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dc.contributorDepartment of Applied Physicsen_US
dc.creatorLau, Sien-ting-
dc.identifier.urihttps://theses.lib.polyu.edu.hk/handle/200/2894-
dc.languageEnglishen_US
dc.publisherHong Kong Polytechnic University-
dc.rightsAll rights reserveden_US
dc.titleStudy of proton-irradiated poly(vinylidene fluoride-trifluoroethylene) copolymersen_US
dcterms.abstractIn 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 the materials useful for specific applications. In this project, high energy proton irradiation with a broad range of doses (20 - 250 Mrad) have been carried out to investigate the potential for modifying the properties of poly(vinylidene fluoride-trifluoroethylene) [P(VDF-TrFE)] copolymers. Three types of copolymers, with different VDF contents 56, 70 and 80 mol%, were prepared by compression moulding and then subjected to 3 MeV proton (H+) irradiation at room temperature. The effects of irradiation on both structures and properties of the copolymers have been investigated. These include the change in lattice spacing, molecular conformation, microstructure, phase transitional behaviour, relative permittivity and electric field induced strain. The irradiated copolymers show many features of relaxor ferroelectrics such as slim hysteresis loop in polarization measurement and dispersion in the frequency plot of the relative permittivity. It is believe that the high-energy protons can break up the coherent polarization domains in the normal ferroelectric copolymer into nano-sized regions. For copolymers with higher VDF content, a higher proton dose is required to convert them to relaxor ferroelectrics. Besides, the XRD data show that the proton irradiation induces a solid-state phase transformation from the ferroelectric (polar) phase to the paraelectric (nonpolar) phase in which the lattice constant increases significantly. The electric field induced phase transformation between these two phase leads to an ultrahigh electrostrictive strain response observed in the irradiated copolymers. With this outstanding electromechanical property, the irradiated copolymers can be a promising material for transducer and actuator applications. Thick irradiated 70/30 mol% copolymer films with thickness of- 35 um have been used as the active element for fabricating high frequency ultrasonic transducers. The transducers were characterized and their transmitting voltage response, bandwidth and centre frequency were measured. Thin irradiated 70/30 mol% copolymer films with a few um-thick were also prepared in this work for actuator applications in microelectromechanical systems (MEMs). A suspended silicon nitride/silicon dioxide (Si3N4/SiO2) membrane was prepared by etching the Si substrate using bulk micromachining technique. A copolymer thin film was then deposited on the membrane by the spin-coating method. After proton irradiation treatment, the performance of the copolymer based MEMs actuator was characterized.en_US
dcterms.extentxix, 216 leaves : ill. ; 30 cmen_US
dcterms.isPartOfPolyU Electronic Thesesen_US
dcterms.issued2004en_US
dcterms.educationalLevelAll Doctorateen_US
dcterms.educationalLevelPh.D.en_US
dcterms.LCSHHong Kong Polytechnic University -- Dissertationsen_US
dcterms.LCSHCopolymers -- Mechanical propertiesen_US
dcterms.LCSHMicroelectromechanical systemsen_US
dcterms.LCSHProton magnetic resonance spectroscopyen_US
dcterms.LCSHElectron microscopyen_US
dcterms.accessRightsopen accessen_US

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