In addition to electrical polarization (piezoelectric effect) caused by mechanical stress, some crystals can also be caused by temperature Change and produce electric polarization. The phenomenon that the surface charge of the medium changes due to temperature is called the pyroelectric effect.
1. The structure and performance of pyroelectric ceramics
The pyroelectric effect is caused by the spontaneous polarization in the crystal. Spontaneous polarization is inherent because the structure of the crystal itself does not overlap the positive and negative electrical centers in a certain direction. The direction of the spontaneous polarization vector is from the negative electrical center to the positive electrical center. When the temperature changes, it causes the relative displacement of the positive and negative charge centers on the crystal structure, so that the spontaneous polarization of the crystal changes.
Usually, the bound charge generated by spontaneous polarization comes from the air, and the free charge accumulated on the outer surface of the crystal and the free charge inside the crystal are shielded, and the charge cannot be revealed, only when the crystal is heated or cooled, that is, when the temperature changes. , The electric charge displayed at both ends of the crystal can only be displayed when the change in the electrical torque caused cannot be compensated.
Since the necessary condition for a crystal to have a pyroelectric effect is spontaneous polarization, a crystal with a symmetry center cannot have a pyroelectric effect, which is the same as a piezoelectric crystal. However, a piezoelectric crystal does not necessarily have pyroelectricity. This is because when the piezoelectric effect occurs, the mechanical force can act in a certain direction, and the relative displacement of the positive and negative electric centers caused by this is generally unequal in different directions, and the crystal expands when heated evenly. It happened simultaneously in all directions. In addition, they must have equal linear expansion coefficients in mutually symmetrical directions.
2. The main application of pyroelectric ceramics
Pyroelectric ceramics are mainly used to detect infrared radiation, remotely measure surface temperature and heat regeneration pyroelectric heaters.
Infrared radiation detection has been widely used in various (industrial and space technology, etc.) radiometers, spectrometers, infrared laser detection and thermal imaging tubes.
With the widespread application of microcomputer information systems and control systems, pyroelectric ceramics are used as sensors for medical, civil and safety protection. This kind of sensor can be used as a non-contact temperature sensor to provide signals to microcomputers and control systems at room temperature.
The main uses of pyroelectric ceramic sensors:
(1) As a circuit breaker;
(2) Room temperature compensator;
(3) Access sensors used in buildings and elevators;
(4) Fire alarm;
(5) Atmospheric environment monitor;
(6) Sensing elements for detecting the position of objects;
(7) The imaging element that detects the temperature distribution of the human body;
(8) Components such as infrared radiation counting and temperature change measurement of related substances.
Pyroelectric infrared ceramic sensor
3. Several typical pyroelectric ceramics
Pyroelectric crystal materials have been widely used, but since ceramic materials are easier to prepare than single crystal materials, the cost is correspondingly lower. The following briefly discusses PbTiO3 and PZT-based pyroelectric ceramics.
(1) PbTiO3 ceramics
PbTiO3 is a piezoelectric ceramic with good dielectric constant and electromechanical coupling coefficient, and it is also a good pyroelectric material. PbTiO3 has a high Curie point, and its pyroelectric coefficient changes little with temperature. It can be used as a stable infrared detector.
(2) PZT ceramics
PbZr1-xTixO3 ceramics have been widely used as piezoelectric materials. As the Curie point decreases as x decreases, the pyroelectric coefficient increases, but the dielectric constant also increases, failing to change the performance as a pyroelectric element. However, by using the complex phase transition of PbZr1-xTixO3 near x=0.1, a pyroelectric element with better performance can be produced.