Thermal Conductivity and Multiferroics Of Electroactive polymers and Polymer Composites
Open Access
- Author:
- Jin, Jiezhu
- Graduate Program:
- Materials Science and Engineering
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- January 17, 2012
- Committee Members:
- Md Amanul Haque, Dissertation Advisor/Co-Advisor
Qing Wang, Dissertation Advisor/Co-Advisor
Ralph H Colby, Committee Member
Adrianus C Van Duin, Committee Member - Keywords:
- thermal conductivity
conducting polymer
multiferroic
piezoelectric
PVDF-based polymer - Abstract:
- Electronically conducting polymers and electromechanical polymers are the two important branches of the cutting-edge electroactive polymers. They have shown significant impact on many modern technologies such as flat panel display, energy transport, energy conversion, sensors and actuators. To utilize conducting polymers in microelectronics, optoelectronics and thermoelectrics, it is necessary to have a comprehensive study of their thermal conductivity since thermal conductivity is a fundamental materials property that is particularly important and sometimes a determining factor of the device performance. For electromechanical polymers, larger piezoelectric effect will contribute to the magnetoelectric (ME) coupling efficiency in their multiferroic composites. The first objective of this dissertation is to characterize electronic conducting polymers for their electrical and thermal conductivity. The analytical model and experimental technique are presented to measure the in-plane thermal conductivity of polyaniline thin films doped by camphorsulfonic acid. Measurements of 20 nm - 1 μm thick films reveal a strong thickness dependence of thermal conductivity below 130 nm. The size effect rapidly diminishes for films thicker than 800 nm. The size effect on thermal conductivity is attributed to the restrictions of the phonon mean free path length and heat capacity. The same technique is extended to measure the electrical and thermal conductivity of 55 nm thick polyaniline thin films doped with different levels of camphorsulfonic acid. Results indicate that the effect of the doping level (camphorsulfonic acid/polyaniline ratio) is more pronounced on electrical conductivity than on thermal conductivity. It is suggested that polarons are the charge carriers responsible for the electrical conduction, while phonons play a dominant role in the heat conduction in doped polyaniline thin films. The scientific outcome of this study provides a fundamental understanding of heat conduction mechanism in nanoscale polyaniline thin films. The second objective of this dissertation is to develop new classes of electromechanical polymers and strain-mediated electromechanical polymer-based multiferroic ME composites. One of them is based on chain-end cross-linked ferroelectric poly(vinylidene fluoride-co-trifluoroethylene) (PVDF-TrFE). With a low dc bias magnetic field, the ME coefficient of this composite is 17.7 V/cm Oe at non-resonance and 383 V/cm Oe at resonance. This remarkable result arises from the introduction of chain-end cross-linking and polysilsesquioxane structures into the ferroelectric P(VDF-TrFE) films, which leads to the formation of larger β phase crystallite size and concurrent improvement in the polarization ordering and consequently, better piezoelectricity in comparison to those of the pristine P(VDF-TrFE) copolymers. ME composite based on poly(vinylidene fluoride-co-hexafluoropropylene) P(VDF-HFP) are also developed. Crystalline β phase structure in P(VDF-HFP) is produced by uniaxially stretching of pre-melted and quenched films. It is found that there is a linear relationship between the ME voltage coefficient of the composite and the poling field applied to the polymer films. Moreover, an electric field-induced phase transition is observed in P(VDF-HFP), manifested by the evolution of a double hysteresis D-E loop at intermediate poling field to a single loop at high poling field. The double hysteresis loop is due to the antiferroelectric phase that will transform into the ferroelectric phase when poled at high electric field. To enhance the piezoelectric effect of P(VDF-HFP) copolymer, chemical modifications and various processing methods are utilized to alter the crystalline structure. For uniaxially stretched, hydrated and post-annealed P(VDF-HFP), the non-resonance ME voltage coefficient is measured to be 22.0 V/cm Oe and the resonance ME voltage coefficient is 310 V/cm Oe, which place them among the best multiferroic ME composites reported to date and also imply the commercial device potential. It is found that the intermolecular interaction between P(VDF-HFP) and hydrate salt provides an efficient way to lead P(VDF-HFP) to crystallize into the β phase. The hydrate salt functions as the nucleation site and the hydrogen bonding between C-F bond from P(VDF-HFP) and O-H bond from the hydrate salt tend to produce ferroelectric β phase. It is suggested that the hydrogen bonds can also stabilize the chain orientation created by uniaxial stretching. Therefore, both enhancement of the crystallization in the β phase, and chain and crystal orientation contribute to the improvement of piezoelectric performance of P(VDF-HFP) and therefore the ME voltage coefficient in the multiferroic composite.