Sound and heat revolutions in phononics

Electron-Transfer-Induced Thermal and Thermoelectric Rectification

Controlling the direction and magnitude of both heat and electronic currents using rectifiers has significant implications for the advancement of molecular circuit design. In order to facilitate the implementation of new transport phenomena in such molecular structures, we examine thermal and thermoelectric rectification effects that are induced by an electron transfer process that occurs across a temperature gradient between molecules. Historically, the only known heat conduction mechanism able to generate thermal rectification in purely molecular environments is phononic heat transport. Here, we show that electron transfer between molecular sites with different local temperatures can also generate a thermal rectification effect and that electron hopping through molecular bridges connecting metal leads at different temperatures gives rise to asymmetric Seebeck effects, that is, thermoelectric rectification, in molecular junctions.

Highly Stretchable Micro/Nano Wrinkle Structures for Infrared Stealth Application

We demonstrate a novel infrared stealth structure consisting of SiO₂/TiO₂ film, which was manufactured as the highly stretchable triangular wrinkle structures. The triangular wrinkle structures have firstly been transferred to the flexible substrate from the surface of Si-substrate, which was manufactured by the MEMS technology. Then, the infrared reflective film have been manufactured to be the triangular wrinkle structures by depositing the materials (noble metal (Ag or Au) or multilayer oxide (SiO₂/TiO₂)) on the surface of flexible substrate. Due to the lower reflection effect of curved surface, the infrared reflectivity of these structures has been tuned down to 5%. And, compared to the flat surface, the reflection-to-diffuse ratios improved approximately one order of magnitude. These structures can adapt to the environment by changing the reflectivity of triangular wrinkle structures under stretching. Finally, an Au-modified infrared stealth structure has been fabricated as the array structures, which disappeared and then display by stretching the triangular wrinkle structures at room temperature. It features high reflection-to-diffuse ratios, stable repeatability, low-cost, and easy to manufacture. It may open opportunities for infrared camouflage for military security and surveillance field application.

Thermal transport crossover from crystalline to partial-crystalline partial-liquid state

Phase-change materials (crystalline at low temperatures and partial-crystalline partial-liquid state at high temperatures) are widely used as thermoelectric converters and battery electrodes. Here, we report the underlying mechanisms driving the thermal transport of the liquid component, and the thermal conductivity contributions from phonons, vibrations with extremely short mean free path, liquid and lattice-liquid interactions in phase-changed Li₂S. In the crystalline state (T ≤ 1000 K), the temperature dependent thermal conductivity manifests two different behaviors, i.e., a typical trend of 1/T below 800 K and an even faster decrease between 800 and 1000 K. For the partial-crystalline partial-liquid Li₂S when T ≥ 1100 K, the contributions of liquid and lattice-liquid interactions increase significantly due to the fluidization of Li ions, and the vibrations with extremely short mean free path, presumably assimilated to diffusons, can contribute up to 46% of the total thermal conductivity at T = 1300 K.

Phase-change materials are applied as thermoelectric converters and battery electrodes, but underlying mechanisms are not fully understood. Here, the authors comprehensively describe thermal transport mechanisms of lithium sulfide based on molecular dynamics and first-principles simulations.

Phonon and heat transport control using pillar-based phononic crystals

Phononic crystals have been studied for the past decades as a tool to control the propagation of acoustic and mechanical waves. Recently, researchers proposed that nanosized phononic crystals can also control heat conduction and improve the thermoelectric efficiency of silicon by phonon dispersion engineering. In this review, we focus on recent theoretical and experimental advances in phonon and thermal transport engineering using pillar-based phononic crystals. First, we explain the principles of the phonon dispersion engineering and summarize early proof-of-concept experiments. Next, we review recent simulations of thermal transport in pillar-based phononic crystals and seek to uncover the origin of the observed reduction in the thermal conductivity. Finally, we discuss first experimental attempts to observe the predicted thermal conductivity reduction and suggest the directions for future research.

On-chip valley topological materials for elastic wave manipulation

Valley topological materials, in which electrons possess valley pseudospin, have attracted a growing interest recently. The additional valley degree of freedom offers a great potential for its use in information encoding and processing. The valley pseudospin and valley edge transport have been investigated in photonic and phononic crystals for electromagnetic and acoustic waves, respectively. In this work, by using a micromanufacturing technology, valley topological materials are fabricated on silicon chips, which allows the observation of gyral valley states and valley edge transport for elastic waves. The edge states protected by the valley topology are robust against the bending and weak randomness of the channel between distinct valley Hall phases. At the channel intersection, a counterintuitive partition of the valley edge states manifests for elastic waves, in which the partition ratio can be freely adjusted. These results may enable the creation of on-chip high-performance micro-ultrasonic materials and devices.

Experimental Demonstration of an Ultrabroadband Nonlinear Cloak for Flexural Waves

Broadband cloaking of flexural waves is a major challenge since the governing equation is not form invariant under coordinate transformations. We fabricate a flexural cloaking structure using only a single material composed of homogeneous and isotropic layers, and then present experimental evidence of the first near-ideal broadband cloak in thin plates. The 3D-printed structure is shown to effectively disguise an object over a broad frequency range (2 kHz-11 kHz). The proposed cloak has potential applications in shielding sensors and sensitive components from vibrations in bridges, automobiles, and aircraft.

Damping Off Terahertz Sound Modes of a Liquid upon Immersion of Nanoparticles

The control of phonon propagation in nanoparticle arrays is one of the frontiers of nanotechnology, potentially enabling the discovery of materials with unknown functionalities for potential innovative applications. The exploration of the terahertz window appears quite promising as phonons in this range are the leading carriers of heat transport in insulators and their control is the key to implement devices for heat flow management. Unfortunately, this scientific field is still in its infancy, and even a basic topic such as the influence of floating nanoparticles on the terahertz phonon propagation of a colloidal suspension still eludes a firm answer. Shedding some light on this topic is the main motivation of the present work, which focuses an inelastic X-ray scattering (IXS) measurements on a dilute suspension of Au nanospheres in water. Measured spectra showed a nontrivial shape displaying multiple inelastic features that, based on a Bayesian inference analysis, we assign to phonon modes propagating throughout the nanoparticle interior. Surprisingly, the spectra bear no evidence of propagating modes, which are known to dominate the spectrum of pure water, owing to the scattering that these modes suffer from the sparse nanoparticles in suspension. In perspective, this finding may inspire simple routes to manipulate high-frequency acoustic propagation in hybrid-liquid and solid-materials.

Intrinsic spin of elastic waves

Unveiling spins of physical systems usually gives people a fundamental understanding of the geometrical properties of waves from classical to quantum aspects. A great variety of research has shown that transverse waves can possess nontrivial spins and spin-related properties naturally. However, until now, we still lack essential physical insights about the spin nature of longitudinal waves. Here, demonstrated by elastic waves, we uncover spins for longitudinal waves and the mixed longitudinal-transverse waves that play essential roles in spin-momentum locking. Based on this spin perspective, several abnormal phenomena beyond pure transverse waves are attributed to the hybrid spin induced by mixed longitudinal-transverse waves. The unique hybrid spin reveals the complex spin essence in elastic waves and advances our understanding about their fundamental geometrical properties. We also show that these spin-dependent phenomena can be exploited to control the wave propagation, such as nonsymmetric elastic wave excitation by spin pairs, a unidirectional Rayleigh wave, and spin-selected elastic wave routing. These findings are generally applicable for wave cases with longitudinal and transverse components.

Frequency domain transformation optics for diffusive photon density waves' cloaking

We make use of transformation optics technique to realize cloaking operation in the light diffusive regime, for spherical objects. The cloak requires spatially heterogeneous anisotropic diffusivity, as well as spatially varying speed of light and absorption. Analytic calculations of Photon's fluence confirm minor role of absorption in reduction of far-field scattering, and a monopole fluence field converging to a constant in the static regime in the invisibility region. The latter is in contrast to acoustic and electromagnetic cloaks, for which the field vanishes inside the core. These results are finally discussed in the context of mass diffusion, where cloaking can be achieved with a heterogeneous anisotropic diffusivity.

Elastic Purcell Effect

In this work, we introduce an elastic analog of the Purcell effect and show theoretically that spherical nanoparticles can serve as tunable and robust antennas for modifying the emission from localized elastic sources. This effect can be qualitatively described by introducing elastic counterparts of the familiar electromagnetic parameters: local density of elastic states, elastic Purcell factor, and effective volume of elastic modes. To illustrate our framework, we consider the example of a submicron gold sphere as a generic elastic GHz antenna and find that shear and mixed modes of low orders in such systems offer considerable elastic Purcell factors. This formalism opens pathways towards extended control over dissipation of vibrations in various optomechanical systems and contributes to closing the gap between classical and quantum-mechanical treatments of phonons localized in elastic nanoresonators.

Coherent control of thermal phonon transport in van der Waals superlattices

van der Waals (vdW) heterostructures are a central focus of materials science and condensed matter physics due to the novel physical phenomena and properties obtained by precisely stacking heterogeneous atomically thin layers. vdW heterostructures are expected to allow for the coherent manipulation of THz lattice vibrations and hence heat conduction due to the ability to precisely control chemical composition at the atomic scale, but little work has focused on thermal transport in these materials. Here, we report an ab initio study of thermal transport in vdW superlattices consisting of alternating transition metal dichalcogenide atomic layers. Our calculations show that the lattice vibrational spectrum and scattering rates can be precisely manipulated by the choice of each atomically thin layer, resulting in materials with novel properties such as large thermal anisotropies approaching 200 and ultralow cross-plane thermal conductivities comparable to those of amorphous materials. Our work demonstrates how coherent manipulation of phonons in vdW superlattices can expand the property space beyond that occupied by natural materials and suggests an experimental route to realize these properties.

Single-Mode Phononic Wire

Photons and electrons transmit information to form complex systems and networks. Phonons on the other hand, the quanta of mechanical motion, are often considered only as carriers of thermal energy. Nonetheless, their flow can also be molded in fabricated nanoscale circuits. We design and experimentally demonstrate wires for phonons by patterning the surface of a silicon chip. Our device eliminates all but one channel of phonon conduction, allowing coherent phonon transport over millimeter length scales. We characterize the phononic wire optically, by coupling it strongly to an optomechanical transducer. The phononic wire enables new ways to manipulate information and energy on a chip. In particular, our result is an important step towards realizing on-chip phonon networks, in which quantum information is transmitted between nodes via 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.

Unusually low thermal conductivity of atomically thin 2D tellurium

Tellurium is a high-performance thermoelectric material due to its superior electronic transport and low lattice thermal conductivity (κL). Here, we report the ultralow κL in the monolayer tellurium, i.e., tellurene, which has been successfully synthesized in recent experiments. We find that tellurene has a compellingly low room temperature κL of 2.16 and 4.08 W m-1 K-1 along the armchair and zigzag directions, respectively, which is lower than any reported values for other 2D materials. We attribute this unusually low κL to the soft acoustic modes, the extremely low-energy optical modes and the strong scattering among optical-acoustic phonons, which place tellurene as a potential novel thermoelectric material. Finally, we show that κL is proportional to the largest acoustic phonon frequency (ωaD) and the lowest optical phonon frequency at the Γ point (ωoΓ) in 2D materials, which reflect both harmonic and anharmonic thermal properties, respectively.

Metastable modular metastructures for on-demand reconfiguration of band structures and nonreciprocal wave propagation

We present an approach to achieve adaptable band structures and nonreciprocal wave propagation by exploring and exploiting the concept of metastable modular metastructures. Through studying the dynamics of wave propagation in a chain composed of finite metastable modules, we provide experimental and analytical results on nonreciprocal wave propagation and unveil the underlying mechanisms that facilitate such unidirectional energy transmission. In addition, we demonstrate that via transitioning among the numerous metastable states, the proposed metastructure is endowed with a large number of bandgap reconfiguration possibilities. As a result, we illustrate that unprecedented adaptable nonreciprocal wave propagation can be realized using the metastable modular metastructure. Overall, this research elucidates the rich dynamics attainable through the combinations of periodicity, nonlinearity, spatial asymmetry, and metastability and creates a class of adaptive structural and material systems capable of realizing tunable bandgaps and nonreciprocal wave transmissions.

Solitons as candidates for energy carriers in Fermi-Pasta-Ulam lattices

Currently, effective phonons (renormalized or interacting phonons) rather than solitary waves (for short, solitons) are regarded as the energy carriers in nonlinear lattices. In this work, by using the approximate soliton solutions of the corresponding equations of motion and adopting the Boltzmann distribution for these solitons, the average velocities of solitons are obtained and are compared with the sound velocities of energy transfer. Excellent agreements with the numerical results and the predictions of other existing theories are shown in both the symmetric Fermi-Pasta-Ulam-β lattices and the asymmetric Fermi-Pasta-Ulam-αβ lattices. These clearly indicate that solitons are suitable candidates for energy carriers in Fermi-Pasta-Ulam lattices. In addition, the root-mean-square velocity of solitons can be obtained from the effective phonons theory.

Acoustic properties of porous microlattices from effective medium to scattering dominated regimes

Microlattices are architected materials that allow for an unprecedented control of mechanical properties (e.g., stiffness, density, and Poisson's coefficient). In contrast to their quasi-static mechanical properties, the acoustic properties of microlattices remain largely unexplored. This paper analyzes the acoustic response of periodic millimeter-sized microlattices immersed in water using experiments and numerical simulations. Microlattices are fabricated using high-precision stereolithographic three-dimensional printing in a large variety of porosities and lattice topologies. This paper shows that the acoustic propagation undergoes a frequency dependent transition from a classic poroelastic behaviour that can be described by Biot's theory to a regime that is dominated by scattering effects. Biot's acoustic parameters are derived from direct simulations of the microstructure using coupled fluid and solid finite elements. The wave speeds predicted with Biot's theory agree well with the experimental measures. Within the scattering regime, the signals show a strong attenuation and dispersion, which is characterized by a cut-off frequency. The strong dispersion results in a frequency dependent group velocity. A simplified model of an elastic cylindrical scatterer allows predicting the signal attenuation and dispersion observed experimentally. The results in this paper pave the way for the creation of microlattice materials for the control of ultrasonic waves across a wide range of frequencies.

Elastic waves in periodically heterogeneous two-dimensional media: locally periodic and anti-periodic modes

Propagation of anti-plane waves through a discrete square lattice and through a continuous fibrous medium is studied. In the long-wave limit, for periodically heterogeneous structures the solution can be periodic or anti-periodic across the unit cell. It is shown that combining periodicity and anti-periodicity conditions in different directions of the translational symmetry allows one to detect different types of modes that do not arise in the purely periodic case. Such modes may be interpreted as counterparts of non-classical waves appearing in phenomenological theories. Dispersion diagrams of the discrete square lattice are evaluated in a closed analytical from. Dispersion properties of the fibrous medium are determined using Floquet–Bloch theory and Fourier series approximations. Influence of a viscous damping is taken into account.

Thermal Studies of Nanoporous Si Films with Pitches on the Order of 100 nm -Comparison between Different Pore-Drilling Techniques

In recent years, nanoporous Si films have been widely studied for thermoelectric applications due to the low cost and earth abundance of Si. Despite many encouraging results, inconsistency still exists among experimental and theoretical studies of reduced lattice thermal conductivity for varied nanoporous patterns. In addition, divergence can also be found among reported data, due to the difference in sample preparation and measurement setups. In this work, systematic measurements are carried out on nanoporous Si thin films with pore pitches on the order of 100 nm, where pores are drilled either by dry etching or a focused ion beam. In addition to thermal conductivity measurements, the specific heat of the nanoporous films is simultaneously measured and agrees with the estimation using bulk values, indicating a negligible change in the phonon dispersion. Without considering coherent phonon transport, the measured thermal conductivity values agree with predictions by frequency-dependent phonon Monte Carlo simulations assuming diffusive pore-edge phonon scattering. In Monte Carlo simulations, an expanded effective pore diameter is used to account for the amorphization and oxidation on real pore edges.

On the dynamic homogenization of periodic media: Willis' approach versus two-scale paradigm

When considering an effective, i.e. homogenized description of waves in periodic media that transcends the usual quasi-static approximation, there are generally two schools of thought: (i) the two-scale approach that is prevalent in mathematics and (ii) the Willis' homogenization framework that has been gaining popularity in engineering and physical sciences. Notwithstanding a mounting body of literature on the two competing paradigms, a clear understanding of their relationship is still lacking. In this study, we deploy an effective impedance of the scalar wave equation as a lens for comparison and establish a low-frequency, long-wavelength dispersive expansion of the Willis' effective model, including terms up to the second order. Despite the intuitive expectation that such obtained effective impedance coincides with its two-scale counterpart, we find that the two descriptions differ by a modulation factor which is, up to the second order, expressible as a polynomial in frequency and wavenumber. We track down this inconsistency to the fact that the two-scale expansion is commonly restricted to the free-wave solutions and thus fails to account for the body source term which, as it turns out, must also be homogenized-by the reciprocal of the featured modulation factor. In the analysis, we also (i) reformulate for generality the Willis' effective description in terms of the eigenfunction approach, and (ii) obtain the corresponding modulation factor for dipole body sources, which may be relevant to some recent efforts to manipulate waves in metamaterials.

Interaction-tailored organization of large-area colloidal assemblies

Colloidal lithography is an innovative fabrication technique employing spherical, nanoscale crystals as a lithographic mask for the low cost realization of nanoscale patterning. The features of the resulting nanostructures are related to the particle size, deposition conditions and interactions involved. In this work, we studied the absorption of polystyrene spheres onto a substrate and discuss the effect of particle-substrate and particle-particle interactions on their organization. Depending on the nature and the strength of the interactions acting in the colloidal film formation, two different strategies were developed in order to control the number of particles on the surface and the interparticle distance, namely changing the salt concentration and absorption time in the particle solution. These approaches enabled the realization of large area (≈cm2) patterning of nanoscale holes (nanoholes) and nanoscale disks (nanodisks) of different sizes and materials.

Phonon Networks with Silicon-Vacancy Centers in Diamond Waveguides

We propose and analyze a novel realization of a solid-state quantum network, where separated silicon-vacancy centers are coupled via the phonon modes of a quasi-one-dimensional diamond waveguide. In our approach, quantum states encoded in long-lived electronic spin states can be converted into propagating phonon wave packets and be reabsorbed efficiently by a distant defect center. Our analysis shows that under realistic conditions, this approach enables the implementation of high-fidelity, scalable quantum communication protocols within chip-scale spin-qubit networks. Apart from quantum information processing, this setup constitutes a novel waveguide QED platform, where strong-coupling effects between solid-state defects and individual propagating phonons can be explored at the quantum level.

Spiral-Based Phononic Plates: From Wave Beaming to Topological Insulators

Phononic crystals and metamaterials can sculpt elastic waves, controlling their dispersion using different mechanisms. These mechanisms are mostly Bragg scattering, local resonances, and inertial amplification, derived from ad hoc, often problem-specific geometries of the materials' building blocks. Here, we present a platform that ultilizes a lattice of spiraling unit cells to create phononic materials encompassing Bragg scattering, local resonances, and inertial amplification. We present two examples of phononic materials that can control waves with wavelengths much larger than the lattice's periodicity. (1) A wave beaming plate, which can beam waves at arbitrary angles, independent of the lattice vectors. We show that the beaming trajectory can be continuously tuned, by varying the driving frequency or the spirals' orientation. (2) A topological insulator plate, which derives its properties from a resonance-based Dirac cone below the Bragg limit of the structured lattice of spirals.

Anomalous Nanoscale Optoacoustic Phonon Mixing in Nematic Mesogens

Recent inelastic X-ray scattering (IXS) experiments on mesogens have revealed entirely new capabilities with regards to their nanoscale phonon-assisted heat management. Mesogens such as nematic liquid crystals (LCs) are appealing systems for study because their structure and morphology can easily be tuned. We report on Q-resolved ultra-high-resolution IXS, X-ray diffraction, and THz time-domain spectroscopy experiments combined with large-scale all-atom molecular dynamics simulations on the dynamic properties of 5CB LCs. For the first time, we observe a strong mixing of phonon excitations originating from independent in-phase and out-of-phase van-der-Waals-mediated displacement patterns. The coexistence of transverse acoustic and optical modes of 5CB LCs at near room temperature is revealed through the emergent transverse phonon gap and THz light-phonon coupling taking place within the same energy range. Furthermore, our experimental observations are supported by analysis showing correlations of spontaneous fluctuations of LCs on picosecond time scales. These findings are significant for the design of a new generation of soft molecular vibration-sensitive nanoacoustic and optomechanical applications.

Acoustic nonreciprocity in a lattice incorporating nonlinearity, asymmetry, and internal scale hierarchy: Experimental study

In linear time-invariant systems acoustic reciprocity holds by the Onsager-Casimir principle of microscopic reversibility, and it can be broken only by odd external biases, nonlinearities, or time-dependent properties. Recently it was shown that one-dimensional lattices composed of a finite number of identical nonlinear cells with internal scale hierarchy and asymmetry exhibit nonreciprocity both locally and globally. Considering a single cell composed of a large scale nonlinearly coupled to a small scale, local dynamic nonreciprocity corresponds to vibration energy transfer from the large to the small scale, but absence of energy transfer (and localization) from the small to the large scale. This has been recently proven both theoretically and experimentally. Then, considering the entire lattice, global acoustic nonreciprocity has been recently proven theoretically, corresponding to preferential energy transfer within the lattice under transient excitation applied at one of its boundaries, and absence of similar energy transfer (and localization) when the excitation is applied at its other boundary. This work provides experimental validation of the global acoustic nonreciprocity with a one-dimensional asymmetric lattice composed of three cells, with each cell incorporating nonlinearly coupled large and small scales. Due to the intentional asymmetry of the lattice, low impulsive excitations applied to one of its boundaries result in wave transmission through the lattice, whereas when the same excitations are applied to the other end, they lead in energy localization at the boundary and absence of wave transmission. This global nonreciprocity depends critically on energy (i.e., the intensity of the applied impulses), and reduced-order models recover the nonreciprocal acoustics and clarify the nonlinear mechanism generating nonreciprocity in this system.

Illusion Thermotics

"Fata Morgana" or "Mirage" phenomena have long been captivated as optical illusions, which actually relies on gradient-density air or vapor. Man-made optical illusions have witnessed significant progress by resorting to artificially structured metamaterials. Nevertheless, two long-standing challenges remain formidable: first, exotic parameters (negative or less than unity) become inevitable; second, the signature of original object is altered to that of a virtual counterpart. It is thus not able to address the holy grail of illusion per se, since a single virtual object still exposes the location. In this study, those problems are successfully addressed in a particular setup-illusion thermotics, which identically mimics the exterior thermal behavior of an equivalent reference and splits the interior original heat source into many virtual signatures. A general paradigm to design thermal illusion metadevices is proposed to manipulate thermal conduction, and empower robust simultaneous functions of moving, shaping, rotating, and splitting heat sources of arbitrary cross sections. The temperature profile inside the thermal metadevice can mislead the awareness of the real location, shape, size, and number of the actual heat sources. The present concept may trigger unprecedented development in other physical fields to realize multiple functionalized illusions in optics, electromagnetics, etc.

Mix-and-Melt Colloidal Engineering

Increasing significance is being placed on the synthesis of smart colloidal particles, since the route to various meta-materials has been outlined through their bottom-up self-assembly. Unfortunately, making particles with well-defined shape and surface chemistry often requires considerable effort and time, and as such, they are available only in restrictive yields. Here we report a synthetic methodology, which we refer to as mix-and-melt reactions (MMR), that allows for rapid prototyping and mass production of anisotropic core-shell colloids. MMR take advantage of the synergistic properties between common colloidal suspensions by aggregating then reconfiguring polystyrene shell particles onto core particle substrates. By systematically exchanging cores and shells, the resultant core-shell particle's properties are manipulated in a modular fashion. The influence of the constituent particles' size ratio is extensively explored, which is shown to tune shell thickness, change the aspect ratio of shells on anisotropic cores, and access specific shapes such as tetrahedra. Beyond particle shape, mixed shell systems are utilized to create regular surface patches. Surface Evolver simulations are used to demonstrate how randomly packed clusters melt into regular shapes via a shell compartmentalization mechanism.

Low Thermal Conductivity through Dense Particle Packings with Optimum Disorder

Heat transport plays a critical role in modern batteries, electrodes, and capacitors. This is caused by the ongoing miniaturization of such nanotechnological devices, which increases the local power density and hence temperature. Even worse, the introduction of heterostructures and interfaces is often accompanied by a reduction in thermal conductivity, which can ultimately lead to the failure of the entire device. Surprisingly, a fundamental understanding of the governing heat transport processes even in simple systems, such as binary particle mixtures is still missing. This contribution closes this gap and elucidates how strongly the polydispersity of a model particulate system influences the effective thermal conductivity across such a heterogeneous system. In a combined experimental and modeling approach, well-defined mixtures of monodisperse particles with varying size ratios are investigated. The transition from order to disorder can reduce the effective thermal conductivity by as much as ≈50%. This is caused by an increase in the thermal transport path length and is governed by the number of interparticle contact points. These results are of general importance for many particulate and heterostructured materials and will help to conceive improved device layouts with more reliable heat dissipation or conservation properties in the future.

Nonreciprocal Thermal Material by Spatiotemporal Modulation

The thermal properties of a material with a spatiotemporal modulation, in the form of a traveling wave, in both the thermal conductivity and the specific heat capacity are studied. It is found that these materials behave as materials with an internal convectionlike term that provides them with nonreciprocal properties, in the sense that the heat flux has different properties when it propagates in the same direction or in the opposite one to the modulation of the parameters. An effective medium description is presented which accurately describes the modulated material, and numerical simulations support this description and verify the nonreciprocal properties of the material. It is found that these materials are promising candidates for the design of thermal diodes and other advanced devices for the control of the heat flow at all scales.

Preparation and Sound Absorption Properties of a Barium Titanate/Nitrile Butadiene Rubber-Polyurethane Foam Composite with Multilayered Structure

Barium titanate/nitrile butadiene rubber (BT/NBR) and polyurethane (PU) foam were combined to prepare a sound-absorbing material with an alternating multilayered structure. The effects of the cell size of PU foam and the alternating unit number on the sound absorption property of the material were investigated. The results show that the sound absorption efficiency at a low frequency increased when decreasing the cell size of PU foam layer. With the increasing of the alternating unit number, the material shows the sound absorption effect in a wider bandwidth of frequency. The BT/NBR-PU foam composites with alternating multilayered structure have an excellent sound absorption property at low frequency due to the organic combination of airflow resistivity, resonance absorption, and interface dissipation.

Thermal and shape topological robustness of heat switchers using nematic liquid crystals

One interesting way to control heat is to use devices designed by transformation thermics, where artificial media are used. However, once manufactured (either repelling or concentrating heat, for example), besides being mono-purpose, such devices are designed according to a specific geometric boundary conditions. Another problem is the temperature dependence of the materials employed, since their properties are sometimes considered temperature-invariant. In this paper, we show that a previously proposed bi-objective heat switcher (Phys. Rev. E 89, 020501(R) (2014)) is in fact robust against temperature and geometric deformations, due to the topological properties of the molecular nematic orientation. Using a geometrical approach for heat propagation, by performing finite element simulations, we show that a device made by concentric cylinders with thermotropic nematic liquid crystal between them, sustains its functionality even with their molecular thermal conductivities depending on the temperature, achieving a 60% increase and a 44% decrease in the heat flux for each mode. Utilizing topological arguments we show that deformations on the surface of the outer cylinder do not break the operating mode (repeller or concentrator). We present a comparison between our geometrical approach and the transformation thermodynamics to give an additional explanation for the obtained results. We hope the presented device is useful for heat control under mechanical and thermal influence of the external environment.

Discrete breathers assist energy transfer to ac-driven nonlinear chains

A one-dimensional chain of pointwise particles harmonically coupled with nearest neighbors and placed in sixth-order polynomial on-site potentials is considered. The power of the energy source in the form of single ac driven particle is calculated numerically for different amplitudes A and frequencies ω within the linear phonon band. The results for the on-site potentials with hard and soft anharmonicity types are compared. For the hard-type anharmonicity, it is shown that when the driving frequency is close to (far from) the upper edge of the phonon band, the power of the energy source normalized to A^{2} increases (decreases) with increasing A. In contrast, for the soft-type anharmonicity, the normalized power of the energy source increases (decreases) with increasing A when the driving frequency is close to (far from) the lower edge of the phonon band. Our further demonstrations indicate that in the case of hard (soft) anharmonicity, the chain can support movable discrete breathers (DBs) with frequencies above (below) the phonon band. It is the energy source quasiperiodically emitting moving DBs in the regime with driving frequency close to the DB frequency that induces the increase of the power. Therefore, our results here support the mechanism that the moving DBs can assist energy transfer from the ac driven particle to the chain.

Structured thermal surface for radiative camouflage

Thermal camouflage has been successful in the conductive regime, where thermal metamaterials embedded in a conductive system can manipulate heat conduction inside the bulk. Most reported approaches are background-dependent and not applicable to radiative heat emitted from the surface of the system. A coating with engineered emissivity is one option for radiative camouflage, but only when the background has uniform temperature. Here, we propose a strategy for radiative camouflage of external objects on a given background using a structured thermal surface. The device is non-invasive and restores arbitrary background temperature distributions on its top. For many practical candidates of the background material with similar emissivity as the device, the object can thereby be radiatively concealed without a priori knowledge of the host conductivity and temperature. We expect this strategy to meet the demands of anti-detection and thermal radiation manipulation in complex unknown environments and to inspire developments in phononic and photonic thermotronics.

Thermal camouflaging techniques typically use bulky structures and require a well-defined and unchanging background. Here, the authors propose a strategy for thermal camouflage using a structured thermal surface, independent of the background material for many practical situations.

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

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

Nanoscale diffractive probing of strain dynamics in ultrafast transmission electron microscopy

The control of optically driven high-frequency strain waves in nanostructured systems is an essential ingredient for the further development of nanophononics. However, broadly applicable experimental means to quantitatively map such structural distortion on their intrinsic ultrafast time and nanometer length scales are still lacking. Here, we introduce ultrafast convergent beam electron diffraction with a nanoscale probe beam for the quantitative retrieval of the time-dependent local deformation gradient tensor. We demonstrate its capabilities by investigating the ultrafast acoustic deformations close to the edge of a single-crystalline graphite membrane. Tracking the structural distortion with a 28-nm/700-fs spatio-temporal resolution, we observe an acoustic membrane breathing mode with spatially modulated amplitude, governed by the optical near field structure at the membrane edge. Furthermore, an in-plane polarized acoustic shock wave is launched at the membrane edge, which triggers secondary acoustic shear waves with a pronounced spatio-temporal dependency. The experimental findings are compared to numerical acoustic wave simulations in the continuous medium limit, highlighting the importance of microscopic dissipation mechanisms and ballistic transport channels.

Nonreciprocity in the dynamics of coupled oscillators with nonlinearity, asymmetry, and scale hierarchy

In linear time-invariant dynamical and acoustical systems, reciprocity holds by the Onsager-Casimir principle of microscopic reversibility, and this can be broken only by odd external biases, nonlinearities, or time-dependent properties. A concept is proposed in this work for breaking dynamic reciprocity based on irreversible nonlinear energy transfers from large to small scales in a system with nonlinear hierarchical internal structure, asymmetry, and intentional strong stiffness nonlinearity. The resulting nonreciprocal large-to-small scale energy transfers mimic analogous nonlinear energy transfer cascades that occur in nature (e.g., in turbulent flows), and are caused by the strong frequency-energy dependence of the essentially nonlinear small-scale components of the system considered. The theoretical part of this work is mainly based on action-angle transformations, followed by direct numerical simulations of the resulting system of nonlinear coupled oscillators. The experimental part considers a system with two scales-a linear large-scale oscillator coupled to a small scale by a nonlinear spring-and validates the theoretical findings demonstrating nonreciprocal large-to-small scale energy transfer. The proposed study promotes a paradigm for designing nonreciprocal acoustic materials harnessing strong nonlinearity, which in a future application will be implemented in designing lattices incorporating nonlinear hierarchical internal structures, asymmetry, and scale mixing.

Breaking the barriers: advances in acoustic functional materials

Acoustics is a classical field of study that has witnessed tremendous developments over the past 25 years. Driven by the novel acoustic effects underpinned by phononic crystals with periodic modulation of elastic building blocks in wavelength scale and acoustic metamaterials with localized resonant units in subwavelength scale, researchers in diverse disciplines of physics, mathematics, and engineering have pushed the boundary of possibilities beyond those long held as unbreakable limits. More recently, structure designs guided by the physics of graphene and topological electronic states of matter have further broadened the whole field of acoustic metamaterials by phenomena that reproduce the quantum effects classically. Use of active energy-gain components, directed by the parity–time reversal symmetry principle, has led to some previously unexpected wave characteristics. It is the intention of this review to trace historically these exciting developments, substantiated by brief accounts of the salient milestones. The latter can include, but are not limited to, zero/negative refraction, subwavelength imaging, sound cloaking, total sound absorption, metasurface and phase engineering, Dirac physics and topology-inspired acoustic engineering, non-Hermitian parity–time synthetic active metamaterials, and one-way propagation of sound waves. These developments may underpin the next generation of acoustic materials and devices, and offer new methods for sound manipulation, leading to exciting applications in noise reduction, imaging, sensing and navigation, as well as communications.

Phononic Crystal Waveguide Transducers for Nonlinear Elastic Wave Sensing

Second harmonic generation is one of the most sensitive and reliable nonlinear elastic signatures for micro-damage assessment. However, its detection requires powerful amplification systems generating fictitious harmonics that are difficult to discern from pure nonlinear elastic effects. Current state-of-the-art nonlinear ultrasonic methods still involve impractical solutions such as cumbersome signal calibration processes and substantial modifications of the test component in order to create material-based tunable harmonic filters. Here we propose and demonstrate a valid and sensible alternative strategy involving the development of an ultrasonic phononic crystal waveguide transducer that exhibits both single and multiple frequency stop-bands filtering out fictitious second harmonic frequencies. Remarkably, such a sensing device can be easily fabricated and integrated on the surface of the test structure without altering its mechanical and geometrical properties. The design of the phononic crystal structure is supported by a perturbative theoretical model predicting the frequency band-gaps of periodic plates with sinusoidal corrugation. We find our theoretical findings in excellent agreement with experimental testing revealing that the proposed phononic crystal waveguide transducer successfully attenuates second harmonics caused by the ultrasonic equipment, thus demonstrating its wide range of potential applications for acousto/ultrasonic material damage inspection.

Spanning the scales of mechanical metamaterials using time domain simulations in transformed crystals, graphene flakes and structured soils

We begin with a brief historical survey of discoveries of quasi-crystals and graphene, and then introduce the concept of transformation crystallography, which consists of the application of geometric transforms to periodic structures. We consider motifs with three-fold, four-fold and six-fold symmetries according to the crystallographic restriction theorem. Furthermore, we define motifs with five-fold symmetry such as quasi-crystals generated by a cut-and-projection method from periodic structures in higher-dimensional space. We analyze elastic wave propagation in the transformed crystals and (Penrose-type) quasi-crystals with the finite difference time domain freeware SimSonic. We consider geometric transforms underpinning the design of seismic cloaks with square, circular, elliptical and peanut shapes in the context of honeycomb crystals that can be viewed as scaled-up versions of graphene. Interestingly, the use of morphing techniques leads to the design of cloaks with interpolated geometries reminiscent of Victor Vasarely's artwork. Employing the case of transformed graphene-like (honeycomb) structures allows one to draw useful analogies between large-scale seismic metamaterials such as soils structured with columns of concrete or grout with soil and nanoscale biochemical metamaterials. We further identify similarities in designs of cloaks for elastodynamic and hydrodynamic waves and cloaks for diffusion (heat or mass) processes, as these are underpinned by geometric transforms. Experimental data extracted from field test analysis of soil structured with boreholes demonstrates the application of crystallography to large scale phononic crystals, coined as seismic metamaterials, as they might exhibit low frequency stop bands. This brings us to the outlook of mechanical metamaterials, with control of phonon emission in graphene through extreme anisotropy, attenuation of vibrations of suspension bridges via low frequency stop bands and the concept of transformed meta-cities. We conclude that these novel materials hold strong applications spanning different disciplines or across different scales from biophysics to geophysics.

Origami-based tunable truss structures for non-volatile mechanical memory operation

Origami has recently received significant interest from the scientific community as a method for designing building blocks to construct metamaterials. However, the primary focus has been placed on their kinematic applications by leveraging the compactness and auxeticity of planar origami platforms. Here, we present volumetric origami cells-specifically triangulated cylindrical origami (TCO)-with tunable stability and stiffness, and demonstrate their feasibility as non-volatile mechanical memory storage devices. We show that a pair of TCO cells can develop a double-well potential to store bit information. What makes this origami-based approach more appealing is the realization of two-bit mechanical memory, in which two pairs of TCO cells are interconnected and one pair acts as a control for the other pair. By assembling TCO-based truss structures, we experimentally verify the tunable nature of the TCO units and demonstrate the operation of purely mechanical one- and two-bit memory storage prototypes.Origami is a popular method to design building blocks for mechanical metamaterials. Here, the authors assemble a volumetric origami-based structure, predict its axial and rotational movements during folding, and demonstrate the operation of mechanical one- and two-bit memory storage.

Reprogrammable Phononic Metasurfaces

Phononic metamaterials rely on the presence of resonances in a structured medium to control the propagation of elastic waves. Their response depends on the geometry of their fundamental building blocks. A major challenge in metamaterials design is the realization of basic building blocks that can be tuned dynamically. Here, a metamaterial plate is realized that can be dynamically tuned by harnessing geometric and magnetic nonlinearities in the individual unit cells. The proposed tuning mechanism allows a stiffness variability of the individual unit cells and can control the amplitude of transmitted excitation through the plate over three orders of magnitude. The concepts can be extended to metamaterials at different scales, and they can be applied in a broad range of engineering applications, from seismic shielding at low frequency to ultrasonic cloaking at higher frequency ranges.

Nanolattices: An Emerging Class of Mechanical Metamaterials

In 1903, Alexander Graham Bell developed a design principle to generate lightweight, mechanically robust lattice structures based on triangular cells; this has since found broad application in lightweight design. Over one hundred years later, the same principle is being used in the fabrication of nanolattice materials, namely lattice structures composed of nanoscale constituents. Taking advantage of the size-dependent properties typical of nanoparticles, nanowires, and thin films, nanolattices redefine the limits of the accessible material-property space throughout different disciplines. Herein, the exceptional mechanical performance of nanolattices, including their ultrahigh strength, damage tolerance, and stiffness, are reviewed, and their potential for multifunctional applications beyond mechanics is examined. The efficient integration of architecture and size-affected properties is key to further develop nanolattices. The introduction of a hierarchical architecture is an effective tool in enhancing mechanical properties, and the eventual goal of nanolattice design may be to replicate the intricate hierarchies and functionalities observed in biological materials. Additive manufacturing and self-assembly techniques enable lattice design at the nanoscale; the scaling-up of nanolattice fabrication is currently the major challenge to their widespread use in technological applications.

Thermal Conductivity of Graphene-Polymer Composites: Mechanisms, Properties, and Applications

With the integration and miniaturization of electronic devices, thermal management has become a crucial issue that strongly affects their performance, reliability, and lifetime. One of the current interests in polymer-based composites is thermal conductive composites that dissipate the thermal energy produced by electronic, optoelectronic, and photonic devices and systems. Ultrahigh thermal conductivity makes graphene the most promising filler for thermal conductive composites. This article reviews the mechanisms of thermal conduction, the recent advances, and the influencing factors on graphene-polymer composites (GPC). In the end, we also discuss the applications of GPC in thermal engineering. This article summarizes the research on graphene-polymer thermal conductive composites in recent years and provides guidance on the preparation of composites with high thermal conductivity.