Investigation of the structure-property-processing relationships in paper and carbon nanotube composite materials

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Muhlbauer, Rachel Lynn
Gerhardt, Rosario A.
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In this research, multiwalled carbon nanotube (MWNT) and paper composite materials were fabricated by dropcasting aqueous dispersions containing MWNTs onto filter paper using vacuum filtration, a highly unidirectional drying technique. By varying the pore size of the paper backbone as well as the number of deposited MWNT layers, composites with distinct architectures and properties were created. This thesis provides numerous examples that show how the processing methodology used influences the location of the MWNTs, the amount of MWNTs deposited, and the interaction between the MWNTs and the paper backbone. These three factors work in tandem to form the structures and properties presented. Understanding how the structures and properties come about allows for the tailorability of these composites for different applications and devices. The pore size of the backbone material combined with the directionality of the drying methodology controlled the location of MWNT deposition. MWNT deposition occurred in three ways: on the paper surface only, within the paper material only, or combined surface and internal deposition. By varying the number of deposition steps, the properties of the composite could be altered in the location of deposition. Surface charge, dispersion concentration, paper pore size, drying methodology, MWNT length, the number of deposited MWNT layers, and post-processing techniques were all factors studied in this thesis which could successfully vary the interaction between the MWNTs and between the MWNT and paper materials and, ultimately, alter the properties of the composite. Regardless of the processing methodology employed and the starting materials used, structure and property evolutions in the composite materials were characterized using impedance spectroscopy, optical microscopy, scanning electron microscopy, and Current-AFM. Combining equivalent circuit fitting of the impedance data with the information obtained from the imaging techniques allowed for the elucidation of structural mechanisms which contribute to the electronic response measured for each composite. An overall equivalent circuit was built for each composite plane which could then be used to extract the electrical properties of the individual conduction mechanisms within the composite. In the in-plane, the electrical properties of the paper backbone, MWNT-MWNT junctions, MWNT bundles, and MWNT curved bundles could be determined. In the thru-plane, the electrical properties within the paper thickness, either paper-dominated or MWNT-dominated, could be measured. The resistance through the thickness of a bulk MWNT surface network could be also measured when the density of the MWNT network is sufficiently high.
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