Piezoelectric materials are functional materials that realize the conversion between mechanical energy and electrical energy(1)

Piezoelectric materials are functional materials that realize the conversion between mechanical energy and electrical energy. Its development has a long history. Since the discovery of the piezoelectric effect on quartz crystals by the CURIE brothers in the 1880s, piezoelectric materials have begun to attract widespread attention. With the deepening of research, a large number of piezoelectric materials have continuously emerged, such as piezoelectric functional ceramic materials, piezo film, piezoelectric composite materials, etc. These materials piezo ceramic disc have a very wide range of uses, and play an important role in functional conversion devices such as electricity, magnetism, sound, light, heat, humidity, gas, and force.

PVDF piezoelectric film

PVDF piezoelectric film is polyvinylidene fluoride piezoelectric film. In 1969, the Japanese discovered the polymer material polyvinylidene fluoride (polyvinylidene fluoride polymer) referred to as PVDF, which has a very strong piezoelectric effect. The PVDF film mainly has two crystal types, namely, α type and β type. The α type crystal does not have piezoelectricity, but after the PVDF film is rolled and stretched, the original α type crystal in the film becomes a β type crystal structure. When the stretched and polarized PVDF film is subjected to external force or deformation in a certain direction, the polarized surface of the material will generate a certain electric charge, namely the piezoelectric effect piezo ceramic disc crystal.

Compared with piezoelectric ceramics and piezoelectric crystals, piezoelectric films have the following advantages:

(1) Light weight, its density is only a quarter of the commonly used piezoelectric ceramic PZT, pasted on the measuring object has almost no effect on the original structure, high elastic flexibility, can be processed into a specific shape can be any measuring surface is completely fitted, with high mechanical strength and impact resistance;

(2) High voltage output, under the same stress conditions, the output voltage is 10 times higher than piezoelectric ceramics;

(3) High dielectric strength can withstand the effect of strong electric field (75V/um), at this time most piezoelectric ceramics have been depolarized;

(4) The acoustic impedance is low, only one tenth of the piezoelectric ceramic PZT, close to water, human tissue and viscous body;

(5) The frequency response is wide, and the electromechanical effect can be converted from 10-3Hz to 109, and the vibration mode is simple.

Therefore, stress and strain can be measured in mechanics, accelerometers and vibration modal sensors can be made in vibration, acoustic radiation modal sensors and ultrasonic transducers can be made acoustically and used in active control, and can be used in robot research Used as a tactile sensor, also has applications in medical and vehicle weight measurement,

At present, the research on thin film materials is developing in various directions, high performance, new processes, etc., and its basic research is also deep in the molecular level, atomic level, nano level, mesoscopic structure, etc., so the research of functional thin film materials is of great significance .

Piezo film properties

1. Dielectric constant

Although the piezoelectric film is a single crystal film or a polycrystalline film with preferred orientation, the atomic packing in it is not as tight and ordered as in a crystal, so the dielectric constant value of the piezoelectric film is different from the value of the crystal . In addition to this, there are also large residual internal stresses often found in thin films and reasons for measurement, which also cause the dielectric constant value of the thin film to be different from the corresponding value of the crystal.

Existing studies have shown that the dielectric constant of the piezoelectric film is not only related to the crystal orientation, but also depends on the test conditions. The dielectric constant of the piezoelectric film has considerable dispersion. In addition to the difference in internal stress and test conditions, the difference between the chemical composition ratio and the film thickness of the film composition is generally believed to decrease with the thickness of the film. In addition, the dielectric constant of the piezoelectric film will also change significantly with the change of temperature and frequency.

2. Volume resistivity

From the perspective of reducing the dielectric loss and relaxation frequency of the piezoelectric film, it is expected that it has a high resistivity, at least ρv≥108Ω•cm. The resistivity of AlN film is 2×1014~1×1015Ω·cm, which is much higher than 108Ω·cm, so in this respect, AlN is a very excellent film. In addition, the change in electrical conductivity of AlN piezoelectric films with temperature also follows the 1nσ∝1/T law. None of the crystals with piezoelectric effect have a center of symmetry, so their electron mobility is also anisotropic and their electrical conductivity is also different. The electrical conductivity of the AlN piezoelectric film along the C-axis direction is different from the direction perpendicular to the C-axis. The former is about 1 to 2 orders of magnitude smaller.

3. Loss angle tangent

The dielectric loss tangent of the AlN piezoelectric film is tanδ=0.003～0.005, and the tanδ of the ZnO film is larger, which is 0.005～0.01. The reason why the tanδ of these films is so large is that in addition to the conductance process, these films also have significant relaxation phenomena. Similar to the dielectric thin film, the tan δ of the piezoelectric thick film increases gradually with the increase of temperature and frequency and the increase of humidity. In addition, as the film thickness decreases, tan δ tends to increase. Obviously, the increase in tanδ with temperature is due to the increase in conductance and the increase in relaxors. It increases with frequency because the number of relaxation times in time increases.

4. Breakdown strength

Because the dielectric breakdown field strength is a strength parameter, and various defects of piezoelectric hemisphere transducer are unavoidable in the film, the breakdown field strength of the piezoelectric film is quite dispersive; the breakdown theory of dielectrics, for a complete and intact film , The breakdown field strength should gradually increase as the film thickness decreases. But in fact, because the film contains many defects, the effect of the defect is more significant as the thickness is smaller, so when the thickness is reduced to a certain value, the breakdown field strength of the film becomes sharply smaller. In addition to the film's own cause, the breakdown field strength of the film is also affected by the edge of the electrode during the test. Since the thicker the film, the more uneven the electric field at the edge of the electrode, so as the film thickness increases, its breakdown field strength gradually decreases.

In addition to the above factors, the breakdown field strength of the dielectric film also depends on the film structure. For the piezoelectric film, its breakdown field strength also depends on the direction of the electric field, that is, it is also anisotropic in the breakdown field strength. Due to the existence of grain boundaries in the polycrystalline film, its breakdown field strength is lower than that of the amorphous film; for similar reasons, the breakdown field strength of the preferentially oriented piezoelectric film in the orientation direction is higher than that in the perpendicular direction .The breakdown field strength is lower.

Like other dielectric films, the breakdown field strength of the piezoelectric film also depends on some external factors, such as voltage waveform, frequency, temperature and electrodes. Because the breakdown field strength of the piezoelectric film is related to many factors, for the same film, the breakdown field strength values reported in the relevant literature are often inconsistent, and even vary greatly. For example, the breakdown field strength of the ZnO film is 0.01 ~0.4MV/cm, AlN film is 0.5~6.0MV/cm.

5. Bulk acoustic wave performance

The most important characteristic parameters of bulk acoustic wave piezoelectric transducers are resonance frequency f0, acoustic impedance Za and electromechanical coupling coefficient K, so the sound velocity υ and temperature coefficient of piezoelectric film, acoustic impedance and electromechanical coupling coefficient are particularly strict. These properties of the film not only depend on the elasticity, dielectric, piezoelectric and thermal properties of the crystal grains in the film, but also are closely related to the structure of the piezoelectric film such as the degree of compactness of the grains and the degree of preferred orientation. In the piezoelectric film, due to the defects and strain of the crystal grain, it is not a good single piezo crystal, so the physical constant of the film is slightly different from the crystal value.

Because the structure of the piezoelectric film is closely related to the preparation process, even for the same piezoelectric film, the performance values reported in various literatures are often inconsistent. Among all inorganic non-ferrous piezoelectric films, the AlN film has a large elastic constant, but a low density and the highest sound velocity. Therefore, the film is most suitable for UHF and microwave devices.

6. Surface acoustic wave performance

When the surface acoustic wave propagates in the piezoelectric medium, its amplitude of particle displacement attenuates rapidly as the distance from the surface of the medium increases, so the surface acoustic wave energy is mainly concentrated in the next two wavelengths on the surface.

The surface acoustic wave performance of thin film materials can be expressed as the following functional formula:surface acoustic wave performance = F (raw material, substrate, film structure, wave mode, propagation direction, interdigitated electrode form, thickness wave number product)

Therefore, any surface acoustic wave performance parameter of the piezoelectric film cannot be represented by a single value. Another acoustic wave property of piezoelectric films is transmission loss. Because piezoelectric films are often used as acoustic transmission media in surface wave devices, the source of transmission loss is mainly the scattering of acoustic waves in the piezoelectric film and the substrate.