Last Updated on June 16, 2021 by You Ling
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Piezoelectric effect: When some dielectrics are deformed by external forces in a certain direction, polarization occurs inside them, and positive and negative opposite charges appear on the two opposite surfaces. When the external force is removed, it will return to the uncharged state. This phenomenon is called the positive piezoelectric effect. When the direction of the force changes, the polarity of the charge also changes. On the contrary, when an electric field is applied in the polarization direction of the dielectric, these dielectrics are also deformed, and the deformation of the dielectric disappears after the electric field is removed. This phenomenon is called an inverse piezoelectric effect. A type of sensor developed based on the dielectric piezoelectric effect is called a piezoelectric sensor.
The piezoelectric body is polarized by external mechanical force and causes the oppositely bound charge in the surface of the piezoelectric body. The charge density is proportional to the external mechanical force. This phenomenon is called positive piezoelectric effect. The electric body is deformed by the external electric field, and its shape variable is proportional to the strength of the external electric field. This phenomenon is called the inverse piezoelectric effect. The solid with positive piezoelectric effect must also have the inverse piezoelectric effect, and vice versa. The positive piezoelectric effect and the inverse piezoelectric effect are collectively referred to as the piezoelectric effect. Whether the crystal has a piezoelectric effect is determined by the symmetry of the crystal structure.
The principle of the piezoelectric effect is that if a pressure is applied to the piezoelectric material, it generates a potential difference (referred to as a positive piezoelectric effect), and vice versa, a mechanical stress (called an inverse piezoelectric effect) is generated. If the pressure is a high frequency vibration, then a high frequency current is produced. When a high-frequency electric signal is applied to a piezoelectric ceramic, a high-frequency acoustic signal (mechanical vibration) is generated, which is what we usually call an ultrasonic signal. That is to say, piezoelectric ceramics have the function of conversion and inverse conversion between mechanical energy and electrical energy, and this mutual relationship is indeed very interesting.
Piezoelectric materials can generate electric fields due to mechanical deformation, or mechanical deformation due to electric field. This inherent mechanical-electrical coupling effect makes piezoelectric materials widely used in engineering. For example, piezoelectric materials have been used to make intelligent structures. In addition to self-supporting capabilities, such structures have functions such as self-diagnosis, self-healing, and self-healing, and they play an important role in future aircraft design.
The piezoelectric effect can be divided into a positive piezoelectric effect and an inverse piezoelectric effect.
It means that when the crystal is subjected to an external force in a fixed direction, the internal phenomenon is generated, and at the same time, opposite signs are generated on one of the two surfaces; when the external force is removed, the crystal returns to the uncharged state; When the direction of action changes, the polarity of the charge also changes; the amount of charge generated by the force of the crystal is proportional to the magnitude of the external force. Piezoelectric sensors are mostly made using the positive piezoelectric effect.
It refers to the phenomenon that the alternating electric field is applied to the crystal to cause mechanical deformation of the crystal. Transmitters made with the inverse piezoelectric effect can be used for electroacoustic and ultrasonic engineering. The piezoelectric deformation of the piezoelectric sensitive element has five basic forms: thickness deformation type, length deformation type, volume deformation type, thickness shear type, and plane shear type. Piezoelectric crystals are anisotropic, and not all crystals can produce piezoelectric effects in these five states. For example, quartz crystals have no volumetric deformation piezoelectric effect, but have good thickness deformation and length deformation piezoelectric effect.
One type of sensor developed based on the dielectric piezoelectric effect is called a piezoelectric sensor.
Here again introduce the electrostrictive effect. The electrostrictive effect, that is, the dielectric is strained by the induced polarization under the action of the electric field, and the magnitude of the strain is proportional to the square of the electric field, independent of the direction of the electric field. The piezoelectric effect exists only in crystals without a center of symmetry. The electrostrictive effect exists for all dielectrics, whether amorphous or crystalline, whether it is a centrally symmetric crystal or a polar crystal.
Below we use piezoelectric ceramics to test the piezoelectric effect and the inverse piezoelectric effect.
Commonly used piezoelectric ceramics are made of lead zirconate titanate (PZT) material. A piezoelectric ceramic piece made of a PZT material is adhered to a circular brass piece to form a piezoelectric ceramic element. It has a pronounced piezoelectric effect.
First, the two leads of the piezoelectric ceramic sheet A are connected to the signal generator through a push button switch. The two leads of the piezoelectric ceramic sheet B are connected to the input end of a loudspeaker (with a horn). The two piezoelectric ceramic sheets A and B were fixed on the same wooden board with black sealant. When the observer presses the button switch to turn on the signal generator and the piezoelectric ceramic A, due to the inverse piezoelectric effect, A starts to vibrate and transmits the vibration to the wooden box, and the vibration of the wooden box is transmitted to the piezoelectric ceramic B. Due to the piezoelectric effect, a changing electrical signal is generated on both sides of B, and then transmitted to the loudspeaker to make the horn sound, so this experiment simultaneously demonstrates the piezoelectric effect and the inverse piezoelectric effect
Discovery of piezoelectric effect
In 1880, Pierre Curie and the Jacques Curie brothers discovered that the tourmaline had a piezoelectric effect. In 1881, they verified the inverse piezoelectric effect through experiments and obtained the positive and negative piezoelectric constants. In 1984, the German physicist Wardmar Voigt (Goldenmar Voigt) inferred that only crystals of the 20 midpoint group with no symmetry center could have a piezoelectric effect.
When you gently press the button, the gas stove will immediately ignite the blue flame. Have you ever realized what brought this convenience to you? Connect a seemingly bland ceramic to the wire and ammeter, and use a hand on it. The pointer of the ammeter will also oscillate – even if it produces a current, is it a strange thing? In fact, this is a piezoelectric ceramic, a functional ceramic material that converts mechanical energy and electrical energy. What kind of material is piezoelectric ceramic? This is a material with a piezoelectric effect. The so-called piezoelectric effect refers to the deformation of some media under the action of force, causing the surface of the medium to be charged, which is a positive piezoelectric effect. Conversely, when an excitation electric field is applied, the medium will undergo mechanical deformation, which is called the inverse piezoelectric effect. This wonderful effect has been applied by scientists in many fields closely related to people’s lives to achieve energy conversion, sensing, driving, frequency control and other functions.
Piezoelectric ceramics have sensitive characteristics and can convert extremely weak mechanical vibration into electrical signals, which can be used in sonar systems, weather detection, telemetry environmental protection, household appliances, etc. Earthquakes are devastating disasters, and the source of the earthquake begins in the depths of the earth’s crust, which was previously difficult to predict, putting humanity in an embarrassing situation.
Piezoelectric ceramics produce small deformation variables under the action of an electric field, up to a fraction of a millionth of their own size. Don’t underestimate the slight change, the precise control mechanism based on this principle–piezoelectric actuator, for Precision instrumentation and mechanical control, microelectronics, bioengineering, etc. are all great news.
Frequency control devices such as resonators and filters are the key devices that determine the performance of communication devices. Piezoelectric ceramics have obvious advantages in this respect. It has good frequency stability, high precision and wide applicable frequency range, and is small in size, non-hygroscopic, and long in life. Especially in multi-channel communication equipment, it can improve the anti-interference, so that the previous electromagnetic equipment can not be expected to face it. The fate of substitution.
Let’s look at a new type of bicycle damping controller. It is difficult to achieve a smooth effect with a general shock absorber. This ACX damping controller provides a continuously variable damping function for the first time through the use of piezoelectric materials. A sensor monitors the movement of the impact piston at a rate of 50 times per second. If the piston moves quickly, it is usually caused by a rapid impact caused by uneven ground. At this time, the maximum shock absorption function needs to be activated; if the piston moves slowly, then Indicates that the road surface is flat and only needs to use a weaker shock absorption function.
It can be said that although piezoelectric ceramics are new materials, they are quite civilian. It is used in high technology, but it is more in life to create a better life for people.
The application of piezoelectric materials can be roughly divided into two categories: vibration energy and ultrasonic vibration energy – electrical energy transducer applications, including electroacoustic transducers, hydroacoustic transducers and ultrasonic transducers, and other sensors. And drive applications.
1, the transducer
A transducer is a device that converts mechanical vibration into an electrical signal or mechanical vibration driven by an electric field.
The piezoelectric polymer electroacoustic device utilizes the transverse piezoelectric effect of the polymer, while the transducer design utilizes the bending vibration of the polymer piezoelectric bimorph or piezoelectric single wafer driven by an external electric field, and the above principle can be used to produce electricity. Acoustic devices such as microphones, stereo headphones, and tweeters. At present, the research on piezoelectric polymer electroacoustic devices mainly focuses on the characteristics of piezoelectric polymers, and develops devices that are difficult to realize by other current technologies and have special electroacoustic functions, such as anti-noise telephones and broadband ultrasonic signal transmission systems. Wait.
At the beginning of the research of piezoelectric polymer underwater acoustic transducers, they are aimed at military applications, such as large-area sensor arrays and monitoring systems for underwater detection. Later, the application fields have gradually expanded to geophysical detection and acoustic testing equipment. Various prototype hydroacoustic devices developed to meet specific requirements, using different types and shapes of piezoelectric polymer materials such as sheets, sheets, laminations, cylinders and coaxial wires to fully utilize piezoelectric polymers High elasticity, low density, easy to prepare for large and small components with different cross sections, and the same acoustic impedance and water order. The last feature makes the hydrophone prepared by piezoelectric polymer can be placed in the measured sound field, and the sound field is perceived. The sound pressure inside is not disturbed by the measured sound field due to its own existence. The high elasticity of the polymer reduces transient oscillations in the hydrophone, further enhancing the performance of the piezoelectric polymer hydrophone.
Piezoelectric polymer transducers have achieved the most successful applications in biomedical sensors, especially in ultrasound imaging. The excellent flexibility and formability of PVDF films make them easy to apply to many sensor products.
2, piezoelectric actuator
Piezoelectric actuators use the inverse piezoelectric effect to convert electrical energy into mechanical or mechanical motion. Polymer drivers are based primarily on polymer bimorphs, including driver applications based on polymer bimorphs using both lateral and longitudinal effects. Research includes display device control, micro-displacement generation systems, and the like. A lot of research is needed to make these creative ideas practical. Electron beam irradiation of P(VDF-TrFE) copolymers gives the material the ability to produce large stretch strains, creating favorable conditions for the development of new polymer drivers. Under the impetus of potential defense applications, the use of radiation-modified copolymers to prepare all-polymer materials for underwater acoustic emission devices is being systematically carried out with the support of the US military. In addition, the use of radiation-modified copolymers for excellent properties, research and development in medical ultrasound, vibration and noise reduction applications, but also a lot of exploration.
3. Application on the sensor
Piezoelectric pressure sensor
Piezoelectric pressure sensors are made using the piezoelectric effect of piezoelectric materials. The basic structure of the piezoelectric pressure sensor is shown in the right figure. Since the amount of charge of the piezoelectric material is constant, special care must be taken when connecting to avoid leakage.
The piezoelectric pressure sensor has the advantages of self-generated signal, large output signal, high frequency response, small volume and firm structure. The disadvantage is that it can only be used for kinetic energy measurements. A special cable is required, and self-recovery is slow when subjected to sudden vibration or excessive pressure.
Piezoelectric acceleration sensor
The piezoelectric element is generally composed of two piezoelectric wafers. Electrodes are plated on both surfaces of the piezoelectric wafer, and leads are drawn. A mass is placed on the piezoelectric wafer, and the mass is generally made of a relatively large metal tungsten or a high specific gravity alloy. The mass is then preloaded with a hard spring or bolt, and the entire assembly is housed in a metal housing of the original base. In order to isolate any strain of the test piece from being transmitted to the piezoelectric element to avoid false signal output, it is generally necessary to thicken the base or use a material with a relatively high rigidity. The weight of the housing and the base is almost the weight of the sensor. half.
When measuring, the sensor base and the test piece are rigidly fixed together. When the sensor is subjected to a vibration force, since the rigidity of the susceptor and the mass is relatively large, and the mass of the mass is relatively small, the inertia of the mass can be considered to be small. Therefore, the mass is subjected to the same motion as the susceptor and is subjected to an inertial force opposite to the direction of acceleration. Thus, the mass has a strain force proportional to the acceleration acting on the piezoelectric wafer. Since the piezoelectric wafer has a piezoelectric effect, an alternating charge (voltage) is generated on its two surfaces. When the acceleration frequency is much lower than the natural frequency of the sensor, the output voltage of the sensor is proportional to the force, that is, It is proportional to the acceleration of the test piece. The output power is led out from the sensor output. After input to the preamplifier, the acceleration of the test piece can be tested with a common measuring instrument. If an appropriate integrating circuit is added to the amplifier, it can be tested. The vibration speed or displacement of the test piece.
4. Application in robot proximity (ultrasonic sensor)
The main purpose of the robot installation proximity sensor is as follows: First, before the object is contacted, the necessary information is obtained to prepare for the next movement; second, the movement space of the robot hand and the foot is accessible. Object. If there are obstacles, take certain measures in time to avoid collisions; third, to obtain general information about the surface shape of the object.
Ultrasound is a kind of mechanical wave that the human ear can’t hear. The frequency is above 20KHZ. The sound that can be heard by the human ear, the vibration frequency range is only 20HZ-20000HZ. Ultrasonic waves can become sonic rays and directionally propagate because of their short wavelength and small diffraction. The purpose of the ultrasonic sensor is to detect the distance between the presence of surrounding objects and the measuring object. Generally used to detect large objects in the surrounding environment, can not measure objects with a distance less than 30mm.
The ultrasonic sensor includes four main parts: an ultrasonic transmitter, an ultrasonic receiver, a timing circuit, and a control circuit. Its working principle is roughly as follows: First, the ultrasonic transmitter emits pulsed ultrasonic waves in the direction of the object to be measured. After the transmitter sends out a series of ultrasonic waves, it turns itself off and stops transmitting. At the same time, the ultrasonic receiver starts to detect the echo signal, and the timing circuit also starts timing. When the ultrasonic wave encounters an object, it is reflected back. After the ultrasonic receiver receives the echo signal, the timing circuit stops counting. At this time, the time recorded by the timing circuit is the propagation time from the start of the transmission of the ultrasonic wave to the receipt of the echo signal. Using the propagation time value, the distance between the measured object and the ultrasonic sensor can be converted. The formula for this conversion is simple, that is, the product of half the time of sound wave propagation and the speed of sound wave propagation in the medium. The entire working process of the ultrasonic sensor is performed sequentially under the control of the control circuit.
In addition to the above applications, piezoelectric materials have a wide range of other applications. Such as frequency discriminators, piezoelectric oscillators, transformers, filters, etc.
Here are a few of the developing piezoelectric ceramic materials and several new applications.
1. Fine grain piezoelectric ceramics
Conventional piezoelectric ceramics are polycrystalline materials composed of multi-domain crystal grains of several micrometers to several tens of micrometers, and the size is no longer sufficient. Reducing the particle size to sub-micron can improve the processability of the material, making the substrate thinner, increasing the array frequency, reducing the loss of the transducer array, improving the mechanical strength of the device, and reducing the multilayer device. The thickness of the layer, thereby reducing the drive voltage, is beneficial for improving laminated transformers and brakes. Reducing the particle size has so many benefits as described above, but at the same time it also has the effect of reducing the piezoelectric effect. In order to overcome this effect, the conventional doping process has been changed to increase the piezoelectric effect of fine-grain piezoelectric ceramics to a level comparable to that of coarse-grain piezoelectric ceramics. The cost of making fine-grained materials is now competitive with ordinary ceramics. In recent years, people have studied cutting and grinding with fine-grained piezoelectric ceramics, and produced some high-frequency transducers, micro-brakes and thin buzzers (porcelain sheets 20-30um thick), which proved the fine grain pressure. The superiority of electric ceramics. With the development of nanotechnology, the research and application development of fine-grained piezoelectric ceramic materials is still a hot topic in the near future.
2. PbTiO3 piezoelectric material
PbTiO3 piezoelectric ceramics are most suitable for making high-frequency high-temperature piezoelectric ceramic components. Although there is a problem that PbTiO3 ceramics are difficult to be fired, polarization is difficult, and production of large-sized products is difficult, people have done a lot of work on modification to improve their sinterability. The grain growth is suppressed to obtain a fine, anisotropic modified PbTiO3 material of each crystal grain. In recent years, there have been many reports on improved PbTiO3 materials, which have been widely used in metal flaw detection and high frequency devices. At present, the development and application development of this material is still a topic of concern for many piezoelectric ceramic workers.
3. Piezoelectric ceramic-polymer composite
A piezoelectric composite material composed of an inorganic piezoelectric ceramic and an organic polymer resin has both the properties of inorganic and organic piezoelectric materials, and can produce characteristics that are not present in both phases. Therefore, it is possible to combine the advantages of the two-phase material as needed to produce transducers and sensors with good performance. Its receiving sensitivity is very high, and it is more suitable for underwater acoustic transducers than ordinary piezoelectric ceramics. Piezoelectric composites also have major advantages in other ultrasonic transducers and sensors. Domestic scholars are also very interested in this field, doing a lot of process research, and doing some useful basic research work on the structure and performance of composite materials, and are currently working on the development of piezoelectric composite materials.
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4. Piezoelectric specific multi-crystal single crystal piezoelectric body
Conventional piezoelectric ceramics have stronger piezoelectric effects than other types of piezoelectric materials, and thus have been widely used. However, as a large-energy, high-energy conversion material, the piezoelectric effect of conventional piezoelectric ceramics still cannot meet the requirements. In recent years, people have done a lot of work to develop new piezoelectric materials with better piezoelectricity. It has been discovered and developed Pb(A1/3B2/3)PbTiO3 single crystals (A=Zn2+, Mg2+). ). This type of single crystal d33 can be up to 2600pc / N (piezoelectric ceramic d33 up to 850pc / N), k33 can be as high as 0.95 (piezoelectric ceramic K33 up to 0.8), its strain >1.7%, almost than the piezoelectric ceramic strain An order of magnitude higher. The energy storage density is as high as 130 J/kg, while the piezoelectric ceramic energy storage density is within 10 J/kg. Iron voltage economists say that the emergence of such materials is another leap in the development of piezoelectric materials. Nowadays, the United States, Japan, Russia and China have begun to study the production process of such materials, and the success of its mass production will surely bring about the rapid development of piezoelectric materials.
In recent years, people have developed a number of polymer materials with piezoelectric effects and inverse piezoelectric effects, and these materials are called “artificial muscles.” Researchers around the world have launched a challenge: to see who can make artificial robotic arms first, and must win in one-on-one wrists with human arms.
The piezoelectric effect is a phenomenon in which a certain kind of medium has a different charge on the surface of the medium when it is deformed under the action of force. Experiments show that the amount of this bound charge is proportional to the force, and the more the amount of electricity, the larger the potential difference (voltage) between the two surfaces. This magical effect has been applied to many areas closely related to people’s production, life, military, and technology to achieve power-electrical conversion and other functions. For example, piezoelectric ceramics can be used to convert external forces into electrical energy, and piezoelectric flashers, gas stove ignition switches, and projectile trigger fuzes without flint can be produced. In addition, piezoelectric ceramics can also be used as sensitive materials for electroacoustic devices such as loudspeakers and phono heads; for piezoelectric seismographs, it can monitor subtle vibrations that humans cannot perceive, and accurately measure the source orientation and intensity. To predict earthquakes and reduce losses. Piezoelectric actuators made with piezoelectric effects have precise control functions and are important devices in the fields of precision mechanics, microelectronics and bioengineering. It can be said that piezoelectric ceramics and other devices are not only widely used in the field of science and technology, but also quite “civilian”. For the majority of “smokers”, there is a “zero contact” with piezoelectric ceramics every day, but they ignore it.
A large number of popular disposable plastic lighters are made of piezoelectric ceramic devices. Take out the piezoelectric ignition component,
What kind of instrument can the instantaneous voltage generated by the voltage ceramic component of the machine be measured? At first, we tried to measure the DC high-voltage gear of the ordinary pointer type multi-meter. It was found that the voltage of the two electrodes can only slightly shake the pointer when the black plastic pressure bar of the ignition element is pressed. The reason for the analysis is that because the voltage pulse duration is very short, the pointer inertia is large, and the pointer cannot synchronously reflect the change of the voltage to make a large deflection.
Switching to the digital display type multimeter, in order to have no pointer inertia effect, it should be able to read the instantaneous high voltage. Whoever knows what is going wrong, we can’t see the expected high voltage reading, we can only see some variable low voltage. data. In analysis, this is due to the slow response of the liquid crystal display, the ignition pulse duration is very short, and it is too late to display the highest instantaneous voltage. It can only display some random voltage readings during the voltage drop (smooth phase).
Finally, we moved out of the lab’s “heavy weapon” – the oscilloscope, and then did a try. We used the most common J2459 student oscilloscope in the lab, and the connecting wires were two ordinary wires with a final fish clip. In theory, the oscilloscope uses the electron beam to deflect and slide on the screen to show the movement of the spot. The electron beam has very little inertia and should be able to “track” the change of the ignition high voltage pulse. The experimental results are not unexpected.
Put the oscilloscope AC/DC selector switch in the “DC” position, and the scanning range is placed in the “10～100kHz” block. Move the horizontal bright line to the center of the square coordinate with X shift and Y shift, and set it on the X axis. In order to estimate the maximum voltage amplitude of the piezoelectric effect, we must first determine the voltage scale by using the square coordinate system in front of the screen: using two wires connected to the yoke input Y of the oscilloscope, 1.5 of a dry battery The V voltage is added to the oscilloscope, the attenuation is placed at 1, and the Y gain is placed at the lowest. It can be found that the horizontal jump line (or the next jump) is just two grids, that is, the two grids represent 1.5V. In the case where the Y gain is constant, the Y attenuation is placed in the 1000 (ie, one thousandth) block, and the two grids in front of the screen can represent 1500V.
Connect the crocodile clips of the two feeders on the Y input terminal to the two electrodes of the piezoelectric lighter piezoelectric element, and quickly press the black plastic pressure rod to see that the horizontal bright line is at the center height. Or down) and then return to the original position. Due to the afterglow effect of the fluorescent screen, the horizontal bright line appears on the oscilloscope as a bright band with a height of four squares, which indicates that the voltage amplitude of the pulse is above 3000V.
If you want to observe the waveform of this voltage pulse, you can carefully adjust the oscilloscope “scan fine adjustment” knob (switch the range to “10～100Hz” beforehand), and we can see it on the screen. The waveform shown in Figure 2 has a steeper voltage rise, a lowerer gradual decrease, and a peak value of more than four squares.
Set the attenuation of the oscilloscope to 1000, the scanning range to “10～100Hz”, the “scanning fine adjustment” to the left, ie the scanning frequency is 10Hz, adjust the “X gain” and “X shift” knobs to make the X axis The scanning line is filled with 10 grids, then each grid represents 1/10×1/10s, that is, 0.01 pressing the black plastic pressure rod of the piezoelectric element, it can be seen that the piezoelectric pulse continues for one grid, as shown in Fig. 3, that is, corresponding At 0.01 s, that is, the pulse duration is approximately 0.01 s.
There is a very interesting kind of crystal that produces different charges at both ends when you squeeze or stretch it. This effect is called the piezoelectric effect. A crystal that produces a piezoelectric effect is called a piezoelectric crystal. Crystal (α-quartz) is a well-known piezoelectric crystal.
If pressure is applied to the sliced piece on the crystal crystal in a certain direction, an electric charge will be generated on the sheet. If the sheet is stretched in the opposite direction, a charge will also appear on the sheet, but the sign is reversed. The greater the force of extrusion or stretching, the more charge there will be on the crystal. If the electrodes are plated on both ends of the sheet and are supplied with alternating current, the sheet will be periodically elongated or shortened, i.e., begins to vibrate. This inverse piezoelectric effect has been widely used in science and technology. Piezoelectric quartz flakes can be made from crystal, which is only a few square millimeters in area and only a few tenths of a millimeter in thickness. Don’t underestimate this small chip, it plays a huge role in radio technology. As mentioned earlier, in an alternating electric field, the vibration frequency of such a sheet is not changed at all. This kind of stable vibration is necessary for controlling the frequency in radio technology. In many electrical appliances such as color TVs in your home, there are filters made of piezoelectric wafers to ensure the clarity of images and sounds. There is a core component in the quartz electronic watch that you wear on your hand called a quartz vibrator. It is this key component that guarantees a higher travel accuracy of quartz watches than other mechanical watches.
Instruments equipped with piezoelectric crystal components enable technicians to study the changes in pressure in steam engines, internal combustion engines and various chemical equipment. The piezoelectric crystal can even measure the pressure of the fluid in the pipeline, the pressure that the cannon barrel can withstand when launching the projectile, and the instantaneous pressure when the bomb explodes.
Piezoelectric crystals are also widely used for sound reproduction, recording, and transmission. A piezoelectric wafer mounted on a microphone converts the vibration of the sound into a change in current. As soon as the sound wave hits the piezoelectric sheet, an electric charge is generated on the electrodes at both ends of the sheet, and the size and sign thereof change with the sound. The change in charge on the piezoelectric wafer can be turned into a distant place by radio waves through electronic devices. These radio waves are received by the radio and are reverberated in the air by the vibration of the piezoelectric crystal sheets placed on the radio speakers. Is it possible to say that the piezoelectric wafer in the microphone can “sound” the sound, and the piezoelectric crystal on the speaker will “speak” or “sing”.
Piezoelectric phenomenon is a process in which stress is applied to a material to induce charge on the surface of the material. Generally, this process is reversible, that is, when the material is subjected to electrical parameters, the material also generates deformation energy. Wood cellulose, strontium collagen and various polyamino acids are common polymeric piezoelectric materials, but their piezoelectricity is too low and has no use value. In organic polymer materials, compounds such as polyvinylidene fluoride have strong piezoelectric properties. The magnitude of the piezoelectricity depends on whether the orientation of the dipoles contained in the molecules is uniform. In addition to polyvinylidene fluoride compounds containing C-F bonds having a large dipole moment, many polymers containing other strongly polar bonds also exhibit piezoelectric properties. Copolymers such as vinylidene diacetate with vinyl acetate, isobutylene, methyl methacrylate, vinyl benzoate and the like all exhibit strong piezoelectric properties. And high temperature stability is better. Mainly used as a transducing material, such as the preparation of acoustic components and control displacement components. The more common examples of the former are transducers in sonic sensors, sonar, headphones, microphones, telephones, sphygmomanometers, and the like. The two piezoelectric films are bonded together, and opposite voltages are applied, respectively, and the film is bent to constitute a displacement control element. Using this principle, aligning members for optical fiber alignment devices, automatically opening and closing curtains, phonographs, and video recorders can be made.
Piezoelectric ceramics are one of the most widely used functional ceramics. The “electronic lighter” used by many people in daily life and the electronic igniter on the gas stove are an application of piezoelectric ceramics. The igniter uses the piezoelectric characteristics of the piezoelectric ceramic to apply a force to it to generate a high voltage of more than ten kV, thereby generating a spark discharge and achieving the purpose of ignition.
Piezoelectric ceramics are actually a polarized ferroelectric ceramic with a piezoelectric effect. It was first introduced in 1946 when it was confirmed that barium titanate ceramics were ferroelectric: almost ten years later, Jaffe and others discovered the PbTi03-PbZrO2 system (the so-called PZT system) and later discovered The mPZT-based ternary piezoelectric ceramic and the citrate piezoelectric ceramic. The performance and applicability of piezoelectric ceramics have been greatly improved. In particular, the appearance of ternary piezoelectric ceramics has made the piezoelectric ceramics have a large room for selecting a certain coupling coefficient and temperature characteristics, which can meet the requirements of various electronic instruments, thereby greatly increasing the application range of piezoelectric ceramics. It is. For example, ceramic filters and ceramic discriminators, electroacoustic transducers, underwater acoustic transducers, acoustic wave devices, electro-optic devices, infrared detectors and piezoelectric gyros are all piezoelectric ceramics in modern electronic technology. Applications.
What is piezoelectric ceramics? In fact, it is a functional ceramic material that can convert mechanical energy and electrical energy into each other. The so-called piezoelectric effect means that some media are subjected to mechanical pressure, even if the pressure is as small as the vibration of the sound wave, it will produce a shape change such as compression or elongation, causing the surface of the medium to be charged, which is a positive piezoelectric effect. Conversely, when an excitation electric field is applied, the medium will undergo mechanical deformation, which is called the inverse piezoelectric effect.
In 1880, the French Curie brothers discovered the “piezoelectric effect.” In 1942, the first piezoelectric ceramic material, barium titanate, was made in the United States, the former Soviet Union, and Japan. In 1947, the barium titanate pickup – the first piezoelectric ceramic device was born. In the early 1950s, another piezoelectric ceramic material with a performance superior to that of barium titanate, lead zirconate titanate, was successfully developed. Since then, the development of piezoelectric ceramics has entered a new stage. From the 1960s to the 1970s, piezoelectric ceramics continued to improve and became perfect. For example, lead zirconate titanate piezoelectric ceramics modified with various elements, ternary and quaternary piezoelectric ceramics based on lead zirconate titanate have also emerged. These materials are excellent in performance, simple in manufacturing, low in cost, and widely used.
Piezoelectric igniters, mobile X-ray power supplies, and projectile igniters can be fabricated by using piezoelectric ceramics to convert external forces into electrical energy. By replacing the ordinary flint with two piezoelectric ceramic columns of 3 mm in diameter and 5 mm in height, a gas electronic lighter capable of continuously firing tens of thousands of times can be made. Using piezoelectric ceramics to convert electrical energy into ultrasonic vibration, it can be used to explore the position and shape of underwater fish, non-destructive testing of metals, ultrasonic cleaning, ultrasonic medical treatment, and various ultrasonic cutters and welding devices. Soldering iron, processing plastics and even metals.
The sensitivity of the piezoelectric ceramic to external forces makes it possible to sense the disturbance of the air by the flying insects flapping their wings over a dozen meters and convert the extremely weak mechanical vibration into an electrical signal. This feature of piezoelectric ceramics can be applied to sonar systems, meteorological detection, telemetry environmental protection, and household appliances.
Today, piezoelectric ceramics have been used by scientists in defense construction, scientific research, industrial production, and many fields closely related to people’s lives, becoming a versatile person in the information age.
In the aerospace field, piezoelectric gyros made of piezoelectric ceramics are the “rudders” of spacecraft and artificial satellites flying in space. Relying on the “rudder”, spacecraft and satellites to ensure their intended position and route. The traditional mechanical gyro has short life, poor precision and low sensitivity, which can not meet the requirements of spacecraft and satellite systems. The compact piezoelectric gyro has high sensitivity and good reliability.
On the submarine that sneaked into the deep sea, there is a sonar system called the underwater scout. It is an indispensable equipment for underwater navigation, communication, reconnaissance of enemy ships, cleaning of enemy water mines, and a powerful tool for the development of marine resources. It can detect fish stocks and survey the topography of the seabed. In this sonar system, there is a pair of bright “eyes” – piezoelectric ceramic underwater acoustic transducers. When the acoustic signal emitted by the underwater acoustic transducer hits a target, a reflected signal is generated, which is received by another receiving type underwater acoustic transducer, and the target is found. Currently, piezoelectric ceramics are one of the best materials for making underwater acoustic transducers.
In medicine, the doctor puts the piezoelectric ceramic probe on the inspection site of the human body, and sends an ultrasonic wave after being energized. When the human body touches the tissue of the human body, it generates an echo, and then receives the echo, which is displayed on the fluorescent screen, and the doctor will Can understand the internal conditions of the human body.
In the industry, there are piezoelectric ceramic components in the geological detector, which can be used to judge the geological conditions of the formation and identify underground deposits. There is also a transformer in the TV set – a voltage ceramic transformer, which is smaller in size, lighter in weight, and has an efficiency of 60% to 80%. It can withstand a high voltage of 30,000 volts, keeping the voltage stable and completely eliminating the blur of the TV image. Deformation defects. Most of the TV sets produced abroad now use piezoelectric ceramic transformers. A 15-inch picture tube, using a 75 mm long piezoelectric ceramic transformer. This makes the TV set smaller and lighter.
Piezoelectric ceramics are also widely used in everyday life. Two piezoelectric ceramic columns with a diameter of 3 mm and a height of 5 mm were used to replace the gas electronic lighter made of ordinary flint, which can be fired tens of thousands of times. Electronic igniters made using the same principle are excellent tools for igniting gas stoves. There is also a children’s toy made of piezoelectric ceramic components, such as a buzzer made of piezoelectric ceramics in the belly of a toy puppy, and the toy will make a realistic and interesting sound.
With the development of high technology, the application of piezoelectric ceramics will become more and more extensive. In addition to being used in high-tech fields, it is more about making people’s eyes in daily life and creating a better life for people.
Editor by Ms.Liao of He-Shuai in 2018/07/01