Extrusion Printing of Carbon Nanotube Inks, from Rheology to Electronics

Abstract

Printed electronics rely on the deposition of conductive liquid-based inks into continuous lines and films, typically on polymeric substrates. Among candidate conductive fillers for use in electronic inks, carbon nanotubes (CNTs) have high conductivity, low density, processability at ambient temperatures, and intrinsic mechanical flexibility, showing their potential to serve as a material of choice in the manufacturing of electronics. However, printed CNT structures have been limited to date in electrical conductivity by manufacturing constraints, typically necessitating nonconductive modifiers to render ink suitable for extrusion, as well as by imperfect CNT quality and low concentration. The goal of this thesis is to surpass the limitations of current manufacturing processes using CNTs in ad-hoc mixtures, and instead to explore printing and electronic properties in relation to CNT-based solution composition and rheology in order to lay the framework for intelligent, fit-for-purpose ink design. In Part One, a short overview of CNT ink rheology is presented, particularly focusing on elastoviscoplastic yield stress behavior, observing scaling laws for flow behavior, and denoting rheometric signatures of ink phase, solution quality, and printability. With further attention to rheometry, a modified vane tool is introduced that has an optimized fractal cross section to improve measurements of this class of slip-prone yield stress fluids.

In Part Two, printing methods and ink designs are developed in coordination to realize three demonstrations of CNT artifact production in 2D and 3D shapes, with the objective in all cases of maximizing electrical performance while attaining “good enough” geometric resolution. By using an aqueous CNT ink with moderate yield stress (2 Pa) for feature fidelity and considering wetting interactions between the ink and a substrate, the printing of thin CNT lines onto paper and polymer substrates is achieved to create flexible electronics, exhibiting conductivity up to 10 kS/m and specific conductivity tailorable from 0.004 to 140 S.m^2/kg. As a demonstration of the process, printed lines serve as interconnects to power embedded LEDs while flexing; as contact sensors; and as interdigitated capacitors for liquid imbibition. Next, taking inspiration from wet fiber-spinning processes, a method of extruding CNT inks into a coagulating liquid bath of non-solvent is introduced, generating rapidly solidifying, lightweight fibers with specific conductivity up to 7,000 S.m^2/kg, comparable with copper (6,600 S.m^2/kg), and conductivity up to 200 kS/m. Harnessing a fluid mechanical coiling instability observed during immersed printing, extensible CNT coils for strain sensing are produced. Third, a family of inks is created with yield stress in the range of 500 Pa and CNT concentrations around 15% to print arrays of cold cathode field electron emitters with freestanding miniature conical shapes having sub-100-micrometer tip diameters onto addressable gridpoints of a PCB. By modifying the ink further to increase extensional viscosity, individual 100-micrometer-scale freestanding cylinders are fabricated that surpass state-of-the-art CNT field emission performance.

Finally, in Part Three, to overcome fundamental limitations of the conductivity of CNT-based macrostructures, a method is developed to enhance the conductivity of existing CNT structures using copper electrodeposition. By understanding the role of CNT wettability for homogeneous nucleation of metal, CNT-copper composites are formed with final conductivity up to 2,000 kS/m for a lightweight composite with density less than 0.8 g/cm^3, yielding a specific conductivity of 2,500 S.m^2/kg. The contributions presented in this thesis provide the means to develop new electronics using CNT-based conductors, and guidelines drawn from the properties of yield stress fluids apply more broadly to the design of solution-processable conductive inks.