What Is the Piezoelectric Effect?

Piezoelectric effect: When some dielectric material is deformed by an external force in a certain direction, polarization will occur inside it, and positive and negative charges will appear on its two opposite surfaces. When the external force is removed, it will return to an uncharged state. This phenomenon is called the positive piezoelectric effect. When the direction of the force changes, the polarity of the charge changes. Conversely, when an electric field is applied in the direction of polarization of the dielectric, these dielectrics also deform. When the electric field is removed, the deformation of the dielectric disappears. This phenomenon is called the inverse piezoelectric effect. A type of sensor developed based on the dielectric piezoelectric effect is called a piezoelectric sensor.

Chinese name: Piezoelectric effect

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

Definition of piezoelectric effect

Piezoelectric body is polarized by external mechanical force, which causes bound charges with opposite signs to appear on the two ends of the piezoelectric body. The charge density is proportional to the external mechanical force. This phenomenon is called the positive piezoelectric effect. The electric body is deformed by the external electric field, and its deformation is proportional to the intensity of the external electric field. This phenomenon is called the inverse piezoelectric effect. A solid with a positive piezoelectric effect must also have an inverse piezoelectric effect, and vice versa. The positive piezoelectric effect and the inverse piezoelectric effect are collectively referred to as the piezoelectric effect. Whether a crystal has a piezoelectric effect is determined by the symmetry of the crystal structure. ^[1]

Principle of Piezoelectric Effect

The principle of the piezoelectric effect is that if pressure is applied to a piezoelectric material, it will generate a potential difference (referred to as the positive piezoelectric effect); otherwise, when a voltage is applied, it will generate mechanical stress (referred to as the inverse piezoelectric effect). If the pressure is a high-frequency vibration, a high-frequency current is generated. When high-frequency electrical signals are applied to piezoelectric ceramics, high-frequency acoustic signals (mechanical vibrations) are generated. This is what we usually call ultrasonic signals. In other words, piezoelectric ceramics have the function of conversion and inverse conversion between mechanical energy and electrical energy. This mutually corresponding relationship is indeed very interesting.

Piezoelectric materials can generate electric fields due to mechanical deformation or mechanical deformation due to the effect of electric fields. 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-bearing capabilities, such structures also have self-diagnostic, adaptive, and self-healing functions, and will play an important role in future aircraft design.

Classification of piezoelectric effects

The piezoelectric effect can be divided into a positive piezoelectric effect and a reverse piezoelectric effect.

Piezoelectric effect

Piezoelectric effect It means: when the crystal is subjected to a certain external force in the fixed direction, the electrode will be generated inside, and at the same time two opposite charges will be generated on the two surfaces; when the external force is removed, the crystal will return to an uncharged state; when the external force When the direction of action changes, the polarity of the charge changes accordingly; 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.

Piezoelectric effect

It refers to the phenomenon that the crystal is mechanically deformed by applying an alternating electric field to the crystal. Transducers made with the inverse piezoelectric effect can be used in electroacoustic and ultrasonic engineering. There are five basic forms of piezoelectric deformation: thickness deformation, length deformation, volume deformation, thickness shear, and plane shear. Piezoelectric crystals are anisotropic, and not all crystals can produce piezoelectric effects in these five states. For example, quartz crystal has no piezoelectric effect of volume deformation, but has good piezoelectric effect of thickness deformation and length deformation.

A type of sensor developed based on the dielectric piezoelectric effect is called a piezoelectric sensor.

Here we introduce the electrostrictive effect. Electrostrictive effect, that is, the dielectric strain under the action of the electric field due to induced polarization, the strain is proportional to the square of the electric field, and has nothing to do with the direction of the electric field. The piezoelectric effect exists only in crystals with no symmetrical center. The electrostrictive effect exists for all dielectrics, whether it is amorphous or crystalline, whether it is a crystal with central symmetry 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 PZT material is bonded to a round brass plate 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 a signal generator through a push button switch. Connect the two leads of the piezoelectric ceramic sheet B to the input of the loudspeaker (with horn). The two piezoelectric ceramic pieces A and B are fixed on a box made of the same wooden board with black sealing mud. When the observer presses the button switch to turn on the signal generator and piezoelectric ceramic A, due to the inverse piezoelectric effect, A starts to vibrate and transmits the vibration to the wooden box. Due to the piezoelectric effect, a variable electrical signal is generated on both sides of B, and then transmitted to the loudspeaker to make the horn sound, so this experiment demonstrated both the piezoelectric effect and the inverse piezoelectric effect.

Discovery of Piezoelectric Effect

In 1880 brothers Pierre Curie and Jacques Curie discovered that tourmaline has a piezoelectric effect. In 1881, they verified the inverse piezoelectric effect experimentally and obtained the forward and inverse piezoelectric constants. In 1984, German physicist Woldermar Voigt (German: Woldermar Voigt) concluded that only crystals with a group of 20 midpoints without a symmetric center could have a piezoelectric effect.

Application Status of Piezoelectric Effect

When you press the button lightly, the gas stove ignites a blue flame immediately. Have you ever realized what this convenience is for you? Connect a piece of seemingly bland ceramic to the wire and the ammeter, hold it with your hand, and the pointer of the ammeter will also swing-it is strange that a current is generated? In fact, this is a piezoelectric ceramic, a functional ceramic material that can convert mechanical energy and electrical energy into each other. What kind of material is piezoelectric ceramics? 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 media to be charged. This is the positive piezoelectric effect. Conversely, when the excitation electric field is applied, the medium will produce 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 functions such as energy conversion, sensing, driving, and frequency control.

Piezoelectric ceramics have sensitive characteristics and can convert extremely weak mechanical vibrations into electrical signals. They can be used in sonar systems, meteorological detection, telemetry and environmental protection, and household appliances. The earthquake was a devastating disaster, and the source of the earthquake started deep in the earth's crust. It was previously difficult to predict, putting humans in an embarrassing situation with nothing to do.

Piezoelectric ceramics produce a small amount of deformation under the action of an electric field, which does not exceed one ten millionth of its size at most. Don't underestimate this small change. An accurate control mechanism based on this principle, a piezoelectric actuator, The control of precision instruments and machinery, microelectronics, and bioengineering are all good news.

Frequency control devices such as resonators and filters are key devices that determine the performance of communication equipment. Piezoelectric ceramics have obvious advantages in this regard. It has good frequency stability, high accuracy, and wide applicable frequency range, and is small in size, non-absorbent, and long in life. Especially in multi-channel communication equipment, it can improve the anti-interference performance. Destiny of substitution.

Let s take a look at a new type of bicycle shock absorber. The general shock absorber is difficult to achieve a smooth effect, and this ACX shock absorber uses a piezoelectric material to provide a continuously variable shock absorber function for the first time. A sensor monitors the movement of the impacting piston at a rate of 50 times per second. If the piston moves quickly, it is usually caused by rapid impact caused by driving on uneven ground. At this time, the maximum shock absorption function needs to be activated; if the piston moves slowly, It means that the road is flat and only weak vibration damping is needed.

It can be said that although piezoelectric ceramics are new materials, they are quite commonplace. It is used for high-tech, but it is more for the eyes of people in life, to create a better life.

Application of piezoelectric effect

The application fields of piezoelectric materials can be roughly divided into two categories: vibratory energy and ultrasonic vibration energy-electric energy transducer applications, including electroacoustic transducers, hydroacoustic transducers and ultrasonic transducers, and other sensors. And drive applications.

1.Transducer

A transducer is a device that converts mechanical vibrations into electrical signals or generates mechanical vibrations when driven by an electric field

Piezoelectric polymer electroacoustic devices use the lateral piezoelectric effect of polymers, while the transducer design uses the bending vibration of polymer piezoelectric bimorphs or piezoelectric monoliths driven by an external electric field. Electricity can be produced using the above principles Acoustic devices such as microphones, stereo headphones, and tweeters. The current research on piezoelectric polymer electroacoustic devices is mainly focused on the use of the characteristics of piezoelectric polymers to develop devices that have special electroacoustic functions that are difficult to achieve with other current technologies, such as anti-noise phones, broadband ultrasonic signal transmission systems Wait.

Piezoelectric polymer underwater acoustic transducers were initially aimed at military applications, such as large-area sensor arrays and monitoring systems for underwater detection, and their application fields gradually expanded to geophysical detection and acoustic testing equipment. Various prototype underwater acoustic devices developed to meet specific requirements use different types and shapes of piezoelectric polymer materials, such as sheets, sheets, laminates, cylinders and coaxial wires, to fully utilize piezoelectric polymers High elasticity, low density, easy to prepare components with different cross sections, large and small, and the same acoustic impedance as the order of magnitude of water. The last feature allows hydrophones made of piezoelectric polymers to be placed in the sound field to be measured and sense the sound field. The sound pressure inside, and will not disturb the measured sound field due to its own existence. The high elasticity of the polymer can reduce the transient oscillation in the hydrophone, thereby further enhancing the performance of the piezoelectric polymer hydrophone.

Piezoelectric polymer transducers have obtained the most successful applications in the field of biomedical sensors, especially in ultrasound imaging, and 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 energy or mechanical motion. Polymer actuators are mainly based on polymer bimorphs, including two ways of using lateral effects and longitudinal effects. The research includes display device control, micro-displacement generation system and so on. In order for these creative ideas to be put to practical use, much research is needed. The electron beam irradiated P (VDF-TrFE) co-polymer has given the material the ability to generate large stretching strains, thereby creating favorable conditions for the development of new polymer actuators. Driven by the prospect of potential defense applications, the research on the use of radiation-modified copolymers to make all-polymer materials underwater acoustic emission devices is being systematically carried out with the strong support of the US military. In addition, using the excellent properties of radiation-modified copolymers to research and develop its applications in medical ultrasound, vibration and noise reduction, and much more research is needed.

3.Applications on sensors

Piezo pressure sensor

Piezoelectric pressure sensors are made using the piezoelectric effect of piezoelectric materials. The basic structure of a piezoelectric pressure sensor is shown in the figure on the right. Since the charge of the piezoelectric material is constant, special attention should be paid when connecting to avoid leakage.

The advantages of the piezoelectric pressure sensor are that it has a self-generating signal, a large output signal, a high frequency response, a small volume, and a sturdy structure. The disadvantage is that it can only be used for kinetic energy measurement. Special cables are required, and self-healing is slower 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 out. A mass is placed on the piezoelectric wafer. 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 nut, and the entire assembly is housed in a metal housing on 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, the base is generally thickened or made of a material with greater stiffness. The weight of the housing and the base almost accounts for the weight of the sensor. half.

During measurement, 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 base and the mass is relatively large, and the mass of the mass is relatively small, it can be considered that the mass's inertia is small. Therefore, the mass undergoes the same motion as the base, and is subject to an inertial force opposite to the acceleration direction. In this way, a mass force that is proportional to the acceleration acts on the piezoelectric chip. Because the piezoelectric chip has a piezoelectric effect, alternating charges (voltages) are generated on its two surfaces. When the acceleration frequency is much lower than the natural frequency of the sensor, the output voltage given by the sensor is proportional to the force, that is, It is proportional to the acceleration of the test piece. The output power is drawn from the sensor output terminal. After inputting to the preamplifier, the acceleration of the test piece can be tested with ordinary measuring instruments; if an appropriate integration circuit is added to the amplifier, the test The speed or displacement of the test piece.

4. Application in robot proximity (ultrasonic sensors)

The main purpose of the robot to install the proximity sensor is to have the following three: First, to obtain the necessary information before contacting the target object, to prepare for the next movement; second, to detect the presence of obstacles in the robot's hand and foot movement space Thing. If obstacles are found, certain measures should be taken in time to avoid collisions. Third, to obtain approximate information about the surface shape of the object.

Ultrasound is a mechanical wave inaudible to the human ear, with a frequency above 20KHZ. The sound that the human ear can hear, the vibration frequency range is only 20HZ-20000HZ. Because of its short wavelength and small diffraction, ultrasonic waves can become sonic rays and propagate directionally. The purpose of the robot using ultrasonic sensors is to detect the existence of surrounding objects and measure the distance of objects. It is generally used to detect larger objects in the surrounding environment, and cannot 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, an ultrasonic transmitter transmits a pulsed ultrasonic wave toward the measured object. 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 an ultrasonic wave encounters an object, it is reflected back. When the ultrasonic receiver receives the echo signal, the timing circuit stops timing. The time recorded by the timing circuit at this time is the propagation time from the time when the ultrasonic wave is transmitted to the time when the echo wave signal is received. Using the propagation time value, the distance from the measured object to the ultrasonic sensor can be converted. The formula for this conversion is simple, that is, the product of half the propagation time of the sound wave and the propagation speed of the sound wave in the medium. The entire working process of the ultrasonic sensor is sequentially performed under the control of a control circuit.

In addition to the above applications, piezoelectric materials have other widely used applications. Such as frequency discriminator, piezoelectric oscillator, transformer, filter, etc.

Status of Piezoelectric Effect

The following introduces several piezoelectric ceramic materials in development and several new applications.

1.Fine-grain piezoelectric ceramics

In the past, piezoelectric ceramics are polycrystalline materials composed of multi-domain grains of several micrometers to several tens of micrometers, and the size is no longer sufficient. Reducing the particle size to the sub-micron level can improve the processability of the material, make the substrate thinner, increase the array frequency, reduce the loss of the transducer array, increase the mechanical strength of the device, and reduce the The thickness of the layer, thereby reducing the driving voltage, is beneficial for improving laminated transformers and brakes. There are so many benefits to reducing particle size, but it also brings the effect of reducing the piezoelectric effect. In order to overcome this effect, people have changed the traditional doping process to increase the piezoelectric effect of fine-grained piezoelectric ceramics to a level comparable to that of coarse-grained piezoelectric ceramics. The cost of producing fine-grained materials is now competitive with ordinary ceramics. In recent years, people have used fine-grained piezoelectric ceramics for cutting and grinding research, and produced some high-frequency transducers, micro-brakes and thin buzzers (ceramics 20-30um thick), which proved the fine-grain pressure Superiority of electric ceramics. With the development of nanotechnology, research and application development of fine-grained piezoelectric ceramic materials is still a recent hot spot.

2.PbTiO3 series piezoelectric materials

PbTiO3 series piezoelectric ceramics are most suitable for making high frequency and high temperature piezoelectric ceramic components. Although PbTiO3 ceramics are difficult to be fired, difficult to polarize, and difficult to make large-sized products, people have done a lot of work in modification to improve their sinterability. The growth of the grains is suppressed, so that each grain is fine and anisotropically modified PbTiO3 material is obtained. In recent years, there have been many reports of improved PbTiO3 materials, which have been widely used in metal flaw detection and high-frequency devices. At present, the development and application of this material is still a topic of concern for many piezoelectric ceramics workers.

3.Piezoelectric ceramic-polymer composites

A piezoelectric composite material composed of an inorganic piezoelectric ceramic and an organic polymer resin has the properties of both inorganic and organic piezoelectric materials, and can produce characteristics that are not found in both phases. Therefore, according to the needs, the advantages of the two-phase material can be combined to make a good performance transducer and sensor. Its receiving sensitivity is very high, and it is more suitable for underwater acoustic transducers than ordinary piezoelectric ceramics. In other ultrasonic transducers and sensors, piezoelectric composites also have great advantages. Domestic scholars are also interested in this field, have done a lot of process research, and have done some useful basic research work on the structure and properties of composite materials. Currently, they are working on the development of piezoelectric composite products.

Please add a caption 4. Piezoelectricity-specific multi-crystal piezoelectric body

Traditional piezoelectric ceramics have a stronger piezoelectric effect than other types of piezoelectric materials, and have been widely used. However, as a large application and high-energy conversion material, the piezoelectric effect of traditional piezoelectric ceramics still cannot meet the requirements. Therefore, in recent years, people have done a lot of work in order to research new piezoelectric materials with better piezoelectricity. Now Pb (A1 / 3B2 / 3) PbTiO3 single crystal (A = Zn2 +, Mg2 +) has been discovered and developed. ). The single crystal d33 can reach up to 2600pc / N (piezo ceramic d33 is up to 850pc / N), k33 can be as high as 0.95 (piezoceramic K33 is up to 0.8), and its strain is> 1.7%, which is almost higher than that of piezoelectric ceramic 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. Ferroelectric scholars claim that the emergence of such materials is another leap in the development of piezoelectric materials. Now the United States, Japan, Russia and China have begun research on the production process of such materials, and the success of its mass production will inevitably lead to the rapid development of piezoelectric material applications.

New field of piezoelectric effect

In recent years, many polymer materials with piezoelectric effect and inverse piezoelectric effect have been developed by synthetic methods, and these materials have been named "artificial muscles". Researchers around the world have launched a challenge: see who can first use artificial muscles to create robotic arms and must win in a one-on-one wrist competition with human arms.

Historical application of the piezoelectric effect

Transducer 2006 is the 126th anniversary of the discovery of the piezo electric effect (Note 1) by the Curie brothers P. Curie and J. Curie. Piezoelectricity was discovered in Jacks' laboratory before 1880. At first, Peel worked on the relationship between pyroelectric effect (Note 2) and crystal symmetry, but later the two brothers discovered that when a pressure is applied to a certain type of crystal, electricity will be generated. They also systematically studied the relationship between the direction of pressure and the electric field strength, and predicted that certain crystals have a piezoelectric effect. After their experiments, they found that piezoelectric materials are: zincblende, sodiumchlorate, tourmaline, quartz, tartaricacid, canesuger, Boracite, calamine, topaz and Rochellesalt. These crystals all have anisotropic structures, and isotropic materials are not piezoelectric.

In an amorphous cubic crystal, an external force is applied to deform the crystal, and an electric displacement is generated due to the local uneven charge distribution in the crystal due to the movement of the charges in the crystal lattice. The displacement of charges is caused by the movement of all ions inside the crystal, or due to the ion polarization caused by the deformation of the electron distribution in the atomic orbit. These charge displacement phenomena exist in all materials. Can cause an effective net electric bipolar moment change per unit volume of the material. Whether such a change is possible depends on the symmetry of the lattice structure. The theory of piezoelectric phenomenon was first discovered by Lippmann when studying the principles of thermodynamics. Later, in the same year, the Curie brothers experimentally proved this theory and established the relationship between piezoelectricity and crystal structure. In 1894, W. Voigt more rigorously determined the relationship between crystal structure and piezoelectricity. He discovered that 32 crystal classes may have piezoelectric effects (21 of the 32 classes without a center of symmetry have 21 (One of which has a piezoelectric constant of zero and the remaining 20 have a piezoelectric effect).

Piezo actuator Today, we all know that piezo-transistors can be used as generators and receivers of sound waves. They have a wide range of uses in military (such as sonar), industry, and engineering. But as early as one-third of the century after the Curie brothers discovered piezoelectricity, the piezoelectric effect received little attention in terms of application. Even Peel himself only used it to measure the charge radiated by radium. In World War I, allied warships were heavily damaged by German submarine attacks, so they sought to find effective ways to detect submarines. Because electromagnetic waves could not effectively penetrate sea water, and sound waves could easily travel through the sea, P. Langevin at the time developed a quartz pressure transistor as a sound wave generator. It is a pity that by the end of the war, it is too late to use it. A steel sheet is affixed to each side of the quartz to reduce its oscillation frequency to 50KHz. In addition to an electrical pulse signal, it is converted into a sound wave by the transducer and transmitted to the ocean floor. After a period of time, the transducer receives a reflection from the ocean floor. The wave, from the round-trip time and the speed at which the wave travels in the sea, determines the distance from the transducer to the sea floor. This principle can also measure the position of the submarine.

Soon after the First World War, two important applications were developed for quartz transducers. First of all, Professor GWPierce of Harvard University uses quartz crystal to make an ultrasonic interferometer. The ultrasonic waves generated by quartz and the echoes reflected by the acoustic wave reflector in the figure are mixed to produce a maximum value. Backward, another maximum value can be obtained, and the wavelength of the acoustic wave can be measured from the distance between the two maximum values, that is, the distance that the reflector moves between two adjacent maximum values. Because the frequency is known, the velocity of the wave in the gas medium can be determined by the product of frequency and wavelength. At the same time, the reduction coefficient of the wave in the gas can be obtained from the amplitude reduction rate between several maxima. At that time, it was used to measure the relationship between the speed of sound waves in carbon dioxide and frequency, and to find the dispersion relationship of wave speed. In this way, the wave velocity and attenuation rate of sound waves at different mixing ratios and temperatures can be studied.

In 1927, RWWood and ALLoomis first used high-power ultrasound. Using Lan Jiewen-type quartz transducers combined with high-power vacuum tubes, high energy is generated in the liquid, causing the liquid to cause the so-called cavitation phenomenon. The effects of high-power ultrasound on biological samples were also studied.

In the study of underwater sound, it was found that the 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. This high frequency-to-temperature stability is most useful for controlling the frequency of oscillators and some filters. In 1919, Professor Cady first used quartz as a frequency controller. Figure 4 is the earliest crystal-controlled oscillator circuit. Because the crystal has a very high Q value (Note 3), the frequency of the oscillator is controlled by the resonant frequency of the crystal, and the frequency does not change with temperature. Later, Pierce and Pierce-Miller invented a crystal-controlled oscillator circuit that was widely used in the future. In World War II, approximately 10 million crystal oscillators were used to establish tank-to-tank communication and between ground and aircraft.

Another important application of quartz crystals is to obtain highly frequency-selective oscillators. Quartz crystal is a high Q value piezoelectric chip. High Q value means low sound wave energy loss (its attenuation rate is proportional to the square of frequency); high Q value also means narrow frequency band, so it is not suitable for sound transmission circuit . For use in carrier communication systems, a series inductor (see Figure 5) can be used to obtain broadband operation. The structure diagram of this type of filter is often used in wired communication systems, microwave communication systems, etc.

The material used for the sonar drums of World War II was Rochelle salt instead of quartz crystals. Although the Rochelle salt has a high electromechanical coupling efficiency, it is relatively unstable, with a low withstand voltage, and it is difficult to operate at too high power. In theory, the Rochelle salt was the first material to have ferroelectricity, with a spontaneous polarization along the crystal axis. Figure 7 shows the relationship between the amount of polarization and the temperature measured along the X axis. It has two Curie temperatures. The amount of polarization is zero at the Curie temperature, and the maximum polarization is between the two temperatures. To commemorate Dr. Seignette, who was born in Rochelle, this effect is called the Seignette ferroelectric effect, and is generally referred to simply as the ferroelectric effect to indicate its similarity to the ferromagnetic effect. In ferroelectric materials, when the temperature is lower than the Curie temperature, there is an electric dipole inside the material. Most hydrogen-bonded electric bipolars, such as Rochelle salts, have a regular arrangement of the bipolar electrodes, and generally have only one Curie temperature. However, Rochelle salts have two Curie temperatures. The difference is mainly due to the difference in the negative ions at the hydrogen bond terminals. The potentialwell distribution of a general hydrogen-bonded crystal is shown in Figure 8. There are two positions where hydrogen ions can exist between two oxygen ions. The hydrogen-bond electric bipolar value is equal to the product of the charge and the difference between the distances between the two groups of ions. . The application of an electric field can cause the hydrogen ions to jump from one position to another and change the direction of the electric bipolar. At high temperatures, there is an equal chance of hydrogen turbulence filling the positions of the two wells, so no natural polarization exists. When the temperature decreases, the two electric bipolars attract each other, and the bipolar direction arrangement tends to be regular. At Curie temperature, the two electric bipolars cancel each other out, but a small external force at Curie temperature can cause a large bias. If the temperature is lower than the Curie temperature, spontaneous partial polarity occurs. For a hydrogen-bonded crystal that generally has a potential well as shown in Figure 8, its bias polarity can be increased until saturation occurs. However, for the Rochelle salt, the polarities begin to decrease to zero after reaching a maximum value. The reason can be illustrated by the potential well distribution diagram in Figure 8. At very low temperatures, all hydrogen ions are completely distributed in the two low-energy wells, and there is no spontaneous partial polarity. As the temperature rises, some hydrogen ions gain thermal energy and jump to higher energy levels. The higher the temperature, the greater the chance of such a transition. The two electric bipolar electrodes generate a lower Curie temperature due to mutual attraction. Figure 9 shows the X-ray diffraction crystal structure of the Rochelle salt. What causes the ferroelectric effect is a hydrogen bond composed of an oxygen molecule numbered 1 and a water molecule numbered 10. For hydrogen ions, these two molecules are two different ions at the endpoints, so two potential wells with different names are formed as shown in FIG.

Rochelle salt has been the only ferroelectric material known in the past, but now we know that there are more than 100 kinds of ferroelectric materials. Ferroelectric materials have spontaneous partial polarity, and can generate induced partial polarity by applying an electric field, 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 It is slightly inferior to the voltage transistor. Gradually, people used ferroelectric ceramic magnets as transducers. The earliest use was barium titanate (BaTiO3), which was discovered by MIT von Hippel and Soviet and Russian scientists Vul and Goldman, respectively. The unpolarized ceramic magnet has no regular polarization direction in the domain (Note 5). The entire ceramic magnet is like a capacitor with a high dielectric constant, because it only needs a small volume to be large. The electric capacity is therefore used in televisions. If a high voltage is applied at a temperature above 120 ° C, the electrical coupling in some domains is regularly arranged, and there is a net bias polarity, which has a piezoelectric effect. We can produce longitudinal waves (the electric field is parallel to the thickness direction) or transverse waves (the electric field is perpendicular to the thickness direction) due to the different directions of the applied AC electric field. Longitudinal waves can travel in water and generate high energy in solids. Shear waves are slower and are suitable for making delay lines. At present, the best piezoelectric ceramic magnetism is PZT (lead-zirconate-titanate).

Recently, two important ferroelectric materials can be used to make sonic transducers, one is a polymer film, polyvinylidenefluoride (PVF2 or PVDF for short), and the other is lithium niobate (LiNbO3). Polyvinylidene fluoride has strong piezoelectricity after stretching and increasing the DC voltage. It has many advantages: its acoustic characteristic impedance is close to water, the impedance is naturally matched, it is easy to obtain broadband operation, and it is suitable for non-destructive testing and medical diagnosis. And sonar and hydrophone use, especially it has a very high acoustic wave reception coefficient, used to make passive sonar (passivesonar) hydrophoneassay is very important. Besides, it is flexible and can withstand high voltage (its breakdown voltage is about 100 times higher than PZT). The lithium niobium oxide single crystal has high electromechanical coupling and extremely low acoustic wave attenuation coefficient, and is easy to excite high-frequency surface acoustic waves (Rayleighwave). It is the best material for making surface acoustic wave (SAW) components. These components have an irreplaceable position in signal processing systems and communication systems. Figure 11 shows a surface wave pass filter using lithium niobium oxide. Generate a surface acoustic wave (a so-called interdigital transducer, or IDT for short) with a set of positive and negative voltage interdigital transducers. The center frequency of the excited acoustic wave is determined by the distance between the positive and negative electrodes. . Figure 12 shows another surface acoustic wave pulse wave stretching and compression filter, which can be used in the CHIRP radar system to improve the search range and resolution.

Another important and unique study is on so-called acoustic microscopy. This microwave-frequency component uses a sputtered piezoelectric film as an acoustic wave transducer to generate several GHz (1GHz = 109) by vibration. (Cycles / second) sound waves, corresponding to a wavelength of about one micron (10-6 meters). Because the vibration frequency of the transducer is inversely proportional to the thickness of the piezo transistor, thin-film piezoelectric materials such as zinc oxide or cadmium sulfide are required to generate such high-frequency sound waves.

The centennial of the discovery of the time-dependent piezoelectric effect, this article is written with reference to the work of WPMon, which introduces the history of piezoelectricity and its application. The early piezoelectric effect was limited to academically interesting research, but now it has become a very useful effect. It has been used to make a variety of acoustic and electrical transducers, and its operating spectrum can range from 100Hz to several GHz. There are different uses depending on the frequency. Sonar, anti-submarine, subsea communication, telephone communication, etc. are the most typical applications of low frequency (audio, AF band) signals. In several MHz range, its wavelength is in the millimeter range, suitable for non-destructive testing (NDT) and medical diagnosis, so-called ultrasound imaging, holography, computer-assisted acoustic tomography, etc. It is for these purposes. In the VHF and UHF bands, surface acoustic wave electronic components developed using piezoelectricity are used. Signal processing components such as delay lines, various filters, convolvers, and correlators have important applications in communication and signal processing. When the frequency is from high to low microwave, its corresponding wavelength is in the micrometer range. It is used to make acoustic microscopes. Its resolution is comparable to that of traditional optical microscopes. The unique properties of mechanical waves instead of electromagnetic waves can make up for the application of optical microscopes insufficient.

Note 1: Applying pressure or pulling force to certain materials, in addition to changes in the material's shape (so-called strain), because the lattice structure of such materials has a certain asymmetry (so-called inversionasymmetry), the shape is deformed The internal electron distribution is locally uneven and a net electric field distribution is generated. Conversely, the application of a periodic voltage or electric field change can cause deformation of the material and a corresponding stress. The change in shape changes with the frequency of the applied voltage signal, which can generate a periodic elastic wave or acoustic wave. This effect is called Piezoelectric effect, these materials are called piezoelectric materials.

Note 2: In some ferroelectric materials, when its temperature changes, it will cause its spontaneous bias moment to change, and it will show a net charge distribution on the surface of the material. This effect is called coke effect. With this effect, temperature changes can be detected or so-called thermalwaves can be measured.

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

Note 4: The insertion loss indicates the total loss of an electronic component or component, that is, the difference between the output signal and the input signal, which is generally expressed in decibels (dB).

Note 5: In ferromagnetic materials, when the temperature is far below the Curie point, from a microscopic point of view, the magnetic moments of all electrons should be aligned in the same direction, but this is not true. In fact, this material is divided into many small areas inside, and the magnetic moments are arranged regularly in each area. However, the arrangement direction of the magnetic moments between small areas and small areas is different, so that the magnetic moment of the entire material is much smaller than that. Saturation magnetic moment. These small regions are referred to as domains or domains for short. Domains also exist in antiferromagnetic materials, ferroelectric materials, antiferroelectric materials, ferroelastics, and superconductor materials.

Piezoelectric effect lighter

The piezoelectric effect is a phenomenon in which a heterogeneous charge appears on the surface of a medium when some medium deforms under the action of a 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 corresponding potential difference (voltage) between the two surfaces. This miraculous effect has been applied to many fields closely related to people's production, life, military, and science and technology to achieve functions such as power conversion. For example, piezoelectric ceramics can be used to convert external force into electrical energy, which can produce piezoelectric lighters that do not use flint, gas stove ignition switches, cannonball trigger fuze, and so on. In addition, piezoelectric ceramics can also be used as sensitive materials in electroacoustic devices such as loudspeakers and electro-phonics; used in piezoelectric seismographs, it can monitor subtle vibrations that humans cannot perceive, and accurately measure the orientation and intensity of seismic sources To predict earthquakes and reduce losses. Piezoelectric actuators made using the piezoelectric effect have the function of precise control and are important devices in the fields of precision machinery, microelectronics, and biological engineering. 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 are blind to their existence.

Quite a few of the currently popular disposable plastic lighters use piezoelectric ceramic devices to light. Take out the piezoelectric ignition element,

Piezoelectric effect measuring instrument

Piezo ignition

What instrument can be used to measure the instantaneous voltage generated by the voltage ceramic element of the machine? At first, we tried to measure with the high-voltage DC block of a common pointer multimeter, and found that each time the black plastic pressure lever of the ignition element is pressed, the pointer can only slightly shake because of the voltage connected to the two electrodes. The reason for analysis is that because the duration of the voltage pulse is very short and the pointer has a large inertia, the pointer cannot simultaneously reflect the change in voltage and do a large deflection.

Switch to a digital display type multimeter, which is supposed to have no pointer inertia effect. It should be able to read the instantaneous high voltage. If the governor does not wish, we ca nt see the expected high voltage reading at all, we can only see some low voltage that is uncertain. data. For analysis, this is due to the slow response of the liquid crystal display and the short duration of the ignition voltage pulse. It is too late to display the highest instantaneous voltage, and can only display some random voltage readings during the voltage drop (more gentle phase).

Finally, we moved out of the laboratory's "heavy weapon" oscilloscope and tried again. We use the most common J2459 student oscilloscope in the laboratory. The connecting wire is two ordinary wires with a fish clip. Theoretically, the oscilloscope uses the electron beam to deflect and display the light spot movement on the fluorescent screen. The electron beam inertia is extremely small, and it should be able to "track" the change of the ignition high-voltage pulse. The experimental results are not unexpected.

Method for estimating piezoelectric effect

Set the oscilloscope AC / DC selection switch to the DC position and the scanning range to the 10 to 100 kHz position. Use the X shift and Y shift to move the horizontal bright line to the center of the grid coordinates and set it on the X axis. In order to estimate the maximum voltage amplitude of the piezoelectric effect, we must first use the grid coordinate system in front of the phosphor screen to determine the voltage scale: using two wires connected to the Y input terminal of the oscilloscope, 1.5 times of a dry battery The V voltage is added to the oscilloscope, the attenuation is set to 1, and the Y gain is set to the lowest. It can be found that the horizontal bright line just jumped (or jumped down) about two divisions, that is, the two divisions at this time represent 1.5V voltage. With the Y gain unchanged, set the Y attenuation to the 1000 (one-thousandth) block, and the two grids in front of the phosphor screen can represent 1500V.

Connect the two crocodile clips of the two feeders on the Y input terminal to the two electrodes of the piezoelectric lighter's piezoelectric element, and quickly press the black plastic pressure lever, you can see that the horizontal bright line originally at the center height is upward ( (Or down) to the beating 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 divisions, 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 "scanning fine adjustment" knob of the oscilloscope every time you press the pressure lever (change the scanning range to "10 ~ 100Hz" in advance). The waveform shown in Figure 2 has a steep rise in voltage and a gentle fall, with peaks above four divisions.

Piezoelectric effect pulse time

Set the attenuation range of the oscilloscope to 1000, and the scanning range to 10 ~ 100Hz. Turn Scan Fine Adjustment to the left, that is, the scanning frequency is 10Hz. The scanning line is filled with 10 divisions, then each division represents 1/10 × 1 / 10s, ie 0.01. Press the black plastic pressure lever of the piezoelectric element, and you can see that the piezoelectric pulse continues for one division, as shown in Figure 3, which corresponds At 0.01s, that is, the duration of the pulse is about 0.01s.

Piezoelectric effect piezoelectric crystal

There is a very interesting type of crystal. When you squeeze or stretch it, it will generate different charges at its two ends. This effect is called the piezoelectric effect. A crystal that produces a piezoelectric effect is called a piezoelectric crystal. Crystal (-quartz) is a well-known piezoelectric crystal.

If pressure is applied to a slice cut from a crystal in a certain direction, a charge will be generated on the slice. If the sheet is stretched in the opposite direction, a charge will also appear on the sheet, but with the opposite sign. The greater the force of squeezing or stretching, the more charge will be on the crystal. If electrodes are plated on both ends of the sheet and alternating current is applied, the sheet will periodically expand or contract, that is, it will start to vibrate. This reverse piezoelectric effect has been widely used in science and technology. Crystals can be used to make piezoelectric quartz flakes with an area of just a few square millimeters and a thickness of only a few tenths of a millimeter. 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 stable and constant vibration is exactly what is necessary to control the frequency in radio technology. Many electrical appliances such as color televisions in your home have filters made of piezoelectric chips to ensure the clarity of images and sound. The quartz electronic watch you wear has a core component called a quartz oscillator. It is this key component that ensures higher accuracy of the quartz watch than other mechanical watches.

Instruments equipped with piezoelectric crystal elements make it possible for technicians to study pressure changes in steam engines, internal combustion engines and various chemical equipment. With piezoelectric crystals, you can even measure the pressure of fluid in the pipeline, the pressure on the cannon barrel when firing the shell, and the instantaneous pressure when the bomb explodes.

Piezoelectric crystals are also widely used in sound reproduction, recording, and transmission. The piezoelectric chip mounted on the microphone converts the vibration of sound into a change in current. As soon as a sound wave hits a piezoelectric sheet, an electric charge is generated on the electrodes at both ends of the sheet, and its size and sign change with the change of sound. This change in the charge on the piezoelectric chip can be turned into radio waves and transmitted to remote places through electronic devices. These radio waves are received by the radio, and through the vibration of the piezoelectric crystal sheet placed on the radio horn, they become sounds echoing in the air. Can it be said that the piezoelectric chip in the microphone can "hear" the sound, and the piezoelectric crystal sheet on the speaker "speaks" or "sings".

Piezoelectric effect piezoelectric polymer

piezoelectric polymer

1946JaffePbTi03-PbZrO2PZTmPZT

Conversely, when the excitation electric field is applied, the medium will produce mechanical deformation, which is called the inverse piezoelectric effect.

18801942194750--6070

X35

In medicine, the doctor places the piezoelectric ceramic probe on the examination part of the human body, sends out an ultrasonic wave after being energized, and transmits an echo after the human body touches the tissue of the human body. Then the echo is received and displayed on a fluorescent screen. Can understand the internal conditions of the human body.

In the industry, there are piezoelectric ceramic elements in the geological detector, which can be used to judge the geological conditions of the stratum and identify the underground mineral deposits. There is also a transformer in the TV, a voltage ceramic transformer, which is smaller in size and lighter in weight, and has an efficiency of 60% to 80%. Defects in deformation. Most of the TV sets produced abroad now use piezoelectric ceramic transformers. A 15-inch picture tube uses a 75-mm piezoelectric ceramic transformer. This makes the TV smaller and lighter.

Piezoelectric ceramics are also widely used in daily life. The gas electronic lighter made of ordinary flint was replaced by two piezoelectric ceramic pillars with a diameter of 3 mm and a height of 5 mm, which can be used for tens of thousands of times. An electronic ignition gun made using the same principle is an excellent tool for igniting a gas stove. 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. The toy will make a realistic and interesting sound.

With the development of high and new technology, the application of piezoelectric ceramics will be more and more broad. In addition to being used in high-tech fields, it is more for the eyes of people in daily life, creating a better life for people.

Piezoelectric effect piezoelectric material

Piezoelectric materials have a piezoelectric effect because of the special arrangement of atoms in the crystal lattice, which makes the material have the effect of coupling stress fields and electric fields. According to the types of materials, piezoelectric materials can be divided into four types: piezoelectric single crystals, piezoelectric polycrystals (piezoelectric ceramics), piezoelectric polymers, and piezoelectric composite materials. According to the specific material form, it can be divided into two categories: piezoelectric materials and piezoelectric thin films.

Piezoelectric effect polymer

As early as 1940, the Soviet Union had discovered that wood was piezoelectric. Later, piezoelectricity was found in ramie, silk bamboo, animal bones, skin, blood vessels and other tissues. In 1960, the piezoelectricity of synthetic polymers was discovered. In 1969, it was found that the polarized polyvinylidene fluoride has strong piezoelectricity. Materials with strong piezoelectric properties include PVDF and its copolymers, polyvinyl fluoride, polyvinyl chloride, poly--methyl-L-glutamate, and nylon-11.

Piezoelectric effect composite

Piezoelectric composite material is a piezoelectric material that is a composite of two or more materials. Common piezoelectric composite materials are two-phase composite materials of piezoelectric ceramics and polymers (such as polyvinylidene fluoride living epoxy resin). This composite material has the advantages of both piezoelectric ceramics and polymers, has good flexibility and processing properties, and has a lower density, which can easily achieve acoustic impedance matching with air, water, and biological tissues. In addition, piezoelectric composites are also characterized by high piezoelectric constants. Piezoelectric composite materials have a wide range of applications in medical, sensing, and measurement fields.