# Piezoelectric Composite Vibrator with a Bilaminated Structure for Bending Vibration
## 1 Introduction
However, the improvement of a vibrator's piezoelectric properties by changing the material structure, especially via fabricating a bilaminated vibrator through piezoelectric composite materials, have yet to be reported. Piezoelectric composite material is a new material with piezoelectric effect, which is composed of at least one piezoelectric phase material and non-piezoelectric phase material in a certain connecting way. Piezoelectric composites have been widely studied and applied since their advent because of their advantages. Piezoelectric composites are classified according to their connectivity. Each component phase can be self-connected in a 0-, 1-, 2-, or 3-dimensional manner. If the composite consists of two component phases, there are 10 kinds of connectivity, namely 0-0, 0-1, 0-2, 0-3, 1-1, 1-2, 1-3, 2-2, 2-3, 3-3 [13,14]. So far, the development and application of piezoelectric composites have a history of nearly 50 years. In the mid-1970s, the Material Laboratory of Pennsylvania State University began to study the application of piezoelectric composites in underwater acoustics, and developed 1-3 piezoelectric composites. [PERSON], [PERSON], [PERSON], [PERSON], and [PERSON] have done a lot of theoretical and experimental research, and tested the properties of piezoelectric composites with different volume percentages [15; 16; 17]. Since the early 1980s, [PERSON] and [PERSON] of Stanford University in California, USA, have established the theoretical model of 1-3 piezoelectric composites with periodic arrangement of PZT (Lead zirconate titanate) columns, and analyzed the transverse structural modes [18]. [PERSON] of Philips Laboratory in New York, USA, has done similar work [19]. At the same time, many countries have also carried out piezoelectric composite research, such as [PERSON] in Australia [20] and [PERSON] in Japan [21]. Some researchers have also fabricated transducers using piezoelectric composites, such as [PERSON] in Japan, [PERSON] and [PERSON] in Britain. [PERSON] et al. developed 3-3 piezoelectric composites for broadband transducers [22]. [PERSON] et al. participated in the production of 1-3 piezoelectric composites. [PERSON] et al. made hydrophones with 0-3 piezoelectric composites [23]. [PERSON] developed 1-3 and 2-2 piezoelectric composites and made them for nondestructive testing and water immersion detection, P-wave and S-wave transducers for damage and lithology measurement [24]. An underwater parametric array source transducer composed of PZT/thin-polymer composite were fabricated by [PERSON] et al. [25]. [PERSON] et al. used a nanocomposite polymeric layer to increase the efficiency of ultrasound radiation into the liquid by antisymmetric zero-order (A0) Lamb waves propagating in piezoelectric plates [26]. Among many piezoelectric composites, 1-3 and 2-2 piezoelectric composites have been extensively studied. Piezoelectric composites have many advantages that ordinary piezoelectric materials do not possess [27]: (1) The acoustic impedance of piezoelectric materials with low acoustic impedance is much larger than that of water, which greatly limits its application in water and ultrasound. The acoustic impedance of composites can be greatly reduced due to the presence of polymer with very low density, which is about a quarter of that of ordinary PZT materials. When piezoelectric composites are used to make transducers, it is easy to find the corresponding sound absorbing materials as backing, and make the transducers easy to match with water or human tissues. In addition, the acoustic impedance of piezoelectric composites can be controlled by changing the volume fraction of PZT phase to match different media. Therefore, piezoelectric composites have broad application prospects in water immersion testing, oil logging, and medical ultrasound. (2) The low quality factor. The piezoelectric composites with low quality factor is 2-3 orders of magnitude lower than that of ordinary PZT, which is due to the greater attenuation of polymers.