Development and application of piezoelectric ceramics.

Piezoelectric ceramics refer to polycrystalline materials formed by high-temperature sintering and the solid-phase reaction of oxide mixtures (zirconium oxide, lead oxide, titanium oxide, etc.), and are treated by DC high-voltage polarization to make them have piezoelectric effect. They are functional ceramic materials that can convert mechanical energy and electrical energy into each other.

Due to their good mechanical properties and stable piezoelectric properties, piezoelectric ceramics, as an important force, heat, electricity, and light sensitive functional material, have been widely used in sensors, ultrasonic transducers, micro-displacers, and other electronic components.

With the continuous research and improvement of material technology, as well as the rapid development of high-tech fields such as electronics, information, and aerospace, the production technology and application development of piezoelectric ceramics as new materials containing high intelligence are hot topics of concern.

Piezoelectric ceramics Piezoelectric ceramics are a type of electronic ceramic material with piezoelectric properties. The main difference from typical piezoelectric quartz crystals that do not contain ferroelectric components is that the crystal phases that constitute their main components are all ferroelectric grains.

Since ceramics are polycrystalline aggregates with randomly oriented grains, the spontaneous polarization vectors of each ferroelectric grain are also chaotically oriented. In order to make ceramics exhibit macroscopic piezoelectric properties, it is necessary to place the piezoelectric ceramics under a strong DC electric field for polarization after they are fired and the end faces are double-electroded, so that the originally chaotically oriented spontaneous polarization vectors are preferentially oriented along the direction of the electric field. After the polarization treatment, the piezoelectric ceramics will retain a certain macroscopic residual polarization intensity after the electric field is cancelled, so that the ceramics have certain piezoelectric properties.

Development History

In 1880, the Curie brothers first discovered the piezoelectric effect of tourmaline, and the history of piezoelectricity began.

In 1881, the Curie brothers experimentally verified the inverse piezoelectric effect and gave the same forward and inverse piezoelectric constants as quartz.

In 1894, Voigt pointed out that only crystals with twenty point groups without a symmetric center can have piezoelectric effects. Quartz is a representative of piezoelectric crystals and has been applied. During the First World War, Curie's successor Lang Zhiwan first used the piezoelectric effect of quartz to make underwater ultrasonic detectors for detecting submarines, thus opening a chapter in the history of piezoelectric applications.

The epoch-making progress of piezoelectric materials and their applications should be attributed to the discovery of BaTiO3 ceramics during the Second World War. In 1947, Roberts of the United States applied high voltage to BaTiO3 ceramics for polarization treatment and obtained the voltage properties of piezoelectric ceramics. Subsequently, Japan actively carried out application research on the use of BaTiO3 piezoelectric ceramics to make various piezoelectric devices such as ultrasonic transducers, high-frequency transducers, pressure sensors, filters, resonators, etc. This research continued until the mid-1950s.

In 1955, B. Jaffe et al. of the United States discovered PZT piezoelectric ceramics that were superior to BaTiO3 in piezoelectricity, which greatly advanced the application research of piezoelectric devices. Some uses that were difficult to be put into practical use in the BaTiO3 era, especially piezoelectric ceramic filters and resonators, have been rapidly put into practical use with the advent of PZT. SAW devices such as filters, delay lines and oscillators using surface acoustic waves (SAW) have also been put into practical use in the late 1970s.

Since the late 1980s, people have developed relaxor ferroelectric ceramic materials, and on this basis, relaxor ferroelectric single crystal materials have been successfully developed, laying the foundation for three-dimensional ultrasonic imaging. At present, people have made new breakthroughs in the application of nanotechnology to the production process of piezoelectric materials.

At present, countries around the world are vigorously developing lead-free piezoelectric ceramics to protect the environment and pursue health...

Application

Since the birth of the first ceramic piezoelectric material barium titanate in 1942, piezoelectric ceramics have been used as application products in all aspects of people's lives. As a link for electromechanical coupling, the application of piezoelectric materials can be roughly divided into two aspects: the application of piezoelectric ceramic frequency control devices represented by piezoelectric resonators and the quasi-static application of mutual conversion between mechanical energy and electrical energy.

1. Piezoelectric vibrators and piezoelectric ceramic frequency control devices

Polarized piezoelectric ceramics, i.e. piezoelectric vibrators, have a natural vibration frequency determined by their size. Stable electrical oscillations can be obtained by using the natural vibration frequency of piezoelectric vibrators and the piezoelectric effect. When the frequency of the applied voltage is the same as the natural vibration frequency of the piezoelectric vibrator, resonance will occur and the amplitude will be greatly increased. In this process, the alternating electric field generates strain through the inverse piezoelectric effect, and the strain generates current through the positive piezoelectric effect, realizing the maximum mutual conversion of electrical energy and mechanical energy. Using this feature of piezoelectric vibrators, various filters, resonators and other devices can be manufactured.

These devices have low cost, small size, no moisture absorption, long life, good frequency stability, higher equivalent quality factor than LC filters, wide applicable frequency range, high precision, especially in multi-channel communication, amplitude modulation reception and various radio communications and measuring instruments to improve anti-interference ability. Therefore, it has replaced a considerable part of electromagnetic oscillators and filters, and this trend is still developing.

2. Piezoelectric transformer

The piezoelectric transformer is made by utilizing the characteristics of the mutual conversion of electrical energy and mechanical energy of the piezoelectric effect. It consists of two parts, the input end and the output end, and their polarization directions are perpendicular to each other. The input end is polarized along the thickness direction, and after applying the alternating voltage, it vibrates longitudinally. Due to the inverse piezoelectric effect, the output end will have a high voltage output.

The piezoelectric ceramic transformer is a new type of solid-state electronic device. Compared with the traditional electromagnetic transformer, it has the advantages of simple structure, small size, light weight, large transformation ratio, good stability, no electromagnetic interference and noise, high efficiency, high energy density, high safety, no winding, non-flammable, no leakage magnetic phenomenon and electromagnetic radiation pollution.

According to the working mode of the piezoelectric ceramic transformer, it can be divided into the following categories: Rosen-type piezoelectric ceramic transformer, thickness vibration mode piezoelectric ceramic transformer, radial vibration mode piezoelectric ceramic transformer, etc.

In recent years, some piezoelectric transformers with better performance have appeared, such as the third-order vibration mode Rosen-type piezoelectric ceramic transformer with two input ends and high-power multilayer piezoelectric ceramic transformer. At present, piezoelectric ceramic transformers are mainly used in power devices such as AC-DC, DC-DC and high-voltage generating devices, such as cold cathode tubes, neon tubes, laser tubes and small X-ray tubes in liquid crystal displays, high-voltage electrostatic spraying, high-voltage electrostatic flocking and radar display tube drivers.

3. Piezoelectric transducers

Piezoelectric transducers use the piezoelectric effect and inverse piezoelectric effect of piezoelectric ceramics to achieve the mutual conversion of electrical energy and acoustic energy. Piezoelectric ultrasonic transducers are one of them. They are underwater acoustic devices that transmit and receive ultrasonic waves. Under the action of sound waves, the piezoelectric transducer in the water will induce charges at both ends of the transducer, which is the sound wave receiver; if an alternating electric field is applied to the piezoelectric ceramic sheet, the ceramic sheet will become thinner and thicker from time to time, and at the same time generate vibrations and emit sound waves, which is the ultrasonic transmitter.

Piezoelectric transducers are also widely used in industry for underwater navigation, ocean detection, precision measurement, ultrasonic cleaning, solid flaw detection, medical imaging, ultrasonic diagnosis, ultrasonic disease treatment, etc. Another application area of piezoelectric ultrasonic transducers today is telemetry and remote control systems. Its specific application examples mainly include: piezoelectric ceramic buzzers, piezoelectric igniters, ultrasonic microscopes, etc.

4. Piezoelectric ultrasonic motor

Piezoelectric ultrasonic motor is a new type of micromotor that uses the inverse piezoelectric effect of piezoelectric ceramics to generate ultrasonic vibrations, amplifies the micro-deformation of the material through resonance, and is driven by the friction between the vibrating part and the moving part, without the need for a conventional electromagnetic coil.

Compared with traditional electromagnetic motors, it has the characteristics of low cost, simple structure, small size, high power density, good low-speed performance (can achieve low-speed operation without a deceleration mechanism), large torque and braking torque, fast response, high control accuracy, no magnetic field and electric field, no electromagnetic interference and electromagnetic noise.

Due to its own characteristics and performance advantages, piezoelectric ultrasonic motors are widely used in precision instruments, aerospace, automatic control, office automation, micro-mechanical systems, micro-assembly, precision positioning and other fields. At present, Japan is in a leading position in technology in this field, and has widely used piezoelectric ultrasonic motors for automatic focusing of cameras and video cameras, and has formed a large-scale series of products.

Development Trends

1. Lead-free piezoelectric ceramics

Lead-free piezoelectric ceramics, also known as environmentally coordinated piezoelectric ceramics, require that ceramic materials do not produce substances that may be harmful to the environment during preparation, use, and waste treatment to avoid harm to human health and reduce environmental pollution. However, the piezoelectric ceramic materials currently used are mainly based on PZT, whose piezoelectric properties are much superior to other piezoelectric ceramic materials, and the electrical properties of the materials can be adjusted through doping modification and process control to meet various application requirements.

Among the various lead-containing piezoelectric ceramic materials currently used in industry, the content of lead oxide accounts for more than 60% of the total mass of the material. The harm caused to the human body and the environment by these materials during component manufacturing, processing, storage, transportation, use and waste treatment is self-evident. Therefore, lead-free environmentally friendly piezoelectric ceramic materials are an important direction and hot topic for research and development in recent years.

At present, the research on lead-free piezoelectric materials has mainly gone through the research process of barium titanate-based, sodium bismuth titanate-based, bismuth layered structure, niobate-based and tungsten bronze structure lead-free piezoelectric ceramics. Among them, niobate-based lead-free piezoelectric ceramics are the most promising lead-free piezoelectric materials. Although the development and research of lead-free piezoelectric ceramics have made great progress, it is still impossible for lead-free piezoelectric ceramics to completely replace lead-based piezoelectric ceramics. The research and development of lead-free piezoelectric ceramics will still have a long way to go.

2. Piezoelectric composite materials

In order to play a role in the application of hydrophones, piezoelectric composite materials have gradually developed in the 1970s. Piezoelectric composite materials are functional composite materials with piezoelectric effect composed of piezoelectric ceramic phases and polymer phases in a certain connection method.

Due to the addition of flexible polymer phases, the density, acoustic impedance and dielectric constant of piezoelectric composite materials are reduced, while the figure of merit and electromechanical coupling coefficient of the composite materials are improved, overcoming the brittleness of pure piezoelectric ceramics and the high cost of piezoelectric polymers. In addition to being used as hydrophones, piezoelectric composite materials are also used in industry, medicine and communications. After more than 40 years of continuous research, piezoelectric composite materials have made considerable progress in their application research, but their complete theory has not yet been established, and their application development needs to be fully explored. At present, the research on piezoelectric composite materials mainly focuses on developing connection types, improving molding processes, and preparing multifunctional devices.

3. Nano-piezoelectric ceramics

In recent years, with the rapid development of nanotechnology, nano-ceramics have gradually attracted people's attention. Nano-powders are formed and sintered to form dense and uniform bulk nano-ceramics. The toughness, strength and superplasticity of the material are greatly improved, overcoming many shortcomings of engineering ceramics, and having an important impact on the mechanical, electrical, thermal, magnetic, optical and other properties of the material.

By selecting the material composition system and adding nano-scale particles, whiskers, chip fibers, etc. for modification, nano-piezoelectric ceramic materials with both high performance and low-temperature sintering can be obtained. By controlling the growth of nano-grains, quantum confinement effects and ferroelectrics with unique properties can be obtained to improve the electromechanical conversion and thermal release properties of piezoelectric pyrolytic materials. Various types of piezoelectric transformers, piezoelectric drivers, high-power ultrasonic welding technology, piezoelectric vibrating feeders, new ultrasonic CVD processes and high-power ultrasonic projects supporting nuclear power plants that have developed rapidly in recent years are all applications of nanoceramics in piezoelectricity.

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