Pyroelectricity is the movement of positive and negative charges to opposite ends of a crystal’s polar axis when heated or cooled, changing the polarization and generating a temporary voltage. The changes in temperature are what cause the atoms to modify themselves, leading to a voltage that runs throughout the entire body of the crystal. When the temperature remains constant, however, the voltage generated by pyroelectricity will slowly begin to disappear. This disappearance is caused by leakage current, which can happen for any number of reasons, including the movement of electrons throughout the crystal’s polar axis, ions moving through the air or current leaking through an attached voltmeter.
To determine the level of pyroelectricity in a material, those in the semiconductor industry compare the change in net polarization to the change in temperature. By taking the piezoelectric contribution from heat sink compounds and adding it to the pyroelectric coefficients, these semiconductor industry workers are provided with the total pyroelectric coefficient at a constant level of stress.
Through these measures, it has been proved that pyroelectricity has some presence in all thirty-two crystal classes, but the voltage generated is strongest in ten, which are known as the pyroelectric classes. These ten classes share similar properties, including spontaneous polarization, piezoelectricity and a dipole in their unit cell. As polar crystals, they have a natural charge separation without requiring the application of an electric field.
The following are the ten pyroelectric crystal classes:
While these ten crystal classes do exhibit pyroelectricity, they do not have net dipole moments under normal conditions. This class of crystals only exhibits the properties they hold when they are disturbed, creating an unbalance in the charges running along the surface of the crystal body. However, these disturbances can occur easily as even the smallest change in temperature can produce significant voltage.
As more research is completed, pyroelectric materials will have more of a common place in semiconductor industry functions. Most recently, there has been research and development of artificial pyroelectric materials spread into a thin film. To generate the most power, pyroelectric materials are subjected to heat similar to that produced by heat sink compounds. Due to their beneficial properties, pyroelectric materials will continue to be created for use in the semiconductor industry.
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