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What is a piezoelectric device? What happens when pressure is applied? Top image | Matsusada Precision

About Piezoelectric Devices

Some dielectrics, such as quartz and tourmaline, have a phenomenon called the piezoelectric effect, in which a voltage is generated when pressure is applied. Conversely, when voltage is applied to these dielectrics, they deform (reverse piezoelectric effect). Passive devices that utilize the piezoelectric effect are called "piezoelectric devices".

Dielectrics that can produce the piezoelectric effect have an asymmetrical lattice-like crystal structure. As a result, the centers of positive and negative charges are unevenly distributed, and polarization is constantly occurring within the crystal.

Under normal conditions, the crystal adsorbs ions from the air, and the charge on the crystal surface is neutralized, so no voltage is generated. However, when the crystal is deformed by external pressure, the bias between positive and negative charges is displaced, and the state of polarization changes. This is how voltage is generated.

This is called dielectric (electric) polarization, and the material that causes this phenomenon is called a piezoelectric material.

The piezoelectric effect was discovered in 1880 by Nobel laureate in physics Pierre Curie and his brother Jacques Curie. An example of the use of the piezoelectric effect that we are familiar with is the ignition device for gas stoves and lighters.

In ignition devices that use piezoelectric devices, when the switch is pressed, the parts inside the switch strike the piezoelectric element with a "click" sound. The voltage generated by this pressure creates a spark, which ignites the gas.

The reverse piezoelectric effect is also used in inkjet printers, SLR autofocus, and image stabilization.

Piezoelectric devices are also called "piezoelectric devices," derived from the Greek word piezein, meaning "to apply pressure".

Structure of piezoelectric devices

Piezoelectric devices have a very simple structure, consisting of a piezoelectric material sandwiched between electrodes. The advantage of this structure is that it is strong and lightweight.

Structure of a piezoelectric (piezo) element

Quartz (synthetic quartz) and ferroelectric ceramics are mainly used as piezoelectric materials. In recent years, ferroelectric ceramics such as barium titanate and barium zirconate have been widely used because single crystals such as quartz take a long time to manufacture, and their composition varies.

Ferroelectric ceramics are made by baking the material at high temperature and then applying a strong electric field of several kV/mm. By utilizing the piezoelectric effect, piezoelectric elements can be used as ignition devices as well as vibration and pressure sensors.

In addition, the reverse piezoelectric effect allows physical motion to be obtained by simply applying a voltage without the need for mechanical components such as motors or gears. It is quick to respond and can control smooth and precise movements of less than one micron. It is also easy to miniaturize, making it an excellent component for precision machinery.

On the other hand, the range that can be moved by a single element is narrow, only a few tens of microns. For applications that require a larger variation, a stacked type is used.

Applications of the piezoelectric effect

Piezoelectric elements are used in a variety of products other than the gas stoves and lighters mentioned at the beginning of this article. For example, facilities have been installed in many places where the vibration and weight of a person walking can be converted into electricity by piezoelectric elements and used for nighttime lighting and opening and closing of doors.

In addition, piezoelectric elements themselves are small and can be precisely controlled, so they are often used as components in precision equipment that requires precision. Products that use piezoelectric elements include the following

Vibration sensors
A sensor that utilizes the piezoelectric effect. 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, and are used not only in industrial applications but also in 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, and when a signal is sent to the piezoelectric element, the expansion and contraction of the piezoelectric element causes the metal plate to vibrate, which turns into sound. The advantage is that it is lightweight and consumes little power.
Inkjet printers
Like 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 ink. The amount of ink can be precisely controlled, and printing at high speed is possible.
Piezo driver
A piezo driver is required to drive the piezoelectric element. Piezoelectric elements have capacitance, so they bounce back input signals under certain conditions. Therefore, normal DC current is not enough to drive the piezoelectric element optimally. A power supply that can suck in as well as spit out the output power is needed. A piezo driver is a high-voltage amplifier-type power supply that can stably drive a piezoelectric element according to its purpose.
Actuator
An actuator is a device that converts electrical energy into mechanical energy in general. Piezoelectric devices are used as actuators in a variety of situations because they can convert electrical energy into mechanical energy through the inverse piezoelectric effect.

Actuators using piezoelectric elements have the following advantages

  1. Fast response time
  2. Smooth movement
  3. Precise movement within a few microns is possible
  4. High load capacity
  5. High durability

Piezoelectric actuators are also used in the base that supports the primary mirror of large astronomical telescopes for observing the universe. Since 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 in smartphones, where only the fingertips tremble when the touch panel is pressed in.

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 you try to take a picture with a camera, the movement of your hand to press the shutter or the minute movements of your body will cause a camera shake. Image stabilization is used to quickly move the light-receiving part of the camera according to the magnitude of the blur. Piezoelectric actuators are also suitable here because they need to move accurately with a very fast response time.