain crystal axes, positively and negatively charged crystal lattice points shift. If the resulting total dipole moment is greater than zero, it can be used as an electrical voltage on electrodes attached to the outside. The mode of operation of all piezo sensors is based on the direct piezo effect. They are used, for example, in electronic scales, to generate ignition sparks in lighters, piezo microphones and pickups in stringed instruments, in measuring devices for determining speed and acceleration and for surface scanning.

Direct longitudinal piezo effect

The force acting in the direction of a polar x-axis causes a longitudinal piezo effect. The electrical voltage builds up in the direction of force in which the crystal lattice is compressed.

Direct transverse piezo effect

The force is applied along a non-polar y-axis of the crystal lattice. The piezo voltage builds up along the polar x-axis. The grid is stretched in this direction and the polarity of the voltage is opposite to the longitudinal piezo effect.

The video clip shows the principle of the two direct piezo effects. Individual control is possible with the fade-in control bar. The similarly charged crystal building blocks of the unit cell form the corner points of a triangle. The center of gravity of the charge is at the intersection of the bisector of a triangle. Charge shifts with generation of tension can also occur when shear forces act parallel to the polar or non-polar crystal axis.

Inverse piezo effect

The mechanical electrostatic coupling works in both directions. When a voltage is applied to the piezo material, its geometry changes. The component becomes a piezo actuator and is used, for example, in piezo loudspeakers, injection nozzles and as a reading aid for displaying Braille.

Inverse longitudinal piezo effect

The voltage is applied in the direction of the non-polar crystal axis. The crystal changes its geometry in the direction of the polar axis, for example through expansion.

Inverse transverse piezo effect

The voltage is applied in the direction of a polar crystal axis. The crystal changes its geometry and expands, for example, in the direction of the non-polar axis. If two crystal plates are attached to a reference electrode on both sides, a bending moment occurs when a voltage is applied to the piezocrystals.

Electric piezo elements

Piezo elements in electronic circuits work in combination with the direct and inverse piezo effect. Quartz crystals in oscillators as well as quartz or ceramic filters are the most popular applications. In delay lines, SAW (surface acoustic wave) assemblies are attached as piezo-ultrasonic transmitters and receivers to a transmission medium, often a glass plate. Special bandpass filters of high quality work as surface acoustic wave filters. With these SAW filters, comb-like electrodes are applied to a piezoelectric single crystal on the input and output sides. The filter effect is based on signal interference and time differences in the piezo crystal between the electrically uncoupled input transmitter and output receiver. The working frequencies reach into the gigahertz range.

Ferroelectricity and ferroelectrics

Many chemical compounds form heteropolar bonds which, in the solid state, lead to crystalline ion lattices. External electric fields can cause displacement polarization. The centers of the anions and cations in the crystal lattice are distorted against each other and dipoles are created. The outwardly effective total dipole moment is largely linearly dependent on the excitation field. The influence of temperature is negligible.

Many substances are dipoles due to their atomic or ionic geometry. The best-known example is the water molecule (H2O), a dipole, in contrast to methane (CH4) without dipole properties. The permanent dipoles form aligned areas in the crystal lattice of the solid. In the case of natural crystal growth with impurities and fault zones, these areas are mostly statistically distributed macroscopically and no dipole moment can be seen on the outside. By applying an electric field, these areas can be reoriented so that a total dipole moment remains even after the field is switched off. The process is known as orientation polarization and is heavily dependent on the field and temperature.

Ferroelectricity can only occur in solids in whose crystal lattice there is at least one polar axis. Thus, all piezoelectric materials are also ferroelectrics. At the same time, they are also pyroelectric, as they react to changes in temperature with a charge shift and the formation of an external electric field. A property observed for the first time in the tourmaline crystal.

Ferroelectrics

Materials with the properties just described are called ferroelectrics. The naming with Ferro- was based on the comparable behavior to ferromagnetic materials. In the ferroelectric state, the dipoles are grouped together in crystal areas, the domains. An outwardly acting sum dipole moment arises. The areas are comparable to the white areas of the elementary magnets and are also referred to as such here. In the ferroelectrics, the Weiss domains are smaller than in the ferromagnetics.

The dipole moment can be influenced by a variable electric field. As with the magnetization of a ferromagnetic material, the changes do not run linearly along a hysteresis curve. If all internal dipole areas are aligned, then the ferroelectric saturation has been reached. When the external field is switched off, the material remains ferroelectric. If the exciting field is slowly reduced to zero, a residual dipole moment remains, which is only canceled by a field reversal and is rebuilt in reverse polarity when the field strength is increased further.

Above a characteristic temperature, the Curie temperature, the material loses its ferroelectricity. This behavior also corresponds to ferromagnetism. The material is in this state after technical production by sintering processes. The cooling takes place during the action of a strong electrostatic field, which aligns the dipoles. The alignment is retained below the Curie temperature. The ferroelectric properties can be adjusted in a targeted manner in this way.

These special properties were first observed in the Seignette salt and tourmaline crystal. Many inorganic and some organic compounds with dielectric and piezoelectric properties are now known. Ferroelectrics are used as single crystals or as polycrystalline ceramics.

All organic electret materials are also ferroelectrics. The most commonly used semi-crystalline polymer material is PVDF, polyvinylidene fluoride (CF2-CH2) n. The film retains its ferroelectric properties when it is cooled in a strong electrostatic field. The permittivity values ​​are 10. The material is preferably used as a pressure-voltage converter in hydrophones.

application areas

Ferroelectrics are used in a variety of ways in electronics. Many of the crystal ceramics have very high permittivities εr, also known as the relative dielectric constant. This makes them interesting for the miniaturization of capacitors, as they can be used to produce very small ceramic capacitors with high capacitance. You can replace electrolytic capacitors up to 10 μF. In addition, they are characterized by much lower parasitic series resistance and inductance values. However, their capacitance values ​​are not as temperature stable. The table shows the range of achievable permittivity numbers compared to air or vacuum.

In connection with semiconductor circuits, ferroelectrics are used as programmable, non-volatile ferroelectric FeRAM memories. As an insulator between the controlling gate electrode and the semiconductor track in MOS transistors, the capacitance increases and, with a somewhat thicker layer, the insulation behavior increases as well. The charge is retained without being refreshed. An insulation layer that is a few nanometers thick reduces the tunnel effects that occur with increasing miniaturization.