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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.

This review summarizes our recent developments in capacitor-type heat flow switching devices that enable active control of heat flow magnitude through the modulation of electron thermal conductivity. We initially demonstrated the feasibility of a capacitor-type heat flow switching device using silver chalcogenides, Ag2S1–xSex, as an electrode material with very low lattice thermal conductivity (≤0.5 W m− 1 K− 1). We achieved significant enhancements in heat flow switching performance through subsequent improvements, including electrode thinning and the implementation of an electric double-layer capacitor structure with ionic liquids. The switching ratio improved from an initial value of 1.1 at the bias voltage of VB = +3 V to 1.9 at VB = +2.4 V, while response times were estimated to be less than 0.2 s. This review discusses the operating principles, experimental methods, and performance metrics across different device configurations, highlighting the critical role of electrode materials with extremely low lattice thermal conductivity. Our findings establish a promising candidate for practical thermal management applications that require rapid and reliable heat flow control without mechanical components.

HighlightsBottlenecks in the field of thermally conductive polymer composites are raised, and corresponding reasons are analysed.Three possible directions for breaking through such bottlenecks are put forward, and current advances in these three directions are illustrated.Future development trends and demands are foreseen to help the development of thermally conductive polymers and their composites.Rapid development of energy, electrical and electronic technologies has put forward higher requirements for the thermal conductivities of polymers and their composites. However, the thermal conductivity coefficient (λ) values of prepared thermally conductive polymer composites are still difficult to achieve expectations, which has become the bottleneck in the fields of thermally conductive polymer composites. Aimed at that, based on the accumulation of the previous research works by related researchers and our research group, this paper proposes three possible directions for breaking through the bottlenecks: (1) preparing and synthesizing intrinsically thermally conductive polymers, (2) reducing the interfacial thermal resistance in thermally conductive polymer composites, and (3) establishing suitable thermal conduction models and studying inner thermal conduction mechanism to guide experimental optimization. Also, the future development trends of the three above-mentioned directions are foreseen, hoping to provide certain basis and guidance for the preparation, researches and development of thermally conductive polymers and their composites.

Highly thermal conductivity materials with excellent electromagnetic interference shielding and Joule heating performances are ideal for thermal management in the next generation of communication industry, artificial intelligence and wearable electronics. In this work, silver nanowires (AgNWs) are prepared using silver nitrate as the silver source and ethylene glycol as the solvent and reducing agent, and boron nitride (BN) is performed to prepare BN nanosheets (BNNS) with the help of isopropyl alcohol and ultrasonication-assisted peeling method, which are compounded with aramid nanofibers (ANF) prepared by chemical dissociation, respectively, and the (BNNS/ANF)-(AgNWs/ANF) thermal conductivity and electromagnetic interference shielding composite films with Janus structures are prepared by the “vacuum-assisted filtration and hot-pressing” method. Janus (BNNS/ANF)-(AgNWs/ANF) composite films exhibit “one side insulating, one side conducting” performance, the surface resistivity of the BNNS/ANF surface is 4.7 × 10 13 Ω, while the conductivity of the AgNWs/ANF surface is 5,275 S/cm. And Janus (BNNS/ANF)-(AgNWs/ANF) composite film with thickness of 95 µm has a high in-plane thermal conductivity coefficient of 8.12 W/(m·K) and superior electromagnetic interference shielding effectiveness of 70 dB. The obtained composite film also has excellent tensile strength of 122.9 MPa and tensile modulus and 2.7 GPa. It also has good temperature-voltage response characteristics (high Joule heating temperature at low supply voltage (5 V, 215.0 °C), fast response time (10 s)), excellent electrical stability and reliability (stable and constant real-time relative resistance under up to 300 cycles and 1,500 s of tensile-bending fatigue work tests).

A common route to obtain efficient thermoelectrics is to optimize the ratio between electrical and thermal conductivity. Typically, materials with a complex, glass-like phonon structure and therefore a very low thermal conductivity are studied. Now, a route showing that solid ions in a liquid-like state can have a low enough thermal conductivity to compete with the best existing thermoelectrics is proposed. Advanced thermoelectric technology offers a potential for converting waste industrial heat into useful electricity, and an emission-free method for solid state cooling 1 , 2 . Worldwide efforts to find materials with thermoelectric figure of merit, zT values significantly above unity, are frequently focused on crystalline semiconductors with low thermal conductivity 2 . Here we report on Cu 2− x Se, which reaches a zT of 1.5 at 1,000 K, among the highest values for any bulk materials. Whereas the Se atoms in Cu 2− x Se form a rigid face-centred cubic lattice, providing a crystalline pathway for semiconducting electrons (or more precisely holes), the copper ions are highly disordered around the Se sublattice and are superionic with liquid-like mobility. This extraordinary ‘liquid-like’ behaviour of copper ions around a crystalline sublattice of Se in Cu 2− x Se results in an intrinsically very low lattice thermal conductivity which enables high zT in this otherwise simple semiconductor. This unusual combination of properties leads to an ideal thermoelectric material. The results indicate a new strategy and direction for high-efficiency thermoelectric materials by exploring systems where there exists a crystalline sublattice for electronic conduction surrounded by liquid-like ions.

Filled skutterudites R x Co 4 Sb 12 are excellent n-type thermoelectric materials owing to their high electronic mobility and high effective mass, combined with low thermal conductivity associated with the addition of filler atoms into the void site. The favourable electronic band structure in n-type CoSb 3 is typically attributed to threefold degeneracy at the conduction band minimum accompanied by linear band behaviour at higher carrier concentrations, which is thought to be related to the increase in effective mass as the doping level increases. Using combined experimental and computational studies, we show instead that a secondary conduction band with 12 conducting carrier pockets (which converges with the primary band at high temperatures) is responsible for the extraordinary thermoelectric performance of n-type CoSb 3 skutterudites. A theoretical explanation is also provided as to why the linear (or Kane-type) band feature is not beneficial for thermoelectrics. It is shown that the large thermoelectric capability of CoSb 3 skutterudite can be associated with a secondary conduction band with high valley degeneracy, which can converge with the light conduction band at high temperatures.

The control of heat conduction through the manipulation of phonons as coherent waves in solids is of fundamental interest and could also be exploited in applications, but coherent heat conduction has not been experimentally confirmed. We report the experimental observation of coherent heat conduction through the use of finite-thickness superlattices with varying numbers of periods. The measured thermal conductivity increased linearly with increasing total superlattice thickness over a temperature range from 30 to 150 kelvin, which is consistent with a coherent phonon heat conduction process. First-principles and Green's function-based simulations further support this coherent transport model. Accessing the coherent heat conduction regime opens a new venue for phonon engineering for an array of applications.

A high density of strong hydrogen bonds connecting two polymers that are homogeneously mixed in a thin film is shown to enhance the intrachain thermal conductance, boosting thermal transport in fully organic layers. Thermal conductivity is an important property for polymers, as it often affects product reliability (for example, electronics packaging), functionality (for example, thermal interface materials) and/or manufacturing cost 1 . However, polymer thermal conductivities primarily fall within a relatively narrow range (0.1–0.5 W m −1 K −1 ) and are largely unexplored. Here, we show that a blend of two polymers with high miscibility and appropriately chosen linker structure can yield a dense and homogeneously distributed thermal network. A sharp increase in cross-plane thermal conductivity is observed under these conditions, reaching over 1.5 W m −1 K −1 in typical spin-cast polymer blend films of nanoscale thickness, which is approximately an order of magnitude larger than that of other amorphous polymers.

From an environmental perspective, lead-free SnTe would be preferable for solid-state waste heat recovery if its thermoelectric figure-of-merit could be brought close to that of the lead-containing chalcogenides. In this work, we studied the thermoelectric properties of nanostructured SnTe with different dopants, and found indium-doped SnTe showed extraordinarily large Seebeck coefficients that cannot be explained properly by the conventional two-valence band model. We attributed this enhancement of Seebeck coefficients to resonant levels created by the indium impurities inside the valence band, supported by the first-principles simulations. This, together with the lower thermal conductivity resulting from the decreased grain size by ball milling and hot pressing, improved both the peak and average nondimensional figure-of-merit (ZT) significantly. A peak ZT of ∼1.1 was obtained in 0.25 atom % In-doped SnTe at about 873 K.

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.