Pyroelectricity (Greek pyrein, πυρος pyro = " burn, I burn ", also: pyroelectric effect, pyroelectric, polarization ) is the property of some piezoelectric crystals, to respond to a temperature change over time? T with charge separation.
In pyroelectric crystals are ionic crystals with permanent electric polarization. If you heat it or cool it off, then the opposing surfaces of opposite charge to electric, which is called the positive charging by heating the end of analog, the other antiloger pole.
The resulting voltage difference can be tapped at the corresponding crystal edges ( surfaces) of the electrode.
The polarization is:
Wherein the pyro-electric constant and the temperature difference.
The existing surface charges were compensated for by recorded from the area charge carriers, such as free electrons. (Apparent) surface charges therefore occur only for a change in temperature. With a thermoelectric material, in contrast, results in a temperature difference between two sides of a workpiece to a constant voltage.
The change in temperature of a pyroelectric crystal causes a change in the spacing of the lattice ions. This causes on the one hand a change in length ( thermal expansion ) in the crystal axis whose direction coincides with the direction of polarization. According to the piezoelectricity this results in a charge. On the other hand, the permanent polarization changes with temperature. Both effects are in the same direction and lead to a charging of the crystal from the outside. If caused by the pyroelectric effect of polarization can be reversed by an external electric field, the term of ferroelectricity.
The pyroelectric effect was first observed in the tourmaline. The reversal of this effect is the electrocaloric effect, that is, the generation of heat ( cold) when creating ( collapse ) of an electric field.
There are two forms of the pyroelectric effect:
In summary, with the above-mentioned equation is thus:
Often being predominate.
Besides the tourmaline exhibit other materials this effect, including triglycine (TGS ), often in deuterated form ( DTGS ), sometimes even with L- alanine doped ( LATGS, DLATGS ), lithium, or polyvinylidene fluoride (PVDF).
More pyroelectric materials are:
- Minerals of the tourmaline
- Strontium barium niobate ( SrBaNbO3 )
- Lead titanate ( PbTiO3 )
- Barium titanate ( BaTiO3 ) in the ferroelectric state, i.e., below the ferroelectric Curie temperature because the material has to have spontaneous polarization.
- Sodium nitrite ( NaNO2 )
- Lithium niobate ( LiNbO3 )
- Also on bones and tendons can be observed a pyroelectric effect
For a qualitative determination of one crystal will be briefly immersed in liquid nitrogen. Then it is observed as condensed on the cold ice crystal from the humidity of the surrounding air. If charges have been moved to the surface, thread-like ice particles form from which follow the electric field lines.