Author ORCID Identifier

https://orcid.org/0000-0002-9728-5246

Year of Publication

2020

Degree Name

Doctor of Philosophy (PhD)

Document Type

Doctoral Dissertation

College

Arts and Sciences

Department

Physics and Astronomy

First Advisor

Dr. Joseph W. Brill

Abstract

The main thrust of this research was to develop new probes to measure thermal conductivities (κ) of small-molecule crystals, as well as polymer blends of organic semiconductors, both to screen these for possible applications, e.g. as thermoelectric power generators, and to gain an understanding of thermal transport in them. Emphasis has been on the crystals of “TIPS pentacene” [TIPS = 6,13 bis(triisopropylsilylethynyl), and free-standing films of PEDOT:PSS [poly(3,4-ethylenedioxythiophene) polystyrene sulfonate] for different electrochemical and thermoelectric applications. Separate techniques were used for in-plane and transverse thermal conductivities in which 𝜅𝜅 is determined indirectly from measurements of the thermal diffusivity (D ≡ κ/ρc, where ρ is the density and c the specific heat). For in-plane measurements, we used a position dependent ac-calorimetric technique in which, long, thin samples are illuminated with light chopped at a low frequency along part of their length, and the temperature oscillations on the opposite surface measured with a thermocouple. For the transverse, we have developed a simplified ac-photothermal apparatus for measurement of the transverse thermal (i.e through-plane) diffusivity of small samples with a typical area of 7 𝑚𝑚2. Our technique is essentially the Fourier transform of the laser flash method. The sample is heated on its front side with chopped light, and we measure the frequency-dependence of thermal radiation from the sample by mounting it in front of a mercury-cadmium-telluride (MCT) infrared detector inside the detector dewar. For optically opaque samples, a simple analysis of the complex frequency dependence of the detector signal gives the transverse diffusivity. For samples which are not opaque, the same analysis, overlooking the finite optical absorption length, can lead to a very large overestimate of the diffusivity. We have shown in this research how the technique can be adapted and present a more complete analysis for less absorbing samples. Since new electrically conducting and mechanically robust fibers and yarns are needed as building blocks for emerging textile devices, we have measured the longitudinal thermal conductivities of PEDOT:PSS fibers fabricated by a continuous wet-spinning process. These fibers have high electrical conductivity, excellent mechanical properties, and moderate thermoelectric performance by including a stage in which they are drawn through dimethyl sulfoxide. Drawing the fibers induces preferential orientation of the polymer chains in the fiber axis direction. A self-heating technique is used to conduct direct measurements on the thermal conductivity of the fibers and the result indicates drawn fibers have thermal conductivities several times larger than most films, consistent with better alignment of the polymer strands and grains. For all samples, the measured total thermal conductivity was about a factor of 20 larger than the electronic thermal conductivity calculated from the Wiedemann−Franz law using the Sommerfeld value for 𝐿0, which could be indicating that the lattice contribution dominated despite the large electrical conductivities observed. This result suggests that drawing the material not only aligns the conducting grains but also improves conduction through the PSS matrix.

Digital Object Identifier (DOI)

https://doi.org/10.13023/etd.2020.284

Funding Information

My research was supported by the following grant from the U.S. National Science Foundation from 2014-2019

DMR-1262261.

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