1312 ieee transactions on ultrasonics, ferroelectrics, and frequency control, vol. 49, no. 9, september 2002 Design, Fabrication, and Performance of a Flextensional Transducer Based on Electrostrictive Polyvinylidene Fluoride-Trifluoroethylene Copolymer ZhongYang Cheng, Tian-Bing Xu, Member, IEEE, Qiming Zhang, Senior Member, IEEE, Richard Meyer, Jr., David Van Tol, and Jack Hughes Abstract—Taking advantage of the high electrostrictive strain and high elastic energy density of a newly developed electrostrictive polymer, modified poly(vinylidene fluoride- trifluoroethylene) [P(VDF-TrFE)] based polymers, a flex- tensional transducer was designed, and its performance was investigated. The flextensional transducer consists of a mul- tilayer stack made of electrostrictive P(VDF-TrFE) poly- mer films and two flextensional shells fixed at the ends to the multilayer stack. Because of the large transverse strain level achievable in the electrostrictive polymer and the displacement amplification of the flextensional shells, a device of a few millimeters thick and lateral dimension about 30 mm 25 mm can generate an axial displacement output of more than 1 mm. The unique flextensional con- figuration and the high elastic energy density of the active polymer also enable the device to offer high-load capability. As an underwater transducer, the device can be operated at frequencies below 1 kHz and still exhibit relatively high transmitting voltage response (TVR), very high source level (SL), and low mechanical quality factor ( ). I. Introduction S olid state electromechanical actuators and transduc- ers have been widely used in many civilian and mili- tary applications, including active vibration control, un- derwater navigation and surveillance, microphones, etc. [1]–[3]. In many of those applications, transducers and ac- tuators with high power and high displacement output are required. In the past several decades, a great deal of effort has been devoted to the development of electromechanical materials with those desired features [4]–[10]. In parallel, different device configurations also have been exploited to achieve an amplification of the relatively small strain lev- els in most commonly used piezoelectric, electrostrictive, and ferroelectric materials [11]–[15]. Recently, it was re- ported that a massive electrostrictive response could be obtained by modifying polyvinylidene (PVDF)-based fer- roelectric copolymers with high-energy electron irradiation or by adding another bulkier termonomer to form a ter- polymer [16]–[19]. In this paper, we investigate a flextensional transducer Manuscript received January 10, 2002; accepted April 5, 2002. This work was supported by DARPA and ONR. The authors are with The Pennsylvania State University, Univer- sity Park, PA 16802 (e-mail: ZXC7@psu.edu). that takes advantage of the large strain and high elastic en- ergy of this new class of electrostrictive polymer, especially the actuation along stretching direction for the stretched polymeric film that is of high electromechanical coupling factor [20]. It will be shown that, for a flextensional trans- ducer of a few millimeter thick and 30 mm × 25 mm lat- eral dimension, a displacement output of more than 1 mm can be generated with a relatively high-loading capabil- ity (>10 N). One of the unique factors of this device is that, because of low acoustic velocity of the active poly- mer (∼1400 m/s) and the flextensional configuration, such a small-sized device can be operated at frequencies below 2 kHz. This small device was capable of generating a high- source level and a transmitting-voltage response (TVR) of 123 dB re 1 μPa/V@1m. This paper is organized as follows. The recent results on the electromechanical properties of the electrostric- tive poly(vinylidene fluoride-trifluoroethylene) [P(VDF- TrFE)]-based polymers will be summarized briefly to pro- vide background of this newly developed material. The results on the fabrication and characterizations of these multilayer stacks used as the active element in flextensional transducer will be presented. The actuator performance of flextentional transducer will be analyzed, and the exper- imental results are presented. The results of underwater performance of the flextentional transducer also will be discussed. In this investigation, the field-induced transverse strain of the multilayer stacks was characterized by a set-up de- scribed in [21]. The displacement generated by the flexten- sional actuator was evaluated by a differential variable re- luctance transducer (DVRT, MicroStrain, Inc, Burlington, VT). The electric impedance as a function of frequency of multilayer stacks and flextensional transducers under dif- ferent direct current (DC) biased field was measured by an HP 4192 (Hewlett-Packard, Santa Clara, CA) impedance analyzer equipped with a blocking circuit. The blocking circuit is used to protect the impedance analyzer from the high DC voltage applied to the multilayer stack. For the underwater performance of the flextensional transducer, the transducer was placed at a depth of 2.78 m in an ane- choic water tank measuring 5.3 m wide, 7.9 m long, and 5.5 m deep. The transmitting voltage response (TVR), the free-field voltage sensitivity (FFVR), the directivity pat- 0885–3010/$10.00 c  2002 IEEE