Slight Symmetry Reduction in Thermoelectrics

Thermoelectric materials with crystal-amorphicity duality induced by large atomic size mismatch

Recently, there has been renewed interest in thermoelectric materials based on germanium telluride, which demonstrate high efficiency in mid-temperature range. This paper discusses the theoretical description of the phonon spectrum and lattice thermal conductivity in GeTe using ab initio methods. Using these methods, the temperature dependence of the lattice thermal conductivity in the rhombohedral phase was calculated and effects of scattering by point defects and nanostructuring were estimated. The modification of the phonon spectrum upon the transition to the high-temperature cubic phase is investigated. The calculated temperature dependences of the lattice thermal conductivity are compared with the available experimental data on GeTe and its solid solutions. Keywords: germanium telluride, ab initio calculations, phonon spectrum, lattice thermal conductivity, nanostructuring.

Recently, there has been renewed interest in thermoelectric materials based on germanium telluride, which demonstrate high efficiency in mid-temperature range. This paper discusses the theoretical description of the phonon spectrum and lattice thermal conductivity in GeTe using ab initio methods. Using these methods, the temperature dependence of the lattice thermal conductivity in the rhombohedral phase was calculated and effects of scattering by point defects and nanostructuring were estimated. The modification of the phonon spectrum upon the transition to the high-temperature cubic phase is investigated. The calculated temperature dependences of the lattice thermal conductivity are compared with the available experimental data on GeTe and its solid solutions.

Germanium telluride (GeTe) and its variants are promising compounds as high figure of merit thermoelectric materials due to their low lattice thermal conductivity. The strong anharmonicity and the intrinsic Ge vacancies are shown to be the origin of the low thermal conductivity. While the anharmonic effect on the lattice thermal conductivity has been systematically studied using the perturbation theory, the vacancy disorder has been treated perturbatively mostly as an extreme case of phonon-isotope scattering. This simplification ignores realistic features such as the nonbonding character and the detailed local environments near the vacancies. In this study, we calculate the lattice thermal conductivity of the cubic GeTe by considering the anharmonicity and the vacancy disorder on the same footing using the machine-learning potential molecular dynamics. We obtain the spectral function via the nonperturbative approaches, the velocity autocorrelation function, and the phonon unfolding scheme to investigate the effect of the vacancies on the lattice thermal conductivity. We find that the reduction in the lattice thermal conductivity by the vacancies originates from the momentum-dependent scattering of the acoustic phonon modes and the momentum-independent scattering of the optical phonon modes as determined by the strength of the anharmonicity.

Synergistic effect of indium doping on thermoelectric performance of cubic GeTe-based thin films

Electronic band convergence and the introduction of doping-induced midgap states near the Fermi level offer a compelling mechanism for modulating the electronic structure to achieve high thermoelectric performance. Germanium telluride (GeTe), with its unique crystal and electronic structure, holds great promise for thermoelectric (TE) power generation. However, a high p-type carrier concentration coupled with high lattice thermal conductivity limits its TE performance. Herein, we report an impressive thermoelectric figure of merit (zT) of ∼2.4 (∼2.6 with Dulong-Petit Cp) at 727 K in Hg and Sb codoped GeTe, achieved through the synergistic effects of electronic structure optimization and lattice thermal conductivity reduction. Hg doping in GeTe boosts the Seebeck coefficient by facilitating the valence band convergence. Importantly, a hybridized midgap band emerges upon Hg doping, through the antibonding interaction of Hg 6s and p orbitals of Te and Ge. Further codoping of Sb makes the midgap state more localized and shifts the EF up to pin the midgap electronic band, resulting in an enhanced electronic density of states near EF, as validated by first-principles density functional theory (DFT) calculations of the electronic band structure and experimental Pisarenko analysis. This leads to a significant enhancement of the Seebeck coefficient in Hg and Sb codoped GeTe. Further, when Hg doping exceeds the solid solution limit, it forms HgTe nanoprecipitates in the GeTe matrix, suppressing the lattice thermal conductivity. We have constructed a double-leg TE device using the developed material as a p-type leg, which exhibits a promising output power density of 0.77 W/cm2 for the ΔT = 440 K, underscoring the material's potential for high-performance TE applications.

Machine learning for next-generation thermoelectrics

Germanium telluride (GeTe)-based materials, which display intriguing functionalities, have been intensively studied from both fundamental and technological perspectives. As a thermoelectric material, though, the phase transition in GeTe from a rhombohedral structure to a cubic structure at ∼700 K is a major obstacle impeding applications for energy harvesting. In this work, we discovered that the phase-transition temperature can be suppressed to below 300 K by a simple Bi and Mn codoping, resulting in the high performance of cubic GeTe from 300 to 773 K. Bi doping on the Ge site was found to reduce the hole concentration and thus to enhance the thermoelectric properties. Mn alloying on the Ge site simultaneously increased the hole effective mass and the Seebeck coefficient through modification of the valence bands. With the Bi and Mn codoping, the lattice thermal conductivity was also largely reduced due to the strong point-defect scattering for phonons, resulting in a peak thermoelectric figure of merit (ZT) of ∼1.5 at 773 K and an average ZT of ∼1.1 from 300 to 773 K in cubic Ge0.81Mn0.15Bi0.04Te. Our results open the door for further studies of this exciting material for thermoelectric and other applications.

Defect engineering, at the core of the field of thermoelectric studies, serves as a scaffold for engineering the intrinsic electrons' and phonons' behaviors to tailor thermoelectric parameters through the direct impacts of band engineering and phonon engineering, which can modify electronic band structure and phonon transport behavior to enhance the power factor (PF = σS2) and reduce the lattice thermal conductivity (κl). By virtue of the implementation of defect engineering, the past decades have witnessed great progress in thermoelectric research through synergistic optimization of the inter-correlated transport parameters, and substantial enhancement has been achieved in the performance of various thermoelectric materials. However, current established optimization strategies based on defect engineering are mainly focused on tuning the electronic and phonon structures, while modulation by additional degrees of freedom caused by defects has long been neglected. In this Perspective, we focus on our interest in the under-exploited aspects of defect engineering, which include defect-related spin effects, defect-mediated atom or charge migration effects, and defect-related interface effects. Through these new points of view, we hope to arouse intense attention to the overlooked parts of defect engineering and combine them with current optimization strategies from the perspective of multiple degrees of freedom modulation, to enable the full potential of defect engineering for boosting thermoelectric performance. Finally, based on the discussion herein and current achievements in thermoelectric research, some personal perspectives on the future of this field are also presented.

Thermoelectric power factor: Enhancement mechanisms and strategies for higher performance thermoelectric materials

ABSTRACTOver a decade ago it was predicted that nano-scaled thermoelectric (TE) materials might have superior properties to that of their bulk counterparts. Subsequently, a significant increase in the figure of merit, ZT (ZT > 2), has been reported for nano-scaled systems such as superlattice and quantum dot systems constituently based on those more commonly used bulk TE materials (e.g., Bi2Te3 and PbTe). However, the challenge remains to achieve these higher performance results in bulk materials in order to more rapidly incorporate them into standard TE devices. Recent theoretical work on boundary scattering of phonons in amorphous materials indicates that micron and submicron grains could be very beneficial in order to lower the lattice thermal conductivity and yet not deteriorate the electron mobility. The focus in this paper will be to highlight some of our new directions in bulk thermoelectric materials research. Thermoelectric materials are inherently difficult to characterize and these difficulties are magnified at high temperatures. Specific materials will be discussed, especially those bulk materials that exhibit favorable properties for potential high temperature power generation capabilities. One potentially fruitful research direction is to explore whether hybrid TE materials possess possible enhanced TE properties. These “engineered” hybrids include materials that exhibit sizes from on the order of a few nanometers to hundreds of nanometers of the initial materials. These initial materials are then incorporated into a bulk structure. A discussion of some of the future research directions that we are pursuing is highlighted, including some bulk materials, which are based on nano-scaled or hybrid composites. The synthesis techniques and the synthesis results of many of these nano-scale precursor materials will be a primary focus of this paper.

Enhanced thermoelectric performance of MXene/GeTe through a facile freeze-drying method

Thermoelectric materials have demanded a significant amount of attention for their ability to convert waste heat directly to electricity with no moving parts. A resurgence in thermoelectrics research has led to significant enhancements in the thermoelectric figure of merit, zT, even for materials that were already well studied. This thesis approaches thermoelectric zT optimization by developing a detailed understanding of the electronic structure using a combination of electronic/thermoelectric properties, optical properties, and ab-initio computed electronic band structures. This is accomplished by applying these techniques to three important classes of thermoelectric materials: IV-VI materials (the lead chalcogenides), Half-Heusler’s (XNiSn where X=Zr, Ti, Hf), and CoSb3 skutterudites. In the IV-VI materials (PbTe, PbSe, PbS) I present a shifting temperature-dependent optical absorption edge which correlates well to the computed ab-initio molecular dynamics result. Contrary to prior literature that suggests convergence of the primary and secondary bands at 400 K, I suggest a higher convergence temperature of 700, 900, and 1000 K for PbTe, PbSe, and PbS, respectively. This finding can help guide electronic properties modelling by providing a concrete value for the band gap and valence band offset as a function of temperature. Another important thermoelectric material, ZrNiSn (half-Heusler), is analyzed for both its optical and electronic properties; transport properties indicate a largely different band gap depending on whether the material is doped n-type or p-type. By measuring and reporting the optical band gap value of 0.13 eV, I resolve the discrepancy in the gap calculated from electronic properties (maximum Seebeck and resistivity) by correlating these estimates to the electron-to-hole weighted mobility ratio, A, in narrow gap materials (A is found to be approximately 5.0 in ZrNiSn). I also show that CoSb3 contains multiple conduction bands that contribute to the thermoelectric properties. These bands are also observed to shift towards each other with temperature, eventually reaching effective convergence for T>500 K. This implies that the electronic structure in CoSb3 is critically important (and possibly engineerable) with regards to its high thermoelectric figure of merit.

In thermoelectric research, the introduction of a dopant can suppress lattice thermal conductivity (κ1) through phonon scattering and optimize the power factor (PF) by changing the behavior of carriers, which are the key prerequisites for high thermoelectric performance. However, the electrical thermal conductivity (κe) can also increase with the increase of electrical conductivity (σ), which may override the optimization in PF and be detrimental to the improvement of final ZT. In this work, we highlight an amphoteric doping method by using Mn atoms to substitute both Ag and Sb atoms in AgSbTe2. The MnSb positive doping in p-type AgSbTe2 can improve the σ through increasing the hole concentration while maintaining a relative high Seebeck coefficient (S), thus substantially improving the PF. On the other hand, the MnAg negative doping can introduce electrons into the matrix, which will recombine with the major hole carriers and lead to a decrease of σ to suppress exorbitant κe induced by the MnSb doping. The combination of the both functions by Mn amphoteric doping can further improve the thermoelectric property through charge compensation modulation. By virtue of amphoteric doping, though σ is decreased, PF is further optimized because of increased S, while the total thermal conductivity (κtotal) is further decreased due to suppressed κe and additional phonon scattering, which are beneficial for the improvement of the final ZT value. As a result, 5 mol % MnAg-MnSb amphoteric doping AgSbTe2 sample achieves a maximum ZT value of ∼0.74 at 550 K, which is higher than that of the pristine sample and other Mn monodoped counterparts. The present work suggests charge compensation modulation via amphoteric doping as an effective avenue to simultaneously achieve low thermal conductivity and high power factor for better thermoelectric performance.

More than 50% of the primary energy input in the global economy is lost to waste heat, a substantial fraction of which is recoverable in principle. For example, approximately half of the waste heat in a car or truck is in the form of high temperature exhaust gas. It is therefore no surprise that there has been a resurgence of interest in thermoelectric materials. Thermoelectrics provide a modular, solid-state technology for extracting electrical energy from heat. The key limitation has been low efficiency limited by the performance of known materials. This is characterized by a figure of merit, ZT. A decade ago the best practical materials had ZT of roughly unity, a figure that had not changed for many years. Efforts over the past decade have led to a doubling of this figure. This accelerating progress underlies the scientific excitement around this field. This topical issue contains a selection of papers covering various aspects of thermoelectric research, including both characterization techniques and thermoelectric materials. The issue emphasizes green materials, which are particularly important for many energy applications. This includes a paper on characterization of oxide-based modules, as well as papers on silicides, chalcogenides, and skutterudites including both theory and experiment. Nano-structure is a key theme in thermoelectrics research, both for control of thermal conductivity and for modification of electronic properties. Several papers in this issue concern aspects of nanostructure and its effect on thermoelectricity. Finally, we remark that thermoelectric performance depends on simultaneous optimization of normally contradictory properties such as conductivity and thermopower. This leads to a particularly fruitful interplay of theory and experiment as well as the exploration of unusual materials that show a fascinating variety of physical properties. We hope that this issue captures the excitement that results.