Issue link: https://iconnect007.uberflip.com/i/359006
36 The PCB Magazine • August 2014 PIEzOELECTRIC, PyROELECTRIC, AND FERROELECTRIC MATERIALS continues Typically, you would see piezoelectric mate- rials in items like printed sensors (particularly, vibrational sensors). Perhaps the most common use of piezoelectric materials is found in inkjet printers themselves. By sending an electric sig- nal to a piezoelectric crystal located in the print heads, the crystal expansion creates a pressure wave that forces inks through the orifice of the inkjet printing heads. So while piezoelectric materials can be printed for various devices, perhaps the most important use of these mate- rials is for the printing platform itself. Pyroelectric Materials Pyroelectric materials are a subset of piezo- electric materials. In other words, if a material is pyroelectric, then it is also piezoelectric, but not necessarily the other way around. Out of the 20 naturally occurring crystal classes, 10 also ex- hibit pyroelectric properties. Pyroelectric ma- terials generate a temporary voltage when the material is heated or cooled. This is not to be confused with thermoelectric materials, which exhibit a permanent voltage change when sub- jected to temperature change. One way to vi- sualize the relationship of piezoelectric materi- als and pyroelectric materials is in the diagram below Pyroelectric materials were first noted in 314 B.C. by the Greek philosopher Theophrastus. While he was more of a botanist and ecologist, as witnessed by the books that he wrote, he was the first one to note that the stone "lyngourion" (most likely tourmaline) became charged when heated. This charged crystal then had the abil- ity to attract bits of straw and wood while other heated rocks did not have any such attraction. In 1707, Johann Georg Schmidt studied tourma- line and noted that it was not attracted to cold ashes, but only the hot ones. Further studies in 1747 lead to tourmaline being called lapidem electricum, or the "electric stone." Interest in the electric properties of some crystals in rela- tion to temperature change led the Curie broth- figure 1: the relationship of piezoelectric materials and pyroelectric materials.