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Thermoelectric power generation represents a promising approach to utilize waste heat. The most effective thermoelectric materials exhibit low thermal conductivity κ. However, less than 5% out of about 105 synthesized inorganic materials are documented with their κ values, while for the remaining 95% κ values are missing and challenging to predict. In this work, by combining graph neural networks and random forest approaches, we predict the thermal conductivity of all known inorganic materials in the Inorganic Crystal Structure Database, and chart the structural chemistry of κ into extended van-Arkel triangles. Together with the newly developed κ map and our theoretical tool, we identify rare-earth chalcogenides as promising candidates, of which we measured ZT exceeding 1.0. We note that the κ chart can be further explored, and our computational and analytical tools are applicable generally for materials informatics.

Understanding and optimizing the key mechanisms used in the synthesis of pitch-based carbon fibers (CFs) are challenging, because unlike polyacrylonitrile-based CFs, the feedstock for pitch-based CFs is chemically heterogeneous, resulting in complex fabrication leading to inconsistency in the final properties. In this work, we use molecular dynamics simulations to explore the processing and chemical phase space through a framework of CF models to identify their effects on elastic performance. The results are in excellent agreement with experiments. We find that density, followed by alignment, and functionality of the molecular constituents dictate the CF mechanical properties more strongly than their size and shape. Last, we propose a previously unexplored fabrication route for high-modulus CFs. Unlike graphitization, this results in increased sp 3 fraction, achieved via generating high-density CFs. In addition, the high sp 3 fraction leads to the fabrication of CFs with isometric compressive and tensile moduli, enabling their potential applications for compressive loading.

Molecular ferroelectrics combine electromechanical coupling and electric polarizabilities, offering immense promise in stimuli-dependent metamaterials. Despite such promise, current physical realizations of mechanical metamaterials remain hindered by the lack of rapid-prototyping ferroelectric metamaterial structures. Here, we present a continuous rapid printing strategy for the volumetric deposition of water-soluble molecular ferroelectric metamaterials with precise spatial control in virtually any three-dimensional (3D) geometry by means of an electric-field-assisted additive manufacturing. We demonstrate a scaffold-supported ferroelectric crystalline lattice that enables self-healing and a reprogrammable stiffness for dynamic tuning of mechanical metamaterials with a long lifetime and sustainability. A molecular ferroelectric architecture with resonant inclusions then exhibits adaptive mitigation of incident vibroacoustic dynamic loads via an electrically tunable subwavelength-frequency band gap. The findings shown here pave the way for the versatile additive manufacturing of molecular ferroelectric metamaterials.

Vertical van der Waals (vdWs) heterostructures based on layered materials are attracting interest as a new class of quantum materials, where interfacial charge-transfer coupling can give rise to fascinating strongly correlated phenomena. Transition metal chalcogenides are a particularly exciting material family, including ferromagnetic semiconductors, multiferroics, and superconductors. Here, we report the growth of an organic-inorganic heterostructure by intercalating molecular electron donating bis(ethylenedithio)tetrathiafulvalene into (Li,Fe)OHFeSe, a layered material in which the superconducting ground state results from the intercalation of hydroxide layer. Molecular intercalation in this heterostructure induces a transformation from a paramagnetic to spin-glass-like state that is sensitive to the stoichiometry of molecular donor and an applied magnetic field. Besides, electron-donating molecules reduce the electrical resistivity in the heterostructure and modify its response to laser illumination. This hybrid heterostructure provides a promising platform to study emerging magnetic and electronic behaviors in strongly correlated layered materials.

Half-Heusler (HH) compounds are among the most promising thermoelectric (TE) materials for large-scale applications due to their superior properties such as high power factor, excellent mechanical and thermal reliability, and non-toxicity. Their only drawback is the remaining-high lattice thermal conductivity. Various mechanisms were reported with claimed effectiveness to enhance the phonon scattering of HH compounds including grain-boundary scattering, phase separation, and electron–phonon interaction. In this work, however, we show that point-defect scattering has been the dominant mechanism for phonon scattering other than the intrinsic phonon–phonon interaction for ZrCoSb and possibly many other HH compounds. Induced by the charge-compensation effect, the formation of Co/4d Frenkel point defects is responsible for the drastic reduction of lattice thermal conductivity in ZrCoSb1−xSnx. Our work systematically depicts the phonon scattering profile of HH compounds and illuminates subsequent material optimizations.

The competing and non-equilibrium phase transitions, involving dynamic tunability of cooperative electronic and magnetic states in strongly correlated materials, show great promise in quantum sensing and information technology. To date, the stabilization of transient states is still in the preliminary stage, particularly with respect to molecular electronic solids. Here, a dynamic and cooperative phase in potassium-7,7,8,8-tetracyanoquinodimethane (K-TCNQ) with the control of pulsed electromagnetic excitation is demonstrated. Simultaneous dynamic and coherent lattice perturbation with 8 nspulsed laser (532nm, 15 MWcm-2 , 10 Hz)in such a molecular electronic crystal initiates a stable long-lived (over 400 days) conducting paramagnetic state (≈42 Ωcm), showing the charge-spin bistability over a broad temperature range from 2 to 360 K. Comprehensive noise spectroscopy, in situ high-pressure measurements, electron spin resonance (ESR), theoretical model, and scanning tunneling microscopy/spectroscopy (STM/STS) studies provide further evidence that such a transition is cooperative, requiring a dedicated charge-spin-lattice decoupling to activate and subsequently stabilize nonequilibrium phase. The cooperativity triggered by ultrahigh-strain-rate (above 106 s- 1 ) pulsed excitation offers a collective control toward the generation and stabilization of strongly correlated electronic and magnetic orders in molecular electronic solids and offers unique electro-magnetic phases with technological promises.

Ultrathin transition metal carbides with high capacity, high surface area, and high conductivity are a promising family of materials for applications from energy storage to catalysis. However, large-scale, cost-effective, and precursor-free methods to prepare ultrathin carbides are lacking. Here, we demonstrate a direct pattern method to manufacture ultrathin carbides (MoCx, WCx, and CoCx) on versatile substrates using a CO2 laser. The laser-sculptured polycrystalline carbides (macroporous, ~10–20 nm wall thickness, ~10 nm crystallinity) show high energy storage capability, hierarchical porous structure, and higher thermal resilience than MXenes and other laser-ablated carbon materials. A flexible supercapacitor made of MoCx demonstrates a wide temperature range (−50 to 300 °C). Furthermore, the sculptured microstructures endow the carbide network with enhanced visible light absorption, providing high solar energy harvesting efficiency (~72 %) for steam generation. The laser-based, scalable, resilient, and low-cost manufacturing process presents an approach for construction of carbides and their subsequent applications., Nature Communications, 10, ISSN:2041-1723

In hybrid materials, a high-symmetry lattice is decorated by low-symmetry building blocks. The result is an aperiodic solid that hosts many nearly-degenerate disordered configurations. Using the perovskite methylammonium lead iodide (MAPbI3) as a prototype hybrid material, we show that the inherent disorder renders the conventional phonon picture of transport insufficient. Ab initio molecular dynamics and analysis of the spectral energy density reveal that vibrational carriers simultaneously exhibit features of both classical phonons and of carriers typically found in glasses. The low frequency modes retain elements of acoustic waves but exhibit extremely short lifetimes of only a few tens of picoseconds. For higher frequency modes, strong scattering due to rapid motion and reconfiguration of the organic cation molecules induces a loss of definition of the wave vector. Lattice dynamics shows that these carriers are more akin to diffusons – the nonwave carriers in vitreous materials – and are the dominant contributors to thermal conduction near room temperature. To unify the framework of glassy diffusons with that of phonons scattered at the ultimate limit, three-phonon interactions resolved from first-principles expose anharmonic effects two orders of magnitude higher than in silicon. The dominant anharmonic interactions occur within modes of the PbI6 octahedral framework itself, as well as between modes of the octahedral framework and modes localized to the MA molecules. The former arises from long-range interactions due to resonant bonding, and the latter from polar rotor scattering of the MA molecules. This establishes a clear microscopic connection between symmetry-breaking, dynamical disorder, anharmonicity, and the loss of wave nature in MAPbI3.

Two-dimensional (2D) materials have received considerable attention as possible electrodes in Li-ion batteries (LIBs), although a deeper understanding of the Li adsorption behavior as well as broad screening of the materials space is still needed. In this work, we build a high-throughput screening scheme that incorporates a learned interaction. First, density functional theory and graph convolution networks are utilized to calculate minimum Li adsorption energies for a small set of 2D metallic materials. The data is then used to find a dependence of the minimum Li adsorption energies on the sum of ionization potential, work function of the 2D metal, and coupling energy between Li+ and substrate. Our results show that variances of elemental properties and density are the most correlated features with coupling. To illustrate the applicability of this approach, the model is employed to show that some fluorides and chromium oxides are potential high-voltage materials with adsorption energies < -7 eV, and the found physics is used as the design principle to enhance the Li adsorption ability of graphene. This physics-driven approach shows higher accuracy and transferability compared with purely data-driven models.

In this work, we discover anomalously low lattice thermal conductivity (

Thermal management is of vital importance in various modern technologies such as portable electronics, photovoltaics, and thermoelectric devices. Impeding phonon transport remains one of the most challenging tasks for improving the thermoelectric performance of certain materials such as half-Heusler compounds. Herein, a significant reduction of lattice thermal conductivity (κL ) is achieved by applying a pressure of ≈1 GPa to sinter a broad range of half-Heusler compounds. Contrasting with the common sintering pressure of less than 100 MPa, the gigapascal-level pressure enables densification at a lower temperature, thus greatly modifying the structural characteristics for an intensified phonon scattering. A maximum κL reduction of ≈83% is realized for HfCoSb from 14 to 2.5 W m-1 K-1 at 300 K with more than 95% relative density. The realized low κL originates from a remarkable grain-size refinement to below 100 nm together with the abundant in-grain defects, as determined by microscopy investigations. This work uncovers the phonon transport properties of half-Heusler compounds under unconventional microstructures, thus showing the potential of high-pressure compaction in advancing the performance of thermoelectric materials.

The electron charge density distribution of materials is one of the key quantities in computational materials science as theoretically it determines the ground state energy and practically it is used in many materials analyses. However, the scaling of density functional theory calculations with number of atoms limits the usage of charge-density-based calculations and analyses. Here we introduce a machine-learning scheme with local-environment-based graphs and graph convolutional neural networks to predict charge density on grid points from the crystal structure. We show the accuracy of this scheme through a comparison of predicted charge densities as well as properties derived from the charge density, and that the scaling is $O$($N$). More importantly, the transferability is shown to be high with respect to different compositions and structures, which results from the explicit encoding of geometry.

2D crystals are typically uniform and periodic in-plane with stacked sheet-like structure in the out-of-plane direction. Breaking the in-plane 2D symmetry by creating unique lattice structures offers anisotropic electronic and optical responses that have potential in nanoelectronics. However, creating nanoscale-modulated anisotropic 2D lattices is challenging and is mostly done using top-down lithographic methods with ≈10 nm resolution. A phase transformation mechanism for creating 2D striated lattice systems is revealed, where controlled thermal annealing induces Se loss in few-layered PdSe

Lead Iodide (PbI2) is a large bandgap 2D layered material that has potential for semiconductor applications. However, atomic level study of PbI2 monolayer has been limited due to challenges in obtaining thin crystals. Here, we use liquid exfoliation to produce monolayer PbI2 nanodisks (30-40 nm in diameter and > 99% monolayer purity) and deposit them onto suspended graphene supports to enable atomic structure study of PbI2. Strong epitaxial alignment of PbI2 monolayers with the underlying graphene lattice occurs, leading to a phase shift from the 1 T to 1 H structure to increase the level of commensuration in the two lattice spacings. The fundamental point vacancy and nanopore structures in PbI2 monolayers are directly imaged, showing rapid vacancy migration and self-healing. These results provide a detailed insight into the atomic structure of monolayer PbI2, and the impact of the strong van der Waals interaction with graphene, which has importance for future applications in optoelectronics., Imaging liquid phase exfoliated nanosheets on suspended graphene via annular dark-field STEM can enable identification of various defects, vacancies and their migration. Here, the authors report matching of zigzag edges of monolayer PbI2 with graphene arm-chairs leading to a phase shift from 1 T to 1 H structure to maximize commensuration of the lattices.

Vacancy-ordered lead-free perovskites with more-stable crystalline structures have been intensively explored as the alternatives for resolving the toxic and long-term stability issues of lead halide perovskites (LHPs). The dispersive energy bands produced by the closely packed halide octahedral sublattice in these perovskites are meanwhile anticipated to facility the mobility of charge carriers. However, these perovskites suffer from unexpectedly poor charge carrier transport. To tackle this issue, we have employed the ultrafast, elemental-specific X-ray transient absorption (XTA) spectroscopy to directly probe the photoexcited electronic and structural dynamics of a prototypical vacancy-ordered lead-free perovskite (Cs3Bi2Br9). We have discovered that the photogenerated holes quickly self-trapped at Br centers, simultaneously distorting the local lattice structure, likely forming small polarons in the configuration of V-k center (Br-2(-) dimer). More significantly, we have found a surprisingly long-lived, structural distorted state with a lifetime of, similar to 59 mu s, which is similar to 3 orders of magnitude slower than that of the charge carrier recombination. Such long-lived structural distortion may produce a transient "background" under continuous light illumination, influencing the charge carrier transport along the lattice framework.

Recently, the domains of low-dimensional (low-D) materials and disordered materials have been brought together by the demonstration of several new low-D, disordered systems. The thermal transport properties of these systems are not well-understood. Using amorphous graphene and glassy diamond nanothreads as prototype systems, we establish how structural disorder affects vibrational energy transport in low-dimensional, but disordered, materials. Modal localization analysis, molecular dynamics simulations, and a generalized model together demonstrate that the thermal transport properties of these materials exhibit both similarities and differences from disordered 3D materials. In analogy with 3D, the low-D disordered systems exhibit both propagating and diffusive vibrational modes. In contrast to 3D, however, the diffuson contribution to thermal transport in these low-D systems is shown to be negligible, which may be a result of inherent differences in the nature of random walks in lower dimensions. Despite the lack of diffusons, the suppression of thermal conductivity due to disorder in low-D systems is shown to be mild or comparable to 3D. The mild suppression originates from the presence of low-frequency vibrational modes that maintain a well-defined polarization and help preserve the thermal conductivity in the presence of disorder.

In this study, nitrogen and sulfur co-doped mesoporous carbon (N-S-MPC) materials are selected as the platform to demonstrate the potential of N-S-MPC to replace precious metal catalyst for fuel cell cathode oxygen reduction. Using both N-S-MPC and commercial available 40%Pt/C as cathode catalysts, the effects of catalyst and bonding layer in the catalyst layer (CL) on the power generation performances are thoroughly investigated for alkaline membrane fuel cells (AMFCs). Through single cell tests, several observations are reached as follows: (1) For N-S-MPC cathode, with increasing N-S-MPC loading from 1.00 to 5.00 mg cm−2, the power density reached the maximum (21.7 mW cm−2) when the catalyst loading is 3 mg cm−2. However, for Pt/C cathode the power density reached the maximum (21.3 mW cm−2) for a catalyst loading of 0.5 mg cm−2, with increasing loading from 0.3 to 0.5 mg cm−2; (2) Increasing the thickness of catalyst layer resulted in an increase in power density. Thus, raising the local hydroxyl ion concentration was in favor of the process of oxygen reduction reaction. (3) The bonding layer also has a significant influence on the MEA fabrications, where the MEA using 30 μL bonding layer produced a maximum power density of 20.8 mW cm−2.

Nitrogen and sulfur dual-doped mesoporous carbons (NSMCs) have been fabricated through a facile template-mediated pyrolyzing method using poly(ethyleneimine) (PEI) as sources of nitrogen (N) and carbon (C), ferrous sulfate(FeSO 4 7H 2 O) as both precursor of sulfur (S) and activation reagent along with nanoscaled silica as sacrificial supports. The composition, morphology, and microstructure of the products are characterized using X-ray diffraction, scanning electron microscopy, nitrogen sorption analysis and X-ray photoelectron spectroscopy. It reveals that these NSMCs possess a BET surface area over 1064 m 2 g −1 and abundant mesoporous structure with pore size ranged from 4 to 20 nm. The atomic percentages of N and S functionalities are found to be 4.00 at.% N and 0.83 at.% S, indication of successful incorporation of both nitrogenand sulfur into carbon network. Benefiting from the aforementioned characteristics, these NSMCs show perfect supercapacitive performances, which have been demonstrated by cyclic voltammetry and constant-current charge/discharge cycling techniques. In 0.5 M H 2 SO 4 electrolyte, the specific capacitance (SC) of the as-prepared NSMCs electrode can reach 280 F/g at a current density of 1 A/g. Even at a high rate capability of 100 A/g, the NSMCs electrode still shows the SC value as high as 232 F/g, retaining 83% of that at 1 A/g. Also, the electrode exhibits excellent charge/discharge cycling stability, and no measurable capacitance losses is observed even after 5000 cycles, making them potentially promising for high-performance energy storage devices.

Non precious metal (NPM) catalysts are considered as promising candidates ofreplacing Pt for the oxygen reduction reaction (ORR) inpolymer electrolyte membrane fuel cells for its low cost and rich resource. It has been reported that the substitution of some carbon atoms withheteroatoms, such as N, S, P, I and B, is an effective way totailor NPM catalysts’ electron-donor properties thus effectively weakening the O−O bonding1 and leading to a high ORR catalytic activity. Up to now, extensive research efforts have been made to explore the N doped non-noble metal oxygen reduction catalysts2 since nitrogen has electronegativity higher than other elements3. In our recent work, we found that sulfur-doping could lead to increased catalyst porosity and therefore promote mass transport4.5. As yet, the S doped ones, in particular the S and N co-doped carbon materials are less reported6-8. On the other hand, the specific surface area and porous structure, which determine the accessible part of active sites and the transport properties of ORR-relevant species (H+, e−, O2, H2O), are believed to play the important role in the performance of NPM catalysts.In this regard, template method has drawn great attention to obtain the specified morphologyand predetermined microstructure9,10. Based on the above conceptions, in this work, we report a novel kind of heteroatom-doped carbon catalyst from N-containing polymer and sulfate by template method and acid leaching.Poly(diallyldimethylammonium chloride)(abbreviate as PDDA) was employed as sources of nitrogen and carbon, ferrous sulfate as precursor of sulfur and metal, while the nanoscale silica as sacrificial supports to create pores.

In the past four decades, non-precious metal catalysts (NPMCs) have been extensively studied as low-cost catalyst alternatives to Pt for the oxygen reduction reaction (ORR) in polymer electrolyte membrane (PEM) fuel cells. However, the role of transition metal playing in the catalysts' active sites is still a subject of controversy. In order to further clarify the nature of the active sites of NPMCs, in this work, using aminopyrine (Apyr) as the nitrogen precursor, Co-, Fe-, Cu-, Mn-, and Ni-incorporated nitrogen-containing electrocatalysts are synthesized for fuel cell ORR in alkaline media. The catalysts' ORR performance can be significantly improved by pyrolysis when the catalysts are incorporated by different transition metals. The observed catalytic activity order is: Co ≫ Fe ∼ Cu > Mn ≫ Ni. However, with respect to the electron transfer numbers (selectivity), the order is: Fe > Mn > Co ≫ Cu > Ni. XRD results reveal that Mn and Fe are more likely to be combined with S than Co, Ni and Cu. XPS analysis indicates that N concentration has a negative correlation with S concentration in the pyrolyzed catalysts, indicating a competitive mechanism between N and S on catalyst surfaces when metal sulfate is applied as the transition metal precursor. For ORR active site identification, the surface N species analysis reveals that catalyst containing more M–N group would give a higher catalytic ORR activity, while the metal incorporation is essential in the ORR active site structure, forming the M–Nx/C catalysts rather than just serving to catalyze the formation of N/C active sites.

Although non-precious metal catalysts (NPMCs) have been extensively studied as low-cost catalyst alternatives to Pt, in particular for the oxygen reduction reaction (ORR) in polymer electrolyte membrane fuel cells (PEMFCs), the nature of the active ORR catalytic sites is still a subject of controversy. In this work, using carbon-supported copper phthalocyanine tetrasulfonic acid tetrasodium salt (CuTSPc/C) nanoparticles as the target catalyst, the effects of the transition metal Cu on the ORR active sites are systematically studied using both rotating disk electrode (RDE) and rotating ring disk electrode (RRDE) techniques in alkaline electrolyte. The results show that acid-leaching can significantly decrease the ORR activity of the CuTSPc/C catalyst, with the half-wave potential negatively shifted by more than 50 mV compared to the catalyst before acid-leaching. The electron transfer number of the ORR process catalyzed by the catalyst before acid-leaching remained at about 3.85 over the whole tested potential range from −0.6 to −0.1 V, while this number greatly decreased from 3.82 at −0.55 V to 3.53 at −0.1 V after acid-leaching. The H2O2 produced accordingly increased sharply from 7.8% to 22%. XRD and TEM results indicate that acid-leaching is an effective method to remove metal-Cu. XPS analysis reveals that metal-Cu is essential in the ORR active site structure, and also plays a key part in the stabilization of the active N and S species.

Ion beam irradiation has recently emerged as a versatile approach to functional materials design. We show in this work that patterned defective regions generated by ion beam irradiation of silicon can create a phonon glass electron crystal (PGEC), a longstanding goal of thermoelectrics. By controlling the effective diameter of and spacing between the defective regions, molecular dynamics simulations suggest a reduction of the thermal conductivity by a factor of $\approx$20 is achievable. Boltzmann theory shows that the thermoelectric power factor remains largely intact in the damaged material. To facilitate the Boltzmann theory, we derive an analytical model for electron scattering with cylindrical defective regions based on partial wave analysis. Together we predict a figure of merit of ZT$\approx$0.5 or more at room temperature for optimally patterned geometries of these silicon metamaterials. These findings indicate that nanostructuring of patterned defective regions in crystalline materials is a viable approach to realize a PGEC, and ion beam irradiation could be a promising fabrication strategy., 21 pages, 4 figures

Classical molecular dynamics calculations were used to investigate the formation of defects produced during irradiation of energetic ions on silicon. The aim of this study was to characterize the nature of defects and defective regions formed through ion irradiation and to establish a connection between the ion irradiation parameters, lattice defect configurations, and the resulting modified lattice thermal conductivity of silicon. The defective regions were characterized according to the total number of defects generated, the size and the density of the defective region, and the longitudinal and radial distribution of defects along the ion impact path. In addition, the clustering of the defects into amorphous pockets is analyzed and the effect of these processing parameters on the properties of the clusters is also studied. Further, the lattice defect configurations produced during continuous bombardment of multiple ions are directly investigated and compared to the single-ion impact results. A range of irradiation parameters including ion species, ion energies, fluence, and beam width have been explored to elucidate the dependence of the resulting defect configurations on these experimental design parameters. High density defective regions are found to be produced by low-energy ions with high atomic number. Analysis of the defects produced under varying beam diameters indicates that the beam diameter, rather than the beam energy, is the more prominent factor in determining the extent of the defective region. We demonstrate that the thermal conductivity of the material is most significantly influenced by the effective diameter of the defective region, making the beam diameter the most influential experimental parameter for tuning the lattice thermal conductivity. A reduction in thermal conductivity of up to 80% from pristine silicon was achieved with the processing parameters used in this work. This study indicates that ion beam irradiation can be a realizable manufacturing process with high tunability and control to achieve desired material properties.

2D crystals are typically uniform and periodic in-plane with stacked sheet-like structure in the out-of-plane direction. Breaking the in-plane 2D symmetry by creating unique lattice structures offers anisotropic electronic and optical responses that have potential in nanoelectronics. However, creating nanoscale-modulated anisotropic 2D lattices is challenging and is mostly done using top-down lithographic methods with ≈10 nm resolution. A phase transformation mechanism for creating 2D striated lattice systems is revealed, where controlled thermal annealing induces Se loss in few-layered PdSe2 and leads to 1D sub-nm etched channels in Pd2 Se3 bilayers. These striated 2D crystals cannot be described by a typical unit cells of 1-2 A for crystals, but rather long range nanoscale periodicity in each three directions. The 1D channels give rise to localized conduction states, which have no bulk layered counterpart or monolayer form. These results show how the known family of 2D crystals can be extended beyond those that exist as bulk layered van der Waals crystals by exploiting phase transformations by elemental depletion in binary systems.

We present a generalized model to describe the lattice thermal conductivity of low-dimensional (low-D) and disordered systems. The model is a straightforward generalization of the Debye-Peierls and Allen-Feldman schemes to arbitrary dimensions, accounting for low-D effects such as differences in dispersion, density of states, and scattering. Similar in spirit to the Allen-Feldman approach, heat carriers are categorized according to their transporting capacity as propagons, diffusons, and locons. The results of the generalized model are compared to experimental results when available, and equilibrium molecular dynamics simulations otherwise. The results are in very good agreement with our analysis of phonon localization in disordered low-D systems, such as amorphous graphene and glassy diamond nanothreads. Several unique aspects of thermal transport in low-D and disordered systems, such as milder suppression of thermal conductivity and negligble diffuson contributions, are captured by the approach., Comment: 7 figures

This paper presents a gas-kinetic scheme to solve the multiple temperature kinetic model (MTKM), which was proposed in J. Comput. Math. 29(6) (2011) 639-660, for the study of non-equilibrium flows. The MTKM is a two-stage particle collision model possessing an intermediate quasi-equilibrium state with a symmetric second-order temperature tensor. A gas-kinetic finite volume scheme is developed for the numerical solution of the MTKM in the continuum and transition flow regimes. The gas-kinetic scheme is designed for the updating of macroscopic variables, which include the conservative flow variables and the multiple temperature field. In order to validate the kinetic model, the gas-kinetic scheme is used in the study of lid-driven cavity flows in both continuum and transition flow regimes. The numerical results predicted by the MTKM are compared with those from the direct simulation Monte Carlo (DSMC) method, the Navier-Stokes equations (NSE), and the early three-temperature kinetic model (TFKM) proposed in Phys. Fluids 19, 016101(2007). It is demonstrated that the MTKM has obvious advantages in comparison with the NSE and the TTKM in capturing the non-equilibrium flow behavior in the transition flow regime. One distinguishable phenomenon captured by the MTKM is that in the transition flow regime the heat flux direction can be from a low temperature to a high temperature region, which violates the Fourier's law of continuum flows. The MTKM provides a more accurate physical model than the NSE for the non-equilibrium flows.

This article presents a comprehensive study of various modeling techniques for noncontinuum gas-phase heat conduction encountered in micro/nano devices over a broad range of Knudsen number. A new slip model is proposed for slip flows and an analytical approach is developed for collisionless steady-state heat conduction inside a fully diffuse enclosure. Excellent agreements with direct simulation Monte Carlo (DSMC) simulations have been achieved for both of them. For problems in the transition regime and/or with partially thermal accommodated walls, the DSMC method is employed. Some noncontinuum phenomena such as the steady gas flows induced by the nonuniform temperature field are observed.

Vibrational energy transport in disordered media is of fundamental importance to several fields spanning from sustainable energy to biomedicine to thermal management. This work investigates hybrid ordered/disordered nanocomposites that consist of crystalline membranes decorated by regularly patterned disordered regions formed by ion beam irradiation. The presence of the disordered regions results in reduced thermal conductivity, rendering these systems of interest for use as nanostructured thermoelectrics and thermal device components, yet their vibrational properties are not well understood. Here, the mechanism of vibrational transport and the reason underlying the observed reduction is established in detail. The hybrid systems are found to exhibit glass-crystal duality in vibrational transport. Lattice dynamics reveals substantial hybridization between the localized and delocalized modes, which induces avoided crossings and harmonic broadening in the dispersion. Allen/Feldman theory shows that the hybridization and avoided crossings are the dominant drivers of the reduction. Anharmonic scattering is also enhanced in the patterned nanocomposites, further contributing to the reduction. The systems exhibit features reminiscent of both nanophononic materials and locally resonant nanophononic metamaterials, but operate in a manner distinct to both. These findings indicate that such “patterned disorder” can be a promising strategy to tailor vibrational transport through hybrid nanostructures.

Using nonequilibrium molecular dynamics and lattice dynamics, we investigate phonon conduction on two-dimensional graphene/boron nitride superlattices with varying periods and interface structures. As the period of superlattice increases to a critical value near 5 nm the lattice thermal conductivity drops sharply to a minimum, and beyond that it smoothly increases with the period. We show that the minimum in the thermal conductivity arises from a competition between lattice dispersion and anharmonic effects such as interface scattering. The initial reduction of thermal conductivity can partially be accounted for by harmonic wave effects induced by interfacial modulation, such as the opening of phononic band gaps and reduction of group velocity. Beyond the minimum, reduced inelastic interface scattering is responsible for the recovery. The overall range of thermal conductivity exhibited by the superlattices is substantially reduced with respect to the parent materials. A universal scaling of the thermal conductivity with total superlattice length is found, suggesting that the critical period is independent of total length and that long-wavelength phonons are dominant carriers. Furthermore, we demonstrate the ultrasensitivity of thermal conductivity to interfacial defects and superlattice periodicity disorder.

A theoretical approach based on gaskinetic theory is described and applied for the modeling of steady-state free-molecule gaseous heat conduction within a diffusive enclosure. With a representative model of microelectromechanical system (MEMS) devices with integrated heaters, the heat transfer between the heated component and its gaseous ambient enclosed in a high vacuum is studied in detail. A molecular simulation based on the direct simulation Monte Carlo (DSMC) method is also employed to validate the theoretical solutions and to study the effects of incomplete thermal accommodation. The impacts of the finite size of the heated beam as well as the gap between the beam and a substrate on the heat transfer are investigated to examine the appropriateness of the common assumptions employed in the modeling of Pirani sensors. Interesting phenomena that are unique in the free-molecule regime are observed and discussed. These studies are valuable to the design of MEMS devices with microheaters.

Knudsen force acting on a heated microbeam adjacent to a cold substrate in a rarefied gas is a mechanical force created by unbalanced thermal gradients. The measured force has its direction pointing towards the side with a lower thermal gradient and its magnitude vanishes in both continuum and free-molecule limits. In our previous study, negative Knudsen forces were discovered at the high Knudsen regime before diminishing in the free-molecule limit. Such a phenomenon was however not observed in the experiment. In this paper, the existence of such a negative Knudsen force is further confirmed using both numerical simulation and theoretical analysis. The asymptotic order of the Knudsen force near the collisionless limit is analyzed and the analytical expression of its leading term is provided, from which approaches for the enhancement of negative Knudsen forces are proposed.Copyright © 2012 by ASME

Knudsen force acting on a heated microbeam adjacent to a cold substrate in a rarefied gas is a mechanical force created by unbalanced thermal gradients. The measured force has its direction pointing towards the side with a lower thermal gradient and its magnitude vanishes in both continuum and free-molecule limits. In our previous study, negative Knudsen forces were discovered at the high Knudsen regime before diminishing in the free-molecule limit. Such a phenomenon was, however, neither observed in experiment [A. Passian et al., Phys. Rev. Lett. 90, 124503 (2003)], nor captured in the latest numerical study [J. Nabeth et al., Phys. Rev. E 83, 066306 (2011)]. In this paper, the existence of such a negative Knudsen force is further confirmed using both numerical simulation and theoretical analysis. The asymptotic order of the Knudsen force near the collisionless limit is analyzed and the analytical expression of its leading term is provided, from which approaches for the enhancement of negative Knudsen forces are proposed. The discovered phenomenon could find its applications in novel mechanisms for pressure sensing and actuation.

The presented work probes the fundamentals of Knudsen forces. Using the direct simulation Monte Carlo (DSMC) method, the flows induced by temperature inhomogeneity within a representative configuration and the Knudsen force acting on a heated microbeam are captured as functions of Knudsen number in the entire flow regime. Both flow strength and Knudsen force peak in the transition regime and negative Knudsen force absent in experimental data is observed. The mechanisms of the thermally induced flows and Knudsen forces are studied. It has been found that thermal edge flow is the main driven source for the formation of the Knudsen force on microbeams and domain configuration plays an important role in the process.

The modeling of heat transfer inside a vacuum packaged MEMS devices has been performed by several researchers mostly through Monte Carlo simulations. In this work, we employ an analytical approach to study the heat transport of gas inside a high vacuum enclosure. In this pressure range, the interaction between gas molecules is negligible compared to their interaction with the walls, and hence the gas is treated as the free-molecule gas. The heated cantilever is modeled as a uniform beam with a rectangular cross section located at a certain distance away from the bottom wall which could represent a substrate in the real device. To account for various situations, the temperatures of the surrounding walls are allowed to be different from each other and different from that of the beam and the substrate. The temperature contour and the heat flux are obtained from the analytical approach. A molecular simulation code based on the direct simulation Monte Carlo (DSMC) has been developed and employed to validate the analytical results and excellent agreements have been obtained. The effects of incomplete thermal accommodation are also investigated. It is anticipated that the developed analytical solutions would be very valuable to the design of Pirani sensors and other MEMS devices utilizing micro heaters, for example, the thermal sensing atomic force microscope.

Supercapacitors are one of the most promising electrochemical energy-storage device and have been widely applied due to their advantages of large power density, long cycle stability and environmentally friendly[1]. In the past few decades, many metal oxides were investigated as active electrode material for supercapacitor, among which ruthenium oxide (RuO2) is the best candidate[ 2-3] . However, the high cost and toxic nature of RuO2 has limited its practical production and commercial applications. Aiming at new cost-effective, high performance electrode material for supercapacitors, partial or complete replacement of precious metal has been under great effort by many research groups[4]. Manganes oxide (MnO2) is generally considered to be the most promising transition metal oxide for the next-generation supercapacitors by virtue of its high energy density, low cost, environmental friendliness, and natural abundance. For obtaining the specified morphology with increased surface area and predetermined microstructure, the MnO2 has been proposed to be synthesized by various methods such as hydrothermal method[5] and microwave method[6] to realize the improved reaction kinetics. However, these methods are relatively complex, which inevitably adds the cost. In this work, use glucose reducing potassium ermanganate, the mesoporous MnO2 can be easily prepared at ambient conditions with high production rate. Thus a very simple, economic and green synthesis procedure is realized compared to other procedures reported in the literature. In addition, the as-prepared mesoporous MnO2 exhibits high-performance in electrochemical capacitors.