Study of proton irradiated 56/44 mol% P(VDF-TrFE) and P(VDF-TrFE) MEMS transducers

Abstract

Polyvinylidene fluoride-trifluoroethylene [P(VDF-TrFE)] is a well-known ferroelectric polymer. As it is easily processed, lightweight, mechanically flexible and can conform to any shapes and surfaces, it has been studied for nearly three decades for applications in electromechanical devices. In recent years, irradiation treatment in polymeric materials has attracted considerable interest as it can change both the structures and properties of the materials significantly and makes them useful for specific applications. In this project, proton irradiation with doses of 10-50 Mrad was carried out to investigate the potential for modifying the properties of P(VDF-TrFE). Copolymer films with VDF contents of 56 mol% were prepared by hot pressing. The effects of irradiation on both structures and properties of the copolymer were investigated by studying the change in lattice spacing, phase transitional behaviour, relative permittivity and electric field induced strain. It was found that irradiation can convert the copolymer from a ferroelectric to a relaxor material and the improved electrostrictive coefficients M33 made the proton irradiated P(VDF-TrFE) a potential candidate for sensor and actuator applications. The application of P(VDF-TrFE) films in the fabrication of microelectromechanical systems (MEMS) devices was investigated. 56/44 mol% P(VDF-TrFE) was spin-coated on a silicon wafer containing a layer of thermally oxidized silicon dioxide. The SiO2/Si was back-etched to produce a membrane structure. Some of the 56/44 mol% P(VDF-TrFE) MEMS were also subjected to proton irradiation with doses of 10-50 Mrad and their effective electrostrictive coefficient M33 was compared with that of free bulk P(VDF-TrFE) thick films. The clamping effect of substrate reduces the M33 but the bending effect of the Si membrane enhances the mechanical properties. Micromachined piezoelectric 70/30 mol% P(VDF-TrFE) MEMS for air transducer application was prepared. The mechanical behaviour of the MEMS was analysed by means of a laser vibrometer. The resonance frequency was found to depend on the structure and the thickness of the membrane. Both the transmitting and receiving acoustic responses of the copolymer MEMS were studied. The result suggested that the piezoelectric P(VDF-TrFE) has potential applications as actuators and in monitoring ultrasound emission.