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Thermal percolation in polymer nanocomposites─the rapid increase in thermal transport due to the formation of networks among fillers─is the subject of great interest in thermal management ranging from general utility in multifunctional nanocomposites to high-conductivity applications such as thermal interface materials. However, It remains a challenging subject encompassing both experimental and modeling hurdles. Successful reports of thermal percolation are exclusively found in high-aspect-ratio, conductive fillers such as graphene, albeit at filler loadings significantly higher than the electrical percolation threshold. This anomaly was attributed to the lower filler-matrix thermal conductivity contrast ratio

Soft ionic materials combine charged mobile species and tailored polymer structures in a manner that enables a wide array of functional devices. Traditional metal and silicon electronics are limited to two charge carriers: electrons and holes. Ionic devices hold the promise of using the wide range of chemical and molecular properties of mobile ions and polymer functional groups to enable flexible conductors, chemically specific sensors, bio-compatible interfaces, and deformable digital or analog signal processors. Stand alone ionic devices would need to have five key capabilities: signal transmission, energy conversion/harvesting, sensing, actuation, and signal processing. With the great promise of ionically-conducting materials and ionic devices, there are several fields working independently on pieces of the puzzle. These fields range from waste-water treatment research to soft robotics and bio-interface research. In this review, we first present the underlying physical principles that govern the behavior of soft ionic materials and devices. We then discuss the progress that has been made on each of the potential device components, bringing together findings from a range of research fields, and conclude with discussion of opportunities for future research.

Two-dimensional (2D) hybrid organic-inorganic perovskites consisting of alternating organic and inorganic layers are a new class of layered structures. They have attracted increasing interest for photovoltaic, optoelectronic, and thermoelectric applications, where knowing their thermal transport properties is critical. We carry out both experimental and computational studies on thermal transport properties of 2D butylammonium lead iodide crystals and find their thermal conductivity is ultralow (below 0.3 W m-1 K-1) with very weak anisotropy (around 1.5) among layered crystals. Further analysis reveals that the unique structure with the preferential alignment of organic chains and complicated energy landscape leads to moderately smaller phonon lifetimes in the out-of-plane direction and comparable phonon group velocities in in-plane and out-of-plane directions. These new findings may guide the future design of novel hybrid materials with desired thermal conductivity for various applications.

To enable the on-demand control of heat flow for sustainable energy solutions, we have been longing for functional thermal components at the nanoscale, in analogue to electronic diodes and transistors. Understanding and discovering fundamental mechanisms that drive thermal rectification are critical to advancing this field. Different mechanisms have been proposed for thermal rectification effects in the classical regime. Using anharmonic atomistic Green's function, we discovered a thermal rectification phenomenon in the quantum regime for nanometer-thick three-dimensional solid interfaces. We found that the anharmonic phonon scatterings across the interface act on the temperature-dependent phonon populations on both sides of the interface, generating the necessary nonlinearity to achieve thermal rectification. This intrinsic thermal interface rectification is a universal phenomenon that can be observed and engineered for nanoscale interfaces.

Thermal switches are of great importance to thermal management in a wide variety of applications. However, traditional thermal switches suffer from being large and having slow transition rates. To overcome these limitations, we took advantage of abrupt second-order phase transitions in thermoresponsive polymer aqueous solutions to enable fast thermal switching. While thermoresponsive polymers have been widely studied for biomedical applications, their thermal switching capability has not been studied. In this work, we used poly(N-isopropylacrylamide) (PNIPAM) as a model system to demonstrate abrupt thermal conductivity changes of thermoresponsive polymer aqueous solutions across their transition temperatures by using a powerful approach, the transient thermal grating technique, which has high sensitivity. We observed a thermal switching ratio up to 1.15 in dilute PNIPAM aqueous solutions (up to 0.025 g/mL) across the transition. This work may provide new opportunities to engineer thermal switches using se...

Molecularly confined polymer fillers in nanopores were found to give superior mechanical properties of polymer nanocomposites. In this work, we study the thermal conductivity of such nanocomposites and unveil the effect of polymer confinement on thermal conductivity. Using the time-domain thermoreflectance method, we measure the cross-plane thermal conductivity of polymer nanocomposites that consist of polystyrene fillers confined within a nanoporous organosilicate matrix. Compared to unconfined bulk polystyrene fillers, we find that pore-confined polystyrene fillers enhance the thermal conductivity of the polymer nanocomposites. This enhancement is attributed to the better aligned and less entangled chains in the confined phase, where chain-chain phonon scatterings are reduced. Our work provides essential insights into the thermal conductivity of polymer nanocomposites for multifunctional thermal and mechanical applications.

Graphamid is a two-dimensional (2D) polymer. It was predicted to have superior mechanical properties owing to its strong hydrogen bonds. The knowledge of thermal and electrical transport properties...

The thermal conductivity of crystalline polymers is higher than their amorphous counterparts yet still spans three orders of magnitude. In this study, by quantifying intrinsic properties including ...

Additive manufacturing (AM) affords the opportunity to print tailored pharmaceuticals to customize the quantity of medicine, number of medicines, and time and rate of release to be adapted for an individual's needs. A series of sulfonated poly(ethylene glycol) (SPEG) polymers for fused filament fabrication (FFF) that (i) dissolves quickly (

Graphene-C60 heterostructures assembled by van der Waals (vdW) interactions between graphene and C60 have shown exciting potential for multifunctional devices. Understanding thermal transport in graphene-C60 heterostructures is the key to guiding the design of vdW heterostructures with desired thermal transport properties. In this work, we report the first study of thermal transport in a graphene-C60 heterostructure and elucidate the importance of vdW interactions to heat conduction using molecular dynamics simulations. We find that the in-plane thermal conductivity of the graphene-C60 heterostructure is as high as about 234 W/(mK) at room temperature, exceeding those of most pure metals. As the vdW interaction parameter, χ , varies from 0.1 to 2, the in-plane thermal conductivity first increases then decreases. On the other hand, as vdW interactions increases, the interfacial thermal conductance between graphene and C60 is enhanced. Our study demonstrates that graphene-C60 heterostructures have high in-plane thermal conductivity and their interfacial thermal conductance is comparable to that of graphene-hexagonal boron-nitride (hBN) heterostructure. Graphene-C60 heterostructures are promising candidates for multifunctional devices with inherent heat dissipation capability.

Thermoelectric generators (TEG) convert heat into electricity and offer a green option for renewable energy generation. The Thomson effect has been proven to impact the overall performance of TEGs negatively but is ignored in the widely-used formula for TEG maximum efficiency calculation. In this study, a new dimensionless number called the Ury number (Ur) is introduced to capture the influence of the Thomson effect on TEG power output and maximum efficiency. Ur is defined as the ratio between the Thomson coefficient and the Seebeck coefficient. It is found that as Ur increases both TEG output power and maximum efficiency decrease. Ur combined with the device figure of merit, ZT, can better dictate the overall performance of a TEG.

Hybrid organic–inorganic perovskites were reported to have ultralow thermal conductivity in recent studies. In this Letter, we report the first experimental phonon dispersion and lifetimes of tetra...

Highlighted by the discovery of high-temperature superconductivity, strongly correlated oxides with highly distorted perovskite structures serve as intriguing model systems for pursuing emerging materials physics and testing technological concepts. While 3d correlated oxides with a distorted perovskite structure are not uncommon, their 4d counterparts are unfortunately rare. In this work, we report the tuning of the electrical and optical properties of a quasi-2D perovskite niobate CsBiNb2O7 via hydrogenation. It is observed that hydrogenation induces drastic changes of lattice dynamics, optical transmission, and conductance. It is suggested that changing the orbital occupancy of Nb d orbitals could trigger the on-site Coulomb interaction in the NbO6 octahedron. The observed hydrogen doping-induced electrical plasticity is implemented for simulating neural synaptic activity. Our finding sheds light on the role of hydrogen in 4d transition metal oxides and suggests a new avenue for the design and development of novel electronic phases.

In this article, we review thermal transport in polymers with different morphologies from aligned fibers to bulk amorphous states. We survey early and recent efforts in engineering polymers with high thermal conductivity by fabricating polymers with large-scale molecular alignments. The experimentally realized extremely high thermal conductivity of polymer nanofibers are highlighted, and understanding of thermal transport physics from molecular simulations are discussed. We then transition to the discussion of bulk amorphous polymers with an emphasize on the physics of thermal transport and its relation with the conformation of molecular chains in polymers. We also discuss the current understanding of how the chemistry of polymers would influence thermal transport in amorphous polymers and some limited, but important chemistry-structural-property relationships. Lastly, challenges, perspectives and outlook of this field are presented. We hope this review will inspire more fundamental and applied research in the polymer thermal transport field to advance scientific understanding and engineering applications., Comment: 98 pages, 22 figures

This review introduces relevant nanoscale thermal transport processes that impact thermal abatement in power electronics applications. Specifically, we highlight the importance of nanoscale thermal transport mechanisms at each layer in material hierarchies that make up modern electronic devices. This includes those mechanisms that impact thermal transport through: (1) substrates, (2) interfaces and two-dimensional materials, and (3) heat spreading materials. For each material layer, we provide examples of recent works that (1) demonstrate improvements in thermal performance and/or (2) improve our understanding of the relevance of nanoscale thermal transport across material junctions. We end our discussion by highlighting several additional applications that have benefited from a consideration of nanoscale thermal transport phenomena, including radio frequency (RF) electronics and neuromorphic computing.

Phonon Anderson localization has been receiving increasing interest in creating unique thermal transport properties. However, due to challenges to resolve and track the mode-specific transmission, the existence of phonon Anderson localization has only been inferred from the decay of overall thermal conductivity versus device length. In this work, we present direct evidence of phonon Anderson localization using the exponential decay of their mode-resolved transmissions. We clearly capture localized modes in different structures, even with a monotonically upward thermal conductivity trend.

One of the next innovations in thermoelectric (TE) device design is the implementation of organic materials in manufacturing thermoelectric generators including flexible and wearable ones. While organic materials pose to be very beneficial due to versatility, pliability, and availability, the main challenge is to tailor their TE properties to match or even exceed the performance of traditional inorganic TE materials. Although a number of experiments and measurements have been performed on organic materials, the dominant mechanisms that control those TE properties are still unclear. Therefore, it is necessary to gain better understanding through simulation, modeling, and design of organic materials. This chapter reviews the most recent advances made in the simulation of organic TE materials. Through continued work in this field, the understanding of organic TE material properties is expected to be significantly advanced. This will allow a new wave of innovation in the field of thermoelectric materials.

As electronic devices shrink down to their ultimate limit, the fundamental understanding of interfacial thermal transport becomes essential in thermal management. However, a comprehensive understanding of phonon transport mechanisms that drive interfacial thermal transport is still under development. The thermal transport across interfaces can be strongly affected by factors such as crystalline structure, surface roughness, chemical diffusion, etc. These complications lead to a significant quantitative uncertainty between experimentally measured thermal boundary conductance (TBC) across real material interfaces and theoretically calculated TBCs that are often predicted on structurally and/or chemically ideal interfaces. In this paper, we report on the thermal conductance across interfaces between various epitaxially grown metal films (Co, Ru, and Al) and $c$-plane sapphire substrates via time-domain thermoreflectance over the temperature range of \ensuremath{\sim}80 to \ensuremath{\sim}500 K. The room-temperature interface conductances of Al/sapphire, Co/sapphire, and Ru/sapphire are all $\ensuremath{\sim}350\phantom{\rule{0.16em}{0ex}}\mathrm{MW}\phantom{\rule{0.16em}{0ex}}{\mathrm{m}}^{\ensuremath{-}1}\phantom{\rule{0.16em}{0ex}}{\mathrm{K}}^{\ensuremath{-}1}$ despite the phonon spectra differences among the metals. We compare our results to predictions of TBC using atomistic Green's function calculations and the modal nonequilibrium Landauer method with transmission from the diffuse mismatch model. We found a consistent quantitative agreement between the experimentally measured TBCs and the predictions using the modal nonequilibrium Landauer model for the $\mathrm{Al}/{\mathrm{Al}}_{2}{\mathrm{O}}_{3}$, $\mathrm{Co}/{\mathrm{Al}}_{2}{\mathrm{O}}_{3}$, and $\mathrm{Ru}/{\mathrm{Al}}_{2}{\mathrm{O}}_{3}$ interfaces. This result suggests that interfacial elastic phonon thermal transport dominates TBC for the various epitaxial metal/sapphire combinations of interest in this work, while other mechanisms are negligible.

A unified understanding of interfacial thermal transport is missing due to the complicated nature of interfaces which involves complex factors such as interfacial bonding, interfacial mixing, surface chemistry, crystal orientation, roughness, contamination, and interfacial disorder. This is especially true for metal nonmetal interfaces which incorporate multiple fundamental heat transport mechanisms such as elastic and inelastic phonon scattering as well as electron phonon coupling in the metal and across the interface. All these factors jointly affect thermal boundary conductance (TBC). As a result, the experimentally measured interfaces may not be the same as the ideally modelled interfaces, thus obfuscating any conclusions drawn from experimental and modeling comparisons. This work provides a systematic study of interfacial thermal conductance across well controlled and ultraclean epitaxial (111) Al parallel (0001) sapphire interfaces, known as harmonic matched interface. A comparison with thermal models such as atomistic Green s function (AGF) and a nonequilibrium Landauer approach shows that elastic phonon scattering dominates the interfacial thermal transport of Al sapphire interface. By scaling the TBC with the Al heat capacity, a nearly constant transmission coefficient is observed, indicating that the phonons on the Al side limits the Al sapphire TBC. This nearly constant transmission coefficient validates the assumptions in AGF and nonequilibrium Landauer calculations. Our work not only provides a benchmark for interfacial thermal conductance across metal nonmetal interfaces and enables a quantitative study of TBC to validate theoretical thermal carrier transport mechanisms, but also acts as a reference when studying how other factors impact TBC.

Single-crystal tin selenide (SnSe), a record holder of high-performance thermoelectric materials, enables high-efficient interconversion between heat and electricity for power generation or refrigeration. However, the rigid bulky SnSe cannot satisfy the applications for flexible and wearable devices. Here, a method is demonstrated to achieve ultralong single-crystal SnSe wire with rock-salt structure and high thermoelectric performance with diameters from micro- to nanoscale. This method starts from thermally drawing SnSe into a flexible fiber-like substrate, which is polycrystalline, highly flexible, ultralong, and mechanically stable. Then a CO2 laser is employed to recrystallize the SnSe core to single-crystal over the entire fiber. Both theoretical and experimental studies demonstrate that the single-crystal rock-salt SnSe fibers possess high thermoelectric properties, significantly enhancing the ZT value to 2 at 862 K. This simple and low-cost approach offers a promising path to engage the fiber-shaped single-crystal materials in applications from 1D fiber devices to multidimensional wearable fabrics. Ministry of Education (MOE) Nanyang Technological University National Research Foundation (NRF) This work was supported in part by the Singapore Ministry of Education Academic Research Fund Tier 2 (MOE2019-T2-2-127), the Singapore Ministry of Education Academic Research Fund Tier 1 (MOE2019-T1-001-103 and MOE2019-T1-001-111) and the Singapore National Research Foundation Competitive Research Program (NRF-CRP18-2017-02). This work was also supported in part by Nanyang Technological University. This work was also supported in part by the Basic Science Center Program for Ordered Energy Conversion of the National Natural Science Foundation of China (No. 51888103) and the Chinese Academy of Sciences Talents Program (No. E0290706). This work was partially supported by the National Natural Science Foundation of China (11804354) and Shenzhen Basic Research Grant: JCYJ20180507182431967.

The traditional atomistic Green's function (AGF) was formulated in the harmonic regime, preventing it from capturing the role of anharmonicity in interfacial thermal transport. Incorporating anharmonicity into the AGF has long been desired but remains challenging. In this Rapid Communication, we developed an anharmonic AGF model to incorporate anharmonicity at interfaces in three-dimensional structures rigorously. Without any fitting parameters, we can obtain the thermal interface conductance using first-principles force constants as the only input. This Rapid Communication represents a significant step forward for the AGF.

Wurtzite aluminum nitride (AlN) has attracted increasing attention for high-power and high-temperature operations due to its high piezoelectricity, ultrawide-bandgap, and large thermal conductivity k. The k of epitaxially grown AlN on foreign substrates has been investigated; however, no thermal studies have been conducted on homoepitaxially grown AlN. In this study, the thickness dependent k and thermal boundary conductance G of homoepitaxial AlN thin films were systematically studied using the optical pump–probe method of frequency-domain thermoreflectance. Our results show that k increases with the thickness and k values are among the highest reported for film thicknesses of 200 nm, 500 nm, and 1 μm, with values of 71.95, 152.04, and 195.71 W/(mK), respectively. Our first-principles calculations show good agreement with our measured data. Remarkably, the G between the epilayer and the substrate reported high values of 328, 477, 1180, and 2590 MW/(m2K) for sample thicknesses of 200 nm, 500 nm, 1 μm, and 3 μm, respectively. The high k and ultrahigh G of homoepitaxially grown AlN are very promising for efficient heat dissipation, which helps in device design and has advanced applications in micro-electromechanical systems, ultraviolet photonics, and high-power electronics.

Kevlar (polyparaphenylene terephthalamide) and PBDT (poly(2,2′-disulfonyl-4,4′-benzidine terephthalamide))-derivatives have very similar chemical structures with aromatic rings. In this study, thermal conductivities of their single chains were calculated using molecular dynamics simulations. Chain rotation was found to be the key to reducing the thermal conductivity. By introducing a new chain rotation factor (CRF), we can easily quantify chain rotation level of single-chain polymers. We demonstrated that thermal conductivity decreases as the CRF increases. We performed further calculations on phonon properties and unveiled that the small thermal conductivity led by large chain rotation can be attributed to reduced phonon group velocities and shortened phonon mean free paths. Insights obtained in this study can be used for tuning thermal conductivity of various polymers and facilitating their various applications including thermal energy conversion and management.

Modeling thermoelectric generator (TEG) performances plays an important role in guiding the design of TEGs to achieve better efficiency. However, a rigorous 1-D TEG modeling performance has not yet been conducted, which prevents reliable prediction of TEG performance. In this work, a detailed 1-D model has been developed to take into account temperature-dependent thermoelectric material properties, heat loss due to radiation and conduction, and Thomson effect. A Lead Telluride (PbTe) TEG was chosen as a sample module and the modeling results agree very well with the experimental results, which proves how powerful the presented detailed 1-D model can be used to predict and validate TEG experimental results. TEG power and efficiency were found to have a respective decrease of 10% and 31% from the simplified model at a temperature gradient of 570 K. While heat loss attributable to conduction and radiation were found to be small, the Thomson effect, which is often neglected, was found to significantly reduce TEG performances. The deep analysis enabled by the new model provides useful guidelines to improve the performance of TEGs.

Two-dimensional (2D) polymers are organic analogues of graphene. Compared to graphene, 2D polymers offer a higher degree of tunability in regards to structure, topology, and physical properties. The thermal transport properties of 2D polymers play a crucial role in their applications, yet remain largely unexplored. Using the equilibrium molecular dynamics method, we study the in-plane thermal conductivity of dubbed porous graphene that is comprised of π-conjugated phenyl rings. In contrast to the conventional notion that π-conjugation leads to high thermal conductivity, we demonstrate, for the first time, that π-conjugated 2D polymers can have either high or low thermal conductivity depending on their porosity and structural orientation. The underlying mechanisms that govern thermal conductivity were illustrated through phonon dispersion. The ability to achieve two orders of magnitude variance in thermal conductivity by altering porosity opens up exciting opportunities to tune the thermal transport properties of 2D polymers for a diverse array of applications.

Covalent organic frameworks (COFs) are a new, growing class of crystalline polymers with exceptional characteristics such as low density, mechanical strength, and flexibility. Taking three-dimensional (3D) COF-300 derivatives as an example, we apply molecular dynamics simulations to show that ultrahigh thermal conductivity (>15 W/(mK)) is achieved at a pore size of d = 0.63 nm. This is the first observation of such an outstanding intrinsic thermal conductivity of a 3D polymer in all directions. Further temperature-dependent thermal conductivity calculations and X-ray diffraction (XRD) patterns highlight the importance of maintaining microscopic order for a large thermal conductivity. We also demonstrate that the tunability and anisotropy in COFs provide sufficient room to engineer and obtain desired thermal conductivity. These findings provide a fundamental understanding of structure-thermal conductivity relation in COFs and shine a light on high-thermal-conductivity COFs for thermal management applications.

In this work, a comparison between the performance of two- and three-stage cascaded thermoelectric generator (TEG) devices is analyzed based on a prescribed maximum hot side temperature of 973 K, an imposed maximum heat input of 505 W, and a fixed cold side temperature of 473 K. Half-Heusler is used as a thermoelectric (TE) material in the top higher temperature stage and skutterudite as a TE in the bottom lower temperature stage for the two-stage structure. Lead telluride is added in the middle stage to form the three-stage structure. Based on the prescribed constraints, the two-stage cascaded TEG is found to produce a power output of 42 W with an efficiency of 8.3%. The three-stage cascaded TEG produces a power output of 51 W with an efficiency of 10.2%. The three-stage cascaded TEG produces 21% more power than the two-stage does; however, if the system complexity, mechanical robustness, manufacturability, and/or cost of three-stage cascaded TEG outweigh the 21% percent power production increase, the two-stage TEG could be preferable.

Black phosphorus (BP) is a layered direct bandgap semiconductor. Few-layer BP, an emergent and promising two-dimensional semiconductor, has attracted increasing interest for novel electronic, photonic, and energy conversion applications due to its unique tunable direct bandgap and anisotropic in-plane transport properties. Understanding the thermal transport properties is critical for thermal management of electronics and thermoelectric applications. In this review, we cover the thermal transport properties of single-layer, few-layer, and bulk BP from most recent theoretical and experimental work, with a particular focus on the anisotropic in-plane thermal conductivity.

In this Letter, we show the phonon dispersion of (CH_{3}NH_{3})_{3}Bi_{2}I_{9} single crystals at 300 K measured by inelastic x-ray scattering. The frequencies of acoustic phonons are among the lowest of crystals. Nanoindentation measurements verified that these crystals are very compliant and considerably soft. The frequency overlap between acoustic and optical phonons results in strong acoustic-optical scattering. All these features lead to an ultralow thermal conductivity. The fundamental knowledge obtained from this study will accelerate the design of novel hybrid materials for energy applications.

In this Letter, we show the phonon dispersion of (CH_{3}NH_{3})_{3}Bi_{2}I_{9} single crystals at 300 K measured by inelastic x-ray scattering. The frequencies of acoustic phonons are among the lowest of crystals. Nanoindentation measurements verified that these crystals are very compliant and considerably soft. The frequency overlap between acoustic and optical phonons results in strong acoustic-optical scattering. All these features lead to an ultralow thermal conductivity. The fundamental knowledge obtained from this study will accelerate the design of novel hybrid materials for energy applications.

Solid-state thermoelectrics are of great interest because they directly convert between thermal and electrical energy. However, wider application of thermoelectrics is dependent upon improving their performance. Anderson localization of charge carriers has been generally perceived as detrimental to thermoelectrics, but in their paper in this issue of ACS Nano, Lee et al. propose a concept of using selective charge Anderson localization for synergetic enhancement of thermoelectric performance. To facilitate discussions and research on Anderson localization to improve thermoelectrics, this Perspective shares potential directions to explore the viability of using Anderson localization or related strategies to drive thermoelectric performance.

Nanostructured materials are of great interest for many applications because of their special properties. Understanding the effect of grain boundaries on phonon transport in polycrystals is important for engineering nanomaterials with desired thermal transport properties. The phonon transport properties of Σ3 grain boundaries in silicon are investigated by employing atomistic Green's function method. Results show that similar to electron transport, the perfect grain boundary does not significantly reduce the thermal conductance, while defective grain boundaries can dramatically reduce the thermal conductance. This work may be helpful for the understanding of the underlying thermal transport mechanism across grain boundaries and the design of grain boundaries for energy applications.

Hybrid organic-inorganic perovskites were reported to have ultralow thermal conductivity in recent studies. In this Letter, we report the first experimental phonon dispersion and lifetimes of tetragonal CH

Boron suboxides (B6O) were identified to be a superhard material, and their stiff bonds are expected to give large thermal conductivity. But their complex crystal structures suggest otherwise. Using first-principles calculations, we show that both α- and β-B6O have unusually high lattice thermal conductivities of 284.9 and 207.1 W/(m K), respectively, at room temperature, despite their complex structures. Our detailed phonon analysis attributed the dominant factor of its large thermal conductivity to the strong bond strength. Their large group velocities result from the strong bonding and light atomic mass, while their large phonon lifetimes can be explained by small anharmonicity and limited scattering phase space. Our results show that materials with complex unit cells like α- and β-B6O can still have high thermal conductivity. The combination of large thermal conductivity and an excellent physical hardness makes B6O a promising material for lightweight, multifunctional thermal management applications.

The thermal energy represents a significant portion of energy potential in municipal wastewater and may be recovered as electricity by a thermoelectric generator (TEG). Converting heat to all-purpose electricity by TEG has been demonstrated with large heat gradients, but its application in waste heat recovery from wastewater has not been well evaluated. Herein, a bench-scale Bi2Te3-based waste heat recovery system was employed to generate electricity from a low temperature gradient through a combination of experiments and mathematical modeling. With an external resistance of 7.8 Ω and a water (hot side) flow rate of 75 mL min-1, a maximum normalized energy recovery of 4.5 × 10-4 kWh m-3 was achieved under a 2.8 °C temperature gradient (ΔT). Model simulation indicated a boost in both power output and energy conversion efficiency from 0.76 mW and 0.13% at ΔT = 2.8 °C to 61.83 mW and 1.15% at ΔT = 25 °C. Based on the data of two-year water/air temperature obtained from the Christiansburg Wastewater Treatment Plant, an estimated energy generation of 1094 to 70,986 kWh could be expected annually with a saving of $163 to $6076. Those results have revealed a potential for TEG-centered direct electricity generation from low-grade heat towards enhanced resource recovery from wastewater and encouraged further exploration of this approach.

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.

Coherent phonon heat conduction has recently been confirmed experimentally in superlattice structures. Such traveling coherent phonon waves in superlattices lead to a linear increase in thermal conductivity as the number of periods increases. For applications such as thermal insulation or thermoelectrics, minimization of the phonon coherent effect is desirable. In this work, we use molecular dynamics simulations to study how to control coherent heat conduction in superlattices (SLs). It is found that either aperiodic SLs or SLs with rough interfaces can significantly disrupt coherent heat conduction when the interface densities are high. For sample thickness less than 125 nm, aperiodic SLs with perfect interfaces are found to have the lowest thermal conductivity. We use the atomic Green’s function method to examine the phonon dynamics. The impact of either aperiodicity or interface roughness is attributed to reduced transmittance. Such impact diminishes as the interface density reduces., United States. Department of Energy (Office of Science, Office of Basic Energy Sciences, Grant No. DE-SC0001299)

Graphene has excellent mechanical, thermal and electrical properties. However, there are limitations in utilizing monolayers of graphene for mechanical engineering applications due to its atomic thickness and lack of bending rigidity. Synthesizing graphene aerogels or foams is one approach to utilize graphene in three-dimensional bulk forms. Recently, graphene with a gyroidal geometry has been proposed. A gyroid is a triply periodic minimal surface that allows graphene sheets to form a three-dimensional structure. Its light weight and high mechanical strength suggests that the graphene that constitutes this geometry can synergistically contribute to the mechanics of the bulk material. However, it is not clear whether gyroid graphene can preserve the high thermal conductivity of pristine graphene sheets. Here, we investigate the thermal conductivities of gyroid graphene with different porosities by using full-atom molecular dynamics simulations. In contrast to its excellent mechanical properties, we find that the thermal conductivity of gyroid graphene is more than 300 times lower than that of pristine graphene, with a bulk density of only about one-third of that of graphene. We derive a scaling law showing that the thermal conductivity does not vary much with different bulk densities, which contrasts the behavior of conventional porous materials. Our analysis shows that the poor thermal conductivity of gyroid graphene can be attributed to defects and curvatures of graphene, which increase with the density, resulting in the reduction of a phonon mean free path by phonon scattering. Our study shows that three-dimensional porous graphene has potential that may be utilized in designing new lightweight structural materials with low and density-insensitive thermal properties and superior mechanical strength.

Thermal interface materials with high thermal conductivity can remove heat efficiently from electronic devices. Carbon nanotube/polymer composite is a good candidate for thermal interface materials. We calculated the thermal conductivity of vertically aligned carbon nanotube/polyethylene (CNT/PE) composite and compared it with vertically aligned CNT and PE based on equilibrium molecular dynamics. We found that the thermal conductivity of CNT/PE composite along the alignment direction can be as high as 470.1±45.1 W/(mK), which is about 40% of that of CNT and about 16 times larger than that of PE. This can be well explained by vibrational density of states. The ultrahigh thermal conductivity of aligned CNT/PE composite may open up exciting opportunities towards enhancing the cross-plane thermal conductivity of polymer-based thermal interface materials for efficient microelectronics cooling.

Cubic boron arsenide (BAs) was predicted to have an exceptionally high thermal conductivity $(k)\ensuremath{\sim}2000\phantom{\rule{0.16em}{0ex}}\mathrm{W}\phantom{\rule{0.16em}{0ex}}{\mathrm{m}}^{\ensuremath{-}1}{\mathrm{K}}^{\ensuremath{-}1}$ at room temperature, comparable to that of diamond, based on first-principles calculations. Subsequent experimental measurements, however, only obtained a $k$ of $\ensuremath{\sim}200\phantom{\rule{0.16em}{0ex}}\mathrm{W}\phantom{\rule{0.16em}{0ex}}{\mathrm{m}}^{\ensuremath{-}1}{\mathrm{K}}^{\ensuremath{-}1}$. To gain insight into this discrepancy, we measured phonon dispersion of single-crystal BAs along high symmetry directions using inelastic x-ray scattering and compared these with first-principles calculations. Based on the measured phonon dispersion, we have validated the theoretical prediction of a large frequency gap between acoustic and optical modes and bunching of acoustic branches, which were considered the main reasons for the predicted ultrahigh $k$. This supports its potential to be a super thermal conductor if very-high-quality single-crystal samples can be synthesized.

In this study, non-equilibrium molecular dynamics (NEMD) simulations are performed to determine the effective thermal conductivity of nanocomposites embedded with a variety of nanoparticles. The effects of orientation and arrangement of asymmetric nanoparticles, thermal property mismatch at the interface, interface density per unit volume of nanocomposite, and polydispersity of nanoparticles on the effective thermal conductivity of nanocomposites are investigated. Simulation results are compared with existing model predictions based on the effective medium approach. Results indicate that, with the same particle volume fraction, the nanocomposites that have a larger interfacial area perpendicular to heat flow or a larger interface density yield a smaller effective thermal conductivity. In addition, a larger mismatch at the interface between the host material and nanoparticle inclusions leads to a smaller effective thermal conductivity. Finally, it is found that the effective thermal conductivity of nanocomposites decreases with increasing polydispersity of embedded nanoparticles.

The wide bandgap semiconductor, ZnO, has gained interest recently as a promising option for use in power electronics such as thermoelectric and piezoelectric generators, as well as optoelectronic devices. Though much work has been done to improve its electronic properties, relatively little is known of its thermal transport properties with large variations in measured thermal conductivity. In this study, we examine the effects of a Hubbard corrected energy functional on the lattice thermal conductivity of wurtzite ZnO calculated using density functional theory and an iterative solution to the Boltzmann transport equation. Showing good agreement with existing experimental measurements, and with a detailed analysis of the mode-dependence and phonon properties, the results from this study highlight the importance of the Hubbard correction in calculations of thermal transport properties of materials with strongly correlated electron systems. Virginia Polytechnic Institute; State University; National Science Foundation [ACI-1053575] This work is supported by the startup fund from Virginia Polytechnic Institute and State University. The computational resources and support used were provided by the Advanced Research Computing at Virginia Tech and the Extreme Science and Engineering Discovery Environment (XSEDE), which is supported by National Science Foundation grant number ACI-1053575. The authors would also like to thank Dr. Arrigo Calzolari, Dr. Wu Li and Dr. Jesus Carrete for their much appreciated guidance.

PbTe1-x Se x alloys are of special interest to thermoelectric applications. Inelastic x-ray scattering determination of phonon dispersion and lifetimes along the high symmetry directions for PbTe1-x Se x alloys are presented. By comparing with calculated results based on the virtual crystal model calculations combined with ab initio density functional theory, the validity of virtual crystal model is evaluated. The results indicate that the virtual crystal model is overall a good assumption for phonon frequencies and group velocities despite the softening of transverse acoustic phonon modes along [1 1 1] direction, while the treatment of lifetimes warrants caution. In addition, phonons remain a good description of vibrational modes in PbTe1-x Se x alloys.

The effects of polymer topology and morphology on thermal transport are of great importance to both fundamental understanding and practical applications. Bottlebrush (BB) polymers are unique polymers that feature a highly extended backbone conformation stabilized by densely packed side-chain polymers, which offer a rich platform to study those effects. In this paper, we report the thermal transport properties of BB polymers using equilibrium molecular dynamics. We find that sidechains dominate heat conduction in aligned BB chains and lead to lower k than aligned linear polymers due to strong localization, while backbones form an effective heat conduction network in amorphous BB polymers and lead to higher k than their linear polymer counterparts. The effect of morphology on k is also discussed.

Understanding and manipulating coherent phonon transport in solids is of interest both for enhancing the fundamental understanding of thermal transport as well as for many practical applications, including thermoelectrics. In this study, we investigate phonon transmission across Si/Ge superlattices using the Green's function method with first-principles force constants derived from ab initio density functional theory. By keeping the period thickness fixed while changing the number of periods, we show that interface roughness partially destroys coherent phonon transport, especially at high temperatures. The competition between the low-frequency coherent modes and high-frequency incoherent modes leads to an optimum period length for minimum thermal conductivity. To destroy coherence of the low-frequency modes, scattering length scale on the order of period length is required. This finding is useful to guide the design of superlattices to reach even lower thermal conductivity.