Abstract

Metallized carbon nanotube (CNT) networks aim to achieve conductivities competitive with bulk metals, while retaining the favorable temperature coefficient of resistance (TCR) for CNT materials. Cu is the predominant metal for high conductivity applications due to cost and availability. However, microscopy shows that Cu poorly wets a CNT surface and requires an adhesion metal for an improved physical and electrical interface. The present dissertation utilizes thermal evaporation as a direct method to evaluate 2-D coatings of Ti as an interfacial metal for bulk Cu-CNT hybrids. Specifically, a 10 nm Ti layer maintained a continuous and uniform coating after annealing to 400 °C for an hour, demonstrating the temperature stability of Ti on a bulk CNT network. Additionally, Ti successfully suppressed the delamination of a Cu overcoat and achieved a 12% decrease in resistance for Cu-Ti-CNT hybrids after annealing at 400 °C. The benefits observed with thermally evaporated Ti adhesion layers motivated the development of a 3-D deposition approach using a novel joule-heated driven CVD technique, which can deposit metal throughout the entire bulk volume. Specifically, an oxygen-free precursor, cyclopentadienyl(cycloheptatrienyl) titanium(II), was used in the process under an inert/reducing atmosphere (95% Ar/ 5% H2) to promote a pure Ti metal deposition. Cross-sectional EDX mapping revealed that CVD successfully achieved diffusion of Ti throughout the entirety of a ~30 μm-thick, porous CNT conductor, demonstrating the capability of CVD as a method to fabricate bulk integrated Ti-CNT conductors for the first time. CVD coating morphology is shown to be tunable via the amount of precursor used, reactor pressure, and temperature, ranging from coatings localized along the individual bundles within the network to a fully connected film formation. Additionally, modification of reactor environment provides control over metal oxidation during growth onto the CNTs, achieving oxide-free to mixed Ti-oxide depositions as validated via Raman spectroscopy. The effectiveness of pure Ti as an adhesion metal on CNTs is benefitted from its wettability, temperature stability, and low contact resistance to CNTs; which can motivate investigating other potential adhesion metals that typically produce stable oxides like tungsten. Modeling of the temperature dependent electrical characteristics indicates an increase in metallic conduction behavior for the Ti-CNT conductors, with a decrease in the tunneling barrier between CNTs after Ti deposition, demonstrating the benefits of nanometal interconnection and showcasing the utility of temperature dependent modeling as a tool to assess nanoscale interaction of metallized CNT networks. CVD deposited Ti-CNT conductors electroplated with Cu, annealed, densified and then annealed a second time, realize conductivities as high as 43.1 MS/m, which is the highest conductivity reported for a bulk metal-CNT conductor at 98% weight loading. A Ti seeded CNT conductor (~9% w/w) electroplated to 98% total metal mass was demonstrated to achieve a specific conductivity of 6257 Sm2/kg, with a TCR (from 300-600 K) of 3.49 × 10-3 K-1, which combined result in a surpassing of the specific conductivity of pure Cu at temperatures above 250 °C. Thus, the overall impact of this work is demonstration of advanced conductors with a combined high conductivity and low TCR, which can provide direct energy savings at elevated temperature operation for applications such as high efficiency motors.

Library of Congress Subject Headings

Carbon nanotubes--Electric properties; Metal coating; Titanium dioxide

Publication Date

8-9-2021

Document Type

Dissertation

Student Type

Graduate

Degree Name

Microsystems Engineering (Ph.D.)

Department, Program, or Center

Microsystems Engineering (KGCOE)

Advisor

Brian J. Landi

Advisor/Committee Member

Ivan Puchades

Advisor/Committee Member

Parsian Mohseni

Campus

RIT – Main Campus

Plan Codes

MCSE-PHD

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