Phonon Transport Properties Research Articles

Phonon transport properties of particulate physical gels.

Particulate physical gels are sparse, low-density amorphous materials in which clusters of glasses are connected to form a heterogeneous network structure. This structure is characterized by two length scales, ξs and ξG: ξs measures the length of heterogeneities in the network structure and ξG is the size of glassy clusters. Accordingly, the vibrational states (eigenmodes) of such a material also exhibit a multiscale nature with two characteristic frequencies, ω* and ωG, which are associated with ξs and ξG, respectively: (i) phonon-like vibrations in the homogeneous medium at ω<ω*, (ii) phonon-like vibrations in the heterogeneous medium at ω*<ω<ωG, and (iii) disordered vibrations in the glassy clusters at ω > ωG. Here, we demonstrate that the multiscale characteristics seen in the static structures and vibrational states also extend to the phonon transport properties. Phonon transport exhibits two distinct crossovers at frequencies ω* and ωG (or at wavenumbers of ∼ξs -1 and ∼ξG -1). In particular, both transverse and longitudinal phonons cross over between Rayleigh scattering at ω<ω* and diffusive damping at ω>ω*. Remarkably, the Ioffe-Regel limit is located at the very low frequency of ω*. Thus, phonon transport is localized above ω*, even where phonon-like vibrational states persist. This markedly strong scattering behavior is caused by the sparse, porous structure of the gel.

Superior efficiency of BN/Ce2O3/TiO2 nanofibers for photocatalytic hydrogen generation reactions

One-dimensional BN/Ce2O3/TiO2 heteroarchitectures with high visible-light photocatalytic activity have been successfully obtained by an electrospinning technique. The properties of the prepared nanofibers were controlled using different ratios of cerium. The results of XPS characterization revealed that the Ce element is present in the valence state + III on the surface of the TiO2 nanofibers. Rietveld refinement of XRD data reveals that introducing small concentrations of Ce3+ (<3.0 at%) into the electrospinning solution leads to the formation of small amounts of rutile and brookite phases, a decrease in the crystallite size of the main anatase phase and a slight decrease in the c parameter and unit cell volume of the anatase phase. In contrast, only the anatase phase, which has a similar crystallite size and lattice parameters to that formed in the undoped sample, is observed when 3 at.% Ce is used. These results suggest that Ce3+ ions did not substitute Ti4+ ions in the lattice of both anatase and rutile phases, which can be explained by the large difference in the ionic radius of Ti4+ and Ce3+ ((r (Ti4+) = 60.5 pm, r (Ce3+) = 101.7 pm). Scanning electron microscopy demonstrates that the diameter of the obtained nanofibers decreases from 240 nm for pristine TiO2 to 101 nm, 70 nm, 40 nm for 2% Ce/TiO2 (CET2), BN/TiO2 (BT) and BN/2% Ce/TiO2 (BCET2), respectively. The d-spacing decreased by 0.3 nm after Ce incorporation as demonstrated by high-resolution transmission electron microscopy. XPS proved the presence of BN nanosheets along with Ce2O3 and TiO2 into the nanofibers. Compared with the pure TiO2 nanofibers (NF), the obtained Ce/Ti ratio of 2% (molar ratio) showed an enhancement of the visible-light photocatalytic activity to water splitting, which is 46 times higher than that of bare TiO2 NF. The Ce2O3 might improve the separation of photogenerated electrons and holes derived from the coupling effect of TiO2 and cerium oxide. This work also examines the modification of TiO2 (anatase) properties with hexagonal boron nitride (h-BN) and the impact this coupling has on photocatalytic activity. XPS proved the presence of BN nanosheets along with Ce2O3 and TiO2 into the nanofibers. The electron transfer rate was improved by BN exfoliation. The photocatalytic results indicated that the BN/Ce2O3/TiO2 nanofibers improve hydrogen production up to 5100 μmol/g for 6 h under visible light, which could be due to the presence of BN sheets that enhanced the separation of the photo-induced electron–hole pairs in TiO2 and increased the specific surface area compared to pure TiO2 and Ce2O3/TiO2 nanofibers. Moreover, the BN/Ce2O3/TiO2 could be easily recycled without decrease in the photocatalytic activity because of its one-dimensional nanostructure nature. Moreover, the present work focuses on the ternary composite of BN/Ce2O3/TiO2 for optimization of electronic and phonon transport properties using the state-of-the-art density functional theory (DFT). The calculated electronic density of states (DOS) modulus shows an excellent agreement compared with available theoretical and experimental data for hydrogen production.

In Situ Evolution of Secondary Metallic Phases in Off-Stoichiometric ZrNiSn for Enhanced Thermoelectric Performance.

The full-Heusler (FH) inclusions in the half-Heusler (HH) matrix is a well-studied approach to reduce the lattice thermal conductivity of ZrNiSn HH alloy. However, excess Ni in ZrNiSn may lead to the in situ formation of FH and/or HH alloys with interstitial Ni defects. The excess Ni develops intermediate electronic states in the band gap of ZrNiSn and also generates defects to scatter phonons, thus providing additional control to tailor electronic and phonon transport properties synergistically. In this work, we present the implication of isoelectronic Ge-doping and excess Ni on the thermoelectric transport of ZrNiSn. The synthesized ZrNi1.04Sn1-xGex (x = 0-0.04) samples were prepared by arc-melting and spark plasma sintering, and were extensively probed for microstructural analysis. The in situ evolution of minor secondary phases, i.e., FH, Ni-Sn, and Sn-Zr, primarily observed post sintering resulted in simultaneous optimization of the electrical power factor and lattice thermal conductivity. A ZT of ∼1.06 at ∼873 K was attained, which is among the highest for Hf-free ZrNiSn-based HH alloys. Additionally, ab initio calculations based on density functional theory (DFT) were performed to provide comparative insights into experimentally measured properties and understand underlying physics. Further, mechanical properties were experimentally extracted to determine the usability of synthesized alloys for device fabrication.

Temperature- and pressure-dependent phonon transport properties of SnS across phase transition from machine-learning interatomic potential

Recently, tin sulfide (SnS) has attracted particular attention in thermoelectrics due to its non-toxicity and earth-abundant elements. However, the phonon and thermal transport mechanisms in SnS under high temperatures and pressures are yet to be fully understood because of the strong lattice anharmonicity and structural phase transition. Herein, we construct a first-principles-based machine learning potential of SnS, which can predict the lattice dynamical evolution of the structural phase transition at different temperatures and pressures. We unveil that SnS exists an abnormal decrease of the Γ4 and Y1 phonon frequencies with increasing pressure at the low-temperature regime, which is attributed to the structural phase transition. We explore the temperature- and pressure-dependent lattice thermal conductivity of SnS. Our results pave the way for further phonon and thermal transport engineering of SnS under high temperatures and pressures.

Electron delocalization enhances the thermoelectric performance of misfit layer compound (Sn1−x Bi x S)1.2(TiS2)2

The misfit layer compound (SnS)1.2(TiS2)2 is a promising low-cost thermoelectric material because of its low thermal conductivity derived from the superlattice-like structure. However, the strong covalent bonds within each constituent layer highly localize the electrons thereby it is highly challenging to optimize the power factor by doping or alloying. Here, we show that Bi doping at the Sn site markedly breaks the covalent bonds networks and highly delocalizes the electrons. This results in a high charge carrier concentration and enhanced power factor throughout the whole temperature range. It is highly remarkable that Bi doping also significantly reduces the thermal conductivity by suppressing the heat conduction carried by phonons, indicating that it independently modulates phonon and charge transport properties. These effects collectively give rise to a maximum ZT of 0.3 at 720 K. In addition, we apply the single Kane band model and the Debye–Callaway model to clarify the electron and phonon transport mechanisms in the misfit layer compound (SnS)1.2(TiS2)2.

Scanning Electron Thermal Absorbance Microscopy for Light Element Detection and Atomic Number Analysis.

Recent developments in nanoscale thermal metrology using electron microscopy have made impressive advancements in measuring either phononic or thermal transport properties of nanoscale samples. However, its potential in material analysis has never been considered. Here we introduce a direct thermal absorbance measurement platform in scanning electron microscope (SEM) and demonstrate that its signal can be utilized for atomic number (Z) analysis at nanoscales. We prove that the measured absorbance of materials is complementary to signals of backscattering electrons but exhibits a much higher collection efficiency and signal-to-noise ratio. Thus, it not only enables successful detections of light elements/compounds under low acceleration voltages of SEM but also allows quantitative Z analyses in agreement with simulations. The direct thermal absorbance measurement platform would become an ideal tool for SEM, especially for thin films, light elements/compounds, or biological samples at nanoscales.

Synergistically optimizing carrier and phonon transport properties in n-type PbTe through I doping and SnSe alloying

PbTe is a promising thermoelectric material, but the properties in n-type PbTe need further improvement to match its advanced p-type material. In this work, the thermoelectric performance in n-type PbTe is synergistically enhanced with iodine (I) doping and SnSe alloying. The experiments show that I element is a good donor dopant to tune the carrier density and SnSe alloying can effectively balance the triple-relations between carrier mobility, carrier effective mass, and lattice thermal conductivity. Thus, high carrier mobility of ∼755 cm2V−1s−1 results in an optimum power factor (PF) of ∼27.0 μWcm−1K−2 in PbTe–2%SnSe at 573 K. Microstructure observation reveals the dispersively embedded Sn-rich nanoprecipitates in PbTe–2%SnSe, which largely suppresses lattice thermal conductivity at room temperature, from ∼3.41 Wm−1K−1 in PbTe to ∼1.01 Wm−1K−1 in PbTe–2%SnSe. Combined the optimized PF and decreased thermal conductivity, the peak ZT value of ∼1.5 can be obtained at 773 K in PbTe–2%SnSe. The obtained high thermoelectric performance in our work is comparable with other advanced n-type PbTe thermoelectrics and has great application potential.

Strategies for boosting thermoelectric performance of PbSe: A review

• General physical properties of typical IV-VI compounds are summarized. • Phase diagram, crystal and electrical band structures of PbSe are presented. • Recent ways of adjusting charge and phonon transport processes in PbSe are updated. • Typical preparation methods for PbSe and their scalability are discussed. • Key challenges and future research directions of PbSe are discussed. Thermoelectric materials enable the direct conversion between waste heat and electric energy, playing an important role in alleviating energy crisis. Many excellent thermoelectrics developed so far contain expensive and scarce Te element, largely limiting their applications. Therefore, exploring Te-free compounds with extraordinary thermoelectric performance becomes a vital topic in thermoelectric community in recent years. PbSe is an ideal candidate that meets above criteria and has advanced rapidly in the last decade with reported peak ZT s close to 2.0. Herein we review the recent research progress of PbSe-based thermoelectric materials. This review article starts with a general introduction of the properties of IV-VI semiconductors as advanced thermoelectric materials by comparing their cost, crustal abundance, mechanical strength and chemical bonding. Following that, phase diagram, crystal and electronic band structures of PbSe are comprehensively summarized. Then we discuss how the frequently used dopants regulate its carrier concentrations. Subsequently, electronic band structure engineering (including resonant levels, band flattening, band convergence, band inversion, etc. ) and microstructural architecturing (including atomic arrangements, dislocation arrays, nanoscale precipitates, etc. ) approaches and their impacts on charge and phonon transport properties of PbSe are elaborated. Finally, we summarize the typical production processes of PbSe and comment on their scalability. The future directions for how to further improve the thermoelectric properties of PbSe and promote its applications are discussed at the end of this article.

Electron mean-free-path filtering in n-type SnSe for improved thermoelectric performance at room temperature

Based on the fact that the mean free path of phonon is larger than that of the electron, the improvement of ZT could be obtained by adjusting the size of grain boundary. Pushing the size of grain boundary down to the length ranges stated by the electron MFP, both electron transport and phonon transport are affected strongly, but the reduced scale of phonon transport is larger than that of electron transport, leading to the enhancement of ZT. By combining all electron and phonon transport property from full electron-phonon interactions, the room-temperature thermoelectric property of n-type SnSe under different electron/phonon transport mechanism is evaluated. The h-LO dominates the scattering process of electron transport, while the optic branch controls the scattering process of phonon transport. Compared to the bulk ZT (0.26), the room temperature ZT of 0.62 with the size of 7 nm is obtained, which is nearly three times higher than that of bulk value. Our work not only reveals the enhancement of ZT by the electron filtering effect, but also exhibits the computational framework for accurately calculating the thermoelectric performance under different electron/phonon transport mechanisms.

Influence of the hBN Dielectric Layers on the Quantum Transport Properties of MoS2 Transistors.

The encapsulation of single-layer 2D materials within hBN has been shown to improve the mobility of these compounds. Nevertheless, the interplay between the semiconductor channel and the surrounding dielectrics is not yet fully understood, especially their electron–phonon interactions. Therefore, here, we present an ab initio study of the coupled electrons and phonon transport properties of MoS-hBN devices. The characteristics of two transistor configurations are compared to each other: one where hBN is treated as a perfectly insulating, non-vibrating layer and one where it is included in the ab initio domain as MoS. In both cases, a reduction of the ON-state current by about 50% is observed as compared to the quasi-ballistic limit. Despite the similarity in the current magnitude, explicitly accounting for hBN leads to additional electron–phonon interactions at frequencies corresponding to the breathing mode of the MoS-hBN system. Moreover, the presence of an hBN layer around the 2D semiconductor affects the Joule-induced temperature distribution within the transistor.

Outstanding CdSe with Multiple Functions Leads to High Performance of GeTe Thermoelectrics

AbstractThermoelectric materials can achieve the direct conversion between electricity and heat, which has drawn extensive attention in recent decades. Understanding the chemical nature of band structure and microstructure is essential to boost the thermoelectric performance of given materials. Herein, CdSe alloying promotes the evolution of multiple valence bands in GeTe, resulting in the contemporaneous appearance of band convergence and density of state distortion, which benefits the sharply enhanced effective mass from 2.3 m0 to 5.0 m0. The carrier mobility and effective mass are well optimized via CdSe alloying, contributing to the ultrahigh weighted mobility of ≈199 cm2 V−1 s−1 at 300 K in CdSe‐alloyed GeTe. Accordingly, a superior power factor of ≈41 µW cm−1 K−2 is attained at 673 K. Meanwhile, the nanoprecipitates, strain, and mass field fluctuations introduced by CdSe alloying result in a significantly decreased lattice thermal conductivity. A highest figure of merit (ZT) of ≈2.3 at 673 K and the ultrahigh ZTave of ≈1.46 at 303–773 K are achieved in CdSe‐alloyed GeTe. This work illustrates that the charge and phonon transport properties of GeTe can be simultaneously optimized through integrating band engineering and all‐scale defects incorporation via CdSe alloying.

Phonon Transport Properties and Polarization Branch Contributions of Graphene Ribbons With Different Edge Termination States

Phonon Transport Properties and Polarization Branch Contributions of Graphene Ribbons With Different Edge Termination States

A first-principles and machine-learning investigation on the electronic, photocatalytic, mechanical and heat conduction properties of nanoporous C5N monolayers.

Carbon nitride nanomembranes are currently among the most appealing two-dimensional (2D) materials. As a nonstop endeavor in this field, a novel 2D fused aromatic nanoporous network with a C5N stoichiometry has been most recently synthesized. Inspired by this experimental advance and exciting physics of nanoporous carbon nitrides, herein we conduct extensive density functional theory calculations to explore the electronic, optical and photocatalytic properties of the C5N monolayer. In order to examine the dynamic stability and evaluate the mechanical and heat transport properties under ambient conditions, we employ state of the art methods on the basis of machine-learning interatomic potentials. The C5N monolayer is found to be a direct band gap semiconductor, with a band-gap of 2.63 eV according to the HSE06 method. The obtained results confirm the dynamic stability, remarkable tensile strengths over 10 GPa and a low lattice thermal conductivity of ∼9.5 W m-1 K-1 for the C5N monolayer at room temperature. The first absorption peak of the single-layer C5N along the in-plane polarization is predicted to appear in the visible range of light. With a combination of high carrier mobility, appropriate band edge positions and strong absorption of visible light, the C5N monolayer might be an appealing candidate for photocatalytic water splitting reactions. The presented results provide an extensive understanding concerning the critical physical properties of the C5N nanosheetsand also highlight the robustness of machine-learning interatomic potentials in the exploration of complex physical behaviors.

Routes to High-Ranged Thermoelectric Performance

Thermoelectric technology has immense potential in enabling energy conversion between heat and electricity, and its conversion efficiency is mainly determined by the wide-temperature thermoelectric performance in a given material. Therefore, it is more meaningful to pursue high ZT values in a wide temperature range (namely high average ZT) rather than the peak ZT value at a temperature point. Herein, taking lead chalcogenides as paradigm, some rational routes to high average ZT value in thermoelectric materials are introduced, such as bandgap tuning and dynamic doping. This perspective will emphasize the importance of dynamically optimizing carrier and phonon transport properties to high-ranged thermoelectric performance, which could judiciously be extended to other thermoelectric systems.

Antibonding induced anharmonicity leading to ultralow lattice thermal conductivity and extraordinary thermoelectric performance in CsK2X (X = Sb, Bi)

A first principles-based machine learning potential is developed to model the phonon transport properties in full Heusler CsK2Sb and CsK2Bi taking account of all higher-order phonon interactions.

Significantly suppressed thermal transport by doping In and Al atoms in gallium nitride.

Thermal transport plays a key role in the working stability of gallium nitride (GaN) based optoelectronic devices, where doping has been widely employed for practical applications. However, it remains unclear how doping affects thermal transport. In this study, based on first-principles calculations, we studied the doping effect on the thermal transport properties of GaN by substituting Ga with In/Al atoms. The thermal conductivities at 300 K along the in-plane(out-of-plane) directions of In- and Al-doped GaN are calculated to be 7.3(8.62) and 12.45(11.80) W m-1 K-1, respectively, which are more than one order of magnitude lower compared to that of GaN [242(239) W m-1 K-1]. From the analysis of phonon transport properties, we find that the low phonon group velocity and small phonon relaxation time dominate the degenerated thermal conductivity, which originated from the strong phonon anharmonicity of In/Al-doped GaN. Furthermore, by examining the crystal structure and electronic properties, the lowered thermal conductivity is revealed lying in the strong polarization of In-N and Al-N bonds, which is due to the large difference in electronegativity of In/Al and N atoms. The results achieved in this study have guiding significance to the thermal transport design of GaN-based optoelectronic devices.

Finite-momentum excitons and the role of electron-phonon couplings in the electronic and phonon transport properties of boron arsenide.

The emerging semiconductor boron arsenide (BAs) with high thermal conductivity has attracted much attention recently, due to its promising application to overcome the bottleneck of high-density heat generated in power electronics and optoelectronic devices. In this work, based on first-principles calculations, we find that cubic BAs possesses high intrinsic electron/hole mobilities and the ionized impurity scattering plays a more important role in carrier scattering, compared with other scattering processes. The mobilities can be significantly enhanced by 14.9% and 76.2% for electrons and holes, respectively, by strain engineering. The investigation of the optoelectronic properties of indirect semiconductor cubic BAs by considering the many-body excitonic effects reveals that the contribution from finite-momentum excitons to optical properties is larger for photon energy ranging from 2.25 eV to 3.50 eV, compared with that from zero-momentum excitons. Finally, we observe that the phonon-electron couplings to total lattice thermal conductivities are non-trivial at low temperatures. These findings provide new insight into the transport and optoelectronic properties of cubic BAs, which are beneficial for the acceleration of the application of this revolutionary thermal management material.

Anisotropy of thermal transport in phosphorene: a comparative first-principles study using different exchange–correlation functionals

Using 12 different exchange–correlation functionals, the phonon transport properties of phosphorene are systematically studied by solving the Boltzmann transport equation.

Abnormally Low Lattice Thermal Conductivity in ABX Honeycomb Compounds

Planar structure and small atomic mass usually induce high lattice thermal conductivity in a crystalline solid. Here, we present a first-principles study on the lattice dynamics and phonon-transport properties of a series of ABX honeycomb compounds and show that replacing cations with lighter atoms may yield lower lattice thermal conductivity $({\ensuremath{\kappa}}_{l})$. The anomalous mass-trend ${\ensuremath{\kappa}}_{l}$, which is experimentally observed, is found to be inherent to the lattice vibration of the specific layered honeycomb structure, using A$\mathrm{Cu}\mathrm{Sb}$ (A = $\mathrm{Ca}$, $\mathrm{Sr}$, and $\mathrm{Ba}$) as prototypical compounds. We reveal that a lighter A atom bonds more weakly with the $\mathrm{Cu}\text{\ensuremath{-}}\mathrm{Sb}$ honeycomb ring, giving rise to stronger vibrational anharmonicity, which induces ultralow ${\ensuremath{\kappa}}_{l}$ in $\mathrm{Ca}\mathrm{Cu}\mathrm{Sb}$ that is unexpected of light elements and a high-symmetry planar structure. Combined with the high degeneracy and strong anisotropy of the bottom conduction band, $\mathrm{Ca}\mathrm{Cu}\mathrm{Sb}$ exposes an ultrahigh n-type zT of about 2.2 at 700 K. These findings elucidate the mechanism governing anomalous phonon-transport behavior, which also offers guidance in discovering low-cost and low-mass-density materials for advanced thermoelectric applications.

Thermal conductivity prediction by atomistic simulation methods: Recent advances and detailed comparison

Atomistic simulation methods, including anharmonic lattice dynamics combined with the Boltzmann transport equation, equilibrium and non-equilibrium molecular dynamics simulations, and Landauer formalism, are vital for the prediction of thermal conductivity and the understanding of nanoscale thermal transport mechanisms. However, for years, the simulation results using different methods, or even the same method with different simulation setups, lack consistency, leading to many arguments about the underlying physics and proper numerical treatments on these atomistic simulation methods. In this perspective, we review and discuss the recent advances in atomistic simulation methods to predict the thermal conductivity of solid materials. The underlying assumptions of these methods and their consequences on phonon transport properties are comprehensively examined. Using silicon and graphene as examples, we analyze the influence of higher-order phonon scatterings, finite-size effects, quantum effects, and numerical details on the thermal conductivity prediction and clarify how to fairly compare the results from different methods. This perspective concludes with suggestions on obtaining consistent thermal conductivity prediction of different material systems and also provides perspective on efficient and accurate simulations of thermal transport in more complex and realistic conditions.