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On the Effects of Material Properties and Parameters on the Triboelectric Output of Filler-Modified Polymers

Author
Zhao, Xiaoyue
Published
[University Park, Pennsylvania] : Pennsylvania State University, 2023.
Physical Description
1 electronic document
Additional Creators
Lanagan, Michael
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Restricted (PSU Only).
Summary
The triboelectricity effect, which is when two materials become electrically charged after coming in contact with each other, is usually seen as a detrimental side effect in industry. For example, it can cause ignition hazard in the processing industry, engine element destruction in the aircraft industry, and electrostatic discharge (ESD) problem in the electronics industry. More recently, researchers began to develop devices that take advantage of the electro-mechanical conversion in the triboelectric effect by harvesting otherwise-wasted mechanical energy from the ambient environment to produce and store electrical charges. To date, the power density of these triboelectric nanogenerators, TENGs, have been reported in the range of Milliwatts per meter square to tens of Watts per meter square, with one study claiming a value as high as 500 W/m2 ; the reported power values are possibly sufficient to power small electronic devices. Efforts are focusing on further enhancing the triboelectric output and improving the performance of TENGs, which include device structural modification, system and circuit design, surface texturing, and material optimization. Among the strategies mentioned above, material optimization is especially important since the triboelectric output is highly dependent on the materials involved. So far, the material optimization is purely based on a trial-and-error type approach since the fundamental physics behind the triboelectric effect is still in debate and not fully understood yet. Therefore, a fundamental and comprehensive understanding of the effects of material properties and geometric parameters on the triboelectric output is needed for making materials-driven discoveries, which is essential for developing and improving TENG. In this dissertation, the effects of material properties, i.e., electrical and mechanical properties, and the effects of thickness and surface roughness on the triboelectric output are investigated experimentally and computationally. Three major objectives are established: The first objective is to develop a fundamental understanding of the impact of material's electrical properties, i.e., the dielectric permittivity and the electrical conductivity, on the triboelectric output. The second objective is to investigate the effect of material's mechanical property and thickness on the triboelectric output. The third objective builds on the understanding developed from the above two objectives to fabricate and study a multi-layer TENG, and then develops an analytical model to predict the triboelectric output of multi-layer TENGs. To achieve the first objective, multi-wall carbon nanotubes (MWCNT), single-wall carbon nanotubes (SWCNT), and ionic liquid-coated single-wall carbon nanotubes (SWCNT-IL) are used as fillers to tune the electrical properties of Polydimethylsiloxane (PDMS) (Chapter 2). It is found that the triboelectric polarity and surface roughness of the different composites change significantly above a critical filler's weight percent, and there is an optimum weight percent (0.1 wt%) for the SWCNT-PDMS composites where the triboelectric output is maximized. The tradeoff between dielectric permittivity and electrical conductivity could account for the existence of optimum filler content. As for the application evaluation, the optimum matching resistance of the TENG with 20:1 SWCNT-IL-PDMS sample is around 180 M[omega] which is about 42% of that of the control PDMS (430 M[omega]), and the corresponding maximum power of the 20:1 SWCNT-IL-PDMS sample is 0.12 W/m2. In addition, the 20:1 SWCNT-IL-PDMS composite achieves a relatively high electrical conductivity that enables the transfer of charges without additional electrodes, which could simplify the structure of TENG. Lastly, the 20:1 SWCNT-IL-PDMS is employed in the triboelectric portion of a hybrid Triboelectric-Dielectric Elastomer Generator (DEG) to prime the DEG in a self-priming circuit configuration. The resulting Triboelectric-DEG can offset the shortcoming of conventional DEGs and significantly increase the feasibility of such devices in wearable and mobile applications. In the second objective (Chapter 3), PDMS is used as a model material and the mechanical properties of PDMS are tuned by changing the crosslinking content and thermal treatment conditions. It is found that the triboelectric polarity of the thicker, thermally-treated PDMS samples is different from that of PDMS without thermal treatment. The change in the triboelectric polarity is proposed to be related to the bond breaking and material transfer between the contacting materials. The triboelectric series of the PDMS samples is established by comparing the open-circuit voltage (Voc) resulting from PDMS contacting with Teflon vs. that of PDMS in contacting with another PDMS, and it is found that more flexible PDMS samples tend to behave more triboelectrically positive. Based on the experimental results, it is proposed that the output of TENG is affected by both the triboelectric charges generated at the contact surface and the charges accumulated at the interface between the electrode and the triboelectric material. Further, it is found that the elastic modulus is a means to control these charges at both the contacting surface and the electrode-material interface. In the third objective (Chapter 4), a multi-layer TENG that contains a contact layer (PDMS), an interlayer ( P(VDF-TrFE)), and an electrode (Cu) is fabricated and studied. It is found that the PDMS with negatively poled P(VDF-TrFE) has the highest Voc, while the PDMS with positively poled P(VDF-TrFE) has the lowest Voc when Teflon is the reference material. In addition, the difference in short-circuit current (Isc) among the samples is not significant. It is assumed that the triboelectric output change results from the space charges accumulated at the interface between PDMS and P(VDF-TrFE) when the material is in contact with and separates from Teflon; the dipole orientation of the poled P(VDF-TrFE) affects the sign of the charges accumulated at the interface, affecting the triboelectric output. When the theoretical calculations of the Voc and Isc account for a layer of charges at the interface, the results agree with the experimental data, which supports this hypothesis. Based on the findings mentioned above, the five main conclusions of this dissertation are listed as follows: 1) There is a tradeoff between high dielectric permittivity and high electrical conductivity for conductive filler-modified polymers, so an optimum filler weight percent exists, below which the addition of conductive fillers increases the dielectric permittivity of the composite such that the triboelectric output is enhanced; and above which the high electrical conductivity leads to a lower triboelectric output. 2) The addition of fillers changes the triboelectric polarity of the composite, so the reduction of the output caused by increased electrical conductivity could be compensated by increasing the triboelectric polarity difference between the two contacting surfaces. 3) By selecting an appropriate conductive filler, polymer matrix, and reference material, a high triboelectric output can be achieved from conductive composites without electrode, simplifying the structure, decreasing the internal impedance, and improving TENG's flexibility. 4) The triboelectric polarity of two identical polymers can be changed by tuning their mechanical properties, where the more flexible one tends to behave more triboelectrically positive. This result is proposed to be related to both the triboelectric charges generated at the contact surface and the charges accumulated at the interface between the electrode and the triboelectric material; thus, the elastic modulus is a means to control these charges at both the contacting surface and the electrode-material interface. 5) The enhancement of the triboelectric output of multi-layer TENGs is proposed to result from the space charges accumulated at the interface between the different layers when the TENG is in operation. This dissertation generalizes approaches for increasing the triboelectric output of polymers and their composites and also proposes the mechanisms of the change in the triboelectric output of multi-layer materials with different mechanical properties, paving the way for the practical use and wide adoption of TENGs in real-world applications.
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Dissertation Note
Ph.D. Pennsylvania State University 2023.
Technical Details
The full text of the dissertation is available as an Adobe Acrobat .pdf file ; Adobe Acrobat Reader required to view the file.
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