Decrease In Lattice Thermal Conductivity Research Articles

Study on thermoelectric properties and atomic-scale mechanism of B-site molybdenum-modified La0.1Sr0.9TiO3

Extensive studies on strontium titanate thermoelectric materials have shown that their poor conductivity results in a low ZT value. However, elemental doping can effectively improve the thermoelectric properties of these materials. In this study, a two-step sintering process was applied, using physical mixing to introduce 1 % molybdenum powder as a modification to the lanthanum-doped strontium titanate solid powder (La0.1Sr0.9TiO3). The obtained sample exhibited a maximum conductivity of 7830 S/m, with a maximum ZT of 0.12. Furthermore, this study combined experimental data with first-principles simulation calculations to elucidate the mechanism at the atomic scale. The incorporation of Mo6+ in place of Ti4+ at the B-site by molybdenum (Mo) adds two free electrons, leading to an elevated carrier concentration. This change concurrently reduces the Fermi level of the material and improves mobility, collectively resulting in a substantial increase in electrical conductivity. Phonon simulation indicated that Mo doping increased structural defects, thereby reducing lattice thermal conductivity. This significant enhancement in electrical conductivity and decrease in lattice thermal conductivity collectively increased the ZT value of the material.

Boosting thermoelectric performance of polycrystalline SnSe by controlled in-situ Ag2Se precipitates in grain boundaries

Boundary engineering has proven effective in enhancing the thermoelectric performance of materials. SnSe, known for its low thermal conductivity, has garnered significant interest; however, its application is hindered by poor electrical conductivity. Herein, the Ag8GeSe6 is introduced into the p-type polycrystalline SnSe matrix to optimize the thermoelectric performance, and the in-situ Ag2Se precipitates are formed in grain boundaries, which play dual roles, acting as an electron attraction center for improving hole concentration and a phonon scattering center for reducing lattice thermal conductivity. It effectively decouples the thermal and electrical transport properties to optimize the thermoelectric performance. Importantly, the amount of Ag2Se can be controlled by adjusting the amount of Ag8GeSe6 added to the SnSe matrix. The introduction of Ag8GeSe6 enhances electrical conductivity due to the increased hole carrier caused by the introduced Ag+ and the formed electron attraction center (in-situ Ag2Se precipitates). Based on the DFT calculations, the band gap of the Ag8GeSe6-doped samples is considerably decreased, facilitating carrier transport. As a result, the electrical transport properties increase to 808 μW m−1 K−2 at 823 K for SnSe + 0.5 wt% Ag8GeSe6. In addition, in-situ Ag2Se precipitates in grain boundaries strongly enhance phonon scattering, causing a decrease in lattice thermal conductivity. Furthermore, the presence of defects contributes to a reduction in lattice thermal conductivity. Specifically, the thermal conductivity of SnSe + 1.0 wt% Ag8GeSe6 decreases to 0.29 W m−1 K−1 at 823 K. Consequently, SnSe + 0.5 wt% Ag8GeSe6 obtains a high ZT value of 1.7 at 823 K and maintains a high average ZT value of 0.57 over the temperature range of 323−773 K. Additionally, the mechanical properties of Ag8GeSe6-doped also show an improvement. These advancements can be applied to energy supply applications during deep space exploration.

Chiral Twist Interface Modulation Enhances Thermoelectric Properties of Tellurium Crystal.

Manipulating the grain boundary and chiral structure of enantiomorphic inorganic thermoelectric materials facilitates a new degree of freedom for enhancing thermoelectric energy conversion. Chiral twist mechanisms evolve by the screw dislocation phenomenon in the nanostructures; however, contributions of such chiral transport have been neglected for bulk crystals. Tellurium (Te) has a chiral trigonal crystal structure, high band degeneracy, and lattice anharmonicity for high thermoelectric performance. Here, Sb-doped Te crystals are grown to minimize the severe grain boundary effects on carrier transport and investigate the interface of chiral Te matrix and embedded achiral Sb2Te3 precipitates, which induce unusual lattice twists. The low grain boundary scattering and conformational grain restructuring provide electrical-favorable semicoherent interfaces. This maintains high electrical conductivity leading to a twofold increase in power factor compared to polycrystal samples. The embedded Sb2Te3 precipitates concurrently enable moderate phonon scattering leading to a remarkable decrease in lattice thermal conductivity and a high dimensionless figure of merit(zT)of 1.1 at 623 K. The crystal growth and chiral atomic reorientation unravel the emerging benefits of interface engineering as a crucial contributor to effectively enhancing carrier transport and minimizing phonon propagation in thermoelectric materials.

Improving Thermoelectric Performance of AgSbTe2 through Suppression of Ag2Te and Band Convergence via Mg and Ti Codoping.

In this study, the impact of codoping Mg and Ti on the thermoelectric performance of AgSbTe2 materials was investigated. Through a two-step synthesis process involving slow cooling and spark plasma sintering, AgSb0.98-xMg0.02TixTe2 samples were prepared. The introduction of Mg and Ti dopants effectively suppressed the formation of the undesirable Ag2Te phase. Density functional theory (DFT) calculations confirmed that Ti doping facilitated the band convergence, leading to a reduction in the effective mass of the carriers. This optimization enhanced carrier mobility and, consequently, electrical conductivity. Additionally, the codoping strategy resulted in the reinforcement of point defects, which contributed to a decrease in lattice thermal conductivity. The AgSb0.98-xMg0.02TixTe2 sample achieved a maximum figure of merit (ZT) value of 1.45 at 523 K, representing an 87% improvement over the undoped AgSbTe2 sample. The average ZT value over the temperature range of 323-573 K was 1.09, marking a significant enhancement in thermoelectric performance. This research demonstrates the potential of Mg and Ti codoping as a strategy to improve the thermoelectric properties of AgSbTe2-based materials.

Thermoelectric properties and effective medium theory analysis on the (GeTe)1-x(InTe)x composites

We employed the p-type semiconductor InTe as an extrinsic phase mixing in the GeTe matrix for thermoelectric properties investigations on the (GeTe)1-x(InTe)x (x=0, 0.02, 0.04, 0.06, and 0.08) composites. We observed notable phenomena that a small amount of InTe has a significant influence on the thermoelectric properties as the primary host matrix in the (GeTe)1-x(InTe)x (x=0.02, 0.04, 0.06, and 0.08) composites. Even for small concentrations of InTe which possess lower thermal and electrical conductivities compared to those of GeTe, the thermoelectric properties of the (GeTe)1-x(InTe)x (x=0.02, 0.04, 0.06, and 0.08) composites are primarily influenced by the properties of InTe. We systematically elucidated the electrical conductivity of the (GeTe)1-x(InTe)x (x=0.02, 0.04, 0.06, and 0.08) composites through the application of effective medium theory (EMT). It is found that the effective media of the (GeTe)1-x(InTe)x (x=0.02, 0.04, 0.06, and 0.08) composites is an asymmetric medium insulator. The addition of InTe into GeTe reveals a significant decrease in total thermal conductivity due to reductions in both electronic and lattice thermal conductivity. Particularly, the decrease in lattice thermal conductivity was attributed to an increase in internal strain within the lattice induced by the addition of InTe. Consequently, the ZT values of the composite significantly increase across all temperature ranges. This study suggests that developing composites is an effective approach for enhancing thermoelectric performance, comparable to elemental doping, and demonstrates the ability to analyze the electrical properties of composite materials using EMT.

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 460 K, 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 + 0.25 wt% InP3 at 490 K. Importantly, the average ZT value within the 300 K - 600 K 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.

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.

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.