Effect of strain and defects on the thermal conductance of the graphene/hexagonal boron nitride interface

Thermal Rectification in Graphene-Boron Nitride Nanotube Hybrid Structures: An Independent Control Mechanism for Forward and Backward Heat Flux

The weak van der Waals interactions in the out-of-plane direction result in markedly low thermal conductivity in one-dimensional (1D) and two-dimensional (2D) materials, which substantially restricts their applications. Developing three-dimensional (3D) columnar hybrid structures, featuring high thermal conductivity both within and beyond the plane, effectively addresses this challenge. This study investigated a 3D hybrid structure composed of graphene and boron nitride nanotubes (GR-BNNTs) using non-equilibrium molecular dynamics simulations. This approach allowed the examination of the formation mechanisms and key factors influencing thermal rectification (TR) in these materials. Our findings reveal a novel mechanism for independently regulating forward and backward heat fluxes in GR-BNNTs. By manipulating the thermal properties of the BNNTs and the graphene layer, the TR ratio can be controlled flexibly. Additionally, we identify specific strategies for independently adjusting the heat flux, such as altering the intercolumn distance of BNNTs, which impacts the backward flux merely, while applying strain to affect the forward flux merely. This research introduces a novel concept of independent regulation of forward and backward heat fluxes, providing significant insights into phonon thermal transport in 3D hybrid structures.

Thermal Rectification across an Asymmetric Layer Carbon Nanotube van der Waals Heterostructure

The exceptional thermal conductivity and strength of carbon nanotubes (CNTs) position them as outstanding materials for thermal conduction. The intriguing properties introduced by van der Waals (vdW) heterojunctions have also captured the interest of researchers. However, further refinement of the research concerning the integration of these two elements is required. In our study, a vdW heterostructure with asymmetric layer nesting of multiwalled CNTs (ALCNTs) is devised, with a specific focus on the model's heat flux and thermal rectification (TR) properties, which are analyzed using nonequilibrium molecular dynamics (NEMD). Notably, the greatest TR ratio is observed in the connection of three-layer and single-layer ALCNTs. Moreover, multilayer variable-length nested models exhibit a sluggish TR ratio. An examination of the interface thermal resistance (ITR) reveals that the maximum ITR in the multilayer nested model resides at the rightmost interface. However, it is essential to highlight that the determinant of the TR ratio and heat flux in the multilayer nested model is not the maximum ITR of the rightmost interface but rather the ITR of the outermost layer on the left. Additionally, the impacts of the defect density, length, temperature difference, and hydrogenation on the model's heat flux and TR are explored, yielding noteworthy conclusions. For instance, defects in the outer CNT have a minimal influence on the heat flux and TR compared with those in the inner CNT. As the length increases, the heat flux initially decreases and then increases. Hydrogenation significantly enhances the model's heat flux but does not favor the TR. Our study contributes to advancing the understanding of CNT vdW heterojunctions and offers valuable insights for their practical applications.

Thermal transport properties of graphene aerogel as an advanced carrier for enhanced energy storage

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.

Si/Ge interfacial thermal conductance enhancement through Sn nanoparticle embedding

The improvement of interfacial thermal conductance (ITC) is a crucial aspect of the thermal management of nanodevices. In this paper, the effect of embedding Sn nanoparticles at the Si/Ge interface on ITC was investigated using non-equilibrium molecular dynamics (NEMD) simulations. It was found that although Sn has a higher atomic weight than both silicon and germanium, the ITC can be enhanced by 1.95 times when the nanoparticles reach a suitable number and diameter. The phonon transmission functions and density of states clearly indicate that an increased ITC can be attributed to the enhanced inelastic phonon scattering facilitated by Sn nanoparticles. This enhancement opens up novel channels for interfacial phonon transport. However, when the number of nanoparticles surpasses a suitable value, elastic phonons begin to dominate heat transport, leading to a subsequent decrease in the ITC. Sensitivity analysis further underscores that the ITC exhibits greater responsiveness to changes in diameter. In addition, it is also shown that with increasing temperature, a higher frequency phonon excitation occurs, increasing phonon inelastic scattering and interface transmission. These findings offer a novel strategy for enhancing ITC and deepening our comprehension of both elastic and inelastic phonon processes in interfacial phonon transport.

Modulation of the thermal conductivity, interlayer thermal resistance, and interfacial thermal conductance of C2N

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, k_i 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.

An insight into thermal properties of BC3-graphene hetero-nanosheets: a molecular dynamics study

Simulation of thermal properties of graphene hetero-nanosheets is a key step in understanding their performance in nano-electronics where thermal loads and shocks are highly likely. Herein we combine graphene and boron-carbide nanosheets (BC3N) heterogeneous structures to obtain BC3N-graphene hetero-nanosheet (BC3GrHs) as a model semiconductor with tunable properties. Poor thermal properties of such heterostructures would curb their long-term practice. BC3GrHs may be imperfect with grain boundaries comprising non-hexagonal rings, heptagons, and pentagons as topological defects. Therefore, a realistic picture of the thermal properties of BC3GrHs necessitates consideration of grain boundaries of heptagon-pentagon defect pairs. Herein thermal properties of BC3GrHs with various defects were evaluated applying molecular dynamic (MD) simulation. First, temperature profiles along BC3GrHs interface with symmetric and asymmetric pentagon-heptagon pairs at 300 K, ΔT = 40 K, and zero strain were compared. Next, the effect of temperature, strain, and temperature gradient (ΔT) on Kaptiza resistance (interfacial thermal resistance at the grain boundary) was visualized. It was found that Kapitza resistance increases upon an increase of defect density in the grain boundary. Besides, among symmetric grain boundaries, 5-7-6-6 and 5-7-5-7 defect pairs showed the lowest (2 × 10-10 m2 K W-1) and highest (4.9 × 10-10 m2 K W-1) values of Kapitza resistance, respectively. Regarding parameters affecting Kapitza resistance, increased temperature and strain caused the rise and drop in Kaptiza thermal resistance, respectively. However, lengthier nanosheets had lower Kapitza thermal resistance. Moreover, changes in temperature gradient had a negligible effect on the Kapitza resistance.

Influence of Surface Defects on the Thermal Conductivity of Hexagonal Boron Nitride/Poly(dimethylsiloxane) Nanocomposites: A Molecular Dynamics Simulation

In this simulation, the reverse nonequilibrium molecular dynamics simulation is employed to explore how the surface defects in hexagonal boron nitride (h-BN) influence the thermal conductivity of poly(dimethylsiloxane) (PDMS)-based composites. First, the interfacial thermal conductivity and the intrinsic thermal conductivity of h-BN are obtained by tuning the defect density, the inhomogeneity of the defect distribution, and the number of h-BN layers. The defects enhance the interfacial thermal conductivity, especially for h-BNs with high inhomogeneity of the defect distribution and multilayer. However, the intrinsic thermal conductivity of h-BN is declined significantly by the defects. They can be explained well by the vibrational density of states of PDMS and h-BNs and their overlap. Then, by combining the effective medium approximation model with the simulation, the overall thermal conductivity of composites is obtained. It exhibits a gradual decrease with increasing defect density or reducing the inhomogeneity of the defect distribution. Meanwhile, the enhancement extent of the overall thermal conductivity by improving the concentration and size of h-BNs depends on the defect density and the defect distribution. Finally, the comparison between the simulation and experiment is discussed. In summary, our work provides some valuable insights into how the defect density, the defect distribution, and the number of layers influence the thermal conductivity of the PDMS-based composite.