In 1880 brothers Pierre Curie and Jacques Curie were working as laboratory assistants at the Faculty of Sciences of Paris. They discovered that applying pressure to crystals such as quartz, tourmaline and Rochelle salt generates electrical charges on the surface of these materials. This conversion of mechanical energy into electrical energy is called the direct piezoelectric effect. “Piezo” is derived from the Greek for “to press”.
The converse effect, that the application of an electrical field to these materials causes the internal generation of a mechanical strain, was predicted by Gabriel Lippman in 1881, via mathematical deduction from fundamental thermodynamic principles. This inverse piezoelectric effect was quickly demonstrated by the Curies via experimentation.
The discovery of piezoelectricity generated significant interest in the European scientific community, and piezoelectricity developed as a new field of research in the last quarter of the 19th century. This research culminated in Woldemar Voigt’s Lehrbuch der Kristallphysik (Textbook on Crystal Physics), published in 1910, which described the 20 natural crystal classes in which piezoelectric effects occur.
Despite the scientific interest that followed the Curie’s discovery, it was some time before the first practical application of piezoelectric materials.
Sonar and Other Applications
The first practical application was sonar, and it was developed in France during World War I, by Paul Langevin and his co-workers. They built an ultrasonic submarine detector consisting of a transducer, made of thin quartz crystal glued between two steel plates, and a hydrophone. The transducer emitted a high-frequency pulse into the water, while the hydrophone detected the returned echo. By measuring the time it took to hear the echo, it was possible to calculate depth.
The detector’s design was not perfected until after the war. Nevertheless, in industrial nations the project’s success led to intense interest in piezoelectric devices. As a result, many new applications for piezoelectric crystals were developed in the years between World War I and World War II, such as microphones, accelerometers, phonograph pick-ups and signal filters.
Ultrasonic transducers were also further developed and used to measure the elasticity and viscosity of materials. This led to huge advances in materials research. In addition, previously invisible flaws in cast metal and stone objects became detectable through the development of time-domain reflectometers, leading to improvements in structural safety.
World War II and Beyond
The piezoelectric effect of natural materials (such as quartz, tourmaline and Rochelle salt) is relatively small. Certain synthetic materials known as ferroelectrics exhibit piezoelectric constants many times higher than natural materials.
The ferroelectric ceramic material barium titanate (BaTiO3) was discovered independently by research groups in three countries during World War II (the United States, Japan and Russia). Lead zirconate titanate (PZT), which exhibits even greater sensitivity and has a higher operating temperature, was developed by physicists at the Tokyo Institute of Technology in 1952.
In the United States significant technical developments were made in the decades following World War II, but the market development for piezoelectric devices lagged behind this technical development. This can be attributed to a nature of secrecy that operated within the companies doing the development. This was likely partly because of the wartime beginnings of the field and also partly because of a belief that patents and secret processes would lead to high profits.
In contrast, in Japan manufacturers shared information, leading to technical challenges quickly being overcome and the creation of new markets. Materials research also led to the creation of new piezoceramic families that were free of patent restrictions. Developments by Japanese manufacturers included signal filters for the television and radio markets, as well as piezoceramic igniters.
The commercial success of Japanese companies attracted the attention of many other nations. As such, it is no surprise that companies continue to search out new piezoelectric product opportunities to this day.