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34 The PCB Magazine • August 2014 Another way to save on production costs is to use well established materials in novel ways. Some polymeric materials have been used for decades for products such as battery covers, paint additives, and insulated foams and seal- ants. By investigating these materials' proper- ties, we find that some of them have uses out- side of their intended function. Piezoelectric, pyroelectric, and ferroelectric materials have been around for quite a long time, but are con- tinuing to find useful niches in emerging elec- tronics markets. Piezoelectric Materials To understand how a piezoelectric mate- rial functions, a general understanding of the piezoelectric effect is needed. The piezoelectric effect is the relationship between mechanical stress and electrical voltage. As stress is applied to a material (usually, but not always, through compression), electrons are bumped off the molecular structure, thus generating an electrical current. The reverse of this process is also true. As electrical current is introduced to the molecule, it causes the solid in most cases to expand. The piezoelectric effect was first demonstrated through experiments by Jacques and Pierre Curie in 1880 based on some initial work by Rene Just Hauy and Antoine Cesar Bec- querel. The Curie experiments noted the piezoelectricity with materials such as quartz and rochelle salt (sodium potassium tatrate tetrahydrate). The next year, Gabriel Lippmann mathemat- ically deduced that the reverse piezo- electric effect should also be true. Experimen- tally, this was then proven by the Curies. Over the next couple of decades, materials exhibiting piezoelectric effect were characterized but not utilized in any practical applications. In 1910, Woldemar Voigt published the "Textbook on Crystal Physics," which describes that 20 out of 32 naturally occurring classes of crystals have the proper symmetry to exhibit piezoelectricity. The first practical use for piezoelectric mate- rials came in WWI. Paul Langevin, a French sci- entist and student of Pierre Curie, developed an ultrasonic submarine detector, known today as sonar. The success of sonar led to other develop- ments using piezoelectric materials. Ultrasonic devices helped to dramatically further materi- als research with more accurate instruments to measure the viscosity and elasticity of liquids. Better solid characterization led to safer build- ing materials, because cracks and air bubbles were easier to find. During WWII, several countries indepen- dently researched piezoelectric materials, lead- ing them to develop synthetic materials such as PZT (lead zirconate titanate), a major piezoelec- tric material still used today. After the war, due to restrictive patent laws and lack of informa- tional sharing in the United States, piezoelectric materials research stalled. However, Japan, not suffering such restric- tions, picked up materials re- search and took the forefront of the device research for a number of years. This lead to the development of a number of piezoelectric devices includ- ing igniters used in gas grills and some of the first television remote controls. In 1969, the synthetic polymer polyvinylidene fluo- ride (PVDF) was discovered to have very strong piezoelectric properties, as well as pyro- electric and ferroelectric prop- erties. This material is com- monly found in additives for paints, electrical wiring insula- tion, aerospace materials, chemi- cal bottle liners, headphones, and bat- teries. PVDF is one of the few materials that ex- hibit the reverse piezoelectric property. In other words, it compresses in volume when an electri- cal field is applied to the material. PVDF is im- portant to the printed electronics world in that it is a polymeric material that can be incorpo- rated into solvent-based systems and made into an ink, making it particularly useful in inkjet formulations. PIEzOELECTRIC, PyROELECTRIC, AND FERROELECTRIC MATERIALS continues PVDF is important to the printed electronics world in that it is a polymeric material that can be incorporated into solvent-based systems and made into an ink, making it particularly useful in inkjet formulations. " "