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Voon, Kevin

University of British Columbia

Thermal energy conversion promises readily available sources of clean, renewable energy through technologies such as thermionic or thermoelectric devices, which can enhance existing sources such as solar or co-generation. Such devices require sustained high temperature gradients between an electron emitter material and an electron collector, which presents challenges due to factors such as heat dissipation. Carbon nanotube (CNT) forests are a promising candidate for efficient thermal energy conversion due to the highly localized heating (“Heat Trap”) that occurs when they are spot heated by a focused light beam. Because of this, temperatures over 1700 °C and temperature gradients of 10 °C per micrometre are achievable with powers less than 50 W/cm², making CNT forests a promising material for low-cost, miniaturized thermal energy conversion. However, there is much mystery regarding the mechanisms underlying the Heat Trap, particularly anomalous effects such as temperature decay and recovery over a period of hours, and how they are connected to the complex, multiscale structure of the CNT forest. In order to maximize the potential of the Heat Trap, it is important to understand how these various internal interactions come together in assemblies of one-dimensional (1-D) nanoscale objects to produce and influence such phenomena. This thesis outlines the development of a semi-empirical computational model for simulating these 1-D nanomaterial systems, incorporating various models for near-field radiation and analogues to structural features in CNT forests such as defects and density variations. This model showed how the forest structure was a major contributor to the Heat Trap due to nanoscale effects such as near-field radiative energy exchange, which also showed that nanoengineering the structure through methods such as increased CNT density could improve heat retention and thermal conversion efficiency. In addition, simulations were able to explain the observed temperature decay and recovery, attributing it to slow diffusion of adsorbates such as oxygen. This process was non-destructive, a promising sign for long-term device stability. These results demonstrate the potential of the model as a framework for improving the effectiveness of thermal energy conversion and related applications, and for further study into thermal applications of general multiscale nanomaterial systems.

Text

2023-09-30

Attribution-NonCommercial-NoDerivatives 4.0 International

http://hdl.handle.net/2429/79412

Electrical and Computer Engineering

University of British Columbia

UBCV

http://creativecommons.org/licenses/by-nc-nd/4.0/