Path Of Phonons Research Articles

Colloidal synthetic environmental design towards high-density twin boundaries and boosted thermoelectric performance in Cu5FeS4 icosahedrons

High-density twin boundaries can significantly influence materials’ functional and mechanical properties. In thermoelectrics, twin engineering is able to manipulate the carrier and phonon transport in semiconductors, leading to improved dimensionless figure of merit (zT). However, obtaining twin boundaries with tunable density and uniform distribution in thermoelectric semiconductors is rather challenging. Here, by precisely controlling the temperature of sulfur precursor, the particle size of synthesized Cu5FeS4 icosahedrons can be decreased to produce higher-density twin boundaries. Accordingly, the optimized average distance between two adjacent twin boundaries matches the mean free path of phonons but is much larger than that of carriers, thereby efficiently lowering the lattice thermal conductivity without affecting the carrier mobility significantly. In combination with the enhanced power factor caused by increased carrier concentration, a maximal zT of 0.95 is achieved at 773 K in a p-type Cu5FeS4 derivative material, which is one of the highest values among those in Earth-abundant, environmentally-friendly thermoelectric sulfides. This work uncovers the crucial role of high-density twin boundaries in decoupling the carrier and phonon transport for improved thermoelectric performance in Cu5FeS4-based materials.

Enhancing the thermal conductivity of semiconductor thin films via phonon funneling

The second law of thermodynamics asserts that energy diffuses from hot to cold. The resulting temperature gradients drive the efficiencies and failures in a plethora of technologies. However, as the dimensionalities of materials shrink to the nanoscale regime, proper heat dissipation strategies become more challenging since the mean free paths of phonons become larger than the characteristic length scales. This leads to temperature gradients that are dependent on interfaces and boundaries, which ultimately can lead to severe thermal bottlenecks. Herein, we uncover a phenomenon which we refer to as ‘phonon funneling’, that allows the control of phonon transport to preferentially direct phonon energy away from geometrically confined interfacial thermal bottlenecks and into localized colder regions. This phenomenon supersedes heat diffusion based on the macroscale temperature gradients, thus introducing a nanoscale regime in which boundary scattering increases the phonon thermal conductivity of thin films, an opposite effect than what is traditionally realized. This work advances the fundamental understanding of phonon transport at the nanoscale and the role of efficient scattering methods for enhancing thermal transport.

The low-temperature thermal conduction and millimeter-wave dielectric properties of β-Si3N4 as a heat-dissipation substrate

Increasing operating frequency is required in wireless communication systems. In use, this generates heat, which results in serious problems for semiconductor devices. Silicon nitride (Si3N4) has high thermal conductivity and fracture strength. Therefore, it is expected to be suitable for heat dissipation in active circuit substrates for wireless communication. In this study, Si3N4 with varying thermal conductivities were prepared by controlling starting material and sintering conditions and their low-temperature thermal conductivities were measured. The mean free path of phonons in β-Si3N4 influences its thermal conductivity. The temperature dependence of the Debye–Waller factor obtained by synchrotron radiation scattering supported the high thermal conductivity of the β-Si3N4. The degradation of thermal conductivity was interpreted in terms of the full width at half maximum of Raman spectra. Further, infrared reflectivity and first-principles calculation for β-Si3N4 confirmed their superior dielectric properties compared with other high thermal conductive materials.

Effects of ballistic transport on the thermal resistance and temperature profile in nanowires

Effects of ballistic transport on the temperature profiles and thermal resistance in nanowires are studied. Computer simulations of nanowires between a heat source and a heat sink have shown that in the middle of such wires the temperature gradient is reduced compared to Fourier’s law with steep gradients close to the heat source and sink. In this work, results from molecular dynamics and phonon Monte Carlo simulations of the heat transport in nanowires are compared to a radiator model which predicts a reduced gradient with discrete jumps at the wire ends. The comparison shows that for wires longer than the typical mean free path of phonons the radiator model is able to account for ballistic transport effects. The steep gradients at the wire ends are then continuous manifestations of the discrete jumps in the model.Graphical abstract

Monolayer 1T-Ag6S2 with Excellent Thermoelectric Properties.

We obtained a new material called monolayer 1T-Ag6S2 by replacing metal atoms in 1T phase transition-metal dichalcogenide sulfides (TMDs) with octahedral Ag6 clusters. Subsequently, the thermoelectric transport properties of monolayer 1T-Ag6S2 were systematically investigated using first-principles calculations and the generalized gradient approximation (GGA-PBE) exchange correlation functional. The findings demonstrate that monolayer 1T-Ag6S2 displays characteristics of a wide-bandgap semiconductor, with a bandgap of 2.48 eV. Notably, the incorporation of Ag6 clusters disrupts the structural symmetry, effectively enhancing the electronic structure and phonon properties of the material. Due to the flat valence band near the Fermi level, the extended relaxation time of the hole results in a greater effective mass compared to the electron, leading to a significant increase in the Seebeck coefficient. Under optimal doping conditions, the power factor of monolayer 1T-Ag6S2 can achieve 14.9 mW/mK2 at 500 K. The intricate crystal structure induces phonon path bending, reduces the overall frequency of phonon vibrations (<10 THz), and causes hybridization of low-frequency optical and acoustic branches, resulting in remarkably low lattice thermal conductivity (0.20 and 0.17 W/mK along the x and y axes at 500 K, respectively). The monolayer 1T-Ag6S2 demonstrates a remarkably high figure of merit ZT of 3.14 (3.15) on the x (y) axis at 500 K, significantly higher than those of conventional TMD materials. Such excellent thermoelectric properties suggest that monolayer 1T-Ag6S2 is a promising thermoelectric (TE) material. Our work reveals the deep mechanism of cluster substitution to optimize the thermoelectric properties of materials and provides a useful reference for subsequent research.

A comprehensive experimental study on the effects of hexagonal boron nitride particle size and loading ratio on thermal and mechanical performance in epoxy composites

Harnessing the potential of hexagonal boron nitride (h-BN) in epoxy composites for tailoring thermal conductivity is a promising avenue in materials science. However, achieving balanced enhancements in both in-plane and through-plane directions remains a challenge that requires innovative solutions. The primary objective of this research is to evaluate how thermal and mechanical characteristics of an epoxy matrix are affected by the size and amount of h-BN particles. To achieve this goal, h-BN particles with varying sizes (micro and nano) are incorporated into the epoxy matrix at different weight ratios spanning from 0.5 wt % to 20 wt % using a pre-dispersion technique. The epoxy composites reinforced with h-BN through a molding process exhibits enhanced mechanical and thermal performance in contrast to the pristine epoxy material. During the flexural test, acoustic emission data is collected to identify the initiation and progression of damage within the specimens under testing conditions. The most notable enhancement in thermal conductivity is observed when incorporating 20 wt% of micron-sized h-BN particles. This leads to a remarkable 107% increase in the in-plane direction and an impressive 112% increase in the through-plane direction. These results can be attributed to the formation of a three-dimensional thermally conductive network by the larger h-BN particles, which extends the path of phonon scattering. Furthermore, there are significant improvements in both flexural modulus and tensile modulus. Epoxy composites containing 10 wt% of micron-sized h-BN experiences an approximate 42% increase, while those with 20 wt% of the same particles displays a substantial 47% rise in these properties. This study effectively addresses the challenges associated with tailoring the thermal properties of epoxy composites, opening up new opportunities for applications in various industries, including electronics, aerospace and thermal management systems.

Electrical and thermal properties of bio-inspired laminated Cu/Ti3SiC2/C composites reinforced by graphene nanoplatelets

Taking inspiration from the unique hierarchical structure of bio-materials like pearl oyster, this study employs bio-inspired composites technological route using flake powder metallurgy to prepare laminated Cu/Ti3SiC2/C composites. The prepared composites have clear laminated structure, uniform dispersion of reinforcing phase and clean interface. At a Cu-plated GNPs content of 0.5 wt%, the composites displayed tensile strength of 300.65 MPa, compressive strength of 575.32 MPa, electrical conductivity of 26.31 MS/m, and thermal conductivity of 285.87 W·m−1·K−1. The reinforcement of composites is caused by the load transfer effect of the laminated structure. During the loading process, the laminated structure creates multiple paths for deflecting cracks, which leads to the redistribution of the applied load. Moreover, the homogeneous distribution of GNPs throughout the laminated structure extends the mean free path of both free electrons and phonons within the material, consequently bolstering the electrical and thermal conductivity of the composites.

Assessing Lattice Thermal Conductivity of Topological Insulator Bi2Se3 by Raman Thermometry

Lattice thermal conductivity (κL) of the hexagon‐shaped nanocrystals cluster of Bi2Se3, prepared by the hot‐injection technique using nontoxic solvents, is studied. From the temperature‐dependent Raman spectra of Bi2Se3 nanocrystals, the average Debye temperature (θD) and Gruneisen parameter (γ) are calculated by adopting the bond‐order–length–strength correlation theory. The average room temperature κL of Bi2Se3 nanocrystals evaluated from the Slack model using θD and γ is ≈1.1 Wm−1K−1. The κL of Bi2Se3 nanocrystals is larger than out‐of‐plane κL (≈0.4 Wm−1K−1) but close to the in‐plane κL (≈1.4 Wm−1K−1) simulated using the Boltzmann transport equation for phonon with three‐phonon scatterings. Nanostructuring introduces grain boundaries in the Bi2Se3 that block the long mean free path of phonons physically, reduces the phonon mean free path, and decreases the κL. The anisotropic phonon scattering introduced by the weak van der Waals force between adjacent quintuple layers in the out‐of‐plane direction, in addition to the acoustic–optical phonon scattering and anharmonicity, hinders the efficient transport of thermal energy in the Bi2Se3 and results in a lower κL. By utilizing materials with anisotropic thermal conductivity, thermoelectric devices can be designed to preferentially conduct heat in specific directions while minimizing heat loss in others.

Preparation, thermophysical properties, and thermal shock resistance of Yb2O3-doped YSZ coatings

The yttria-stabilized zirconia (YSZ) coatings were doped with various concentrations of Yb2O3 to build a multi-component solid solution system. The effects of concentration on the phase composition, microstructure, phase stability at high temperatures, thermal conductivity, thermal expansion, fracture toughness, elastic modulus, and other thermophysical properties of the coatings were systematically analyzed. Furthermore, the mechanism through which Yb2O3 doping impacts the thermal shock resistance of YSZ coatings was elucidated. The results reveal that doping with Yb3+, which has a similar radius to Zr4+, allows the YSZ coatings to retain the tetragonal phase structure at low doping concentrations. The Yb2O3 doping can reduce the thermal conductivity of YSZ coatings due to replacement atom and oxygen vacancy defects enhancing phonon scattering and decreasing the mean free path of phonons. The 1YbYSZ coating demonstrates better thermal shock resistance owing to its high CTE and high fracture toughness resulting from its tetragonal phase, high elastic modulus and high grain boundary density. The Yb2O3 doping can form defect clusters and increase the cation diffusion path due to oxygen vacancies, which inhibits the uneven diffusion of Y3+. Therefore, Yb2O3-doped YSZ coatings demonstrate remarkable phase stability at high temperatures of 1700 °C.

Influence of High Surfactant Concentrations on the Morphology and Crystallinity of Electrodeposited Antimony and Bismuth Nanowires

Antimony and bismuth nanowires are excellent model systems to investigate the interplay of various size-dependent transport phenomena relevant for applications in thermoelectrics, spintronics, or catalysis. The properties of these nanowires can be significant influenced by size effects due to the large mean free path and Fermi wavelength of charge carriers and phonons. Since antimony and bismuth possess highly anisotropic Fermi surfaces, the transport is also strongly dependent on the crystallographic orientation. Therefore, it is very important to develop and control suitable electrodeposition processes that allow us to understand the nanowire growth and to accordingly adjust preferred orientation and morphology of the synthesized nanowires.The so called template route employed in the presented work allows to control number density, alignment, geometry, length and diameter of the nanowires. The electrolytes and plating processes influence crystallinity, crystal orientation, and morphology of the electrodeposited materials. Here, antimony and bismuth nanowires are grown by potentiostatic electrodeposition in polycarbonate etched ion-track membranes using SbCl3 and BiCl3 in aqueous electrolytes.In this talk, we will show the influence of the presence of nonionic and anionic surfactants in high concentrations in the electrolyte during the electrodeposition of Bi and Sb nanowires. The recorded current-time curves of the plating process will be presented. Systematic characterization of the nanowires by scanning- and transmission electron microscopy, and X-ray diffraction reveal a strong change in crystallinity as a function of surfactant concentration, which will be discussed in detail.

Low-dimensional heat conduction in surface phonon polariton waveguide

Heat conduction in solids is typically governed by the Fourier’s law describing a diffusion process due to the short wavelength and mean free path for phonons and electrons. Surface phonon polaritons couple thermal photons and optical phonons at the surface of polar dielectrics, possessing much longer wavelength and propagation length, representing an excellent candidate to support extraordinary heat transfer. Here, we realize clear observation of thermal conductivity mediated by surface phonon polaritons in SiO2 nanoribbon waveguides of 20-50 nm thick and 1-10 μm wide and also show non-Fourier behavior in over 50-100 μm distance at room and high temperature. This is enabled by rational design of the waveguide to control the mode size of the surface phonon polaritons and its efficient coupling to thermal reservoirs. Our work laid the foundation for manipulating heat conduction beyond the traditional limit via surface phonon polaritons waves in solids.

Effect of Bi3+, Ti4+ and Ta5+ doping on the thermoelectric properties of KSr2Nb5O15: A first-principles investigation

The enhancement of thermoelectric characteristics in tungsten bronze structural materials is contingent upon the attainment of a high figure of merit (ZT) value. The first-principles calculations are used to investigate the thermoelectric characteristics of KSN doped with Bi3+, Ti4+ and Ta5+. The findings indicate that the electrical conductivity of KSN can be enhanced more effectively by doping Bi3+ that replacing K+ due to the larger disparity in valence electrons between Bi3+ and K+. When KSN is doped with Ti4+ and Ta5+, the thermal conductivity of KSN is decreased, especially for KSN-Ti, it is shown to fall from 1.643 W/m/K to 0.906 W/m/K at a temperature of 1200 K. That is due to the presence of Ti4+ and Ta5+ in the framework site B of the KSN structure results in significant lattice distortion, and the distortion is particularly pronounced when Ti4+ is introduced as dopants. Consequently, stronger phonon scattering occurs, the structure becomes more unstable due to the presence of defects. Simultaneously, the presence of Ti4+ leads to the generation of oxygen vacancies, resulting in a notable increase in the scattering interactions between phonons and defects, thus reducing the average free path of phonons, thereby greatly reducing the thermal conductivity of the material as a result. Following the process of cation doping, the electrical conductivity of KSN is enhanced while its thermal conductivity is lowered, resulting in a notable increase in the ZT. Notably, the ZT of KSN experienced a significant rise from 0.434 to 0.586 (1200 K) with the introduction of Ti4+ doping. This study offers theoretical insights on enhancing the thermoelectric efficiency of KSN.

Effect of nanostructuring on thermoelectric performance of SiGe thin films

We experimentally investigated the effect of nanostructuring on the thermoelectric performance of SiGe thin films. Nanoscale porous structures were fabricated using lithography in a top-down approach to reduce the thermal conductivity of the thin films and the thermoelectric figure of merit (ZT) was evaluated. The thermal conductivity of nanostructured SiGe thin films is up to 24% lower than that of thin film without nanostructure while the electrical conductivity is up to about 19% lower, resulting in a 4% increase in ZT value to 0.041 at RT. Since the mean free path (MFP) of phonons in SiGe is short compared to the characteristic length of the nanostructures, the effect of nanostructuring on thermoelectric performance is limited. Nanostructuring is known as a promising method to increase ZT values. However, it is only effective when the thermal phonon MFPs are comparable to the characteristic length of the nanostructure.

Simultaneously improved surface hardness and thermal diffusivity of carbon nanotube/zinc silicate composites via colloidal processing

Materials with ideal thermal dissipation can prevent electronic devices from overheating and sustaining internal damage. However, obtaining passive heat sink materials with a well-distributed thermal transport network remained challenging. In this study, carbon nanotubes (CNTs) reinforced zinc silicate (Zn2SiO4) composites (CNTs/ZS) were fabricated via colloidal processing and pressureless sintering. The CNTs was first dispersed in 1,2,3,4-tetrahydronapathalene (C10H12) through sonication, before composite-mixing and densification with zinc silicate ceramic. It revealed that the dispersed CNTs simultaneously improved the mechanical and thermal properties of the composites, along with reducing relative density. The Vickers hardness, thermal diffusivity, and thermal conductivity of the optimized composite containing 1.0 wt% CNTs increased by 42%, 179%, and 537%, respectively, compared to the pristine ZS. The SEM observation on the 1.0 wt% CNTs/ZS composite surface determined that the area fraction of the CNTs inclusions was approximately 60%, which was advantageous for phonon propagation and electron transmission path. The combined effect of the phonon and electron transmission path outperformed the polycrystalline-induced Umklapp scattering and porosity-induced phonon-pore scattering, which were identified as obstacles to the heat-transfer process. Thus, this CNTs reinforcement through colloidal processing can be a strategy for designing of efficient silicate-based heatsink candidates in optoelectronic applications.

Long-range polaritonic heat conduction in asymmetric surrounding media

Surface Phonon Polaritons (SPhPs) as an evanescent electromagnetic surface wave supports long propagation which readily surpasses the mean free path of classical heat carriers, e.g., phonons and electrons in solids. SPhPs have emerged as a promising candidate to dominate heat transfer in thin films. Polaritonic heat transfer has two distinct advantages: superior thermal conduction and a wide range of manipulation. Here, we study the upper limit of the thermal conductivity mediated by long-range polaritons in asymmetric surrounding media, where its surface effect overwhelms the volumetric one. The thin film structure strengthens the interactions of two surface waves at the top and bottom surfaces, and the asymmetric surrounding media makes an evanescent surface wave to further penetrate the free space with a higher refractive index, but it requires a fine tuning of asymmetric permittivity of the surrounding media to reach the upper limit of energy transmission efficiency near to the modal cut-off, where the transverse wavevector becomes zero. Both analytical and numerical simulations were introduced to investigate dispersion in asymmetric surrounding media and to model the thermal conductivity of glass thin films. Anomalously high thermal conductivity of 248 W/m-K was achieved with a 50 nm thick SiO2 film in asymmetric surrounding media, yet subtly dissimilar.

Incorporation of FexOy nanoparticles into 3D interlinked porous carbon nanofiber networks to synergistically enhance the electrical insulation, electromagnetic wave absorbing/shielding performance and thermal conductivity

The property incompatibility among heat conduction, electromagnetic wave (EMW) absorption/shielding, and electrical insulation has restricted the synchronous enhancement and application of polymer-based composites. Herein, we first synthesized porous 3D interlinked C nanofiber/FexOy (CNF/FexOy) wafers for significantly enhanced properties by a simple sol–gel and annealing route. The texture, composition, defects and the resulting properties can conveniently be manipulated via altering Fe3+ concentration and annealing temperature. The CNF/FexOy wafers exhibit a high heat conductivity (3.22 W/mK) at a low loading (30 wt%) thanks to the shorter continuous thermal transfer paths and the increased phonon mean free path and thermal capacity of phonons per volume. Moreover, CNF/FexOy wafers possess a wide absorption band (9.36 GHz), prominent EMI shielding effectiveness (SET ≥ 20 dB over C, X, and Ku bands), excellent electrical insulation (σ = 0.00991 S/m), and enhanced mechanical/hydrophobicity properties, surpassing most reported fillers. These enhancements could be explained by the fact that the cooperative action of porous structures, defects, and magnetic/dielectric FexOy hinder electron migration and improve the impedance matching and multiple-scattering of materials. The silicone composites supported by 3D interlinked CNF/FexOy wafers have great potential as a filler with high-performance EMI shielding/absorption and thermal control in next-generation flexible electronics.

Lifetime of photoexcited carriers in space-controlled Si nanopillar/SiGe composite films investigated by a laser heterodyne photothermal displacement method

Thermal management has become more critical as semiconductor devices are miniaturized. In metal–oxide–semiconductor field-effect transistors, the problem is the reduction in electron mobility in the channel layer owing to the temperature rise caused by heat generation near the channel-drain region. Focusing on the mean free paths of phonons and electrons in Si, nanostructures of a few 10 nm may only hinder heat propagation without affecting electron transportation. Therefore, inserting nanostructures into the channel layer may prevent a temperature rise and maintain a higher electron mobility. To discuss the relationship between the spacing between the nanopillars (NPs) and the heat generation and carrier behavior of the Si-NP/SiGe composite film, samples with NP spacings of 13, 27, or 47 nm were prepared. We previously confirmed that the thermal conductivity of the Si-NP/SiGe composite film decreased as NP spacing narrowed. The NPs scattered phonon propagation and suppressed heat propagation. However, carrier transport properties such as electrical conductivity, carrier mobility, and carrier lifetime have never been discussed. The laser heterodyne photothermal displacement method was used to examine the effect of nanostructures on carrier mobility and carrier lifetime of Si-NP/SiGe composite films. We observed that the carrier lifetime became longer when the NP spacing was comparable to the electron mean-free path of approximately 27 nm.

Microwave synthesis and excellent thermoelectric performances of Yb-filled CoSb3 skutterudites

The nearly single-phase Yb-filled and Te-doped YbxCo4Sb11.5Te0.5 are prepared by microwave, and the skutterudites are further densification by spark plasma sintering. The experimental results show that Yb filling and Te doping further improve the electrical properties and confine the mean free path of phonons, resulting in a significant reduction the lattice thermal conductivity. And the maximum power factor of Yb0.2Co4Sb11.5Te0.5 is 4364.45 μWm−1K−2 at 675 K. Analysis believes that the resonance scattering root from Yb filling and the defect scattering and stress field fluctuation introduced by Te doping can effectively limit the thermal transport performance. In 300∼775 K, the total thermal conductivity of Yb0.2Co4Sb11.5Te0.5 is 2.68∼3.74 Wm−1K − 1, and the maximum ZT is 1.25 at 775 K, revealing Yb filling and Te doping improve the electrical properties and reduce the thermal conductivity of by limiting the mean free path of phonons.

Thermal management of chips by a device prototype using synergistic effects of 3-D heat-conductive network and electrocaloric refrigeration

With speeding up development of 5 G chips, high-efficient thermal structure and precise management of tremendous heat becomes a substantial challenge to the power-hungry electronics. Here, we demonstrate an interpenetrating architecture of electrocaloric polymer with highly thermally conductive pathways that achieves a 240% increase in the electrocaloric performance and a 300% enhancement in the thermal conductivity of the polymer. A scaled-up version of the device prototype for a single heat spot cooling of 5 G chip is fabricated utilizing this electrocaloric composite and electromagnetic actuation. The continuous three-dimensional (3-D) thermal conductive network embedded in the polymer acts as nucleation sites of the ordered dipoles under applied electric field, efficiently collects thermal energy at the hot-spots arising from field-driven dipolar entropy change, and opens up the high-speed conduction path of phonons. The synergy of two components, thus, tackles the challenge of sluggish heat dissipation of the electroactive polymers and their contact interfaces with low thermal conductivity, and more importantly, significantly reduces the electric energy for switching the dipolar states during the electrocaloric cycles, and increases the manipulable entropy at the low fields. Such a feasible solution is inevitable to the precisely fixed-point thermal management of next-generation smart microelectronic devices.

Analytical models for phonon mean free path in polycrystalline nanostructures based on mean square displacement

In this study, a numerical simulation method and analytical models for predicting the boundary scattering mean free path (MFP) of phonons in polycrystalline nanostructures are developed. The grain morphologies are assumed to be approximately equiaxed, i.e., forbidding needle-like or pancake-like morphologies. Adapting a technique from rarefied gas dynamics, the method evaluates the MFP from the mean square displacements of phonons that experience random motion and interface collisions in nanostructures. We confirm that the MFP in simple cubic polycrystalline nanostructures obtained by the simulations agrees with that reported in a previous study; this result supports the validity of the method. Two analytical models for high and low interfacial transmission probabilities at the crystal interfaces are also derived by considering the mean square displacements. We find that the grain-boundary intercept length distribution of polycrystalline structures is an essential parameter for determining this boundary scattering MFP. These analytical models reproduce the MFPs in simple cubic and Voronoi diagram polycrystalline nanostructures calculated by the numerical simulations. This result indicates that the boundary scattering MFP of phonons in polycrystalline nanostructures can be obtained once the intercept length distribution is evaluated, without any additional numerical simulations.