Discovery And Insights: 1880 - 1882
The first experimental demonstration of a connection between macroscopic piezoelectric phenomena and crystallographic structure was published in 1880 by Pierre and Jacques Curie. Their experiment consisted of a conclusive measurement of surface charges appearing on specially prepared crystals (tourmaline, quartz, topaz, cane sugar and Rochelle salt among them) which were subjected to mechanical stress. These results were a credit to the Curies' imagination and perseverance, considering that they were obtained with nothing more than tinfoil, glue, wire, magnets and a jeweler's saw.
In the scientific circles of the day, this effect was considered quite a "discovery," and was quickly dubbed as "piezoelectricity" in order to distinguish it from other areas of scientific phenomenological experience such as "contact electricity" (friction generated static electricity) and "pyroelectricity" (electricity generated from crystals by heating).
The Curie brothers asserted, however, that there was a one-to-one correspondence between the electrical effects of temperature change and mechanical stress in a given crystal, and that they had used this correspondence not only to pick the crystals for the experiment, but also to determine the cuts of those crystals. To them, their demonstration was a confirmation of predictions which followed naturally from their understanding of the microscopic crystallographic origins of pyroelectricity (i.e., from certain crystal asymmetries).
The Curie brothers did not, however, predict that crystals exhibiting the direct piezoelectric effect (electricity from applied stress) would also exhibit the converse piezoelectric effect (stress in response to applied electric field). This property was mathematically deduced from fundamental thermodynamic principles by Lippmann in 1881. The Curies immediately confirmed the existence of the "converse effect," and continued on to obtain quantitative proof of the complete reversibility of electro-elasto-mechanical deformations in piezoelectric crystals.
At this point in time, after only two years of interactive work within the European scientific community, the core of piezoelectric applications science was established: the identification of piezoelectric crystals on the basis of asymmetric crystal structure, the reversible exchange of electrical and mechanical energy, and the usefulness of thermodynamics in quantifying complex relationships among mechanical, thermal and electrical variables.
In the following 25 years (leading up to 1910), much more work was done to make this core grow into a versatile and complete framework which defined completely the 20 natural crystal classes in which piezoelectric effects occur, and defined all 18 possible macroscopic piezoelectric coefficients accompanying a rigorous thermodynamic treatment of crystal solids using appropriate tensorial analysis. In 1910 Voigt's "Lerbuch der Kristallphysik" was published, and it became the standard reference work embodying the understanding which had been reached.
During the 25 years that it took to reach Voigt's benchmark, however, the world was not holding its breath for piezoelectricity. A science of such subtlety as to require tensorial analysis just to define relevant measurable quantities paled by comparison to electro-magnetism, which at the time was maturing from a science to a technology, producing highly visible and amazing machines. Piezoelectricity was obscure even among crystallographers; the mathematics required to understand it was complicated; and no publicly visible applications had been found for any of the piezoelectric crystals.
The first serious applications work on piezoelectric devices took place during World War I. In 1917, P. Langevin and French co-workers began to perfect an ultrasonic submarine detector. Their transducer was a mosaic of thin quartz crystals glued between two steel plates (the composite having a resonant frequency of about 50 KHz), mounted in a housing suitable for submersion. Working on past the end of the war, they did achieve their goal of emitting a high frequency "chirp" underwater and measuring depth by timing the return echo. The strategic importance of their achievement was not overlooked by any industrial nation, however, and since that time the development of sonar transducers, circuits, systems, and materials has never ceased.
The success of sonar stimulated intense development activity on all kinds of piezoelectric devices, both resonating and non-resonating. Some examples of this activity include:
- Megacycle quartz resonators were developed as frequency stabilizers for vacuum-tube oscillators, resulting in a ten-fold increase in stability.
- A new class of materials testing methods was developed based on the propagation of ultrasonic waves. For the first time, elastic and viscous properties of liquids and gases could be determined with comparative ease, and previously invisible flaws in solid metal structural members could be detected. Even acoustic holographic techniques were successfully demonstrated.
- Also, new ranges of transient pressure measurement were opened up permitting the study of explosives and internal combustion engines, along with a host of other previously unmeasurable vibrations, accelerations, and impacts.
In fact, during this revival following World War I, most of the classic piezoelectric applications with which we are now familiar (microphones, accelerometers, ultrasonic transducers, bender element actuators, phonograph pick-ups, signal filters, etc.) were conceived and reduced to practice. It is important to remember, however, that the materials available at the time often limited device performance and certainly limited commercial exploitation.
During World War II, in the U.S., Japan and the Soviet Union, isolated research groups working on improved capacitor materials discovered that certain ceramic materials (prepared by sintering metallic oxide powders) exhibited dielectric constants up to 100 times higher than common cut crystals. Furthermore, the same class of materials (called ferroelectrics) were made to exhibit similar improvements in piezoelectric properties. The discovery of easily manufactured piezoelectric ceramics with astonishing performance characteristics naturally touched off a revival of intense research and development into piezoelectric devices.
The advances in materials science that were made during this phase fall into three categories:
- Development of the barium titanate family of piezoceramics and later the lead zirconate titanate family.
- The development of an understanding of the correspondence of the perovskite crystal structure to electro-mechanical activity.
- The development of a rationale for doping both of these families with metallic impurities in order to achieve desired properties such as dielectric constant, stiffness, piezoelectric coupling coefficients, ease of poling, etc.
All of these advances contributed to establishing an entirely new method of piezoelectric device development - namely, tailoring a material to a specific application. Historically speaking, it had always been the other way around.
This "lock-step" material and device development proceeded the world over, but was dominated by industrial groups in the U.S. who secured an early lead with strong patents. The number of applications worked on was staggering, including the following highlights and curiosities:
- Powerful sonar - based on new transducer geometries (such as spheres and cylinders) and sizes achieved with ceramic casting.
- Ceramic phono cartridge - cheap, high signal elements simplified circuit design.
- Piezo ignition systems - single cylinder engine ignition systems which generated spark voltages by compressing a ceramic "pill".
- Sonobouy - sensitive hydrophone listening/radio transmitting bouys for monitoring ocean vessel movement.
- Small, sensitive microphones - became the rule rather than the exception.
- Ceramic audio tone transducer - small, low power, low voltage, audio tone transducer consisting of a disc of ceramic laminated to a disc of sheet metal.
- Relays - snap action relays were constructed and studied, at least one piezo relay was manufactured
It is worth noting that during this revival, especially in the U.S., device development was conducted along with piezo material development within individual companies. As a matter of policy, these companies did not communicate. The reasons for this were threefold: first, the improved materials were developed under wartime research conditions, so the experienced workers were accustomed to working in a "classified" atmosphere; second, post war entrepreneurs saw the promise of high profits secured by both strong patents and secret processes; and third, the fact that by nature piezoceramic materials are extraordinarily difficult to develop, yet easy to replicate once the process is known.
From a business perspective, the market development for piezoelectric devices lagged behind the technical development by a considerable margin. Even though all the materials in common use today were developed by 1970, at that same point in time only a few high volume commercial applications had evolved (phono cartridges and filter elements, for instance). Considering this fact with hindsight, it is obvious that while new material and device developments thrived in an atmosphere of secrecy, new market development did not - and the growth of this industry was severely hampered.
In contrast to the "secrecy policy" practiced among U.S. piezoceramic manufacturers at the outset of the industry, several Japanese companies and universities formed a "competitively cooperative" association, established as the Barium Titanate Application Research Committee, in 1951. This association set an organizational precedent for successfully surmounting not only technical challenges and manufacturing hurdles, but also for defining new market areas.
Beginning in 1965 Japanese commercial enterprises began to reap the benefits of steady applications and materials development work which began with a successful fish-finder test in 1951. From an international business perspective they were "carrying the ball," i.e., developing new knowledge, new applications, new processes, and new commercial market areas in a coherent and profitable way.
Persistent efforts in materials research had created new piezoceramic families which were competitive with Vernitron's PZT, but free of patent restrictions. With these materials available, Japanese manufacturers quickly developed several types of piezoceramic signal filters, which addressed needs arising in television, radio, and communications equipment markets; and piezoceramic igniters for natural gas/butane appliances.
As time progressed, the markets for these products continued to grow, and other similarly valuable ones were found. Most notable were audio buzzers (smoke alarms, TTL compatible tone generators), air ultrasonic transducers (television remote controls and intrusion alarms) and SAW filter devices (devices employing Surface Acoustic Wave effects to achieve high frequency signal filtering).
By comparison to the commercial activity in Japan, the rest of the world was slow, even declining. Globally, however, there was still much pioneering research work taking place as well as device invention and patenting.
The commercial success of the Japanese efforts has attracted the attention of industry in many other nations and spurred a new effort to develop successful piezoceramic products. If you have any doubts about this, just track the number of piezo patents granted by the U.S. Patent Office every year - there has been a phenomenal rise. Another measure of activity is the rate and origin of article publication in the piezo materials/applications area - there has been a large increase in publication rate in Russia, China and India.
Solid state motion is presently the single most important frontier. The technical goals of the frontier are to obtain useful and reasonably priced actuators which are low in power and consumption and high in reliability and environmental ruggedness; or, more simply stated, "solenoid replacements," or "electrostatic muscles."
The search for perfect piezo product opportunities is now in progress. Judging from the increase in worldwide activity, and from the successes encountered in the last quarter of the 20th century, important economic and technical developments seem certain.