Views:1 Author:Site Editor Publish Time: 2019-09-12 Origin:Site
Smart materials include sensing materials and driving materials. Perceptual materials are a class of materials that have a sensing function for external or internal stress, strain, heat, light, electricity, magnetism, radiant energy, and chemical quantities. They can be used to make various sensor devices; Materials that respond to environmental conditions or internal changes and perform actions that can be used to make a variety of drive devices. The smart device is an piezo actuator with a sensing drive function made of smart materials. The intelligent structure is composed of materials and devices. It integrates sensing, signal processing, control and driving into a material system or structural system. It can sense the environment or internal parameters, process information, issue commands, execute and complete actions. to achieve the self-diagnosis, self-healing and adaptive functions. The application of intelligent material systems and structures is very extensive, not only in defense-defense weapons such as airplanes, warships, etc., but also in strategically important areas of the national economy, especially in high-tech fields. The main materials currently completing smart material systems and structures are shape memory materials, piezoelectric materials (including piezoelectric ceramics, piezoelectric polymers), electrostrictive materials, optical fibers and electrorheological variants, magnetorheological variants, and the like. The use of these smart materials, combined with clever and sophisticated composite design and fabrication,which results in a material system and structure that is driven, sensed and controlled.
Piezoelectric materials are a major class of materials in smart material systems and structures. A dielectric crystal of piezoelectric ceramics transducer with a piezoelectric effect will be polarized and form a surface charge under the action of mechanical stress. If such a dielectric crystal is placed in an electric field, the action of the electric field will cause a relative displacement of the positive and negative charge centers inside the dielectric to cause deformation. . Since the piezoelectric material has the above characteristics, the uniformity of the sensing piezo element and the action element can be achieved. Piezoelectric materials can be widely used in smart materials and structures, especially for material damage self-diagnosis, self-adaptation, vibration reduction and noise control. The types of piezoelectric materials developed including single crystal, polycrystalline, microcrystalline glass, organic polymers, and composite materials. Since the 1980s, with the end of the climax of piezoelectric ceramic materials from the development of binary systems to ternary and multi-component systems, the research on piezoelectric materials has been slow. With the rapid development of science and technology, the development and exploration under the demand of application has given new impetus to the research of piezoelectric materials. Together with the unremitting efforts of science and technology workers in basic research and production process improvement, the new type of pressure has been in the past decade. The continuous emergence of electrical materials has made the research of piezoelectric materials.
2 Overview of piezoelectric materials
In the piezoelectric ceramics crystal, the asymmetry of the arrangement of the positive and negative ions and the non-coincidence of the center of gravity of the positive and negative charges of the unit form an electric dipole moment. These electric dipole moments are aligned in a certain direction to become a domain structure, and the domains are disordered on the crystal. the polarization effects cancel each other , the polarization in the material is zero, and the polarization direction of the domain polarized by the DC electric field tends to be in the same direction. When an external force acts on the piezoelectric material to cause deformation, the material is positively and negatively bound. The pitch of the charge becomes smaller, and the polarization intensity also becomes smaller. The free charge originally adsorbed on the electrode is partially released, and the discharge phenomenon occurs, which is called the positive piezoelectric effect; a certain intensity electric field is applied to the two poles of the piezoelectric material, and the on-chip is the positive and negative charge spacing becomes larger, the polarization intensity also becomes larger, and some free charges are adsorbed on the electrodes to cause charging phenomenon. The electric charge moves in the circuit to output mechanical energy externally, which is called on the inverse piezoelectric effect.
2.3 Preparation method of piezoelectric material
For different piezoelectric materials, a suitable preparation method is selected according to its application, characteristics . The preparation method is divided into a solid phase method, a liquid phase method and a gas phase method according to the phase of the phase which occurs during preparation.
2.3.1 Solid phase method
When PZT piezo is prepared by the traditional solid phase method, the sintering temperature higher than 1200 °C will cause the volatilization of PbO. It is difficult to control the stoichiometric ratio, which makes the microstructure and electrical properties of the material difficult to control. It is suitable for raw materials, simple process and piezoelectric materials. Where performance requirements are not high.
2.3.2 Liquid phase method
The preparation of piezoelectric materials by liquid phase method is currently the most commonly used method, including coprecipitation method, hydrothermal synthesis method, sol-gel method, alkoxide hydrolysis method and the like. The coprecipitation method enables low-temperature sintering to obtain a piezoelectric material having a higher density than the theoretical density. The coprecipitation method used a 700 degree temperature programmed roasting method to prepare BaT iO3 powder with a particle size of 60n m. The researchersin the United States used a coprecipitation method combined with a freeze-drying process to synthesize nano-sized PZ T powder at 800 degrees. Sintering gave a material with a theoretical density of 98%. In the study, N b2 O 5 and T a 2 O5 were used as precursor reactants, and K T aN b O3 ceramic powders were prepared by hydrothermal method and solvent thermal method. The sintered piezoelectric ceramics were studied. The coupling coefficient reaches 0.5, and the piezoelectric coefficient d 33 is between 150 ~ 450p C / N. However, the hydrothermal method requires higher temperature and pressure, and the equipment investment is large, which limits the application of the method. The sol-gel method is the most commonly used method in the liquid phase method. High performance films can be prepared by sol-gel combined with various molding and sintering processes.
2.3. 3 gas phase method
The gas phase method is suitable for the preparation of nano-scale piezoelectric films, mainly physical vapor deposition and chemical vapor deposition. Among them, the sputtering method is the most commonly used method. A P t / T i bottom electrode was deposited on the Si 2 / S i substrate by a target sputtering method, and a PZT film having a thickness of about 800mm was prepared by radio frequency (RF ) sputtering. Chemical vapor deposition can precisely control the chemical composition of the reaction product, and it is convenient to dope, but it is difficult to obtain a suitable gas source material, which is not suitable for low-cost, large-volume preparation of a film, and is practically used less.