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Phonon Transport in Ultrahigh and Ultralow Thermal Conductivity Materials

  • Author(s): Kang, Joonsang
  • Advisor(s): Hu, Yongjie
  • et al.
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Abstract

Advanced materials with extreme thermal conductivity are critically important for various technological applications including energy conversion, storage, and thermal management. Low thermal conductivity is needed for thermal insulation and thermoelectric energy harvesting, while high thermal conductivity is desirable for efficient heat spreading in electronics. However, practical application deployments are usually limited by the materials availability in nature. Moreover, understanding the fundamental origins for extreme thermal conductivity still remains challenging. My PhD research focuses on finding new thermal materials and unveiling fundamental phonon transport mechanisms in extreme thermal conductivity matters to push the frontier of thermal science.

My dissertation is composed of three topics. The first topic is focused on developing and investigating a new group of ultrahigh conductivity materials. High-quality boron phosphide (BP) and boron arsenide (BAs) crystal are synthesized and measured with thermal conductivities of 460 and 1300 W/mK, respectively. In particular, our result shows that BAs is the best thermal conductor among common bulk metals and semiconductors. To better understand the fundamental origin of such an ultrahigh thermal conductivity, advanced phonon spectroscopy and temperature dependent characterizations are performed. Our measurements, in conjunction with atomistic theory, reveal that, unlike the commonly accepted rule for most materials near room temperature, high-order anharmonicity through the four-phonon process is significant in BA because of its unique band structure. Our result underscores the promise of using BP and BAs for thermal management and develops microscopic understanding of the phonon transport mechanisms.

The second topic of my thesis is to investigate phonon transport in ultralow thermal conductivity material with a focus on tin selenide (SnSe). SnSe is a recently discovered material for high performance thermoelectricity. However, the thermal properties of intrinsic SnSe remain elusive in literature. To understand the dominant phonon transport mechanisms for the extremely low thermal conductivity of SnSe, temperature-dependent sound velocity, lattice expansion, and Gr�neisen parameter was measured. The measurement result shows that high-order anharmonicity introduces strong phonon renormalization and the ultralow thermal conductivity.

The third topic of the thesis is to investigate in-situ dynamic tuning of thermal conductivity in layered materials. A novel device platform based on lithium ion battery is developed to characterize the interactions between ions and phonons of layered materials. We observe a highly reversible modulation and anisotropy of thermal conductivity from phonon scattering introduced by ionic intercalation in the interspacing layers. This study provides a unique approach to explore the fundamental energy transport involving lattices and ions and open up new opportunities in thermal engineering.

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This item is under embargo until December 16, 2021.