In some dielectrics the electrical polarization can exist in long absence of an applied external electric field
Биография Дэвида Моррела
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In some dielectrics the electrical polarization can exist in long absence of an applied external electric field. This polarized state may be energetically favorable (stable and highly resistant to external influences), and exist in a metastable state (which may be broken). In the first case the polarization is called "spontaneous" and in the second case - the "residual".
In spontaneously polarized dielectric thermal energy can be converted directly into electrical energy by the pyroelectric effect, i.e. E. By changing the intrinsic polarization of dielectrics when heated or cooled.
Thus, pyroelectric as piezoelectric is a solid state energy converter, is just a piezoelectric electromechanical one (or, conversely, mechanic-electrical) converter and a thermoelectric power pyroelectric (or electrothermal) converter. This transformation of energy in a solid is only possible if the insulator (crystal, polycrystal or polymer) is electrically polarized. However, this polarization in the absence of external influences noticeably not shown. The fact that at a constant temperature "private" (spontaneous) polarization generally fully compensated electric charges deposited on the surface of a polar or dielectric electrodes. Private (internal) pyroelectric polarization manifests itself only in dynamic (temporary) changes in external conditions. Pyroelectric polarization manifests itself when the temperature of the polar substance. Raising or lowering the temperature of the polar dielectric changes the intensity of the thermal motion of particles in it, and thus changes the orientation of the polar complex, and the distance between them, leading to a change in the spontaneous polarization of. Consequently, on the surface of a polar dielectric appear not compensated electric charges. If the pyroelectric element is connected to the "load" (for example, to the input impedance of the amplifier), then the resistance of the pyroelectric current flows. In the case of open-crystal it appears a pyroelectric voltage. However, if the temperature of the pyroelectric not subsequently changed, the pyroelectric current (or pyroelectric capacity) gradually decreases.
Pyroelectric effect has been described in ancient scientific works (mentioned about 2,000 years ago by the Greek philosopher Pliny). Observe this effect, usually in semi-precious mineral tourmaline (later these crystals were called "electric"). The term "pyro" comes from the Greek word for "fire" because the effect manifested itself when heated tourmaline in the fire. With this heat on the surface of the crystal appeared electric charges, which was accompanied by the sounds of "cracking" - electric discharges. Moreover, there was heated attraction tourmaline fine particles. Recent measurements have shown that the change in temperature of only 10 degrees leads to a thin (~ 1 mm) plate of tourmaline crystal an electric potential of about 1 kV. This tourmaline is a relatively weak pyroelectric. The pyroelectric crystal with large pyroelectric coefficient, even at a small crystal is heated electric breakdown can occur. In addition to tourmaline crystal group, the occurrence of "electrical response" when the temperature changes observed in quartz, in which there is a vector polar electric moment (quartz - not pyroelectric, but piezoelectric). Explanation "pyroelectric effect in quartz" was given only about 15 years ago when it was found that "artificial Pyro" can occur under certain boundary conditions for all classes of piezoelectric crystals as a result of artificial dissymmetry.
As in minerals and among artificially synthesized crystals pyroelectrics are relatively rare. By pyroelectric - minerals are mainly tourmaline (alyumoborosilikates type NaMg [Al3B3. SiO6 (OOH) 30] with different impurities) and synthetic pyroelectric - lithium sulfate (LiSO4 .H2O), lithium niobate LiNbO3, potassium tart rate (K4C8O12. H2O), and others pyroelectric material are all types of semiconductors with wide bottom AIIB VI (CdS, ZnO, etc.), pyroelectric effect but they are usually small. It is interesting to note that as a pyroelectric crystal sugar (S12N12O11) As a rule, pyroelectric effect caused by the temperature change in the spontaneous (spontaneous) polarization of polar crystals, but a similar effect can be artificially induced in all solid dielectrics, if they applied the electric field from the outside. Without an external electric field and in the absence of mechanical influences change with the polarization of the temperature is possible only in crystals, where the spontaneous polarization exists: Pc. So, under normal conditions (temperature near 300 K), the spontaneous polarization specially synthesized crystals - pyroelectric is:
In lithium niobate Pc = 100 SCLC / cm2
A lithium sulfate Pc = 6 SCLC / cm2
A potassium tart rate Pc = 80 SCLC / cm2.
Spontaneous polarization natural tourmaline crystal is 17 SCLC / cm2.
Furthermore crystals pyroelectricity has polarized ferroelectric ceramics: it at elevated temperature and under the action of an externally applied electric field during manufacture oriented domains (spontaneously polarized micro regions), so that, after cooling to normal temperature, and turning off the field is polarized pyro active texture. Currently, there are several ferroelectrics, many of which can be produced in the form of polycrystalline materials for ceramic technology. The most studied is barium titanate BaTiO3.
In practice the dielectric active elements – variconds are applied.
Variconds called ferroelectric ceramic capacitors pronounced nonlinear dependence of polarization and the dielectric constant of the electric field. Variconds are ferroelectrics and have the property of spontaneous, t. E. Spontaneous electric polarization that exists independently of the external field. The most studied is barium titanate BaTiO3.
A polarized ferroelectric ceramics is also used in pyrometry, although its sensitivity can significantly inferior to some polar crystals - ferroelectrics (any ferroelectric in which the application of an external field to orient manage domains, it becomes after such polarization and pyroelectric and piezoelectric).
Among the new materials that can efficiently convert mechanical and thermal effects in the electric charge, the increasing attention of researchers attract electro active polymers. In particular, the most promising is the polyvinylidene fluoride (PVDF) and its copolymers.
We made a capacitor, where we used as a dielectric film polyvinylidene fluoride (PVDF). It showed only the piezoelectric effect, as ordinary film PVDF, which is commercially available, is not polarized in the manufacture, and for the manifestation of the pyroelectric effect it is necessary to activate an electric field at an elevated temperature.
Video of the piezoelectric effect can be seen here:
The prospect of the use of this polymer is shown in the works of Zaeva N.E., to convert thermal energy into electrical energy. Applying polyvinylidene fluoride as an active insulator which specific power 1000 times greater than the barium titanate BaTiO3, possible practical and commercially viable production and use of autonomous electrical battery.
Reverse pyroelectric effect electro caloric effect affects the value permittivity. When thermal equilibrium has time set on the frequency of the applied electric field, the pyroelectric completely absorbed (gives) applying an electric energy converted to heat them. From the circuit, this process is reduced to enhance both the electrical capacity of the cell, i.e. it increases the dielectric constant.
At higher frequencies, the electric field energy exchange is adiabatic (thermal equilibrium is established does not have time). It looks like a decrease in capacitance pyroelement, so that a higher frequency is determined by the dielectric constant of the adiabatic already pyroelectric. The difference between them depends on the magnitude of the pyroelectric coefficient and heat capacity. Pyroelectric effect can be used primarily for the detection and measurement of heat fluxes, and subject to certain conditions - for the direct conversion of thermal energy into electrical energy. Electro caloric effect may be used to lower the temperature of the electrically controlled (e.g., to achieve a better
cryogenic cooling technology, domestic and industrial refrigeration machines and units).
Thus, pyroelectric - solid thermal energy converter in electrical and vice versa. Electro caloric effect, in which electrical energy is linearly converted into heat, cooling allows pyroelectric material by applying an electric field to it a certain polarity.
Using the principle of multi-devices with optimization of conditions of energy between successive stages chain can count on the creation of pyroelectric transducers of thermal energy into electrical energy competitive with other types of similar devices. For comparison, the data type of power converters:
- Radioisotope Thermoelectric-mass 200 kg / kW, efficiency to 3%;
--Semiconductor photovoltaic (solar cells), weight 10 kg / kW, efficiency to
- Pyroelectric multistage cascading mass of 4 kg / kW, efficiency to 20%.
Known record values of electro caloric effect (2,6 ° C) near the phase transition of the anti-ferroelectric ceramics system Pb (Zr, Sn, Ti) O3, as well as in ceramics Pb (Sc, Nb) O3. It was shown a probability achievable thermal efficiency cycle of about 10% with the expected output energy up to 2 kW / L energy source that allows you to experience the real future competitiveness pyroelectric converters with classical autonomous power plants.
Pyroelectric, piezoelectric and how is a solid-state transmitter
energy. Piezoelectricity is associated with an electromechanical conversion, and Pyro - a thermoelectric power. This transformation of energy in a solid is only possible if the insulator (crystal, polycrystal or polymer) electrically polarized; i.e. pyroelectric has spontaneous polarization. When thermoelectric power conversion appears pyroelectric effect, and the reverse it for electric energy conversion - electro caloric effect.