Thermal Behaviors in Multi-scale Nanostructures of 2-dimensional Nanomaterials Dong Jun Kang

Abstract

Technological developments in various engineering fields, such as the advanced electronics, metallurgy industry, wearable garments, housing, and aerospace, have an urgent problem in common, dealing a greater amount of energy efficiently within spatial limitedness. In this context, thermal management, ensuring the effective and stable operation of those electrical and biological system, commences with controlling heat, an inevitable manifestation of energy flow. The existing three major materials class in material science field, e.g. metals, ceramics, and polymers, have intrinsic limitations in solving current thermal challenges due to the substandard thermal, electrical, and mechanical properties of those classical engineering materials. In particular, the thermal conductivity of metal at the level up to ~500 W m-1 K-1 in room temperature can’t afford the heat flow from advanced electronic devices. Also, the metal and ceramic heating wires for heating systems are too heavy and fragile to use as a wearable or portable units. On the contrary, low-dimensional nanomaterials, which have a relatively shorter research history than materials thereof, have emerged as an alternative due to its unprecedented thermal, electrical and mechanical properties. However, optimization research on synthesis and purification is still in progress. Additionally, their superior characteristics couldn’t be applicable directly to the macroscale structure unless they are properly assembled. Nevertheless, considering the full potential of nanomaterials, they must be studied for thermal management system of next-generation system, with further researches on strategic assembly methods. By considering both thermal and electrical conductivity, mainly 2-D nanomaterials such as graphene, graphene oxide, reduced graphene oxide and MXene are studied here as representative candidates for thermal management application. For the macroscale usage, nanomaterials should be assembled into optimized multi- scale structure depending on the purpose. The final application can be categorized to address three urgent thermal challenges, regarding advanced heat dissipation, energy efficient heating process and wearable heating units with low voltage operation. When the assembled structure being constructed in a densely packed form, the movement of internal electrons and phonons becomes ballistic, resulting in excellent thermal, electrical, and mechanical properties. For that purpose, 1-D wet spinning is recommended method for neatly aligned structure. In this thesis, the precise control of the 1-D wet spinning of graphene oxide through programmed mechanical alignment process are reported, and successfully correlated structural contributions on the movement of internal heat carriers for designing high performance heat dissipators. The assembly of nanomaterials into a 2- D film form also demonstrates additional possibilities of electrothermal heating materials for heat management. Following 1-D assembly strategy, a graphene-metal hybrid film has been introduced to create a high-efficiency electrothermal heater. The strategy presented in this study involves inducing heat concentration at the interface between mixed metal and graphene interfaces, forming localized thermal gradients to enhance the transfer of heat to the object. The development of low-voltage-driven flexible heating elements was achieved using MXene. MXene, possessing high electrical conductivity without further reduction steps, is suitable for low-voltage heating, but it also has the drawback of being oxidative, making it vulnerable to external environments. To overcome this, a multi-layered structure was proposed to protect the MXene from external environment, using a graphene skin. The graphene skin is thermally stable and exhibits excellent oxidation stability with a low convective heat loss coefficient. This thesis deals with functional nanostructures for advanced thermal management for various systems. Appropriate nanomaterials, and structures are selected and designed to explore the potential of multi-scale nanostructure through appropriate thermal analysis. Electrical Joule heating analysis was focused on this thesis, which can delicately control the input power applied to the material to determine various thermal characteristics of it, using direct current and alternating current. The goal of this thesis is the identification of the thermal phenomena of low-dimensional materials and its assembly to improve a comprehensive understanding of thermal management materials.