Piezo History
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Piezoelectricity was discovered in 1880 by Jacques and Pierre Curie when studying how pressure generates electrical charge in crystals (such as quartz and tourmaline). Its use in submarine sonar in World War 1 generated intense developmental interest in piezoelectric devices.
Most modern piezoelectric materials are ceramics, although polymeric and single crystals also exist and are used commercially in many applications.
The meaning of the term ceramics has evolved a great deal from its Greek root keramos, meaning pottery (or potter’s clay). Ceramic materials are now broadly considered to be all inorganic non-metallic materials. However, it is more useful to classify them as polycrystalline non-metallic materials that acquire mechanical strength through a sintering process. The inherent physical properties of ceramics has made them desirable for use in a wide range of industries. The first applications in the electronics sector made use of their inherently high electrical resistivity and intrinsic stability for fabrication into insulating bodies that is needed to carry and isolate electrical conductors.
However, these immediately apparent properties exploited in the first half of the twentieth century are only the most obvious of a wide range of properties. The discovery of materials with unusually high dielectric constants (er > 2000-10,000) in the 1940s, and the realisation that this phenomenon was a result of their ferroelectric nature, led to an explosion of their use. These materials where first employed in high-dielectric capacitors (barium titanate [BaTiO3] based), and later developed into:
- piezoelectric transducers
- positive temperature coefficient (PTC) devices
- electro-optic light valves
Some of the more recent developments in the field of ferroelectric ceramics are their use in:
- medical ultrasonic composites
- high displacement piezoelectric actuators
- thick/thin films
The birth of ferroelectric ceramics as a useful class of materials came about as a result of:
- The discovery of an unusually high dielectric constant in barium titanate
- The discovery that the origin of this high dielectric constant was due to a permanent internal dipole moment - ferroelectricity. This allowed the development of ABO3 structure ferroelectrics.
- The discovery of the electrical poling process within ceramics, giving rise to single-crystal like properties.
A review of the history of ferroelectrics can be found at: Gene H. Haertling, “Ferroelectric Ceramics: History and Technology”, J. Am. Ceram. Soc., 82 [4] 797-818 (1999)
Over the next few decades, new piezoelectric materials and applications were explored and developed. Piezoelectric devices began to emerge in many applications. Ceramic phonograph cartridges made record players cheaper to maintain and easier to build. Ultrasonic time-domain reflectometers could find flaws inside cast metal and stone objects, which improved structural safety.
Other developments included new designs for piezoceramic filters used in radios and televisions, and the piezoelectric igniter that generates sparks for gas ignition systems by compressing a ceramic disc.
The most commonly used ferroelectric materials are PZT (Pb[Zr,Ti]O3) ceramics, which are based on cubic perovskite structures. Most commercial materials are based on morphotropic phase boundary (mpb) compositions. (A mpb separates solid solutions of the same prototype structure but with different structural distortions.) The mpb in PZT separates rhombohedral and tetragonal phases.
More recently, it is possible using domain engineering in single crystals by the use of appropriate crystal cuts to maximise piezoelectric properties. However, it is not possible to produce useful size single crystals of PZT because it melts incongruently. However, it is possible to grow large crystals of PMN-PT and PZN-PT (Pb[Zn1/3,Nb2/3]O3 – PbTiO3) with mpb compositions. The pioneering work at Penn State by Tom Shrout has shown that it is possible to achieve piezoelectric properties that are nearly an order of magnitude greater than those achievable with PZT ceramics.
The environmentally-focused agenda of most governments includes the legislated reduction in the industrial use of lead and lead containing materials. For many industrial sectors the use of lead containing piezoelectric materials (such as PZT, or PMN-PT) is permitted because the societal advantages outweigh any perceived dangers. The lead in PZT is chemically bound within its crystalline structure and there is no evidence that this lead can leach out into the environment. However, this has not stopped the aggressive development of lead-free piezoelectric materials – notably by research groups in Japan. Impressive research has uncovered new lead-free compositions that might one day offer potential replacement strategies for many applications.