Lattice Thermal Transport Research Articles

Anharmonic effects on lattice dynamics and thermal transport of two-dimensional InTe monolayer

The lattice thermal conductivity (κl) plays a key role in the performance of thermoelectric (TE) materials, where the lower values lead to higher figure of merit values. Two-dimensional (2D) group III-VI monolayers such as InTe are promising materials for TE energy generation owing to their low κl that lead to high TE figure of merit values. In this work, we investigate the influence of the lattice anharmonicity on the lattice thermal conductivity of InTe monolayer. The thermodynamic parameters are calculated by using the self-consistent phonon (SCP) theory. The κl value of the InTe monolayer is obtained to be 0.30 Wm−1 K−1 by using the standard Boltzmann transport equation (BTE) approach, while it is 3.58 Wm−1 K−1 by using SCP + BTE approach. These results confirm the importance of the anharmonic effects on the κl value, where it was found to be significantly higher (91%) using the SCP + BTE approach than that obtained using the standard BTE approach.

Anharmonicity-induced phonon hardening and phonon transport enhancement in crystalline perovskite BaZrO3

We investigated the effects of cubic and quartic anharmonicity on lattice dynamics and thermal transport in highly anharmonic ${\mathrm{BaZrO}}_{3}$ crystal over a wide temperature range (300--2000 K) by combining the first-principles-based self-consistent phonon theory and a unified theory of thermal transport including population and coherence contributions. By considering the effects from bubble and loop diagrams, the contributions of both three-phonon (3ph) and four-phonon (4ph) interaction processes to phonon scattering rates and energy shifts were clarified. Anharmonic phonon renormalization is found to play a crucial role in determining the finite-temperature phonon energies and lattice thermal conductivity ${\ensuremath{\kappa}}_{\mathrm{L}}$ in ${\mathrm{BaZrO}}_{3}$. Specifically, the lattice anharmonicity induces significant low-frequency optical phonon hardening at elevated temperatures, which is correlated with the U-shaped potential energy surfaces for these modes. The low-frequency optical phonon hardening significantly suppresses phonon scattering rates by altering the phonon weighted phase space of both 3ph and 4ph interaction processes, thereby leading to significant enhancement in the ${\ensuremath{\kappa}}_{\mathrm{L}}$ and weaker temperature dependence of ${\ensuremath{\kappa}}_{L}\ensuremath{\sim}{T}^{\ensuremath{-}0.75}$ than traditional harmonic treatments. Moreover, although the coherent thermal transport channel is suppressed by anharmonic phonon renormalization, it is enhanced by the 4ph scattering processes. The coherence contribution becomes nonnegligible at elevated temperatures and may contribute up to 17.38% of the total ${\ensuremath{\kappa}}_{\mathrm{L}}$ at 1500 K. In this paper, we highlight the strong influence of the lattice anharmonicity on thermal conductivity in severely anharmonic systems and the importance of coherent thermal transport channel at elevated temperatures.

The ultra-low lattice thermal conductivity dominated by the quartic anharmonicity in Bi-based binary compounds A3Bi (A = K, Rb)

In this article, by using self-consistent phonon (SCP) theory, compressed sensing (CS) technology and Boltzmann transport equation (BTE), we studied the role of quartic anharmonicity in the lattice dynamics and thermal transport characteristics of Bi-based binary compounds A3Bi (A = K, Rb), with a particular focus on unraveling the impacts of quartic anharmonicity on lattice thermal conductivity . By analyzing the phonon dispersion curve and the phonon density of states, we found that the strong quartic anharmonicity, which is mainly caused by the alkali metal atoms A1 in A3Bi (A = K, Rb), hardens the vibration frequency of the low-lying phonon branch and ensures the lattice dynamic stability. Through the relaxation time approximation of the Boltzmann transport equation, the calculated considering the three-phonon (3ph) and four-phonon (4ph) scattering process shows an ultra-low value. In addition, we also conducted a detailed analysis of the normalized lattice thermal conductivity, phonon group velocity, phonon scattering rate, and scattering phase space, revealing that the extremely low in A3Bi is mainly due to its relatively strong 4ph scattering process. Our research proves the existence of strong quartic anharmonicity in A3Bi, and we have studied in detail the influence of 3ph and 4ph scattering processes on the thermal transport properties of A3Bi, which has never been reported before.

Scattering lifetime and high figure of merit in CsAgO predicted by methods beyond relaxation time approximation

The electronic transport behaviour of CsAgO has been discussed using the theory beyond relaxation time approximation from room temperature to 800 K. Different scattering mechanisms such as acoustic deformation potential scattering, impurity phonon scattering, and polar optical phonon scattering are considered for calculating carrier scattering rates to predict the absolute values of thermoelectric coefficients. The scattering lifetime is of the order of 10−14 s. The lattice thermal transport properties like lattice thermal conductivity and phonon-lifetime have been evaluated. The calculated lattice thermal conductivity equals 0.12 and 0.18 W mK−1 along ‘a’ and ‘c’ axes, respectively, at room temperature, which is very low compared to state-of-the-art thermoelectric materials. The anisotropy in the electrical conductivity indicates that the holes are favourable for the out-of-plane thermoelectrics while the electrons for in-plane thermoelectrics. The thermoelectric figure of merit for holes and electrons is nearly same with a value higher than 1 at 800 K for different doping concentrations. The value of the thermoelectric figure of merit is significantly higher than the existing oxide materials, which might be appealing for future applications in CsAgO.

Monolayer SnI2: An Excellent p-Type Thermoelectric Material with Ultralow Lattice Thermal Conductivity.

Using density functional theory and semiclassical Boltzmann transport equation, the lattice thermal conductivity and electronic transport performance of monolayer SnI2 were systematically investigated. The results show that its room temperature lattice thermal conductivities along the zigzag and armchair directions are as low as 0.33 and 0.19 W/mK, respectively. This is attributed to the strong anharmonicity, softened acoustic modes, and weak bonding interactions. Such values of the lattice thermal conductivity are lower than those of other famous two-dimensional thermoelectric materials such as MoO3, SnSe, and KAgSe. The two quasi-degenerate band valleys for the valence band maximum make it a p-type thermoelectric material. Due to its ultralow lattice thermal conductivities, coupled with an ultrahigh Seebeck coefficient, monolayer SnI2 possesses an ultrahigh figure of merits at 800 K, approaching 4.01 and 3.34 along the armchair and zigzag directions, respectively. The results indicate that monolayer SnI2 is a promising low-dimensional thermoelectric system, and would stimulate further theoretical and experimental investigations of metal halides as thermoelectric materials.

Remarkable decrease in lattice thermal conductivity of transition metals borides TiB2 by dimensional reduction

Two-dimensional transition metals borides Ti x B x have excellent magnetic and electronic properties and great potential in metal-ion batteries and energy storage. The thermal management is important for the safety and stability in these applications. We investigated the lattice dynamical and thermal transport properties of bulk-TiB2 and its two-dimensional (2D) counterparts based on density functional theory combined with solving phonon Boltzmann transport equation. The Poisson’s ratio of bulk-TiB2 is positive while it changes to negative for monolayer TiB2. We found that dimension reduction can cause the room-temperature in-plane lattice thermal conductivity decrease, which is opposite the trend of MoS2, MoSe2, WSe2 and SnSe. Additionally, the room temperature thermal conductivity of mono-TiB2 is only one sixth of that for bulk-TiB2. It is attributed to the higher Debye temperature and stronger bonding stiffness in bulk-TiB2. The bulk-TiB2 has higher phonon group velocity and weaker anharmonic effect comparing with its 2D counterparts. On the other hand, the room temperature lattice thermal conductivity of mono-Ti2B2 is two times higher than that of mono-TiB2, which is due to three-phonon selection rule caused by the horizontal mirror symmetry.

Visualizing Hot-Carrier Expansion and Cascaded Transport in WS2 by Ultrafast Transient Absorption Microscopy.

The competition between different spatiotemporal carrier relaxation determines the carrier harvesting in optoelectronic semiconductors, which can be greatly optimized by utilizing the ultrafast spatial expansion of highly energetic carriers before their energy dissipation via carrier–phonon interactions. Here, the excited‐state dynamics in layered tungsten disulfide (WS2) are primarily imaged in the temporal, spatial, and spectral domains by transient absorption microscopy. Ultrafast hot carrier expansion is captured in the first 1.4 ps immediately after photoexcitation, with a mean diffusivity up to 980 cm2 s−1. This carrier diffusivity then rapidly weakens, reaching a conventional linear spread of 10.5 cm2 s−1 after 2 ps after the hot carriers cool down to the band edge and form bound excitons. The novel carrier diffusion can be well characterized by a cascaded transport model including 3D thermal transport and thermo‐optical conversion, in which the carrier temperature gradient and lattice thermal transport govern the initial hot carrier expansion and long‐term exciton diffusion rates, respectively. The ultrafast hot carrier expansion breaks the limit of slow exciton diffusion in 2D transition metal dichalcogenides, providing potential guidance for high‐performance applications and thermal management of optoelectronic technology.

Study on the thermoelectric properties of p-type doped CsCdF3 and CsHgF3 with quartic anharmonicity

The cubic perovskite materials CsCdF3 and CsHgF3 have extremely low lattice thermal conductivity, but due to the limitations of the previous calculation methods. The CsHgF3 have been considered unstable for a long time. Through self-consistent phonon theory and compressed sensing technology, we verified the stability of CsHgF3, and studied the thermoelectric transport characteristics of p-type doped CsCdF3 and CsHgF3 based on the first principles calculations combined with the Boltzmann transport equation. In addition, we also discussed the influence of the quartic anharmonicity on the lattice dynamics and thermal transport properties of CsHgF3. The results show that the halogen elements in CsHgF3 have strong quartic anharmonicity and directly affect the stability of the material. The lattice thermal conductivity of CsCdF3 and CsHgF3 calculated by self-consistent phonon theory are 2.03 W/mK and 1.23 W/mK at 300 K, respectively, which are higher than the results obtained by the traditional method, but still lower than most existing classical thermoelectric materials. By combining the obtained thermoelectric coefficients, the calculated thermoelectric figure of merit can reach 0.799 and 1.275 for CsCdF3 and CsHgF3 at 700 K, which shows that CsCdF3 and CsHgF3 have potential development prospects in high-temperature thermoelectric application field.

Topological phonons and electronic structure of Li2BaSi class of semimetals

Extension of the topological concepts to the bosonic systems has led to the prediction of topological phonons in materials. Here we discuss the topological phonons and electronic structure of Li2BaX (X = Si, Ge, Sn, and Pb) materials using first-principles theoretical modelling. A careful analysis of the phonon spectrum of Li2BaX reveals an optical mode inversion with the formation of nodal line states in the Brillouin zone. Our electronic structure results reveal a double band inversion at the Γ point with the formation of inner nodal-chain states in the absence of spin–orbit coupling (SOC). Inclusion of the SOC opens a materials-dependent gap at the band crossing points and transitions the system into a trivial insulator state. We also discuss the lattice thermal conductivity and transport properties of Li2BaX materials. Our results show that coexisting phonon and electron nontrivial topology with robust transport properties would make Li2BaX materials appealing for device applications.

Novel insights into lattice thermal transport in nanocrystalline Mg3Sb2 from first principles: the crucial role of higher-order phonon scattering.

Zintl phase Mg3Sb2, which has ultra-low thermal conductivity, is a promising anisotropic thermoelectric material. It is worth noting that the prediction and experiment value of lattice thermal conductivity (κ) maintain a remarkable difference, troubling the development and application. Thus, we firstly included the four-phonon scattering processes effect and performed the Peierls-Boltzmann transport equation (PBTE) combined with the first-principles lattice dynamics to study the lattice thermal transport in Mg3Sb2. The results showed that our theoretically predicted κ is consistent with the experimentally measured, breaking through the limitations of the traditional calculation methods. The prominent four-phonon scatterings decreased phonon lifetime, leading to the κ of Mg3Sb2 at 300 K from 2.45 (2.58) W m-1 K-1 to 1.94 (2.19) W m-1 K-1 along the in (cross)-plane directions, respectively, and calculation accuracy increased by 20%. This study successfully explains the lattice thermal transport behind mechanism in Mg3Sb2 and implies guidance to advance the prediction accuracy of thermoelectric materials.

Superior thermoelectric properties of ternary chalcogenides CsAg5Q3 (Q = Te, Se) predicted using first-principles calculations.

Tailoring novel thermoelectric materials (TEMs) with a high efficiency is challenging due to the difficulty in realizing both low thermal conductivity and high thermopower factor. In this work, we propose ternary chalcogenides CsAg5Q3 (Q = Te, Se) as promising TEMs based on first-principles calculations of their thermoelectric properties. Using lattice dynamics calculations within self-consistent phonon theory, we predict their ultralow lattice thermal conductivities below 0.27 W m-1 K-1, revealing the strong lattice anharmonicity and rattling vibrations of Ag atoms as the main origination. By using the mBJ exchange-correlation functional, we calculate the electronic structures with the direct band gaps in good agreement with experiments, and evaluate the charge carrier lifetime as a function of temperature within the deformation potential theory. Our calculations to solve Boltzmann transport equations demonstrate high thermopower factors of 2.5 mW m-1 K-2 upon p-type doping at 300 K, comparable to the conventional dichalcogenide thermoelectric GeTe. With these ultralow thermal conductivities and high thermopower factors, we determine a relatively high thermoelectric figure of merit ZT along the z-axis, finding the maximum value of ZTz to be 2.5 at 700 K for CsAg5Se3 by optimizing the hole concentration. Our computational results highlight the great potentiality of CsAg5Q3 (Q = Te, Se) for high-performance thermoelectric devices operating at room temperature.

Anomalous Behavior of Strain Modulated Lattice Thermal Transport in Piezoelectric Crystals and the Effect of Polarization

Anomalous Behavior of Strain Modulated Lattice Thermal Transport in Piezoelectric Crystals and the Effect of Polarization

Modulating thermal transport in a porous carbon honeycomb using cutting and deformation techniques.

During the past few years, there has been a flurry of investigations on the lattice thermal transport of three-dimensional (3D) graphene, however, few studies have detailed how to adjust this property effectively using the presently available engineering technologies. In this work, the thermal transport properties of a porous single layer carbon honeycomb (SL-dCHC-2) and its mechanical response are systematically studied. We show that the thermal conductivity of SL-dCHC-2 can be adjusted effectively by varying the tensile strain, and its value is enhanced by up to 11.3 times with 8% strain as compared to the unstrained case. This value is significantly larger than what was observed for other two-dimensional (2D) materials such as silicene (∼7 times larger). This outstanding behavior is explained by the phonon mode level, indicating that a profound increase of the thermal conductivity under tensile strain is attributed to the enhancement of the phonon lifetime. In addition, the trend for the root mean squared displacement, which is closely related to the phonon anharmonic effect, correlates with the non-monotonic response of the dimerized C-C bonds at the linkage of the structure. These investigations and obtained results provide important guidance to develop 3D carbon honeycombs for several different purposes, such as for use as molecular sieves and in water purification applications.

3-X Structural Model and Common Characteristics of Anomalous Thermal Transport: The Case of Two-Dimensional Boron Carbides.

Improving the reliability of electronic devices requires effective heat management, and the key is the relationship between the thermal transport and temperature. Inspired by synthesized T-carbon and H-boron, the 3-X structural models are proposed to unify the two-dimensional (2D) multitriangle materials. Employing structural searches, we identify the stability of the 3-X configuration in 2D boron carbides as 3-9 BC3 monolayer, which, unexpectedly, exhibits a linear thermal conductivity versus temperature, not the traditional ∼1/T trend. We summarize the common characteristics and explore why this behavior is absent in 3-9 AlC3 and graphene via investigating the optical modes. We show that the linear behavior is a direct consequence of the special oscillation modes by the 3-X model associated with the largest group velocity. We find that 2D materials with such behavior usually share a relatively low thermal conductivity. Our work paves the way to deeply understand the lattice thermal transport and to widen nanoelectronic applications.

Lattice thermodynamic behavior in CeO2 from first principles

By combining the first principles with Boltzmann's transport equation, we have carried out a detailed calculation and analysis of the lattice thermal transport properties of CeO2. We found that as the pressure increases, the acoustic mode and optical mode of CeO2 gradually separate and an obvious bandgap appears. In addition, the lattice thermal conductivity increases significantly with increasing pressure, and can be increased from 16.99 W/mK (0 GPa) to 237.73 W/mK (160 GPa) at 300 K. By studying the contribution of phonons to the lattice thermal conductivity at different frequencies, we found that the lattice thermal conductivity of CeO2 is mainly provided by low-frequency phonons (<400 cm−1). Based on this we calculated and studied the phonon group velocity of low-frequency phonons, scattering rate, three-phonon scattering phase space and Grüneisen parameters. The results show that an increase in pressure significantly reduces the three-phonon scattering rate of CeO2 and the anharmonicity of the system, which in turn affect the lattice thermal conductivity seriously. Our work provides a theoretical basis for the subsequent study of CeO2 under high pressure.

Lattice thermal conductivity of transition metal carbides: Evidence of a strong electron-phonon interaction above room temperature

Strong electron-phonon interaction, can not only control charge carrier mobility, but also impact heat transport by reducing lattice thermal conductivity. In this work, we reveal by experiments a strong electron-phonon interaction in group IV and V transition metal carbides, which results in anomalous weak temperature dependence of lattice thermal conductivity from 300 K up to 773 K. This contrasts with the common belief that electron-phonon interaction would be only important for lattice heat transport at very low temperatures. The other important finding is that strong electron-phonon interaction can notably suppress lattice thermal transport and its temperature dependence provided that free charge carrier density is large enough, whereas Fermi surface nesting may not be necessary. This work provides new perspective on electron-phonon interaction and its consequence on lattice thermal conduction in solids, and it suggests a new avenue to modulate heat transfer above room temperature.

Tuning the Lattice Thermal Conductivity in Bismuth Telluride via Cr Alloying

Decreasing thermal conductivity of a thermoelectric material is always a prerequisite for its potential application. Using first-principle calculations, we examine the magnetism induced change in lattice thermal transport in bismuth telluride. The source of magnetic moment, Cr in the doped system, weakly magnetizes the coordinated Te atoms to make the latter's phonon softer than that in the pure compound. Though the transition metal dopants do not participate directly in the heat conduction process, the anharmonicity induced by them favor in reducing the lattice thermal conductivity. Large anharmonicity in $(Bi_{0.67}Cr_{0.33})_2Te_3$ reduces the in-plane room temperature lattice thermal conductivity by $\sim 79\%$. The thermal conductivity, strictly, does not vary monotonically with doping concentration. Even, for any particular doping level, the thermal conductivity is different for different configurations which is related to the internal energy of the system. We found that the internal energy variance of 0.03 $eV$ would reduce the in-plane thermal conductivity of the room temperature lattice by at least 60$\%$ for 50$\%$ doping.

Low lattice thermal conductivity of pentadiamond

The lattice thermal conductivity of carbon materials is particularly interesting because it can vary within a range spanning five orders of magnitude depending on the atomic configuration. Herein, we systematically study the lattice thermal conductivity and phonon transport properties of pentadiamond, a new three-dimensional carbon allotrope consisting of pentagonal carbon rings. Based on first-principles calculations and an iterative solution to the linearized Boltzmann transport equation, the intrinsic lattice thermal conductivity (kl) is found to be 490.88 W/mK at room temperature, much lower than 2664.93 W/mK of diamond. A detailed analysis of both harmonic and anharmonic properties reveals that the low kl of pentadiamond essentially originates from its large phonon phase space, short phonon relaxation time resulting from strong overlap between the acoustic and low-lying optical phonon branches, and the low phonon group velocity. The distinct thermal transport behavior exhibited in pentadiamond further shows the diversity and complexity in lattice thermal conductivity of carbon allotropes.

Simulation study of effects of grain boundary and helium bubble on lattice thermal resistance of tungsten

Tungsten (W) as plasma facing material (PFM) has been used in ITER divertor. Many experiments indicate that due to helium (He) ion irradiation the thermal conductivities (TCs) of W can be significantly reduced. In this work, the lattice thermal transport property is studied based on the non-equilibrium molecular dynamics (NEMD) method. Firstly, the Kapitza resistance of grain boundaries (GBs) in W is obtained. It can be confirmed that the Kapitza resistance is positively correlated with GB energies, which reflect the lattice distortion degree in a sense. The result indicates that the distortion energy may play a key role in the thermal properties. Further simulation is performed to study the thermal resistance of GBs with nano He bubble. When the number of He atoms in bubble is less than a value, the thermal resistances are stable and change little. It implies that the effect of GBs is more significant than He bubble to block phonons diffusion. With the number of He atoms increasing, the property of heat conduction deteriorates further because the coupling of He bubble and GBs change the lattice distortion degree.

Zintl Phase BaAgSb: Low Thermal Conductivity and High Performance Thermoelectric Material in Ab Initio CalculationSupported by the National Key R&D Program of China (Grant Nos. 2016YFA0401503 and 2017YFA0403700), the National Natural Science Foundation of China (Grant Nos. 11675255, U1932220, 11675195, and U1932220), the Key Research Program of Frontier Sciences, CAS (Grant No. 292016YQYKXJ00135), and the Program of State Key Laboratory

Thermoelectric materials are critical parts in thermal electric devices. Here, Zintl phase BaAgSb in space group of P63/mmc is reported as a promising thermoelectric material in density function theory. The anisotropic lattice thermal conductivity and phonon transport properties are investigated in theory. The strong phonon-phonon scattering in BaAgSb exhibits ultra-low lattice thermal conductivity of 0.59 W⋅m−1⋅K−1 along c-axis at 800 K, and high thermoelectric performance ZT = 0.94 at 400 K. The mix of covalent and ionic bond supports high carrier mobility and low thermal conductivity. The unusual features make BaAgSb a potential thermoelectric material.