In this blog post, we’ll explore how the piezoelectric effect—where pressure is converted into electricity—can be applied to various everyday devices to improve energy efficiency.
When heading to the subway, you often come across interesting staircases. Every time you walk up or down these stairs, the sound of piano keys plays, and a screen displays the number of people who have used the stairs along with the amount of money raised. How does this system work? Also, are there ways to apply this principle in real life? For example, could we use the energy generated by touching a smartphone or exercising as a self-powered source?
The scientific principle at work here is the piezoelectric effect. The piezoelectric effect is a phenomenon where electrical energy is generated when pressure is applied, and it comes in two forms: direct piezoelectricity and inverse piezoelectricity. The former is called the first-order piezoelectric effect, and the latter is called the second-order piezoelectric effect. To explain in detail, while most materials are electrically neutral, materials with specific crystal structures have positive and negative charges slightly misaligned, so the 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 these electric dipoles.
When force is applied to a piezoelectric material, the crystal structure deforms, altering the size of the electric dipole; this, in turn, changes the electric field, generating an electric charge. In the case of inverse piezoelectricity, applying an external electric field alters the arrangement of the electric dipoles, and this structural change causes mechanical deformation depending on the characteristics of the electric field. Here, tension refers to the phenomenon where an external force is applied parallel to the object’s central axis, causing the object to stretch. Tension is classified 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; initially, it was believed that electricity was generated by temperature changes, but in fact, mechanical deformation was the cause. A year later, Lipman predicted this inverse relationship through mathematical reasoning, and subsequently, the Curie brothers were able to calculate the degree of energy conversion. Currently, more than 20 types of piezoelectric materials are classified according to their piezoelectric constants, and their characteristics have been systematically documented.
Devices or components that utilize this piezoelectric effect are called piezoelectric devices, and they are commonly used in everyday life. Examples include airbags, quartz watches, lighters, and gas stoves. Piezoelectric devices are classified as primary or secondary based on the type of piezoelectric effect. Primary devices include lighters, airbags, and microphones, while secondary devices include filters, speakers, and motors. The relationship between forward and reverse devices is similar to that between a motor and a generator. However, while piezoelectric devices deal with the interaction between electrical and mechanical energy, motors and generators deal with the interaction between kinetic and electrical energy.
The advantages of piezoelectric devices are their intuitiveness and speed. Unlike most energy conversion methods, which involve converting thermal energy into kinetic energy by turning a turbine, the piezoelectric effect enables a simpler and more intuitive form of energy conversion. A prime example of this is the airbag. Based on the principle that a device subjected to pressure during a vehicle collision immediately generates the energy needed to deploy the airbag, the airbag inflates to 300 km/h within 0.03 seconds after the collision. Although the energy involved is not as great as in the example above, the piezoelectric element found in the airbag’s acceleration sensor is capable of generating a strong force instantaneously. This piezoelectric element estimates acceleration based on the voltage generated during a collision and uses nitrogen gas produced by the explosion of sodium azide—a compound consisting of sodium and nitrogen—to inflate the airbag.
The airbag example suggests the potential for piezoelectric devices to function as sensors.
Like the reflexes in our bodies, these sensors react rapidly, evoking the image of David, who overpowered his opponent by harnessing the force against him. There are various ways to utilize pressure signals, including microphones and ultrasonic transducers that use sound waves. A microphone is a sensor that converts sound signals into electrical signals; if such piezoelectric devices were applied to communication circuits, they could quickly and easily convey changes to the other party. An ultrasonic transducer is a type of piezoelectric device that generates ultrasonic waves by evaporating water through vibration. Piezoelectric elements are also used in equipment such as high-speed camera shutters, spray nozzles, and X-ray shutters. Because they can detect high pressure more accurately, they are useful in military sensors and can also be applied to medical and industrial non-destructive testing sensors that utilize ultrasonic waves.
Recently, a pacemaker incorporating a flexible piezoelectric element into the heart has been developed. It continuously supplies electricity as long as the heart is beating, and in cases where patients with hypertension or arrhythmia experience irregular heartbeats, it uses electricity to force the heart to beat normally. In this device, both forward and reverse piezoelectric effects occur simultaneously, which can be considered true self-powering. In the field of piezoelectric device research, polymer materials such as piezoelectric polymers are being utilized to manufacture transparent piezoelectric films in Korea, and the functionality and efficiency of these materials continue to improve. Thanks to their intuitive characteristics, they hold high potential for development in various fields such as music, education, and medicine.
The disadvantages of piezoelectric devices include low efficiency and the fact that they generate only a single pulse of current. Furthermore, electricity is not generated simply by the presence of pressure; a single pulse signal is generated only when both pressure and a change in shape occur.
Despite these limitations, primary and secondary piezoelectric devices hold value in their ability to collect and utilize minute amounts of energy. As the saying goes, “A journey of a thousand miles begins with a single step,” this small energy conversion technology can make a significant contribution to energy conservation. Anyone who experiences it will realize the importance of energy conservation and naturally develop a mindset of conservation. In a sense, this small piezoelectric device is a powerful component that proves that the sophisticated, small David is stronger than the giant Goliath.
