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In electrically conductive solids, the Wiedemann-Franz law requires the electronic contribution to thermal conductivity to be proportional to electrical conductivity. Violations of the Wiedemann-Franz law are typically an indication of unconventional quasiparticle dynamics, such as inelastic scattering, or hydrodynamic collective motion of charge carriers, typically pronounced only at cryogenic temperatures. We report an order-of-magnitude breakdown of the Wiedemann-Franz law at high temperatures ranging from 240 to 340 kelvin in metallic vanadium dioxide in the vicinity of its metal-insulator transition. Different from previously established mechanisms, the unusually low electronic thermal conductivity is a signature of the absence of quasiparticles in a strongly correlated electron fluid where heat and charge diffuse independently.

Thermoelectric materials can directly generate electrical power from waste heat but the challenge is in designing efficient, stable and inexpensive systems. Nanostructuring in bulk materials dramatically reduces the thermal conductivity but simultaneously increases the charge carrier scattering, which has a detrimental effect on the carrier mobility. We have experimentally achieved concurrent phonon blocking and charge transmitting via the endotaxial placement of nanocrystals in a thermoelectric material host. Endotaxially arranged SrTe nanocrystals at concentrations as low as 2% were incorporated in a PbTe matrix doped with Na 2 Te. This effectively inhibits the heat flow in the system but does not affect the hole mobility, allowing a large power factor to be achieved. The crystallographic alignment of SrTe and PbTe lattices decouples phonon and electron transport and this allows the system to reach a thermoelectric figure of merit of 1.7 at ~800 K. Developing efficient thermoelectric materials that can directly generate electrical power from heat is a challenge, but now a nanostructured system of SrTe nanocrystals in a Na 2 Te-doped PbTe matrix achieves high efficiency by blocking heat flow without impeding carrier flow.

A large anomalous Nernst effect (ANE) is crucial for thermoelectric energy conversion applications because the associated unique transverse geometry facilitates module fabrication. Topological ferromagnets with large Berry curvatures show large ANEs; however, they face drawbacks such as strong magnetic disturbances and low mobility due to high magnetization. Herein, we demonstrate that YbMnBi 2 , a canted antiferromagnet, has a large ANE conductivity of ~10 A m −1  K −1 that surpasses large values observed in other ferromagnets (3–5 A m −1  K −1 ). The canted spin structure of Mn guarantees a non-zero Berry curvature, but generates only a weak magnetization three orders of magnitude lower than that of general ferromagnets. The heavy Bi with a large spin–orbit coupling enables a large ANE and low thermal conductivity, whereas its highly dispersive p x / y orbitals ensure low resistivity. The high anomalous transverse thermoelectric performance and extremely small magnetization make YbMnBi 2 an excellent candidate for transverse thermoelectrics. The anomalous Nernst effect (ANE) in topological materials with large Berry curvature shows great potential for transverse thermoelectrics, but antiferromagnets typically show small ANEs. The antiferromagnet YbMnBi 2 has an ANE thermopower of 3 μV K −1 , similar to ferromagnets, and a larger ANE conductivity.

The main obstacle to improving the thermoelectric efficiency of a material arises from the common interdependence of electrical and thermal conductivity, whereas one ideally wants to raise the former while lowering the latter: a simple layered crystalline material — SnSe — is now reported that seems to have these qualities built in. Impressive thermoelectric performance from SnSe crystals Thermoelectric materials hold promise as a practical means of converting waste heat into electrical energy, but the energy-conversion efficiency of existing materials tends to be rather low. The main obstacle to improving the thermoelectric efficiency of a material arises from the common interdependence of electrical and thermal conductivity. Thermoelectric efficiency demands high electrical but low thermal conductivity and one route that might provide that combination is nanostructuring. Now Li-Dong Zhao and colleagues describe a simple layered crystalline material, tin selenide (SnSe), that appears to have these qualities built in. The authors identify features in the bonding structure of this material that they believe to be responsible, and suggest that these might help to guide the discovery of other candidate materials for high thermoelectric performance. The thermoelectric effect enables direct and reversible conversion between thermal and electrical energy, and provides a viable route for power generation from waste heat. The efficiency of thermoelectric materials is dictated by the dimensionless figure of merit, ZT (where Z is the figure of merit and T is absolute temperature), which governs the Carnot efficiency for heat conversion. Enhancements above the generally high threshold value of 2.5 have important implications for commercial deployment 1 , 2 , especially for compounds free of Pb and Te. Here we report an unprecedented ZT of 2.6 ± 0.3 at 923 K, realized in SnSe single crystals measured along the b axis of the room-temperature orthorhombic unit cell. This material also shows a high ZT of 2.3 ± 0.3 along the c axis but a significantly reduced ZT of 0.8 ± 0.2 along the a axis. We attribute the remarkably high ZT along the b axis to the intrinsically ultralow lattice thermal conductivity in SnSe. The layered structure of SnSe derives from a distorted rock-salt structure, and features anomalously high Grüneisen parameters, which reflect the anharmonic and anisotropic bonding. We attribute the exceptionally low lattice thermal conductivity (0.23 ± 0.03 W m −1  K −1 at 973 K) in SnSe to the anharmonicity. These findings highlight alternative strategies to nanostructuring for achieving high thermoelectric performance.

The thermoelectric properties of poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) and tellurium-PEDOT:PSS (Te-PEDOT:PSS) hybrid composites were enhanced via simple chemical treatment. The performance of thermoelectric materials is determined by their electrical conductivity, thermal conductivity and Seebeck coefficient. Significant enhancement of the electrical conductivity of PEDOT:PSS and Te-PEDOT:PSS hybrid composites from 787.99 and 11.01 to 4839.92 and 334.68 S cm −1 , respectively was achieved by simple chemical treatment with H 2 SO 4 . The power factor of the developed materials could be effectively tuned over a very wide range depending on the concentration of the H 2 SO 4 solution used in the chemical treatment. The power factors of the developed thermoelectric materials were optimized to 51.85 and 284 μW m −1 K −2 , respectively, which represent an increase of four orders of magnitude relative to the corresponding parameters of the untreated thermoelectric materials. Using the Te-PEDOT:PSS hybrid composites, a flexible thermoelectric generator that could be embedded in textiles was fabricated by a printing process. This thermoelectric array generates a thermoelectric voltage of 2 mV using human body heat.

Layered Ti[sub.3]C[sub.2]T[sub.x] MXene has been successfully intercalated and exfoliated with the simultaneous generation of a 3D silica network by treating its cationic surfactant intercalation compound (MXene-CTAB) with an alkoxysilane (TMOS), resulting in a MXene–silica nanoarchitecture, which has high porosity and specific surface area, together with the intrinsic properties of MXene (e.g., photothermal response). The ability of these innovative MXene silica materials to induce thermal activation reactions of previously adsorbed compounds is demonstrated here using NIR laser irradiation. For this purpose, the pinacol rearrangement reaction has been selected as a first model example, testing the effectiveness of NIR laser-assisted photothermal irradiation in these processes. This work shows that Ti[sub.3]C[sub.2]T[sub.x]-based nanoarchitectures open new avenues for applications that rely on the combined properties inherent to their integrated nanocomponents, which could be extended to the broader MXene family.

This review analyzes thermal and electrically conductive properties of composites and how they can be influenced by the addition of special nanoparticles. Composite functional characteristics—such as thermal and electrical conductivity, phase changes, dimensional stability, magnetization, and modulus increase—are tuned by selecting suitable nanoparticle filler material. The conductivity of composites can be related to the formation of conductive pathways as nanofiller materials form connections in the bulk of a composite matrix. With increasing use of nanomaterial containing composites and relatively little understanding of the toxicological effects thereof, adequate disposal and recyclability have become an increasing environmental concern.

Materials with ultrahigh or low thermal conductivity are desirable for many technological applications, such as thermal management of electronic and photonic devices, heat exchangers, energy converters and thermal insulation. Recent advances in simulation tools (first principles, the atomistic Green’s function and molecular dynamics) and experimental techniques (pump–probe techniques and microfabricated platforms) have led to new insights on phonon transport and scattering in materials and the discovery of new thermal materials, and are enabling the engineering of phonons towards desired thermal properties. We review recent discoveries of both inorganic and organic materials with ultrahigh and low thermal conductivity, highlighting heat-conduction physics, strategies used to change thermal conductivity, and future directions to achieve extreme thermal conductivities in solid-state materials. This Review provides an overview of experimental and theoretical methods for the understanding of thermal transport, summarizes recent progress in materials with ultrahigh (or low) thermal conductivities, and outlines strategies for the engineering of extreme thermal conductivity materials.

Thermal insulation under extreme conditions requires materials that can withstand complex thermomechanical stress and retain excellent thermal insulation properties at temperatures exceeding 1,000 degrees Celsius 1 – 3 . Ceramic aerogels are attractive thermal insulating materials; however, at very high temperatures, they often show considerably increased thermal conductivity and limited thermomechanical stability that can lead to catastrophic failure 4 – 6 . Here we report a multiscale design of hypocrystalline zircon nanofibrous aerogels with a zig-zag architecture that leads to exceptional thermomechanical stability and ultralow thermal conductivity at high temperatures. The aerogels show a near-zero Poisson’s ratio (3.3 × 10 −4 ) and a near-zero thermal expansion coefficient (1.2 × 10 −7 per degree Celsius), which ensures excellent structural flexibility and thermomechanical properties. They show high thermal stability with ultralow strength degradation (less than 1 per cent) after sharp thermal shocks, and a high working temperature (up to 1,300 degrees Celsius). By deliberately entrapping residue carbon species in the constituent hypocrystalline zircon fibres, we substantially reduce the thermal radiation heat transfer and achieve one of the lowest high-temperature thermal conductivities among ceramic aerogels so far—104 milliwatts per metre per kelvin at 1,000 degrees Celsius. The combined thermomechanical and thermal insulating properties offer an attractive material system for robust thermal insulation under extreme conditions. Hypocrystalline ceramic aerogels with a zig-zag architecture show high thermal stability under thermal shock and exposure to high temperature, providing a reliable material system for thermal insulation at extreme conditions.

Soil thermal conductivity (STC) is an important physical parameter in modeling land surface processes. Previous studies on evaluations of STC schemes are mostly based on direct measurements of local conditions, and their recommendations cannot be used as a reference in selecting STC schemes for land modeling use. In this work, seven typical STC schemes are incorporated into the Common Land Model to evaluate their applications in land surface modeling. Statistical analyses show that the Johansen (1975) scheme and its three derivatives, Côté and Konrad (2005, https://doi.org/10.1139/t04‐106), Balland and Arp (2005, https://doi.org/10.1139/s05‐007), and Lu et al. (2007, https://doi.org/10.2136/sssaj2006.0041), are significantly superior to the other schemes with respect to both STC estimations and their applications in modeling soil temperature and the partitioning of surface available energy in the Common Land Model. The Balland and Arp (2005, https://doi.org/10.1139/s05‐007) scheme ranks at the top among the selected schemes. Uncertainty analyses based on single‐point and global simulations both show that the differences in STC estimations can induce significant differences in simulated soil temperature in arid/semiarid and seasonally frozen regions, especially at deep layers. The hydrology‐related variables are slightly affected by STC variations, but slight changes in these variables can induce notable changes in soil temperature by altering soil thermal properties. These results emphasize the important role of STC in modeling soil thermodynamics and suggest the necessity of further developing STC schemes based on land modeling applications. According to the evaluation analyses, we recommend the Balland and Arp (2005, https://doi.org/10.1139/s05‐007) scheme as a more suitable selection for use in land surface models. Plain Language Summary Soil thermal conductivity (STC) is essential for simulating soil temperature and heat flux in land surface models. Previous studies have proposed quite a few parameterization schemes to estimate STC. Although many works have tried to determine which scheme performed best, most of them were based on direct measurements from specific experimental conditions or local soil samples, and few studies have considered the performances of STC schemes in land surface modeling. Here, we present an evaluation of seven typical STC schemes based on their land modeling applications. The results show that four of the schemes, the Johansen (1975) scheme, Côté and Konrad (2005, https://doi.org/10.1139/t04‐106) scheme, Balland and Arp (2005, https://doi.org/10.1139/s05‐007) scheme, and Lu et al. (2007, https://doi.org/10.2136/sssaj2006.0041) scheme, perform comparatively better than the other schemes and that the Balland and Arp (2005, https://doi.org/10.1139/s05‐007) scheme ranks at the top. Uncertainty analyses demonstrate that soil thermodynamics are strongly sensitive to STC estimates, especially in dry and partly frozen regions. This relationship highlights the important role of STC in modeling land surface processes. Currently, we recommend the Balland and Arp (2005, https://doi.org/10.1139/s05‐007) scheme as a good selection for use in land surface models. Key Points Seven typical soil thermal conductivity schemes are evaluated by incorporation into the Common Land Model The Johansen (1975) scheme and its three derivatives perform better than the others, with the Balland and Arp (2005, https://doi.org/10.1139/s05‐007) scheme ranking at the top Uncertainty analyses show strong effects of soil thermal conductivity on soil thermodynamics in arid/semiarid regions and seasonally frozen regions