Spectral mapping of heat transfer mechanisms at liquid-solid interfaces

Unraveling the Regimes of Interfacial Thermal Conductance at a Solid/Liquid Interface

The interfacial thermal conductance at a solid/liquid interface (G) exhibits an exponential-to-linear crossover with increasing solid/liquid interaction strength, previously attributed to the relative strength of solid/liquid to liquid/liquid interactions. Instead, using a simple Lennard-Jones setup, our molecular simulations reveal that this crossover occurs due to the onset of solidification in the interfacial liquid at high solid/liquid interaction strengths. This solidification subsequently influences interfacial energy transport, leading to the crossover in G. We use the overlap between the spectrally decomposed heat fluxes of the interfacial solid and liquid to pinpoint when "solid-like energy transport" within the interfacial liquid emerges. We also propose a novel decomposition of G into (i) the conductance right at the solid/liquid interface and (ii) the conductance of the nanoscale interfacial liquid region. We demonstrate that the rise of solid-like energy transport within the interfacial liquid influences the relative magnitude of these conductances, which in turn dictates when the crossover occurs. Our results can aid engineers in optimizing G at realistic interfaces, critical to designing effective cooling solutions for electronics among other applications.

Phonon dynamic behaviors induced by amorphous layers at heterointerfaces

An amorphous layer is commonly found at the interfaces of heterostructures due to lattice and thermal mismatch between dissimilar materials. While existing research has explored the impact of these layers on interfacial thermal transport, a comprehensive understanding of the underlying microscopic mechanisms remains essential for advancing thermal nanodevice development. Through phonon wave packet simulations, we investigated the dynamic behaviors of phonons crossing the amorphous interlayer at the GaN/AlN interface from the mode level. Our results highlight the amorphous layer's capability to notably adjust the polarization properties of incoming phonons, culminating in phonon localization. By examining transmission outcomes on a per-mode basis, we demonstrate the amorphous layer's impediment on phonon transport. Notably, this resistance escalates with an increase in the amorphous layer thickness (L), with certain high-frequency TA phonons showing unexpectedly high transmissivity due to polarization conversion and inelastic scattering at the amorphous interface. In addition, we observe that the amorphous layer prompts multiple reflections of incident phonons, instigating discernible from the two-beam interference equation. Finally, in pursuit of enhanced phonon transport, we employ annealing techniques to optimize the interface morphology, leading to the recrystallization of the amorphous layer. This optimization yields a substantial enhancement of interfacial thermal conductance by up to 38% for L = 3 nm.

Temperature-dependent differential capacitance of an ionic liquid-graphene-based supercapacitor

One of the critical factors affecting the performance of supercapacitors is thermal management. The design of supercapacitors that operate across a broad temperature range and at high charge/discharge rates necessitates understanding the correlation of the molecular characteristics of the device (such as interfacial structure and inter-ionic and ion-electrode interactions) with its macroscopic properties. In this study, we use molecular dynamics (MD) simulations to investigate the influence of Joule heating on the structure and dynamics of the ionic liquid (IL)/graphite-based supercapacitors. The temperature-dependent electrical double layer (EDL) and differential capacitance-potential (CD-V) curves of two different ([Bmim][BF4] and [Bmim][PF6]) IL-graphene pairs were studied under various thermal gradients. For the [Bmim][BF4] system, the differential capacitance curves transition from 'U' to bell shape under an applied thermal gradient (∇T) in the range from 3.3 K nm-1 to 16.7 K nm-1. Whereas in [Bmim][PF6], we find a positive dependence of differential capacitance with ∇T with a U-shaped CD-V curve. We examine changes in the EDL structure and screening potential (ϕ(z)) as a function of ∇T and correlate them with the trends observed in the CD-V curve. The identified correlation between the interfacial charge density and differential capacitance with thermal gradient would be helpful for the molecular design of the IL-electrode interface in supercapacitors or other chemical engineering applications.

Nanoscale Energy Transport Dynamics across Nonbonded Solid-Molecule Interfaces and in Molecular Thin Films

Thermal conductance across a solid-solid interface requires an atomic- or molecular-level understanding, especially when a system is in a non-equilibrium state and/or consists of nanosized materials with prominent differences in structures, properties, and vibrational behaviors. Here, we report the lattice dynamics of graphite-supported molecular thin films of ethanol, whose layers exhibit in-plane hydrogen-bonded chains and out-of-plane van der Waals stacking with clear structural anisotropy. The direct structure-probing method of ultrafast electron diffraction reveals a surprising temperature difference of more than 400 K at pico- to sub-nanosecond times across the graphite-ethanol interface, yet the temporal behavior signifies a reasonably large thermal boundary conductance. This apparent conflict in a non-equilibrium condition can be resolved by considering the coupling of out-of-plane motions, instead of the commonly used temperature-based model, at transient times for energy transport across the interface separated by van der Waals interactions with mismatched unit sizes and no strong bonds. The importance of spatiotemporally resolved structural dynamics at the atomic or molecular level is emphasized.

Molecular Dynamics Simulation on the Heat Transfer in the Cross-Linked Poly(dimethylsiloxane)

In this work, the effect of cross-linking degree and stretching on the thermal conductivity of poly(dimethylsiloxane) (PDMS) is explored by performing a molecular dynamics simulation. Our results demonstrate that the thermal conductivity of PDMS exhibits a monotonous rise with an increase in the cross-linking degree. By decomposing the total heat flux into three microscopic heat transfer modes, the high cross-linking degree improves the contribution from bonding interactions to the heat transfer more than that from the nonbonding interactions. An analysis of the vibrational density of states shows a blue-shift of the vibrational modes at low frequencies, indicating a large phonon group velocity due to the strong interchain bonding interaction. From the spectral distribution of heat flux, the spectral contributions are shifted toward the higher frequencies with the increasing cross-linking degree, which reflects more contribution from the high-frequency modes to the heat transfer. Stretching can improve the thermal conductivity parallel to the tensile direction with the increase in strain. This is mainly due to the further improved contribution of bonding interactions or high-frequency modes to heat transfer. Interestingly, the anisotropy of the thermal conductivity first decreases and then increases with the increasing cross-linking degree. Our study conducts a detailed investigation of the thermal conductivity of cross-linked PDMS, providing guidance on the application of thermal interface 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.

Ultrafast and Nanoscale Energy Transduction Mechanisms and Coupled Thermal Transport across Interfaces

The coupled interactions among the fundamental carriers of charge, heat, and electromagnetic fields at interfaces and boundaries give rise to energetic processes that enable a wide array of technologies. The energy transduction among these coupled carriers results in thermal dissipation at these surfaces, often quantified by the thermal boundary resistance, thus driving the functionalities of the modern nanotechnologies that are continuing to provide transformational benefits in computing, communication, health care, clean energy, power recycling, sensing, and manufacturing, to name a few. It is the purpose of this Review to summarize recent works that have been reported on ultrafast and nanoscale energy transduction and heat transfer mechanisms across interfaces when different thermal carriers couple near or across interfaces. We review coupled heat transfer mechanisms at interfaces of solids, liquids, gasses, and plasmas that drive the resulting interfacial heat transfer and temperature gradients due to energy and momentum coupling among various combinations of electrons, vibrons, photons, polaritons (plasmon polaritons and phonon polaritons), and molecules. These interfacial thermal transport processes with coupled energy carriers involve relatively recent research, and thus, several opportunities exist to further develop these nascent fields, which we comment on throughout the course of this Review.

A Critical Review of Thermal Boundary Conductance across Wide and Ultrawide Bandgap Semiconductor Interfaces

The emergence of wide and ultrawide bandgap semiconductors has revolutionized the advancement of next-generation power, radio frequency, and opto- electronics, paving the way for chargers, renewable energy inverters, 5G base stations, satellite communications, radars, and light-emitting diodes. However, the thermal boundary resistance at semiconductor interfaces accounts for a large portion of the near-junction thermal resistance, impeding heat dissipation and becoming a bottleneck in the devices' development. Over the past two decades, many new ultrahigh thermal conductivity materials have emerged as potential substrates, and numerous novel growth, integration, and characterization techniques have emerged to improve the TBC, holding great promise for efficient cooling. At the same time, numerous simulation methods have been developed to advance the understanding and prediction of TBC. Despite these advancements, the existing literature reports are widely dispersed, presenting varying TBC results even on the same heterostructure, and there is a large gap between experiments and simulations. Herein, we comprehensively review the various experimental and simulation works that reported TBCs of wide and ultrawide bandgap semiconductor heterostructures, aiming to build a structure-property relationship between TBCs and interfacial nanostructures and to further boost the TBCs. The advantages and disadvantages of various experimental and theoretical methods are summarized. Future directions for experimental and theoretical research are proposed.

Effects of interfacial molecular mobility on thermal boundary conductance at solid-liquid interface

The effects of interfacial molecular mobility on the thermal boundary conductance (TBC) across graphene-water and graphene-perfluorohexane interfaces were investigated using non-equilibrium molecular dynamics simulations. The molecular mobility was varied by equilibrating nanoconfined water and perfluorohexane at different temperatures. The long-chain molecules of perfluorohexane exhibited a prominent layered structure, indicating a low molecular mobility, over a wide temperature range between 200 and 450 K. Alternatively, water increased its mobility at high temperatures, resulting in an enhanced molecular diffusion that significantly contributed to the interfacial thermal transport, in addition to the increasing vibrational carrier population at high temperatures. Furthermore, the TBC across the graphene-water interface exhibited a quadratic relationship with the rise in temperature, whereas for the graphene-perfluorohexane interface, a linear relationship was observed. The high rate of diffusion in interfacial water facilitated additional low-frequency modes, and a spectral decomposition of the TBC also indicated an enhancement in the same frequency range. Thus, the enhanced spectral transmission and higher molecular mobility of water with respect to perfluorohexane explained the difference in the thermal transport across the interfaces considered herein.

Reducing interfacial thermal resistance by interlayer

Heat dissipation is crucial important for the performance and lifetime for highly integrated electronics, Li-ion battery-based devices and so on, which lies in the decrease of interfacial thermal resistance (ITR). To achieve this goal, introducing interlayer is the most widely used strategy in industry, which has attracted tremendous attention from researchers. In this review, we focus on bonding effect and bridging effect to illustrate how introduced interlayer decreases ITR. The behind mechanisms and theoretical understanding of these two effects are clearly illustrated. Simulative and experimental studies toward utilizing these two effects to decrease ITR of real materials and practical systems are reviewed. Specifically, the mechanisms and design rules for the newly emerged graded interlayers are discussed. The optimization of interlayers by machine learning algorithms are reviewed. Based on present researches, challenges and possible future directions about this topic are discussed.

Effects of Electrostatic Interactions on Kapitza Resistance in Hexagonal Boron Nitride-Water Interfaces

Electrostatic interactions in nanoscale systems can influence the heat transfer mechanism and interfacial properties. This study uses molecular dynamics simulations to investigate the impact of various electrostatic interactions on the Kapitza resistance (R_k) on a hexagonal boron nitride-water system. The Kapitza resistance at hexagonal boron nitride nanotube (hBNNT)-water interface reduces with an increase in diameter of the nanotube due to more aggregation of water molecules per unit surface area. An increase in the partial charges on boron and nitride caused the reduction in R_k. With the increase in partial charge, a better hydrogen bonding between hBNNT and water was observed, whereas the structure and order of the water molecules remain the same. Nevertheless, the addition of NaCl salt into water does not have any influence on interfacial thermal transport. R_k remains unchanged with electrolyte concentration because the cumulative Coulombic interaction between the ions and the hBNNT is significantly less when compared with water molecules. Furthermore, the effect of electric field strength on interfacial heat transfer is also investigated by providing uniform positive and negative surface charges on the outermost hBN layers. R_k is nearly independent of the practical range of applied electric fields and decreases with an increasing electric field for extreme field strengths until the electrofreezing phenomenon occurs. The ordering of water molecules toward the charged surface leads to an increase in the layering effect, causing the reduction in R_k in the presence of an electric field.

Size-dependent effects of the thermal transport at gold nanoparticle-water interfaces

The transfer of heat from a plasmonic nanoparticle to its water environment has numerous applications in the fields of solar energy conversion and photothermal therapies. We use non-equilibrium molecular dynamics to investigate the size-dependent effects of the interfacial thermal conductance of gold nanoparticles immersed in water and of tunable wettability. The interfacial thermal conductance is found to increase when the nanoparticle size decreases. We rationalize such a behavior with a generalized acoustic model, where the interfacial bonding decreases with the nanoparticle size. The analysis of the interfacial thermal spectrum reveals the importance of the low frequency peak of the nanoparticle spectrum as it matches relatively well the oxygen peak in the vibrational spectrum. However, by reducing the nanoparticle size, the low frequency peak is exacerbated, explaining the enhanced heat transfer observed for small nanoparticles. Finally, we assess the accuracy of continuum heat transferequations to describe the thermal relaxation of small nanoparticles with initial high temperatures.We show that, before the nanoparticle looses its integrity, the continuum model succeed in describing with small percentage deviations the molecular-dynamics data. This work brings a simple methodology to understand, beyond the plasmonic nanoparticles, thermal boundary conductance between a nanopartice and its environment.

Thermal and rheological properties of magnetic nanofluids: Recent advances and future directions

Technological advancement and miniaturization of electronic gadgets fueled intense research on nanofluids as potential candidates for cooling applications as a substitute to conventional heat transfer fluids. Among nanofluids, magnetic nanofluids, traditionally known as ferrofluids have attracted a lot of attention owing to their magnetic field tunable thermal conductivity and rheological properties due to the aggregation of the magnetic nanoparticles into chains or columns in the presence of the magnetic field. The field-induced aggregates act as low resistance pathways thereby improving thermal transport substantially. Recent studies show that ferrofluids with smaller size and narrow size distribution display significant enhancement in thermal conductivity in the presence of a magnetic field with negligible viscosity enhancement, which is ideal for effective thermal management of electronic devices, especially in miniature electronic devices. On the contrary, highly polydisperse ferrofluids containing large aggregates, show modest enhancement in thermal conductivity in the presence of a magnetic field and a huge enhancement in viscosity. The most recent studies show that magnetic field ramp rate has a profound effect on aggregation kinetics and thermal and rheological properties. The viscosity enhancement under an external stimulus impedes their practical use in electronics cooling, which warrants the need to attain a high thermal conductivity to viscosity ratio, under a modest magnetic field. Though there are several reviews on heat transfer in nanofluids and hybrid nanofluids, a comprehensive review on fundamental understanding of field-induced thermal and rheological properties in magnetic fluids is missing in the literature. This review provides a pedagogical description of the fundamental understanding of field-induced thermal and rheological properties in magnetic fluids, with the necessary background, key concepts, definitions, mechanisms, theoretical models, experimental protocols, and design of experiments. Many important case studies are presented along with the experimental design aspects. The review also provides a summary of important experimental studies with key findings, along with the key challenges and future research directions. The review is an ideal material for experimentalists and theoreticians practicing in the field of magnetic fluids, and also serves as an excellent reference for freshers who indent to begin research on this topic.

Thermal transport mechanism at a solid-liquid interface based on the heat flux detected at a subatomic spatial resolution

Heat flux is a fundamental quantity in thermal science and engineering and is essential for understanding thermal transport phenomena. In this study, the heat flux in a solid-liquid interfacial region is obtained in a three-dimensional (3D) space at a subatomic spatial resolution based on classical molecular dynamics, yielding a 3D structure of the heat flux between the solid and liquid layers in contact. The results using the Lennard-Jones potential reveal the directional qualities of the heat flux, which are significantly dependent on the subatomic stresses in the structures of condensed phase systems. The heat flux and stress at the subatomic scale are related to the macroscopic transport quantities, which can be obtained using distribution functions; the stress and energy flux properties at the subatomic scale are comprehensively investigated using a single-interaction-based stress and energy flux to determine the origin of the thermal transport mechanism at the solid-liquid interface. The findings reveal that the density of states due to the stress caused by a single interaction exhibits a bandlike behavior. The net energy transport comprises positive and negative energy transport inside and outside the band. In addition, this is related to the intrinsic transport property of the atoms and molecules at the solid-liquid interface at the subatomic scale. The difference between the energy transport rates when a solid atom in the vicinity of the interface is near to or far from the liquid phase is the origin of the energy transport mechanism at the solid-liquid interface. 3D analysis of the heat flux and stress is carried out by varying the interaction strengths between the liquid molecules and solid atoms at the solid-liquid interface. This reveals that the directional quality of transport quantities is high at strong interaction strengths, thus indicating enhanced thermal transport. Furthermore, the influence of the temperature gradient in the system suggests that the energy transport imbalance between inside and outside the stress band in a high-stress field at the subatomic scale induces the net thermal transport across the interface in the nonequilibrium state.

The interfacial thermal conductance spectrum in nonequilibrium molecular dynamics simulations considering anharmonicity, asymmetry and quantum effects

The thermal conductance spectrum from the left interfacial Hamiltonian can be different from that of the right counterpart, which stems from the asymmetry of anharmonic phonon scatterings.

Reducing Kapitza resistance between graphene/water interface via interfacial superlattice structure

The control of thermal transport across solid/liquid interface has attracted great interests for efficient thermal management in the integrated devices. Based on molecular dynamics simulations, we study the effect of interfacial superlattice structure on the Kapitza resistance between graphene/water interface. Compared to the original interface, introducing interfacial superlattice structure can result in an obvious reduction of Kapitza resistance by as large as 40%, exhibiting a decreasing trend of Kapitza resistance with the decrease of superlattice period. Surprisingly, by analyzing the structure of water block and atomic vibration characteristics on both sides of the interface, we find the interfacial superlattice structure has a minor effect on the water structure and overlap in the vibrational spectrum, suggesting that the improved interfacial heat transfer is not mainly originated from the liquid block. Instead, the spectral energy density analysis reveals that phonon scattering rate in the interfacial graphene layer is significantly enhanced after superlattice decoration, giving rise to the increased thermal resistance between the interfacial graphene layer and its nearest neighboring layer. As this thermal resistance is coupled to the Kapitza resistance due to the local nature of interfacial superlattice decoration, the enhanced thermal resistance in the solid segment indirectly reduces the Kapitza resistance between graphene/water interface, which is supported by the enhancement of the spectral interfacial thermal conductance upon superlattce decoration at microscopic level. Our study uncovers the physical mechanism for controlling heat transfer across solid/liquid interface via interfacial superlattice structure, which might provide valuable insights for designing efficient thermal interfaces.

The effect of interface angle on the thermal conductivity of Si/Ge superlattices

Si/Ge superlattices (SLs) are good candidates for thermoelectric materials because of their remarkable thermal insulating performance compared with their bulk counterparts. In this paper, the non-equilibrium molecular dynamics (NEMD) simulation method was applied to investigate the thermal conductivity of Si/Ge SLs containing tilted interfaces. It was found that the thermal conductivity will be 4-5 times higher than that of other angles when the period length is 4-8 atomic layers and the interface angle is 45°. This phenomenon can be attributed to the smooth arrangement of the 45° interface which induces phonon coherent transport. Meanwhile, the thermal conductivity has not been improved due to the phonon localization although the phonons satisfy the coherent transport when the interface angle is 90°. Interestingly, the thermal conductivity is almost unchanged with the increasing interface angle when the period length is large enough which exceeds 20 atomic layers. The main reason for the unchanged thermal conductivity is due to the period length which is greater than the phonon coherence length inducing the phonon incoherent transport.

Weaker bonding can give larger thermal conductance at highly mismatched interfaces

Thermal boundary conductance is typically positively correlated with interfacial adhesion at the interface. Here, we demonstrate a counterintuitive experimental result in which a weak van der Waals interface can give a higher thermal boundary conductance than a strong covalently bonded interface. This occurs in a system with highly mismatched vibrational frequencies (copper/diamond) modified by a self-assembled monolayer. Using finely controlled fabrication and detailed characterization, complemented by molecular simulation, the effects of bridging the vibrational spectrum mismatch and bonding at the interface are systematically varied and understood from a molecular dynamics viewpoint. The results reveal that the bridging and binding effects have a trade-off relationship and, consequently, that the bridging can overwhelm the binding effect at a highly mismatched interface. This study provides a comprehensive understanding of phonon transport at interfaces, unifying physical and chemical understandings, and allowing interfacial tailoring of the thermal transport in various material systems.

Enhancement of interface thermal conductance between Cr-Ni alloy and dielectric via Cu nano-interlayer

Facilitating interfacial thermal transport is highly desirable for various engineering applications, such as improving heat dissipation in microelectronics and efficiency of electrothermal heating element. Here, the interface thermal conductances (ITCs) of Cr0.22Ni0.78/MgO and Cr0.22Ni0.78/Al2O3interfaces are studied through the non-equilibrium molecular dynamics simulation. It is found that the two ITCs can be hugely enhanced by 3 and 2.4 times, respectively, with the introduction of Cu nano-interlayer of a thickness larger than 7.2 Å. The enhanced ratio is robust and shows weak dependence on temperature. Further vibrational spectral analysis and phonon transmission function reveal that the enhancement in ITC mainly originates from the boosting of inelastic phonon scattering, which is generally considered to contribute a small proportion to ITC. Here, the inelastic scattering contributes as high as 63% to the ITC of Cr0.22Ni0.78/Cu/MgO interface at 300 K. The findings provide an effective strategy to enhance ITC at a wide temperature range and advance our understanding of inelastic scattering in interfacial phonon transport.

Measurement of local thermal contact resistance with a periodic heating method using microscale lock-in thermography

This study proposes a new method for measuring thermal contact resistance using lock-in thermography. By using lock-in thermography with an infrared microscope, the dynamic and spatially resolved temperature behavior of the contact interface was visualized on a microscale with one measurement. In addition, a new thermal contact resistance measurement principle was constructed after solving the three-dimensional thermal conduction equation in the cylindrical coordinates by considering a periodic heat source with a Gaussian intensity distribution and the relative position of the heating point to the sample edge, in the presence of thermal resistance at the contact interface. Consequently, the discontinuous behaviors of the temperature wave, amplitude, and phase lag at the contact interface were observed on a microscale. From that discontinuity, the local thermal contact resistance was analyzed as a fitting parameter by matching the theoretical curve to the measured amplitude and phase lag. Furthermore, the simultaneous analysis of the material thermal diffusivity was demonstrated and the validity of the measurements was confirmed.

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.

Phonon coupling induced thermophoresis of water confined in a carbon nanotube

The phonons in CNT are found to be suppressed by the presence of water, giving new insight into thermophoresis.

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.

Thermal conductivity of suspended few-layer MoS2

Modifying phonon thermal conductivity in nanomaterials is important not only for fundamental research but also for practical applications. However, the experiments on tailoring thermal conductivity in nanoscale, especially in two-dimensional materials, are rare due to technical challenges. In this work, we demonstrate the in situ thermal conduction measurement of MoS₂ and find that its thermal conductivity can be continuously tuned to a required value from crystalline to amorphous limits. The reduction of thermal conductivity is understood from phonon-defect scattering that decreases the phonon transmission coefficient. Beyond a threshold, a sharp drop in thermal conductivity is observed, which is believed to be due to a crystalline-amorphous transition. Our method and results provide guidance for potential applications in thermoelectrics, photoelectronics, and energy harvesting where thermal management is critical with further integration and miniaturization.

Anomalous interfacial temperature profile induced by phonon localization

Through the integration of the power spectral density, we obtain temperature profiles of both multisegment harmonic and anharmonic systems, showing the presence of an anomalous negative temperature gradient inside the interfacial segment. Via investigating patterns of the power spectral density, we found that the counterintuitive phenomenon comes from the presence of interfacial localized phonon modes. Two out-band localized modes of the harmonic model, which make no contributions to local temperature due to the absence of phonon interactions, result in the concave temperature profile and overcooling effect. For the anharmonic model, thanks to the phonon-phonon interactions, the localized modes are excited and make considerable contributions to interfacial temperature, which is clearly shown by examining the temperature accumulation function. When anharmonicity is considerably large, the negative temperature gradient is absent since the localized phonon modes are fully mixed. The presence of localized modes are evidently demonstrated by the inverse participation ratio and normal mode analysis for the isolated harmonic model. The localized modes make contribution to interfacial temperature profiles of the harmonic system when they are excited in initial conditions of simulations.

The role of optical phonons in intermediate layer-mediated thermal transport across solid interfaces

Thermal transport across solid interfaces plays important roles in many applications, especially in the thermal management of modern power electronics. In this study, we use non-equilibrium MD (NEMD) simulations to systematically study a model SiC/GaN interface, which is an important interface in GaN-based power electronics, mated by different intermediate layers (ILs) with the focus on how the atomic masses of the ILs influence the overall thermal conductance. To isolate the mass effect, the Tersoff potential with the same parameters is used to approximate the interatomic interactions between all atoms, with the only differences between materials being their atomic masses. The NEMD results show that the thermal boundary conductance (TBC) of IL-mated interfaces depends not only on the total primitive cell mass of the IL but also on the relative masses of the atoms within the unit cell. By analyzing the vibrational power spectra (VPS) of SiC, IL, and GaN, it is found that the optical phonons play important roles in thermal transport across the solid/solid interfaces. There is an optimal mass ratio of the atoms in the unit cell of the IL that can maximize the overlap of IL optical phonon VPS with those of SiC and GaN. Furthermore, the atomic masses of a number of III-V semiconductor compounds are studied for the ILs. It is shown that when only considering the mass effect, in the classical limit, AlN will be the best IL to enhance thermal transport across SiC/GaN interfaces with an improvement of as much as 27% over that of a pristine SiC/GaN interface. Despite the known limitation of the model (e.g., absence of strain and quantum effects), the results from this work may still provide some useful information for the design of ILs to improve thermal transport across solid/solid interfaces.

Thermal transport at a solid-nanofluid interface: from increase of thermal resistance towards a shift of rapid boiling

We use molecular dynamics simulations to investigate interfacial thermal transport between an ethanol suspension containing gold atomic clusters and a gold surface, using both realistic and simplified molecular models of nanoparticles. The interfacial thermal conductance was determined via a thermal relaxation method for a variety of nanoparticle-nanoparticle and nanoparticle-surface interaction strengths. The Kapitza resistance is found to increase due to the presence of nanoparticles in the vicinity of the solid-liquid interface. The heat flow from the solid to the nanoparticles is separated from its counterpart from the solid to the liquid to discriminate their respective contribution to the total heat current. A per-vibrational-mode analysis highlights a shift of major heat carriers from low frequencies towards higher frequencies due to the coupling of the internal nanoparticle dynamics to the gold surface, in addition to stronger particle-surface interactions. Finally, we demonstrate that the increase of the Kapitza resistance significantly shifts the nanofluid/solid surface explosive boiling temperature to higher temperatures compared to pure ethanol.

Thermal Transport at Solid-Liquid Interfaces: High Pressure Facilitates Heat Flow through Nonlocal Liquid Structuring

The integration of three-dimensional microelectronics is hampered by overheating issues inherent to state-of-the-art integrated circuits. Fundamental understanding of heat transfer across soft-solid interfaces is important for developing efficient heat dissipation capabilities. At the microscopic scale, the formation of a dense liquid layer at the solid-liquid interface decreases the interfacial heat resistance. We show through molecular dynamics simulations of n-perfluorohexane on a generic wettable surface that enhancement of the liquid structure beyond a single adsorbed layer drastically enhances interfacial heat conductance. Pressure is used to control the extent of the liquid layer structure. The interfacial thermal conductance increases with pressure values up to 16.2 MPa at room temperature. Furthermore, it is shown that liquid structuring enhances the heat-transfer rate of high-energy lattice waves by broadening the transmission peaks in the heat flux spectrum. Our results show that pressure is an important external parameter that may be used to control interfacial heat conductance at solid-soft interfaces.

Molecular Tuning of the Vibrational Thermal Transport Mechanisms in Fullerene Derivative Solutions

Control over the thermal conductance from excited molecules into an external environment is essential for the development of customized photothermal therapies and chemical processes. This control could be achieved through molecule tuning of the chemical moieties in fullerene derivatives. For example, the thermal transport properties in the fullerene derivatives indene-C₆₀ monoadduct (ICMA), indene-C₆₀ bisadduct (ICBA), [6,6]-phenyl C₆₁ butyric acid methyl ester (PCBM), [6,6]-phenyl C₆₁ butyric acid butyl ester (PCBB), and [6,6]-phenyl C₆₁ butyric acid octyl ester (PCBO) could be tuned by choosing a functional group such that its intrinsic vibrational density of states bridge that of the parent molecule and a liquid. However, this effect has never been experimentally realized for molecular interfaces in liquid suspensions. Using the pump-probe technique time domain thermotransmittance, we measure the vibrational relaxation times of photoexcited fullerene derivatives in solutions and calculate an effective thermal boundary conductance from the opto-thermally excited molecule into the liquid. We relate the thermal boundary conductance to the vibrational modes of the functional groups using density of states calculations from molecular dynamics. Our findings indicate that the attachment of an ester group to a C₆₀ molecule, such as in PCBM, PCBB, and PCBO, provides low-frequency modes which facilitate thermal coupling with the liquid. This offers a channel for heat flow in addition to direct coupling between the buckyball and the liquid. In contrast, the attachment of indene rings to C₆₀ does not supply the same low-frequency modes and, thus, does not generate the same enhancement in thermal boundary conductance. Understanding how chemical functionalization of C₆₀ affects the vibrational thermal transport in molecule/liquid systems allows the thermal boundary conductance to be manipulated and adapted for medical and chemical applications.