Piezoelectricity is the conversion of pressure applied to an object into electrical energy, and it’s used in a variety of everyday technologies, including piano steps, airbags, and pacemakers. Piezoelectric devices are used to generate power from pressure changes or to transmit signals by detecting vibrations, and while they are less efficient, they are ideal for harvesting small amounts of energy. Recent research has extended to transparent piezoelectric films, high molecular polymers, and more, opening up new possibilities in medical, military, telecommunications, and other fields.
When you take the subway, you often see an interesting staircase. As you go up and down these stairs, you hear the sound of piano keys, and a screen shows the number of people who have climbed the stairs and the amount of donations accumulated. How does this system work, and how can we use it in real life? For example, could we use the energy generated by touching our phones or exercising as a power source?
The scientific principle at work in this case is the piezoelectric effect. Piezoelectricity is the generation of electrical energy when pressure is applied, and it comes in two forms: forward and reverse piezoelectricity. The former is called primary piezoelectricity, while the latter is called secondary piezoelectricity. To elaborate, most materials are electrically neutral, but in certain crystal structures, the positive and negative charges are somewhat misaligned, so that neutrality is not canceled out and an electric field is formed. This is called an electric dipole, and piezoelectric materials are characterized by having a crystal structure with an electric dipole.
When a force is applied to a piezoelectric material, the crystal structure is deformed to change the size of the electric dipole, which in turn changes the electric field and generates electricity. In the case of reverse piezoelectricity, the arrangement of electric dipoles changes when an electric field is applied from the outside, and this structural change causes mechanical changes depending on the characteristics of the electric field. Tension is a phenomenon in which an object is stretched by an external force applied parallel to the object’s center axis. Tension is divided into simple tension and eccentric tension depending on whether the line of action coincides with the central axis.
The piezoelectric effect was discovered by the Curie brothers in the 19th century and was initially thought to be caused by a change in temperature, but it was actually due to mechanical strain. A year later, Lippmann predicted the reverse reaction through mathematical reasoning, and the Curie brothers were able to calculate the degree of energy conversion. Today, more than 20 different piezoelectric materials are classified according to their piezoelectric constants and their properties have been systematically summarized.
Devices or components that utilize piezoelectricity are called piezoelectric devices and are commonly used in everyday life. Examples include airbags, quartz watches, lighters, and gas stoves. Piezoelectric devices are categorized into primary and secondary depending on the type of piezoelectric effect. Primary devices include lighters, airbags, and microphones, while secondary devices include filters, speakers, and motors. The relationship between a forward and reverse element is similar to that of a motor and a generator. However, piezoelectric devices deal with the interaction between electrical energy and mechanical energy, while motors and generators deal with the interaction between kinetic energy and electrical energy.
The advantages of piezoelectrics are intuitiveness and speed. Unlike most energy conversion, which involves turning a turbine to convert heat energy into kinetic energy, the piezoelectric effect allows for a simpler and more intuitive energy conversion. A classic example of this is an airbag. In a car crash, the element under pressure instantly generates the energy needed to deploy the airbag, which inflates to 300 kilometers per hour in just 0.03 seconds after impact. While this is not a lot of energy, the element that can generate this instantaneous force is the piezoelectric in the acceleration sensor in the airbag. This piezoelectric estimates the acceleration from the voltage generated during a collision and inflates the airbag with nitrogen gas from the explosion of sodium azide, a compound of sodium and nitrogen.
The airbag example suggests the potential for piezoelectrics to act as sensors. Like our own reflexes, they respond quickly, conjuring up images of David using his opponent’s strength to overpower him. Pressure signals can be used in a variety of ways, including microphones and ultrasonic vibrators that utilize sound waves. Microphones are sensors that convert voice signals into electrical signals, and if these piezoelectric elements are applied to communication circuits, they can quickly and easily communicate changes to the other party. Ultrasonic vibrators are a type of inverse device that generates ultrasonic waves by evaporating water through vibration. Piezoelectric elements are also used in equipment such as high-speed camera shutters, sprayers, and X-ray shutters. Their ability to detect large pressures more accurately makes them useful for military sensors, and they also have applications in medical and industrial non-destructive sensors that utilize ultrasound.
More recently, pacemakers have been developed with flexible piezoelectric devices inserted into the heart. It provides a continuous supply of electricity as long as the heart doesn’t stop beating, and it can be used to force the heart to beat when it’s not beating normally in people with high blood pressure or arrhythmias. In this device, both forward and reverse piezoelectric reactions occur simultaneously, making it truly self-powered. In the field of piezoelectric device research, transparent piezoelectric films are being manufactured in Korea by utilizing polymeric materials such as piezoelectric polymers, and the functionality and efficiency of the materials are also continuing to improve. Thanks to their intuitive properties, they have great potential for development in various fields such as music, learning, and medicine.
The disadvantages of piezoelectric devices include their low efficiency and the fact that only a one-time current is generated in the static element. Furthermore, pressure does not necessarily generate electricity, but only a one-time signal when pressure and shape change.
Despite these limitations, primary and secondary piezoelectric devices are valuable for harvesting and using microscopic amounts of energy. As the saying goes, “a thousand miles go a long way with a single step,” and this small energy conversion technology can make a big contribution to energy conservation. After experiencing it, people will realize the importance of energy conservation and naturally have a sense of care for it. This tiny piezoelectric device is a powerful component that proves that a sophisticated and small David is stronger than a big Goliath.
