Thermal rectification in novel two-dimensional hybrid graphene/BCN sheets: A molecular dynamics simulation.

The graphene-like monolayer of carbon, boron and nitrogen that maintains the native hexagonal atomic lattice (BCN), is a novel semiconductor with special thermal properties. Herein, with the aid of a non-equilibrium molecular dynamics approach (NEMD), we study phonon thermal rectification in a hybrid system of pure graphene and BCN (G-BCN) in various configurations under a series of positive and negative temperature gradients. We begin by investigating the relation of thermal rectification to sample's mean temperature, T, and the imposed temperature difference, ΔT, between the two heat baths at its ends. We then move to explore the effect of varying strain levels of our sample on thermal rectification, followed by Kapitza resistance calculations at the G-BCN interface, which shed light on the interface effects on thermal rectification. Our simulation results reveal a BCN-configuration-dependent behavior of thermal rectification. Finally, the underlying mechanism leading to a preferred direction for phonons is studied using phonon density of states (DOS) on both sides of the G-BCN interface.

Thermal conductivity and structural behavior of confined H2from molecular dynamics simulation.

In this work, we perform equilibrium molecular dynamics simulation to study the thermal conductivity of hydrogen molecules (H2) under extreme confinement within graphene nanochannel. We analyze the structural behavior of H2molecules inside the nanochannel and also examine the effect of nanochannel height, the number of H2molecules, and temperature of the system on the thermal conductivity. Our results reveal that H2molecules exhibit a strong propensity for absorption onto the nanochannel wall, consequently forming a dense packed layer in close to the wall. This phenomenon significantly impacts the thermal conductivity of the confined system. We made a significant discovery, revealing a strong correlation between the mass density near the nanochannel wall and the thermal conductivity. This finding highlights the crucial role played by the density near the wall in determining the thermal conductivity behavior. Surprisingly, the average thermal conductivity for nanochannels with a height (h) less than 27 Å exhibited an astonishing increase of over 12 times when compared to the bulk. Moreover, we observe that increasing the nanochannel height, while the number of H2molecules fixed, leads to a notable decrease in thermal conductivity. Furthermore, we investigate the influence of temperature on thermal conductivity. Our simulations demonstrate that higher temperature enhance the thermal conductivity due to increased phonon activity and energy states, facilitating more efficient heat transfer and higher thermal conductivity. To gain deeper insights into the factors affecting thermal conductivity, we explored the phonon density of states. Studying the behavior of hydrogen in confined environments can offer valuable insights into its transport properties and its potential for industrial applications.

Thermal rectification in polytelescopic Ge nanowires.

Herein we served non-equilibrium molecular dynamics (NEMD) approach to simulate thermal rectification in the mono- and polytelescopic Ge nanowires (GeNWs). We considered mono-telescopic structures with different Fat-Thin configurations (15-10 nm-nm or Type (I); 15-5 nm-nm or Type (II); and 10-5 or Type (III) nm-nm) as generic models. We simulated the variation of thermal conductivity against interfacial cross-sectional temperature as well as the direction of heat transfer, where a higher thermal conductivity correlating to thicker nanowires, and a more significant drop (or discontinuity) in the average interface temperature in the positive (or negative) direction were detected. Noticeably, interfacial thermal resistance followed the order of Type (II) (48 K/µW, maximal) ˃ Type (III) ˃ Type (I) (5 K/µW, minimal). In the second stage, a series of polytelescopic nanostructures of GeNWs were born with consecutive cross-sectional interfaces. Surprisingly, larger interfacial cross-sectional areas equivalent to smaller diameter changes along the GeNWs were responsible for higher temperature rectification. This led to a very limited thermal conductivity loss or a very high unidirectional heat transfer along the polytelescopic structures - the key for manufacturing next generation high-performance thermal diodes.

Heat transfer through hydrogenated graphene superlattice nanoribbons: a computational study.

Optimization of thermal conductivity of nanomaterials enables the fabrication of tailor-made nanodevices for thermoelectric applications. Superlattice nanostructures are correspondingly introduced to minimize the thermal conductivity of nanomaterials. Herein we computationally estimate the effect of total length and superlattice period ([Formula: see text]) on the thermal conductivity of graphene/graphane superlattice nanoribbons using molecular dynamics simulation. The intrinsic thermal conductivity ([Formula: see text]) is demonstrated to be dependent on [Formula: see text]. The [Formula: see text] of the superlattice, nanoribbons decreased by approximately 96% and 88% compared to that of pristine graphene and graphane, respectively. By modifying the overall length of the developed structure, we identified the ballistic-diffusive transition regime at 120 nm. Further study of the superlattice periods yielded a minimal thermal conductivity value of 144 W m-1 k-1 at [Formula: see text] = 3.4 nm. This superlattice characteristic is connected to the phonon coherent length, specifically, the length of the turning point at which the wave-like behavior of phonons starts to dominate the particle-like behavior. Our results highlight a roadmap for thermal conductivity value control via appropriate adjustments of the superlattice period.

A theoretical insight into phonon heat transport in graphene/biphenylene superlattice nanoribbons: a molecular dynamic study.

Manipulating the thermal conductivity of nanomaterials is an efficacious approach to fabricate tailor-made nanodevices for thermoelectric applications. To this end, superlattice nanostructures can be used to achieve minimal thermal conductivity for the employed nanomaterials. Two-dimensional biphenylene is a recently-synthesized sp2-hybridized allotrope of carbon atoms that can be employed in superlattice nanostructures and therefore further investigation in this context is due. In this study, we first determined the thermal conductivity of biphenylene at 142.8 W mK-1which is significantly lower than that of graphene. As a second step, we studied the effect of the superlattice period (lp) on thermal conductivities of the employed graphene/biphenylene superlattice nanoribbons, using molecular dynamics simulations. We calculated a minimum thermal conductivity of 105.5 W mK-1atlp= 5.066 nm which indicates an achieved thermal conductivity reduction of approximately 97% and 26% when compared to pristine graphene and biphenylene, respectively. This superlattice period denotes the phonon coherent length at which the wave-like behavior of phonons starts prevailing over the particle-like behavior. Finally, the effects of temperature and temperature gradient on the thermal conductivity of superlattice were also investigated.

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.

Graphene-carbon nitride interface-geometry effectson thermal rectification: A molecular dynamicssimulation.

We apply the non-equilibrium molecular dynamics approach (NEMD) to study thermal rectification in a hybrid graphene-carbon nitride system (G - C3N) under a series of positive and negative temperature gradients with varying interface geometries. In this study, we investigate the effects of a) temperature differences, (∆T), between the two employed baths, b) mediainterface geometry, and c) sample size, on thermal rectification. Our simulation results portray a sigmoid relation between thermal rectification and temperature difference, with a sample-size depending upper asymptote occurring at generally large temperature differences. The achieved thermal rectification values are significant and go up to around 120% for ∆T = 150 K. Furthermore, the consideration of varying media-interface geometries yields a non-negligible effect on thermal rectification and highlights areas for further investigation. Finally, calculations of Kapitza resistance at the G - C3N interface are performed for assisting us in the understanding of interface-geometry effects on thermal rectification.