Some science behind the scenes

Pyroelectric crystals

Pyroelectricity is the ability of certain materials to generate a temporary voltage when they are heated or cooled. The change in temperature modifies the positions of the atoms slightly within the crystal structure, such that the polarization of the material changes. This polarization change gives rise to a voltage across the crystal. If the temperature stays constant at its new value, the pyroelectric voltage gradually disappears due to leakage.

Pyroelectricity can be generated from only certain crystal structures. Although artificial pyroelectric materials have been engineered, the effect was first discovered in minerals such as tourmaline.

Crystal structures can be divided into 32 classes, or point groups, according to the number of rotational axes and reflection planes they exhibit that leave the crystal structure unchanged.

Twenty-one of the 32 crystal classes lack a center of symmetry, and of these, 20 are piezoelectric. Of these 20 piezoelectric crystal classes, 10 of them are pyroelectric (polar).

Artificial pyroelectric materials have also been developed,  out of gallium nitride (GaN), caesium nitrate (CsNO3), polyvinyl fluorides, derivatives of phenylpyrazine, and cobalt phthalocyanine. Lithium tantalate (LiTaO3) is a crystal exhibiting both piezoelectric and pyroelectric properties.  Perovskite oxides all demonstrate pyroelectric effects.

Perovskite oxide nanowires: synthesis, property and structural characterization - Zhu X, Liu Z, Ming N; National Laboratory of Solid State Microstructures, Department of Physics, Nanjing University, Nanjing 210093, China.

Perovskite oxide materials display a wide spectrum of functional properties, including switchable polarization, piezoelectricity, pyroelectricity, and non-linear dielectric behavior. These properties are indispensable for application in electronic devices such as non-volatile memories, sensors, microactuators, infrared detectors, microwave phase filters, and so on.

Natural compounds with this structure are perovskite, loparite, and silicate perovskite.