In this blog post, we will look at how the piezoelectric effect, in which pressure is converted into electricity, can be applied to various devices in everyday life to increase energy utilization.
When I take the subway, I often see interesting stairs. Every time I go up and down these stairs, the sound of a piano keyboard plays, and a screen shows the number of people who have climbed the stairs and the amount of donations that have been made. How does this system work? Also, is there any way to use this principle in real life? For example, is it possible to use the energy generated by touching a mobile phone or exercising as its own power source?
The scientific principle that applies in this case is the piezoelectric effect. The piezoelectric effect is a phenomenon in which electrical energy is generated when pressure is applied, and there are two types: normal 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, but materials with a specific crystal structure have a slight offset between the positive and negative charges, which does not cancel out and forms an electric field. This is called an electric dipole, and piezoelectric materials are characterized by the crystal structure of this electric dipole.
When force is applied to the piezoelectric material, the crystal structure is deformed, causing the size of the electric dipole to change, which in turn causes the electric field to change and electricity to be generated. In the case of inverse piezoelectricity, applying an electric field from the outside changes the arrangement of the electric dipoles, and this structural change causes mechanical changes depending on the characteristics of the electric field. Here, the term “tensile stress” refers to the phenomenon in which an external force is applied parallel to the axis of an object, causing the object to stretch. Tensile stress is divided into simple tensile stress and eccentric tensile stress depending on whether the line of action is parallel to the axis.
The piezoelectric effect was discovered by the Curie brothers in the 19th century, and although it was initially thought that electricity was generated due to temperature changes, it was actually caused by mechanical deformation. A year later, Rife predicted this reverse reaction through mathematical reasoning, and the Curie brothers were then 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 are systematically organized.
Devices or components that apply the piezoelectric effect are called piezoelectric devices, and they are commonly used in everyday life. For example, airbags, quartz watches, lighters, and gas ranges. Piezoelectric elements are classified into primary and secondary depending on the type of piezoelectric effect. Primary elements include lighters, airbags, and microphones, while secondary elements include filters, speakers, and motors. The relationship between primary and secondary elements is similar to that between a motor and a generator. However, piezoelectric elements deal with the interaction between electrical and mechanical energy, while motors and generators deal with the interaction between kinetic and electrical energy.
The advantages of piezoelectric elements are intuitiveness and speed. Unlike most energy conversion methods, which turn turbines into kinetic energy by converting thermal energy, the piezoelectric effect enables simpler and more intuitive energy conversion. An example of this is airbags. When a vehicle is involved in a collision, the principle is that the pressurized element immediately generates the energy needed to inflate the airbag. The airbag inflates to 300 km/h in 0.03 seconds after the collision. Although this is not a large amount of energy, the element that can instantaneously generate strong force is the piezoelectric element in the acceleration sensor in the airbag. This piezoelectric element inflates the airbag with nitrogen gas generated by the explosion of sodium azide, which is composed of sodium and nitrogen, by estimating the acceleration from the voltage generated during the collision.
The airbag case suggests the possibility of piezoelectric elements acting as sensors. It is a sensor that responds quickly, like our body’s reflexes, and is reminiscent of the image of a David who uses the opponent’s strength against them to subdue them. There are various ways to use pressure signals, one of which is a microphone using sound waves and an ultrasonic vibrator. A microphone is a sensor that converts voice signals into electrical signals, and if such a piezoelectric element is applied to a communication circuit, changes can be quickly and easily communicated to the other party. Ultrasonic vibrators are one of the piezoelectric elements, which generate 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. They can detect large pressures more accurately, making them useful for military sensors, and can also be applied to medical and industrial non-destructive sensors that use ultrasound.
Recently, a pacemaker that has a flexible piezoelectric element inserted into the heart has been developed. It continuously supplies electricity as long as the heart is beating, and when a patient with hypertension or arrhythmia has a heartbeat that is not normal, it uses electricity to force the heart to beat. In this device, both positive and negative piezoelectric reactions occur simultaneously, which can be said to be a true self-powered device. In the field of piezoelectric device research, transparent piezoelectric films are being manufactured in Korea using polymer materials such as piezoelectric polymers, and the functionality and efficiency of the materials are being continuously improved. Thanks to their intuitive characteristics, they have great potential for development in various fields such as music, learning, and medicine.
The disadvantages of piezoelectric elements include low efficiency and the fact that they only generate one-time currents. Also, electricity does not occur just because there is pressure, but a one-time signal is generated only when there is a change in pressure and shape.
Despite these limitations, primary and secondary piezoelectric elements have value in that they collect and use 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 great contribution to energy conservation. Anyone who has experienced it will realize the importance of energy conservation and naturally develop a sense of saving. This small piezoelectric element is a powerful component that proves that the sophisticated and small David is stronger than the great Goliath.
