This blog post explores how the piezoelectric effect—where pressure converts to electricity—can be applied to various everyday devices to enhance energy utilization.
When heading to the subway, you often see intriguing stairs. Each time someone steps on these stairs, a piano key sound plays, and a screen displays the number of people who have climbed them along with accumulating donations. How does this system work? And could this principle be applied in real life? For example, could the energy generated by touching a phone or exercising be used as a self-powered source?
The scientific principle at work here is the piezoelectric effect. The piezoelectric effect is the 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 primary piezoelectric effect, and the latter is called the secondary piezoelectric effect. To explain in detail, most materials are electrically neutral. However, materials with specific crystal structures have a slight misalignment between the positions of positive and negative charges, preventing neutrality from being canceled out and sometimes forming an electric field. This is called an electric dipole. Piezoelectric materials are characterized by having this electric dipole crystal structure.
When force is applied to a piezoelectric material, its crystal structure deforms, altering the size of the electric dipoles. This change in dipole arrangement causes the electric field to shift, generating electricity. Conversely, in the case of inverse piezoelectricity, applying an external electric field alters the electric dipole arrangement. This structural change then induces mechanical deformation corresponding to the characteristics of the applied electric field. Here, tension refers to the phenomenon where an external force applied parallel to the object’s central axis causes the object to elongate. Tension is classified as either simple tension or 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 thought that electricity was generated due to temperature changes, but the actual cause was mechanical deformation. A year later, Liebig predicted this inverse reaction through mathematical reasoning, enabling the Curie brothers to subsequently calculate the degree of energy conversion. Today, over 20 types of piezoelectric materials are classified based on their piezoelectric constants, with their properties systematically documented.
Devices or components utilizing this piezoelectric effect are called piezoelectric devices and are commonly used in everyday life. Examples include airbags, quartz watches, lighters, and gas stoves. Piezoelectric devices are categorized 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 direct and inverse elements is similar to that between a motor and a generator. However, while piezoelectric elements handle the interaction between electrical energy and mechanical energy, motors and generators handle the interaction between kinetic energy and electrical energy.
The advantages of piezoelectric elements are their intuitiveness and speed. Unlike most energy conversion methods that rely on thermal energy to turn turbines and convert it into kinetic energy, the piezoelectric effect enables a simpler and more intuitive form of energy conversion. A prime example is the airbag. Upon vehicle collision, the pressurized element instantly generates the energy required to deploy the airbag. Within 0.03 seconds after impact, the airbag inflates at a speed of 300 km/h. While not generating such immense energy, the piezoelectric element within the airbag’s accelerometer sensor is a device capable of instantaneously emitting a powerful force. This piezoelectric element estimates the acceleration based on the voltage generated during a collision. It then uses the nitrogen gas produced by the explosion of sodium azide, composed of sodium and nitrogen, to inflate the airbag.
The airbag example suggests the potential for piezoelectric devices to function as sensors. Like the reflexes of our own bodies, it evokes an image of David-like counterforce—a sensor that reacts swiftly and harnesses an opponent’s strength to subdue them. Pressure signals can be utilized in diverse ways, one being microphones and ultrasonic transducers that utilize sound waves. Microphones are sensors that convert voice signals into electrical signals. If such piezoelectric elements are applied to communication circuits, they can quickly and easily convey changes to the other party. An ultrasonic vibrator is a type of transducer that generates ultrasonic waves by evaporating water through vibration. Piezoelectric elements are also used in equipment like high-speed camera shutters, spray nozzles, and X-ray shutters. Their ability to detect high pressures more accurately makes them useful for military sensors, and they can also be applied to medical and industrial non-destructive sensors utilizing ultrasound.
Recently, a pacemaker incorporating a flexible piezoelectric device into the heart has been developed. It continuously supplies electricity as long as the heart beats, using electricity to force the heart to beat when patients with hypertension or arrhythmia experience irregular heart rhythms. This device simultaneously exhibits both direct and reverse piezoelectric effects, representing a true self-powered system. In the field of piezoelectric element research, polymer materials like piezoelectric polymers are being utilized. In Korea, transparent piezoelectric films are being manufactured, and the functionality and efficiency of the materials continue to improve. Due to its intuitive characteristics, it holds high potential for development in various fields such as music, learning, and medicine.
Disadvantages of piezoelectric devices include low efficiency and the generation of only a single pulse of current in direct devices. Furthermore, electricity is not generated unconditionally upon pressure application; a single pulse signal occurs only when both pressure and shape change are present.
Despite these limitations, primary and secondary piezoelectric devices hold value in gathering and utilizing 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. Those who experience it will realize the importance of saving energy and naturally develop a mindset of conservation. This tiny piezoelectric device is a powerful component proving that the small, sophisticated David is stronger than the giant Goliath.
