Electrostrictive polymers and composites

Pao Yue-kong Library Electronic Theses Database

Electrostrictive polymers and composites


Author: Tai, Liang Shiang
Title: Electrostrictive polymers and composites
Degree: M.Phil.
Year: 2001
Subject: Polymers
Composite materials
Hong Kong Polytechnic University -- Dissertations
Department: Dept. of Applied Physics
Pages: ix, 123 leaves : ill. ; 30 cm.
Language: English
InnoPac Record: http://library.polyu.edu.hk/record=b1566410
URI: http://theses.lib.polyu.edu.hk/handle/200/2607
Abstract: The thermoplastic polyurethane (PU) and its composites with ferroelectric lead zirconate titanate (PZT) were prepared by a solution-cast process. Samples of pure PU, and PU mixed with 1, 2, 4, 8, 10 and 16 volume % of PZT were subsequently hot pressed to form thin films and used for the investigation of electrostriction and related properties. A modified Michelson interferometer has been used to measure the displacement of samples along the direction of the applied electric field. The interferometric system is capable of resolving displacement to 10 nm which is sensitive enough to detect the electric field induced strain of the samples. Pure PU has shown a decreasing electrostriction coefficient and relative permittivity after annealing and mechanical drawing. Wide angle X-ray diffraction and orientation determination revealed that the segmented structure of PU under thermal and mechanical process was altered. Pressure dependence of electrostriction gives a converse measurement of the electrostriction coefficient. According to the Landau-Lifshitz equation, the experimental result is comparable to that from direct measurement. It is interesting that the phenomenon of charge injection in PU was observed in which space charges were found accumulated just a few microns under the sample surface. From the study of morphology by SEM, it was found that aggregation of PZT is common in the high volume fraction PZT/PU samples. The relative permittivity of the composites increased drastically at around 8% of PZT; a dramatic rise in the dielectric loss was also detected. This could be related to the phenomenon of percolation of the composite which occurs above a critical volume fraction. A polarization reversal was observed for the 8% to 16% composites such that a strain reversal occurred at a critical electric field. The electrostriction coefficients of the composites decreased linearly from pure PU to the 8% PZT/PU composite. The Maxwell stress contribution and electromechanical coupling coefficient of the composites were also determined and discussed in this thesis.

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