Glass-like Thermal Conductivity Research Articles

Dual-phase zirconate/tantalate high-entropy ceramics boost thermal properties and fracture toughness for thermal barrier coating materials

Working temperatures, thermal insulation performance, and life span of thermal barrier coatings (TBCs) are primarily influenced by their high-temperature stability, thermal expansion coefficients (TECs), thermal conductivity, and fracture toughness. To address the limitations of current zirconate- and tantalate-based oxides, dual-phase zirconate/tantalate high-entropy ceramics (HECs) are designed and synthesized to improve their thermal and mechanical properties. The combined effects of high entropy, a high concentrations of oxygen vacancies, and relatively low phonon velocity result in low glass-like thermal conductivity, with a minimum value of 1.55 W·m-1·K-1 at 1200 °C. The high TECs (10.6∼10.9×10-6 K-1 at 1400 ºC) and exceptional high-temperature stability demonstrate that these materials can withstand 1300 °C for more than 300 h, significantly surpassing the performance of traditional yttria-stabilized zirconia (YSZ). Compared with YSZ (3.6 MPa·m1/2) and YTaO4 (2.5 MPa·m1/2), the increments in fracture toughness (4.4 MPa·m1/2) of dual-phase zirconate/tantalate HECs are as high as 22.2% and 76.0%, respectively. It is evident that the designed dual-phase zirconate/tantalate HECs can effectively promote thermal properties and fracture toughness, positioning them as the next-generation TBCs with high operating temperatures and outstanding thermal insulation performance.

Ultralow two-channel thermal conductivity in aikinite

Realizing glass-like ultralow lattice thermal conductivity κL in an ordered crystalline material is overwhelmingly appealing for various applications in thermoelectrics and thermal barrier coatings. Here, based on first-principles calculations, we take CuPbBiS3 as an example and systematically study the underlying physical mechanism of the ultralow and glass-like κL in aikinite. It is found that the strong anharmonic renormalization induces temperature-driven stiffening of the acoustic modes and low-lying optical modes from Pb, Cu, and Bi atoms, while leading to the softening of high-frequency optical phonon modes from S atoms. Further analyses highlight that CuPbBiS3 hosts metavalent bonding, stereochemical lone pair activity, and loosely bonded rattling atoms. These characteristics arouse partially liquid-like state, synergistically contribute to strong anharmonicity, and result in unexpectedly low thermal conductivity. Finally, our calculated κL at 300 K is 0.68 Wm−1K−1 with a non-standard dependency of κL ∝ T−0.7, which reasonably agrees well with experimental values in both magnitude (0.52 ± 0.05 Wm−1K−1 at 300 K) and temperature-dependence trend (∝ T−0.2). We demonstrate that the anomalous thermal transport originates from the dual particle-wave behavior of the heat-carrying phonons, in which wavelike tunneling contribute over 18 % to the total κL when T exceeds 300 K. Our work underpins the microscopic origins of ultralow κL in CuPbBiS3, providing crucial insights into designing highly efficient aikinite materials for thermoelectric conversion.

Glass-Like Phonon Dynamics and Thermal Transport in a GeTe Nano-Composite at Low Temperature.

In this work, the experimental evidence of glass-like phonon dynamics and thermal conductivity in a nanocomposite made of GeTe and amorphous carbon is reported, which is of interest for microelectronics, and specifically phase change memories. It is shown that, the total thermal conductivity is reduced by a factor of three at room temperature with respect to pure GeTe, due to the reduction of both electronic and phononic contributions. This latter, similarly to glasses, is small and weakly increasing with temperature between 100 and 300 K, indicating a mostly diffusive thermal transport and reaching a value of 0.86(7)Wm-1K-1 at room temperature. A thorough investigation of the nanocomposite's phonon dynamics reveals the appearance of an excess intensity in the low energy vibrational density of states, reminiscent of the Boson peak in glasses. These features can be understood in terms of an enhanced phonon scattering at the interfaces, due to the presence of elastic heterogeneities, at wavelengths in the 2-20 nm range. The findings confirm recent simulation results on crystalline/amorphous nanocomposites and open new perspectives in phonon and thermal engineering through the direct manipulation of elastic heterogeneities.

Optimizing Thermoelectric Properties through Compositional Engineering in Ag-Deficient AgSbTe2 Synthesized by Arc Melting.

Thermoelectric materials offer a promising avenue for energy management, directly converting heat into electrical energy. Among them, AgSbTe2 has gained significant attention and continues to be a subject of research at further improving its thermoelectric performance and expanding its practical applications. This study focuses on Ag-deficient Ag0.7Sb1.12Te2 and Ag0.7Sb1.12Te1.95Se0.05 materials, examining the impact of compositional engineering within the AgSbTe2 thermoelectric system. These materials have been rapidly synthesized using an arc-melting technique, resulting in the production of dense nanostructured pellets. Detailed analysis through scanning electron microscopy (SEM) reveals the presence of a layered nanostructure, which significantly influences the thermoelectric properties of these materials. Synchrotron X-ray diffraction reveals significant changes in the lattice parameters and atomic displacement parameters (ADPs) that suggest a weakening of bond order in the structure. The thermoelectric characterization highlights the enhanced power factor of Ag-deficient materials that, combined with the low glass-like thermal conductivity, results in a significant improvement in the figure of merit, achieving zT values of 1.25 in Ag0.7Sb1.12Te2 and 1.01 in Ag0.7Sb1.12Te1.95Se0.05 at 750 K.

High thermoelectric performance near the Mott–Ioffe–Regel limit in CuxS0.6Te0.4 meta-phases

The meta-phase, characterized by crystal-amorphicity duality in the extremely special case, has garnered considerable interests due to its glass-like thermal conductivity and tunable conduction mechanisms for carriers. However, the impact of crystal-amorphicity duality on the electrical and thermal transports remains elusive, and the carrier concentration in the meta-phase is rarely tuned in a wide range for optimized thermoelectric properties. Herein, through a combination of molecular dynamic simulations, chemical bonding analysis, and structural characterization, we reveal the glass-like distribution of Cu cations modulated by the mismatched S/Te anions in CuxS0.6Te0.4 meta-phase. Such unique atomic structure leads to hopping conduction of carriers and extremely low thermal conductivity. The carrier concentration is effectively optimized by tuning the Cu content in CuxS0.6Te0.4. Finally, we achieve a high zT of 1.4 at 900 K for Cu2.01S0.6Te0.4 sample, which is three times larger than those of Cu2S and Cu2Te matrixes. Remarkably, the high zT value is attained near the Mott–Ioffe–Regel Limit where the carrier mean free path is close to the average atomic distance. Our findings shed light on a new route to discovering high-performance thermoelectric materials.

Glass-like thermal conductivity and phonon transport mechanism in disordered crystals.

Solid materials with ultra-low thermal conductivity (κ) are of great interest in thermoelectrics for energy conversion or as thermal barrier coatings for thermal insulation. Many low-κ materials exhibit unique properties, such as weak or even insignificant dependence on temperature (T) for κ, i.e., an anomalous glass-like behavior. However, a comprehensive theoretical model elucidating the microscopic phonon mechanism responsible for the glass-like κ-T relationship is still absent. Herein, we take rare-earth tantalates (RE3TaO7) as examples to reexamine phonon thermal transport in defective crystals. By combining experimental studies and atomistic simulations up to 1800 K, we revealed that diffusion-like phonons related to inhomogeneous interatomic bonding contribute more than 70% to the total κ, overturning the conventional understanding that low-frequency phonons dominate heat transport. Furthermore, due to the bridging effects of interatomic bonding, the κ of high-entropy tantalates is not necessarily lower than that of medium-entropy materials, suggesting that attempts to reduce κ through high-entropy engineering are limited, at least in defective fluorite tantalates. The new physical mechanism of multimodal phonon thermal transport in defective structures demonstrated in this work provides a reference for the analysis of phonon transport and offers a new strategy to develop and design low-κ materials by regulating the inhomogeneity of interatomic bonding.

High Thermoelectric Performance in Phonon-Glass Electron-Crystal Like AgSbTe2.

Achieving glass-like ultra-low thermal conductivity in crystalline solids with high electrical conductivity, a crucial requirement for high-performance thermoelectrics , continues to be a formidable challenge. A careful balance between electrical and thermal transport is essential for optimizing the thermoelectric performance. Despite this inherent trade-off, the experimental realization of an ideal thermoelectric material with a phonon-glass electron-crystal (PGEC) nature has rarely been achieved. Here, PGEC-like AgSbTe2 is demonstrated by tuning the atomic disorder upon Yb doping, which results in an outstanding thermoelectric performance with figure of merit, zT ≈ 2.4 at 573 K. Yb-doping-induced enhanced atomic ordering decreases the overlap between the hole and phonon mean free paths and consequently leads to a PGEC-like transport behavior in AgSbTe2 . A twofold increase in electrical mobility is observed while keeping the position of the Fermi level (EF ) nearly unchanged and corroborates the enhanced crystalline nature of the AgSbTe2 lattice upon Yb doping for electrical transport. The cation-ordered domains, lead to the formation of nanoscale superstructures (≈2 to 4nm) that strongly scatter heat-carrying phonons, resulting in a temperature-independent glass-like thermal conductivity. The strategy paves the way for realizing high thermoelectric performance in various disordered crystals by making them amorphous to phonons while favoring crystal-like electrical transport.

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.

Two-channel thermal transport and scattering channel of high-temperature phase SnSe using temperature-dependent effective potential

To resolve the imaginary phonon frequency of high-temperature phase SnSe (853 K) caused by ground-state theory, the temperature-dependent effective potential (TDEP) method is used to evaluate the temperature-dependent interatomic force constants (IFCs). Based on the Boltzmann transport equation, both two-channel transport and scattering channel are studied using the temperature-dependent IFCs. Considering three-phonon and four-phonon scattering, the particle-like thermal conductivity along the xx axis, yy axis, and zz axis is 0.66, 0.34, and 0.61 W/mK, respectively. For glass-like thermal conductivity, the value along the xx axis, yy axis, zz axis are 0.0777, 0.0316, and 0.0791 W/mK, respectively. For the three-phonon scattering channel, the scattering channels of X + O/A→O and X→O+O/A play a major role. For four-phonon scattering channel, the major scattering channels are X + O+A→O, X + A+A→O, X + O-O/A→A, X + A-O→A, X-O/A-O/A→O, X-O-A→A. By utilizing the crystal orbital Hamilton population (COHP) analysis, the low lattice thermal conductivity is attributed to the weak chemical bonds from anti-bonding orbitals between Se4p and Sn5s states. This work provides new insight into the physical mechanisms for thermal transport of high-temperature phase SnSe with strong anharmonic effects.

Role of high-order anharmonicity and off-diagonal terms in thermal conductivity: A case study of multiphase CsPbBr3

We investigate the influence of three- and four-phonon scattering, perturbative anharmonic phonon renormalization, and off-diagonal terms of coherent phonons on the thermal conductivity of ${\mathrm{CsPbBr}}_{3}$ phase change perovskite, by using advanced implementations and first-principles simulations. Our study spans a wide temperature range covering the entire structural spectrum. Notably, we demonstrate that the interactions between acoustic and optical phonons result in contrasting trends of phonon frequency shifts for the high-lying optical phonons in orthorhombic and cubic ${\mathrm{CsPbBr}}_{3}$ as temperature varies. Our findings highlight the significance of wavelike tunneling of coherent phonons in ultralow and glasslike thermal conductivity in halide perovskites.

The thermal transport and scattering channel of body centered cubic carbon BC14 using self-consistent phonon theory

Using the self-consistent phonon (SCP) theory, lattice thermal expansion, and Boltzmann transport equation (BTE), both thermal conductivity and scattering channel are studied by considering the cubic and quartic anharmonicity. The increase of phonon group velocity is caused by phonon frequency shifts at about 12 THz, but the phonon group velocity decreases slightly above 15 THz, agreeing with the decrease of phonon frequency induced by quartic anharmonic renormalization. The relaxation time of SCP is larger than that of HA, leading to the increase of thermal conductivity under SCP. The glass-like thermal conductivity of HA+ 3 ph, HA+ 3 ph + 4 ph, SCP+ 3 ph and SCP+ 3 ph + 4 ph is 0.12517 W/mK, 0.15713 W/mK, 0.1187 W/mK, 0.28551 W/mK, respectively. The three-phonon weighted phase space of HA is slightly larger than that of SCP, while the four-phonon weighted phase space of HA is nearly the same as the phase space of SCP. The major contributions to the scattering channels are X + ZA/TA/LA→O, X→O+ZA/TA/LA, X + O+ZA/TA→O, X + O-O/TA/LA→O, X + O-O→ZA/TA/LA, and X-O-ZA/TA→O. The weak C-C bonding is responsible for the low thermal conductivity of BC14. Moreover, these findings help us to understand the physical mechanism of thermal transport with the effect of quartic anharmonic renormalization.

Charge fluctuations above TCDW revealed by glasslike thermal transport in kagome metals AV3Sb5 (A=K,Rb,Cs)

We present heat capacity, electrical, and thermal transport measurements of kagome metals $A{\mathrm{V}}_{3}{\mathrm{Sb}}_{5}$ $(A=\mathrm{K},\mathrm{Rb},\mathrm{Cs})$. In all three compounds, the development of short-range charge fluctuations above the charge density wave (CDW) transition temperature ${T}_{\mathrm{CDW}}$ strongly scatters phonons via the electron-phonon coupling, leading to the glasslike phonon heat transport, i.e., the phonon thermal conductivity decreases weakly upon cooling. Once the long-range charge order sets in below ${T}_{\mathrm{CDW}}$, short-range charge fluctuations are quenched, and the typical Umklapp scattering dominated phonon heat transport is recovered. The charge-fluctuations-induced glasslike phonon thermal conductivity implies sizable electron-phonon coupling in $A{\mathrm{V}}_{3}{\mathrm{Sb}}_{5}$.

Occupational disorder as the origin of flattening of the acoustic phonon branches in the clathrate Ba8Ga16Ge30

In the search for high-performance thermoelectrics, materials such as clathrates have drawn attention due to having both glass-like low phonon thermal conductivity and crystal-like high electrical conductivity. Ba$_{8}$Ga$_{16}$Ge$_{30}$ (BGG) has a loosely bound guest Ba atom trapped inside rigid Ga/Ge cage structures. Avoided crossings between acoustic phonons and the flat guest atom branches have been proposed to be the source of the low lattice thermal conductivity of BGG. Ga/Ge site disorder with Ga and Ge exchanging places in different unit cells has also been reported. We used time-of-flight neutron scattering to measure the complete phonon spectrum in a large single crystal of BGG and compared these results with predictions of density functional theory to elucidate the effect of the disorder on heat-carrying phonons. Experimental results agreed much better with the calculation assuming the disorder than with the calculation assuming the ordered configuration. Although atomic masses of Ga and Ge are nearly identical, we found that disorder strongly reduces phonon group velocities, which significantly reduces thermal conductivity. Our work points at a new path towards optimizing thermoelectrics.

Achieving glass-like lattice thermal conductivity in PbTe by AgBiTe2 alloying

Here, we report the thermal transport properties of lead telluride (PbTe)1−x(AgBiTe2)x (x = 0.05 and 0.15) alloys. It is found that the prominent peak in lattice thermal conductivity at 15 K for PbTe disappears after forming solid solution with AgBiTe2, exhibiting a typical glass-like thermal transport behavior. The high energy phonons are scattered by the point defects induced by cationic disorder, while the appearance of soft vibrational modes arises from Ag atoms acting like Einstein oscillators (ΘE1 = 5.4 K, ΘE2 = 67.3 K), which substantially affects the lattice thermal conductivity. Further with nanostructuring, the mid-frequency phonons are scattered by the high-density disc-like Ag2Te precipitates. As a result, an ultralow lattice thermal conductivity (≤1.0 W m−1 K−1) for (PbTe)0.85(AgBiTe2)0.15 is obtained, which is the lowest value ever reported to date for the PbTe-based TE materials. Our work highlights a synthetic route to achieve glass-like lattice thermal conductivity in PbTe over the entire temperature range.

High-entropy MTiO3 perovskite oxides with glass-like thermal conductivity for thermoelectric applications

High-entropy oxides with complex compositions can be designed to lower the thermal conductivity of SrTiO3-based thermoelectric materials to optimize their properties. High-entropy (Sr0.25Ca0.25Ba0.25RE0.25)TiO3 (RE=Nd, Sm, Eu, Gd, Dy, Ho) ceramics were designed to systematically investigate the effects of critical average ionic radius (rA̅), atomic size disorder factor (δAr), and the mass difference factor (δAm) of A-site elements on the single-phase formation, disorder microstructure, and the thermoelectric properties. The successful synthesis of single-phase perovskite-type ceramics of (Sr0.25Ca0.25Ba0.25Nd0.25)TiO3, (Sr0.25Ca0.25Ba0.25Sm0.25)TiO3, and (Sr0.25 Ca0.25Ba0.25Eu0.25)TiO3 by a solid-state reaction method were reported. When rA̅ ＜1.377 Å, indicator of δAr≥ 12.86, and δAm≥ 18.6, the pyrochlore phase of Gd2Ti2O7, Dy2Ti2O7, and Ho2Ti2O7 precipitated. Importantly, the obtained temperature- independent low glass-like lattice thermal conductivity of 2.27 W/(m∙K) was significantly reduced by 68.2–21.4% compared with the majority values ranging from 7.16 W/(m∙K) at 300 K to 2.89 W/(m∙K) at 1073 K of SrTiO3-based polycrystal system. The temperature independence of thermal conductivity over the whole range of 323–1073 K realizes the concept of ‘phonon-glass electron crystal’. Evidence strongly confirmed that large lattice distortion, TiO6 octahedral twist, dislocations, and complex strain field coexisted by Raman, HRTEM combined with GPA analysis in the high-entropy perovskite structures.

Pyrochlore–fluorite dual-phase high-entropy RE2(Ce0.2Zr0.2Hf0.2Sn0.2Ti0.2)2O7 (RE2HE2O7, RE = La, Nd, Sm, Eu, Gd, Dy) ceramics with glass-like thermal conductivity

Pyrochlore–fluorite dual-phase high-entropy RE2(Ce0.2Zr0.2Hf0.2Sn0.2Ti0.2)2O7 (RE2HE2O7, RE = La, Nd, Sm, Eu, Gd, Dy) ceramics with glass-like thermal conductivity

Effect of yttria on thermal transport and vibrational modes in yttria-stabilized hafnia

Low thermal conductivity plays an essential role in application relevant to thermal energy conversion and management. In this paper, we utilize molecular dynamics to investigate the thermal transport and lattice variation modes in yttria-stabilized hafnia, which only contains binary oxides of Y2O3 and HfO2. It is found that the thermal conductivity κ of yttria-stabilized hafnia decreases significantly with the increase of doping ratio of Y2O3, and then reaches a limiting value (∼2.1 W m−1K−1), because of the strong phonon scattering of oxygen vacancies. Importantly, a glass-like thermal conductivity κ is achieved in yttria-stabilized hafnia samples when the content of Y2O3 exceeds 15 mol%. By decomposing the phonon vibrational modes, we find that most of the heat is transported by diffusive modes. As a result, the κ exhibits a glass-like feature in yttria-stabilized hafnia samples with high content of Y2O3. Notably, the κ of yttria-stabilized hafnia is much lower than those of classical functional ceramics materials. The insight into the κ in yttria-stabilized hafnia system is beneficial for understanding and reducing the κ of materials through defect engineering. Despite its simple composition, yttria-stabilized hafnia with different doping ratios demonstrates unexpected high scattering rate of phonon vibration density states, which is confirmed by the diffused wavevector-frequency dispersion. Eigenvector periodicity and phonon participation ratio of phonon have been visualized to capture the distribution of phonon modes in yttria-stabilized hafnia with various dopant. This work investigates into the details of phonon vibrational modes in yttria-stabilized hafnia, which would be valuable for conducting experiments to acquire low thermal conductivity materials in laboratory.

Fluctuating bonding leads to glass-like thermal conductivity in perovskite rare-earth tantalates

Understanding the glass-like thermal conductivity (κ) in simple-composition crystalline materials is critical for the development and design of low‒κ materials. Here, we report the origin of glass-like low κ in binary crystalline tantalates RETa3O9 (RE = Nd, Sm, Eu, Gd) by atomistic simulation. It is found that the low κ is mainly due to the rattling effect of RE3+ cations arising from the large difference in the interatomic bonding between O‒RE and O‒Ta. The vibrational mode decomposition results reveal that most modes in RETa3O9 fall in the Ioffe–Regel regime and exhibit a strongly diffusive nature due to the fluctuating interatomic force of O‒Ta in the TaO6 octahedron. The dual-phonon model reveals that the vast majority (>97%) of heat in RETa3O9 is transported by diffusive modes rather than propagating modes. Consequently, the thermal conduction in RETa3O9 exhibits a unique glass-like nature. Contrary to conventional belief, the optical phonon modes in RETa3O9 play a significant role in thermal conduction. Overall, the new insight into the link between chemical binding and glass-like κ serves to the development and design of low‒κ materials for thermoelectric and thermal management applications.

Orientation-Controlled Anisotropy in Single Crystals of Quasi-1D BaTiS3

Low-dimensional materials with chain-like (one-dimensional) or layered (twodimensional) structures are of significant interest due to their anisotropic electrical, optical, thermal properties. One material with chain-like structure, BaTiS3 (BTS), was recently shown to possess giant in-plane optical anisotropy and glass-like thermal conductivity. To understand the origin of these effects, it is necessary to fully characterize the optical, thermal, and electronic anisotropy of BTS. To this end, BTS crystals with different orientations (aand c-axis orientations) were grown by chemical vapor transport. X-ray absorption spectroscopy (XAS) was used to characterize the local structure and electronic anisotropy of BTS. Fourier transform infrared (FTIR) reflection/transmission spectra show a large inplane optical anisotropy in the a-oriented crystals, while the c-axis oriented crystals were nearly isotropic in-plane. BTS platelet crystals are promising uniaxial materials for IR optics with their optic axis parallel to the c-axis. The thermal conductivity measurements revealed a thermal anisotropy of ~4.5 between the c- and a-axis. Time-domain Brillouin scattering showed that the longitudinal sound speed along the two axes is nearly the same suggesting that the thermal anisotropy is a result of different phonon scattering rates.

Pyrochlore-Fluorite Dual-Phase High-Entropy Re2(Ce0.2zr0.2hf0.2sn0.2ti0.2)2o7 (Re2he2o7, Re = La, Nd, Sm, EU, Gd, Dy) Ceramics with Glass-Like Thermal Conductivity

Pyrochlore-Fluorite Dual-Phase High-Entropy Re2(Ce0.2zr0.2hf0.2sn0.2ti0.2)2o7 (Re2he2o7, Re = La, Nd, Sm, EU, Gd, Dy) Ceramics with Glass-Like Thermal Conductivity