#ThanksScience pieces are short, informative articles about how science has created aspects of our modern world that we take for granted. Click here for more!
We typically think of crystals and rocks as rigid. Take a hammer to a crystal and you might expect it to break, not bend. Surprisingly, for a lot of the technologies we take for granted, this expectation is incorrect. In this installment of #ThanksScience, we explore the under-appreciated science responsible for winning wars, keeping time, and visualizing fetuses: piezoelectricity.
When some materials absorb mechanical energy (the energy you transfer when you hit that material with a hammer, for example), they deform visibly and irreversibly. Piezoelectric materials, on the other hand, absorb mechanical energy in a way that generates electric charge. Hit the material with rapid waves of mechanical energy, such as sound, and the material responds with an electrical current.
Jacques and Pierre Curie were the first scientists to discover piezoelectricity. The French brothers were familiar with a similar concept—pyroelectricity—in which heat, or thermal energy, converts to electrical energy. With an expert understanding of crystals, they predicted that pyroelectric materials might also respond to mechanical energy.
Few accounts exist that detail the exact moment of discovery, but, as a PhD student born in France, I believe I am apt to paint the scene. The year is 1880, and the smell of camembert wrestles with chemical fumes for complete dominion over the Curie Lab airspace. A long day has already given way to a long night when Pierre Curie tells his brother, “Screw it. I’m just going to squeeze this rock and see what happens. [Belch] Hold my wine.”
Just like that (probably), the Curie brothers discovered the piezoelectric effect. Shortly after, they also proved the inverse: electrical energy induces mechanical changes in piezoelectric materials—a profound discovery that ushered in a wave of applications. This discovery was so profound, in fact, that Pierre Curie earned the eternal nickname, Piezo Pierre. Pierre Curie could not be reached for comment.
A few decades after Piezo Pierre and Jacques Curie detailed the piezoelectric effect, French and British defense agencies developed the first real application. Sonar, used in WWI to locate enemy submarines, employed quartz plates to generate sound. When supplied with current, the plates would oscillate rapidly, producing soundwaves underwater. These soundwaves* would then reflect off submarines and back to a piezoelectric receiver, which converts detected mechanical signal (sound) into an electrical signal. This invention proved so pivotal to victory in WWI that the British gifted the technology to the United States in WWII.
The piezoelectric effect also found similar application on a much smaller scale than war-time submarines. Medical ultrasound also uses the premise of generating sound and detecting the ultimate reflection. In this case, a piezoelectric material is electrically charged to oscillate, emitting ultrasound. Ultrasonic waves are soundwaves with such a high frequency that humans cannot perceive them. In medical applications, the ultrasound reflects off of the internal body part and back to the noninvasive device. There, a different piezoelectric element senses and converts the sound into a digital signal, giving an image. Medical ultrasound has allowed doctors to diagnose organ issues and monitor pregnancies to better screen for complications.
Have you ever wondered why your wristwatch or clock says “quartz”? Despite a large role in matters of life and death, piezoelectricity may be best known for its role in in time-keeping. When supplied with charge from a battery, quartz oscillates predictably—32768 times per second—responding with an electric field each time. A smaller digital counter in the clock keeps track of these oscillations, thereby keeping time without needing any small gears found in other watch movements. This technology not only exists in wristwatches and clocks, but also computers.
Existing piezoelectric applications are impactful, but the behind-the-scenes nature of this science has kept it from broad recognition. Future applications may usher in technologies where the piezoelectric effect is front and center. Labs have made important strides in developing piezoelectric roads, wind harvesting, and biomaterials. For roads and wind harvesting, researchers envision harnessing abundant mechanical energy to generate clean electrical energy. With piezoelectric biomaterials, scientists aim to mimic natural physiological processes, such as the piezoelectricity of bone, which promotes growth and repair.
The Curie brothers, in their early 20s at the time, made a discovery which took decades to mature. But since the first application, piezoelectricity has quietly and consistently influenced the world. With it, humankind has advanced medicine, time-keeping, and technology. Without it, the Germans could have perhaps prevailed in a World War. For that, we say #ThanksScience and, of course, #ThanksPiezoPierre.
By Max Levy
*Not to detract from this piece appreciating an underappreciated scientific phenomena, but some military sonar is known to affect marine species, including whales. “Evidence shows that whales will swim hundreds of miles, rapidly change their depth (sometime leading to bleeding from the eyes and ears), and even beach themselves to get away from the sounds of sonar.” #ThanksScience, but also #SaveTheWhales.