Manipulating the temperature dependence of the thermal conductivity of graphene phononic crystal

Prediction of high thermal rectification behavior in carbon/C3N heteronanotubes based on nonequilibrium molecular dynamics simulations

Carbon/C3N heteronanotubes (CC3NNTs) have garnered significant interest for their distinctive performance and versatility across various applications. However, the understanding of interfacial heat transport within these heterostructures remains limited. This study aims to enrich the field by constructing models of CC3NNTs through the bonding of CNTs and C3NNTs, and employs nonequilibrium molecular dynamics (NEMD) simulations to predict their heat flux and thermal rectification (TR) effects. Placing the heat source in the CNT region induces a stronger heat flux compared to the C3NNT region, thus demonstrating a pronounced TR effect. This effect can be attributed to the mismatch in phonon spectra, as evidenced by the cumulative correlation factor derived from the phonon density of states (phonon DOS). Using this approach, we predict that the TR ratio for zigzag CC3NNTs (ZCC3NNT) significantly exceeds that of armchair CC3NNTs (ACC3NNT). Notably, in contrast to ACC3NNT, ZCC3NNT exhibits the phenomenon of negative differential thermal resistance in the backward heat flux with a temperature difference of Δ = 120 K. This phenomenon can be attributed to a lower phonon participation ratio at Δ = 120 K compared to other values of Δ. Subsequently, given that ZCC3NNT demonstrates the most pronounced TR ratio at room temperature, we explored how stress-strain, system size, defect density, and interface position impact the TR ratio. These insights are invaluable for guiding the design of thermal rectifiers, smart thermal management systems, and microelectronic processor coolers.

Modulating Thermal Conductivity via Targeted Phonon Excitation

Thermal conductivity is a critical material property in numerous applications, such as those related to thermoelectric devices and heat dissipation. Effectively modulating thermal conductivity has become a great concern in the field of heat conduction. Here, a quantum modulation strategy is proposed to modulate the thermal conductivity/heat flux by exciting targeted phonons. It shows that the thermal conductivity of graphene can be tailored in the range of 1559 W m-1 K-1 (decreased to 49%) to 4093 W m-1 K-1 (increased to 128%), compared with the intrinsic value of 3189 W m-1 K-1. The effects are also observed for graphene nanoribbons and bulk silicon. The results are obtained through both density functional theory calculations and molecular dynamics simulations. This novel modulation strategy may pave the way for quantum heat conduction.

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.

Improved thermoelectric properties of WS2-WSe2 phononic crystals: insights from first-principles calculations

Recently, two-dimensional transition metal dichalcogenide (TMDC) monolayers have attracted much attention owing to their excellent physical properties. In the present study, we systematically investigate the thermoelectric properties of different WS₂-WSe₂ phononic crystals by utilizing first-principles calculations. First, the thermal properties of all phononic crystals with superlattices (SL1 and SL2) and their individual components (WS₂ and WSe₂) are evaluated, in which the lattice thermal conductivities (k_ph) of WS₂ and WSe₂ monolayers present isotropic behaviors, while the values of SL1 and SL2 monolayers reveal weak anisotropic behaviors. It can be observed that the k_ph values of WS₂ and WSe₂ monolayers are larger than those of SL1 and SL2 monolayers, which can be attributed to the decreasing phonon group velocity and phonon lifetime. Moreover, we calculate the electronic band structures of all monolayers, indicating that all monolayers are semiconductors. Afterwards, the electrical conductivities, the Seebeck coefficients, the power factors, the electronic thermal conductivities, and the ZT values at different temperatures are evaluated. The ZT_max values of WS₂, WSe₂, SL1, and SL2 monolayers with p-type doping are 0.43, 0.37, 0.95, and 0.66 at 1000 K. It can be proved that the SL1 monolayer possesses the largest ZT, which is at least two times higher than those of the WS₂ and WSe₂ monolayer. Finally, we build two kinds of phononic crystals with periodic holes (PCH1 and PCH2) and evaluate the thermoelectric properties. It can be observed that the PCH2 structure shows the best thermoelectric performance. The ZT_max values of the PCH2 structure can reach 2.53 and 4.54 with p-type doping along the x and y directions, which are 2.66 and 6.75 times higher than those of the SL1 monolayer. This work provides a new strategy to obtain higher thermoelectric performance and demonstrates the potential applications of phononic crystals in TMDC-based nanoelectronic devices.

Graphene-carbon nitride interface-geometry effects on thermal rectification: a molecular dynamics simulation

In this paper we expand our previous study on phonon thermal rectification (TR) exhibited in a hybrid graphene-carbon nitride system (G-C3N) to investigate the system's behavior under a wider range of temperature differences, between the two employed baths, and the effects of media-interface geometry on the rectification factor. Our simulation results reveal a sigmoid relation between TR and temperature difference, with a sample-size depending upper asymptote occurring at generally large temperature differences. The achieved TR values are significant and go up to around 120% for ΔT = 150 K. Furthermore, the consideration of varying media-interface geometries yields a non-negligible effect on TR 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 TR.

Disorder limits the coherent phonon transport in two-dimensional phononic crystal structures

Recently, increasing efforts are being made to control thermal transport via coherent phonons in periodic phononic structures; however, the direct observation of coherent phonon transport is experimentally very difficult at ambient temperature, and the importance of coherent phonons to the total thermal conductivity has not been critically assessed to date. In this study, using the non-equilibrium molecular dynamics simulations, we studied coherent phonon transport in a C3N phononic crystal (CNPnC) structure at room temperature by changing the porosity. When the holes were randomly distributed to construct the disordered C3N (D-C3N) structure, the localization of the coherent phonons was revealed by the phonon transmission coefficient, phonon wave packet simulation, phonon participation ratio and spatial energy density, which led to a significant reduction in the thermal conductivity. Finally, the effects of the length, temperature and strain on the thermal conductivity of CNPnC and D-C3N have also been discussed. Our study provides a solid understanding of the coherent phonon transport behavior, which will be beneficial for phononic-related control based on coherent phonons.

Randomness-Induced Phonon Localization in Graphene Heat Conduction

Through nonequilibrium molecular dynamics simulations, we report the direct numerical evidence of the coherent phonons participating in thermal transport at room temperature in graphene phononic crystal (GPnC) structure and evaluate their contribution to thermal conductivity based on the two-phonon model. With decreasing period length in GPnC, the transition from the incoherent to coherent phonon transport is clearly observed. When a random perturbation to the positions of holes is introduced in a graphene sheet, the phonon wave-packet simulation reveals the presence of notable localization of coherent phonons, leading to the significant reduction of thermal conductivity and suppressed length dependence. Finally, the effects of period length and temperature on the coherent phonon contribution to thermal conductivity are also discussed. Our work establishes a deep understanding of the coherent phonons transport behavior in periodic phononic structures, which provides effective guidance for engineering thermal transport based on a new path via phonon localization.

Generalized Two-Temperature Model for Coupled Phonons in Nanosized Graphene

The design of graphene-based composite with high thermal conductivity requires a comprehensive understanding of phonon coupling in nanosized graphene. We extended the two-temperature model to coupled groups of phonons. The study give new physical quantities, the phonon-phonon coupling factor and length, to characterize the couplings quantitatively. Besides, our proposed coupling length has an obvious dependence on system size. Our studies can not only observe the nonequilibrium between different groups of phonons but explain theoretically the thermal resistance inside nanosized graphene.

Experimental study of thermal rectification in suspended monolayer graphene

Thermal rectification is a fundamental phenomenon for active heat flow control. Significant thermal rectification is expected to exist in the asymmetric nanostructures, such as nanowires and thin films. As a one-atom-thick membrane, graphene has attracted much attention for realizing thermal rectification as shown by many molecular dynamics simulations. Here, we experimentally demonstrate thermal rectification in various asymmetric monolayer graphene nanostructures. A large thermal rectification factor of 26% is achieved in a defect-engineered monolayer graphene with nanopores on one side. A thermal rectification factor of 10% is achieved in a pristine monolayer graphene with nanoparticles deposited on one side or with a tapered width. The results indicate that the monolayer graphene has great potential to be used for designing high-performance thermal rectifiers for heat flow control and energy harvesting.

Thermal rectification is instrumental to achieving active heat flow control and energy harvesting in nanoscale devices. Here, the authors demonstrate thermal rectification in asymmetric graphene nanostructures, achieving a large rectification factor up to 26%.

A Series Circuit of Thermal Rectifiers: An Effective Way to Enhance Rectification Ratio

A novel approachis proposed to enhance the thermal rectification ratio, namely, arranging two thermal rectifiers in series. Through theoretical analysis and molecular dynamics simulations on graphene/phononic crystal structures, the results show that the series thermal rectifiers enhance thermal rectification ratio significantly, compared to a single rectifier. Meanwhile, the results of theoretical prediction match well with simulation results.