Amit Rai

Date of Award

Fall 11-17-2018

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Micro and Nanoscale Systems

First Advisor

Arden L. Moore

Abstract

Recently, there has been a growing interest in flexible electronic devices as they are light, highly flexible, robust, and use less expensive substrate materials. Such devices are affected by thermal management issues that can reduce the device’s performance and reliability. Therefore, this work is focused on the study of the thermal properties of nanomaterials and the methods to address such issues. The goal is to enhance the effective thermal conductivity by adding nanomaterials to the polymer matrix or by structural modification of nanomaterials. The thermal conductivity of copper nanowire/polydimethylsiloxane and copper nanowire/polyurethane composites were measured and showed more than threefold enhancement compared to the thermal conductivity values of the neat polymers. Furthermore, identical heat sources were used on the neat polymer as well as the composite samples, and the resulting thermal images were taken, which showed that the resulting hot spot was significantly less severe for the composite sample, demonstrating the potential of copper nanowire/polymer composite as a substrate for flexible electronics with better heat spreading capability.

In addition, the thermal properties of cellulose nanocrystals-poly (vinyl alcohol) composite films with different structural configurations of cellulose nanocrystals (such as isotropic and anisotropic configurations) were investigated as an alternative to commonly used petroleum-based materials for potential application in the thermal management of flexible electronic devices. Also, the in-plane thermal conductivity of the anisotropic composite film was as high as ~ 3.45 W m-1 K-1 in the chain direction. Moreover, the composite films showed ~ 4-14 fold higher in-plane thermal conductivity than most polymeric materials used as substrates for flexible electronics. A high degree of cellulose nanocrystal orientation and the inclusion of poly (vinyl alcohol) were the reasons for such improvements. In addition, thermal images showed that the cellulose nanocrystals-poly (vinyl alcohol) composite films had better heat dissipation capability compared to the neat poly (vinyl alcohol) films, indicating its potential application for flexible electronic devices.

In another study, thermal properties of nanodiamond films obtained through a solution-based directed covalent assembly were studied as a low-cost and greener alternative to the nanodiamond films grown via chemical vapor deposition method for thermal management of electronics. The results obtained showed cross-plane thermal conductivity as high as 3.50 +/- 0.54 W m-1 K-1 for nanodiamond film of 139.1 +/- 19.5 nm thick. Such a low cross-plane thermal conductivity value can be attributed to higher porosity and poor interface quality compared to that of the nanodiamond films grown via chemical vapor deposition method. Hence, there is still more room for improvement for such nanodiamond films.

The above chapters were focused on the study of the thermal properties of various types of nanomaterials for thermal management of electronic devices. In the next chapter, a technique for the fabrication of a device, that is capable of performing characterization nanomaterials was presented. In this work, suspended beam microdevices for electrothermal characterization of nanomaterials were fabricated through a standard photolithography technique that is less time-consuming, less expensive and much simpler than the methods used by other research groups in the past. The agreement of the measured in-plane thermal conductivity of the suspended central silicon nitride bare bridge with the literature validated the microdevice, setup, and the experimental procedure. Furthermore, these microdevices can be used to measure other important thermoelectric properties of nanomaterials such as the Seebeck coefficient, electrical conductivity, and thermoelectric figure of merit.

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