Anisotropic Lattice Thermal Conductivity Research Articles

Anisotropic Lattice Thermal Conductivity in Highly Ordered PEDOT Fibers

AbstractWhen it comes to sustainable and efficient energy solutions, organic semiconductors can play an important role in thermoelectric applications, since they are non‐toxic, cheap, made of abundant chemical species, and show intrinsically low thermal conductivities. Their electrical conductivity can be optimized via doping. Yet, thermal conduction should be as low as possible and, to this end, the atomic scale mechanisms behind heat transport –e.g. the correlation between morphology and thermal conductivity or the role of doping– should be understood in detail. Fully atomistic molecular dynamics calculations of the lattice thermal conductivity of doped poly(3,4‐ethylenedioxythiophene) (PEDOT) highly ordered, quasi‐crystalline nanofibers are presented here. It is found that the conductivity along the backbone direction is not necessarily the highest, but it depends on the length of the PEDOT chains, thus the degree of anisotropy depends on the the aspect ratio of the nanofiber. Indeed, transport along the lamellar direction can be of the same order or higher than that of the backbone if their lengths are comparable. These results challenge the usual expectation that thermal conduction along the backbone largely exceeds those along the lamellar and π − π direction and have the important consequence that the anisotropy could be leveraged in thermal management applications.

Group III (In/Ga)-V (P/As)-VI (S/Se) Monolayers: A New Class of Auxetic Semiconductors with Highly Anisotropic Electronic/Optical/Mechanical/Thermal Properties.

We present a theoretical design of a class of 2D semiconducting materials, namely, group III (In/Ga)-V (P/As)-VI (S/Se) monolayers, whose global-minimum structures are predicted based on the particle swarm optimization method. Electronic structure calculations suggest that all group III-V-VI monolayers exhibit quasi-direct semiconducting characteristics with desirable band gaps ranging from 1.76 to 2.86 eV (HSE06 functional). Moreover, most group III-V-VI monolayers possess highly anisotropic carrier mobilities with large anisotropic ratios (3.4-6 for electrons, 2.2-25 for holes). G0W0+BSE calculations suggest that these monolayers show high optical anisotropy and relatively large exciton binding energies (0.33-0.75 eV), comparable to that (0.5 eV) of MoS2 monolayer. In particular, the GaPS monolayer manifests strikingly anisotropic I-V curves with a large ON/OFF ratio of ∼105 (106 for the GaPS bilayer) and anisotropic lattice thermal conductivity. Furthermore, the GaPS monolayer is predicted to exhibit both in-plane and out-of-plane negative Poisson ratios (NPRs) and prominent anisotropic Young moduli.

Asymmetrical LaO9 polyhedron inducing anisotropic positive thermal expansion and moderate lattice thermal conductivity in LaBO3 with isolated planar [BO3] group

Thermal physical properties of inorganic borates are important in application of optical device working under high temperature. Herein, we investigate the temperature-dependent thermoelasticity, thermal expansion(α), and lattice thermal conductivity(κ) of simple orthohomibic LaBO3 with a two dimension like crystal structure though first principles for the first time. The calculated rate of temperature-dependent second elastic constant C11 is significantly larger than that of C22 and C33, and the theoretical slopes of corresponding bulk, shear, and Young's moduli in the range of 300–900 K are −0.0289 GPa•K−1, −0.0156 GPa•K−1, and −0.0398 GPa•K−1, respectively. The predicted α is positive and increases with the rise of temperature. The anisotropic ratio αc/αa or αc/αb is approximately 2 at 300 K, which might be attributed to the difference of elastic compliance S12 and S13. Besides, the anisotropic lattice thermal conductivity κa, κb and κc of LaBO3 is predicted to be 1.43, 1.40 and 0.80 W/m-K at 300 K via Debye-Callaway model. Most importantly, the atomic projected Grüneisen parameters γ reveal that asymmetrical LaO9 polyhedra in the crystal lattice dominate the thermal expansion and lattice thermal conductivity. Our results indicate that LaBO3 has good thermal physical properties working under high temperature.

Comprehensive investigation of the extremely low lattice thermal conductivity and thermoelectric properties of BaIn2Te4

Recently, an extremely low lattice thermal conductivity value has been reported for the alkali-based telluride material ${\mathrm{BaIn}}_{2}{\mathrm{Te}}_{4}$. The value is comparable with low-thermal conductivity metal chalcogenides, and the glass limit is highly intriguing. Therefore, to shed light on this issue, we performed first-principles phonon thermal transport calculations. We predicted highly anisotropic lattice thermal conductivity along different directions via the solution of the linearized phonon Boltzmann transport equation. More importantly, we determined several different factors as the main sources of the predicted ultralow lattice thermal conductivity of this crystal, such as the strong interactions between low-frequency optical phonons and acoustic phonons, small phonon group velocities, and lattice anharmonicity indicated by large negative mode Gr\"uneisen parameters. Along with thermal transport calculations, we also investigated the electronic transport properties by accurately calculating the scattering mechanisms, namely the acoustic deformation potential, ionized impurity, and polar optical scatterings. The inclusion of spin-orbit coupling (SOC) for electronic structure is found to strongly affect the $p$-type Seebeck coefficients. Finally, we calculated the thermoelectric properties accurately, and the optimal $ZT$ value of $p$-type doping, which originated from high Seebeck coefficients, was predicted to exceed unity after 700 K and have a direction averaged value of 1.63 (1.76 in the $y$-direction) at 1000 K around $2\ifmmode\times\else\texttimes\fi{}{10}^{20}\phantom{\rule{4pt}{0ex}}{\mathrm{cm}}^{\ensuremath{-}3}$ hole concentration. For $n$-type doping, a $ZT$ around $3.2\ifmmode\times\else\texttimes\fi{}{10}^{19}\phantom{\rule{4pt}{0ex}}{\mathrm{cm}}^{\ensuremath{-}3}$ concentration was predicted to be a direction-averaged value of 1.40 (1.76 in the $z$-direction) at 1000 K, mostly originating from its high electron mobility. With the experimental evidence of high thermal stability, we showed that the ${\mathrm{BaIn}}_{2}{\mathrm{Te}}_{4}$ compound has the potential to be a promising mid- to high-temperature thermoelectric material for both $p$-type and $n$-type systems with appropriate doping.

KP15: Natural van der Waals material with ultra-low thermal conductivity and excellent thermoelectric performance

Recently, a natural van der Waals material KP15 has been successfully synthesized and reported to possess obvious anisotropy, high carrier mobility, and stable chemical properties. Herein, through combining Boltzmann transport equation and first-principles calculations, we systematically investigate the electronic and phononic transport properties and thermoelectric (TE) performance of KP15. Our calculations show that KP15 exhibits obvious anisotropic lattice thermal conductivity with extremely low intertube thermal conductivity of 1.49 W/mK at room temperature. Such low thermal conductivity is mainly originated from the intrinsic weak intertube interactions and beneficial for the TE performance. Meanwhile, KP15 also hosts evident anisotropic electronic transport properties and relatively high Seebeck coefficients over a wide range of carrier concentrations. Based on the electronic relaxation time obtained from the deformation potential theory, the TE conversion efficiency of KP15 is evaluated, and the thermoelectric figure of merit could approach as high as 1.40 at 700 K along the c-axis under n-type doping. The findings presented in this work indicate that KP15 has extremely low intertube thermal conductivity and excellent TE performance, which will stimulate further experimental exploration of similar alkali metal phosphating systems and more van der Waals materials.

Anisotropic Thermal Conductivity of Nickel-Based Superalloy CM247LC Fabricated via Selective Laser Melting

Efforts to enhance thermal efficiency of turbines by increasing the turbine inlet temperature have been further accelerated by the introduction of 3D printing to turbine components as complex cooling geometry can be implemented using this technique. However, as opposed to the properties of materials fabricated by conventional methods, the properties of materials manufactured by 3D printing are not isotropic. In this study, we analyzed the anisotropic thermal conductivity of nickel-based superalloy CM247LC manufactured by selective laser melting (SLM). We found that as the density decreases, so does the thermal conductivity. In addition, the anisotropy in thermal conductivity is more pronounced at lower densities. It was confirmed that the samples manufactured with low energy density have the same electron thermal conductivity with respect to the orientation, but the lattice thermal conductivity was about 16.5% higher in the in-plane direction than in the cross-plane direction. This difference in anisotropic lattice thermal conductivity is proportional to the difference in square root of elastic modulus. We found that ellipsoidal pores contributed to a direction-dependent elastic modulus, resulting in anisotropy in thermal conductivity. The results of this study should be beneficial not only for designing next-generation gas turbines, but also for any system produced by 3D printing.

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.

Correction to Rapid Prediction of Anisotropic Lattice Thermal Conductivity: Application to Layered Materials

Correction to Rapid Prediction of Anisotropic Lattice Thermal Conductivity: Application to Layered Materials

Lattice distortion and anisotropic thermoelectric properties in hot-deformed CuI-doped Bi2Te2·7Se0.3

We investigated anisotropic thermoelectric properties of (CuI)xBi2Te2·7Se0.3 (x = 0.0, 0.3, 0.6, and 0.9 mol.%) compounds, synthesized by the hot-press and hot-deformation process. In spite of polycrystalline compound, the hot-deformed compounds exhibit preferred orientation along the c-axis, parallel with the applied press direction. The sample of x = 0.3 mol.% shows the maximum power factor (3.8 mW m−1 K−2 at 300 K) and ZT value (0.97 at 423 K), which is relatively high thermoelectric performance in n-type thermoelectric materials as a mild-temperature operation. Notably, the in-plane lattice thermal conductivity of the x = 0.3% compound with covalent bonding layer has lower value than the one of out-of-plane lattice thermal conductivity with van der Waals bonding layer. From the high resolution transmission electron microscopy and electron diffraction measurements, we observe the lattice distortion of the x = 0.3% compound. Therefore, the unconventional anisotropic lattice thermal conductivity can be associated with the lattice distortion along the in-plane on the compound driven by the CuI doping.

Rapid Prediction of Anisotropic Lattice Thermal Conductivity: Application to Layered Materials

Thermal conductivity plays a crucial role in many applications; the use of single-crystal and textured polycrystalline materials in such applications necessitates understanding the anisotropy in th...

Two-Dimensional Tellurene as Excellent Thermoelectric Material

We study the thermoelectric properties of two-dimensional tellurene by first-principles calculations and semiclassical Boltzmann transport theory. The HSE06 hybrid functional results in a moderate direct band gap of 1.48 eV at the Γ point. A high room temperature Seebeck coefficient (Sxx = 0.38 mV/K, Syy = 0.36 mV/K) is combined with anisotropic lattice thermal conductivity (κxxl = 0.43 W/m K, κyyl = 1.29 W/m K). Phonon band structures demonstrate a key role of optical phonons in the record low thermal conductivity that leads to excellent thermoelectric performance of tellurene. At room temperature and moderate hole doping of 1.2 × 10–11 cm–2, for example, a figure of merit of ZTxx = 0.8 is achieved.

Comparative study on structural, elastic, dynamical, and thermodynamic properties of Weyl semimetals MX (M = Ta or Nb; X = As or P)

We present a comparative investigation on structural, elastic, dynamical and thermodynamic properties of Weyl semimetals MX (M = Ta or Nb; X = As or P) using density functional theory (DFT) within the generalized gradient approximation. The elastic properties of NbAs, TaP and NbP are obtained for the first time, then we compared them with each other and with some well-studied materials. Among four Weyl semimetals, TaP and NbAs possess the largest and smallest bulk modulus B, shear modulus G, and Young's modulus E, respectively, while NbP and TaAs own the maximum and minimum elastic Debye temperature. Through the analysis of three dimensional (3D) representations and two dimensional (2D) projections of Young's modulus, MX series exhibit distinct elastic anisotropy, especially for TaAs and NbAs. The calculated phonon dispersions of four Weyl semimetals show no imaginary frequency throughout the Brillouin zone, indicating they are dynamically stable. In addition, compared with other theoretical results, our calculated Brillouin-zone-center frequencies of MX series are more in line with experimental data. Furthermore, Phonon velocities are obtained using phonon spectra, and anisotropic phonon group velocities are responsible for their anisotropic lattice thermal conductivity. Additionally, thermodynamic properties are also predicted using the calculated phonon density of states. The results are in good agreement with available experimental values. We expect our work can provide more information for further experimental studies.

High thermoelectric performance of Weyl semimetal TaAs

Abstract The existence of Weyl nodes predicted in TaAs has been confirmed by angle-resolved photoemission spectroscopy, which provides potential applications in thermoelectric devices due to the extraordinary transport properties of TaAs. By using first-principles calculations and semiclassical Boltzmann transport theory, we study the electrical transport properties of TaAs. High anisotropy is observed in the electrical transport of TaAs. The obtained Seebeck coefficients are in good agreement with experimental values. The lattice dynamics properties of TaAs are also investigated and the obtained phonon frequencies agree well with the measurements. The lattice thermal conductivity is calculated using the self-consistent iterative approach. Anisotropic lattice thermal conductivity is observed as well. Maximum thermoelectric figure of merit zT of 0.63 at 900 K is found for n-doping TaAs along zz direction. Finally, the size dependence of lattice thermal conductivity and corresponding thermoelectric properties are investigated for designing thermoelectric nanostructures. The work sheds light on the nature of the thermoelectric response of Weyl semimetal.

Anisotropic Lattice Thermal Conductivity and Suppressed Acoustic Phonons in MOF-74 from First Principles

The thermal transport properties of metal–organic frameworks (MOFs) developed for molecular storage and catalytic separations play an important role in adsorption or catalysis processes but are rarely reported. We calculate the anisotropic thermal conductivities (κ) of water-stable Zn-MOF-74 with the Boltzmann transport equation and the density-functional-based tight-binding (DFTB) method, which allows for a sufficiently large number of atoms in the simulations without much compromise on accuracy. We find an anisotropic κ of 0.44 and 0.68 W/m·K at 300 K, across and along the pore directions, with acoustic contributions of 8% and 30%, respectively. These unusually low acoustic contributions are explained by the rattling-like behavior of phonons with large vibrational amplitude, low group velocity, and large scattering rate, which are caused by the unique 1-D tubing bundle structure. On the other hand, the cylindrical pores enable larger phonon speed and higher directional structural rigidity along the pore...

Anisotropic Lattice Thermal Conductivity in Enstatite as a Function of Pressure and Temperature

Summary Principal lattice thermal conductivity coefficients have been determined from measurements of heat flux and temperature gradient in oriented samples of enstatite single crystals at pressures between 19 and 56 × 1018 Nm−21 and at temperatures between 300° and 400°K. A noticeable change in the rate of increase of conductivity with pressure near 38 × 1018 Nm−2 is interpreted as the onset of a reversible polymorphic transition from orthorhombic enstatite to monoclinic clinoenstatite. The two structures are derivable from one another by twinning and untwinning processes and a gradual transition is considered to proceed by the formation of an increasing number of stacking faults in the orthorhombic structure. This, obviously, increases the amount of phonon scattering, and observed thermal conductivity changes can thus be used to determine stacking fault densities. Figures for the minimum in lattice thermal conductivity have been derived from observed Δλ/ΔP and Δλ/ΔT values terms of hypothetical values of depth and temperature within the Earth. Estimates of ∂T/∂P at constant entropy, as determined from Δλ/ΔP and Δλ/ΔT values, suggest that in the case of enstatite conditions for convection are not fulfilled within the region of appreciable transformation at pressures greater than about 35 × 108 Nm−2. Straight line graphs are obtained if log (Δλ/ΔT) is plotted versus 1/T yielding activation energies of the order of 3.2 × 10−20 J (0.2 ev) for processes controlling the variation of lattice thermal conductivity with temperature. In the Appendix a two layer model is discussed to illustrate possible deviations of the terrestrial heat flow vector from the direction of maximum temperature gradient in the presence of an anisotropic structure such as enstatite.