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Leveraging graphene aerogels as carriers offers innovative avenues for achieving enhanced energy density, thermal conductivity, and stability in energy storage materials due to their unique attributes. This study investigates the thermal transport properties of composite sulfur cathode materials and phase change materials based on graphene aerogels using molecular dynamics simulation. A graphene aerogel model is established, and the effects of sulfur and octadecane content on the thermal transport properties of graphene aerogels and graphene aerogel-based composites are examined. A theoretical model of heat transport is developed to analyze the contribution of fillers and graphene aerogels to the thermal conductivity of the composites. The results show that the theoretical analytical model shows strong agreement with the molecular dynamics results, especially at high filler content. This research provides valuable theoretical guidance for understanding the thermal transport properties of graphene aerogel-based composite sulfur cathode materials and phase change materials.
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C2N, a novel 2D semiconductor with orderly distributed holes and nitrogen atoms, has attracted significant attention due to its possible practical applications. This paper investigates the in-plane thermal conductivity and interlayer thermal resistance of C2N and the interfacial thermal conductance of in-plane heterostructures assembled by C2N and carbonized C2N(C-C2N) using molecular dynamics simulations. The in-plane thermal conductivities of C2N monolayers along zigzag and armchair directions are 73.2 and 77.3 W m-1 K-1, respectively, and can be effectively manipulated by point defects, chemical doping, and strain engineering. Remarkably, nitrogen vacancies have a more substantial impact on reducing the thermal conductivity than carbon vacancies because of the more pronounced suppression of the high-frequency peaks. The difference in doping sites leads to a change in phonon mode localization. When the C2N size is small, as the tensile strain increases, ki is affected by dimensional lengthening due to stretching in addition to tensile strain. The interlayer thermal resistance decreases with increasing layer number and interlayer coupling strength. The AA stacking gives rise to a lower thermal resistance than the AB stacking when the heat flow passes through the multilayer due to the weaker in-plane bonding strength. Moreover, various possible atomic structures of C2N/C-C2N in-plane heterojunctions and the effect of carbon and nitrogen vacancies on interfacial thermal conductance are explored. The results provide valuable insights into the thermal transport properties in the application of C2N-based electronic devices.
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Graphite carbon nitride (GCN), which can be regarded as a nitrogen heteroatom-substituted graphite framework, has attracted great attention as a new 2D layered structure material with semiconductor electronic characteristics. Using molecular dynamics simulations, the in-plane thermal conductivity and cross-plane thermal resistance of two GCN structures (i.e., triazine-based and heptazine-based) are investigated. Our results show that the in-plane thermal conductivities of the triazine-based and heptazine-based GCN monolayers along the armchair direction are 55.39 and 17.81 W m-1 K-1, respectively. The cross-plane thermal resistance decreases with increasing layer number and reaches asymptotic values of 3.6 × 10-10 and 9.3 × 10-10 m2 K W-1 at 40 layers for triazine-based and heptazine-based GCN, respectively. The in-plane thermal conductivity can be effectively manipulated by changing the temperature and applying strain, while it is insensitive to the number of layers, which is in sharp contrast to that of graphene. Moreover, the cross-plane thermal resistance decreases monotonically with temperature and coupling strength, and can be modulated by external strain. Surprisingly, the cross-plane tensile strain can reduce the thermal resistance of the heptazine-based GCN. Our study serves as a guide to groups interested in the physical properties of GCN.
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In-plane heterojunctions, obtained by seamlessly joining two or more nanoribbon edges of isolated two-dimensional atomic crystals such as graphene and hexagonal boron nitride, are emerging as nanomaterials for the development of future multifunctional devices. The thermal transport behavior at the interface of these heterojunctions plays a pivotal role in determining their functional performance. Using molecular dynamics simulations, the interfacial thermal conductance of graphene/hexagonal boron nitride (GE/BN) in-plane heterojunctions was investigated. The GE/BN heterostructure has a remarkably high interfacial thermal conductance, and thermal rectification occurs at the interface. The results also show that the interfacial thermal conductance is effectively modulated by strain and defect engineering. The atomic defect location can affect the phonon transmission at the interface. Interestingly, compared with the nitrogen doping effect, the boron doping defect can more effectively facilitate vibrational coupling at the interface in the graphene sheet. Stress distribution and vibrational spectral analyses are performed to elucidate the thermal transport mechanism. The results of this study may provide a foundation for future research attempting to manipulate the interfacial thermal conductance in other two-dimensional heterostructures.
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The thermal conductivities of single-layer BC3 (SLBC) sheets and their responses to environmental temperature, vacancy defects and external strain have been studied and compared with those of single-layer C3N (SLCN) sheets by molecular dynamics (MD) simulations. We found that SLBC and SLCN are isotropic in the basal plane and that their predicted thermal conductivities for infinite length sheets are 488.54 W m-1 K-1 and 799.87 W m-1 K-1, respectively. Despite many similar features in the structures of these materials, SLBC exhibits a lower thermal conductivity than SLCN due to stronger flexural acoustic phonon-defect scattering rates and weaker interatomic bonding stiffnesses. The vibrational density of states (VDOS) are calculated in both structures to elucidate their thermal conductivity differences. SLBC exhibits a more substantial redshift phenomenon in the high- and low-frequency domains than SLCN. In addition, the thermal conductivities of these materials exhibit decreasing trends in response to increases in temperature and defect ratio, and the temperature effect in SLBC is more substantial than that in SLCN, while the defect effect in SLBC is less substantial than that in SLCN. The influences of uniaxial compressive and tensile strains on the thermal conductivities of these materials are analysed separately. These two deformation modes cause different effects on the thermal transport behaviours of SLBC and SLCN: the effect of uniaxial compressive strain is slightly negative, while the effect of uniaxial tensile strain is initially positive and then negative. Moreover, the biaxial strains result in a more severe reduction in thermal conductivity than the uniaxial strains. Remarkably, the impact of uniaxial and biaxial tensile strains on thermal transport was stronger in SLBC than in SLCN. We propose that SLBC nanomembranes are promising candidates for various thermal applications.
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Silica aerogel (SA), recognized as an efficient insulating material, is characterized by its extremely low thermal conductivity (TC) and high porosity, presenting extensive application potential in aerospace and building energy conservation. In this study, the thermal transport properties of gas-filled SA are explored using molecular dynamics (MD) methods. It is found that an increase in porosity leads to a significant decrease in TC, primarily due to enhanced phonon scattering and reduced material stiffness. Additionally, the TC of SA influenced by gas exhibits a pattern of initial decrease, followed by an increase, and then a decrease again, driven by complex interactions between gas molecules and pore walls, phonon localization, and scattering mechanisms. At a gas concentration of 80%, the TC in confined spaces is significantly increased by nitrogen, attributed to enhanced intermolecular interactions and increased collision frequency. The impact of gases on the TC of gas-solid coupled composite materials is also investigated, revealing that gas molecules serve as a "bridge" for phonons, playing a crucial role in reducing interfacial scattering and enhancing low-frequency vibrational modes, thus further enhancing heat transfer efficiency. The TC of these composite materials is primarily regulated by the gas-phase TC in response to temperature, while the response to strain is predominantly governed by variations in the solid-phase TC. These results provide essential theoretical support and design guidelines for the development and design of new high-efficiency insulating materials.
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The thermal management of lithium-sulfur batteries with high specific energy has become one of the critical issues for their applications. Carbon-based nanotubes are widely used to construct composite sulfur cathodes. This paper focuses on the thermal transport properties of sulfur-coated and sulfur-embedded boron carbide nanotubes (BCNTs) and carbon nanotubes (CNTs) and their composites using molecular dynamics. It is found that phonon softening and localization play a role in making BCNT exhibit a lower thermal conductivity (TC) than CNT. Furthermore, it is discovered that the sulfur embedded inside the carbon-based nanotube has a greater negative impact on carbon-based nanotube phonon transport. Moreover, the effective medium theory model is not suitable for predicting the effective thermal conductivity of coated sulfur composites, in contrast to its good applicability to embedded sulfur composites. These findings provide an in-depth understanding of the thermal transport properties of composite sulfur cathodes in lithium-sulfur batteries.