Materials that transduce both electric and magnetic fields were discovered in the nineteenth century. In 1842, James P. Joule discovered that a bar of iron would constrict under the influence of a magnetic field, giving rise to the term “Joule magnetostriction.” In 1880, Jacques and Pierre Curie discovered that an electric charge could be produced by applying an external mechanical force to quartz crystals.
Soon investigators learned that introducing charge to the same crystal would produce a corresponding mechanical strain. Through World War II, the dominant transduction material was nickel. Known piezoelectric ceramic strain was relatively insignificant until around 1946 when it was discovered that barium titanate could be electrically “poled,” a process similar to magnetizing a permanent magnet.
Lead Zirconate Titanate
Within the next decade, lead zirconate titanate (the acronym PZT is trademarked by Clevite Corp.) was found to have properties superior to nickel and therefore, largely replaced it in sonar and most other applications.
The design limits of nickel and piezoelectric ceramic sonar began to be reached over 25 years ago. At the same time Naval Ordnance Laboratories (NOL), now Naval Surface Warfare Center (NSWC) began to develop metal alloys of the lanthanide elements with “giant” magnetostrictive properties. These materials promised greater acoustic power and other capabilities such as lower frequency, broader bandwidth, and greater reliability.
Since then, magnetostrictive TERFENOL-D has been commercially developed as the lanthanide material best suited to drive maximum acoustic power, relatively low-frequency equipment, including sonar.
TERFENOL-D is an intermetallic alloy of the lanthanide elements terbium and dysprosium combined with iron (Fe) and commercially produced as a near-single crystal. The name combines the symbols for the elements with NOL, derived from the facility of origin. The material is usually supplied to manufacturers ready to assemble into devices, without the need for further processing.