Low Lattice Thermal Conductivity κL Research Articles

Ultralow Lattice Thermal Transport and Considerable Wave-like Phonon Tunneling in Chalcogenide Perovskite BaZrS3.

Chalcogenide perovskites provide a promising avenue for nontoxic, stable thermoelectric materials. Here, the thermal transport and thermoelectric properties of BaZrS3 as a typical orthorhombic perovskite are investigated. An extremely low lattice thermal conductivity κL of 1.84 W/mK at 300 K is revealed for BaZrS3, due to the softening effect of Ba atoms on the lattice and the strong anharmonicity caused by the twisted structure. We demonstrate that coherence contributions to κL, arising from wave-like phonon tunneling, lead to an 18% thermal transport contribution at 300 K. The increasing temperature softens the phonons, thus reducing the group velocity of materials and increasing the scattering phase space. However, it simultaneously reduces the anharmonicity, which is dominant in BaZrS3 and ultimately improves the particle-like thermal transport. In addition, via replacement of the S atom with Se- and Ti-alloying strategy, the ZT value of BaZrS3 is significantly increased from 0.58 to 0.91 at 500 K, making it an important candidate for thermoelectric applications.

Room temperature high thermoelectric performance of Bi-based full-Heusler compounds CsxRb[formula omitted]Bi with strong anharmonicity

Bismuth-based compounds have been identified as a class of promising candidates for the realization of high-performance thermoelectrics, derived from their exceptional electrical conductivity and intrinsic low lattice thermal conductivity κL. Here, we employed first-principles calculations, self-consistent phonon theory, and compressive sensing technique in conjunction with the Boltzmann transport equation to investigate the anharmonic thermoelectric properties of full-heusler bismuth compounds CsxRb3−xBi. As the Cs content increases, the κL of CsxRb3−xBi decreases due to impurity scattering effects, and then increases. Among them, at 300 K, Rb3Bi has the largest κL of 0.69 Wm−1K−1, while Cs2RbBi has the smallest κL of 0.45 Wm−1K−1, both of which are lower than the κL of traditional bismuth-based thermoelectric materials. The ultralow κL originates from the strong lattice anharmonicity. In addition, the coexistence of high dispersion and flat band edges leads to high thermoelectric power factor at optimal doping concentration. As a result, the largest thermoelectric figure of merit ZT values of 2.59 (4.19) at 300 (500) K were obtained for CsRb2Bi at optimal hole doping level. These findings suggest that CsxRb3−xBi compounds are superior for thermal management and thermoelectric applications.

Computational understanding and prediction of 8-electron half-Heusler compounds with unusual suppressed phonon conduction

The conventional thinking of designing materials with low lattice thermal conductivity κL is usually associated with chemical and structural complexity. Here, we proposed a new strategy for establishing the interaction strength between the nested cation and the anionic framework as a control knob for tuning κL in two orders of magnitude in isostructural half-Heusler compounds. A synthesized cubic and light-weight 8-electron half-Heusler compound, namely, MgCuSb, exhibits glass-like thermal conductivity in both magnitude and temperature dependence that seems to contradict common understanding while common 18-electron counterparts are known for high κL. Our studies reveal that both the native strong anharmonicity induced by the tension effect of atomic filling and a low-energy shearing vibration mode triggered by weak Mg–Cu bonding are responsible for the unusual suppressed phonon conduction in MgCuSb. Finally, an analytic model is constructed by machine learning method to predict phonon conduction of both 8- and 18-electron half-Heusler compounds in a unified way, which demonstrates that the interaction between cations and anions is universal by means of adjusting the thermal conductivity of this material family.

Anomalous high [formula omitted] above 2.2 in light element dominated compound Li[formula omitted]OTe due to low thermal conductivity

Thermoelectric materials usually contain heavy metal elements to ensure low lattice thermal conductivity κl. However, such elements like Pb, Bi and Sb have the expenses of high prices and potential risks of pollution. Herein, we propose a novel design strategy for thermoelectrics by constructing new light element dominated materials, such as quasi-antifluorite Li4OTe, with a high figure of merit surpassing 2.2. The introduction of O atom in Li4OTe crystal causes optical phonon softening and large phase space for three-phonon scattering processes, leading to much lower κl (<1 W m−1 K−1) than antifluorite Li2X (X=O, Te). This picture can be extended to other quasi-antifluorites, which will stimulate intense interests in the exploration of the light element dominated compounds, as new environmentally friendly and low-cost thermoelectric materials.

Low Lattice Thermal Conductivity in a Wider Temperature Range for Biphasic-Quaternary (Ti,V)CoSb Half-Heusler Alloys.

Intrinsically high lattice thermal conductivity has remained a major bottleneck for achieving a high thermoelectric figure of merit (zT) in state-of-the-art ternary half-Heusler (HH) alloys. In this work, we report a stable n-type biphasic-quaternary (Ti,V)CoSb HH alloy with a low lattice thermal conductivity κL ≈ 2 W m-1 K-1 within a wide temperature range (300-873 K), which is comparable to the reported nanostructured HH alloys. A solid-state transformation driven by spinodal decomposition upon annealing is observed in Ti0.5V0.5CoSb HH alloy, which remarkably enhances phonon scattering, while electrical properties correlate well with the altering electronic band structure and valence electron count (VEC). A maximum zT ≈ 0.4 (±0.05) at 873 K was attained by substantial lowering of κL and synergistic enhancement of the power factor. We perform first-principles density functional theory calculations to investigate the structure, stability, electronic structure, and transport properties of the synthesized alloy, which rationalize the reduction in the lattice thermal conductivity to the increase in anharmonicity due to the alloying. This study upholds the new possibilities of finding biphasic-quaternary HH compositions with intrinsically reduced κL for prospective thermoelectric applications.

Unveiling the origins of low lattice thermal conductivity in 122-phase Zintl compounds

The intrinsically low lattice thermal conductivity κL is essentially important for thermoelectric technology to maintain large working temperature difference, in order to enlarge the output power. In principle, high density and complex structure at atomic and nano scale are typical features for thermoelectric materials with low lattice thermal conductivities (κLs). However, many 122-phase Zintl compounds with promising thermoelectric properties defy these paradigms. They adopt three relatively simple structures with comparably small mean atomic mass, while these compounds exhibit considerably low lattice thermal conductivities κLs. Herein we unveil the origins of low κLs in 122-phase Zintl compounds, ascribed from the important aspects including atomic constituents, their arrangements and chemical bonding. The coupling between the acoustic branches and low-frequency optical branches, as well as the anisotropic chemical bonding are responsible for the high anharmonicity, favoring for the low phonon velocity and small relaxation time. This work offers a guidance to design high-performance Zintl thermoelectric materials with ultralow lattice thermal conductivity.

Enhancement of thermoelectric properties by lattice softening and energy band gap control in Te-deficient InTe1−δ

The InTe has intrinsically low lattice thermal conductivity κL originating from the anharmonic bonding of In1+ ion in the lattice, which scatters the phonons. Here we report the enhancement of thermoelectric properties in Te-deficient InTe1−δ (δ = 0, 0.01, 0.1, and 0.2) polycrystalline compounds by lattice softening and energy band gap opening. Te-deficiency gives rise to more weak chemical bonding between In1+ atoms and In3+Te2− clusters than those of pristine InTe, resulting in the reduction of κL near the room temperature. The weak ionic bonding is confirmed by the increase of lattice volume from the X-ray diffraction and lattice softening by the decrease of Debye temperature with increasing Te-deficiency. We observed the low lattice thermal conductivity κL of 0.53 W m−1 K−1 at 300 K for InTe0.99, which is about 25 % decreased value than those of InTe. The Te-deficiency also induces energy band gap so that the electrical resistivity and Seebeck coefficient are increased due to the decrease of carrier concentration. Temperature-dependent thermoelectric properties shows the high Seebeck coefficient at high temperature and high electrical conductivity near room temperature, resulting in the temperature-insensitive high power factor S2σ over a wide temperature range. Owing to the temperature-insensitive high power factor and intrinsic low lattice thermal conductivity by Te-deficiency, the thermoelectric performances of figure-of-merit ZT and engineering ZTeng are enhanced at mild temperature range (≤550 K).

Reducing lattice thermal conductivity in schwarzites via engineering the hybridized phonon modes

The cage structure and low lattice thermal conductivity κL make the schwarzites a suitable candidate for building the host-guest system favorable for thermoelectric applications. In host-guest system, the guest atoms in cages play a crucial role for reducing κL. However, the underlying mechanism on the thermal conductivity reduction remains unclear. In this work, the authors unveil, through atomic simulations, the working principles of guest atoms in schwarzites by highlighting the relationship between thermal transport properties and rattling motions of guest atoms. It has been found that local vibration of the “on-center” guest atoms at low temperatures gives rise to a single hybridized mode, while the positions of guest atoms at high temperatures deviate severely from the center of cages yielding blue-shifted hybridized modes. These blue-shifted hybridized modes are responsible for the reduction of the phonon relaxation time in a wide range of frequencies. Based on these findings, the authors propose guidelines for reducing κL in schwarzites by properly tuning the frequency of one hybridized mode. Further reduction of κL can be achieved by simultaneously introducing multiple hybridized modes with different characteristic frequencies. This study provides insights to the controllable thermal transport properties in schwarzites by engineering the hybridized phonon modes.

Phonon thermal transport in a class of graphene allotropes from first principles.

Utilizing first principle calculations combined with the phonon Boltzman transport equation (PBTE), we systematically investigate the phonon thermal transport properties of α, β and γ graphyne, a class of graphene allotropes. Strikingly, at room temperature, a low lattice thermal conductivity κL of 21.11, 22.3, and 106.24 W m-1 K-1 is obtained in α, β and γ graphyne, respectively, which are much lower than that of graphene. We observe contributions from the phonon modes below the specified frequency and find that many optical phonon modes play critical roles in the phonon thermal transport. These optic modes participate in thermal transport, enhancing the phonon scattering process, thus leading to the low κL value. Our results provide insights into the thermal transport of graphyne, and forecast its potential applications for thermoelectric and thermal barrier coatings.

Effect of Isovalent Substitution on the Electronic Structure and Thermoelectric Properties of the Solid Solution α-As2Te3-xSex (0 ≤ x ≤ 1.5).

We report on the influence of Se substitution on the electronic band structure and thermoelectric properties (5-523 K) of the solid solution α-As2Te3-xSex (0 ≤ x ≤ 1.5). All of the polycrystalline compounds α-As2Te3-xSex crystallize isostructurally in the monoclinic space group C2/m (No. 12, Z = 4). Regardless of the Se content, chemical analyses performed by scanning electron microscopy and electron probe microanalysis indicate a good chemical homogeneity, with only minute amounts of secondary phases for some compositions. In agreement with electronic band structure calculations, neutron powder diffraction suggests that Se does not randomly substitute for Te but exhibits a site preference. These theoretical calculations further predict a monotonic increase in the band gap energy with the Se content, which is confirmed experimentally by absorption spectroscopy measurements. Increasing x up to x = 1.5 leaves unchanged both the p-type character and semiconducting nature of α-As2Te3. The electrical resistivity and thermopower gradually increase with x as a result of the progressive increase in the band gap energy. Despite the fact that α-As2Te3 exhibits very low lattice thermal conductivity κL, the substitution of Se for Te further lowers κL to 0.35 W m-1 K-1 at 300 K. The compositional dependence of the lattice thermal conductivity closely follows classical models of phonon alloy scattering, indicating that this decrease is due to enhanced point-defect scattering.