Thesis: Ph. D., Massachusetts Institute of Technology, Department of Materials Science and Engineering, 2015. / Cataloged from PDF version of thesis. / Includes bibliographical references. / Thermoelectric (TE) materials, which can convert unused waste heat into useful electricity or vice versa, could play an important role in solving the current global energy challenge of providing sustainable and clean energy. Nevertheless, thermoelectrics have long been too inefficient to be utilized due to the relatively low energy conversion efficiency of present thermoelectrics. One way to obtain improved efficiency is to optimize the so-called TE figure of merit, ZT = S2[sigma]/[kappa], which is determined by the transport properties of the active layer material. To this end, higher-efficiency thermoelectrics will be enabled by a deep understanding of the key TE properties, such as thermal and charge transport and the impact of structural and chemical changes on these properties, in turn providing new design strategies for improved performance. To discover new classes of thermoelectric materials, computational materials design is applied to the field of thermoelectrics. This thesis presents a theoretical investigation of the influence of chemical modifications on thermal and charge transport in carbon-based materials (e.g., graphene and crystalline C60 ), with the goal of providing insight into design rules for efficient carbon-based thermoelectric materials. We carried out a detailed atomistic study of thermal and charge transport in carbon-based materials using several theoretical and computational approaches - equilibrium molecular dynamics (EMD), lattice dynamics (LD), density functional theory (DFT), and the semi-classical Boltzmann theory. We first investigated thermal transport in graphene with atomic-scale classical simulations, which has been shown that the use of two-dimensional (2D) periodic patterns on graphene substantially reduces the room-temperature thermal conductivity compared to that of the pristine monolayer. This reduction is shown to be due to a combination of boundary effects induced from the sharp interface between sp 2 and sp 3 carbon as well as clamping effects induced from the additional mass and steric packing of the functional groups. Using lattice dynamics calculations, we elucidate the correlation between this large reduction in thermal conductivity and the dynamical properties of the main heat carrying phonon modes. We have also explored an understanding of the impact of chemical functionalization on charge transport in graphene. Using quantum mechanical calculations, we predict that suitable chemical functionalization of graphene can enhance the room-temperature power factor of a factor of two compared to pristine graphene. Based on the understanding on both transport studies we have gained here, we propose the possibility of highly efficient graphene-based thermoelectric materials, reaching a maximum ZT ~ 3 at room temperature. We showed here that it is possible to independently control charge transport and thermal transport of graphene, achieving reduced thermal conductivity and enhanced power factor simultaneously. In addition, we discuss here the broader potential and understanding of the key thermoelectric properties in 2D materials, which could provide new design strategies for high efficient TE materials. Transport properties of crystalline C60 are investigated, and the results demonstrate that these properties can be broadly modified with metal atom intercalation in crystalline C60. In contrast to the case of graphene, where chemical modifications induce structural changes in graphene lattice (from sp 2 C to sp3 C), intercalating metal atoms only modify van der Waals interactions between C60 molecules, but still having a huge impact on both thermal and charge transport. Taken both transport studies together, we suggest that the metal atom intercalation in crystalline C60 could be a highly appealing approach to improve both transports in solid C60, and with appropriate optimization of TE figure of merit, ZT value as large as 1 at room-temperature can be achieved. This dissertation consists of five chapters. Chapter 1 contains a brief review of thermoelectric materials. Chapter 2 introduces the theoretical approaches for computing both thermal (with molecular dynamics and lattice dynamics) and charge transport (with density functional theory and semi-classical Boltzmann approach) in materials. In Chapter 3, our study of thermal transport in functionalized graphene is presented. Chapter 4 describes our results on charge carrier transport in functionalized graphene. Combining these two works, we predict the full ZT values of functionalized graphene. Chapter 5 describes how to optimize ZT value in metal atom intercalated crystalline C60 / by Jeong Yun Kim. / Ph. D.
| Identifer | oai:union.ndltd.org:MIT/oai:dspace.mit.edu:1721.1/98739 |
| Date | January 2015 |
| Creators | Kim, Jeong Yun |
| Contributors | Jeffrey C. Grossman., Massachusetts Institute of Technology. Department of Materials Science and Engineering., Massachusetts Institute of Technology. Department of Materials Science and Engineering. |
| Publisher | Massachusetts Institute of Technology |
| Source Sets | M.I.T. Theses and Dissertation |
| Language | English |
| Detected Language | English |
| Type | Thesis |
| Format | 102 pages, application/pdf |
| Rights | M.I.T. theses are protected by copyright. They may be viewed from this source for any purpose, but reproduction or distribution in any format is prohibited without written permission. See provided URL for inquiries about permission., http://dspace.mit.edu/handle/1721.1/7582 |
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