Performance Research and Application Trend of Piezoelectric Ceramics

Piezoelectric ceramics are ferroelectric ceramics that are made of mixed oxides (zirconia, lead oxide, titanium oxide, etc.) after high-temperature sintering and solid-phase reaction, and through DC high-voltage polarization treatment to make them have piezoelectric effect collectively. It is a functional ceramic material that can convert mechanical energy and electrical energy. Due to its 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, aerospace, etc., the production technology and application development of piezoelectric ceramics containing highly intelligent new materials are hot topics.

 

The polarization principle of piezoelectric ceramics

The free electrons of piezoelectric ceramics are arranged disorderly before polarization. After the polarization treatment, the residual polarization is generated along the polarization direction to become anisotropic polycrystalline. The free electrons tend to be consistent, and the piezoelectricity is greatly enhanced. As shown in Figure 1 and Figure 2, the piezoelectric ceramic material can be made into any shape and any polarization direction. Piezoelectric ceramic materials before and after polarization have different dielectric constants(ε)and piezoelectric constants(d).

Set the dielectric constant before polarization:

ε11 = ε22 = ε33. If the piezoelectric material is polarized in direction 3, the other two electrode surfaces are perpendicular to the polarization direction. The dielectric constant after polarization: ε11 = ε22 ≠ ε33 and the value of ε33 is much larger than ε11. The piezoelectric constant of piezoelectric ceramics is also anisotropic, and the value of the piezoelectric constant d is also different in different directions. Among them, the value along the direction 3 is the largest, that is, d33> d31 and d32. When measuring with an ammeter, only d33 has current, and no current is generated in the other two directions. The polarization of piezoelectric ceramics is very similar to the magnetization of magnets, and the magnetic field strength before and after magnetization will change greatly.

 

The development status of low-temperature sintering of piezoelectric ceramics

 

The research on low-temperature sintering technology of piezoelectric ceramics began after 1960, usually from the two aspects of adding sintering aid and improving the process to reduce the sintering temperature. Since the 1980s, scholars at home and abroad have conducted extensive research on the low-temperature sintering of piezoelectric ceramics. Li Longshi of Tsinghua University added a co-solvent to the PZT binary system and developed a material with good performance and sintered at a low temperature of 960 degrees Celsius. Q. Yill et al. added sintering aids to KNN-based ceramics to prepare lead-free piezoelectric ceramic materials with excellent performance at low temperatures. In addition, researchers have also carried out many useful explorations in improving the process, and have achieved certain results.

 

Reduce the sintering temperature of piezoelectric ceramic materials

 

Lowering the sintering temperature of piezoelectric ceramic materials is usually carried out from the two aspects of adding co-solvent and improving the process. There are mainly the following four methods:

1. Add co-solvent to reduce the sintering temperature

Adding flux to the base material, there are three low-temperature sintering methods:

The first way is to reduce the sintering temperature by forming a solid solution. Ion replacement causes distortion of the crystal lattice, increases structural defects, and reduces the barrier between electrical domains, thereby facilitating ion diffusion and promoting sintering. The second way is to reduce the sintering temperature by forming the liquid phase sintering. The grain rearrangement and strengthened contact in liquid phase sintering can increase the grain boundary mobility, fully discharge the pores, promote the growth of crystal grains, increase the density of the porcelain body, and achieve the purpose of reducing the sintering temperature. The third way is to reduce the sintering temperature and improve performance through transitional liquid phase sintering. The low melting point additives first form a liquid phase to promote sintering during the sintering process, and then serve as the final phase in the late sintering process, sucking back into the main crystalline phase, and playing a role of doping modification.

 

This "dual effect" of low melting point additives can reduce the sintering temperature by 250-300 ℃ and improve the performance.

 

2. Chemical synthesis method reduces sintering temperature

 

The chemical synthesis method can reduce the sintering temperature, but the cooling range is limited, and the sintering temperature of the material is still higher than 1000 ℃.

 

3. Hot pressing method reduces sintering temperature

 

Hot-pressing sintering can increase the sintering driving force of ceramics, and facilitate the diffusion of pores or vacancies from the grain boundary to the ceramic body, thereby increasing the density of the ceramic body and reducing the sintering temperature. Using hot-press sintered PZT piezoelectric ceramic material, the sintering temperature is reduced by 150-200℃, and the performance is also improved a lot.

 

4. Cold pressing method reduces sintering temperature

 

Under the pressure of hundreds of thousands of atmospheres, the powder can be densified and sintered. For example, the emissive PZT ceramic powder was cold-pressed at 150,000 atmospheres, and as a result, a ceramic body with a density of 7.2g/cm (90% of the theoretical density) was obtained, and the ceramic powder was originally earthy yellow. Cold-pressed and sintered into a gray-black porcelain body.

 

Compare the above research on low-temperature sintering of piezoelectric ceramic materials at home and abroad. There are the following conclusions:

(1) When forming a solid solution to lower the sintering temperature, ion replacement must be carried out under certain conditions, and the resulting structural defects are limited, so the temperature drop is not large, generally within 200 ℃.

(2) The effect of lowering the sintering temperature through the formation of a liquid phase is obvious, but the liquid phase product remains in the ceramic microstructure. The existence of this low melting point product will cause the material's mechanical strength, dielectric properties, and piezoelectric properties to decline

(3) The sintering temperature when the powder is made by chemical synthesis is still higher than 1000 degrees Celsius. In addition, due to the different compounding capabilities of various metal ions in the solution, during the dehydration or calcining process, compounds may separate or form other compounds. It can be seen that not all raw materials can be prepared by chemical synthesis.

(4) During the hot-pressing sintering process, crystal grain orientation will be produced to make its piezoelectric properties directional. The ceramic body will be cooled in the mold to produce greater internal stress, which will affect the piezoelectric properties, and the sintering temperature cannot be lowered too low.

(5) The use of the "dual effect" of low melting point additives can greatly reduce the sintering temperature while improving the piezoelectric properties of the material, with low cost and simple process. This is an ideal method for low-temperature sintering of piezoelectric ceramics.

 

Application of piezoelectric ceramics

 

Since the birth of the first ceramic piezoelectric material barium titanate in 1942, as an application product of piezoelectric ceramics, it has spread throughout all aspects of people's lives. The application of piezoelectric materials as the link of electromechanical coupling can be roughly divided into two aspects: the application of piezoelectric ceramic frequency control devices represented by piezoelectric resonators and the application of quasi-static applications that convert mechanical energy and electrical energy.

 

1. Piezoelectric vibration and piezoelectric ceramic frequency control device

 

The polarized piezoelectric ceramic, that is, the piezoelectric vibrator, has the natural vibration frequency determined by its size and the piezoelectric effect can obtain stable electric oscillation. When the frequency of the applied voltage is the same as the natural vibration frequency of the piezoelectric vibrator, resonance will be caused, 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 a current through the positive piezoelectric effect. Realize the maximum mutual conversion between electrical energy and mechanical energy. Using the characteristics 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 frequency range and high accuracy, especially used in multi-channel communication and amplitude modulation reception And various radio communication and measuring instruments can improve the anti-interference ability. So it has replaced a considerable part of electromagnetic oscillators and filters, and this trend is still developing.

 

2. Piezoelectric Transformer

Piezoelectric transformers are made by using the characteristics of the mutual conversion of electrical energy and mechanical energy of the piezoelectric effect. It is composed of two parts, an input end, and an output end, and the polarization directions are perpendicular to each other. The input end is polarized in the thickness direction, and the alternating voltage is applied for longitudinal vibration. Due to the inverse piezoelectric effect, there will be a high voltage output at the output. The piezoelectric ceramic transformer is a new type of solid-state electronic device. Compared with the traditional electromagnetic transformer, it has a simple structure, small size, lightweight, large transformation ratio, good stability, no electromagnetic interference and noise, high efficiency, high energy density, high safety, no winding, no The advantages of combustion, no magnetic leakage 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. In recent years, some piezoelectric transformers with better performance have appeared, such as the third-order vibration mode Rosen piezoelectric ceramic transformer with two input terminals and the high-power multilayer piezoelectric ceramic transformer. At present, piezoelectric ceramic transformers are mainly used for AC-DC, DC-DC, and other power devices and high voltage generating devices, such as cold cathode tubes, neon tubes, laser tubes, and small x-ray tubes, high voltage electrostatic spraying, high voltage Electrostatic flocking and driving of radar display tube, etc.

 

3. Piezoelectric transducer

The piezoelectric transducer uses the piezoelectric effect of piezoelectric ceramics and the reverse piezoelectric effect to realize the mutual conversion of electric energy and sound energy. The piezoelectric ultrasonic transducer is one of them. It is an underwater acoustic device that transmits and receives ultrasonic waves underwater. Under the action of sound waves, the piezoelectric transducer in the water induces electric charges at both ends of the transducer. This is the sound wave receiver. If an alternating electric field is applied to a piezoelectric ceramic sheet, the ceramic sheet will become thinner and thicker from time to time, and it will vibrate and emit sound waves. This is an ultrasonic transmitter. Piezoelectric transducers are also widely used in the industry for underwater navigation, ocean exploration, precision measurement, ultrasonic cleaning, solid detection, medical imaging, ultrasonic diagnosis, and ultrasonic disease treatment. Another application field of today's piezoelectric ultrasonic transducers is telemetry and remote control systems. Specific application examples include piezoelectric ceramic buzzers, piezoelectric igniters, ultrasonic microscopes, etc. 

4. Piezoelectric ultrasonic motor

The piezoelectric ultrasonic motor is a new type of micromotor that uses the inverse piezoelectric effect of piezoelectric ceramics to generate ultrasonic vibration, 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 usual electromagnetic coil. Compared with traditional electromagnetic motors, it has low cost, simple structure, small size, high power density, good low-speed performance (low-speed operation can be achieved without deceleration mechanism), large torque and braking torque, fast response, and control accuracy High, no magnetic field and electric field, no electromagnetic interference and electromagnetic noise. Piezoelectric ultrasonic motors are widely used in precision instruments, aerospace, automatic control, office automation, micro-mechanical systems, micro-assembly, precision positioning, and other fields due to their own characteristics and performance advantages. At present, Japan is in the leading position of technology in this field. Piezoelectric ultrasonic motors have been widely used for automatic focusing of cameras and video cameras, and large-scale series of products have been formed.

 

The development trend of piezoelectric ceramics

 

1. Lead-free piezoelectric ceramics

 

Lead-free piezoelectric ceramics are also called environmentally compatible piezoelectric ceramics. It requires that ceramic materials do not produce substances that may be harmful to the environment in the process of preparation, use, and disposal, so as to avoid harm to human health and reduce environmental pollution. 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. It is self-evident that these materials cause harm to the human body and the environment in the process of component manufacturing, processing, storage and transportation, use, and waste disposal. Therefore, lead-free environmentally friendly piezoelectric ceramic materials are an important direction of research and development in recent years. However, the piezoelectric ceramic materials currently used are mainly based on PZT, and its piezoelectric performance is much better than other piezoelectric ceramic materials. Moreover, the electrical properties of the material can be adjusted through doping modification and process control to meet various application requirements.

 

2. Piezoelectric composite materials

In order to play a role in the application of hydrophones, piezoelectric composite materials were gradually developed in the 1970s. The piezoelectric composite material is a kind of functional composite material with a piezoelectric effect composed of piezoelectric ceramic phase and polymer phase in a certain connection mode. Due to the addition of the flexible polymer phase, the density, acoustic impedance, and dielectric constant of the piezoelectric composite material is reduced, while the figure of merit and the electromechanical coupling coefficient of the composite material are improved, which overcomes the brittleness and piezoelectricity of simple piezoelectric ceramics. Disadvantages of the high cost of polymers. In addition to being used as hydrophones, piezoelectric composites are also used in the industrial, medical, and communications fields. After more than 40 years of continuous research on piezoelectric composites, its application research has made considerable progress, but its complete theory has not yet been established, and its application development has yet to be explored. At present, the research of piezoelectric composite materials mainly focuses on the development of connection types, improvement of molding processes, and preparation of multifunctional devices.

 

3. Nano piezoelectric ceramics

 

With the rapid development of nanotechnology in recent years, nanoceramics have gradually attracted people's attention. The nanopowder is formed and sintered to form a dense and uniform bulk nano ceramic. The toughness, strength, and superplasticity of the material have been greatly improved, which overcomes many shortcomings of engineering ceramics, and has an important impact on the mechanical, electrical, thermal, magnetic, and optical properties of the material. By selecting the material composition system and adding nano-scale particles, whiskers, wafer fibers, etc. to modify it, nano piezoelectric ceramic materials with high performance and low-temperature sintering can be obtained. By controlling the growth of nanocrystalline grains, quantum confinement effects and ferroelectrics with strange properties can be obtained to improve the electromechanical conversion and thermal release properties of piezoelectric pyrolysis materials. Various types of piezoelectric transformers, piezoelectric drivers, high-power ultrasonic welding technology, piezoelectric vibrating feeders, new ultrasonic CVD technology, and high-power ultrasonic engineering supporting nuclear power plants that have developed rapidly in recent years are all nano-ceramics in the piezoelectricity.

 

With the in-depth understanding of the material structure and the research and expansion of application technology, piezoelectric ceramic materials will be widely used in high-tech fields such as electronic technology, communication technology, laser technology, and biotechnology. With the rapid development of these fields and new economic and social development needs, there will be higher requirements for the performance of piezoelectric ceramics, such as high Curie temperature, high electromechanical coupling coefficient, and mechanical quality factor.