Important Applications of Piezoelectric Ceramics

                           Important Applications of Piezoelectric Ceramics

Piezoelectric ceramics have been widely used because of their piezoelectricity and the diversity of electromechanical properties caused by piezoelectricity. Due to the wide variety of piezoelectric ceramic devices and their wide range of applications, it is difficult to strictly classify them using a simple method. General applications can be broadly divided into two categories: piezoelectric vibrators and piezoelectric transducers.

1. Transducer

The application of the piezoelectric effect is varied, and one of the most important ones is to use it’s transducers characteristics. It’s energy conversion characteristic,it is the effect of electricity is applied to the piezoelectric ceramic, The electrical energy can be converted into mechanical energy through the reverse voltage effect; The electrical effect converts mechanical energy into electrical energy. People make use of this physical property of piezoelectric ceramics to manufacture many kinds of piezoelectric devices, which are widely used in underwater communication, ultrasound, high-voltage ignition and other fields.

1, Piezoelectric ceramic igniter

This is a device that converts mechanical force into electric sparks to ignite combustibles. It is an electromechanical transducer. In 1958, he pioneered the use of piezoelectric effect of barium titanate (BaTiO3) ceramics for ignition. The ignition rate of this piezo element piezo rod is not high, and the noise is large. In 1962, lead zirconate titanate (PZT) piezoelectric ceramics were used to make igniters. This kind of igniter is widely used in daily life, industrial production and military affairs, and is used for igniting and detonating gases, various explosives and rockets.

(1) Basic principle: The working process of the igniter is divided into three stages: high voltage generation, discharge ignition and ignition of combustible gas. High voltage generation - Take cylindrical piezoelectric ceramic elements as an example, when the mechanical force F acts on the cylinder, the crystal will be distorted, causing the center of positive and negative charges in the crystal to shift, so that a large amount of free charges will appear on the upper and lower surfaces of the cylinder accumulate, it is producing a high voltage output. The output voltage is: V=ga3Fh/A, where A——the cross-sectional area of the cylinder; h——the height of the cylinder; ga3——the piezoelectric voltage constant. Discharge ignition - put the piezoelectric ceramic element in a closed circuit and leave a proper gap. When the voltage rises to the discharge voltage of the gap, a discharge spark will be generated in the gap. Ignition of combustible gas - general fuel gas is not easy to burn, so ethane which is easy to gasify is mostly used. In order to prolong the discharge time and prevent the spark from being extinguished too quickly, to increase the ignition rate. An appropriate resistor can be connected in series at the discharge end.

(2) The structure and working principle of the igniter,there are many kinds of igniters, and the household piezoelectric igniter is taken as an example to illustrate its structure and working principle. It can be fixed on the household cooker to ignite the gas, turn the cam switch,it use the protruding part of the cam to push the impact block, and compress the spring behind the impact block. When the protruding part of the cam breaks away from the impact block, due to the elastic force of the spring, the impact block gives the piezoceramic .piezoelectric element an impact force, which generates high voltage at both ends of the piezoelectric element, and outputs high voltage from the middle electrode to generate an electric spark to ignite the gas.

2. Underwater acoustic transducer

An underwater acoustic transducer is a transducer device used for underwater communication and detection. People know that air communication and detection mainly rely on electromagnetic waves, such as radio communication and radar equipment, etc., all rely on electromagnetic waves to transmit information in the air. It is not possible to use electromagnetic waves for underwater communication and detection. This is because electromagnetic waves have a large propagation loss in water, and they will be absorbed by someone who does not travel far. However, the propagation loss of sound waves in water is very small, so underwater communication and detection mainly use Sound waves are used to transmit information, and the instruments that generate and detect sound waves are called sonar systems. Sonar systems are indispensable tools for underwater navigation, communication, detection of submarines and fish schools, and marine research. People compare sonar in the water with radar in the air, and the eyes and ears of the sonar system are underwater acoustic transducers. Research on underwater acoustic transducers began in World War I. France's Langevin first used quartz crystals to make underwater acoustic transducers based on the piezoelectric effect. Although the underwater acoustic transducer created by Lang Zhiwan was limited by technical conditions at the time and was not actually used on deep-sea submarines, it made a significant contribution to the development of underwater acoustic science in the future. The Langevin transducer uses the reverse voltage effect of the quartz crystal to emit sound waves into the water, piezo ceramics tube receives the sound waves returned from the water through the positive voltage effect, and performs some underwater measurements according to the reciprocating time of the pulse sound waves.

People have conducted in-depth and systematic research on piezoelectric underwater acoustic transducers to make them practical. However, the main piezoelectric materials used at that time were water-soluble piezoelectric crystals - Roche salt and potassium dioxyphosphate. In the late 1950s, piezoelectric ceramics appeared. Making underwater acoustic transducers with piezoelectric ceramics has almost become the main piezoelectric material chosen by people. Because it has many characteristics that piezoelectric crystals did not have in the past, it has become the most ideal piezoelectric material for making underwater acoustic transducers, and there is no other material that can match it. The main advantages of piezoelectric ceramic underwater acoustic transducers are:

(1) No need for DC bias voltage and coil, the vibration system is simple;

(2) The piezoelectric ceramic transducer is small in size and has excellent characteristics;

(3) Piezoelectric ceramic transducers can be made into any shape as required.

Piezoelectric transducers are the most widely used type of transducers in the field of underwater acoustic technology. The performance indicators of underwater acoustic transducers only need to have operating frequency, electromechanical coupling coefficient, electromechanical conversion coefficient, quality factor, frequency characteristics, impedance characteristics, directional characteristics, amplitude characteristics, transmission sensitivity, reception sensitivity, transmitter power, temperature and time stability properties, mechanical strength and weight etc. However, for a practical transducer, it is not necessary to put forward so many index requirements regardless of the occasion, but to put forward different and representative index requirements according to its use and application occasions.

Second, the piezoelectric vibrator

After the appearance of PZT piezoelectric ceramics, it is possible to make ceramic filters. Ceramic filters with different frequencies can be made by using different vibration modes of piezoelectric vibrators. The earliest applied vibration mode is radial vibration or contour vibration, making 455kHz filter. Later, the frequency of ceramic filters developed to both ends, with the high end reaching 10MHz and the low end reaching below 1kHz. Due to the application of the energy trap mode, the frequency of the ceramic filter is as high as 100MHz, the surface acoustic wave filter excited by the interdigital transducer has reached above 1GHz, and the highest frequency of the surface acoustic wave filter using piezoelectric ceramics as the substrate has been Up to 630MHz.

The piezoelectric transformer is also a vibrator in terms of its application, and its basic structure is to set two sets of electrodes on the piezoelectric ceramic body to form four terminals. Adding an electrical signal to the primary side makes it resonate, and the secondary side has an output. In this way, it works as a transformer at the time of resonance. The research on piezoelectric transformers started earlier. The power and driving voltage of piezoelectric transformers made of monolithic ceramics are not easy to increase. The multi-layer piezoelectric transformer is manufactured with the same multi-layer composite technology as the monolithic capacitor manufacturing technology, and its power and driving voltage are greatly improved, which further expands the application range of the piezoelectric transformer.

1. Piezoelectric Transformer

Piezoelectric transformers have been developed since the 1950s. At that time, barium titanate was used as the main material. The boost ratio is low (only 50~60 times). The output voltage is about 3000V. With the emergence of lead zirconate titanate piezoelectric ceramic materials, the step-up ratio is increased to 300~500 times, and it is gradually popularized and used in televisions, electrostatic copiers, and negative ion generators as high-voltage power supplies.

(1) Basic principles. The electrical vibration energy input to the piezoelectric ceramic is converted into mechanical vibration energy through the inverse piezoelectric effect, and then converted into electrical energy through the positive piezoelectric effect. Impedance conversion (from low impedance to high impedance) is realized during these two energy conversions, so that high piezoelectric output can be obtained at the resonant frequency of the ceramic sheet. Now take the horizontal and vertical transformers with stretching vibration as an example to illustrate the principle of transformers.

The whole ceramic chip is divided into two parts, the left part is the input end (also known as the driving part), there are burnt silver electrodes on the upper and lower sides, polarized along the thickness direction, the right part is the output end (also known as the power generation part), and the right part is the output end (also known as the power generation part). There are burnt silver electrodes on the surface. Polarized along its length. When an alternating voltage is applied to the input end, due to the reverse voltage effect, the ceramic piece will produce stretching vibration along the length direction, which will convert the input electrical energy into mechanical energy; while the power generation part will convert mechanical energy into electrical energy through the positive voltage effect, and then transfer it from the output end The output voltage. When there is no load, the open circuit boost ratio, Qm is the mechanical quality factor of the material; K31, K33 are the longitudinal and transverse electromechanical coupling coefficients of the material; L is the length of the power generation part; t is the thickness of the transformer. Piezoelectric transformers are mainly used in the case of high voltage, low power and sine wave conversion, and have unique advantages such as high output voltage, light weight, small size, no leakage magnetic field, and no combustion. In order to obtain multiple voltage outputs, according to the output voltage of the horizontal-vertical transformer is proportional to the length, the closer to the end of the power generation part, the higher the voltage, and electrodes can be made at different positions of the power generation part as shaft heads to obtain different voltage outputs .

(2) The basic working principle and characteristics of monolithic (multilayer) piezoelectric ceramic transformers. Piezoelectric ceramics are a brittle material. In order to ensure its mechanical strength, the piezoelectric transformer must have a certain thickness, and the driving voltage of the above-mentioned transformer is quite limited. For this reason, the monolithic (multi-layer) piezoelectric ceramic transformer project came into being. After adopting the monolithic (multi-layer) structure, the thickness and number of layers of each single layer can be adjusted, and the driving voltage is no longer limited, so the voltage can be made Electric transformers can work in the best state no matter what driving voltage they are in.

The core technologies of this project are submicron low-temperature sintered piezoelectric ceramic materials, internal electrode co-firing technology, polarization treatment technology and structural design. Monolithic (multilayer) piezoelectric ceramic transformers (MPTs) are the third generation of electronic transformers with the following features.

① Ultra-thin: the thickness generally does not exceed 4mm.

②High conversion efficiency: over 97% at full load (resistive load).

③ It has the self-protection feature of automatic cut-off of load short circuit.

④Resonant transformer: It can realize zero voltage and zero current conversion.

⑤ It has quasi-constant current output characteristics for low impedance loads.

⑥ No reverse peak voltage, reliable protection of the power amplifier circuit.

⑦ No electromagnetic interference.

⑧No coil breakdown, mildew breakage.

⑨Salt spray resistance, good weather resistance, especially suitable for use in marine climates.

2. Piezoelectric ceramic pickup and speaker

Piezoelectric ceramics transducers are widely used in electroacoustic equipment, such as piezoelectric ceramic pickups and speakers.

(1) Double-diaphragm vibrator (Figure 6-16). Electroacoustic equipment requires low mechanical impedance and can match the sound source or vibration source, and the double-diaphragm piezoelectric vibrator can meet these requirements. It is made of two piezoelectric ceramic sheets that are stretchable in length. When one piece is stretched, the other piece is shortened, and the whole bends.

It gives the working principle of the double-diaphragm vibrator. When a piece of piezoelectric ceramic with a certain thickness is bent under force, one side of its thickness is elongated, and the other side is compressed. At this time, charges will be generated inside the ceramic piece. , but because the polarization direction of the entire diaphragm is the same, the upper side is elongated, and the lower side is compressed, which causes the electric dipole moment to be opposite, and the upper and lower sides have the same charge sign, so there is no potential difference, as shown in Figure 6-16 (a ) shown. If a double-diaphragm structure with two superimposed sheets is used instead, a voltage output can be obtained when the force is bent. Figure 6-16(b) uses two diaphragms with opposite polarization directions connected in series. When a force is applied, the upper one stretches and the lower one compresses. Since the polarization directions are opposite, the upper and lower sides of the double diaphragm are charged with opposite signs, and a voltage output can be obtained. Figure 6-16(c) is formed by connecting two diaphragms with the same polarization direction in parallel, and the output voltage can also be obtained.

(2) Piezoelectric ceramic speaker structure and working principle: Piezoelectric ceramic speaker is a simple and lightweight electro-acoustic device with high sensitivity, no magnetic field spillover, no need for copper wires and magnets, low cost, low power consumption, Easy repair, easy mass production, etc.

Its drive system is a piezoelectric ceramic double diaphragm, the vibration system is a paper cone, and the coupling element effectively transfers the energy of the drive system to the vibration system. When working, the electrical energy added to the piezoelectric ceramic double diaphragm is converted into mechanical energy, which is transmitted to the paper cone through the coupling element to make it vibrate and sound. The piezoelectric double-diaphragm has a relatively high impedance, which constitutes a voltage drive. The relationship between the force F and the voltage V is F=KV, and K is the proportional coefficient. If the vibration mechanical impedance including the radiation impedance is Z, the vibration speed is : v=F/Z, the sound pressure P at the center r of the high diaphragm can be obtained. |P|=10fρS/r |v| where: f—frequency; ρ—medium density; S—effective area of vertebral body. In addition, other electro-acoustic energy converters can be made according to the piezoelectric effect of piezoelectric ceramics, such as transmitters, receivers, buzzers, etc....