Piezoelectric effect

Positive piezoelectric effect, inverse piezoelectric effect

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Piezoelectric effect: some dielectric When deformed by external force in a certain direction polarization At the same time, positive and negative charges appear on its two opposite surfaces. When the external force is removed, it will return to the uncharged state, which is called Positive piezoelectric effect 。 When Force When the direction of the charge changes, the polarity of the charge also changes. On the contrary, when dielectric Of Polarization direction These dielectrics will also deform when an electric field is applied to them. After the electric field is removed, the deformation of dielectrics will disappear. This phenomenon is called Inverse piezoelectric effect 。 A type of sensor developed according to the piezoelectric effect of dielectric is called Piezoelectric sensor 。

Chinese name: Piezoelectric effect

Foreign name: piezoelectric effect
Classification: Positive piezoelectric effect, inverse piezoelectric effect

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definition

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Piezoelectric body Under the action of external mechanical force, the electrode will be formed, and the opposite sign will appear on the surface of both ends of the piezoelectric body Bound charge , which charge density It is proportional to the external mechanical force, which is called positive piezoelectric effect The deformation of a piezoelectric body under the action of an external electric field is proportional to the intensity of the external electric field. This phenomenon is called the inverse piezoelectric effect Solids with positive piezoelectric effect must also have negative piezoelectric effect, and vice versa Positive piezoelectric effect and converse piezoelectric effect are collectively called piezoelectric effect. Whether a crystal has piezoelectric effect is determined by the symmetry of the crystal structure^[1]

principle

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The principle of piezoelectric effect is that if Piezoelectric material When pressure is applied, it will produce Potential difference (called positive piezoelectric effect), on the contrary, when voltage is applied Mechanical stress (called inverse piezoelectric effect). If the pressure is a high-frequency vibration, it will produce high-frequency current 。 When high-frequency electrical signals are added to piezoelectric ceramics, high-frequency acoustic signals (mechanical vibration) will be generated, which is what we usually call ultrasonic signals. In other words, piezoelectric ceramics have mechanical energy The function of conversion and inverse conversion with electric energy is really interesting.

Piezoelectric materials can generate electric field due to mechanical deformation, or mechanical deformation due to electric field. This inherent mechatronics coupling effect Piezoelectric materials have been widely used in engineering. For example, piezoelectric materials have been used to make Intelligent structure , this kind of structure has carrying capacity In addition, it is also self diagnostic Adaptability And self-healing, which will play an important role in the future aircraft design.

classification

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Piezoelectric effects can be divided into Positive piezoelectric effect and Inverse piezoelectric effect 。

Positive piezoelectricity

Piezoelectric effect

Means: when crystal Under the action of external force in a fixed direction, the internal electrode phenomenon will be generated, and at the same time, opposite charges will be generated on two surfaces; When the external force is removed, the crystal returns to the uncharged state; When External force action When the direction changes, the polarity of the charge also changes; Produced by force on crystal Charge quantity It is proportional to the external force. Piezoelectric transducer Most are made of positive piezoelectric effect.

Inverse piezoelectricity

It refers to the application of Alternating electric field Causative crystal Mechanical deformation Phenomenon. Use the inverse piezoelectricity Effect manufacturing Transmitter It can be used in electroacoustic and ultrasonic engineering. piezoelectricity Sensitive element There are thickness deformation Length deformation There are five basic types: type A, volume deformation type, thickness shear type and plane shear type. Piezoelectric crystal yes anisotropy Not all crystals can produce piezoelectric effects in these five states. for example quartz The crystal has no volume deformation piezoelectric effect, but has good thickness deformation and length deformation piezoelectric effect.

basis dielectric A type of sensor developed by piezoelectric effect is called Piezoelectric sensor 。

Here's another introduction Electrostriction effect 。 Electrostrictive effect, i.e dielectric Under the action of electric field Polarization The strain is proportional to the square of the electric field Electric field direction irrelevant. Piezoelectric effect only exists in none Symmetrical center In the crystal. The electrostriction effect exists for all dielectrics, whether Amorphous Matter, or crystalline matter, whether in the center Symmetry Crystal, or Polar crystal 。

Now let's use Piezoelectric ceramics Test piezoelectric effect and inverse piezoelectric effect.

The commonly used piezoelectric ceramics are made of zirconium Lead titanate （ PZT ）It is made of materials. Made of PZT material Piezoelectric ceramic sheet Glue to round brass On the film Piezoelectric ceramic element 。 It has obvious piezoelectric effect.

First, pass two leads of piezoelectric ceramic sheet A through one Button switch And Signal generator Connect. Connect the two leads of piezoelectric ceramic sheet B with Loudspeaker (with horn). Apply black to two piezoelectric ceramic chips A and B Lute. It is fixed on the box made of the same board. When Observer Press the button switch and turn on the signal generator and piezoelectric ceramic A. Due to the converse piezoelectric effect, A starts to vibrate and transmits the vibration to the wooden box. The vibration of the wooden box is transmitted to the piezoelectric ceramic B. Due to the piezoelectric effect, changes occur on both sides of B electrical signal , and then it is transmitted to the loudspeaker to make the loudspeaker sound, so this experiment demonstrates the piezoelectric effect and the converse piezoelectric effect at the same time.

Discovery of piezoelectric effect

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1880 Pierre Curie And the Jacques Curie brothers tourmaline It has piezoelectric effect. In 1881, they verified the inverse piezoelectric effect through experiments and obtained the positive and inverse piezoelectric constants. In 1984, the German physicist Woldemar Voigt deduced that there was nothing Symmetrical center Only the crystal of 20 midpoint group of 20 can have piezoelectric effect.

Application status

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When you press the button, gas cooker Immediately ignite the blue flame. Have you ever realized what brought you this convenience? Connect a piece of plain looking ceramic to the wire and ammeter As soon as you press it with your hand, the pointer of the ammeter will also swing - it will produce current, isn't it strange? Actually, this is Piezoelectric ceramics , a function that can convert mechanical energy and electrical energy into each other ceramic material 。 What kind of material is piezoelectric ceramics? This is a material with piezoelectric effect. The so-called piezoelectric effect refers to the deformation of some media under the action of force, causing the surface of the media to be charged, which is a positive piezoelectric effect. On the contrary, when an exciting electric field is applied, the medium will produce mechanical deformation, which is called inverse piezoelectric effect. This wonderful effect has been applied by scientists in many fields closely related to people's life to realize energy conversion , sensing, driving frequency control And other functions.

Piezoelectric ceramics have sensitive characteristics, which can transform extremely weak mechanical vibration Converted into electrical signals, which can be used for Sonar system Weather detection, remote sensing environmental protection, household appliances, etc. Earthquake is a devastating disaster, and Source It started from deep in the earth's crust and was difficult to predict before, which made mankind into an awkward situation where there was no way to do anything.

The deformation of piezoelectric ceramics under the action of electric field is very small, at most not more than one ten thousandth of its own size. Don't underestimate this small change. It is accurate to make based on this principle Control mechanism －－ Piezoelectric actuator It is a great blessing for the control of precision instruments and machinery, microelectronics technology, bioengineering and other fields.

resonator 、 wave filter Equal frequency control device , is the decision Communication equipment Piezoelectric ceramics, the key device of performance, has obvious advantages in this respect. it Frequency stability Good, high precision and applicable frequency range Wide, small Hygroscopic Long service life, especially in multi-channel communication equipment Immunity As a result, the electromagnetic equipment in the past can not match the situation and is facing the fate of being replaced.

Let's look at a new type of bicycle damping controller Shock absorber It is difficult to achieve a stable effect, and this ACX damping controller, through the use of piezoelectric materials, provides a continuously variable damping function for the first time. A sensor monitors the movement of the impact piston at the rate of 50 times per second. If the piston moves quickly, it is generally due to the rapid impact caused by driving on uneven ground. At this time, the maximum shock absorption function needs to be activated; If Piston movement If it is slow, it means that the road surface is flat, and only the weak shock absorption function needs to be used.

It can be said that although piezoelectric ceramics are new materials, they are quite civilian. It is used for high technology, but more for serving people in life and creating a better life.

application

Piezoelectric application area It can be roughly divided into two categories: vibration kinetic energy and ultrasonic vibration energy electric energy transducer Applications, including Electroacoustic transducer ， Underwater acoustic transducer and ultrasonic transducer And other sensors and Driver Application.

1. Transducer

Transducers are devices that convert mechanical vibration into electrical signals or generate mechanical vibration driven by electric fields

Piezoelectric polymer electroacoustic device Utilizing polymer Transverse piezoelectric effect , while the transducer design uses the bending vibration , the above principle can be used to produce Electroacoustic device as Microphone 、 Stereo headphones And tweeters. At present, the research on piezoelectric polymer electroacoustic devices mainly focuses on the use of the characteristics of piezoelectric polymers to develop devices with special electroacoustic functions that are difficult to achieve with other existing technologies, such as anti noise telephones, broadband ultrasonic signal transmission systems, etc.

Piezoelectric polymer underwater acoustic transducers were initially targeted at military applications, such as large-area sensor arrays and Monitoring system Later, the application field gradually expanded to geophysical Detection, acoustic wave Test equipment And so on. Various prototype underwater acoustic devices developed to meet specific requirements adopt different types and shapes of piezoelectricity Polymer materials , such as sheet, sheet Lamination , cylinder and Coaxial line And so on to give full play to piezoelectric polymers High elasticity , low density, easy to prepare components with large and small different cross sections, and acoustic impedance And water Order of magnitude The last feature makes the piezoelectric polymer hydrophone It can be placed in the measured sound field to sense the sound pressure And the measured sound field will not be disturbed due to its own existence. The high elasticity of polymer can reduce the transient Oscillation, thereby further enhancing the performance of the piezoelectric polymer hydrophone.

Piezoelectric polymer transducers are used in the field of biomedical sensors, especially Ultrasonic imaging The most successful application PVDF Film excellent flexibility And formability, making it easy to apply to many sensor products.

2. Piezoelectric actuator

Piezoelectric actuators convert electrical energy into mechanical energy or mechanical movement ， polymer The driver is mainly based on polymer bimorph, including the use of Transverse effect And longitudinal effect. The driver application research based on polymer bimorph includes Display device Control, micro displacement generation system, etc. A lot of research is needed to make these creative ideas practical. Electron beam irradiation P (VDF TrFE) copolymer enables the material to produce large tensile strain, thus creating favorable conditions for the development of new polymer actuators. Under the promotion of potential national defense application prospect, radiation modification copolymer Preparation Polymer materials Underwater sound launcher With the strong support of the United States military, the research of the. In addition, by taking advantage of the excellent properties of irradiation modified copolymers, Research and development It is in Medical ultrasound , vibration and noise reduction and other fields of application, still need a lot of exploration.

3. Applications on sensors

Piezoelectric pressure sensor

Piezoelectric pressure sensor is made of piezoelectric effect of piezoelectric material. Of piezoelectric pressure sensor Basic structure As shown in the right figure. Due to the Charge quantity It is certain, so pay special attention to avoid leakage when connecting.

The advantage of piezoelectric pressure sensor is that it has a self generated signal, output signal Large frequency response Small size and solid structure. Its disadvantage is that it can only be used for kinetic energy measurement. Special cables are required, and self recovery is slow when subjected to sudden vibration or excessive pressure.

Piezoelectric accelerometer

Piezoelectric elements are generally composed of two piezoelectric chips. Electrodes are plated on two surfaces of the piezoelectric chip, and leads are led out. Place a mass block on the piezoelectric chip. The mass block is usually made of relatively large metal tungsten Or high gravity alloy. Then use one Hard spring Or bolts, Nut Preload the mass block, and install the whole assembly in a metal shell of the original base. To isolate Test piece Any strain of shall be transmitted to the piezoelectric element to avoid generating False signal Therefore, the base is generally thickened or made of materials with high rigidity. The weight of the shell and base accounts for almost half of the weight of the sensor.

During measurement, the sensor base and the test piece shall be rigidly fixed together. When the sensor is subjected to the vibration force, the inertia of the mass block can be considered as very small because the stiffness of the base and the mass block is quite large, while the mass of the mass block is relatively small. Therefore, the mass block is subjected to the same motion as the base, and acceleration Opposite inertia force Role of. In this way, the mass block has one Proportional Acceleration dependent Strain force It acts on the piezoelectric chip. Because the piezoelectric chip has piezoelectric effect, alternating charges are generated on its two surfaces（ Voltage ）, when the acceleration frequency is much lower than the natural frequency The sensor gives output voltage And Force It is proportional to the acceleration of the test piece. The output power is led from the output terminal of the sensor and input to Preamplifier Then the acceleration of the test piece can be measured with ordinary measuring instruments; If the appropriate Integrating circuit The vibration speed or displacement of the test piece can be tested.

4. The Application of GPS in Robot Proximity Sense（ ultrasonic sensor ）

There are three main purposes for the robot to install proximity sensors: first, before contacting the object, obtain the necessary information to prepare for the next movement; Second, detect whether there are obstacles in the movement space of robot hands and feet. If obstacles are found, take certain measures in time to avoid collision; Third, to obtain the approximate information of the object surface shape.

Ultrasound is a kind of sound that cannot be heard by human ears Mechanical wave , the frequency is above 20KHZ. The vibration frequency range of the sound that can be heard by the human ear is only 20HZ － 20000HZ. Because of its short wavelength diffraction Small, but can become acoustic ray and directional propagation. The robot uses ultrasonic sensors to detect the existence of surrounding objects and measure the distance between objects. Generally used to detect surrounding environment Medium and large objects, cannot Measuring distance Objects smaller than 30mm.

Ultrasonic sensors include ultrasonic transmitter, ultrasonic Acceptor , timing circuit and control circuit. Its working principle is roughly as follows: first, the ultrasonic transmitter emits pulsed ultrasonic waves in the direction of the measured object. The transmitter will automatically shut down and stop transmitting after sending out a series of ultrasonic waves. At the same time, the ultrasonic receiver starts to detect the echo signal, and the timing circuit also starts timing. When ultrasonic waves encounter objects, they are reflected back. After the ultrasonic receiver receives the echo signal, the timing circuit stops timing. At this time, the time recorded by the timing circuit is from the time when the ultrasonic wave is transmitted to the time when the echo signal is received Propagation time 。 Using the propagation time value, the distance between the measured object and the ultrasonic sensor can be converted. The conversion formula is very simple, that is, half of the propagation time of the sound wave and the sound wave in the medium propagation velocity The product of. Ultrasonic sensor working process They are carried out in sequence under the control of the control circuit.

In addition to the above applications, piezoelectric materials are also widely used. as Frequency discriminator Piezoelectric oscillator, transformer, filter, etc.

present situation

Several piezoelectric ceramic materials in development and several new applications are introduced below.

1. Fine grain piezoelectric ceramics

In the past, piezoelectric ceramics were composed of multi domain grains ranging from several microns to tens of microns Polycrystalline material , the size can no longer meet the needs. Reduce particle size to Submicron Grade, which can improve the Processability , the substrate can be made thinner, the array frequency can be increased, the loss of transducer array can be reduced, and the mechanical strength , reduce the thickness of each layer of multilayer devices, thereby reducing driving voltage , which improves Stack Transformer Brake All are beneficial. Reducing the particle size has so many benefits as mentioned above, but it also has the effect of reducing the low-voltage electrical effect. In order to overcome this effect, people have changed the traditional doping process to increase the piezoelectric effect of fine grain piezoelectric ceramics to the level equivalent to that of coarse grain piezoelectric ceramics. Now the cost of making fine grain materials is comparable to Ordinary ceramics Competition. In recent years, people have studied cutting and grinding with fine grain piezoelectric ceramics, and made some high-frequency transducers, micro brakes and thin Buzzer (20~30um thick), which proves the advantages of fine grain piezoelectric ceramics. along with Nanotechnology The research and application of fine grain piezoelectric ceramic materials is still a hot topic in recent years.

2. PbTiO3 series piezoelectric materials

PbTiO3 series piezoelectric ceramics are most suitable for making high-frequency high-temperature piezoelectric ceramic components. Although PbTiO3 is present Ceramic firing People have done a lot of work on modification to solve the problems of difficult to polarize and make large-sized products Sinterability 。 inhibition grain growth Therefore, the fine and anisotropic modified PbTiO3 materials are obtained. In recent years, there have been many reports on improved PbTiO3 materials metal defect detection High frequency devices have been widely used. At present, the development and application of this material is still a concern of many piezoelectric ceramic workers.

3. Piezoelectric ceramics- Polymer composites

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Inorganic piezoelectric ceramics and organic polymer resin Piezoelectric composite , which combines the properties of inorganic and organic piezoelectric materials, and can produce characteristics that neither of them has. Therefore, transducers and sensors with good performance can be made according to the needs and the advantages of two-phase materials. its Receiving sensitivity Very high, more suitable for Underwater acoustic transducer 。 In other Ultrasonic transducer And sensors, piezoelectric compound material It also has great advantages. Domestic scholars are also very interested in this field, have done a lot of process research, and have done some useful work in the structure and properties of composite materials fundamental research At present, he is committed to the development of piezoelectric composite products. 4、 Piezoelectricity Specific multicomponent single crystal Piezoelectric body

Traditional piezoelectric ceramics have stronger piezoelectric effect than other types of piezoelectric materials, so they are widely used. However, as a large edge, high-energy energy transfer material, the piezoelectric effect of traditional piezoelectric ceramics still can not meet the requirements. So in recent years, in order to find out more excellent Piezoelectricity A lot of work has been done, and Pb (A1/3B2/3) PbTiO3 single crystal (A= Zn 2+, Mg2+). Energy storage density Up to 130J/kg, while the energy storage density of piezoelectric ceramics is within 10J/kg. Ferrovoltaics scholars said that the emergence of such materials is another leap in the development of piezoelectric materials. Now the United States, Japan Russia And China has begun to carry out such materials Production process Research shows that the success of its batch production will bring about the rapid development of piezoelectric material applications.

New areas

In recent years, many polymer materials with piezoelectric effect and converse piezoelectric effect have been developed by synthetic methods, and these materials are named“ Artificial muscle ”。 Researchers all over the world have launched a challenge: see who can first use artificial muscles to produce Robot arm And must be one-on-one with the human arm Break wrist Win the game.

Historical application

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transducer

In 2006 Curie Brother Peel (P· Curie ）The 126th anniversary of the discovery of piezoelectric effect with J. Curie. Found in Jakes' lab 1880 years ago Piezoelectricity 。 At first, Peel devoted himself to the coke phenomenon（ pyroelectric effect， Note 2) and Crystal symmetry Later, the two brothers found that electricity would be generated when pressure was applied to a certain type of crystal. They also systematically studied the pressure direction and electric field intensity And predict that a certain kind of crystal has piezoelectric effect. Through their experiments, they found that Piezoelectricity The materials of are: sphalerite （zinc blende ）, Sodium Chlorate (sodiumchlorate), tourmaline（ tourmaline ）、 quartz （ quartz ）、 tartaric acid （tartaricacid）、 sucrose （canesuger）、 Boralite （boracite）、 Heteropolar ore （calamine）、 Xanthine （ topaz ）And Rochelle salt. These crystals have anisotropy (anisotropic) structure, Isotropy (isotropic) material will not produce Piezoelectricity Of.

In an amorphous square crystal, if an external force is applied to deform the crystal, the internal localization of the crystal will be caused by the movement of charges in the lattice Uneven charge distribution , resulting in an electrical displacement. The displacement of charge is due to the movement of all ions in the crystal, or because Atomic orbital upper Electron distribution These charge displacement phenomena exist in all materials. However, in order to have piezoelectric effect, it must be able to cause effective and net electric bipolar moment change per unit volume of materials. Check whether there is such a change Lattice structure It depends on the symmetry. Piezoelectricity The theory was originally Lippmann (Lippmann) is studying Thermodynamic principle It was discovered at that time. Later in the same year, Curie brothers did experiments to prove this theory and established Piezoelectricity And crystal structure Relationship. In 1894, W. Voigt determined the crystal structure and Piezoelectricity He found 32 relationships Crystalloid (class) may have piezoelectric effect (21 of 32 types do not have symmetric center, one of which Piezoelectric constant 0, the other 20 have piezoelectric effect).

Piezoelectric actuator

Today, we all know that pressure transistor It can be used as a sound wave generator and receiver , whether militarily (e.g sonar ）It has a wide range of applications in industry and engineering. However, as early as a third century after Curie brothers discovered piezoelectricity, the piezoelectric effect has hardly received any attention in application. Even Pierre himself just used it to measure radium The electric charge emitted by the element. here we are the First World War Allied warships were heavily damaged by German submarines, so they tried to find effective methods to detect submarines. because electromagnetic wave Unable to effectively penetrate the sea water, while sound waves could easily travel in the sea. Therefore, P. Langevin developed a quartz piezotransistor as a sound wave generator at that time. Unfortunately, by the time we got a good result, the war was drawing to a close and it was too late to use it. Paste one on both sides of quartz Steel sheet , make it oscillation frequency Reduce to 50KHz and apply a power supply pulse wave Signal transducer It is converted into sound waves and transmitted to the seabed; After a period of time, the transducer receives the reflection from the seabed echo The distance from the transducer to the seabed can be determined by the return time and the speed of the wave traveling in the sea. This principle can also measure the position of submarines.

Shortly after World War I, quartz transducers developed two important applications. first, Harvard University Of peirce Professor (G.W. Pierce) makes ultrasonic waves with quartz crystal Interferometer , the ultrasonic wave generated by quartz and the sound wave in the figure reflector Reflected echoes are mixed to produce maximum value If the reflector is slightly adjusted to move forward or backward, another maximum value can be obtained. The wavelength of sound wave can be measured from the distance between the two maxima, that is, the distance the reflector moves between two adjacent maxima. Because the frequency is known, the product of frequency and wavelength, Determinable The velocity of the outgoing wave in the gas medium. At the same time amplitude The apparent attenuation coefficient of the wave in the gas can be obtained by the reduction rate. It was used to measure sound waves carbon dioxide in wave velocity To the frequency, and calculate the dispersion relation 。 In this way, it is possible to study the gas Mixing ratio And the velocity of sound wave at temperature Attenuation rate 。

In 1927, Wood (R.W. Wood) and A.L. Loomis first used high-power ultrasound. Use Langevin type quartz transducer with high power Vacuum tube , generating high energy in the liquid, causing the liquid to cause the so-called cavity（ cavitation ）Phenomenon. At the same time, the effects of high power ultrasound on biological samples were also studied.

In the research of underwater sound, it is found that quartz crystal is not a good transducer material, but its oscillation frequency does not change with temperature, that is, it has a low temperature coefficient 。 The high stability of this frequency to temperature is used for control oscillator Frequency, and some filters are most useful. In 1919, Professor Cady used quartz as a frequency controller for the first time. Figure 4 shows the earliest crystal controlled oscillator circuit. Because the crystal has a very high Q value (Note 3), the frequency of the oscillator is affected by the crystal resonance frequency And the frequency does not change with the temperature. Later, Pierce and Pierce Miller invented a widely used crystal control Oscillating circuit 。 stay the Second World War About 10 million crystal oscillator To establish communication between tanks and between ground and aircraft.

Another important application of quartz crystal is to obtain height Frequency selectivity Oscillator for. Quartz crystal is a piezoelectric chip with high Q value, and high Q value means low sound wave energy loss (its Attenuation rate Is proportional to the square of frequency); High Q also means narrow frequency band , so it is not suitable for the use of sound transmission circuits. In order to be able to Carrier communication For use in the system, a series inductor (see Figure 5) can be used to obtain broadband operation. The Structure diagram It is often used in wired communication systems Microwave communication System, etc.

The material used in the World War II sonar drum was Rochelle salt instead of quartz crystal. Although the Rochelle salt has high electromechanical coupling efficiency, it is unstable, and its withstand voltage is not high, so it is difficult to operate at too high power. In theory, if the Sherr salt is the first one with Ferroelectricity (ferroelectricity) materials have a spontaneous polarization along the crystal axis. Figure 7 shows the relationship between the measured polarization along the X-axis and temperature. It has two Curie temperature (Curie temperature), the polarization amount is zero at Curie temperature, and the polarization is maximum between two temperatures. In memory of Dr. Seignette, who was born in Rochelle, this effect is called Seignette ferroelectric effect, generally referred to as ferroelectric effect, to express its relationship with ferromagnetic effect Similarity 。 stay Ferroelectric material When the temperature is lower than the Curie temperature, the material has an electric bipolar inside. Most of the electric bipolars bonded by hydrogen bonds, such as the sherr salt, have regularity Arranged, and generally only has one Curie temperature, but if the sherr salt has two Curie temperatures, the difference between the two types is mainly due to the hydrogen bond terminal anion Is different. commonly Hydrogen bonded crystal The distribution of potential well is shown in Figure 8, between two oxygen ions Hydrogen ion There are two possible positions, hydrogen bond electric double extremum It is equal to the product of the charge and the distance difference between two groups of ions. Applying an electric field can make the hydrogen ion jump from one position to another, and change the direction of the electric bipolar. At high temperature, the thermal disturbance will make the hydrogen ions fill the positions of the two wells equally, so there is no natural polarization. When the temperature decreases, the two electric bipoles are attracted to each other and the bipolar direction arrangement tends to be regular. At the Curie temperature, the two electric bipolars cancel each other, but at the Curie temperature, a small external force can cause a large polarization. When the temperature is lower than the Curie temperature, spontaneous polarization occurs. For hydrogen bonded crystals with potential wells as shown in Figure 8, the polarization can be increased until saturation occurs. However, for Joscher salt, the polarity will begin to decrease to zero after reaching a maximum. The reason can be seen from the potential well in Figure 8 Distribution map It means that at very low temperature, all hydrogen ions are completely distributed in two low-energy There is no spontaneous polarity bias in the well. When the temperature rises, some hydrogen ions get heat energy and jump to a higher energy level. The higher the temperature is, the greater the chance of such a transition is. The two electric bipoles attract each other to produce a lower Curie temperature. Figure 9 shows the X-ray of Rochelle salt diffraction crystal structure 。 What causes the ferroelectric effect is grade 1's Oxygen molecule And label 10 water Hydrogen bond formed by molecules. yes Hydrogen ion In other words, these two molecules are two different ions on the end point, thus forming two potential wells with different names as shown in Figure 8.

Previously, Rochelle salt was the only known ferroelectric material, but now we know that there are more than 100 kinds of ferroelectric materials. Ferroelectric materials have spontaneous polarization, and Electric field energy It generates induced polarization, so it is used as a transducer. This general piezoelectric single crystal such as quartz has higher electromechanical coupling efficiency and sensitivity, but its stability is slightly inferior to that of piezoelectric transistors. Gradually, people used ferroelectric ceramic magnets as transducers. What was first used barium titanate (BaTiO3), which is Massachusetts Institute of Technology Von Hippel and Soviet Russia It was discovered by scientists Vul and Goldman respectively. The unpolarized ceramic magnetism is biased in the domain (Note 5) Polarization direction Irregular, the whole piece of ceramic magnet is like a piece of high Dielectric constant Of capacitor Because it only needs a small volume and has enough capacitance, it is used on TV sets. If the temperature is above 120 ℃, add one high voltage , then the electric coupling in some domains is regularly arranged, and there is a net polarity, with piezoelectric effect. We can generate longitudinal wave (electric field parallel to thickness direction) or transverse wave (The electric field is perpendicular to the thickness direction). longitudinal wave It can travel in water and generate high energy in solid. The shear wave is suitable for making because of its slow speed delay line 。 At present, the best piezoelectric ceramics belong to PZT (lead- zirconate -titanate）。

Recently, two important ferroelectric materials can be used to make acoustic transducers. One is polymer film, polydifluoroalkylene ethylene (PVF2 or PVF2 for short) PVDF ）First, oxidation lithium Niobium (LiNbO3) DC voltage It has many advantages: sound wave characteristic impedance It is close to water, impedance Natural matching , easy access to broadband operation, suitable for non-destructive detection, medical diagnosis, sonar and hydrophone use, especially its high acoustic reception coefficient, used to make passive sonar（ passive Sonar) hydrophone Array (hydrophone array) is of great importance. Except that it is soft and Tolerable High voltage (its collapse Voltage ratio PZT is about 100 times higher). Lithium oxide Niobium single crystal has high electromechanical coupling and extremely low acoustic wave Attenuation coefficient , easy to excite high frequency Surface acoustic wave Rayleigh wave is used to produce surface acoustic wave SAW ）Best material for components. These components are used in the signal processing system and communication system It has an irreplaceable position. Figure 11 shows the use of Lithium oxide niobium surface wave Direct filter. Use a set of plus and minus Voltage The interphase cross toe transducer generates surface acoustic waves (so-called interdigital transducer, Or short for IDT ）, of the excited sound wave center frequency Determined by the distance between positive and negative electrodes bandwidth Is inversely proportional to the number of electrodes. Figure 12 shows another surface acoustic wave pulse stretch and compression filter, which can be used in the CHIRP radar system to improve Search scope And Resolving power 。

Another important and unique research is that on the so-called acoustic microscope, this microwave frequency module uses a sputtered piezoelectric film as an acoustic transducer to generate several GHz (1GHz=109 cycles/second) sound wave, whose corresponding wavelength is about one micron (10-6 meters). Because the transducer Vibration frequency It is inversely proportional to the thickness of the piezotransistor, and thin films are required to produce such high frequency sound waves Piezoelectric material , such as zinc oxide or Cadmium sulfide Etc.

On the centenary of the discovery of piezoelectric effect, this article is written with reference to W. P. Mason's work, introducing the history and application of piezoelectricity. In the early days, piezoelectric effect was only an interesting academic study, but now it has become a very useful effect. It can be used to make a variety of acoustic electric transducers, whose operating spectrum can range from 100Hz to several GHz, and has different uses according to different frequencies. Sonar, anti submarine, submarine communication, telephone communication, etc. are low frequency（ audio frequency , AF band). In several MHz Range, its wavelength is in the millimeter range, suitable for non-destructive testing materials（ nondestructive testing, NDT for short) and medical diagnosis, the so-called ultrasonic imaging, holography Photography Technology, computer-aided acoustic tomography, etc. are studied for these purposes. Frequency at VHF 、 UHF The surface acoustic wave (SAW) electronic components developed by piezoelectricity are used in the waveband. Such as delay line, various filters Gyrator （convolver）、 Correlator (correlator) and other signal processing components have important applications in communication and signal processing. When the frequency is high to low microwave band, its corresponding wavelength is in the micron range. It is used to make acoustic microscope, and its image resolving power can be similar to that of traditional optical microscope Compared with the United States Mechanical wave The unique properties of non electromagnetic waves can make up for the deficiencies in the application of optical microscopy.

Note 1: If a pressure or tension is applied to some materials, in addition to the change of material shape (so-called strain), the lattice structure of such materials has some Asymmetry (The so-called inversion symmetry), the deformation of the shape makes the internal electron distribution locally uneven and produces a net electric field distribution. On the contrary, add a periodicity Voltage Or the change of electric field can make the material deform, and a corresponding stress. The shape change varies with the frequency of the applied voltage signal, which can produce a periodicity elastic wave Or acoustic wave, this effect is called piezoelectric effect, and these materials are called piezoelectric materials.

Note 2: In some ferroelectric materials, when the temperature changes, the spontaneous polarization moment will change Material surface It is distributed as net charge, and this effect is called coke Electric effect 。 This effect can be used to detect temperature changes or measure so-called Thermal wave （ thermal wave）。

Note 3: The definition of the oscillator Q value (qualityfactor) is the power lost by the oscillation wave per unit period. Sometimes we use Q=center frequency/bandwidth. bandwidth The narrower the oscillator, the higher the Q value, such as the quartz oscillator.

Note 4: Insertion loss refers to the Total loss Quantity, that is, the difference between the output signal and the input signal. Generally Decibel (dB).

Note 5: In Ferromagnetic material When the temperature is much lower than the Curie point, from a microscopic point of view, the magnetic moments of all electrons should be arranged in the same direction, which is not the case. In fact, this material is divided into many small areas inside magnetic moment It is regularly arranged, but the magnetic moment arrangement direction between small regions is different, so that the magnetic moment of the whole material is far less than its saturation magnetic moment 。 These small regions are called domains or domains for short Ferroelectric material , antiferroelectric materials, ferroelastic materials Superconductor Fields also exist in materials.

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Piezoelectric effect is the phenomenon of heterogeneous charges on the surface of some media when they are deformed under the action of force. Experiments show that this Bound charge The electric quantity of is proportional to the force, and the more electric quantity, the corresponding electric potential difference between two surfaces（ Voltage ）The bigger it is. This magical effect has been applied to many fields closely related to people's production, life, military, science and technology to realize the function of force electricity conversion. For example, piezoelectric ceramics can be used to convert external force into electric energy, which can produce piezoelectric lighters without flint, ignition switches for gas stoves, and shells Trigger fuze Etc. In addition, piezoelectric ceramics can also be used as Sensitive materials , for amplifiers Phonograph head etc. Electroacoustic device ； be used for Piezoelectric seismometer , which can monitor and accurately measure the subtle vibration that human cannot perceive Source Direction and strength, so as to predict earthquakes and reduce losses. The piezoelectric actuator made of piezoelectric effect has the function of precise control. It is a precision machine microelectronics And important devices in the fields of 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 "zero contact" with piezoelectric ceramics every day, but they turn a blind eye to it.

Currently popular disposable plastic lighter A considerable part of them use piezoelectric ceramic devices to ignite. Take out the pressure Electric spark Element,

Measuring instrument

Schematic diagram of piezoelectric effect measuring instrument

What instrument can measure the instantaneous voltage generated by the voltage ceramic element of the piezoelectric lighter? At first, we tried to use ordinary pointer Multipurpose meter When measuring the DC high voltage gear, it is found that the pointer can only shake slightly because of the voltage connected from the two electrodes when the black plastic pressure bar of the ignition element is pressed. The reason is that because the duration of voltage pulse is very short and the pointer inertia is large, the pointer cannot synchronously reflect the voltage change and make a large deflection.

The digital display multi-purpose ammeter was originally thought to have no influence of pointer inertia and should be able to read the instantaneous high voltage Come on, who knows? We can't see the expected high voltage reading at all. We can only see some uncertain low voltage data. Analyzed, this is due to the LCD display response speed Slow, the duration of ignition voltage pulse is too short to display Highest Moment Voltage, can only be displayed Voltage Drop Some random voltage readings in the process (more gentle stage).

Finally, we moved out of the laboratory's "heavy weapons" - Oscilloscope , try again. We use the most common J2459 student oscilloscope in the laboratory, and the connecting wires are two ordinary wires with terminal fish clips. In theory, oscilloscope is used Electron beam deflection After hitting Fluorescent screen Show on light spot Moving, electron beam The inertia is very small, and it should be able to "track" the change of the ignition high-voltage pulse. The experimental results are as expected.

Estimation method

Connect the oscilloscope AC and DC Selector switch Place on“ DC ”Block, Scanning range Place it in the "10~100kHz" gear, and use X shift and Y shift to move the horizontal bright line to the Central Department , placed on the X axis. In order to estimate the highest piezoelectric effect Voltage Amplitude, we must first use the square in front of the fluorescent screen Coordinate system , fixed voltage Ruler : Use the Y input connected to the oscilloscope Terminal The two wires on the A dry battery The 1.5V voltage of is applied to the oscilloscope, the attenuation is set at 1, and the Y gain is set at the lowest. It can be found that the horizontal bright line just now jumps (or jumps down) about two grids, that is, the two grids represent 1.5V voltage. With the Y gain unchanged, put the Y attenuation at 1000 (i.e. one thousandth) gear, and the two grids of the grid coordinates in front of the fluorescent screen can represent 1500V.

Input Y into two terminals feeder Of crocodile clip Respectively connect to the two electrodes of the piezoelectric element of the piezoelectric lighter, and quickly press the black plastic Pressure bar You can see that the horizontal bright line originally located at the central height jumps up (or down) and returns to its original position. Due to the afterglow effect of the fluorescent screen, the horizontal bright line appears on the oscilloscope as a bright band up to four grids high, 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 while pressing the pressure bar each time (change the scanning range to the "10~100Hz" gear in advance). We can see the waveform shown in Figure 2 on the fluorescent screen. The voltage rises steeply, decreases gently, and the peak value is more than four grids.

Pulse time

Set the attenuation gear of the oscilloscope to 1000 gear, set the scanning range to "10~100Hz" gear, and turn the "scanning fine adjustment" to the left, that is Scanning frequency 10 Hz, adjust the "X gain" and "X shift" knobs to make the X axis Scanline When 10 grids are filled, each grid represents 1/10 × 1/10s, that is, 0.01. Press the black plastic pressure rod of the piezoelectric element, you can see Electric pulse The duration is one grid, as shown in Figure 3, which corresponds to 0.01s, that is, the pulse duration is about 0.01s.

Piezoelectric crystal

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There is a very interesting crystal When you squeeze or stretch it, its two ends will generate different charges. This effect is called piezoelectric effect. The crystal that can produce piezoelectric effect is called Piezoelectric crystal 。 Crystal (α - quartz) is a famous piezoelectric crystal.

If it is aligned in a certain direction Crystal crystal If a pressure is applied to a slice cut from the top, an electric charge will be generated on the slice. If the sheet is stretched in the opposite direction, an electric charge will also appear on the sheet, but the sign is opposite. The greater the squeezing or stretching force, the more charge will be on the crystal. If electrodes are plated on both ends of the sheet alternating current Then the sheet will periodically elongate or shorten, that is, start to vibrate. such Inverse piezoelectric effect It has been widely used in science and technology. It can be made with crystal Piezoelectric quartz The area of the sheet is only a few square millimeters, and the thickness is only a few tenths of a millimeter. Don't underestimate this small chip, it is radio technology China has played a huge role. As previously mentioned, in the Alternating electric field Medium, this thin Vibration frequency Nothing changed. This steady vibration is exactly what radio technology Control frequency What you need, your family colour tv set Piezoelectric chips are used in many electrical equipment wave filter , ensuring the image and sound definition 。 It's on your hand Quartz electronic watch There is a core component called Quartz oscillator 。 This is it Key components Guaranteed quartz watch More than others Mechanical watch Higher travel time Accuracy 。

Instruments equipped with piezoelectric crystal elements enable technicians to study steam engine , internal combustion engine and various Chemical equipment The change of pressure becomes a reality. Piezoelectric crystals can even be used to measure the pressure of the fluid in the pipeline, the pressure borne by the gun barrel when launching shells, and bombs During explosion Instantaneous pressure, etc.

piezoelectricity Crystals are also widely used for sound reproduction, recording and transmission. Installed on Microphone The piezoelectric chip on the can transform the vibration of sound into the change of current. When sound waves touch a piezoelectric sheet, they will generate charges on the electrodes at both ends of the sheet, and their size and symbols will change with the change of sound. The change of charge on the piezoelectric chip can be changed into radio waves To distant places. These radio waves are radio The vibration of the piezoelectric crystal sheet that is received and placed on the radio horn turns into sound reverberating in the air. It can be said that the piezoelectric chip in the microphone can "hear" the sound, while the piezoelectric chip on the speaker can "talk" or "sing".

Piezoelectric polymer

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piezoelectric polymer

Piezoelectricity is due to Stress action In material, in Material surface The process of inducing charge generation is generally reversible, that is, when the material is subject to the action of electrical parameters, the material will also generate Deformation energy 。 Wood cellulose, tendon collagen and various polymers amino acid Are common polymers Piezoelectricity Material, but its piezoelectric rate is too low use value 。 stay Organic polymer materials in Polyvinylidene fluoride Such compounds have strong piezoelectric properties. The piezoelectricity rate depends on the dipole Whether the arrangement direction of is consistent. In addition to containing larger dipole moment Many polyvinylidene fluoride compounds with C-F bond Polar bond The polymer also exhibits piezoelectric properties. Ruya Vinyl Dicyanogen and Vinyl acetate 、 isobutene 、 Methyl methacrylate 、 Vinyl benzoate Waiting copolymer , all show strong piezoelectric characteristics. And high temperature stability is good. It is mainly used as energy exchanging material, such as the preparation of sound elements and displacement control elements. The common examples of the former are the probe of ultrasonic diagnostic instrument sonar , headset, microphone, telephone sphygmomanometer And other energy exchange components in the device. Put two piezoelectric films together and apply opposite Voltage , the film will bend to form displacement Control elements 。 This principle can be used to make Optical fibre Alignment device, automatic opening and closing curtain a record player and video recorder Alignment piece of.

Piezoelectric ceramics

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Piezoelectric ceramics are Functional ceramics It is widely used. Used by many people in daily life“ Electronic lighter ”And gas cooker On Electronic igniter Is an application of piezoelectric ceramics. Igniter It is to use the piezoelectric characteristics of piezoelectric ceramics to apply force on them, so as to generate more than 10 kV high voltage, thus producing Spark discharge To achieve the purpose of ignition.

Piezoelectric ceramics are actually Polarization treatment With piezoelectric effect ferroelectric ceramics 。 It was in 1946 when it was confirmed Barium titanate ceramics It began to appear after ferroelectricity: 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 ceramics and Niobic acid Salt based piezoelectric ceramics. The properties and applicability of piezoelectric ceramics have been greatly improved. In particular, the emergence of ternary piezoelectric ceramics makes the selection of piezoelectric ceramics coupling coefficient 、 temperature characteristic It can meet the requirements of a variety of electronic instruments, thus making piezoelectric ceramics Scope of application Greatly increased. for example Ceramic filter and Ceramic frequency discriminator ， Electroacoustic transducer ， Underwater acoustic transducer , wave device of sound meter, electro-optic device, infrared detector device and Piezoelectric gyroscope They are all applications of piezoelectric ceramics in modern electronic technology.

What is piezoelectric ceramics? In fact, it is a way to mechanical energy Function of mutual conversion with electric energy ceramic material 。 The so-called piezoelectric effect means that when some medium is subjected to mechanical pressure, even if the pressure is as small as the acoustic vibration, it will produce compression or elongation and other shape changes, causing the surface of the medium to be charged. This is Positive piezoelectric effect 。 On the contrary, if an exciting electric field is applied, the medium will produce mechanical deformation, which is called Inverse piezoelectric effect 。

The French Curie brothers discovered the "piezoelectric effect" in 1880. In 1942, the first piezoelectric ceramic material—— barium titanate In the United States Soviet Union and Japan make. In 1947, barium titanate Pickup ——The first piezoelectric ceramic device was born. In the early 1950s, another kind of performance was much better than barium titanate Zirconium Lead titanate Successful development. Since then, the development of piezoelectric ceramics has entered a new stage. From the 1960s to the 1970s, piezoelectric ceramics continued to improve and become perfect. For example, zirconium improved by multiple elements Lead titanate Binary system Piezoelectric ceramics based on lead zirconate titanate Ternary system 、 Quaternary system Piezoelectric ceramics also came into being. these ones here Material properties Excellent, simple manufacturing, low cost and wide application.

By using the characteristics of piezoelectric ceramics to convert external force into electric energy Piezoelectric igniter Mobile X-ray power supply and shell detonating device. Using two piezoelectric ceramic columns with a diameter of 3 mm and a height of 5 mm instead of ordinary flint, a gas electronic lighter that can ignite tens of thousands of times continuously can be made. Using piezoelectric ceramics to convert electric energy into ultrasonic vibration can be used to explore the position and shape of underwater fish NDT , and Ultrasonic cleaning Ultrasonic medical treatment can also be made into various kinds of ultrasound Cutter Welding device and Soldering iron , processing plastics and even metals.

Piezoelectric ceramics are sensitive to external forces so that it can even sense the disturbance of flying insects flapping their wings more than ten meters away to the air, and will mechanical vibration Convert to electrical signal. This characteristic of piezoelectric ceramics can be applied to Sonar system Meteorological detection, remote sensing environmental protection, household appliances, etc.

Now piezoelectric ceramics have been applied by scientists to national defense construction, scientific research industrial production As well as in many fields closely related to people's life, he has become a generalist in the information age.

In the aerospace field, piezoelectric gyroscopes made of piezoelectric ceramics fly in space spacecraft 、 Artificial satellite The rudder of the. Only by relying on "rudder", spacecraft and artificial satellites can their established positions and routes be guaranteed. The traditional mechanical gyroscope has short life, poor accuracy and low sensitivity, which can not meet the requirements of spacecraft and satellite systems. The compact piezoelectric gyro has high sensitivity and good reliability.

The submarines that dive into the deep sea are all equipped with people called underwater Scout Sonar system. It is an indispensable equipment for underwater navigation, communication, reconnaissance of enemy ships, and clearance of enemy laid mines. It is also a development tool ocean resources It is a powerful tool for detecting fish and exploring the seabed topographic features Etc. In this sonar system, there is a pair of bright "eyes" - piezoelectric ceramics Underwater acoustic transducer 。 When Underwater acoustic transducer When the emitted acoustic signal encounters a target, a reflected signal will be generated. This reflected signal is received by another receiving type underwater acoustic transducer, and then the target is found. At present, piezoelectric ceramics are made Underwater acoustic transducer Is one of the best materials.

In medicine, the doctor places the piezoelectric ceramic probe on the examination part of the human body, sends out ultrasonic waves after being powered on, transmits them to the human body and produces echoes after it touches the human tissue, and then receives the echoes and displays them on the Fluorescent screen So that doctors can understand the internal conditions of the human body.

In industry, Geological detector There are Piezoelectric ceramic element It can be used to judge the geological condition of the stratum and find out the underground mineral resources. And the transformer in the TV—— Voltage Ceramic transformer, which is smaller in size, lighter in weight, and has an efficiency of 60%~80%, can withstand 30000 V high voltage, keep the voltage stable, and completely eliminate the defects of fuzzy deformation of TV images. Now? Foreign production Most of our televisions use Piezoelectric ceramic transformer 。 A 15 inch Kinescope , a 75mm long piezoelectric ceramic transformer is enough. This makes the TV smaller and lighter.

Piezoelectric ceramics are also widely used in daily life. Two piezoelectric ceramic columns with a diameter of 3 mm and a height of 5 mm are used to replace the gas electronic lighter made of ordinary flint, which can ignite tens of thousands of times continuously. utilize Same principle Electronic point made Musket It is an excellent tool for igniting gas stove. Another is made of piezoelectric ceramic elements Children's Toys , such as installing piezoelectric ceramics in the belly of a toy dog Buzzer , toys will make lifelike and interesting sounds.

With the development of high-tech, the application of piezoelectric ceramics will be more and more extensive. In addition to being used in the high-tech field, it serves people in their daily life and creates a better life for them.

Piezoelectric material

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The piezoelectric effect of piezoelectric materials is due to the special arrangement of atoms in the lattice, which makes the materials have Stress field And Electric field coupling The effect of. According to the type of materials, piezoelectric materials can be divided into piezoelectric materials single crystal , piezoelectricity Polycrystal (piezoelectric ceramics), piezoelectric polymers and piezoelectric composites. According to the specific material morphology, it can be divided into piezoelectric materials and piezoelectric films.

polymer

As early as 1940, Soviet Union It was found that wood has piezoelectricity. And then successively ramie Piezoelectricity was found in tissues such as silk bamboo, animal bones, skin and blood vessels. In 1960, he discovered the synthetic Polymer Piezoelectricity of. After the discovery of electropolarization in 1969 Polyvinylidene fluoride It has strong piezoelectricity. Materials with stronger piezoelectric properties include PVDF and its copolymers Polyfluoroethylene 、 polyvinyl chloride , poly - γ - methyl-L- Glutamate ester and Nylon-11 Etc.

compound material

Piezoelectric composites are piezoelectric materials composed of two or more materials. Common piezoelectricity compound material For piezoelectric ceramics and polymers (e.g Polyvinylidene fluoride live epoxy resin ）Two phase composite material. This composite material has the advantages of piezoelectric ceramics and polymers, has good flexibility and processing performance, and has low density, easy to mix with air, water and biology organization Realization sound impedance matching 。 In addition, piezoelectric composites also have Piezoelectric constant High characteristics. Piezoelectric composites have been widely used in medical treatment, sensing, measurement and other fields.

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