Pyroelectricity happens when certain crystals become electrically polarized via natural processing, resulting in large electric fields. The name pyroelectricity results from two words; the Greek word pyr, meaning fire, and electricity. It can be described as the ability of certain materials to generate a temporary voltage when they are heated or cooled.
How it works is that the change in temperature will slightly modify the positions of the atoms within the crystal structure, and with that comes a change in the polarization of the materials. This polarization change increases the voltage across the crystal. If the temperature doesn’t change and remains constant at the new value, leakage current will cause the pyroelectric voltage to gradually disappear. This leakage is caused by ions moving through the air, electrons moving through the crystal, or current leaking through a voltmeter which might be attached across the crystal.
Pyroelectric Effect
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The pyroelectric effect is often connected to the piezoelectric effect and is largely exploited for pyroelectric infrared temperature sensors. The fact that thermoelectric generators need a temperature (spatial) gradient and pyroelectric material need a temporal temperature change open the door to a variety of different application fields, where temperature gradients are not possible, and the temperature is not static.
Pyroelectric materials are part of a group of noncentrosymmetric polar crystals that show a form of correlation between electrical polarization and temperature. The correlation is in such a way that a change in temperature results in a change in the electric dipole moment, which brings about the pyroelectric effect discussed earlier.
Pyroelectricity is easily recognized by a three-pointed star shape, each point representing kinetic, electrical, and thermal energies (the three energy states comprised in the crystal). The side between thermal and electric energies represents the pyroelectric effect and produces no kinetic energy, while the other side between electrical and kinetic energies represents the piezoelectric effect and produces no heat.
Pyroelectric charge in minerals tends to develop on opposite faces of the asymmetric crystals. The direction to where the dissemination of the charge occurs is usually constant throughout a pyroelectric material. However, it is worth noting that in some materials, this direction can be changed if there’s an electric field close by. Such materials are said to exhibit ferroelectricity.
It is also important to keep in mind that pyroelectric materials are classified as piezoelectric. Despite being pyroelectric, novel materials such as boron gallium nitride and boron aluminum nitride have zero piezoelectric response for strain along the c-axis at certain compositions, which is quite peculiar considering that the two properties are closely related. Nevertheless, some piezoelectric materials have a crystal symmetry that does not allow pyroelectricity.
To measure pyroelectricity, we take the change in net polarization proportional to a change in temperature. The sum of the piezoelectric contribution from thermal expansion (secondary pyroelectric effect) and the pyroelectric coefficients at constant strain (primary pyroelectric effect) will give the total pyroelectric coefficient measured at constant stress. Polar materials do not display a net dipole moment in regular circumstances.
As a result, the intrinsic dipole moment is neutralized by a free electric charge that builds upon the surface through internal conduction or from the ambient atmosphere, which means there are no electric dipole equivalents of bar magnets. To reveal the true nature of polar crystals, they have to be perturbed in some fashion that momentarily upsets the balance with the compensating surface charge.
It is worth noting that spontaneous polarization is temperature-dependent. Therefore, a good perturbation probe should include a change in temperature, which induces a flow of charge to and from the surfaces. This is the pyroelectric effect. Since all polar crystals are pyroelectric, the ten polar crystal classes are sometimes referred to as the pyroelectric classes.
Piezoelectricity is a form of electric charge that accumulates in certain solid materials. Examples of these materials include crystals, certain ceramics, and biological matter such as bone, DNA, and various proteins. The accumulation of charge is usually in response to applied mechanical stress. To put it simply, the piezoelectric effect is the ability of certain materials to generate an electric charge as a response to applied mechanical stress.
Ferroelectricity, on the other hand, is a property of certain nonconducting crystals, or dielectrics, whereby they exhibit spontaneous electric polarization. This is the separation of the center of positive and negative electric charge, resulting in one side of the crystal having a positive charge and the other one having a negative charge. The electric polarization can be reversed in direction through the application of an appropriate electric field. It is also worth remembering that all ferroelectrics are pyroelectric, but the fact that their electrical polarization is reversible is what makes them different.
Pyroelectric Materials
As stated earlier, most pyroelectric materials are hard crystals, but soft pyroelectricity can be achieved by using electrets. Despite most of them being artificial through the invention of modern-day technology, the first effect was discovered in minerals. A good example is a tourmaline. It is also present in bone and tendon.
Application of Pyroelectric Effect
The pyroelectric effect has numerous applications, with the most notable one being in heat sensors. This is made possible because minute temperature changes can produce pyroelectric potential. Most infrared sensors are usually made from pyroelectric material since animal heat from a few meters away is enough to generate some voltage.
Other Applications of the Pyroelectric Effect
There are plenty of other ways in which the pyroelectric effect can be applied in today’s world. These applications range from advanced technology, a good example being interplanetary probes and radiometers in weather satellites, to simple thermal sensors that have hundreds of different commercial and industrial uses.
The pyroelectric effect is also useful in infrared spectroscopy, absolute radiometry, and also laser research. Beyond the thermal imaging systems use, the pyroelectric vidicon is now widely used in laser interferometers and other applications that require a two-dimensional detector.
Certain pyroelectric materials can also be heated and cooled repeatedly to generate electrical power. They can therefore be used to make power engines as has been seen in the past.