Last Updated on March 23, 2023 by You Ling
The importance of piezoelectric composite materials
Piezoelectric materials are also constantly evolving. However, each piezoelectric material has its own advantages and disadvantages, and it is difficult to simultaneously meet the different performance requirements of piezoelectric transducers. Therefore, piezoelectric composite materials have been developed.
- What are Piezoelectric Composite Materials
- History of piezoelectric composite materials
- Types of piezoelectric composite materials
- Characteristics of piezoelectric composite materials
- How to prepare piezoelectric composite materials
- Main Applications of Piezoelectric Composite Materials
- How to Choose Piezoelectric Composite Materials
- Case Pictures of Piezoelectric Composite Materials from He-Shuai Company
- The Future of Piezoelectric Composite Materials
1、What are Piezoelectric Composite Materials?
They are made by compounding piezoelectric ceramics and polymers (such as epoxy resin or active polymers) through a certain volume ratio, mass ratio, and different connection methods. The characteristic of these materials is that they have both excellent piezoelectric properties of inorganic piezoelectric materials and excellent processing properties of polymeric piezoelectric materials, and they do not need to undergo stretching or other treatments to obtain piezoelectricity. Moreover, this piezoelectricity is isotropic within the thin film, and thus displays the same piezoelectric properties in any direction.
2、History of piezoelectric composite materials
Piezoelectric ultrasonic transducers have been widely used in industrial non-destructive testing, medical diagnosis, and biomedical engineering. The core part of the piezoelectric transducer, the piezoelectric material, has continuously improved in terms of electromechanical coupling coefficient, electrical quality factors, anti-aging, and frequency band characteristics. However, different application fields have different requirements for performance. For example, industrial ultrasonic testing requires high sensitivity and signal-to-noise ratio, while hydrophones require strong hydrostatic pressure performance and impedance matching with water. In response to the different requirements of different fields, piezoelectric materials are also constantly developing. However, each piezoelectric material has its own advantages and disadvantages, and it is difficult to simultaneously meet the different performance requirements of piezoelectric transducers. It was not until 1972 that Nakamura Kitayama of Japan developed PVDF-BaTiO3 composite materials, which marked the beginning of the history of piezoelectric composite materials. The combination of the two materials amplified the advantageous properties of the materials. Later, R.E. Newnham of the Materials Experiment 1R.E. at the State University of Binghamton in the United States also proposed piezoelectric ceramic-polymer composite materials and put forward the concept of connectivity. The piezoelectric composite materials are made by compounding piezoelectric ceramics and polymers through a certain volume ratio, mass ratio, and different connectivity methods.
3、Types of piezoelectric composite materials
Based on different connectivity methods, there are ten types of piezoelectric composite materials: 0-0, 0-1, 0-2, 0-3, 1-1, 1-2, 1-3, 2-2, 2-3, and 3-3. The first digit represents the connectivity dimension of the piezoelectric material, and the second digit represents the connectivity dimension of the polymer.
The 1-3 type piezoelectric composite material is currently widely used. It is formed by parallel arrangement of one-dimensional connected piezoelectric ceramics and three-dimensional connected polymers. By adding polymers, the weak points of ceramics in terms of strength and brittleness are effectively reduced, the lateral coupling of ceramics is reduced, and the longitudinal electromechanical conversion efficiency of the composite material is increased. It has low acoustic impedance, making it easy to match the impedance with water and skin, and low dielectric constant and small static capacitance, requiring a higher input impedance during the operation of the transducer and therefore having high sensitivity to received voltage. It is suitable for manufacturing hydrophones with high static hydrostatic piezoelectric constant gh=dh/ε, low equivalent noise sound pressure level, and high sensitivity. However, due to the large attenuation of polymers, it is suitable for making broadband narrow pulse transducers with low Qm values.
4、Characteristics of piezoelectric composite materials:
(1) Weak transverse vibration and small crosstalk sound pressure;
(2) Low mechanical quality factor Q value;
(3) Large bandwidth (80% ~ 100%);
(4) Large electromechanical coupling coefficient;
(5) High sensitivity, signal-to-noise ratio better than ordinary PZT probes;
(6) Stable characteristics within a large temperature range;
(7) Probes with complex shapes can be processed with simple cutting and filling techniques;
(8) The speed of sound, acoustic impedance, relative dielectric constant, and electromechanical coefficient are easy to change (because these parameters are related to the volume fraction of the ceramic material);
(9) It is easy to match with materials with different acoustic impedances (from water to steel);
(10) Ultrasonic sensitivity can be adjusted by changing the ceramic volume fraction.
5、How to prepare piezoelectric composite materials?
Common preparation methods for 1-3 type piezoelectric composite materials include arrangement casting, cutting and filling, demolding, injection, continuous lamination, electric dielectric, extrusion, and laser ultrasonic cutting. Among them, two methods are mainly used. The first is arrangement casting, which has a mature process. Ceramic small columns are flexibly distributed on the template according to the requirements. Epoxy resin or other polymers are poured under vacuum, cured at high temperature, cut or ground to the required thickness, coated with electrodes, polarized, and made into 1-3 type piezoelectric composite materials. The arrangement of piezoelectric ceramic columns can also be irregular and flexible, but the material yield is low due to the ceramic’s brittleness. The second is cutting and filling, which is a relatively simple process. Polarized piezoelectric ceramics are cut into uniformly arranged columns, filled with epoxy resin, vacuumed, cured, and ground to remove the uncut part. The resulting ceramic columns are as fine as 75-100μm, and the thickness of the ceramic columns can be relatively flexible. The disadvantage is that the process cost is high, and there is waste of raw materials.
6、Main Applications of Piezoelectric Composite Materials
Non-destructive testing (NDT) ultrasound transducers
Ultrasonic air transducers
Sonar, phased array
Medical ultrasound transducers
Piezoelectric composite materials have been widely used in medical ultrasound transducers and underwater applications. In different application fields, piezoelectric composite transducers have advantages over those made of conventional ceramics, such as high sensitivity and large static hydrostatic pressure piezoelectric constant for waterborne transducers. In photoacoustic imaging, the transducer signal-to-noise ratio and bandwidth are significantly improved, and in medical ultrasound, the piezoelectric composite shell focusing transducer reduces impedance and improves focusing intensity, among other advantages.
7、How to Choose Piezoelectric Composite Materials
Although piezoelectric composite ultrasound transducers have outstanding advantages and a wide range of applications, their fabrication process is complicated. Differences in the volume fraction and structure of the two-phase materials in different application fields can lead to different performance characteristics. Therefore, the optimal values of material parameters in various fields need to be determined to achieve the best material performance. Additionally, when subjected to external forces or heat, the material is susceptible to deformation, which can seriously affect its acoustic radiation characteristics. Also, the presence of non-conductive polymers in the structure and difficulties in electrode lead-out can also constrain its application.
When designing composite materials, it is important to select a suitable piezoelectric composite material frequency and wavelength that match those of the piezoelectric ceramics and polymers.
8、Case Pictures of Piezoelectric Composite Materials from He-Shuai Company
9、The Future of Piezoelectric Composite Materials
The application of piezoelectric composite materials involves various disciplines such as materials science, acoustics, biomedical science, and mechanics. The research on phononic crystals has aroused great interest among researchers. Piezoelectric composite materials have gradually attracted more and more attention from researchers, showing good application prospects. Therefore, we should fully utilize the various advantages of piezoelectric composite materials, further optimize the performance parameters of piezoelectric transducers, and expand their application fields more widely to maximize their industrial value.