Author: Ullah, Sana
Title: Dielectric, piezoelectric and pyroelectric properties of ferroelectric composites containing two-dimensional materials
Advisors: Zheng, Guang Ping (ME)
Degree: Ph.D.
Year: 2021
Subject: Ferroelectric crystals
Nanocomposites (Materials)
Hong Kong Polytechnic University -- Dissertations
Department: Department of Mechanical Engineering
Pages: xviii, 156 pages : color illustrations
Language: English
Abstract: The ferroelectric copolymer has attracted great attention due to their ferroelectric, pyroelectric and piezoelectric properties. Due to the flexible nature and easy fabrication techniques, ferroelectric copolymer and its two-dimensional (2D) nanocomposites lead to their outstanding technological applications in infrared detection, sensor and actuators, and infrared imaging. The dielectric materials are also desirable for application in energy storage devices. In this thesis, ferroelectric copolymer and its nanocomposites and 2D metal-organic frameworks are investigated and their dielectric properties, electrical energy storage density, electrocaloric effect, pyroelectric and piezoelectric properties are investigated systematically. In order to tune and improve the ferroelectric response and electrocaloric effect, ECE, two-dimensional (2D) graphitic-C3N4 (g-C3N4) nano-fillers are introduced into P(VDF-TrFE) copolymer. It is the first time, to the best of our knowledge that 2D g-C3N4 are used as nano-fillers in the P(VDF-TrFE)-based nanocomposites. The nanocomposites with various contents of g-C3N4 are characterized by different techniques. The differential scanning calorimetry (DSC) is employed to explore the effects of g-C3N4 on the ferroelectric-to-paraelectric (FE-to-PE) phase transition and the kinetics of the crystallization process in the nanocomposites. More importantly, the negative ECE mechanisms in the nanocomposites are elucidated from the experimental results. The maximum value of ECE 5.4 K is achieved at 322 K by applying an electric field of about 0.45 MV/cm. The enhanced dielectric and negative electrocaloric make them more potential candidates for electrocaloric refrigeration applications. The nanocomposites consisting of graphene oxide (GO) and quasi-2D ferroelectric copolymer P(VDF-TrFE) have been successfully synthesized by a co-evaporation method. The structural, dielectric and ferroelectric properties of the composite papers are investigated. The Raman spectroscopy analyses on the nanocomposites GO/P(VDF-TrFE) reveal that the defects in GOs are reduced significantly by the loading of ferroelectric P(VDF-TrFE). The IG/ID ratio increases from 1.02 (for pure GO) to 1.17 (for GO-10%P(VDF-TrFE)), revealing that the defects are reduced by the introduction of the nano-fillers due to a strong interaction between GO and P(VDF-TrFE) in the nanocomposites. It is worth noted that nanocomposites with 10% of P(VDF-TrFE) enhanced the permittivity by almost 3-times as compared to that of the pristine GOs. The nanocomposites show a notably raised polarization with a high applied electric field. Furthermore, due to the high dielectric constants, the electrical energy storage density of the nanocomposites is as high as ~39.89 J/cm3 at 2.8 MV/cm. The high energy density and high dielectric breakdown strength suggest that GO/P(VDF-TrFE) could be the promising novel materials for electrical energy storage. The piezoelectric effect is constrained by the intrinsic crystal structure of the constituent material. Herein we describe the designed formation of multicomponent-metal organic frameworks (M-MOFs) 2D piezoelectric materials by tuning the metallic constituent elements, which lead to a non-centrosymmetric structure and tunable effective d33 piezoelectric coefficients. In this work, MOFs and M-MOFs such as Ni_MOFs, Cu_MOFs, NixCu1-x_MOFs, and Co0.25Ni0.25Cu0.25Zn0.25_MOFs are obtained via a well-developed bottom-up method; meanwhile, their piezoelectric performance is measured for the first time by a dual AC resonance tracking piezo-response force microscopy. The results show that bimetal M-MOFs and quaternary-metal M-MOFs obtained displayed anomalous piezoelectric behaviors rather than centrosymmetric single-metal MOFs. The research will pave a new avenue for flexible piezoelectric material development.
Rights: All rights reserved
Access: open access

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