Piezoelectric ceramics materials are functional materials that realize the conversion between mechanical energy and electrical energy (2)

Hydrothermal preparation of thick films has many advantages:

① This process is completed in the liquid phase at one time, and no later crystallization heat treatment is required, thus avoiding defects such as cracking, grain coarsening, reaction with the substrate or atmosphere that may be cause during the heat treatment process;
②Inorganic substances are used as precursors, and water is used as the reaction medium, the raw materials are easily available, the cost of film preparation is reduced, and the environmental pollution is less;
③ The equipment is simple and the hydrothermal treatment temperature is low, which avoids the interdiffusion of the film and substrate components before and after hydrothermal treatment. The resulting film has high purity and good uniformity . In addition, when this method is used to prepare thick films, thick films can be deposited on substrate surfaces of various complex shapes. The resulting thick films have certain advantages of spontaneous polarization, low hysteresis, and good bonding with substrates. At present, this method has attracted more and more attention.

6. Electrophoretic deposition method
Electrophoretic deposition (EPD) refers to dispersing the prepared fine powder with the same composition as the thick film in the suspension to form a suspension with different concentrations of piezoelectric plate crystal. The pH value of the suspension is adjusted with an acid-base solution. Stable suspension is obtained through ultrasonic dispersion and magnetic stirring, and under the constant pressure, the charging particles are directionally moved under the action of the electric field, thereby obtaining a thick film with a certain thickness. The thick film prepared by this method has the advantages of simple equipment, fast film formation, unlimited shape of the plated parts, uniform and controllable film thickness, etc. The resulting thick film can reach tens of microns, and the composition is uniform and dense.

Application of piezoelectric thin film materials
Piezoelectric films are widely used due to their excellent performance. Piezoelectric thick films can be used to manufacture a variety of micro devices, such as micropumps, ultrasonic motors, resonators, pyroelectric thick film sensors, thick film actuators, micro-energy pickups, etc.
 
1. Hydrophone and sonar
General electric company's single-diaphragm hydrophone with a 2.5 μm thick PVDF piezoelectric film as a substrate, they can be used in medical and NDE transducers, and can be used in the range of 0.5 ~ 50 HZ Characterization and calibration,Due to the long-term stability and repeatability of these devices, these characteristics of hydrophones have been utilized to develop a new multi-element instrument. underwater scanning sonar system is composed of 100 PVDF substrate hydrophones for underwater safety / rescue devices. The system uses passive mode with an operating frequency of 1 to 1 000 HZ. It can also be operated in active mode at three different frequencies. It can be detected with this system. Small submarines up to 3 km away can also detect engines up to 600 m, and the angle deviation is less than 5mm. The recent hydrophone calculation model shows that if the PVDF element is properly designed in the system demonstration, the hydrophone can detect signals over 10 dB.

2. Antifouling
Piezoelectric ultrasonic transducer has a very broad prospect in the antifouling applications. At present, the National Institute of Research and the University of Montrich in France have conducted research on the anti-fouling of piezoelectric films. Piezoelectric film has been proved to be more effective for measuring the vibration of the polymer in the shell. Therefore, it can be used to prevent the approach of most marine organisms that will cause pollution of ships. In addition, the Delft Scientific Research Institute in the Netherlands is extending the conclusions it has obtained to larger metal structures, such as the shells of merchant ships or ships. At the same time, the same principle is being studied on how to produce antifreeze surfaces on airplanes.

3. Medicine
At present, people are actively studying the application of PVD F in medicine. In many countries, many babies die of SIDS or other syndromes every year. In order to reduce the sudden death rate of infants, at least three companies in the Netherlands, Germany, and the United States are producing a respiratory monitor. This monitor puts a mat with a PVDF piezoelectric film under the baby's body to continuously monitor the slight vibration caused by breathing and heartbeat (especially at night). At a length of time, it will trigger an alarm, which can prevent the suffocation death of the baby in time and effectively.

4. Electrodes
If the outer surface of the electrode is covered with a layer of PVDF piezoelectric polymer, it can convert information such as vibration, impact, pressure, stress and strain into electrical signals. For this type of electrode, for the quasi-static input from low frequency to high frequency, its output can always retain its frequency characteristics, and the output of the electrical signal is linearly related to the applied pressure.At the same time, this electrode also has good temperature stability, suitable for various environments of-40 ~ 70 ℃. The range is generally from 0.25 to 25 p C / N. By winding the narrow piezoelectric sheet into a circle, a sensitive piezoelectric electrode with a value of 40 p C / N can be obtained.

5. Aviation and Navigation
The "Portable Automatic Remote Inspection System" developed by Sigma Research, abbreviated as PAR IS, is one of the first advanced devices to enter the commodity era. This system is specifically designed for in-situ inspection of large-area layered or composite structures.

The key element of piezo electric sensor is a 200 × 200 mm deformable PVDF piezoelectric film, which contains 1024 transducers. The flexibility of this film is very good, and it can completely fit the curved surface with a radius of curvature of 4. The device connected with it also has a hand-held controller, data sampler and display device. This receiver has a total signal-to-noise ratio of 100 dB and a center frequency of 2.5 MHz. Graphite-epoxy composites on airplanes or large structures on ships can be used for convenient testing. For aluminum and steel, satisfactory results have already been obtained in this regard. For this particular application of PVDF, it has been found that a film with biaxial polarity is more suitable than a uniaxial polarity.

6. Structural monitoring
The PVD F piezoelectric film also has certain applications in structural monitoring. A systematic study on the basic characteristics of the PVDF piezoelectric film monitoring structure was carried out, and a dynamic and static response test of the PVDF piezoelectric film monitoring structure was done. At the same time, preliminary research on the impact load of the structure using PVDF piezoelectric film was also carried out. The results show that the use of PVD F piezoelectric film can monitor the impact load on the structure in real time and can reflect the impact damage on the structure in time . As the application of piezoelectric thin film materials is more and more extensive, it will be well developed in the future. The manufacturing technology of piezoelectric thin film will also develop towards high efficiency, low cost and high quality. Because the CVD method has better coverage, it can be deposited on deep holes, steps, depressions or other complex three-dimensional shapes. In addition, the chemical vapor deposition method can also control the stoichiometric ratio of the prepared film in a wide range, which is very prominent compared with other methods.
The cost and operating cost of chemical vapor deposition equipment are relatively low, which is suitable for mass production and continuous production, and has good compatibility with other processing procedures. Therefore, in the future, the use of CVD method to prepare piezoelectric films will be well applied, and piezoelectric films will be better utilized.