1. Introduction
Sonar is an electronic system that detects and identifies underwater objects by means of acoustic waves. The underwater acoustic transducer is the key component of the sonar system. In recent years, the application of Unmanned Underwater Vehicle (UUV) has made rapid development of medium- and high-frequency underwater acoustic transducers. Generally, the high Q value of the high-frequency transducer results in its narrow working bandwidth and less information acquisition. However, due to the small beam angle of the planar transducer, the angle of the transmitting and receiving signals is limited. Thus, the hot topic of scholars at home and abroad is how to get wider bandwidths and larger beam angles of transducers.
In terms of expanding the bandwidth of the transducer, S. Cochran and others in Britain fabricated a transducer with a bandwidth of more than one octave by adding a matching layer to the 1-3 type piezoelectric composite material. Zhang Kai and others fabricated a dual matching layer high-frequency broadband transducer with a working range of 43–155 kHz. Although the above-mentioned transducer had expanded bandwidth, its beam angle was small, so it was difficult to realize the transmitting or receiving of large-angle underwater acoustic signals [
1,
2,
3]. In terms of enlarging the transducer’s beam angle, MSI company had made 6 rows and 4 columns of arc-shaped transducer array; its working range was 8–16 kHz, and the horizontal opening angle was 150° degrees. Zhang Kai and others fabricated a high-frequency broadband transducer whose radial vibration frequency was 47.5 kHz and the working range was 40–80 kHz. Such transducers expanded the beam angle, but the bandwidth was relatively small [
4,
5,
6].
Combined with most of the current research ideas, this paper combines the two most commonly used methods of expanding transducer bandwidth:
1. Composite materials
The concept of composite materials was proposed in the 1970s. It was defined as a material that combines a piezoelectric ceramic and a polymer in a certain communication mode, a certain volume or weight ratio, and a certain spatial geometric distribution. Adding a three-dimensionally connected polymer material to a one-dimensional piezoelectric material to fabricate a 1-3 type piezoelectric composite material can increase the loss of the transducing material and increase the bandwidth [
7,
8]. The mechanical quality factor of the transducing material can be expressed as:
where
is the mechanical quality factor of the transducing material,
is the circular frequency, M is the equivalent mass of the transducing material, and R is the sum of the loss resistances of the transducing material. From Equation (1), it can be seen that increasing the loss of the transducing material can reduce the mechanical quality factor of the transducing material. In addition, the mechanical quality factor can be expressed as:
where
is the resonant frequency of the transducing material and
is the frequency bandwidth of the 3 dB drop in conductance response. From Equation (2), it can be seen that the decrease of the mechanical quality factor of the transducing material expands the bandwidth of the transducing material. So, adding a flexible polymer to the piezoelectric material can extend the bandwidth of the transducing material.
2. Adding a matching layer
When no matching layer is added, the impedance of the water load is the impedance of the surface of the transducing material. After adding a matching layer with a specific acoustic characteristic impedance to the acoustic radiation surface of the transducing material, the impedance of the water load has the impedance generated by the matching layer in addition to the impedance of the surface of the transducing material. The difference in the two impedances produces two resonant frequencies. Adjusting the thickness of the matching layer brings the two resonant frequencies close enough to couple to expand the working bandwidth of the transducing material [
9,
10].
At the same time, using a curved-surface forming process to fabricate a circular piezoelectric composite can increase the beam angle of the transducer.
The PZT-5A (produced by Risheng Electronics Co., Ltd., Kunshan, China) was selected as a piezoelectric material to fabricate a transducer. At present, in addition to the traditional piezoelectric materials, the high-performance requirements of the transducers have led to the development of new piezoelectric materials, including piezoelectric composites, relaxed ferroelectric single crystals, etc. At this stage, the relaxation ferroelectric single crystal has been a hot spot due to its high piezoelectric coefficient (
) and high mechanical quality factor (
) [
11,
12]. The transducer based on the relaxed ferroelectric single crystal can increase the sensitivity by 12 dB, the bandwidth by 2–3 times, and the sound source level by 12 dB. In terms of the conventional PZT piezoelectric ceramic, it is relatively hard and has the ability to exert and withstand a large stress in physical properties. From a chemical point of view, it is inert and unaffected by moisture and other atmospheric conditions. The manufacturing method is also relatively simple, and as a transducing material, it also has excellent piezoelectric properties. Compared with the relaxed ferroelectric single crystal, the PZT-5A piezoelectric material has a higher Curie temperature, so its temperature stability is high, the aging rate is small, the time constant is large, the manufacturing cost is low, it can be molded in a large area, and commercialized application is more mature. Compared to the transducers designed here, the cost of preparing a transducer using the same size single crystal material can be 5–6 times higher. A relaxation ferroelectric single crystal PMN-PT29 material (produced by Materials Research Institute, Pennsylvania State University, State College, PA, USA) was selected and compared with the piezoelectric ceramic PZT-5A material. The material parameters of the two are shown in
Table 1:
For transmitting transducers, mechanical loss (larger ) is generally required to improve the efficiency of the emission. However, sometimes it needs to increase the bandwidth and requires a smaller material. In summary, PZT-5A was chosen as the transducer material to fabricate the transducer. Furthermore, the PZT-5A material was used to prepare the 1-3 type piezoelectric composite. Compared with pure PZT-5A piezoelectric ceramics, the hydrostatic pressure constant is 1–2 orders of magnitude higher. Due to the addition of the flexible polymer, the equivalent density of the transducing material is reduced, and the acoustic medium has a good acoustic matching. As a “soft” piezoelectric material, PZT piezoelectric ceramics can be prepared into a desired shape by adding a flexible polymer. This soft nature makes it more resistant to vibration and mechanical shock, which can increase the service life of the transducer in a complex seawater environment.
From the beginning to the present, piezoelectric composite materials have been a research hotspot. Many scholars did a large number of theoretical and experimental research on piezoelectric composites, and also tested the impact of ceramic volume fraction on the performance of piezoelectric composites. For transducer applications, the key material parameter is the electromechanical coupling factor and variation of charge constant, which are closely related to device bandwidth and sensitivity. For example, Smith W.A. and Auld B.A. et al. studied the impact of different piezoelectric ceramic volume fractions on electromechanical coupling factor. The results showed that the electromechanical coupling coefficient shows an upward trend within 20% of the volume fraction; it remains stable in the range of 20% to 80%; and it shows a downward trend in the range of 80% to 100% [
13]. Chan H.L.W. and Unsworth J. et al. studied the impact of different piezoelectric ceramic volume fractions on the piezoelectric charge constant
. The results showed that the piezoelectric charge constant
shows an upward trend within 40% of the ceramic volume fraction, and basically stabilizes after 40% [
14]. T.R. Gururaja et al. tested the impact of different volume fractions on mechanical quality factor. It was found that the composite material has a lower mechanical quality factor than the pure piezoelectric ceramic material, which is advantageous for expanding the bandwidth of the transducer [
15]. Based on the above studies, we found that the volume fraction of piezoelectric ceramics is in the range of 40–60%, and its comprehensive performance is optimal. Therefore, we chose to prepare a 1-3 type piezoelectric composite with a piezoelectric ceramic volume fraction of 50%.
Finally, the experimental results show that the piezoelectric composite transducer fabricated in this paper has achieved the target of high-frequency wideband and wide beam angle. The design theory and fabrication process will greatly promote the study of the extended bandwidth and the beam angle of the high-frequency transducer.
2. Structure of a High-Frequency Wideband Composite Cylindrical Transducer
The structure of the high-frequency broadband composite cylindrical transducer is shown in
Figure 1.
It consists of piezoelectric composite material, matching layer, hard foam backing, waterproof sound transmission layer, and electrode lead. The piezoelectric composite material is composed of piezoelectric ceramic and flexible polymer. One-dimensional connected piezoelectric ceramic is arranged in a three-dimensional connected flexible polymer to form a 1-3 type piezoelectric composite. Its advantage is that it has a purer thickness vibration mode and can realize the transducer working in the high-frequency range. At the same time, this composite structure provides the possibility for the curved-surface forming of the transducer. Therefore, we use the 1-3 type piezoelectric composite material as the sensitive component of the transducer to realize the performance of high-frequency broadband and wide beam angle. Adding a matching layer to the acoustic radiation surface of the sensitive component forms two kinds of vibration modes. Adjusting the thickness of the matching layer enables the coupling of two vibration modes in the water to expand the bandwidth of the transducer. The matching layer can also produce the effect of prestress, which makes the amplitude difference of each point on the vibration surface smaller. The most important part of the transducer is the 1-3 type piezoelectric composite material with matching layer. Its structure with the individual ceramic dimensions and the dimensions of the polymer part is shown in
Figure 2.
Piezoelectric ceramics are used as active components and their size determines the parameters of the transducer. Since the thickness vibration mode of the piezoelectric ceramic is used in this design, the influence of the thickness of the piezoelectric ceramic on the frequency of the transducer is mainly considered. The finite element simulation of piezoelectric ceramics with different thicknesses was carried out by ANSYS software, and the variation curve of thickness resonance frequency with ceramic thickness was obtained as shown in
Figure 3:
It can be seen from
Figure 3 that as the thickness increases, the thickness resonance frequency decreases. A piezoelectric ceramic of 5 mm thickness was selected based on design requirements near 300 kHz.
The height of the transducer is determined by the directivity requirements of the transducer in the vertical direction. For composite materials, the vertical direction directivity calculation formula is shown in Equation (3):
where
is the directivity angle in the vertical direction, h is the height in the vertical direction,
λ is the wavelength of the sound wave in the water, and
θ is the calculation range. The variation of the directivity angle of the vertical direction with the vertical direction can be obtained by Matlab software calculation, as shown in
Figure 4:
The vertical directivity angle required for the transducer of this design is about 5°, so the height selected by
Figure 4 is 50 mm.