Date of Award

Spring 2016

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Micro and Nanoscale Systems

First Advisor

Shengnian Wang

Abstract

A novel flow guided assembly approach is presented to well align and accurately position nanowire arrays in pre-defined locations with high throughput and large scale manufacturing capability. In this approach, polyacrylonitrile (PAN)/N, N-dimethylformamide (DMF) solution is first filled in an array of microfluidic channels. Then a gas flow is introduced to blow out most solutions while pushing a little leftover against the channel wall to assemble into polymer nanowires. In this way, highly-ordered nanowires are conveniently aligned in the flow direction and patterned along both sides of the microchannels. In this study, we demonstrated this flow-guided assembly process by producing millimeter-long nanowires across 5 mm x 12 mm area in the same orientation and with basic "I-shape", "T-shape", and "cross" patterns. The assembled polymer nanowires were further converted to conductive carbon nanowires through a standard carbonization process. After integrated into electronic sensors, high sensitivity was found in model protein sensing tests. This new nanowire manufacturing approach is anticipated to open new doors to the fabrication of nanowire-based sensing systems and serve as the Good Manufacturing Practices (GMP) (a system for ensuring that products are consistently produced and controlled according to quality standards) for its simplicity, low cost, alignment reliability, and high throughput.

By using the same polymer solution (polyacrylonitrile (PAN)/N, N- dimethylformamide (DMF) solution), a new, simple, and low-cost method has been developed in the production of porous composite nanofibers via a one-step foaming and electrospinning process. Sublimable aluminum acetylacetonate (AACA) was dissolved into polyacrylonitrile (PAN)/N, N-dimethylformamide (DMF) solution as the foaming agent. Silicon nanoparticles were then added and the resulting suspension solution was further electrospun to produce PAN/silicon composite nanofibers. The PAN nanofibers were then foamed during a thermal stabilization treatment and further carbonized into carbon/silicon composite nanofibers. Such mesoporous composites nanofiber mats were explored as the anode material for lithium ion batteries. Within this composite of nanofiber electrode, carbon nanofibers serve as the conductive media, while silicon nanoparticles ensure high lithium ion capacity and electrical density. The inter-fiber macrovoids and intra-fiber mesopores provide the buffering space to accommodate the huge volume expansion and consequent stress in the composite anode during the alloying process to mitigate electrode pulverization. Its high surface-to-volume ratio helps facilitate lithium ion transport between electrolytes and the active materials. Our electrochemical tests demonstrated higher reversible capacity and better capacity retention with this porous carbon/silicon composite nanofiber anode when compared with that made of nonporous composite nanofibers and CNF alone with similar treatments.

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