Decrease In Lattice Thermal Conductivity Research Articles

Effect of biaxial tensile strain on the thermoelectric properties of monolayer ZrTiCO2

Using ab-initio calculation principle calculations, we investigate the effect of biaxial tensile strain on the stability and electronic properties of the two-dimensional double transition metal MXenes ZrTiCO2. The monolayer ZrTiCO2 has stability and semiconducting properties under different strain conditions. The results of thermoelectric properties under other strain conditions calculated by Boltzmann transport theory and Slack model show that biaxial tensile strain can improve the electrical transport properties of monolayer ZrTiCO2 materials and lead to the decrease of lattice thermal conductivity. The thermoelectric efficiency of a material can be evaluated using the figure of merit ZT. The n-type ZrTiCO2 has a maximum ZT value of 1.49 at 900 K without adding biaxial strain and reaches a ZT value of 2.86 with 2% biaxial strain. The monolayer ZrTiCO2 material has the potential to become a thermoelectric material, and its thermoelectric properties can be improved by biaxial tensile strain.

InP3 facilitating high thermoelectric performance in Te-based composites via high pressure

Thermoelectric devices as eco-friendly candidates for energy conversion have attracted increasing interest as a sustainable and emission free solution to solve the environmental problem. Tellurium, is expected to be a suitable candidate, nevertheless, due to the poor electrical property, it has severely hampered the further development in practical application. This work proposes a new fabrication route for bulk Te via high pressure and high temperature method to enhance its power factor by compositing a certain quality of InP3. As InP3 decomposed into indium and black phosphorus under high pressure conditions, both the carrier concentration and Seebeck coefficient was enhanced significantly, leading to a maximum power factor of 11.4 μWcm-1K-2 at 460K, which increase by 44.2% compared to the peak value of pristine Te. Moreover, the induced defects, such as point defects, abundant grain boundaries and heterojunctions, strongly influence the thermoelectric performance, with a substantial decrease in lattice thermal conductivity. Consequently, the ZT values are sharply enhanced by InP3 compositing with the highest value reaching ∼0.76 of Te + 0.25wt.% InP3 at 490K. Importantly, the average ZT value within the 300K - 600K range reaches to 0.48, which is 3.5 times that of the pristine Te. This work mark a step toward developing high performance thermoelectric devices for energy conversion, making elemental Te more competitive with high-performance in medium temperature range.

Ultrahigh average zT realized in polycrystalline SnSe0.95 materials through Sn stabilizing and carrier modulation

The average zT determines the conversion efficiency, and the power factor plays an important role in average zT value. However, the inadequate electrical conductivity of SnSe materials seriously limits its application. Herein, the TaCl5-doped in polycrystalline SnSe0.95 materials synthesized using the melting method and combined with spark plasma sintering technology achieves a zT value of 1.64 at 773 K and a record zTave of 0.62 from 323 K to 773 K. The electrical conductivity increases due to the released electron carrier induced by effective TaCl5 doping. According to the DFT calculation, the energy band of TaCl5-doped samples is narrowed, which can enhance the electron transport. Besides, the Seebeck coefficient is maintained at an elevated level as a result of the incorporation of the heavy element Ta. Due to the significantly enhanced electrical conductivity and maintained high Seebeck coefficient, the power factor reaches to 622 μW·m–1·K–2 at 773 K for the SnSe0.95 + 1.75% (in mass) TaCl5 sample, which is almost 21 times higher than that of the pristine sample. Simultaneously, a high average power factor value of 334 μW·m–1·K–2 for the SnSe0.95 + 1.75% (in mass) TaCl5 sample from 323 to 773 K was obtained. It is surprisingly found that the Ta element plays another important role to improve the stability of SnSe0.95 by forming Ta2Sn3 and removing the low melting point Sn, which usually existed in n-type SnSe samples, resulting in the decreased lattice thermal conductivity. A low lattice thermal conductivity value of 0.24 W·m–1·K–1 was also obtained for the SnSe0.95 + 2.0% (in mass) TaCl5 sample at 773 K due to the multiscale defects. Consequently, the SnSe0.95 + 2.0% (in mass) TaCl5 sample obtains a peak zT value of 1.64 at 773 K and a record zTave of 0.62 from 323 to 773 K, and the theoretically calculated conversion efficiency reaches 11.2%, it can be utilized for power generation and/or cooling at a broad temperature range. This strategy of introducing high-valence halides with heavy element can optimize the thermoelectric performance for other material systems.

Enhancing Thermoelectric Performance of n-Type Bi2Te2.7Se0.3 through Incorporation of Amorphous Si3N4 Nanoparticles.

Bi2Te3-based thermoelectric (TE) materials are the state-of-the-art compounds for commercial applications near room temperature. Nevertheless, the application of the n-type Bi2Te2.7Se0.3 (BTS) is restricted by the comparatively low figure of merit (ZT) and intrinsic embrittlement. Here, we show that through dispersion of amorphous Si3N4 (a-Si3N4) nanoparticles both 14% increase in power factor (at 300 K) and 48% decrease in lattice thermal conductivity are simultaneously realized. The increased power factor comes from enhanced thermopower and reduced electrical resistivity while the reduced lattice thermal conductivity originates mainly from scattering of middle- and low-frequency phonons at the incorporated a-Si3N4 nanoparticles. As a result, a large ZTmax = 1.19 (at 373 K) and an average ZTave ∼ 1.12 (300-473 K) with better mechanical properties are achieved for the BTS/0.25 wt % Si3N4 sample. Present results demonstrate that the incorporation of a-Si3N4 is a promising way to improve TE performance.

High thermoelectric properties in polycrystalline SnSe materials realized by rare earth halide Co-doping

SnSe materials have garnered significant interest due to the intrinsic low thermal conductivity. However, its limited electrical conductivity hinders their practical implementation. This study achieved a high ZT value of 1.40 at 773 K in the samples of n-type polycrystalline SnSe0.95 + x wt% SmCl3 + y wt% ErCl3 (x = 0, 0.75, 1.0, 1.25, and 1.5; y = 0, 0.0675, 0.125, and 0.25) since (SmCl3, ErCl3) co-doping decoupled the electrical-thermal transport properties. First, the released electron carrier concentration due to SmCl3 doping enhanced the electrical transport properties. The SnSe0.95 + 1.25 wt% SmCl3 sample exhibited a high power factor of 563 μWm−1K−2 at 773 K. Then, the lattice thermal conductivity markedly decreased with further ErCl3 doping due to the high-density dislocations and coherent boundary, resulting in effective the phonon scattering. Additionally, the special microtopography was conducive to the decreased lattice thermal conductivity. Therefore, the SnSe0.95 + 1.25 wt% SmCl3 + 0.125 wt% ErCl3 sample achieved a low lattice thermal conductivity value of 0.28 Wm−1K−1 at 773 K. Simultaneously, the (SmCl3, ErCl3) co-doped samples maintained the electrical transport properties than the SmCl3-doped samples. Consequently, the SnSe0.95 + 1.25 wt% SmCl3 + 0.125 wt% ErCl3 sample obtained a peak ZT value of 1.40 at 773 K and a competitive average ZT value of 0.37 from 323 to 773 K, and a theoretically calculated conversion efficiency reached 7.2%, which can be applied in the deep space for power generation. This strategy can be utilized to optimize the thermoelectric performance of other thermoelectrical material systems.

High-Performance p-Type Bi2Te3-Based Thermoelectric Materials Enabled via Regulating Bi-Te Ratio.

Bi2Te3-based alloys, as the sole commercial thermoelectric (TE) material, play an irreplaceable role in the thermoelectric field. However, the low TE efficiency, poor mechanical properties, and high cost have limited its large-scale applications. Here, high-performance p-type Bi2Te3-based materials were successfully prepared by ball milling and hot pressing. The optimized p-type Bi0.55Sb1.45Te3 + 2.5 wt % Bi shows a peak zT value of 1.45 at 360 K, and the average zT value of up to 1.24 at 300-480 K, which is completely comparable with previously reported Bi2Te3-based alloys with excellent performance. Such performance mainly results from the enhanced electrical conductivity and decreased lattice thermal conductivity via regulating carrier and phonon transport. Furthermore, this material shows good mechanical properties, in which the Vickers hardness and compressive strength are up to 0.95 GPa and 94.6 MPa, respectively. Overall, both the thermoelectric and mechanical performance of the materials fabricated by our processing technology are quite competitive. This may enlighten researchers concentrating on Bi2Te3-based alloys, thus further promoting their industrial applications.

Gradient Nanotwins and Enhanced Weighted Mobility Synergistically Upgrade Bi0.5Sb1.5Te3 Thermoelectric and Mechanical Performance

AbstractBi2Te3‐based alloys have historically dominated the commercial realm of near room‐temperature thermoelectric (TE) materials. However, the more widespread application is currently constrained by its mediocre TE performance and inferior mechanical properties resulting from intrinsic hierarchical structure. Herein, microstructure modulation and carrier transport optimization strategies are employed to efficiently balance the electro‐thermal transport performance. Specifically, the weighted mobility increases by 24%, while the lattice thermal conductivity decreases by 31% at 350 K compared to the matrix. Consequently, the Bi0.5Sb1.496Cu0.004Te2.98 sample attains a peak ZT of 1.45 at 350 K and an average ZT of 1.20 (300–500 K). Moreover, intricated microstructure design, exemplified by the gradient twin structure, significantly enhances the mechanical performance metrics, including Vickers hardness, compressive strength, and bending strength, to notable levels of 0.94 GPa, 224 MPa, and 58 MPa, respectively. Consequently, the constructed 17‐pair TE modules demonstrate a maximum conversion efficiency of 6.5% at ΔT = 200 K, surpassing the majority of reported Bi2Te3‐based modules. This study provides novel insights into the synergistic enhancement of TE and mechanical properties in Bi2Te3‐based materials, with potential applicability to other TE systems.

Substrate-induced asymmetric charge distribution tuning the thermal transport and electronic properties of two-dimensional GaX (X=S and Se)

GaS and GaSe, as members of the III-VI group compounds, have garnered significant attention due to their excellent optoelectronic properties. Recent studies have highlighted the low lattice thermal conductivity (TC) and excellent thermoelectric properties of two-dimensional (2D) GaX (X=S and Se). However, these investigations have focused on free-standing GaX, overlooking the substrate effect. In our study, we conducted first-principles calculations to assess the thermal transport and electronic properties of 2D free-standing and supported GaX (X=S, Se). In terms of thermal transport properties, our findings reveal a significant reduction in the lattice TC of 2D GaS and GaSe when supported on substrates. When supported on the h-BN (0001) substrate, the reductions are 68.01 % and 80.34 %, respectively. Similarly, when supported on the β-Si3N4 (0001) substrate, the lattice TC of 2D GaS and GaSe decreases by 74.60 % and 88.73 %, respectively. The significant decrease in lattice TC is primarily from the asymmetric bonding of GaX induced by the substrates, which enhances phonon anharmonicity and reduces phonon lifetime. In the realm of electronic properties, our investigations reveal that the influence exerted by the h-BN substrate upon the energy band structure of supported GaX is minimal, whereas β-Si3N4 substrate has a greater effect on the energy band structure of the supported GaX due to its high and low undulating atomic surface. Our findings underscore the remarkable tunability of both lattice TC and electronic properties in 2D GaX by selecting different substrates, which holds significant promise for the future applications of 2D GaX in emerging technologies.

Construct Amorphous Polymer Interface to Enhance the Thermoelectric Performance of Commercial Bi0.5Sb1.5Te3 Materials.

Interfaces, such as grain boundaries and phase boundaries in thermoelectric (TE) materials, play a crucial role in the carrier/phonon transport. Accurate control of the features of interfaces, including composition, crystalline nature, and thickness may give rise to a promising pathway to break the trade-off between phonon and carrier transport properties, which is essential to design high-performance TE materials. In this work, the amorphous polymer interface (API) layer is introduced to the p-type commercial Bi0.5Sb1.5Te3 (BST) TE material by the liquid-phase sintering process. Due to the larger mismatch in the acoustic impedance or phonon spectra between the amorphous polymer layer and the BST phase, the additional interfacial thermal resistance is introduced, which results in a large decrease in lattice thermal conductivity. It is found that the interfacial thermal resistance at the API is much higher than that of normal grain boundary and hetero interface reported in the literature. Conversely, taking advantage of the strong electron and phonon scattering, a large net get of ZT was achieved. A maximum ZT of ∼1.22 at 350 K was obtained in the BST/polyimide-0.5% sample, which is considerably greater than that of the commercial BST matrix (∼0.99 at 350 K). Furthermore, the optimized BST/polymer sample also exhibited almost 20% enhancement in hardness compared with the pure BST sample. This work has opened a new window for designing high-performance TE composites, which may extend to other material systems.

The role of pressure on lattice thermal conductivity and its related thermodynamical parameters in In0.53Ga0.47As nanofilms.

Lattice thermal conductivity (LTC) for In0.53Ga0.47As alloy films, with thicknesses ranging from 10 nm to 1.4 μm, was investigated under pressures of up to 11 GPa and temperatures between 1 and 450 K, utilizing the modified Debye-Callaway model. The effects of structural and thermodynamical parameters, as well as phonon interactions, on LTC were examined. The Clapeyron, Murnaghan, and Post equations were applied to determine the pressure dependence of the melting temperature, lattice volume, and Debye temperature, respectively. A novel derivative form of the bulk modulus, suitable for nanomaterials, has been introduced. It was found that decreasing the film thickness increases the Gruneisen parameter, while increasing pressure decreases it. The LTC of nanofilms is significantly affected by their thickness and pressure strength; notably, under 11 GPa, films with a thickness of 10 nm exhibit a substantial decrease in LTC. LTCmax declines due to the greater influence of boundary scattering compared to dislocations. These findings suggest potential applications in managing nanofilm temperature and size, which are key to advancing nanomaterials and enhancing the efficiency of electronic devices.

Boosting room-temperature thermoelectric performance of Mg3Sb1.5Bi0.5 material through breaking the contradiction between carrier concentration and carrier mobility

Recently, low-cost Mg3Sb2-yBiy-based materials with high performance near room temperatures have been reported, which are considered to be promising candidates for replacing commercial Bi2Te3-based materials. Herein, we report a high power factor of ∼3000 μW m−1 K−1 and a ZT value of 0.82 at room temperature in Ti0.05Mg3.15Sb1.5Bi0.49Te0.01. Ti dopant entering into the Mg1 sublattice slightly increases the carrier concentration by the introduction of impurity states and enhances the carrier mobility by the promotion of grain size growth. The two-donor doping strategy breaks the contradiction between carrier concentration and carrier mobility, thus significantly improving the electrical transport properties near room temperatures. In addition, the introduction of defects by Ti dopant contributes to the decreased lattice thermal conductivity. Consequently, a ZTavg value of 1.19 is obtained at 50–250 ℃, ranking the top among reported n-type thermoelectric materials. Moreover, the as-fabricated single-leg device shows an high engineering efficiency considering the radiation calibration, i.e., 7.2 % and 11.8 % at temperature differences of 250 ℃ and 500 ℃ with Tc = 0 ℃, respectively, demonstrating the great potential of substituting the commercial Bi2Te3-family.

Room‐Temperature Cubic Ag2S1−2xSexTex with Promising Ductility and Thermoelectric Properties Enabled by Entropy Engineering

AbstractSince the discovery of superior ductility in semiconducting Ag2S at room temperature, Ag2S‐based inorganics attract ever‐increasing attention as ductile thermoelectrics (TEs) for flexible electronics, while the monoclinic to cubic structure transition near room temperature (≈455 K for Ag2S) of these materials leads to instability of their structures and properties. In this work, single‐phase cubic Ag2S1−2xSexTex (x = 0.13–0.33) samples are stabilized at room temperature via entropy engineering. In comparison with pure Ag2S, the random mixing of S, Se, and Te at the anion site results in increased configuration entropy, improved electrical conductivity, decreased lattice thermal conductivity, and thus significantly enhanced TE properties of cubic Ag2S1−2xSexTex samples. By further optimizing the carrier concentration through introducing Ag vacancies, the slightly Ag‐deficient Ag1.98S0.34Se0.33Te0.33 sample achieves a power factor of 6.1 µW cm−1 K−2 and a dimensionless figure of merit zT of 0.4 at room temperature. In the measured temperature range of 300–500 K, this cubic sample with excellent ductility shows not only a record average zT value of 0.62 in ductile inorganics but also very stable TE properties, demonstrating the great potential of entropy engineering in the design of high‐performance ductile TE inorganics.

Constructing cell-membrane-mimic grain boundaries for high-performance n-type Ag2Se using high-dielectric-constant TiO2

The coupling of charge carrier and phonon transport limits the application of Ag2Se as a low-toxic near-room-temperature thermoelectric material. Strategies that reduce the thermal conductivity via enhancing the phonon scattering usually lead to reduced carrier mobility due to high grain boundary potential barrier. In this study, we developed a cell-membrane-mimic grain boundary engineering strategy for decoupling the charge carrier and phonon scattering through decorating high-dielectric-constant rutile TiO2 at Ag2Se grain boundaries to enable the charge carrier/phonon selective permeability. The nano-sized TiO2 with high dielectric permittivity can secure the charge carrier transport by shielding the interfacial Coulomb potential to lower the energy barrier of grain boundaries, rendering an enhanced power factor. Additionally, benefited from the enhanced phonon scattering by TiO2 nanoparticles, a significantly decreased lattice thermal conductivity of ∼0.20 W m−1 K−1 and a high zT of ∼0.97 at 390 K are obtained in the Ag2Se-based nanocomposites. This work demonstrates that such cell-membrane-mimic grain boundary engineering strategy may shed light on developing high-performance thermoelectric materials.

Mechanism of Thermoelectric Performance Enhancement in CaMg2 Bi2 -Based Materials with Different Cation Site Doping.

Chemical bonds determine electron and phonon transport in solids. Tailoring chemical bonding in thermoelectric materials causes desirable or compromise thermoelectric transport properties. In this work, taking an example of CaMg2 Bi2 with covalent and ionic bonds, density functional theory calculations uncover that element Zn, respectively, replacing Ca and Mg sites cause the weakness of ionic and covalent bonding. Electrically, Zn doping at both Ca and Mg sites increases carrier concentration, while the former leads to higher carrier concentration than that of the latter because of its lower vacancy formation energy. Both doping types increase density-of-state effective mass but their mechanisms are different. The Zn doping Ca site induces resonance level in valence band and Zn doping Mg site promotes orbital alignment. Thermally, point defect and the change of phonon dispersion introduced by doping result in pronounced reduction of lattice thermal conductivity. Finally, combining with the further increase of carrier concentration caused by Na doping and the modulation of band structure and the decrease of lattice thermal conductivity caused by Ba doping, a high figure-of-merit ZT of 1.1 at 823 K in Zn doping Ca sample is realized, which is competitive in 1-2-2 Zintl phase thermoelectric systems.

The role of nanostructuring strategies in PbTe on enhancing thermoelectric efficiency

A nanostructured n-type thermoelectric (TE) material based on lead telluride (PbTe) (doped with 0.2 wt% PbI2 and 0.3 wt% Ni) was fabricated and investigated. Fabricating technology included the synthesis of PbTe by direct alloying of components, the grinding of synthesized PbTe in a planetary ball mill, and the compaction of the nanopowders by spark plasma sintering (SPS). Complex investigations of the structure and composition of powders and nanostructured PbTe and the thermoelectric parameters of nanostructured PbTe were carried out. The particle sizes of the powders ground for 20 and 60 min varied from 36 to 378 nm and from 29 to 210 nm, respectively. The maximum value of dimensionless thermoelectric figure of merit (ZT) = 1.35 for nanostructured PbTe was obtained for a sample compacted by SPS after grinding synthesized PbTe for 60 min, which is 14% higher than ZT for TE material obtained by hot pressing. Recrystallization of crystalline particles during SPS leads to an increase of the grain sizes in nanostructured PbTe by approximately 3 times and the elimination of microdeformations that appear during grinding. Analysis of the temperature dependencies of thermoelectric parameters showed that an increase in ZT for nanostructured PbTe is achieved due to the decrease in lattice thermal conductivity.

Modulation doping of p-type Cu12Sb4S13 toward improving thermoelectric performance

The commercial viability of thermoelectric (TE) devices relies heavily on two factors: cost reduction and efficiency enhancement. In this study, we first produce p-type Cu12Sb4S16–x (x = 0, 3, 4) using a low-temperature bottom-up approach and demonstrate Cu12Sb4S13 to show the best TE performance among the three tested compositions. Subsequently, the TE energy conversion efficiency of Cu12Sb4S13 is further enhanced by optimizing its electronic band structure through the incorporation of small amounts of tellurium. At an optimal Te content of 5 mol%, more than a twofold increase in the TE figure of merit (zT) is obtained. To gain insight into the mechanism of improvement on the transport properties of the material, we compare the interphase transport mechanism by incorporating nanodomains of different metals (Ag and Cu) into the Cu12Sb4S13 matrix. The improved electrical conductivity obtained with Cu12Sb4S13-Te nanocomposites is attributed to a charge flooding of the Cu12Sb4S13 surface. In contrast, excessive downward band-bending at the interphases of Ag/Cu metal-semiconductor drastically reduces the electrical conductivity. Besides, a weighted mobility (μw) analysis shows a dominant thermal activation of carriers in Cu12Sb4S13-Te nanocomposites. In this material, a strong decrease in lattice thermal conductivity is also found, which is associated with a phonon-carrier scattering mechanism. Our work shows the importance of proper band-engineering in TE nanocomposites to decouple electrical and thermal transport to enhance TE performance, and the efficacy of μw for electrical and thermal transport analysis.

Effects of Double Doping Germanium and Indium on the Thermoelectric Properties of Permingeatite

Cu3Sb1–x–yGexInySe4 (0.02 ≤ x ≤ 0.12; 0.04 ≤ y ≤ 0.08) permingeatite compounds doped with Ge and In were prepared using solid-state synthesis. The phases and microstructures were analyzed, and the charge transport and thermoelectric properties were evaluated according to the Ge and In doping content. Most of the samples contained a single phase of permingeatite with a tetragonal structure; however, secondary phases (Cu0.875InSe2, In2Se3, and InSb) were detected in the samples with x = 0.12 and y = 0.08. Both the a-axis and c-axis lattice constants of permingeatite were increased by Ge and In doping, with a = 0.5651–0.5655 nm and c = 1.1249–1.1255 nm, but the change in lattice constant due to the change in doping amount was insignificant. The melting point of the sample double-doped with Ge and In was determined to be 736 K, which was lower than the melting point (741 K) of pure Cu3SbSe4. This lowering of the melting point was attributed to the formation of a solid solution. The electrical conductivity exhibited degenerate semiconductor behavior, decreasing with increasing temperature. As the Ge and In doping content increased, the carrier concentration and electrical conductivity increased; however, when x ≥ 0.12, the electrical conductivity did not increase further. The Seebeck coefficient exhibited positive values and p-type conduction characteristics. In addition, intrinsic transitions did not occur in the measurement temperature range, and the Seebeck coefficient increased as the doping level increased. The power factor exhibited a positive temperature dependence, and Cu3Sb0.86Ge0.08In0.06Se4 exhibited the highest value of 0.89 mWm–1K–2 at 623 K. As the temperature increased, the thermal conductivity tended to decrease because of the decreased lattice thermal conductivity and slightly increased electronic thermal conductivity. All the samples exhibited minimum thermal conductivities of 0.94–1.11 Wm–1K–1 at 523 K. At high temperatures, the double doping of Ge and In improved the thermoelectric performance; thus, the dimensionless figure of merit obtained at 623 K for Cu3Sb0.86Ge0.08In0.06Se4, was 0.47.

Tuning of lattice thermal conductivity of amorphous Fe0.85Zr0.15 by nanostructured voids, pressure and temperature

Metallic glasses are known as one of the favorable amorphous materials with their remarkable stiffness, durability and low thermal conductivities. Furthermore, manipulation of lattice thermal conductivity by forming nanostructures is an important route to enhance the performance for several industrial applications. Here, equilibrium molecular dynamics simulations were performed for the generation of metallic glass (MG) Fe0.85Zr0.15 followed by the calculation of lattice thermal conductivity by Green–Kubo method. The amorphicity of our simulated MG is verified through the analysis of radial distribution functions and Voronoi tessellations. Nanostructured spherical voids with varying sphere radii were introduced. We have found that with increasing porosity, the lattice thermal conductivity decreases. We have also studied the dependence of temperature and pressure on thermal conductivity. Finally, our analysis of calculated vibrational densities of states shows interesting behavior of extended and localized modes in various situations of pressure, temperature and size of the nanostructured voids.

Enhanced thermoelectric properties of NbCoSn half-Heuslers through in-situ nanocrystallization of amorphous precursors during the consolidation process

Tailoring nanostructures is a general approach used to obtain enhanced thermoelectric properties for half-Heusler compounds because the wide areas of grain and phase boundaries could be scattering centers that lower lattice thermal conductivity. However, a common fabrication method based on the sintering of crystalline precursors crushed from as-cast alloy ingots has limitations in obtaining a homogeneous microstructure without microsized impurity phases, owing to residual elemental segregation from casting. In this study, we used amorphous NbCoSn alloys as a precursor for the sintered specimen to obtain a homogeneous NbCoSn bulk specimen without microsized impurity phases and segregation, which led to the enhanced Seebeck coefficient due to the high purity of the half-Heusler phase after crystallization. Moreover, superplasticity originating from amorphous features enabled the powders to be largely deformed during the sintering process, even at a low sintering temperature (953 K). This resulted in less oxidation at both, the grain boundary and the interior, as the O diffusion pathway was blocked during the sintering process. As a result, the NbCoSn0.95Sb0.05 specimen using an amorphous precursor exhibited an enhanced zT of 0.7, due to the increase in the power factor and a decrease in lattice thermal conductivity compared to the specimen using a crystalline precursor.

Enhancing thermoelectric performance of n-type AgBi3S5 through synergistically optimizing the effective mass and carrier mobility

AgBi3S5 is a new n-type thermoelectric material that is environmentally friendly and composed of elements of earth-abundant, non-toxic and high performance-cost ratio. This compound features an intrinsically low thermal conductivity derived from its complex monoclinic structure. However, the terrible electrical transport properties greatly limited the improvement of thermoelectric performance. Most previous studies considered that carrier concentration is the main reason for low electrical conductivity and focused on improving carrier concentration by aliovalent ion doping. In this work, we found that the critical parameter that restricts the electric transport performance of AgBi3S5 was the extremely low carrier mobility instead of the carrier concentration. According to the Pisarenko relationships and density functional theory calculations, Nb doping can sharpen the conduction band of AgBi3S5, which contributes to reducing the effective mass and improving the carrier mobility. With a further increase of the Nb doping content, the conduction band convergence can enlarge the effective mass and preserve the carrier mobility. Combined with the decrease in lattice thermal conductivity due to the intensive phone scattering, a maximum ZT value of ∼0.50 at 773 K was achieved in Ag0.97Nb0.03Bi3S5, which was ∼109.6% higher than that of pure AgBi3S5. This work will stimulate the new exploration of high-performance thermoelectric materials in ternary metal sulfides.