MULTIFUNCTIONAL COMPOSITES OF HEXAGONAL BORON NITRIDE FOR THERMAL SWITCH, LUNAR HEAT DISSIPATION, AND HIGH-TEMPERATURE ENERGY STORAGE A Thesis Presented to the Faculty of the Graduate School of Cornell University in Partial Fulfillment of the Requirements for the Degree of Master of Science by Yu-Han Wu May 2025 © 2025 Yu-Han Wu ALL RIGHTS RESERVED ABSTRACT Thermal management is a critical challenge in electronics, aerospace, energy storage, and automotive applications, where inadequate heat dissipation can lead to overheating and reduced reliability. Hexagonal boron nitride (h-BN)- based polymer nanocomposites offer a promising solution due to their excep- tional thermal properties. This thesis focuses on developing multifunctional h-BN composites for thermal switching, high-voltage insulation, and thermo- chemical energy storage, characterized using Differential Scanning Calorime- try (DSC), Laser Flash Analysis (LFA), and Thermomechanical Analysis (TMA). Thermal interface materials (TIMs) incorporating mixed-size h-BN particles (5 µm + 30 µm) achieved an optimal 40 wt% composition, reaching a thermal conductivity of 1.157 W/m·K. While higher h-BN content reduced thermal ex- pansion (270.6 ppm/°C), adjusting the TIM’s thickness ensured stable thermal switching over multiple cycles, making these materials suitable for battery ther- mal management and power electronics. Additionally, incorporating h-BN, silver, and multi-walled carbon nanotube fillers into polytetrafluoroethylene (PTFE) matrices enhanced thermal dissipation while maintaining high-voltage insulation. A 30% CNT (3-layer) composite achieved a thermal conductivity of 1.05 W/m·K, demonstrating potential for lunar power transmission. Further- more, metal hydride composites with h-BN were fabricated for thermochemical energy storage, though alternative characterization methods are needed due to LFA limitations. These findings provide valuable insights into the scalable de- velopment of multifunctional h-BN composites for extreme environments. BIOGRAPHICAL SKETCH Yu-Han Wu was born in Kaohsiung, Taiwan, in May 2000 and immigrated to South Africa with his family in 2011. After graduating from Edenvale High School in 2018, he pursued his undergraduate studies at the University of Jo- hannesburg. During his fourth year, Yu-Han conducted research on convert- ing sewage sludge and fly ash into sustainable green binders via a carbonation process under the mentorship of Dr. Tebogo Mashifana. In his final year, he was recognized with the Recognition of Exceptional Academic Achievement in Honours Studies and earned his Bachelor of Engineering Technology Honours degree in Chemical Engineering in 2023. Driven by a deep interest in materials science, Yu-Han pursued a Master of Science degree in Materials Science and Engineering at Cornell University un- der the mentorship of Dr. Zhiting Tian. His research focused on fabrication and characterization of multifunctional composites for diverse thermal management challenges, including thermal switching applications, lunar power systems, and high-temperature energy storage. Yu-Han is eager to further explore the world of materials science, aspiring to contribute to the development of sustainable energy devices that address global energy challenges. iii I dedicate this work to my family, whose constant love and encouragement have been my foundation and strength throughout this journey. iv ACKNOWLEDGEMENTS I would like to express my heartfelt gratitude to my advisor, Dr. Zhiting Tian, for her invaluable guidance and support throughout my research. Her men- torship has been instrumental in my academic development, and I am truly grateful for the opportunities she has provided me to deepen my knowledge in thermal transport. I would also like to extend my sincere thanks to my committee member, Dr. Lenan Zhang, for his insightful feedback and direction. His guidance has been crucial in helping me refine my research and explore new avenues to excel in this work. A special thanks to Dr. Liam Alexis, whose mentorship has been a constant source of support. His willingness to assist me whenever needed and his pa- tience in sharing his expertise has made a significant difference in my research experience. I am deeply grateful to work with Dr. Prithwish Biswas on the metal hy- dride composites and providing me all the necessary assistance throughout this research. His contributions have been vital to the success of this project. Lastly, I would like to thank Jaejun Lee, Dimitrios Koumoulis and Mark Pfeifer for their technical assistance with laser flash analysis (LFA), Differen- tial Scanning Calorimetry (DSC) and thermomechanical analysis (TMA). Their expertise played an essential role in ensuring the accuracy and precision of my experiments. v TABLE OF CONTENTS Biographical Sketch . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . iii Dedication . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . iv Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . v Table of Contents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . vi List of Figures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . viii 1 Introduction 1 1.1 Background . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 1.2 Motivation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 1.3 Research Objectives . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 1.4 Thesis Outline . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 2 Literature Review 5 2.1 Hexagonal Boron Nitride (h-BN) as a Filler Material . . . . . . . . 6 2.2 Polymer Matrices for Thermal Management . . . . . . . . . . . . 7 2.2.1 Silicone Elastomer . . . . . . . . . . . . . . . . . . . . . . . 7 2.2.2 Polytetrafluoroethylene (PTFE) . . . . . . . . . . . . . . . . 8 2.3 Thermal Interface Materials for Thermal Switching Applications . 9 2.4 Thermal Management for Aerospace and Lunar Applications . . 10 2.5 Thermochemical Energy Storage . . . . . . . . . . . . . . . . . . . 11 3 Methodology 13 3.1 Thermal Conductivity Measurement . . . . . . . . . . . . . . . . . 13 3.2 Heat Capacity Measurement . . . . . . . . . . . . . . . . . . . . . . 14 3.3 Thermal Diffusivity Measurement . . . . . . . . . . . . . . . . . . 15 3.4 Coefficient of Thermal Expansion (CTE) Measurement . . . . . . 15 4 THERMAL SWITCHING IN SILICONE ELASTOMER-BASED HEXAG- ONAL BORON NITRIDE COMPOSITE 17 4.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17 4.2 Experimental Methods . . . . . . . . . . . . . . . . . . . . . . . . . 19 4.3 Results and Discussion . . . . . . . . . . . . . . . . . . . . . . . . . 19 4.3.1 Thermal Conductivity and Thermal Expansion of h- BN/Silicone Elastomer Composites . . . . . . . . . . . . . 20 4.3.2 Thermal Switch System Design and Working Principal . . 21 4.3.3 Thermal Cycling and Thermal Switching Behavior . . . . 22 4.3.4 Thermal Regulation Performance . . . . . . . . . . . . . . . 24 4.4 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28 5 THERMAL CHARACTERIZATION OF PTFE-BASED HEXAGONAL BORON NITRIDE COMPOSITE 29 5.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29 5.2 Experimental Methods . . . . . . . . . . . . . . . . . . . . . . . . . 29 vi 5.3 Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31 5.3.1 Thermal Characterization of Ag/PTFE Composites . . . . 31 5.3.2 Thermal Conductivity of Laminated Ag/h-BN/PTFE Composite . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32 5.3.3 Thermal Conductivity of Single-Layer and Laminated MWCNT/h-BN/PTFE Composites . . . . . . . . . . . . . 33 5.4 Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35 5.5 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36 6 THERMAL CHARACTERIZATION OF METAL HYDRIDE-BASED HEXAGONAL BORON NITRIDE COMPOSITE 37 6.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37 6.2 Experimental Methods . . . . . . . . . . . . . . . . . . . . . . . . . 37 6.3 Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38 6.4 Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40 6.5 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42 7 Conclusion and Recommendations 43 7.1 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43 7.2 Recommendations . . . . . . . . . . . . . . . . . . . . . . . . . . . 44 A Matlab code 46 vii LIST OF FIGURES 1 A simple schematic illustrating the working principles of LFA and DSC thermal characterization. . . . . . . . . . . . . . . . . . . 14 2 A simple schematic illustrating the working principles of TMA thermomechanical characterization. . . . . . . . . . . . . . . . . . 16 3 (a) Thermal conductivity of the h-BN/silicone elastomer TIM. (b) Coefficient of thermal expansion (CTE) of the h-BN/silicone elastomer TIM. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21 4 (a) Illustration of thermal switch system design. (b)-(c) ON and OFF state of h-BN/silicone elastomer thermal switch. . . . . . . . 22 5 (a) Constant power supply over time. (b) Thermal cycling over 27,000 seconds to evaluate the thermal switching behavior of h- BN/silicone elastomer TIM. (c) Zoom-in plot of thermal cycling data for the first cycle from 0 seconds to 4,800 seconds. . . . . . . 24 6 (a) Constant power supply over time. (b) Thermal cycling over 18,000 seconds to evaluate the thermal regulation performance of the h-BN/silicone elastomer thermal interface material (TIM). (c) First thermal cycle from 0 to 6,000 seconds. (d) Zoomed-in view of the first thermal cycle from 0 to 900 seconds. (e) Tem- perature difference (∆T = Tn − T0) during the first thermal cycle, showing data from 0 to 6,000 seconds. (f) Zoomed-in view of ∆T for the heat sink temperature during the first cycle, from 0 to 3,000 seconds. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27 7 Thermal characterization data for Ag/PTFE composites (a) heat capacity, (b) density, (c) thermal diffusivity, and (d) thermal Con- ductivity. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31 8 Thermal conductivity data of laminated Ag/BN/PTFE compos- ites. (a) Sample preparation for LFA and DSC using different cut- ting methods. (b) Effect of different adhesive powders between the laminated layers. . . . . . . . . . . . . . . . . . . . . . . . . . . 33 9 Thermal conductivity data for single-layer and laminated CNT/BN/PTFE composites. . . . . . . . . . . . . . . . . . . . . . 34 10 (a) Nano-TiH2 on graphene sheet, (b) Nano-TiH2 on conductive carbon cloth, (c) h-BN/ EGaIn on glass disk, and (d) TiH2/h- BN/ EGaIn on glass disk. . . . . . . . . . . . . . . . . . . . . . . . 39 11 Thermal diffusivity data for (a) h-BN/EGaIn on glass disk, and (b) TiH2/h-BN/EGaIn on glass disk. . . . . . . . . . . . . . . . . . 40 viii CHAPTER 1 INTRODUCTION Thermal management remains a critical challenge across industries such as elec- tronics, aerospace, energy storage, and automotive applications, where effi- cient heat dissipation is essential to prevent overheating and ensure reliabil- ity. Hexagonal boron nitride (h-BN)-based polymeric nanocomposites have emerged as promising materials for thermal management, offering a combina- tion of exceptional thermal and mechanical properties. This thesis focuses on advancing the development and optimization of h-BN composites to address thermal management challenges in these industries. 1.1 Background Efficient thermal management is crucial in ensuring the performance, reliability, and longevity of devices and systems in demanding environments. Materials used in thermal management applications must not only dissipate heat effec- tively but also exhibit properties such as electrical insulation, chemical stability, and mechanical robustness. Hexagonal boron nitride (h-BN) is an ideal filler material for polymer ma- trices due to its high thermal conductivity, chemical inertness, thermal stability, electrical insulation, and corrosion resistance [1, 2]. These characteristics en- able h-BN to enhance the thermal regulation performance of polymeric systems, making it suitable for various applications. Despite its potential, optimizing the integration of h-BN in polymer matrices to balance thermal and mechanical 1 properties remains a significant challenge requiring further exploration. 1.2 Motivation The increasing demand for high-performance materials in industries such as aerospace, electronics, and energy storage has driven the need for advanced thermal management solutions [3]. For instance, electronic devices generate sig- nificant heat during operation, which must be efficiently dissipated to prevent performance degradation or failure. Similarly, energy storage systems and au- tomotive applications require materials that can operate reliably under extreme thermal conditions [4]. h-BN-based polymeric nanocomposites provide an avenue for addressing these challenges. However, challenges such as achieving uniform dispersion of h-BN within polymer matrices, optimizing filler-matrix interactions, and tailor- ing the composite structure to specific applications remain unresolved [5]. This research aims to address these challenges and advance the development of mul- tifunctional thermal management materials tailored to real-world applications. 1.3 Research Objectives This thesis hypothesizes that tailoring the composition and structure of h-BN- based composites can significantly improve their thermal management perfor- mance and multifunctionality. The specific research objectives are: 1. Development of Thermal Interface Materials (TIMs): Designing TIMs us- 2 ing silicone elastomer and h-BN for thermal switch applications, with a focus on optimizing thermal conductivity and thermal expansion to enhance their ther- mal switching capabilities. 2. Enhancement of Thermal Management in Lunar Power Systems: Integrat- ing h-BN, silver, and carbon nanotubes into PTFE matrices to enhance thermal conductivity for high-voltage insulation applications in lunar power transmis- sion. 3. Optimization of Thermochemical Energy Storage Materials: : Investigat- ing the thermal conductivity of metal hydride composites containing h-BN to optimize thermal transport for high-temperature energy storage. 1.4 Thesis Outline This thesis is organized into seven chapters. Chapter 2 provides a comprehen- sive literature review on h-BN as a thermally conductive material and explores various polymer matrices for thermal management applications. It also presents background information on thermal interface materials, their applications in lu- nar environments, and thermochemical energy storage systems, enhancing the understanding of their potential and challenges. Additionally, this chapter re- views commonly used fabrication methods for h-BN-based polymer compos- ites, highlighting key techniques and advancements. Chapter 3 outlines the experimental methodologies used in this study, including thermal and mechan- ical characterization techniques such as differential scanning calorimetry (DSC), laser flash analysis (LFA), and thermomechanical analysis (TMA). Chapter 4 in- vestigates the fabrication and characterization of silicone elastomer-based com- 3 posites embedded with h-BN and evaluates their performance as a thermal switch. Chapter 5 examines the fabrication and characterization of PTFE-based composites with h-BN, focusing on the thermal performance of layered PTFE composites. Chapter 6 explores the fabrication and characterization of metal hydride-based composites with h-BN. Finally, Chapter 7 presents the conclu- sion and recommendations, summarizing the key findings of this research and providing recommendations for future research directions. 4 CHAPTER 2 LITERATURE REVIEW Effective thermal management is a significant challenge in many modern applications, including microelectronics, aerospace systems, and energy stor- age devices. As these technologies continue to be optimized, pushing the limits of performance and miniaturization, managing heat becomes crucial to ensur- ing reliability, efficiency, and safety. In this context, the development of multi- functional materials shows great potential to provide promising solutions that address the complex requirements of specific conditions in various applications and environments, enabling effective thermal regulation and mechanical stabil- ity. Among these materials, hexagonal boron nitride (h-BN) stands out as an excellent choice of filler due to its unique combination of being thermally con- ductive and electrically insulating. When integrated with polymer matrices, h- BN facilitates the fabrication of lightweight, flexible thermal interface materials (TIMs) capable of withstanding extreme environments, such as those encoun- tered in aerospace and lunar applications. Additionally, incorporating other ad- vanced materials, such as carbon nanotubes (CNTs) and metal hydrides, opens new opportunities for enhancing thermal conductivity and enabling thermo- chemical energy storage. This literature review examines the state-of-the-art advancements in h-BN- based composites, focusing on their thermal and mechanical performance. It highlights current fabrication techniques and identifies opportunities for fur- ther research. This review aims to provide a comprehensive understanding and foundation for advancing h-BN multifunctional materials to develop novel ther- 5 mal management solutions. 2.1 Hexagonal Boron Nitride (h-BN) as a Filler Material Boron nitride (BN) is an inorganic material widely used in various applications due to its exceptional properties, including high thermal and chemical stability, mechanical robustness, and a low dielectric constant, which ensures excellent electrical insulation. Its remarkable thermal stability at extreme temperatures further enhances its suitability for applications requiring effective heat dissipa- tion. There are four types of boron nitride: amorphous boron nitride (a-BN) and three crystalline forms—hexagonal boron nitride (h-BN), cubic boron nitride (c- BN), and wurtzite boron nitride (w-BN). Among these, h-BN is the most stable form [6]. Often referred to as ”white graphite,” h-BN consists of flat layers of boron and nitrogen atoms arranged in a hexagonal lattice, held together by van der Waals forces between the layers [7]. With its wide bandgap, excellent flexi- bility, and high thermal conductivity (ranging from 300 to 2000 W/m·K), h-BN is a strong candidate for heat dissipation applications [8]. When comparing h-BN to other fillers such as graphene and alumina (Al2O3), significant differences emerge. While alumina is an electrical insula- tor, its thermal conductivity is relatively low, at approximately 30–35 W/m·K. In contrast, both h-BN and graphene exhibit high thermal conductivity. How- ever, graphene, being a zero-bandgap material, is highly electrically conductive, whereas h-BN is an exceptional electrical insulator due to its wide bandgap and low electron mobility [9]. This makes h-BN an ideal material for heat dissi- 6 pation in microelectronic devices or high-voltage applications where electrical insulation is critical for safety and uninterrupted device performance. 2.2 Polymer Matrices for Thermal Management The choice of polymer matrices is critical as it directly influences the perfor- mance of polymer composites for thermal management. Different polymer ma- trices are required for various applications, depending on environmental con- ditions such as ambient temperature, electrical insulation requirements, and chemical stability. These factors significantly impact the performance, lifespan, and reliability of systems and devices like microelectronics and batteries [10]. 2.2.1 Silicone Elastomer Silicone polymers, particularly silicone elastomers, are widely used to produce silicone-based thermally conductive composites that serve as thermal interface materials (TIMs). These materials are highly valued for their exceptional ther- mal and chemical stability, as well as their flexibility [11, 12]. When silicone elastomers are reinforced with inorganic fillers or chemically modified, they can withstand temperatures exceeding 250°C [13]. Additionally, silicone elastomers exhibit relatively high dielectric breakdown strength compared to silicone gels, maintaining stable electrical insulation properties even after 500 hours of expo- sure to 250°C [14]. Due to their unique properties, silicone elastomers are commonly used as polymer matrices in the fabrication of h-BN composites. For instance, silicone 7 elastomers have been utilized to create self-healing silicone/BN composites and flexible boron nitride/silicone rubber composites. Both materials exhibit high thermal conductivity and low electrical conductivity, making them ideal for applications requiring effective thermal management and electrical insulation [15, 16]. 2.2.2 Polytetrafluoroethylene (PTFE) Fluoropolymers, especially polytetrafluoroethylene (PTFE), have also proven to be excellent candidates for use as polymer matrices in thermal management applications. PTFE is particularly suitable for extreme environments, such as aerospace conditions, where high thermal and chemical stability are required. Its low dielectric constant makes it ideal for electrically insulating purposes, such as power transmission in lunar environments [1]. In conclusion, although most polymer matrices, such as silicone elastomers and PTFE, have low intrinsic thermal conductivities ranging between 0.1 and 0.5 W/m·K, their unique properties combined with the incorporation of h-BN particles enable the fabrication of multifunctional materials. These composites can be tailored for various applications, offering a promising approach to effec- tive thermal management. 8 2.3 Thermal Interface Materials for Thermal Switching Appli- cations Thermal interface materials (TIMs) are used to facilitate heat transfer between two surfaces of different materials by filling air gaps and improving contact between them, effectively dissipating heat generated by the heat source. Due to this capability, TIMs hold significant potential for use as thermal switches. To create effective TIMs, several properties of the polymer composites must be considered. These include high cross-plane thermal conductivity, flexibility, and a low modulus. Polymer composites exhibiting high out-of-plane thermal conductivity are particularly suitable for such applications [17]. Silicone elastomers are an ideal polymer matrix for fabricating h-BN com- posites due to their softness, flexibility, and ability to operate at high tempera- tures compared to other polymers. Consequently, extensive research has been conducted on the fabrication of silicone-based h-BN composites. The most common fabrication methods for h-BN composites involve me- chanical mixing techniques to align h-BN particles in the same direction, thereby enhancing their thermal conductivity. This process is often followed by de- gassing and curing the composites. For example, one study fabricated oriented BN/silicone rubber composites using a rod-milling process, achieving an or- derly arrangement of BN particles that resulted in an out-of-plane thermal con- ductivity of 7.62 W/m·K ([17]. Other mechanical mixing methods, such as using an overhead stirrer with rotational motion to uniformly mix silicone rubber with h-BN, have also been employed [18]. 9 2.4 Thermal Management for Aerospace and Lunar Applica- tions In the lunar environment, the materials used for power transmission cables are critical as they must meet several requirements to ensure optimal operating con- ditions and longevity. Heat dissipation is one of the key considerations for high- voltage cables. Polymer composites are particularly suitable for such applica- tions as they combine the beneficial properties of both the polymer matrix and the filler materials. The combination of a PTFE matrix and a thermally conductive filler such as h-BN is essential for effective heat dissipation and high-voltage insulation. Ad- ditionally, this combination offers other crucial characteristics for the lunar en- vironment, including lightweight properties, chemical inertness, excellent abra- sion resistance, and corrosion resistance [1]. h-BN/PTFE composites can be fabricated using either solid or liquid prepa- ration methods, such as solvent dispersion techniques [1]. Recent studies have demonstrated that the thermal conductivity of BN/PTFE composites can reach up to approximately 1.14 W/m·K at 30 wt% h-BN [19]. Furthermore, the particle size of h-BN significantly influences the thermal conductivity of the composite. Studies indicate that smaller h-BN filler particles, around 5 µm in size, can be more effectively incorporated into the PTFE matrix compared to larger particles, such as 30 µm h-BN. This results in a higher thermal conductivity of 3.5 W/m·K at 40 wt% h-BN [1]. Electrically conductive fillers, such as carbon nanotubes (CNTs), are also cru- 10 cial for electromagnetic interference (EMI) shielding to ensure the optimal oper- ation of cables [20]. The extraterrestrial lunar environment presents challenges far more complex than those typically encountered on Earth. To address these challenges, an integrated multi-layer design is needed. The outer layer should consist of h-BN composites to ensure efficient heat dissipation, while the mid- dle layer should be composed of a combination of electrically conductive and thermally conductive fillers. This design would provide EMI shielding effects while maintaining adequate thermal conductivity to transfer heat to the outer h-BN composite layer for dissipation. 2.5 Thermochemical Energy Storage Thermochemical energy storage systems rely on reversible chemical reactions. During the daytime, metal hydrides absorb heat from solar energy, undergo- ing an endothermic decomposition that separates the hydrides into metal and hydrogen. The hydrogen is then stored in a container for later use. At night, the process is reversed, resulting in an exothermic reaction that releases thermal energy in the absence of sunlight [21]. Metal hydrides are particularly promis- ing for thermochemical energy storage due to their high energy density, which is ten times greater than the molten salts currently used in concentrated solar power plants [21]. Developing a novel composite that incorporates h-BN into metal hydride nanoparticles could significantly enhance heat transfer efficiency. This approach would improve thermal conductivity and hydrogen transport while preventing hotspot formation. 11 Recent studies have successfully utilized liquid metals, such as eutectic gallium-indium (EGaIn), to create self-standing boron nitride bulk structures. EGaIn offers a high thermal conductivity of 26.6 W/m·K, far surpassing that of conventional polymers [22]. This demonstrates great potential for using liquid metals to bind metal hydrides, such as TiH2, with h-BN to develop advanced thermochemical energy storage materials. 12 CHAPTER 3 METHODOLOGY This chapter describes the thermal characterization techniques employed to evaluate the h-BN composite materials, including differential scanning calorimetry (DSC), laser flash analysis (LFA), and thermomechanical analysis (TMA). 3.1 Thermal Conductivity Measurement Thermal conductivity (κ) is determined using the equation: κ = ρCpα (1) where ρ represents density, Cp stands for heat capacity, and α represents ther- mal diffusivity. As shown in Figure 1, the heat capacity was measured using Differential Scanning Calorimetry (DSC), while the thermal diffusivity was ob- tained via Laser Flash Analysis (LFA). To determine the sample density, each specimen was cut into a circular shape with a diameter of 0.5 inches. The weight and thickness of the sample were then measured using an analytical balance and a caliper, respectively. These values were used to calculate the density. 13 Figure 1: A simple schematic illustrating the working principles of LFA and DSC thermal characterization. 3.2 Heat Capacity Measurement Heat capacity values were determined using a TA Instruments DSC Auto 2500. An empty pan and lid were prepared as reference materials. The DSC under- went a heat-cool-heat cycle, starting at 0◦C and heating at a rate of 10◦C per minute until reaching 40◦C. It then cooled at the same rate back to 0◦C. Through- out this process, the DSC measured the heat flow difference between the sample and the reference material as a function of temperature to obtain the heat capac- ity. Measurements were taken at three different locations on the same specimen to ensure accuracy. 14 3.3 Thermal Diffusivity Measurement Under adiabatic conditions, the thermal diffusivity (α) is determined using the following equation derived by Parker et al. [23]: α = 1.38L2 π2t 1 2 (2) where L represents the sample thickness, and t 1 2 denotes the half-rise time. This model assumes a short laser pulse that is uniformly absorbed and re- emitted by the sample surface. To ensure accurate measurements, both sides of the sample should be coated with a black layer to enhance absorption, block laser light, and minimize radiative heat transfer [23]. Thermal diffusivity was then measured using the Linseis XFA 500 Laser Flash Analyzer. In this method, a laser beam is directed onto the sample surface, causing a localized and rapid temperature increase. An infrared (IR) sensor con- tinuously monitors the temperature change on the opposite side of the sample, providing real-time thermal diffusivity data. To enhance laser energy absorp- tion and minimize surface reflection, both sides of the sample were coated with a thin layer of graphite. Measurements were taken at 3–5 different spots on each specimen to ensure data reliability. 3.4 Coefficient of Thermal Expansion (CTE) Measurement The coefficient of thermal expansion (CTE) is defined as: 15 CTE = ∆L Lo × ∆T (3) where ∆L represents the change in length, Lo is the initial length of the sam- ple, and ∆T is the change in temperature. Thermomechanical analysis (TMA) was performed using a TA Instruments Q400EM to determine the CTE. The schematic illustration for TMA was shown in Figure 2. First, the initial length of each sample was recorded. The temper- ature was then increased from 0°C to 220°C at a rate of 10°C per minute while continuously monitoring the sample length. The change in length (∆L) was used to calculate the CTE based on the equation above. Figure 2: A simple schematic illustrating the working principles of TMA thermomechanical characterization. 16 CHAPTER 4 THERMAL SWITCHING IN SILICONE ELASTOMER-BASED HEXAGONAL BORON NITRIDE COMPOSITE 4.1 Introduction Efficient thermal management is crucial for ensuring the performance, reli- ability, and longevity of modern electronic devices, energy storage systems, and aerospace technologies [24]. As these systems become more compact and power-intensive, the challenge of dissipating excess heat grows, necessitating advanced materials that can dynamically regulate heat flow. One promising approach to adaptive thermal regulation is the use of ther- mal switches—materials or devices that modulate heat transfer in response to temperature variations. These systems have been widely investigated for appli- cations such as microelectronics cooling, spacecraft thermal control, and energy- efficient building materials [25, 26, 27]. Thermal switches operate by detecting temperature fluctuations and transitioning between ON and OFF states, either enhancing or restricting heat flow to maintain safe operating conditions and prevent thermal runaway [28, 29]. While conventional thermal switches of- ten rely on mechanical actuation or phase change materials, polymer compos- ites with tunable thermal properties offer a scalable and lightweight alternative [1, 30, 31]. Among these materials, thermal interface materials (TIMs) provide a promising platform for thermal switching applications. TIMs enhance heat transfer between surfaces by filling air gaps and improving contact, effectively 17 dissipating heat from the source [24]. Given their ability to mediate interfacial thermal resistance, TIMs can be engineered to dynamically modulate heat dissi- pation in response to temperature changes. For this function, polymer compos- ites used in TIMs must exhibit high cross-plane thermal conductivity, mechani- cal flexibility, and a low modulus, ensuring efficient and adaptive heat transport [17]. To achieve these properties, we explore the integration of hexagonal boron nitride (h-BN) into a polymer matrix. h-BN is an ideal filler for thermally con- ductive TIMs due to its high in-plane thermal conductivity, chemical inertness, and electrical insulation properties [32, 1]. When embedded in a silicone elas- tomer matrix, h-BN enhances thermal transport while simultaneously enabling thermal switching through a unique thermal expansion-driven mechanism. Sil- icone elastomers, known for their flexibility and thermal stability, have been used in thermally actuated devices and thermal sensors [33, 34, 35, 36]. Thus, silicone elastomers provide an ideal platform for fabricating responsive TIMs that adapt to thermal fluctuations. In this work, we present a h-BN/silicone elastomer TIM with intrinsic ther- mal switching ability, where thermal expansion modulates interfacial contact, dynamically opening and closing air gaps to regulate heat dissipation. By em- bedding h-BN within a soft elastomeric matrix, we create a TIM that exhibits high out-of-plane thermal conductivity while maintaining mechanical adapt- ability. This approach offers a scalable, lightweight, and energy-efficient solu- tion for smart thermal management in advanced electronics and energy storage systems. 18 4.2 Experimental Methods The silicone elastomer SYLGARDTM 184 (Part A and Part B) was obtained from Dow, and hexagonal boron nitride (h-BN) particles (5 µm and 30 µm) were sourced from Panadyne Inc. To prepare h-BN/silicone elastomer TIM, Part A was dissolved in hexane at a 1:1 ratio and vortexed for 2 minutes using a Greiner Bio-One vortex mixer. In a separate beaker, 5 µm h-BN particles at 20, 30, and 40 wt% compositions were mixed with hexane at a mass-to-10 mL ra- tio and stirred at 1200 rpm on a hot plate. The curing agent (Part B) was then added to the silicone elastomer solution, followed by vortexing for 2 minutes. The elastomer mixture was poured into the h-BN dispersion and sonicated for 20 minutes at 40–50 amps. The combined mixture was subsequently stirred at 1200 rpm and 60°C until it thickened, after which it was poured into a mold to achieve a 1 mm thickness and cured on a hot plate at 45°C for 2 days. The same hexane dispersion method was used to fabricate mixed-particle h-BN/silicone elastomer composites, where the filler consisted of 50% 5 µm h-BN and 50% 30 µm h-BN. 4.3 Results and Discussion There are two key factors to consider when fabricating an h-BN/silicone elas- tomer thermal interface material (TIM) that functions as a thermal switch. First, the thermal conductivity of the TIM should be enhanced through h-BN fillers to improve heat dissipation. Second, the polymer matrix should have high ther- mal expansion to ensure proper contact with the heat sink, facilitating its ability to switch between ON and OFF modes. 19 4.3.1 Thermal Conductivity and Thermal Expansion of h- BN/Silicone Elastomer Composites Figure 3(a) presents the thermal conductivity (κ) of the h-BN/silicone elastomer TIM. Pure silicone elastomer has a κ value of 0.218 W/m·K, and the thermal conductivity increases with the h-BN weight percentage, reaching a maximum of 0.898 W/m·K at 40 wt%. Notably, TIMs containing mixed-size h-BN parti- cles (5 µm + 30 µm) exhibit higher thermal conductivity than those with only 5 µm particles. This enhancement is attributed to the synergistic effect of large and small h-BN particles, which improves particle-to-particle contact, thereby enhancing thermal connectivity and facilitating heat transfer. The smaller par- ticles effectively fill the voids between the larger ones, creating a more efficient thermal pathway and reducing thermal resistance. As a result, the 40 wt% h- BN TIM with mixed-size particles achieves the highest κ value of 1.157 W/m·K, which aligns with previous literature that demonstrates the benefits of incorpo- rating mixed-size fillers for improving thermal conductivity [37]. Figure 3(b) shows that the coefficient of thermal expansion (CTE) decreases upon incorporating h-BN fillers into the silicone elastomer. However, the varia- tion in CTE between 20 wt% and 40 wt% h-BN is relatively small. Interestingly, the 30 wt% mixed-size h-BN TIM (5 µm + 30 µm) exhibits the highest CTE at 369.4 ppm/°C, followed by a significant reduction to 270.6 ppm/°C at 40 wt%. This trend is likely due to the initial mismatch between the 5 µm and 30 µm h-BN particles at 30 wt%, which creates more voids where the elastomer can expand freely. However, as the filler content increases to 40 wt%, the binary packing effect becomes more pronounced, with the smaller 5 µm particles fit- ting between the larger 30 µm ones [38]. This reduces the polymer-rich regions, 20 thereby limiting thermal expansion. Figure 3: (a) Thermal conductivity of the h-BN/silicone elastomer TIM. (b) Coefficient of thermal expansion (CTE) of the h-BN/silicone elastomer TIM. 4.3.2 Thermal Switch System Design and Working Principal The thermal switch system is designed to evaluate the thermal switching capa- bility of h-BN/silicone elastomer TIMs, as shown in Figure 4(a). This system incorporates a mechanical jack to precisely adjust the height between the TIM and the heat sink, accommodating the millimeter-scale thermal expansion of the TIM. To minimize heat loss, the heater was enclosed in polystyrene and further wrapped with aerogel tape for thermal insulation. The TIM was placed in di- rect contact with the heat source. The outer structure, a 3D-printed polycar- bonate frame, was used to suspend the heat sink. Two thermocouples were positioned—one on the heater surface next to the TIM and another inside the 21 heat sink—to measure the temperatures of the heater (Theater) and the heat sink (Tsink). Figure 4(b)-(c) illustrates the working principle of the h-BN/silicone elas- tomer TIM as a thermal switch. In the OFF state, an air gap exists between the TIM and the heat sink, minimizing heat transfer. In the ON state, the TIM ther- mally expands upon heating, bridging the gap and facilitating heat dissipation to the heat sink. Figure 4: (a) Illustration of thermal switch system design. (b)-(c) ON and OFF state of h-BN/silicone elastomer thermal switch. 4.3.3 Thermal Cycling and Thermal Switching Behavior A thermal cycling test was performed to evaluate the thermal switching per- formance of the h-BN/silicone elastomer thermal interface material (TIM). The experiment involved monitoring the temperatures of both the heat source and the heat sink at 30-second intervals. As shown in Figure 5(a), a constant power 22 of 1.633 W was applied to the heat source for 4,800 seconds (80 minutes) to allow the TIM to fully transition from the OFF state to the ON state and reach ther- mal equilibrium. The power was then switched off, and temperature monitor- ing continued until both the heat source and heat sink cooled to approximately room temperature. Figure 5(b) shows the TIM’s transition from the OFF to the ON state, reach- ing a steady state where the heat source temperature stabilizes at around 62°C and the heat sink temperature stabilizes at 32°C. The thermal switching behav- ior, driven by thermal expansion, is evident throughout the thermal cycling data and is consistent across all three cycles. As seen in Figure 5(c), the TIM activates and switches from the OFF to the ON state after 390 seconds, when the heat source temperature reaches 36°C. Steady state is achieved around 4,260 seconds. 23 Figure 5: (a) Constant power supply over time. (b) Thermal cycling over 27,000 seconds to evaluate the thermal switching behavior of h- BN/silicone elastomer TIM. (c) Zoom-in plot of thermal cycling data for the first cycle from 0 seconds to 4,800 seconds. 4.3.4 Thermal Regulation Performance Since the thermal cycling data confirms that the TIM successfully switches OFF and ON, it is essential to further investigate its thermal regulation capability. 24 To assess this, an additional set of thermal cycling tests was conducted in- side a refrigerator to evaluate the thermal regulation performance of the h- BN/silicone elastomer TIM. The experimental procedure remained the same, where the temperatures of both the heat source and the heat sink were moni- tored at 30-second intervals. As shown in Figure 6(a), a constant power of 4.14 W was applied to the heat source for 900 seconds (15 minutes). The power was then switched off, allowing the system to cool down for 5,100 seconds (85 min- utes). Figure 6(b) presents three different cases. The red line represents the tradi- tional TIM, where the h-BN/silicone elastomer TIM remains in direct contact with the heat sink throughout the experiment. The blue line corresponds to the thermal switch being evaluated, where an initial air gap exists between the h- BN/silicone elastomer TIM and the heat sink. Upon heating, the TIM thermally expands to make contact with the heat sink, demonstrating its switching capa- bility. The green line represents a control case, where only the heat source and heat sink are present, with an air gap in between and no TIM. Figures 6(c-d) show that initially, the heater temperature for the Thermal Switch case (blue line) follows the No TIM trend (green line), indicating that the switch remains in the OFF state. This also demonstrates the material’s ability to retain heat within the heat source in a cooler environment. After 450 seconds of heating, the heater temperature for the Thermal Switch begins to deviate from the No TIM trend, showing a more gradual increase. This indicates the transition to the ON state, as shown in the yellow region, where heat dissipation occurs as the TIM transfers heat to the heat sink. 25 Figures 6(e-f) show the temperature difference (∆T ), which is calculated by subtracting the initial temperature (T0) from each subsequent temperature mea- surement (Tn), i.e., ∆T = Tn − T0. As a result, the ∆T for all cases begins at zero. In Figure 6(e), the temperature trends of the heat sink correspond to those of the heat source, although they follow an inverse pattern due to the direction of heat flow. In Figure 6(f), the green line exhibits the lowest temperature, as heat is transferred primarily through convection throughout the experiment. In con- trast, the red line, which represents direct contact between the heat source and heat sink, shows the highest temperature due to efficient heat transfer via con- duction. The blue line, representing the actual thermal switch, lies between the two. After the power is switched off, the green line begins to drop immediately, as heat is transferred through convection. However, the red and blue lines con- tinue to rise briefly, as they remain in contact with the heat sink. Subsequently, the red line shows a steeper decline, indicating rapid heat loss through conduc- tion, while the blue line gradually follows the slope of the green line, reflecting a transition from conduction-dominant to convection-like cooling behavior. 26 Figure 6: (a) Constant power supply over time. (b) Thermal cycling over 18,000 seconds to evaluate the thermal regulation performance of the h-BN/silicone elastomer thermal interface material (TIM). (c) First thermal cycle from 0 to 6,000 seconds. (d) Zoomed-in view of the first thermal cycle from 0 to 900 seconds. (e) Tem- perature difference (∆T = Tn − T0) during the first thermal cycle, showing data from 0 to 6,000 seconds. (f) Zoomed-in view of ∆T for the heat sink temperature during the first cycle, from 0 to 3,000 seconds. 27 4.4 Conclusion This study demonstrates that TIMs with mixed-size h-BN particles (5 µm + 30 µm) exhibit higher thermal conductivity than those containing only 5 µm par- ticles. However, there is a trade-off between thermal conductivity and ther- mal expansion in these TIMs—while increasing the h-BN weight percentage enhances thermal conductivity, it reduces thermal expansion performance. The optimal composition identified in this study is the 40 wt% mixed-size h-BN TIM, which achieves the highest thermal conductivity of 1.157 W/m·K while main- taining a slightly lower coefficient of thermal expansion of 270.6 ppm/°C. This limitation can be compensated by adjusting the TIM’s initial thickness. Addi- tionally, the h-BN/silicone elastomer TIM exhibits consistent thermal switching behavior over multiple thermal cycles, effectively retaining heat in cold con- ditions and expanding to dissipate heat at elevated temperatures. This ther- mal regulation mechanism makes it particularly suitable for applications in bat- tery thermal management and power electronics, where gradual and long-cycle thermal regulation is essential for ensuring stable operating temperatures, im- proving safety, and enhancing system efficiency and longevity. 28 CHAPTER 5 THERMAL CHARACTERIZATION OF PTFE-BASED HEXAGONAL BORON NITRIDE COMPOSITE 5.1 Introduction To develop polymer nanocomposites suitable for high-voltage insulation in lunar power transmission, it is essential to consider both heat dissipation to prevent cable overheating and electromagnetic interference (EMI) shielding. Polytetrafluoroethylene (PTFE) serves as the polymer matrix, providing high- voltage insulation, while hexagonal boron nitride (h-BN) is incorporated as a thermally conductive filler to enhance heat dissipation. Additionally, multi- walled carbon nanotubes (MWCNTs) and silver (Ag) nanoparticles are investi- gated as electrically conductive fillers for EMI shielding. This study focuses on the thermal characterization of the fabricated nanocomposites. To achieve this, thermally and electrically conductive poly- mer composites will first be developed separately and then integrated into a layered structure to achieve the desired multifunctional properties. The overall thermal conductivity of the layered composite will then be characterized and evaluated. 5.2 Experimental Methods PTFE powder (polymer matrix) was purchased from Goodfellow Cambridge Limited. Hexagonal boron nitride (h-BN) particles (∼ 5 µm), serving as ther- 29 mally conductive fillers, were sourced from Panadyne Inc. Electrically conduc- tive silver (Ag) particles (5 µm) and multi-walled carbon nanotubes (MWCNTs) (5–20 µm in length) were obtained from US Research Nanomaterials, Inc., and Thermo Scientific. First, 30 g of PTFE powder was mixed with isopropanol (IPA) in a beaker, sealed with Parafilm, and stirred at 800 rpm at 50 ◦C for 1 hour using a VWR 7 × 7 CER hot plate stirrer. After this initial mixing, h-BN particles were added to achieve a 30 wt% composition, and the mixture was stirred for an additional 3 hours at 50 ◦C. The Parafilm was then removed, and the temperature was increased to 80 ◦C while reducing the stirring speed to 500 rpm to facilitate IPA evaporation. As mixing progressed, the increasing viscosity eventually made the mixture too thick for continued stirring. At this stage, the partially dried sample was transferred to a preheated Binder drying oven at 80 ◦C for further drying, followed by vacuum drying to remove any residual IPA. The dried h-BN/PTFE powder was subsequently subjected to RAM mixing at 90G for 6 minutes to break up any remaining agglomerates formed during the mixing and drying processes. Finally, the composite powder was hot-pressed into thin composite layers using a Wabash MPI G30H-IS-CX hot press. The same fabrication methods were applied to PTFE-based composites, with variations in filler content (wt%) and the inclusion of different conductive fillers, such as Ag and MWCNTs. 30 5.3 Results 5.3.1 Thermal Characterization of Ag/PTFE Composites As shown in Figure 7(b), the density of Ag/PTFE composites increases signifi- cantly with silver content, reaching 4.586 g/cm3 at 80 wt% Ag. Figures 7(c-d) illustrates a substantial increase in both thermal diffusivity and thermal conductivity beyond 50 wt% Ag, where the thermal conductivity reaches 0.478 W/m · K. The highest thermal conductivity, 2.527 W/m · K, is ob- served in the 80 wt% Ag composite. Figure 7: Thermal characterization data for Ag/PTFE composites (a) heat capacity, (b) density, (c) thermal diffusivity, and (d) thermal Con- ductivity. 31 5.3.2 Thermal Conductivity of Laminated Ag/h-BN/PTFE Com- posite Figures 8(a-b) presents the thermal conductivity data for single-layer, 3-layer, and 5-layer laminated composites. The 30 wt% BN composite exhibits a thermal conductivity of 2.262 W/m · K, whereas the 50 wt% Ag composite shows a lower value of 0.478 W/m · K. The 3-layer composite consists of a 50 wt% Ag/PTFE middle layer positioned between two 30 wt% BN/PTFE outer layers, while the 5-layer composite follows a similar design. In Figure 8(a), the thermal conductivity of the sample prepared using a hole punch for cutting is approximately 0.66 W/m · K. In contrast, the sample cut with a razor blade shows a higher thermal conductivity of around 0.95 W/m · K. Figure 8(b) shows that both 3-layer and 5-layer laminated composites us- ing an h-BN and PTFE powder mixture as the adhesive exhibit higher ther- mal conductivity, approximately 0.9 W/m · K, compared to those using pure PTFE powder as the adhesive between the layers. The thermal conductivity for the pure PTFE powder adhesive is 0.338 W/m · K for the 3-layer sample and 0.689 W/m · K for the 5-layer sample. 32 Figure 8: Thermal conductivity data of laminated Ag/BN/PTFE compos- ites. (a) Sample preparation for LFA and DSC using different cut- ting methods. (b) Effect of different adhesive powders between the laminated layers. 5.3.3 Thermal Conductivity of Single-Layer and Laminated MWCNT/h-BN/PTFE Composites The results shown in Figure 9 indicate that the single-layer composite consists of PTFE as the polymer matrix, mixed with various filler combinations of h- BN, CNT, and CNT/BN. The laminated composites consist of electrically con- ductive layers and thermally conductive layers, arranged in the same manner as described above. The thermally conductive layers are made of a 40 wt% BN/PTFE composite, while the electrically conductive layers are made with dif- ferent weight percentage combinations of CNT/PTFE or CNT/BN/PTFE. The results in Figure 9 show that the 40 wt% BN composite has a low thermal conductivity of 1.2 W/m · K, compared to the 30 wt% BN composite used in the laminated Ag/h-BN/PTFE composites, which has a thermal conductivity 33 of 2.262 W/m · K. In the case of laminated composites, the 20 wt% CNT:20 wt% BN (5 layers) exhibits better thermal conductivity compared to the 20 wt% CNT:30 wt% BN (5 layers), as shown in Figure 9. Furthermore, the 3-layer laminated composite demonstrates higher thermal conductivity than the 5-layer composite. Finally, the laminated composite with 30 wt% CNT (3 layers) achieves the highest ther- mal conductivity of 1.05 W/m · K. Figure 9: Thermal conductivity data for single-layer and laminated CNT/BN/PTFE composites. 34 5.4 Discussion The results in Figures 7(b-d) show a significant rise in thermal properties be- yond 50 wt% Ag, suggesting that thermal percolation occurs at this threshold. The thermal conductivity of 70 wt% and 80 wt% Ag composites remains high (2.087–2.527 W/m·K), demonstrating their potential for EMI shielding applica- tions while ensuring efficient heat dissipation. However, the increased density at higher Ag content is not ideal for lightweight composites and may require optimization to balance thermal performance and weight reduction. The discrepancy in thermal conductivity values, as shown in Figure 8(a), can be attributed to the damage caused by the hole punch, which results in delam- ination and disruption of the sample’s layered structure. In contrast, using a razor blade for slicing the sample maintains the integrity of the laminated lay- ers, improving thermal transport and yielding higher thermal conductivity. Ad- ditionally, the updated measurement shows a thermal conductivity value that is closer to the average of the single-layer 30% BN/PTFE and 50% Ag/PTFE layers, suggesting that this method provides a more accurate representation of the material’s thermal properties. These findings emphasize the importance of sample preparation techniques, especially for multilayered composites, as they can significantly influence the accuracy of thermal conductivity measurements. Figure 8(b) shows that incorporating h-BN/PTFE powder as an adhesive be- tween composite layers enhances thermal conductivity by forming a more effi- cient thermal pathway. The h-BN network reduces interfacial thermal resistance between h-BN/PTFE and Ag/PTFE layers, improving heat transfer across the multilayer structure. 35 In Figure 9, the unexpectedly low thermal conductivity of the 40% BN com- posite remains unclear and requires further investigation. In contrast, Figure 9 shows that the 20% CNT:20% BN (5 layers) composite exhibits higher ther- mal conductivity compared to the 20% CNT:30% BN (5 layers). This difference is likely due to the higher BN loading in the latter, which makes adhesion be- tween the layers more difficult. Additionally, the 3-layer laminated composite outperforms the 5-layer composite because fewer interfaces reduce thermal re- sistance. The laminated composite with 30% CNT (3 layers) achieves the highest thermal conductivity of 1.05 W/m·K, making it the best-performing composite in this study for heat dissipation. 5.5 Conclusion In conclusion, the combination of BN-PTFE powders for adhesion effectively reduces thermal resistance between layers, enhancing overall thermal conduc- tivity. Higher BN loading in the laminated CNT/BN/PTFE composites reduces PTFE content, which weakens layer adhesion and negatively impacts perfor- mance. Among the composites studied, the 30% CNT (3 layers) exhibited the highest thermal conductivity of 1.05 W/m·K, delivering the best heat dissipa- tion results. 36 CHAPTER 6 THERMAL CHARACTERIZATION OF METAL HYDRIDE-BASED HEXAGONAL BORON NITRIDE COMPOSITE 6.1 Introduction In a thermochemical energy storage (TCES) system, two critical factors to con- sider are the energy generation rate and the energy transfer rate. Achieving a high energy transfer rate to the heat-exchanging medium is essential to mini- mize energy losses and improve overall efficiency. Therefore, this study aims to develop metal hydride-based hexagonal boron nitride (h-BN) composites to enhance the thermal conductivity of metal hydrides, facilitating efficient heat exchange and increasing energy storage efficiency. Titanium hydride (TiH2) is selected as the metal hydride in this study, and it will be incorporated with h-BN to form a composite. Additionally, eutec- tic gallium-indium (EGaIn) will be used to enhance the binding between TiH2 and h-BN, contributing to the development of advanced thermochemical en- ergy storage materials. To evaluate the thermal properties of the composite, laser flash analysis (LFA) will be employed to measure its thermal diffusivity. 6.2 Experimental Methods TiH2 was purchased from US Research Nanomaterials, Inc., while eutectic gallium-indium (EGaIn) was obtained from Beantown Chemical. Hexagonal boron nitride (h-BN) particles ( 5 µm) were sourced from Panadyne Inc. The 37 graphite spray (Graphit 33 Conductive Coating) was purchased from Kontakt Chemie. Preparation of Nano-TiH2 on Graphene Sheets First, TiH2 was added to dimethylformamide (DMF) at a 1:10 mass-to-volume ratio and probe-sonicated at 60 amps for 15 minutes. The solution was then passed through a 200 nm filter. The dispersed nano-TiH2 was deposited onto graphene sheets at 110°C on a hot plate. After the DMF evaporated from the deposited graphene sheets, the deposition process was repeated until the de- sired thickness was achieved. The same method was used to deposit on carbon cloth. Fabrication of TiH2/h-BN/EGaIn Bulk Composites on Glass Disks First, IPA was added to 90 wt% TiH2 and 5 wt% EGaIn in a beaker and then sonicated in a bath for a few minutes using a Vevor Digital Ultrasonic Cleaner until the solution turned gray. Next, 5 wt% h-BN was added to the gray solution and probe-sonicated for 1 hour. Meanwhile, a circular half-inch-diameter glass disk was coated with a thin layer of graphite spray. The prepared solution was then deposited onto the graphite-coated glass disk. After the IPA evaporated, the deposition process was repeated until the desired thickness was achieved. Finally, the sample was coated with an additional layer of graphite. The same method was used to fabricate EGaIn/h-BN bulk composites. 6.3 Results Figures 10(a-b) illustrates that the nano-TiH2 particles deposited on graphene sheets form a very thin layer, approximately 0.034 mm thick. Additionally, only 38 a minimal amount of nano-TiH2 is observed on the carbon cloth. Figures 10(c-d) shows that the graphite coating on the h-BN/EGaIn layer on top of the glass disk tends to peel off more easily compared to the coating on TiH2/h-BN/EGaIn. Both the h-BN/EGaIn and TiH2/h-BN/EGaIn composites exhibit rough surface textures. Figure 10: (a) Nano-TiH2 on graphene sheet, (b) Nano-TiH2 on conductive carbon cloth, (c) h-BN/ EGaIn on glass disk, and (d) TiH2/h- BN/ EGaIn on glass disk. The laser flash results shown in Figures 11(a-b) indicate that both the h- 39 BN/EGaIn samples and the TiH2/h-BN/EGaIn samples exhibit inconsistent trends, with the TiH2/h-BN/EGaIn sample displaying a particularly significant variation in intensity. This leads to a substantial variation in the determined thermal diffusivity values for the same samples. Figure 11: Thermal diffusivity data for (a) h-BN/EGaIn on glass disk, and (b) TiH2/h-BN/EGaIn on glass disk. 6.4 Discussion To ensure accurate thermal diffusivity measurements using laser flash analy- sis (LFA), it is crucial to apply a graphite coating to the sample. In this study, nano-TiH2 was initially deposited on graphene sheets to serve as a reference sample. However, as shown in Figure 10(a), achieving a sufficient thickness was challenging due to liquid flow during deposition. The resulting layer was only 0.034 mm thick, significantly below the 0.1 mm minimum required for LFA. As a result, the thermal diffusivity could not be determined. 40 To address this limitation, a thicker carbon cloth (0.33 mm) was introduced as a substrate to replace the graphene sheets. Despite this adjustment, only ap- proximately 30 wt% of the deposited nano-TiH2 remained on the carbon cloth. This is likely due to the cloth’s porous structure, with pore sizes ranging from 10 to 50 µm, which affects the retention and uniformity of the deposited material. To further improve the measurement setup, a modified approach was imple- mented: first, a glass disk was coated with graphite, followed by the deposition of the sample material, and finally, another layer of graphite was applied on top. This method ensures that the sample is fully enclosed in a conductive graphite layer while using the glass disk solely as a support, since the sample itself is not free-standing. Glass was chosen because it does not absorb the laser energy dur- ing LFA, allowing the laser to pass through without interference. This ensures that the measurement focuses solely on the thermal response of the deposited material, improving the reliability of the results. Figures 10(c-d) highlights differences in graphite adhesion between the h- BN/EGaIn and TiH2/h-BN/EGaIn composites. The graphite coating adhered poorly to the h-BN/EGaIn layer, peeling off more easily than on TiH2/h- BN/EGaIn. This is likely because EGaIn binds more effectively with TiH2 and h-BN, creating a denser surface that enhances graphite adhesion. Without TiH2, the composite is more loosely packed, resulting in a flakier, powdery structure that weakens interfacial bonding. In Figures 11(a-b), the inconsistent intensity versus time curves observed for both the h-BN/EGaIn composite and the large variations in the TiH2/h- BN/EGaIn composite suggest that thermal diffusivity cannot be accurately de- termined using the laser flash method. This issue is due to the convection 41 through the cracks introduced during the thickness measurement. Given these challenges, alternative methods, such as powder sample measurements, may offer a more reliable approach to accurately characterize the thermal diffusivity of these composites. 6.5 Conclusion Laser flash analysis is not a suitable thermal characterization method for these types of composites. Therefore, different thermal characterization method is required to accurately determine the thermal conductivity of the metal hydride- based hexagonal boron nitride composite. 42 CHAPTER 7 CONCLUSION AND RECOMMENDATIONS 7.1 Conclusion This thesis presents the development and characterization of multifunc- tional hexagonal boron nitride (h-BN)-based polymeric composites for thermal switching, high-voltage insulation, and thermochemical energy storage. By in- tegrating mixed-size h-BN particles into silicone elastomer-based thermal inter- face materials (TIMs), an optimal 40 wt% composition was identified, achieving enhanced thermal conductivity of 1.157 W/m·K while maintaining manageable thermal expansion at 270.6 ppm/°C. This composition enables stable and re- peatable thermal switching, making it highly suitable for battery thermal man- agement and power electronics. In addition, laminated composites incorporating h-BN, silver, and multi- walled carbon nanotubes (MWCNTs) into polytetrafluoroethylene (PTFE) ma- trices demonstrated enhanced thermal dissipation while preserving high- voltage insulation. The 30% CNT (3-layer) composite achieved the highest ther- mal conductivity of 1.05 W/m·K, highlighting its potential for aerospace and lunar power transmission applications. Furthermore, the study explored metal hydride composites with h-BN for thermochemical energy storage. However, challenges in thermal characteriza- tion using laser flash analysis (LFA) necessitate alternative methods to accu- rately assess their thermal properties. Overall, these findings contribute valuable insights into the design and scal- 43 ability of multifunctional h-BN-based composites for extreme environments, of- fering pathways toward improved thermal management in energy storage, elec- tronics, and aerospace systems. 7.2 Recommendations Enhancing thermal conductivity and thermal expansion remains essential for more sensitive thermal switching applications. Future research should focus on optimizing hybrid filler systems to improve thermal conductivity by incor- porating additional thermally conductive fillers, such as metal nanoparticles. Additionally, modifying the silicone elastomer or exploring alternative poly- mer matrices could further enhance thermal expansion, improving the overall effectiveness of TIMs in applications like power electronics and battery thermal management. For PTFE-based composites designed for high-voltage insulation, improv- ing layer adhesion in laminated CNT/BN/PTFE structures is a critical area for further study. Determining the optimum filler loading is also necessary to max- imize thermal conductivity while maintaining mechanical integrity. Further- more, long-term stability assessments, including electrical breakdown strength and thermal aging under extreme conditions, will be crucial to validating the feasibility of these materials for aerospace and lunar power transmission appli- cations. Finally, laser flash analysis (LFA) proved unsuitable for characterizing the thermal properties of metal hydride-based composites. It is necessary to ex- plore thermal characterization methods that can accurately measure the ther- 44 mal conductivity of powders, such as the Transient Plane Source (TPS) method. 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