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Pyroelectricity Vs Piezoelectricity

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Pyroelectricity Definition

 It is a Greek word derived, means electricity generated by heat.

Certain materials have the ability to produce temporary voltage when they are heated or cooled.  The change in temperature modifies the position of the atoms slightly within the crystal structure because polarization changes within the material. The change in polarization give rise the potential difference across the material and hence the voltage.

The pyro-electric voltage gradually vanishes due to leakage current. This can happen when the electrons move through the crystal, or moving of the ions through the air.

Pyroelectricity Vs Thermoelectricity Vs Ferroelectricity

Pyroelectricity is different from the thermoelectricity as the whole crystal when induced with the temperature change, conjures the polarity in the crystal and hence the potential difference temporarily.

 While in case of thermo electricity the two dissimilar materials are joined and the joining ends when kept in at a temperature difference, causes a permanent voltage as long as the temperature is maintained across the ends.

Ferroelectricity is the intrinsic property of some materials that have a spontaneous electric polarization that can be reversed by the application of an external electric field. The term is normally used analogous to ferromagnetism, in which a material exhibits a permanent magnetic moment.

Pyroelectric Effect and Pyroelectricity Explanation-

 It can be visualized as sort of one side of a triangle, where each side portrays energy states in the crystal viz. kinetic, electrical and thermal energy.  The side between the electrical and thermal corners shows the pyroelectric effect and generates no kinetic energy. The side between kinetic and electrical corners exhibits the piezoelectric effect and produces no heat.

Artificial materials have been developed by the engineers. The effect was discovered in the mineral Tourmaline for the first time.  You would be surprised that pyroelectric effect is present in our body in both bones and tendons. The pyroelectric charge develops on the opposite faces of asymmetric crystals in minerals.

The direction of propagation of the electric charge is generally constant throughout the pyroelectric material. Though in some materials this direction can be changed by nearby electric fields. These materials are said to exhibit ferroelectricity. All pyroelectric materials are piezoelectric and the two properties are closely related. It is worth noting that some piezoelectric materials have crystal symmetry that does not permit pyroelectricity.

A very small temperature change can produce an electric potential difference due to a material’s pyroelectricity. Passive infrared sensors are often designed around pyroelectric material as the heat of a human or animal from several feet away are enough to produce a difference in charge.

 pyroelectric crystals, classes and piezoelectricity

All kind of crystals that has been discovered can be divided into 32 crystal classes. 21 crystal are non centrosymmetric (does not have the Centre of symmetry) out of 32 crystal structures.  20 crystals out of these 21 exhibits direct piezoelectricity.

10 crystals out of 20 are classes re polar that means they bears a spontaneous polarization, shows piezoelectricity and have a dipole in their unit cell. If somehow the dipole in the unit cell is reversed by the utilization of electric fields then that crystal said to be ferroelectric in nature. We all know that when an external electric field is applied to dielectric material it develops a dielectric polarization i.e. the electrostatics. However if the material has such natural charge separation capability even in the absence of electric fields it is called a polar material.

Only ten crystals out of 32 are grouped as polar crystal and all the polar crystals are pyroelectric. Hence these 10 polar classes are sometimes called as pyroelectric classes.

This property of the pyroelectricity is the measured change in the net polarization and this is proportional to the change in temperature or temperature gradient.

The net pyroelectric coefficient at constant stress is the sum total of pyroelectric coefficients at constant strain and the piezoelectric induced from thermal expansion. Contribution from thermal expansion is called the secondary pyroelectric effect while pyroelectric strain at constant strain is called primary piezoelectric effect.

Under normal conditions, even the polar substances do not exhibit a net dipole moment. As a result there are no dipole equivalents of bar magnets. This is because the intrinsic dipole moment is neutralized by free electric charge that builds up on the surface due to either outer ambient atmosphere or by the internal conduction. Polar crystals only display their nature when altered in some fashion that momentarily upsets the balance with the compensating surface charge.