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Title: Polymer/carbon nanotube composite materials and their electronic applications
Other Titles: Ju he wu/tan na mi guan fu he cai liao ji qi dian zi ying yong
Authors: Tang, Qingyuan ( 唐慶圓)
Department: Department of Electronic Engineering
Degree: Doctor of Philosophy
Issue Date: 2011
Publisher: City University of Hong Kong
Subjects: Nanotubes.
Nanocomposites (Materials)
Notes: CityU Call Number: TA418.9.N35 T37 2011
xx, 155 leaves : ill. 30 cm.
Thesis (Ph.D.)--City University of Hong Kong, 2011.
Includes bibliographical references (leaves 125-153)
Type: thesis
Abstract: In this study, one type of polyimides (PI), which is widely used for flexible substrates and dielectric interlayers in microelectronic devices, was chosen for the polymeric matrix and multiwall carbon nanotube (MWNT) was chosen for the filler. The improvement of dispersion of MWNTs in an organic solvent for in-situ polymerization of the PI, dispersant assistant in-situ polymerization, effects of the MWNTs on thermal imidization and the electrical, mechanical properties of fabricated PI/MWNTs composites were studied. Moreover, application examples of polymer composites for humidity sensors and soft radio frequency (RF) antennas were demonstrated and studied. The main results of above studies are summarized as follows. First of all, with a comparison of the effects of different surfactants: sodium dodecyl benzene sulfonate (SDBS), polyvinylpyrrolidone (PVP), SDBS/PVP, and Triton X-100 on the dispersion of 0.1 wt% MWNTs in dimethylacetamide, an effective dispersant (SDBS/PVP) and its suitable composition (ratio of 1:12) were found. The states of aggregation of MWNTs treated by different surfactants are distinctive, and the electrical properties of MWNTs are strongly related to these states of aggregation. Secondly, PI/MWNTs composites were synthesized by in-situ polymerization with the aid of the SDBS/PVP dispersant. The loading of MWNTs, characterization and comparison of PI/MWNT composites were studied. The microstructures of PI/MWNTs composites were optimized by the presence of a certain amount of the dispersant. The storage modulus of a 2 wt% surfactant-assisted PI/CNTs film is 17% higher than that of a film without the dispersant (at the same content of CNTs). A percolation of electrical conductivity can be obtained at 1 wt% of CNTs, which is capable of decreasing static discharge. Two types of stability test: 85°C/85RH% (360 hours) and a pressure cooker test (PCT, 125°C, 1.5 atm and 100% RH for 100 hours) were employed for stability evaluation. A film incorporating 1 wt% of CNTs with 2 wt% of dispersant shows the smallest change of conductivity after the tests. Thus, this film is suitable for microelectronic applications as it has a fine microstructure, high stability and an ideal conductivity for solving charge accumulation issue. Thirdly, we report the effect of MWNTs and their states of dispersion on the thermal imidization of the precursor films of polyimide (poly(amic acid), PAA) for the first time. The curing process was followed by measuring Fourier transform-infrared (FT-IR) spectra, fluorescence spectra, thermogravimetric-differential scanning calorimeter (TG-DSC) properties and the refractive indices of films. The different states of the distribution of MWNTs were observed by a light microscopy and scanning electron microscopy. It is proved that a better dispersion of 1 wt% of MWNTs dramatically enhanced the speed of imidization, thus the full imidization temperature of the polyimide can be reduced from 300°C to 250°C. Moreover, the DSC results show that lower decomposition temperature of PAA can be obtained with a uniform distribution of MWNTs, which means the process of cyclodehydration of the PAA was accelerated. In the fourth part, a fast response resistive-type PI/MWNT composite film is demonstrated which is able to monitor humidity inside electronic packages. A composite film with a loading of 3 wt% MWNTs possesses a very linear response nature, a linearity correlation (R2) of 0.99157 and a sensitivity of 0.00146/%RH. The response time is less than five seconds. The recoverable and repeatable resistive responses affirm the capability of this film for fast humidity detection. A negative temperature effect was found and proper temperature compensation should be considered in future applications. Moreover, the humidity sensing properties were related to a bulk effect owing to water penetration. The stability of the composites was proven as well, which further confirmed that the composites could be used as reliable sensor materials. The proposed sensing mechanisms are related with tunnel effects, doping of MWNTs by water and a barrier effect between MWNTs. Lastly, patch antennas based on soft polydimethylsiloxane (PDMS) and gold sputtered MWNTs films were fabricated and their return loss, gain and radiation pattern were measured. The fabrication of MWNTs films by filtration and their electrical and radio performance related to the MWNTs films were studied. The working frequency of the probe-feeding antenna matches the simulations very well. Radiation patterns of a PDMS/MWNTs probe-feeding antenna are similar with that of a standard normal metallic antenna. This part showed an effective method to integrate MWNTs films into PDMS, and then fabricate soft antennas. Moreover, the frequency-tuning capability was demonstrated on a PDMS/MWNTs cavity antenna.
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