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38 The PCB Magazine • August 2014 ers to start investigating this group of materials, leading to the discovery of piezoelectricity. Some common pyroelectric materials such as tourmaline, PZT, and PVDF are utilized for devices such as thermal sensors. There have been some studies into these materials for pow- er generation, where the theoretical efficiency of power generation could be at 50% or higher. Additionally, pyroelectric materials have dem- onstrated their use in a process called Pyroelec- tric fusion. The pyroelectric properties of the crystals used in this particular experiment were used to cause nuclear fusion of deuterium into Helium-3. While this makes for a good academ- ic exercise, this method of fusion is very inef- ficient. However, the ability to make a "neutron generator" with these materials could point to other uses as research areas. In the world of printed electronics, materi- als such as PVDF can be utilized in inexpensive temperature sensors. These printed sensors have the potential to be used on such items as pro- duce (to monitor temperature during shipping), pre-packaged items such as those sold in cof- fee shops and corner markets (to track the shelf life of food items or monitor the recommended storage temperatures), and temperature-sensi- tive items such as chemicals. Development is actively being done to bring these types of sen- sors to the marketplace, with some of them test- ing in select markets. Ferroelectric Materials The final group is ferroelectric materials, which make up a very small subset of pyroelec- tric materials. In other words, materials that are ferroelectric are also pyroelectric and piezoelec- tric in nature. While the prefix of "ferro" usu- ally means that a given material contains iron, this is rarely the case with ferroelectric materi- als. Ferroelectric materials exhibit a spontane- ous polarization that can be reversed by apply- ing an electrical field. When ferroelectric ma- terials were discovered in 1920 by the scientist J. Valasek, they were thought to be a subset of ferromagnetic materials (materials that exhibit a permanent magnetic moment). Ferroelectric materials were determined to be a new set of materials, but the "ferro" prefix stuck as a label. To understand ferroelectricity, we need to understand a couple of other types of polariza- tion. The first type, dielectric polarization, oc- curs when polarization is directly proportional to the applied electrical field in a linear relation- ship. Paraelectric polarization is a non-linear re- lationship between the applied electrical field and the polarization response. Finally, ferroelec- tric polarization is characterized by a couple of properties. First, it is polarized even when there is no applied electrical field. When an electri- cal field is applied in the opposite direction, the polarization is reversed, giving what is called a "hysteresis loop." Finally, as the temperature is PIEzOELECTRIC, PyROELECTRIC, AND FERROELECTRIC MATERIALS continues figure 2: the ferroelectric effect.