The effect of temperature oscillations on energy storage rectification in harmonic systems.

Rectification, the preferential transport of a current in one direction through a system, has garnered significant attention in molecules because of its importance for controlling thermal and electronic currents at the nanoscale. Here, we report the presence of energy storage rectification effects in a molecular chain. This phenomenon is generated by subjecting a harmonic molecular chain to an oscillating temperature gradient and showing that the energy absorption rate of the system depends on the direction of the gradient. We examine how the energy storage rectification ratios in the chain are affected by the oscillating gradient, asymmetry in the chain, and the system parameters. We find that energy storage rectification can be observed in harmonic lattice structures with time-dependent temperatures and that, correspondingly, anharmonicity is not required to generate this rectification mechanism in such systems.

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.

Room temperature thermal rectification in suspended asymmetric graphene ribbon.

Thermal rectifiers are essential in optimizing heat dissipation in solid-state devices to enhance energy efficiency, reliability, and overall performance. In this study, we experimentally investigate the thermal rectification phenomenon in suspended asymmetric graphene ribbons (GRs). The asymmetry within the graphene is introduced by incorporating periodic parallel nanoribbons on one side of the GR while maintaining the other side in a pristine form. Our findings reveal a substantial thermal rectification effect in these asymmetric graphene devices, reaching up to 45% at room temperature and increasing further at lower environmental temperatures. This effect is attributed to a significant thermal conductivity contrast between pristine graphene and nanoribbon graphene within the asymmetric structure. We observe that the incorporation of nanoribbons leads to a notable reduction in thermal conductivity, primarily due to phonon scattering and bottleneck effects near the nanoribbon edges. These findings suggest that graphene structures exhibiting asymmetry, facilitated by parallel nanoribbons, hold promise for effective heat management at the nanoscale level and the development of practical phononic devices.

Thermal-Rectified Gradient Porous Polymeric Film for Solar-Thermal Regulatory Cooling.

Solar-thermal regulation concerning thermal insulation and solar modulation is pivotal for cooling textiles and smart buildings. Nevertheless, a contradiction arises in balancing the demand to prevent external heat infiltration with the efficient dissipation of excess heat from enclosed spaces. Here, a concentration-gradient polymerization strategy is presented for fabricating a gradient porous polymeric film comprising interconnected polymeric microspheres. This method involves establishing an electric field-driven gradient distribution of charged crosslinkers in the precursor solution, followed by subsequent polymerization and freeze-drying processes. The resulting porous film exhibits a significant porosity gradient along its thickness, leading to exceptional unidirectional thermal insulation capabilities with a thermal rectification factor of 21%. The gradient porous film, with its thermal rectification properties, effectively reconciles the conflicting demands of diverse thermal conductivity for cooling unheated and spontaneously heated enclosed spaces. Consequently, the gradient porous film demonstrates remarkable enhancements in solar-thermal management, achieving temperature reductions of 3.0 and 4.1 °C for unheated and spontaneously heated enclosed spaces, respectively, compared to uniform porous films. The developed gradient-structured porous film thus holds promise for the development of thermal-rectified materials tailored to regulate solar-thermal conditions within enclosed environments.

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 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 Rectification in Modularly Designed Bulk Metamaterials.

Thermal rectification is a phenomenon of great practical importance where heat transfer is preferential in one direction. Programmable control of heat transfer in 3D space is key to enable thermal rectification at the macroscale but is rarely realized in natural materials or in current existing devices that are constructed at the nano and microscales with high system complexity. Here, modularly designed bulk metamaterials that can break the symmetry of heat transfer from one direction to the other are created, leading to thermal rectification in convergent or divergent states by tuning the metamaterial microstructural design. These thermal metamaterials are microstructured composites made using one material composition, however, they offer sufficient microstructural design freedom to allow tunable local thermal properties for unusual macroscopic heat transfer. The strategy and performance achieved are promising for next-generation thermal management.

Significant thermal rectification induced by phonon mismatch of functional groups in a single-molecule junction.

Single-molecule junctions (SMJs) may bring exotic physical effects. In this work, a significant thermal rectification effect is observed in a cross-dimensional system, comprising a diamond, a single-molecule junction, and a carbon nanotube (CNT). The molecular dynamics simulations indicate that the interfacial thermal resistance varies with the direction of heat flow, the orientation of the crystal planes of the diamond, and the length of the CNT. We find that the thermal rectification ratio escalates with the length of the CNT, achieving a peak value of 730% with the CNT length of 200 nm. A detailed analysis of phonon vibrations suggests that the primary cause of thermal rectification is the mismatched vibrations between the biphenyl and carbonyl groups. This discovery may offer theoretical insights for both the experimental exploration and practical application of SMJs in efficient thermal management strategy for high power and highly integrated chips.

Thermal rectification in mass-asymmetric one-dimensional anharmonic oscillator lattices with and without a ballistic spacer.

In this work we perform a systematic analysis of various structural parameters that have influence on the thermal rectification effect, i.e. asymmetrical heat flow, and the negative differential thermal resistance -reduction of the heat flux as the applied thermal bias is increased- present in a one-dimensional, segmented mass-graded system consisting of a coupled nearest-neighbor harmonic oscillator lattice (ballistic spacer) and two diffusive leads (modeled by a substrate potential) attached to the lattice at both boundaries. At variance with previous works, we consider the size of the spacer as smaller than that of the leads. Also considered is the case where the leads are connected along the whole length of the oscillator lattice; that is, in the absence of the ballistic spacer. Upon variation of the system's parameters it was determined that the performance of the device, as quantified by the spectral properties, is largely enhanced in the absence of the ballistic spacer for the small system-size limit herein considered.

Thermal rectification in ultra-narrow hydrogen functionalized graphene: a non-equilibrium molecular dynamics study.

In this study, the non-equilibrium molecular dynamics simulation (NEMD) has been used to evaluate the thermal properties, especially the rectification of ultra-narrow edge-functionalized graphene with hydrogen atoms. The system's small width equals 4.91 Å (equivalent to two hexagonal rings). The dependence of the thermal rectification on the mean temperature, hydrogen concentration, and temperature difference between the two baths was investigated. Results reveal that the thermal rectification increases to 100% at 550 K by increasing the mean temperature. Also, it is disclosed that hydrogen concentration plays a vibrant role in thermal rectification. As a result of maximum phonon scattering at the interface, a thorough rectification is obtained in a half-fully hydrogenated system. As well, the effects of temperature difference of baths ΔT on thermal rectification has been calculated. As a result, the thermal rectification decreases even though the current heat increases with ΔT. Finally, the thermal resistance at the interface using a mismatching factor between the two-phonon density of states (DOS) on both sides of the interface has been explained.

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.

Phosphorene grain boundary effect on phonon transport and phononic applications.

Grain boundaries (GBs) widely exist in black phosphorene (BP), which plays a vital role in determining the properties of 2D materials. Significant GB effect on the thermal boundary resistance in BP structures is found by using molecular dynamics calculations and lattice dynamic analysis. A remarkably high interface thermal resistance is observed. By analyzing the strain distribution and phonon vibrational spectra, we reveal this high thermal resistance originates from phonon localization and strong phonon boundary scattering induced by the local stress at the GB area. Particularly, it is interesting to find that the partial phonon modes display weak localization when GBs present. The fraction of atoms participating in a particular phonon vibrational mode has been quantified through the calculation of phonon participation ratio. In addition, the thermal boundary resistance is found size-dependent, which further induces interesting thermal rectification effect in the BP structures. A high rectification ratio is obtained by adjusting the structural length and temperature bias. These findings provide a through insight into the GB effects on individual phonon mode transmission across the GBs, and highlight that the GB effect is an important factor and should be taken into account for the applications of BP-based phononic devices.

Thermal Rectifier and Thermal Transistor of 1T/2H MoS2 for Heat Flow Management.

Thermal rectifiers and thermal transistors are expected to be widely used for efficient thermal management and energy cascade utilization due to their excellent directional thermal management. Two-dimensional micro/nano materials have huge potential in the applications of thermal transistors, thermal logic circuits, and thermal rectifiers owing to the phase transition and thermal rectification phenomenon. Herein, a lithium intercalation method was used to transform 2H-MoS2 into the 1T phase with a purity of 76%, and a suspended microelectrode was applied to measure the thermal conductivity and thermal rectification coefficient of the same MoS2 film with 1T and 2H phases in suit. The thermal conductivity and thermal rectification effect of two-phase MoS2 couple with its phase state and structure were also obtained. The results demonstrate that the thermal conductivities of MoS2 in both 1T and 2H phases decrease with increasing temperature. It is also found that the thermal rectification coefficient has no obvious dependence on the temperature and phase change but the asymmetric structure. Furthermore, a thermal rectifier and transistor with a high thermal rectification effect are designed. The direction and magnitude of heat flow through the samples can be effectively controlled and managed by adjusting the phase, size, and structural asymmetry of the different samples. The maximum thermal rectification coefficient of the thermal rectifiers is up to 0.8.

Effect of Inter-System Coupling on Heat Transport in a Microscopic Collision Model.

In this paper we consider a bipartite system composed of two subsystems each coupled to its own thermal environment. Based on a collision model, we mainly study whether the approximation (i.e., the inter-system coupling is ignored when modeling the system-environment interaction) is valid or not. We also address the problem of heat transport unitedly for both excitation-conserving system-environment interactions and non-excitation-conserving system-environment interactions. For the former interaction, as the inter-system interaction strength increases, at first this approximation gets worse as expected, but then counter-intuitively gets better even for a stronger inter-system coupling. For the latter interaction with asymmetry, this approximation gets progressively worse. In this case we realize a perfect thermal rectification, and we cannot find an apparent rectification effect for the former interaction. Finally and more importantly, our results show that whether this approximation is valid or not is closely related to the quantum correlations between the subsystems, i.e., the weaker the quantum correlations, the more justified the approximation and vice versa.

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.

Small-Nanostructure-Size-Limited Phonon Transport within Composite Films Made of Single-Wall Carbon Nanotubes and Reduced Graphene Oxides.

Nanocarbon materials have been widely used for nanoelectronics and energy-related applications. In this work, composite films consisting of reduced graphene oxides (rGOs) and single-wall carbon nanotubes (SWCNTs) are synthesized and studied for their in-plane thermal conductivities. Different from pristine carbon nanotubes or graphene with decreased thermal conductivities above 300 K, the in-plane thermal conductivities of these composite films are found to follow the trend of the specific heat of graphene from 100 to 400 K, i.e., monotonously increasing at elevated temperatures. Such a trend can often be found within amorphous solids but has seldom been observed for nanocarbon. This unique temperature dependence of thermal conductivities is attributed to the largely restricted phonon mean free paths within the graphene sheets that mainly contribute to the in-plane thermal transport. The highest in-plane thermal conductivity among samples with different synthesis conditions is 62.8 W/(m·K) at 300 K. Such a high thermal conductivity, combined with its unique temperature dependency, can be ideal for applications such as flexible film-like thermal diodes based on the junction between two materials with a large contrast for their temperature dependence of the thermal conductivity.

Experimental Study on Thermal Conductivity and Rectification in Suspended Monolayer MoS2.

Thermal rectification is an attractive phenomenon for thermal management, which refers to a specific behavior in a heat transfer system where heat flow in one direction is stronger than that in the opposite direction under the same conditions. Two-dimensional monolayer molybdenum disulfide (MoS2) synthesized by chemical vapor deposition (CVD) has exhibited exceptional thermal, optical, and electrical properties due to its special structure; however, the thermal rectification in monolayer MoS2 is still not achieved by experimental measurement. Here, we successfully transferred monolayer MoS2 samples with three geometrical morphologies to the suspended microelectrodes by the PMMA approach. Through further heating the suspended microelectrodes with AC power in the opposite directions of these three monolayer MoS2 samples, we experimentally measured the thermal conductivity and first obtained the thermal rectification of monolayer MoS2. The rectification coefficients of monolayer MoS2 with three different geometrical morphologies are 10-13, 11-4, and 69-70%. Moreover, a theoretical model was also applied to discuss the dependence of thermal rectification on the geometrical asymmetry (angle and spacing). The results demonstrate that the monolayer MoS2 has an obvious thermal rectification phenomenon owing to the asymmetric structure, and it would have great potentials in the application of thermal energy control and management.

Thermal Rectification in Asymmetric Graphene/Hexagonal Boron Nitride van der Waals Heterostructures.

Graphene/hexagonal boron nitride (h-BN) heterostructures assembled by van der Waals (vdW) interactions show numerous unique physical properties such as quantum Hall effects and exotic correlated states, which have promising potential applications in the design of novel electronic devices. Understanding thermal transport in such junctions is critical to control the performance and stability of prospective nanodevices. In this work, using nonequilibrium molecular dynamics simulations, we systematically investigate the thermal transport in asymmetric graphene/h-BN vdW heterostructures. It is found that the heat prefers to flow from the monolayer to the multilayer regions, resulting in a significant thermal rectification (TR) effect. To determine the optimum conditions for TR, the influences of sample length, defect density, asymmetric degree, ambient temperature, and vdW interaction strength are studied. Particularly, we found that the TR ratio could be improved by about 1 order of magnitude via increasing the coupling strength from 1 to 10, which clearly distinguishes from the commonly held notion that the TR ratio is practically insensitive or even decreasing with the interaction strength. Detailed spectral analysis reveals that this unexpected increase of the TR ratio can be attributed to heavily modified phonon properties of encased graphene due to enhanced interlayer coupling. Our results elucidate the importance of vdW interactions to heat conduction in nanostructures.

A Thermal Diode Based on Nanoscale Thermal Radiation.

In this work we demonstrate thermal rectification at the nanoscale between doped Si and VO2 surfaces. Specifically, we show that the metal-insulator transition of VO2 makes it possible to achieve large differences in the heat flow between Si and VO2 when the direction of the temperature gradient is reversed. We further show that this rectification increases at nanoscale separations, with a maximum rectification coefficient exceeding 50% at ∼140 nm gaps and a temperature difference of 70 K. Our modeling indicates that this high rectification coefficient arises due to broadband enhancement of heat transfer between metallic VO2 and doped Si surfaces, as compared to narrower-band exchange that occurs when VO2 is in its insulating state. This work demonstrates the feasibility of accomplishing near-field-based rectification of heat, which is a key component for creating nanoscale radiation-based information processing devices and thermal management approaches.

Significantly High Thermal Rectification in an Asymmetric Polymer Molecule Driven by Diffusive versus Ballistic Transport.

Tapered bottlebrush polymers have novel nanoscale polymer architecture. Using nonequilibrium molecular dynamics simulations, we showed that these polymers have the unique ability to generate thermal rectification in a single polymer molecule and offer an exceptional platform for unveiling different heat conduction regimes. In sharp contrast to all other reported asymmetric nanostructures, we observed that the heat current from the wide end to the narrow end (the forward direction) in tapered bottlebrush polymers is smaller than that in the opposite direction (the backward direction). We found that a more disordered to less disordered structural transition within tapered bottlebrush polymers is essential for generating nonlinearity in heat conduction for thermal rectification. Moreover, the thermal rectification ratio increased with device length, reaching as high as ∼70% with a device length of 28.5 nm. This large thermal rectification with strong length dependence uncovered an unprecedented phenomenon-diffusive thermal transport in the forward direction and ballistic thermal transport in the backward direction. This is the first observation of radically different transport mechanisms when heat flow direction changes in the same system. The fundamentally new knowledge gained from this study can guide exciting research into nanoscale organic thermal diodes.