Design Of High-performance Thermoelectric Materials Research Articles

Seeking non-Fourier heat transfer with ultrabroad band thermoreflectance spectroscopy

Studying superdiffusive thermal transport is crucial for advanced thermal management in electronics and nanotechnology, ensuring devices run efficiently and reliably. Such study also contributes to the design of high-performance thermoelectric materials and devices, thereby improving energy efficiency. This work leads to a better understanding of fundamental physics and non-equilibrium phenomena, fostering innovations in numerous scientific and engineering fields. We are showing, from a one shot experiment, that clear deviations from classical Fourier behavior are observed in a semiconductor alloy such as InGaAs. These deviations are a signature of the competition that takes place between ballistic and diffusive heat transfers. Thermal propagation is modelled by a truncated Lévy model. This approach is used to analyze this ballistic-diffusive transition and to determine the thermal properties of InGaAs. The experimental part of this work is based on a combination of time-domain and frequency-domain thermoreflectance methods with an extended bandwidth ranging from a few kHz to 100 GHz. This unique wide-bandwidth configuration allows a clear distinction between Fourier diffusive and non-Fourier superdiffusive heat propagation in semiconductor materials. For diffusive processes, we also demonstrate our ability to simultaneously measure the thermal conductivity, heat capacity and interface thermal resistance of several materials over 3 decades of thermal conductivity.

High Ductility and Excellent Thermoelectric Performance in Te-Stabilized Cubic Ag2TexS1-x Solid Solutions.

The stabilization at low temperatures of the Ag2S cubic phase could afford the design of high-performance thermoelectric materials with excellent mechanical behavior, enabling them to withstand prolonged vibrations and thermal stress. In this work, we show that the Ag2TexS1-x solid solutions, with Te content within the optimal range 0.20 ≤ x ≤ 0.30, maintain a stable cubic phase across a wide temperature range from 300 to 773 K, thus avoiding the detrimental phase transition from monoclinic to cubic phase observed in Ag2S. Notably, the Ag2TexS1-x (0.20 ≤ x ≤ 0.30) samples showed no fractures during bending tests and displayed superior ductility at room temperature compared to Ag2S, which fractured at a strain of 6.6%. Specifically, the Ag2Te0.20S0.80 sample demonstrated a bending average yield strength of 46.52 MPa at 673 K, significantly higher than that of Ag2S, whose bending average yield strength dropped from 80.15 MPa at 300 K to 12.66 MPa at 673 K. Furthermore, the thermoelectric performance of the Ag2TexS1-x (0.20 ≤ x ≤ 0.30) samples surpassed that of both InSe and pure Ag2S, with the Ag2Te0.30S0.70 sample achieving the highest ZT value of 0.59 at 723 K. These results indicate substantial potential for practical applications due to enhanced durability and thermoelectric performance.

Suppressed lattice thermal conductivity in porous compounds for high-performance thermoelectric applications

Traditional zinc blende semiconductor materials of groups II–VI and III–V exhibit excellent electrical properties, yet suffer from oversized lattice thermal conductivity, causing poor thermoelectric performance. Herein, we have explored an alternative metastable phase of those materials, namely, porous phase. Compared with the stable zinc blende structure, which has simple crystal structure with nearly isotropic bonding feature, porous compounds exhibit complex bonding hierarchy and softened acoustic phonon modes with strong anharmonicity, reducing the lattice thermal conductivity by nearly two orders of magnitude. As an outstanding representative of porous compound family, the suppressed thermal conductivity [∼0.76 W/(m K) at room temperature] combined with enhanced Seebeck coefficient makes porous MgTe a high-performance thermoelectric material with figure of merit above unity at n-type doping and high temperature. This work highlights the important role of intrinsic porosity in design of high-performance thermoelectric materials with low lattice thermal conductivity.

Achieving Ultralow Lattice Thermal Conductivity and High Thermoelectric Performance in SnTe by Alloying with MnSb2Se4.

The manipulation of defect chemistry is crucial in the design of high-performance thermoelectric materials. Studies have demonstrated that alloying compounds within the I-V-VI2 family, such as AgSbTe2, NaSbTe2, etc., can effectively enhance the thermoelectric performance of SnTe by controlling the hole concentration and reducing the lattice thermal conductivity. In this paper, samples of SnTe alloyed with MnSb2Se4 were prepared, and the microstructure, electrical properties, and thermal properties were thoroughly investigated. Based on SEM and TEM analysis, it was observed that MnSb2Se4 can dissolve into SnTe during the preparation of the samples, which leads to the formation of various secondary phases with different compositions and point defects. Consequently, the lattice thermal conductivity is reduced to 0.44 W m-1 K-1 at 800 K, approaching the amorphous limit. Furthermore, the diffusion of the Mn and Sb elements leads to a significant improvement in the Seebeck coefficient through valence band convergence. The vacancy concentration in SnTe can also be modulated by alloying with MnSb2Se4. The findings indicated that MnSb2Se4 alloying can enhance the thermoelectric performance of SnTe through increasing the vacancy concentration, promoting valence band convergence, and introducing secondary phases. Consequently, a ZT value of 1.36 at 800 K for Sn1.03Te-5%MnSb2Se4 can be achieved.

Boosting thermoelectric performance of carbon nanotube-based materials and devices by radical-containing molecules

Organic thermoelectric materials are attracting growing research interests as they are seen as environmentally friendly functional materials with tremendous potential for use in waste heat recovery. The absence of low electrical conductivity has somewhat slowed down the development of most organic thermoelectric materials. Blending the organic semiconductor materials with single walled carbon nanotube (SWCNT) has become an extremely important strategy to boost the conductivity and thermoelectric performance of organic materials. Therefore, in this paper, novel arylimide compounds NDI-2 T and PDI-2 T with radical substituents of TEMPO are designed from molecular engineering, complexing with SWCNT as organic thermoelectric composites. Compared with the non-radical compounds of NDI-2TP and PDI-2TP, the introduction of radicals in NDI-2 T and PDI-2 T can significantly improve the performance of the thermoelectric composites. Radical-containing composite films exhibit outstanding thermoelectric performance compared to the film based on non-radical molecules. The best power factor reaching 278.2 ± 8.2 μW m−1 K−2 for p-type films, while n-type films can attain 64.6 ± 13.5 μW m−1 K−2. The NDI-2 T/SWCNT devices can obtain a maximum output power of 0.94 μW when the temperature gradient is around 60 K, which is 5 times higher of NDI-2TP/SWCNT device. The significantly enhanced thermoelectric performance of the composite film based on organic radical compounds is resulted from the increased interactions between the radical compounds and SWCNTs and modulated carrier concentrations in the composite films. This work provides a new direction for the design of high-performance thermoelectric materials and devices.

The role of branched alkylthio side chain on dispersion and thermoelectric properties of regioregular polythiophene/carbon nanotubes nanocomposites

Herein, poly(3-alkylthiophene)s (P3ATs) and poly[(3-alkylthio)thiophene]s (P3ATTs) are respectively synthesized using poly(thiophene)s with linear/branched alkyl groups and branched alkylthio moieties with short/long side chain lengths for dispersing single-walled carbon nanotubes (SWCNTs), and the resulting composites are solution-processed for use as p-type thin film thermoelectric devices. With an additional sulfur atom on the side chain, a well-dispersed P3ATTs/SWCNTs nanocomposite is obtained by polymer-wrapping at the SWCNTs surfaces driven by S − π and π − π interactions between the alkylthio side chain substituents and π-conjugated backbones of the P3ATTs and SWCNTs, thereby contributing to greatly improved thermoelectric performance relative to the P3ATs/SWCNTs nanocomposite without the added sulfur. The poly[3-(2-hexyldecylthio)thiophene] (P3HDTT) nanocomposite with the longest alkylthio side chain exhibits the strongest interaction with the SWCNTs, yielding the highest power factor (PF) of 307.7 μW m−1 K−2. The ability of this spray-coated nanocomposite film to generate power is demonstrated with a prototype flexible thermoelectric generator (TEG) that produces 888.1 μW m−2. Thus, the introduction of debundled SWCNTs networks wrapped by P3ATTs with alkylthio side chains suggests a feasible strategy for the design of high-performance thermoelectric materials and opens up new opportunities for capturing waste heat in wearable electronics.

Structural Modularization of Cu2 Te Leading to High Thermoelectric Performance near the Mott-Ioffe-Regel Limit.

To date, thermoelectric materials research stays focused on optimizing the material's band edge details and disfavors low mobility. Here, the paradigm is shifted from the band edge to the mobility edge, exploring high thermoelectricity near the border of band conduction and hopping. Through coalloying iodine and sulfur, the plain crystal structure is modularized of liquid-like thermoelectric material Cu2 Te with mosaic nanograins and the highly size mismatched S/Te sublattice that chemically quenches the Cu sublattice and drives the electronic states from itinerant to localized. A state-of-the-art figure of merit of 1.4 is obtained at 850 K for Cu2 (S0.4 I0.1 Te0.5 ); and remarkably, it is achieved near the Mott-Ioffe-Regel limit unlike mainstream thermoelectric materials that are band conductors. Broadly, pairing structural modularization with the high performance near the Mott-Ioffe-Regel limit paves an important new path towards the rational design of high-performance thermoelectric materials.

Revealing nano-chemistry at lattice defects in thermoelectric materials using atom probe tomography

The population of all non-equilibrium lattice defects in materials is referred to as microstructure. Examples are point defects such as substitutional and interstitial atoms, and vacancies; line defects such as dislocations; planar defects such as interfaces and stacking faults; or mesoscopic defects such as second-phase precipitates. These types of lattice imperfections are usually described in terms of their structural features, breaking the periodicity of the otherwise regular crystalline structure. Recent analytical probing at the nanoscale has revealed that their chemical features are likewise important and characteristic. The structure of the defects as well as their individual chemical composition, that is their chemical decoration state, which results from elemental partitioning with the adjacent matrix, can significantly influence the electrical and thermal transport properties of thermoelectric materials. The emergence of atom probe tomography (APT) has now made routinely accessible the mapping of three-dimensional chemical composition with sub-nanometer spatial accuracy and elemental sensitivity in the range of tens of ppm. Here, we review APT-based investigations and results related to the local chemical decoration states of various types of lattice defects in thermoelectric materials. APT allows to better understand the interplay between thermoelectric properties and microstructural features, extending the concept of defect engineering to the field of segregation engineering so as to guide the rational design of high-performance thermoelectric materials.

Improved thermoelectric property of Ti0.75HfMo0.25CrGe by doping Ti2CrGe Heusler alloy with Hf and Mo: Confirmation of entropy “gene” in thermoelectric materials design

The electronic structure, thermoelectric properties, and thermodynamic entropy of Ti2CrGe-doped Ti0.75HfMo0.25CrGe were investigated using first-principles calculations in combination with the semi-classical Boltzmann transport theory and a common thermodynamic formalism. The band structure was half-metallic with a narrow gap of 0.02 eV in the spin-down channel and metallic character in the spin-up channel. The calculated thermoelectric transport properties revealed that Ti0.75HfMo0.25CrGe exhibited a larger thermoelectric figure of merit ZT with a lower lattice thermal conductivity than its prototype alloy Ti2CrGe. In particular, the entropy of Ti0.75HfMo0.25CrGe was larger than that of Ti2CrGe in the temperature range of 0–1000 K. These results indicate that increasing the entropy is an effective approach for the design of high-performance thermoelectric materials and confirm the entropy “gene” in thermoelectric materials.

Development of A Seebeck Coefficient Prediction Simulator Using Tight-Binding Quantum Chemical Molecular Dynamics

Technologies oriented to the development of thermoelectric materials are of great interest because they can assist in directly tapping the vast reserves of currently underused thermal energy. To improve the rational design of high-performance thermoelectric materials, a new numerical procedure for prediction of Seebeck coefficient based on the electronic information from tight-binding quantum chemical molecular dynamics method, has been developed. The newly developed simulator can evaluate Seebeck coefficient theoretically. The simulator is used for the prediction of the Seebeck coefficients for Pt and Si that are the representative materials for metals and semiconductors, respectively. The results show that both coefficients are in quantitative agreement with experimental data, i.e. in the case of Pt metal the predicted value is 5.62 µV/K while the experimental is -4.45 µV/K. Similarly, in the case of Si semiconductor the predicted value is -324.48 µV/K while the experimental is 300–400 µV/K. This new developed simulator can be used to guide the rational design of high performance thermoelectric materials.

Theoretical studies on the thermopower of semiconductors and low-band-gap crystalline polymers

A numerical procedure has been used for the prediction of the Seebeck coefficient of a crystalline material based on its electronic band structure with the goal of testing this approach on the simple polymers polythiophene and polyaminosquaraine. The investigation of several representative materials, including the crystalline solids $\ensuremath{\beta}\text{\ensuremath{-}}{\mathrm{Zn}}_{4}{\mathrm{Sb}}_{3}$ and $\mathrm{Au}{\mathrm{In}}_{2}$ and these polymers, under ambient or external pressure conditions, indicates that Seebeck coefficients can be calculated within the rigid-band and constant-relaxation-time approximations. The results are in semiquantitative agreement with experiment and provide a basic understanding of the mechanisms for thermopower. These theoretical results together with previous similar studies show that band-structure calculations can be used to guide the rational design of high-performance thermoelectric materials. We also suggest that appropriate and specially engineered doped low-band-gap polymers may be promising candidate materials for thermoelectric applications.