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.
We investigate the effect of non-analytical corrections on the phonon thermal transport properties in two-dimensional indium chalcogenide compounds. The longitudinal optical (LO) and transverse ...optical (TO) branches in the phonon dispersion are split near the Γ-point. The lattice thermal conductivity of monolayer InS is increased by 30.2% under non-analytical corrections because of the large LO-TO splitting at Γ-point. The predicted lattice thermal conductivities with non-analytical corrections at room temperature are 57.1 W/mK, 44.4 W/mK and 33.1 W/mK for the monolayer InS, InSe and InTe, respectively. The lattice thermal conductivity can be effectively reduced by nanostructures because the representative mean free paths are found very large in these monolayers. By quantifying the relative contribution of the phonon modes to the lattice thermal conductivity, we predict that the longitudinal acoustic branch is the main contributor to the lattice thermal conductivity. Due to the low lattice thermalconductivities of these monolayers, they can be useful in the nanoscale thermoelectric devices.
The thermoelectric properties of poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (
PSS) and tellurium-
PSS (Te-
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
PSS and Te-
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 H2SO4. The power factor of the developed materials could be effectively tuned over a very wide range depending on the concentration of the H2SO4 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-
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.
Thermoelectric (TE) materials convert heat energy directly into electricity, and introducing new materials with high conversion efficiency is a great challenge because of the rare combination of ...interdependent electrical and thermal transport properties required to be present in a single material. The TE efficiency is defined by the figure of merit ZT=(S2σ) T/κ, where S is the Seebeck coefficient, σ is the electrical conductivity, κ is the total thermal conductivity, and T is the absolute temperature. A new p‐type thermoelectric material, CsAg5Te3, is presented that exhibits ultralow lattice thermal conductivity (ca. 0.18 Wm−1 K−1) and a high figure of merit of about 1.5 at 727 K. The lattice thermal conductivity is the lowest among state‐of‐the‐art thermoelectrics; it is attributed to a previously unrecognized phonon scattering mechanism that involves the concerted rattling of a group of Ag ions that strongly raises the Grüneisen parameters of the material.
A p‐type thermoelectric material, CsAg5Te3, is presented. It exhibits ultralow thermal conductivity (ϰtol≈0.18 Wm−1 K−1) and a high figure of merit (ZT≈1.5 at 727 K). The low thermal conductivity is attributed to a previously unrecognized phonon scattering mechanism that involves the rattling of Ag ions, strongly raising the Grüneisen parameters of the material.
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.
La(PO3)3aCa(PO3)2 meta-phosphate glassaceramic composites were prepared by quenching from the melt, followed by heating at 800 degC for 20 h, in air. Microstructural characterization showed that ...exposure to water vapor pressures equal to or in excess of 4.2 kPa at as low as 550 degC resulted in the disproportionation of the lanthanum metaphosphates, with formation of some LaPO4. This reaction puts important limitations on the conditions under which the lanthanum metaphosphates could be used as electrolytes in electrochemical devices. The conductivities at uniform air/water partial pressures around 3 kPa, were independent of the water partial pressures used. Comparison of the total ionic effective transference numbers and the proton effective transference number suggested that electronic transport contributed on the average about 7% to the total conductivity in the examined temperature range. The measurements suggested that oxygen ions also participate. The H/D isotope effect, assessed from the ratio IH2O/ID2O ranged from 1.28 at 400 degC to 1.35 at 500 degC, evidencing dominant proton conduction. At 550 degC, the proton effective transference numbers, tH+, determined from the OCVs of cells with oxidizing differential atmospheres were around 0.87, in good agreement with the value derived from the IH2O/ID2O conductivity ratio at 550 degC. However, at lower temperatures such agreement was not found.
Advanced thermoelectric technology offers a potential for converting waste industrial heat into useful electricity, and an emission-free method for solid state cooling. 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. 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.
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, 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.
Urged by the increasing power and packing densities of integrated circuits and electronic devices, efficient dissipation of excess heat from hot spot to heat sink through thermal interface materials ...(TIMs) is a growing demand to maintain system reliability and performance. In recent years, graphene‐based TIMs received considerable interest due to the ultrahigh intrinsic thermal conductivity of graphene. However, the cooling efficiency of such TIMs is still limited by some technical difficulties, such as production‐induced defects of graphene, poor alignment of graphene in the matrix, and strong phonon scattering at graphene/graphene or graphene/matrix interfaces. In this study, a 120 µm‐thick freestanding film composed of vertically aligned, covalently bonded graphene nanowalls (GNWs) is grown by mesoplasma chemical vapor deposition. After filling GNWs with silicone, the fabricated adhesive TIMs exhibit a high through‐plane thermal conductivity of 20.4 W m−1 K−1 at a low graphene loading of 5.6 wt%. In the TIM performance test, the cooling efficiency of GNW‐based TIMs is ≈1.5 times higher than that of state‐of‐the‐art commercial TIMs. The TIMs achieve the desired balance between high through‐plane thermal conductivity and small bond line thickness, providing superior cooling performance for suppressing the degradation of luminous properties of high‐power light‐emitting diode chips.
Graphene nanowalls, composed of high‐quality, vertically aligned, and covalently bonded graphene frameworks, exhibit excellent ability to improve the thermal conductivity of polymer‐based thermal interface materials. The resulting composites show a through‐plane thermal conductivity of 20.4 W m−1 K−1 at a filler content of 5.6 wt%, resulting in ≈1.5 times higher cooling efficiency compared to that of a commercial thermal pad.
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