- Why piezoelectric composite materials are important
- Types of piezoelectric composite materials
- Preparation Method of Piezoelectric Composite Materials
- Production equipment for piezoelectric composite materials
- Applications of Piezoelectric Composite Materials
1、Why piezoelectric composite materials are important
Piezoelectric materials are dielectric materials that can generate charges (positive piezoelectricity) or deformations (inverse piezoelectricity) under external forces or electric fields. Traditional piezoelectric materials such as PZT piezoelectric ceramics, PVDF piezoelectric films, etc., used to make emission-type underwater acoustic transducers, have the disadvantages of low coercive field, small anisotropy, and small bandwidth, which result in low performance of the transducers made of them and make them unsuitable for more complex working frequencies and more accurate imaging scenarios. Therefore, developing a high-power, vibration mode-pure, wide-band, low-loss, and better-matching piezoelectric material is the core of optimizing the design of ultrasonic transducers.
Piezoelectric materials with superior piezoelectric properties can be achieved by combining two or more materials. Piezoelectric single crystals have excellent piezoelectric properties, with very high piezoelectric constants and electromechanical coupling coefficients. The anisotropy of the single crystals enables large-area single crystals to have relatively pure vibration modes. However, the disadvantages of piezoelectric single crystals are their high density and acoustic impedance, poor acoustic matching performance with human tissue and water. Polymer materials have good flexibility, low density, light weight, impedance matching with air, human tissue, and water. The elastic compliance coefficient of polymer materials is two orders of magnitude larger than that of piezoelectric ceramics. However, the disadvantages of polymer materials are their very small piezoelectric and electromechanical coupling coefficients, which make them unsuitable for making emission transducers, and their high dielectric loss. Therefore, to obtain materials with better emission performance, we need to composite polymer materials and high-performance piezoelectric materials to meet the needs of making high-performance emission transducers.
Generally, the cutting type and external dimensions of the material have a significant impact on the electromechanical coupling coefficient of piezoelectric materials. Currently, in the field of transducers, the most commonly used piezoelectric materials are those with thickness vibration mode and those with height vibration mode. The electromechanical coupling coefficient under the thickness vibration mode is generally about 60%, while under the height vibration mode, it is generally about 80%. For example, for PZT ceramic materials, the electromechanical coupling coefficient kt under the thickness vibration mode is generally about 60%, while the electromechanical coupling coefficient k33' under the height vibration mode is generally about 80%. Therefore, in high-end device applications, to obtain higher device sensitivity and frequency domain bandwidth, piezoelectric composite materials are generally used as the core sound emitting material in transducers.
On the other hand, the operating frequency of piezoelectric transducers is related to the thickness of the sound-emitting piezoelectric material, represented by the intrinsic parameter "frequency constant" of a piezoelectric material, satisfying the formula N=ft, where N represents the frequency constant of the piezoelectric material, f represents the operating frequency of the piezoelectric material, and t represents the thickness of the piezoelectric material. Generally, for the same piezoelectric material, the frequency constant N33' of piezoelectric composites under height vibration mode is half of the frequency constant Nt of the piezoelectric material under thickness vibration mode. Therefore, for piezoelectric materials with the same operating frequency, when making transducers using piezoelectric composites, the thickness of the sound-emitting piezoelectric composite material is about 50% smaller than that of the sound-emitting material in the pure piezoelectric material transducer. Furthermore, as the frequency increases, the thickness of the piezoelectric composite material decreases, which further increases the difficulty in the device fabrication process.
In addition, for layered acoustic transducers, when the sound-emitting piezoelectric material converts the received electrical signal into mechanical vibration through the reverse piezoelectric effect, the vibration signal needs to be projected into the external medium through the acoustic transparent material on the front end face of the device to form a radiated sound field, in order to achieve subsequent application conditions. Whether the acoustic waves generated by mechanical vibration can enter the medium in front of the device in a large proportion depends on the design of the acoustic impedance parameters of the acoustic transparent material between the piezoelectric material and the medium. Generally, when the acoustic impedance of the interface between two media is closer, the projection rate will be much greater than the reflection rate when the sound wave passes through the medium interface. Since transducers are generally used to generate radiated sound fields in media with lower acoustic impedance, such as water (1.5MRayl) or coupling agents (1.2MRayl), the acoustic impedance (30MRayl) of the piezoelectric material is severely mismatched with the medium, requiring a certain acoustic transparent material as an acoustic matching transition layer, and the sound transmission rate of the acoustic transparent material is limited. Compared to pure piezoelectric materials, piezoelectric composites have lower acoustic impedance, which is more favorable for the parameter design of the acoustic matching transition layer.
2、Types of piezoelectric composite materials
There are ten types of piezoelectric composite materials based on different connectivity methods, including 0-0 type, 0-1 type, 0-2 type, 0-3 type, 1-1 type, 1-2 type, 1-3 type, 2-2 type, 2-3 type, and 3-3 type. The first number refers to the connectivity dimension of the piezoelectric material, and the second number refers to the connectivity dimension of the polymer.
Among them, 2-2 and 1-3 types are widely used piezoelectric composite materials. Our company's (he-shuai) 1-3 type piezoelectric composite material is formed by arranging one-dimensional connected piezoelectric ceramics parallel to three-dimensional connected polymer. By adding polymer, 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, which is easy to match with media such as water and skin, low dielectric constant, small static capacitance, and higher impedance is required when the transducer is working, which has the advantage of high sensitivity to received voltage. At the same time, the static pressure piezoelectric constant gh=dh/ε is high, which is suitable for manufacturing hydrophones and has the advantages of low equivalent noise sound pressure level and high sensitivity. Due to the high attenuation of the polymer, the Qm value is relatively low, which is suitable for making broadband narrow-pulse transducers.
3、Preparation Method of Piezoelectric Composite Materials
The traditional methods for preparing piezoelectric composite materials are mainly the cutting-filling method and the etching method. The etching method uses ion etching to create grooves for filling, but it has high equipment and production costs, limited processing depth, and side wall corrosion angle. Therefore, it is generally used for processing piezoelectric composite materials with a frequency of 50MHz or higher. The cutting-filling method involves using a precision cutting machine to cut grooves of a certain width and depth into the piezoelectric composite material, and then filling the grooves with polymer to solidify. This is a mature and simple processing method that is widely used in laboratory and commercial production.
The cutting-filling method is mainly used to prepare piezoelectric composite materials with a frequency of 0.5-10MHz. For piezoelectric composite materials above 15MHz, the thin cutting blade of the cutting tool is affected by the processing spacing, processing depth, and processing method, which can easily cause defects such as collapse and fracture of the piezoelectric phase, irregular cutting grooves, and high scrap rate. Even if these defects do not occur, material properties may not meet expectations due to mechanical stress damage and other reasons. Therefore, existing cutting-filling methods cannot meet the processing requirements for the performance and consistency of high-frequency array transducers, severely restricting the production of high-frequency array transducers.
The processing method for high-frequency piezoelectric composite materials involves cutting the first and second cutting grooves on the first and second surfaces of the piezoelectric material, respectively, and making the first and second cutting grooves pass through to achieve cutting of the entire piezoelectric material. This can reduce the cutting depth of the first and second cutting grooves, effectively reducing problems such as the snake-shaped path, material collapse, and material damage caused by small cutting spacing and excessive cutting depth during the processing of piezoelectric composite materials. It can meet the processing requirements for high-frequency piezoelectric composite materials, especially those with a frequency of 15-30MHz. It can also meet the processing requirements for array piezoelectric composite materials, improve the yield and consistency, and has the advantages of simple method and low cost.
4、Production equipment for piezoelectric composite materials（from he-shuai company）
5、Applications of Piezoelectric Composite Materials
Piezoelectric composite materials are widely used in medical ultrasound transducers and underwater applications. In different application fields, piezoelectric composite transducers have more obvious advantages compared to transducers made of ordinary ceramics. For example, the water acoustic transducer made of piezoelectric composite materials has high sensitivity and a large static pressure piezoelectric constant. In photoacoustic imaging, the transducer has significantly improved signal-to-noise ratio and bandwidth. In medical ultrasound, the shell-shaped focusing transducer made of piezoelectric composite materials reduces impedance and improves focusing intensity.
As an important equipment for underwater communication and navigation, ocean resources, and marine geology and landforms, underwater acoustics has become the main equipment for underwater information acquisition. The piezoelectric composite material is the core and key material of the underwater acoustic transducer, directly affecting and restricting the performance of underwater acoustic equipment.
