About Piezoelectric Devices
The piezoelectric effect is a phenomenon occurring primarily in dielectric materials, such as quartz and tourmaline, where mechanical stress generates an electric charge or voltage. Conversely, when a voltage is applied to these materials, they undergo physical deformation (expansion or contraction). This phenomenon is known as the inverse piezoelectric effect. Passive components utilizing these effects are collectively called "piezoelectric devices."
Dielectric materials exhibiting the piezoelectric effect possess an asymmetrical crystal lattice structure. Consequently, the centers of positive and negative charges are unevenly distributed, creating continuous polarization within the crystal.
Under normal conditions, surface charges are neutralized by ions in the air, resulting in no observable voltage. However, when external mechanical pressure deforms the crystal, the balance of positive and negative charges shifts, altering the polarization state and generating a voltage.
This phenomenon is known as dielectric (electric) polarization, and materials exhibiting this property are referred to as piezoelectric materials.
The piezoelectric effect was discovered in 1880 by Pierre Curie, a Nobel laureate in physics, and his brother Jacques Curie. A common application of this effect is the ignition mechanism found in gas stoves and lighters.
In these ignition devices, pressing the switch causes an internal hammer to strike a piezoelectric element. The high voltage generated by this mechanical impact creates a spark, igniting the gas.
The inverse piezoelectric effect is widely applied in precision technologies, including inkjet printer heads, camera autofocus mechanisms, and image stabilization systems.
The term "piezo" is derived from the Greek word "piezein," meaning "to press" or "to squeeze."
Structure of piezoelectric devices
Piezoelectric devices have a simple structure comprising a piezoelectric material sandwiched between electrodes. The advantage of this structure is that it is strong and lightweight.
Ferroelectric ceramics are manufactured by sintering materials at high temperatures and strictly aligning their polarization using a strong electric field (poling). Utilizing the piezoelectric effect, these elements serve as sensors for vibration and pressure, or as high-voltage ignition sources.
Conversely, utilizing the inverse piezoelectric effect allows for precise physical motion simply by applying voltage, eliminating the need for mechanical gears or motors. This enables rapid response times and smooth, sub-micron positioning control. Their compact size makes them ideal for integration into precision machinery.
While a single piezoelectric layer offers limited displacement (only a few tens of microns), stacked piezoelectric actuators--comprising multiple layers--are used to achieve larger displacement ranges at lower driving voltages.
Applications of the piezoelectric effect
Piezoelectric elements are used in a variety of products other than the aforementioned gas stoves and lighters. For example, facilities have been installed in many places where the vibration and weight of a walking person can be converted into electricity via piezoelectric elements and used for night-time lighting and the opening and closing of doors.
Moreover, as piezoelectric elements are small and can be precisely controlled, they are often used as components in high-precision equipment.
- Vibration sensors
- These sensors utilize the piezoelectric effect. Here, a weight is attached to the piezoelectric element, and the force generated by the vibration is converted into a voltage for detection. They are characterized by their small size and relatively low cost. They are used in industrial applications as well as medical applications, such as heart rate sensors.
- Piezoelectric speaker
- Using the inverse piezoelectric effect, piezoelectric materials can be used as the source of vibration for speakers. A plate-shaped piezoelectric element is attached to a metal plate; when a signal is sent to the piezoelectric element, the expansion and contraction of the piezoelectric element cause the metal plate to vibrate, thereby generating sound. These speakers are lightweight and consume little power.
- Inkjet printers
- Similar to piezoelectric speakers, the ink ejection mechanism of inkjet printers consists of a piezoelectric element bonded to a metal plate, which is deflected by the deformation of the piezoelectric element to eject the ink. The amount of ink can be precisely controlled, and printing at a high speed can be accomplished.
- Piezo driver
- A piezo driver is essential for operating piezoelectric elements effectively. Since piezoelectric elements act as capacitive loads, they store electrical energy when voltage is applied. To return the element to its original position quickly, this stored charge must be rapidly discharged. Therefore, a standard DC power supply is often insufficient. Optimal control requires a high-voltage amplifier (piezo driver) capable of both sourcing (outputting) and sinking (absorbing) current to handle the reactive energy and ensure fast, precise response.
- Actuator
- An actuator is a device that converts electrical energy into mechanical energy. Piezoelectric devices are used as actuators in a variety of situations because they can convert electrical energy into mechanical energy via the inverse piezoelectric effect.
Actuators using piezoelectric elements have the following advantages
- Fast response time
- Smooth movement
- Precise movement within a few microns is possible
- High load capacity
- High durability
Piezoelectric actuators are also used in bases that support the primary mirror of large astronomical telescopes used for observing the universe. As the mirror of an astronomical telescope can be as large as 8 meters in diameter, deformation due to its own weight is unavoidable. Therefore, the backside of the mirror is supported by the actuator to correct the distortion of the mirror surface.
Actuators using piezoelectric elements are used not only in cutting-edge science such as in astronomical telescopes but also in familiar applications such as smartphones, especially in touchscreen panels where only fingers are used.
Piezo actuators are also used in semiconductor lithography equipment to expose electronic circuit patterns on silicon plates and optical measuring instruments used in manufacturing and research laboratories to fine-tune the positioning of mirrors, just as in astronomical telescopes.
They are also used for image stabilization in digital cameras. When one attempts to take a picture with a camera, the movement of the hand to press the shutter or the minute movements of the body would cause the camera to shake. Image stabilization is used to quickly move the light-receiving part of the camera based on the magnitude of the blur. Piezoelectric actuators are also suitable in this regard given the requirement to move accurately with an extremely fast response time.
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