History of sciences: From Rochelle salt to PZT
Ferroelectricity was discovered in the beginning of the century by J. Valasek in Rochelle salt (potasium sodium tartrate) which was originally produced in France in 1665 by an apothecary Pierre Seignette. Rochelle salt was originally used in medicine as a mild purgative. Crystals of Rochelle salt were easily grown and were subsequently used in piezoelectric devices such as crystal microphones and phonograph pickup cartridges. Historically, ferroeletricity was discovered after piezoelectricity and pyroelectrici the ceramic BaTiO3 (barium titanate), was found by B. Wul and I. M. Goldman. This discovery triggered considerable efforts in search of additional ferroelectrics having the same structure. A significant progress in applications was made possible after the discovery of lead zirconate titanate - Pb(Zr,Ti)O3 or PZT - with a very strong piezoelectric response, and a large remanent ferroelectric polarization. Lead-based materials have since become the dominant compounds in this field.
Pyroelectricity
Pyroelectricty was probably first observed in tourmaline by ancient Greeks, but quantitatively investigated only in the eighteenth century, during the early studies of electrostatics. Sir David Brewster, a Scottish scientist, was the first to use the term pyro (fire) electricity in 1824 when describing this phenomena in one of his numerous and famous contributions to the Encyclopaedia Britannica. Pyroelectric materials have a spontaneous polarization whose amplitude changes under the influence of temperature gradients. The discovery of PZT triggered many applications based on this phenomenon, such as infrared detection, thermal imaging (absorption of energy resulting in polarization changes) and dielectric bolometers.
Piezoelectricity
Piezoelectricity was discovered later, around 1880, by Pierre and Jacques Curie who were the first to demonstrate the generation of electricity (surface charges) on well prepared crystals of quartz as a result of mechanical pressure. Inversely, when a voltage is applied across a piezoelectric material, it can undergo a mechanical distortion in response. The beginning of the twentieth century gave birth to most of the classic applications of piezoelectrics, such as quartz resonators, accelerometers and those already mentioned above. After world war II and following the discovery of PZT, the advances made in material science allowed the development of numerous applications based on tailored piezoelectric properties.
Ferroelectricity
All ferroelectrics are piezoelectric and pyroelectric, but they additionally possess a reversible, non-volatile macroscopic spontaneous electric dipole moment in the absence of an external electric field. In simple, words ferroelectric crystals can be seen as an assembly of batteries with a particular orientation, which remains stable unless an external electric field is applied to change its direction. Their polar state is a consequence of the structural transition from a high-temperature, high-symmetry paraelectric phase to a low-temperature, low-symmetry ferroelectric phase. These materials also behave as high dielectric-constant insulators useful in the developement of capacitors and energy storage materials.
Some applications of ferroelectrics
Although for the moment several technological issues, such as fabrication costs and read-out times, make the nanoscale control of ferroelectricity described by Prof. Triscone and his collaborators a purely research topic, it could eventually lead to ultra-high density nonvolatile memory devices, capable of retaining data during power loss, and with very short boot-up times. Ferroelectric thin films also play an increasingly important role in other aspects of modern technology. In particular, their piezoelectric, dielectric and pyroelectric properties have been exploited in diverse applications, from accelerometers (airbags), ferroelectric random access memories (FeRAMs), electro-optical devices (thermal imaging), high frequency devices for medical imaging (ultrasonic-based imaging) and surface acoustic wave (SAW) devices (high frequency telecommunication filtering), to embeded Smart Systems (active vibration control) and many more. Our purpose here is not to make an exhaustive list but rather to highlight some of the interesting recent developments.
Ferroelectric random access memories (FeRAMs)
Ferroelectric Random Access Memories (FeRAMs) are non-volatile ferroelectric-based memories. The information is stored using the two stable polarization states of PZT capacitors. Such memories present the advantage of having a higher speed in write mode than today's conventional memories, low power consumption and high endurance. Although their integration in conventional CMOS technology is continuously being improved, another limitation encoutered in processing such memories lies in the difficulty of obtaining reliable preformance and material characteristics when considering ultra-dense/small capacitors. In other words, the best availble FeRAMs on the market are "only" 256Kb, but the demand and related market for low density FeRAM is huge enough to allow their mass production. For example, Fujitsu has already shipped more than 150 millions FeRAM devices to the market since 1999. The CMOS integration is now at 1Mb, with mass produced devices to be made available within the year (now sampling). Several examples of application of FeRAMs are smart card chips (credit cards or prepayment cards), cellular phone (memory, audio/video strorage), play stations and other emerging applications such as electronic tickets or new identification cards.
Ultrasonic device for medica imaging
Today, the universally familiar ultrasound non-invasive imaging system is the preeminent method for in-utero fœtal imaging. This technology, unknown 30 years ago, was again made possible by the development of piezoelectric-based array transducers and their miniaturisation, since increasing resolution requires increasing number of sensors inside a same detector size. Today 1.5D - 2D detector arrays are commercially available, and continuing advances will soon provide real 3D imaging systems. Other novel medical application using ferroelectrics include the development of focused surgery transducers and non-invasive medical therapies.
Active damping
The company Siemens AG in Germany has recently developed damping units based on multilayer piezo actuators for trains using the tilting technology. Active damping of train bogies is necessary to improve the performance of such high speed trains. The optimization of speed versus trajectory control coupled with comfort and security questions represent the key issues of this technology. Although the use of PZT-based multilayered piezolectric damping was not straightforward for such high load applications, significant progress was recently achieved. Other "lighter" applications of active and adaptive embedded damping units have been developed for use in vibration reduction/ control inside tennis racquets and skis.
