Enhancement Of Thermoelectric Performance Research Articles

Tailoring thermoelectric performance of n-type Bi2Te3 through defect engineering and conduction band convergence

Bi2Te3, an archetypical tetradymite, is recognised as a thermoelectric (TE) material of potential application around room temperature. However, large energy gap (ΔEc ) between the light and heavy conduction bands results in inferior TE performance in pristine bulk n-type Bi2Te3. Herein, we propose enhancement in TE performance of pristine n-type Bi2Te3 through purposefully manipulating defect profile and conduction band convergence mechanism. Two n-type Bi2Te3 samples, S1 and S2, are prepared by melting method under different synthesis condition. The structural as well as microstructural evidence of the samples are obtained through powder x-ray diffraction and transmission electron microscopic study. Optothermal Raman spectroscopy is utilized for comprehensive study of temperature dependent phonon vibrational modes and total thermal conductivity ( κ ) of the samples which further validates the experimentally measured thermal conductivity. The Seebeck coefficient value is significantly increased from 235 μVK−1 (sample S1) to 310 μVK−1 (sample S2). This is further justified by conduction band convergence, where ΔEc is reduced from 0.10 eV to 0.05 eV, respectively. To verify the band convergence, the double band Pisarenko model is employed. Large power factor (PF) of 2190 μWm−1K−2 and lower κ value leading to ZT of 0.56 at 300 K is gained in S2. The obtained PF and ZT value are among the highest values reported for pristine n-type bulk Bi2Te3. In addition, appreciable value of TE quality factor and compatibility factor (2.7 V−1) at room temperature are also achieved, indicating the usefulness of the material in TE module.

Enhancement of thermoelectric performance by stacking fault control in (GeTe)1-x(Bi2Te3)x compounds, synthesized by hot press sintering method

We investigated the thermoelectric (TE) properties of GeTe by incorporating Bi2Te3 in the GeTe system. The addition of Bi2Te3 into the GeTe matrix leads to phonon scattering caused by stacking fault or structural defect, resulting in a substantial reduction in lattice thermal conductivity κlat. We observed a significant decrease in total thermal conductivities in the vertical and horizontal pressing directions, respectively. The optimized carrier properties and low thermal conductivity in the composites have led to a remarkable improvement in the dimensionless figure of merit (zT) 1.26 at 650 K. This study highlights the potential for creating high-performance thermoelectric materials through GeTe-Bi2Te3 compounds, emphasizing the importance of defect engineering and stacking faults in reducing thermal conductivity and improving thermoelectric efficiency.

Realizing high average zT in GeTe through band modulation and suppressing Ge vacancies

GeTe is regarded as an excellent thermoelectric material, while its intrinsically high hole concentration impedes its further enhancement in thermoelectric performance. In this work, we introduce a two-step strategy by Bi and Y co-doping and Pb alloying to optimize the thermoelectric properties of the GeTe compound. Aliovalent Bi doping on the Ge site is found to effectively reduce the hole concentration, and a small substitution of Bi by Y atoms can further increase the effective mass, resulting in a high thermoelectric figure of merit (zT) of ∼ 1.92 at 723 K. Alloying Pb with high concentration elaborately decreases the carrier concentration to around 1.09 × 1020 cm−3, leading to an enhancement of Seebeck coefficient. In addition, Pb alloying maintains stable carrier mobility, contributing to a high power factor. PbTe alloying induces a strong point defect scattering and strain field fluctuation, leading to a low lattice thermal conductivity. Benefiting from the synergistically optimized effects of Bi and Y co-doping and Pb alloying, a peak zT of ∼ 2.26 at 723 K and an average zT of 1.64 over 300–773 K are realized in Ge0.8Pb0.15Bi0.04Y0.01Te.

Advancing Thermoelectric Performance of Bi2Te3 below 400 K.

Thermoelectric cooling devices utilizing Bi2Te3-based alloys have seen increased utilization in recent years. However, their thermoelectric performance remains inadequate within the operational temperature range (≤400 K), with limited research addressing this issue. In this study, we successfully modulated the carrier concentration of the sample through Te content reduction, consequently lowering the peak temperature of the zT value from 400 to 300 K. This led to a substantial enhancement in thermoelectric performance at room temperature (≤400 K). Furthermore, by doping with La, the electrical transport properties have been further optimized, and the lattice thermal conductivity has been effectively reduced at the same time; the average zT value was ultimately elevated from 0.69 to 0.9 within the temperature range of 300-400 K. These findings hold significant promise for enhancing the efficacy of existing thermoelectric cooling devices based on Bi2Te3-based alloys.

Enhancing thermoelectric performance of CrN ceramics by optimizing sintering temperature

Chromium nitride (CrN) ceramics were sintered at various temperatures by plasma activated sintering, and their thermoelectric properties were explored. The formation of Cr2N phase can be suppressed by venting N2 gas into the chamber before sintering and increasing the sintering temperature to accelerate the densification process. Sintering temperature significantly impacted the thermoelectric properties. Higher sintering temperature led to an increase in the N content, reducing the electrical and thermal conductivity, and increasing the Seebeck coefficient. The single-phased CrN ceramic sintered at 1573 K exhibited excellent thermoelectric performance with a maximum power factor of 1002 μW·m−1·K−2, a thermal conductivity of 4.78 W·m−1·K−1, and a zT value of 0.20 at 973 K. Furthermore, this CrN ceramic also exhibited a high relative density of 97% and a hardness of ∼11.9 GPa. Enhanced high-temperature thermoelectric properties and excellent mechanical properties make CrN ceramics suitable for thermoelectric applications in extreme environments.

Enhancement of thermoelectric performance of thienoisoindigo based D‐A conjugated polymers through the incorporation of 3,4‐ethylenedioxythiophene as donor unit

AbstractWith the good planarity of conjugated backbone and high charge carrier mobilities, thienoisoindigo (TIIG)‐based polymers show great potential in organic electronic devices. In this work, two TIIG‐based D‐A conjugated polymers (PTIIG‐3T and PTIIG‐2T‐EDOT) were designed and synthesized, which exhibit high lying highest occupied molecular orbital (HOMO) energy level and can be facilely doped by FeCl3. Compared with PTIIG‐3T, one thiophene unit in the donor building block is replaced by 3,4‐ethylenedioxythiophene (EDOT) in PTIIG‐2T‐EDOT, and PTIIG‐2T‐EDOT shows a higher HOMO level with slightly higher coplanarity than PTIIG‐3T, which induce an enhancement in electrical conductivity after oxidation doping. After proper doping, doped PTIIG‐2T‐EDOT film achieved a higher TE power factor of 17.9 μW m−1 K−2 than that of PTIIG‐3T due to its higher conductivity. The results indicate that TIIG unit is a promising building block for future high‐performance conjugated polymers for thermoelectric applications, and incorporation of the EDOT unit into D‐A conjugated polymers could be an effective way to develop high performance thermoelectric polymers.

Enhancement of Phase Stability and Thermoelectric Performance of Meta‐Stable AgSbTe2 by Thermal Cycling Process

AbstractThermoelectric materials are crucial components in thermoelectric devices employed for environmentally friendly solid‐state cooling and waste heat recovery, demanding not only high performance but also superior thermal or phase stability. The high‐performance thermoelectric material AgSbTe2 encounters challenges when utilized in thermoelectric modules due to the precipitation of Ag2Te, resulting in both thermally unstable characteristics and diminished performance. In this study, a thermal cycling process is employed to enhance the thermal stability and thermoelectric performance of AgSbTe2. Through thermal cycling, secondary phases of AgSbTe2 are made uniform, and the undesired Ag2Te is substituted with Sb2Te3 using an optimized thermal cycling process. As a result, the thermal stability of AgSbTe2 is enhanced due to its meta‐stable state, with <5% variance in thermoelectric figure of merit (ZT) measurements, and the ZT value is raised from 0.8 to 1.7 at 643 K through the optimized thermal cycling. The findings indicate that thermal cycling is an effective strategy for enhancing the thermal stability and thermoelectric performance of AgSbTe2, presenting a novel approach for achieving uniform secondary phases in materials with phase separation or phase change characteristics.

Enhancing thermoelectric performance of AB2Sb2-type Zintl phase through band shaping and lattice distortion

AB2Sb2-type Zintl phases, especially Mg3Sb2, have garnered widespread attention in virtue of their nontoxic constituent elements and outstanding thermoelectric performance in n-type materials. However, the p-type counterpart of AB2Sb2 still struggles with a low figure of merit zT, primarily because of the limited valence band degeneracy, large band effective mass, and relatively high thermal conductivity. Here we simultaneously optimize the electrical and thermal transports of p-type AB2Sb2-based materials through shaping the band edge and distorting the crystal lattice. By alloying Zn at Mg2 site and Ca at Mg1 site, we successfully decorate the crystal lattice of Mg3Sb2 in real space and thereby modify the band structure in reciprocal space. Zn alloying promotes the band degeneracy while Ca alloying reduce the single band effective mass, which largely promotes the transport of charge carriers. Moreover, the distorted crystal lattice effectively blocks the heat-carry phonons to reduce the lattice thermal conductivity κL. A high zT of 0.81 is finally realized at 750 K in our Zn/Ca co-alloyed samples, which is three times larger than that of Mg3Sb2 matrix. Our work opens new possibilities for the synthetic optimization of both electrical and thermal transports in Zintl phase and other thermoelectric systems.

Thermoelectric performance enhancement of Pb-doped α -MgAgSb near room temperature

α-MgAgSb is taken as the p-type leg material for recently focused Mg-based thermoelectric devices because of the high thermoelectric performance near room temperature. However, the thermoelectric performance of α-MgAgSb is inhibited by the existence of the Ag-rich second phase. The ordinary methods like carrier concentration optimization and minimizing lattice thermal conductivity were nearly invalid because of the extremely low doping level for heteroatoms and intrinsically low lattice thermal conductivity. The crystal structure of α-MgAgSb can be viewed as Ag atom filled in half distorted hexahedron in the distorted rock salt skeleton formed by the Mg–Sb sublattice. In this work, by replacing the smaller Mg in the sublattice with Pb, the volume of the distorted hexahedron is effectively expanded to accommodate Ag atoms and then lead to the re-dissolution of Ag-rich second phase in the matrix. In addition, as Ag is the main source of low-frequency phonons, the enhanced lattice anharmonicity by Pb doping leads to stronger scattering of phonons in the distorted hexahedron and results in 20% reduction of lattice thermal conductivity in the temperature range of 300–500 K. Finally, the figure of merit zT is enhanced by ∼40% in the whole temperature range, demonstrating that lattice management is a promising method for the optimization of α-MgAgSb materials.

Power factor enhancement in benzodithiophene-based conjugated polymers through controlling solution-state aggregation

Controlling solution-state aggregation of conjugated polymers is crucial for tuning solid-state morphology as well as electrical properties. However, the effect of solution-state aggregation on their thermoelectric performance is rarely reported. Using a series of poly(benzodithiophene-alt-thiophene) conjugated polymers (PBDT-Fm, m is the content of semi-fluorinated groups in polymer chain), the polymer solution aggregations were tuned by the evaporation temperature, which has achieved optimized P(BDT-Fm) films with high degree of molecular order, homogeneous polymer aggregates and small crystalline domains, leading to dramatic enhancement in thermoelectric performance. Compared with the naturally dried polymer films, the heated evaporating process led to at least 49-fold and 15-fold enhancement in the conductivity and power factor, respectively. Optical spectroscopy and microcosmic morphology have shown that the strong molecular motion of polymer chains at high evaporation temperature would disaggregate the polymer pre-aggregations in solution and assist the formation of orderly packed microstructures in solid state, leading to a significant increase in the power factor. Through controlling solution-state polymer aggregations, PBDT-F15 achieved the power factors up to 7.74 ± 0.01 µW m−1 K−2, which is among the highest values in the reported benzodithiophene-based conjugated polymers, highlighting the importance of controlling polymer solution aggregation in optimizing thermoelectric performance.

Enhancement of thermoelectric performance in Mg3(Sb,Bi)2 through engineered lattice strain and controlled carrier scattering

Mg3(Sb,Bi)2 emerges as a promising thermoelectric (TE) material due to its rich elemental composition and high TE performance, offering potential for industrial applications. In this study, we found out that the elevated sintering temperatures can introduce dislocations, vacancies, and nanoscale precipitates in the lattice, inducing desirable lattice strain. Higher lattice strain broadens the phonon dispersion, effectively reducing lattice thermal conductivity. Simultaneously, high-temperature sintering increases grain size, significantly mitigating grain boundary carrier scattering and enhancing carrier mobility. As a result, we observe a 31.4 % increase in lattice strain, a 30 % reduction in lattice thermal conductivity, a 200 % rise in room temperature mobility, ultimately achieving an outstanding figure of merit (zT) of 1.7 at 750 K for Mg3.22Ho0.03Sb1.5Bi0.5. The synergistic effect of engineering lattice strain to minimize lattice thermal conductivity and controlling carrier scattering mechanisms to enhance mobility significantly provides a novel strategy to improve TE performance of Mg3(Sb,Bi)2-based materials.

High performance dual-mode module for power generation and cooling based on optimized BiSbTe

Bismuth telluride is currently the most outstanding thermoelectric (TE) material near room temperature, and the high demand for solid-state refrigeration and low-grade waste heat recovery means there is an urgent need to improve its TE performance. This study investigates the enhanced TE performance of Bi0.4Sb1.6Te3 through the addition of Ag2Se. The introduction of Ag promotes an acceptor doping effect, leading to increased carrier concentration, while inhibiting lattice thermal conductivity together with Se doping. As a result, a notable enhancement in the figure of merit (ZT) to 1.35 at 373 K is achieved, with an average ZT of 1.17 spanning temperatures from 300 K to 523 K. More significantly, a 7-pair TE module was integrated using p-type Bi0.4Sb1.6Te3-0.00125Ag2Se and n-type extruded BiTeSe for cooling and power generation. A competitive conversion efficiency of 5.54 % at a temperature difference of 230 K and a maximum cooling temperature difference of 63.1 K at Th = 300 K are achieved, demonstrating the enormous practical application potential of this dual-mode module.

Synergistic enhancement of thermoelectric performance of n-type PbTe by resonant level and single-atom-layer vacancies

Modulating electrical and thermal transport through zero-dimensional point defects and one-dimensional dislocations are extensively investigated. However, the systematic study of exploring single-atom-layer vacancies for improving thermoelectric (TE) performance remains rare. In this study, we modulate the number density of single-atom-layer two-dimensional (2D) Te vacancies in lead telluride (PbTe) for the first time through bismuth and indium doping. DFT calculations reveal (Bi, In) co-doping lowers the formation energy of such defects, confirming the feasibility of this method. 2D Te vacancies simultaneously enhance phonon scattering and evoke bond softening, significantly lowering the lattice thermal conductivity. Meanwhile, indium doping generates resonant level in the energy band, which increases the carrier concentration without deteriorating the Seebeck coefficient drastically. As a result, a peak TE figure of merit (zT) of ∼1.6 and an average zT (zTavg) of 1.17 are achieved in the (Bi, In) co-doped PbTe. Furthermore, a high conversion efficiency of 6.5 % is obtained for the fabricated 2-pair module at a temperature difference of 500 K.

A Recipe for a Great Meal: A Benchtop Route from Elemental Se to Superior Thermoelectric β-Ag2Se.

The low-temperature modification of β-Ag2Se has proven to be useful as a near-room-temperature thermoelectric material. Over the past years, research has been devoted to interstitial, vacancy, and substitutional doping into the parent β-Ag2Se structure, aiming at tuning the material's charge and heat transport properties to enhance thermoelectric performance. The transformation of β-Ag2Se into α-Ag2Se at ∼134 °C and the low solubility of dopants are the main obstacles for the doping approach. Herein, we report a facile, safe, scalable, and cost-effective benchtop approach to successfully produce metal-doped β-Ag2Se. The doped materials display a remarkable enhancement of thermoelectric performance with a record-high peak zT of 1.30 at 120 °C and an average zT of ∼1.15 in the 25-120 °C range for 0.2 at. % Zn-doped Ag2Se. The enhancement in zT is attributed to point defects created by Zn doping into Ag vacancies/interstitials, which enhances the scattering of phonons and tunes the charge carrier properties, leading to the significant suppression of thermal conductivity. The simplicity of the synthetic method developed herein and the high performance of the final products provide an avenue to produce high-quality Ag2Se-based thermoelectric materials.

Enhancing thermoelectric performance of p-type SnTe through manipulating energy band structures and decreasing electronic thermal conductivity

Enhancing thermoelectric performance of p-type SnTe through manipulating energy band structures and decreasing electronic thermal conductivity

Roles of Cu doping in YbZn2Sb2 for thermoelectric performance enhancement

Roles of Cu doping in YbZn2Sb2 for thermoelectric performance enhancement

Effective Nonstoichiometric Strategy Combined Post-annealing Process for Boosting Thermoelectric Properties in n-Type PbTe.

The intrinsic low electrical properties have hindered the enhancement of thermoelectric performance for n-type PbTe over a long period of time, primarily due to the generation of intrinsic Pb vacancies and other defects. In this work, PbTe samples with nonstoichiometric excess Pb atoms were successfully prepared by a melting reaction followed by spark plasma sintering. First, the introduction of precisely controlled excess Pb atoms has effectively eliminated the typical p-n transition phenomenon in PbTe systems by suppressing the generation of Pb vacancies. Further, the vacuum annealing process employed in nonstoichiometric samples increases the carrier mobility significantly because of the improved crystallinity and the lowered holes. Thus, the Hall mobility was optimized from 754.3 to 1215.9 cm2 V-1 s-1, while the power factor was ultimately elevated from 3087.8 to 4565.7 μW m-1 K-2 for the Pb1.03Te sample at 323 K. Benefited from the enhanced electrical transport properties near room temperature, an average zT ∼ 1.03 ranging from 323 to 723 K was achieved, demonstrating an outstanding performance in n-type nondoped PbTe. This work provides guidance for optimizing the thermoelectric performance of n-type PbTe and relevant telluride by reducing vacancies and other defects.

Enhanced thermoelectric properties of Mg2Si0.3Sn0.7 via Bi-doping under high pressure

A series of n-type Mg2(Si0.3Sn0.7)1-xBix compounds with 0≤x≤0.02 were successfully synthesized through a combined approach involving solid-state reaction, high-pressure synthesis, and spark plasma sintering techniques. The method yielded homogeneously distributed single-phase materials at the micron scale, although minor compositional variations were detected within nanoscale precipitates embedded in the grains. The sample with Bi content of 0.01 exhibited enhanced thermoelectric properties, reaching a peak thermoelectric figure of merit (ZT) of 1.45 at 700 K while maintaining an average ZT of about 1.1 over the temperature range of 300−773 K. The enhancement in thermoelectric performance is ascribed to the optimized carrier concentration, band convergence, and refined microstructure. The findings suggest a simplified approach to doping that could be beneficial in creating high-performance thermoelectric materials for energy conversion.

Enhancement of thermoelectric performance in monolayer AlP3 via Ga and In doping: A first-principles study

Recently, two-dimensional (2D) triphosphides have attracted much attention in the thermoelectric field due to their semiconductor properties and excellent electronic transport properties. In our study, the performances of AlP3, Al0.5Ga0.5P3, and Al0.5In0.5P3 doped monolayers in the thermoelectric transport are studied. First, the dynamic structural stabilities of the three monolayers are verified through phonon dispersion curves. Moreover, all monolayers are indirect band-gap semiconductors, which can be used as thermoelectric materials. The results show that larger electronic relaxation time and carrier mobilities of doped monolayers lead to larger electronic transport parameters and higher dimensionless thermoelectric figure-of-merit (ZT). It is found that after doping via Ga and In, the thermoelectric performances of AlP3 are increased by 1.31–1.79 times. Our findings show that AlP3 and its doped monolayers hold significant promise as thermoelectric materials within the temperature span of 300–700 K and the doping mechanism could significantly improve the thermoelectric properties of 2D structures.

Enhancing thermoelectric performance of single-walled carbon nanotube films by poly(styrene sulfonate acid) dispersing and sequential base treatment

Despite single-walled carbon nanotubes (SWCNTs) have attracted great interest for thermoelectric (TE) conversion, the strong van der Waals forces between SWCNTs cause them to aggregate into bundles with poor dispersibility, which dramatically deteriorates the charge carrier transport, and thereby induces low electrical conductivity and TE power factor. Herein, we report a facile and efficient approach to enhance TE performance of SWCNTs via the addition of poly(styrene sulfonic acid) (PSSH) and sequential base treatment. PSSH is served as a dispersant and dopant to improve the dispersibility of SWCNTs and facilitate charge carrier conduction, contributing to high electrical conductivity of 3749 ± 122 S cm−1 and power factor of 117.7 ± 3.4 μW m−1 K−2 for 20 wt% PSSH/SWCNT composite film. Further base treatment induces partial removal of insulative PSSH and de-doping effect of SWCNTs, which decreases the barriers between adjacent nanotubes junctions and modulates charge carrier transport to synergistically optimize the electrical conductivity and Seebeck coefficient. With fine-tuning base treatment temperature, the PSSH/SWCNT composite film reaches highest power factor of 160.6 ± 7.9 μW m−1 K−2, higher than pristine SWCNTs of 70.2 ± 4.4 μW m−1 K−2. This work demonstrates the feasibility of this approach to boost the TE properties of SWCNTs, exhibiting potential application for high performance renewable energy harvesting.