Dielectric Properties And Morphology Of Ferroelectric Ceramic Polymer Composite Films

Author : Nusrat Jahan
Publisher :
ISBN 13 :
Total Pages : 156 pages
Book Rating : 4.:/5 (114 download)

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Download or read book Development of the Piezoelectric Properties of Poly(vinylidene Fluoride) Based Ferroelectrics and Ferroelectrets Using Fillers and Mechanical Stretching written by Nusrat Jahan and published by . This book was released on 2018 with total page 156 pages. Available in PDF, EPUB and Kindle. Book excerpt: Piezoelectric polymers are very useful in various electromechanical applications because of their strong piezoelectric response, flexibility, lightweight and formability. In addition to ferroelectric ceramics, ferroelectric polymers (such as PVDF) and ferroelectrets are also popular as a new group of piezoelectrics. Ferroelectrics based on b-PVDF are interesting due to the ease of preparation with usable piezoelectric response in addition to the above mentioned advantages of polymers. Incorporating nanofillers (e.g. ceramic filler-BaTiO3, PZT, carbon nanotube, clay, ferrite nanoparticles etc.) into PVDF is one of the facile ways to produce b phase in PVDF often followed by mechanical stretching. While fillers nucleate b phase in the PVDF matrix, subsequent mechanical stretching helps in aligning the dipoles to some extent. Therefore, the aim of the first part of this study is to prepare ferroelectric b-PVDF films by incorporating a hybrid filler (micro-CaCO3+ nanoclay) followed by mechanical stretching. While nanoclay is one of the most widely used cost-effective fillers capable of nucleating b-PVDF, synergistic improvement in b phase formation as well as in mechanical, electrical and piezoelectric properties of hybrid composites have been reported in this part of the study. Additionally, the influence of subsequent mechanical stretching e.g. draw ratio, R (final length/initial length), has also been investigated. FTIR and XRD are employed to determine the b phase content in both stretched and unstretched samples. While the tensile properties show a gradual decrease, dielectric constant increased gradually with increasing CaCO3 content in the hybrid filler. The maximum piezoelectric d33 coefficient of 30 pC/N is obtained for stretched hybrid composite films. Ferroelectrets, on the other hand, are another type of functional polymer films with heterogeneous cellular structure and internal quasi-permanent dipole moments. The piezoelectricity in ferroelectrets originates from the change in dipole moments under an applied mechanical stress. Over the past few decades, cellular polypropylene (PP) films have been the most popular and widely investigated ferroelectrets and are already in commercial applications. While the piezoelectric d33 coefficient of PP ferroelectrets could exhibit up to 1000 pC/N or more, their working temperature is quite low, not higher than 60 °C. Recently developed fluoropolymer i.e. polytetrafluoroethylene (PTFE) and fluoroethylene propylene (FEP) based ferroelectrets exhibit strong piezoelectric response and excellent thermal stability. However, processing these fluoropolymer ferroelectrets using commercially viable manufacturing technique such as cast film extrusion poses few challenges because of their very high melt viscosity, high manufacturing temperature, degradation tendency and low tensile elongation. Therefore, in this part of the study, a new thermally stable ferroelectret has been investigated using commercial processing technique with strong piezoelectric response. Cast extrusion of the polymer/filler mixture followed by mechanical stretching is one of the promising techniques for preparing a cellular polymer. Micron scale fillers are usually employed for creating voids from interfacial delamination under applied stress. Polyvinylidene fluoride (PVDF) is a fluoropolymer which is easily melt-processable than other fluoropolymers and showing excellent physical, chemical and electrical properties. An "extrusion-stretching-inflation" process has been followed to prepare a cellular structure in PVDF films. Solid PVDF/hybrid filler (micro-CaCO3 + nanoclay) films were extruded and subsequently stretched to create the initial cellular structure inside the films. While CaCO3 acts as void nucleation centers, nanoclay increases the stretchability of the highly CaCO3 filled PVDF films. Gas diffusion expansion (GDE) or controlled inflation is preformed to adjust the voids dimensions to lens-shaped voids leading to lower elastic moduli and stronger piezoelectricity. Obviously, the gas permeation characteristics in polar polymers is different than non-polar polymers because of their different type of polymer/gas interaction and the final piezoelectric properties of the ferroelectrets strongly depend on the cellular morphology such as void size, shape and distribution. Therefore, two different inflation gases i.e. neutral N2 and polar CO2 are used with various inflation strategies. Combination of pressure, temperature and their way of application (direct or step-wise) is examined for various times to optimize the inflation level. The inflated films are then subjected to corona charging at room temperature to create the dipoles inside the voids. Finally, PVDF ferroelectrets show piezoelectric d33 coefficient as high as 251 pC/N with N2 inflation and 327 pC/N for CO2 treatment. As expected, PVDF ferroelectrets exhibit better thermal stability than PP, PETP, COP and PEN and as high as Teflon. Samples charged at room temperature have their working temperature up to 120 °C