Maximum ZT Value Research Articles

Effects of pressure and ions doping on the optimization of double filled CoSb3 thermoelectric materials

In order to investigate the synergistic effect of ion doping and pressure regulation, Te has been chosen as the doping element to prepare In0.05Ba0.15Co4Sb11.5Te0.5 materials at various synthesis pressures by high pressure and high temperature (HPHT) method. The electrical transport properties of the samples have been optimized effectively by Te doping, and the lattice thermal conductivities have been further reduced through introducing defects by high synthesis pressure. In the whole synthesis pressure range, the overall performances of samples increase with increasing synthesis pressures. The minimum lattice thermal conductivity of In0.05Ba0.15Co4Sb11.5Te0.5 sample prepared at 2.0 GPa reached 1.02 W m−1 K−1. Combined with the relative high power factor, a maximum ZT value of 1.23 has been reached.

Uniform dispersion of SiC in Yb-filled skutterudite nanocomposites with high thermoelectric and mechanical performance

Although filled skutterudites are very promising thermoelectric (TE) materials at intermediate temperature, most of the high-performance skutterudites reported are multiple-filled materials that tend to have poor stability and machinability. Here, we successfully fabricate single-filled skutterudite based composites with uniformly dispersed β-SiC nanoparticles via cryogenic grinding and spark plasma sintering. The nanocomposites show not only increased Seebeck coefficient owing to a possible energy filtering effect, but also reduced lattice thermal conductivity through enhanced phonon scattering. A maximum ZT value of 1.42 at 850 K with greatly improved mechanical properties is achieved, which is very attractive for the application in TE devices.

High Thermoelectric Performance in Two-Dimensional Tellurium: An Ab Initio Study.

In 2016, bulk tellurium was experimentally observed as a remarkable thermoelectric material. Recently, two-dimensional (2D) tellurium, called tellurene, has been synthesized and has exhibited unexpected electronic properties compared with the 2D MoS2. They have also been fabricated into air-stable and highly efficient field-effect transistors. There are two stable 2D tellurene phases. One (β-Te) has been confirmed with an ultralow lattice thermal conductivity (κL). However, the study of the transport properties of the other more stable phase, α-Te, is still lacking. Here, we report the thermoelectric performance and phonon properties of α-Te using Boltzmann transport theory and first-principles calculations. A maximum ZT value of 0.83 is achieved under a reasonable hole concentration, suggesting that the monolayer α-Te is a potential competitor in the thermoelectric field.

Thermoelectric properties and EPR analysis of Fe doped Cu12Sb4S13

Recently tetrahedrite compounds have attracted great attention due to their potential applications on the mid-temperature thermoelectric technologies. In this work, structural, Electron Paramagnetic Resonance (EPR) and the thermoelectric properties of tetrahedrite (Cu12-xFexSb4S13 where x = 0.0, 1.0, 1.5, 2.5) were investigated. The crystal structural, morphological properties and elemental compositions of the samples were investigated by powder X-ray diffraction (PXRD), scanning electron microscopy (SEM), and energy dispersive X-ray spectroscopy (EDS-EDX) tools respectively. Magnetic properties of the samples were performed by EPR techniques using electron spin resonance (ESR) spectrometer with the model of Bruker EMX series at X-band (9.5 GHz). The thermoelectric properties were assisted based on Seebeck coefficient, electrical resistivity, and thermal conductivity measurement between 300 and 600 K temperature range. PXRD patterns and SEM-EDX analysis confirmed the main phase of tetrahedrite structure for all compounds. The Seebeck coefficient sign indicated that holes were dominant carriers in all compounds. Electrical resistivity measurement showed an increment with increasing Fe concentration likely due to decreasing in carrier density. A typical behavior of intrinsic semiconductor was observed from temperature-dependent electrical resistivity measurements. The maximum ZT value of 0.6 was achieved for the sample Cu11Fe1Sb4S13 at 550 K. Electron Paramagnetic Resonance (EPR) and thermoelectric measurements exhibited that the maximum Fe concentration might be ≤ 1.0 for achieving high ZT value in Cu12-xFexSb4S13 compounds.

Thermoelectric Properties of Co- and Mn-Doped Al2Fe3Si3

Al2Fe3Si3 is a promising semiconductor for application as an environmentally friendly thermoelectric material. Its intrinsic carrier concentration is 5 × 1019 cm−3 and shows p-type conduction. In this article, we report on the thermoelectric properties of carrier-doped Al2Fe3Si3. Co or Mn was substituted for Fe for electron or hole doping, respectively. Al2Fe3−xMxSi3 (M=Co or Mn; x = 0.1–1.0 for Co and 0.1–0.3 for Mn) samples were synthesized by arc melting followed by spark plasma sintering and heat treatment. The Co- and Mn-doped samples displayed Hall carrier concentrations of 1.4 × 1020 cm−3 to 5.1 × 1020 cm−3 for n-type and 1.3 × 1020 cm−3 to 1.3 × 1021 cm−3 for p-type conduction. The n-type Al2Fe3Si3 exhibited a higher absolute value of Seebeck coefficient and lower Hall carrier mobility than p-type Al2Fe3Si3 at the same carrier concentration. The power factor increased with increasing carrier concentration for n-type conduction, and reached 0.65 × 10−3 W/mK at 520 K. On the other hand, the power factor for p-type Al2Fe3Si3 was not enhanced with increasing carrier concentration. The maximum ZT value for Co-substituted Al2Fe3Si3 was 0.09 at 600 K, which is 50% higher than that of pure Al2Fe3Si3.

Enhancement of thermoelectric properties of p-type single-filled skutterudites CexFeyCo4-ySb12 by tuning the Ce and Fe content

CoSb3-based filled skutterudites are known as good n-type thermoelectric (TE) materials. However, the p-type ones have not been intensively investigated. In this study, we focus on Ce-filled Fe-substituted CoSb3 as p-type skutterudites, where Fe will act as a hole dopant. Polycrystalline samples with nominal compositions of CexFeyCo4-ySb12 (x = 0.8, 0.9, 1.0 and y = 3.0, 3.5) are synthesized and the high temperature TE properties are investigated. All the samples show the p-type conduction as we expected. It is found that the samples with y = 3.5 exhibit clearly lower κlat values than those of the samples with y = 3.0, meaning that the Fe/Co substitution is more predominant than the Ce filling for the κlat reduction. The maximum zT value observed in this study is 0.78 at 704 K for the sample with x = 1.0, y = 3.5, which is close to the best value reported so far for Ce-filled Fe-substituted CoSb3-based p-type skutterudites.

Improvement of the Thermoelectric Properties of the Perovskite SrTiO3 by Cr-Doping

In recent years, among different thermoelectric materials, SrTiO3 have been receiving great attention due to their greater capacity and conversion between electrical energy and heat energy. The thermoelectric properties of the SrTiO3 can be improved by substitutional doping on different sites (A-site and B-site) in the lattice. In this study, the improvement of the thermoelectric performance of the perovskite SrTiO3 by Cr-doping has been investigated. The doped SrTiO3 with Cr was synthesized by the conventional solid-state reaction method. The electronic transport properties including Seebeck coefficient, electrical conductivity, and thermal transport properties in a moderate temperature regime from 300 to 900 K have been investigated. The large absolute value of the Seebeck coefficient with low thermal conductivity was achieved by Cr-doping. The electrical conductivity was quite low but increased with increasing doping level up to x = 0.002 mol, and; hence, the power factor increased with increasing doping level up to x = 0.002 mol. The maximum ZT value was observed for SrTi0.998Cr0.002O3 at 773 K by the combination of a high value of the Seebeck coefficient and low thermal conductivity.

Enhancement of the Thermoelectric Properties of Cu1.98Se by Na Doping

Cu1.98−xNaxSe nanopowders were synthesized using a hydrothermal method and hot-pressed into bulk pellets to assess the effects of Na doping on the thermoelectric properties of Cu1.98Se. The original layered microstructure was maintained at low doping levels, although nanopores, together with point defects, appeared to decrease the thermal conductivity. The thermoelectric properties of the material were improved by Na doping, resulting in a maximum ZT value of 1.32 for Cu1.94Na0.04Se at 973 K. The experimental data indicate that an optimal level of Na doping effectively improves the thermoelectric performance of non-stoichiometric Cu1.98Se.

Rapid preparation of Ge0.9Sb0.1Te1+x via unique melt spinning: Hierarchical microstructure and improved thermoelectric performance

Thermoelectric semiconductors based on GeTe has been proved to be an important materials for converting waste heat into electricity. In this study, we demonstrate an enhancement of thermoelectric performance for p-type Ge0.9Sb0.1Te1+x by a unique melt spinning method combined with hot pressing (MS-HP), through which increased Seebeck coefficients and reduced thermal conductivities are achieved simultaneously. Moreover, the MS technique greatly gives rise to a variety of microstructures, such as the fine Ge precipitates, twins and nano-domains, all of which contribute to the significantly reduced thermal conductivities. Finally, the Ge0.9Sb0.1Te1.03 reaches a maximum zT value of ∼1.9 at 760 K and a giant average zT of ∼1.2 between 300 K and 820 K, which is comparable to the reported results for GeTe based materials. Combined with the rapid fabrication, Ge0.9Sb0.1Te1+x prepared by melt spinning process is an attractive p-type material for thermoelectric power generation application.

Approaching Topological Insulating States Leads to High Thermoelectric Performance in n-Type PbTe.

Realizing high thermoelectric performance requires high electrical transport properties and low thermal conductivity, which are essentially determined by balancing the interdependent controversy of carrier mobility, effective mass, and lattice thermal conductivity. Here, we observed an electronic band inversion (approaching topological insulating states) in Sn and Se co-alloyed PbTe, resulting in optimizing effective mass and carrier mobility. The Sn alloying in PbTe(Se) can narrow its band gap due to band inversion and induce a sharper conduction band (equals to lower carrier mass), which further facilitates high carrier mobility, ∼251 cm2 V-1 s-1 in Pb0.89Sn0.11Te0.89Se0.11 at room temperature, thus leading to a high power factor. Meanwhile, we found that the lattice thermal conductivity κl can be reduced from ∼0.77 Wm-1 K-1 in PbTe to ∼0.45 Wm-1 K-1 in (Pb0.91Sn0.09)(Te0.91Se0.09) by producing point defects via Sn and Se co-alloying. Coupling reducing lattice thermal conductivity with integration of optimizing effective mass and carrier mobility bymeans of electronic band inversion, we obtained a maximum ZT value ∼1.4 at 773 K in n-type (Pb0.93Sn0.07)(Te0.93Se0.07).

Enhanced thermoelectric properties of Cu2−δSe nanopowder dispersed Bi2Ba2Co2Oy ceramics

The Bi2Ba2Co2Oy ceramics dispersed with x wt% Cu2−δSe (x = 0.00, 0.25, 0.50, 0.75, 1.00, 1.25 and 1.50) nanopowders were synthesized by the solid-state reaction method and the effects of Cu2−δSe dispersion on the thermoelectric properties were investigated. The thinner and better aligned lamellar structures were observed in the Cu2−δSe dispersed Bi2Ba2Co2Oy samples, which resulted in the significant reducing of thermal conductivities due to the enhanced phonon scattering by the lamellar structures. At the same time, the electrical resistivity was reduced evidently by an appropriate amount of Cu2−δSe dispersion owing to its low electrical resistivity. As a result, all the Cu2−δSe dispersed samples exhibited higher figure of merit (ZT), indicating that Cu2−δSe dispersion is an effective method to raise the thermoelectric properties of Bi2Ba2Co2Oy ceramic. The maximum ZT value 0.21 was achieved at 873 K when the Cu2−δSe amount is 0.5 wt%, which is increased about 35% compared with the undispersed sample.

High Thermoelectric Figure of Merit Achieved in Cu2S1- xTe x Alloys Synthesized by Mechanical Alloying and Spark Plasma Sintering.

Chalcogenides have been considered as promising thermoelectric materials because of their low cost, nontoxicity, and environmental benignity. In this work, we synthesized a series of Cu2S1- xTe x (0 ≤ x ≤ 1) alloys by a facile, rapid method of mechanical alloying combined with spark plasma sintering process. The Cu2S1- xTe x system provides an excellent vision of the competition between pure phase and phase transformation, entropy-driven solid solution, and enthalpy-driven phase separation. When the Te concentration increases, the Cu2S1- xTe x system changed from the pure monoclinic Cu2S at x = 0 to monoclinic Cu2S1- xTe x solid solution at 0.02 ≤ x ≤ 0.06 and then transforms to hexagonal Cu2S1- xTe x solid solution at 0.08 ≤ x ≤ 0.1. The phase separation of hexagonal Cu2Te in the hexagonal Cu2S matrix occurs at 0.3 ≤ x ≤ 0.7 and finally forms the hexagonal Cu2Te at x = 1. Owing to the changed band structure and the coexisted Cu2S and Cu2Te phases, greatly enhanced power factor was achieved in all Cu2S1- xTe x (0 < x < 1) alloys. Meanwhile, the point defect introduced by the substitution of Te/S atoms strengthened the phonon scattering, resulting in a lowered lattice thermal conductivity in most of these solid solutions. As a consequence, Cu2S0.94Te0.06 exhibits a maximum ZT value of 1.18 at 723 K, which is about 3.7 and 14.8 times as compared to the values of pristine Cu2S (0.32) and Cu2Te (0.08), respectively.

Thermoelectric performance of In0.8+yGa0.2Sb (0 ≤ y ≤ 0.06) ternary solid solutions with In excess

N-type In0.8+yGa0.2Sb(y = 0, 0.02, 0.04, 0.06) ternary solid solutions were prepared by B2O3 flux method followed by spark plasma sintering (SPS) technique. X-ray diffraction analysis and scanning electron microscopy observations show that all samples are composites containing both major InSb-rich solution phase and minor GaSb-rich solution phase. We found that increasing In excess content markedly increased the Seebeck coefficient while had a weak influence on the electrical conductivity of In0.8Ga0.2Sb. As a result, a maximum power factor of 5.2 mWm−1 K−2 was obtained for y = 0.06 at 680 K, two times that of the y = 0 sample. Ga alloying obviously reduces the lattice thermal conductivity. The lowest lattice thermal conductivity of 1.98 Wm−1 K−1 was obtained in the sample with y = 0 at 776 K. The phonon scattering mechanism was discussed. Due to the highest Seebeck coefficient, the maximum ZT value of 0.9 was obtained at 770 K for y = 0.06, which is one of the best results for (In, Ga)Sb based TE materials, suggesting application perspectives at intermediate temperatures. These results also demonstrate the crucial importance of controlling the In stoichiometry in the (In, Ga)Sb solid solutions to obtain high TE performance.

Enhanced thermoelectric performance of p-type Mg3Sb2 by lithium doping and its tunability in an anionic framework

Mg3Sb2-based Zintl compounds are considered as promising thermoelectric materials due to their low thermal conductivity and high Seebeck coefficient. In this paper, we studied the thermoelectric properties of p-type lithium (Li)-doped Mg3Sb2 prepared by ball milling and hot pressing. A maximum ZT value ~ 0.6 at about 773 K was achieved in Mg2.975Li0.025Sb2, which was ascribed to the increased carrier concentration by Li doping. Mixed scattering mechanism is dominated at lower temperature, yielding higher carrier mobility. Isoelectronic substitution of Zn on the Mg site further increased the electrical conductivity by narrowing the band gap and decreased the lattice thermal conductivity by double substitution, contributing to an enhanced average ZT in Mg2.475Zn0.5Li0.025Sb2.

Parametric analysis of ball milling condition on thermoelectric performance of Bi0.6FeCo3Sb12 skutterudite

The FeCo3Sb12 skutterudite crystal was doped with Bi to act as a filler material for the Bi0.6FeCo3Sb12 formulation. It was prepared by ball milling and spark plasma sintering. The microstructure and thermoelectric performance of the bulk skutterudite was evaluated as function of its mechanical alloying process, i.e. ball milling. Rietveld analysis of its XRD spectra indicated that The Bi doping on the Co4Sb12 based skutterudite succeeded in partially filling the voids of the skutterudite, whilst the Fe doping partially substituted the Co sites in the lattice. This serves to simultaneously increase the electrical conductivity whilst reducing the thermal conductivity of the resulting skutterudite. The ball milling times was correlated to the resulting microstructure, and ultimately, its thermoelectric performance. It was found that the moderate milling times (at 15 h) resulted in the best electrical conductivity, given the homogenous distribution of particles. A Maximum ZT value was observed 0.18 for 10 h ball milled sample at 673 K, whilst almost the same value was achieved for the 15 h ball milled sample, i.e. ZT = 0.17 at 673 K. Beyond this milling time, agglomeration of particles after ball milling caused degradation in the overall thermoelectric performance. Thus, this paper presents a strategy optimize the mechanical alloying process parameters to provide improved thermoelectric performance.

Enhanced Thermoelectric Performance of Conjugated Polymer/Single-Walled Carbon Nanotube Composites with Strong Stacking

We design and synthesize a novel donor–acceptor-type BDT-based conjugated polymer,PBDTDTffBT, and prepare composite films containing single-walled carbon nanotubes (SWCNTs) and the PBDTDTffBT polymer. Strong π–π interfacial interactions are present between the PBDTDTffBT polymer and the SWCNTs, and we systematically study the effects of different mass ratios of PBDTDTffBT to SWCNTs on the thermoelectric (TE) properties. The maximum electrical conductivity, Seebeck coefficient, power factor, and ZT value of the composite films are 529.3 S cm–1, 68.1 μV K–1, 80.9 μW m–1 K–2, and 0.028 at room temperature, respectively. Additionally, the composite film with a PBDTDTffBT:SWCNT mass ratio of 1:10 exhibits the highest power factor of 116.7 μW m–1 K–2 at approximately 95 °C. This study has a large potential to broaden the research scope of composite TE materials.

Enhanced figure of merit of poly(9,9‐di‐n‐octylfluorene‐alt‐benzothiadiazole) and SWCNT thermoelectric composites by doping with FeCl3

ABSTRACTPoly(9,9‐di‐n‐octylfluorene‐alt‐benzothiadiazole (F8BT) generally has a large Seebeck coefficient, and single‐walled carbon nanotubes (SWCNTs) have high electrical conductivity. In this work, we prepared F8BT/SWCNT composites to combine the good Seebeck coefficient of the polymer and the excellent electrical conductivity of SWCNTs to achieve enhanced thermoelectric properties. For the composite materials, the maximum power factor of 1 μW mK−2 was achieved when the SWCNT content was 60%, with the maximum ZT value of 4.6 × 10−4. After ferric chloride was employed as the oxidative dopant for the composites, the electrical conductivity of the composites improved significantly. The maximum value of power factor (1.7 μW mK−2) was achieved when the SWCNT content was 60%, and the ZT value of 7.1 × 10−4 was about 1.5 times as high as that of the composites with undoped F8BT. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019, 136, 47011.

Enhancement of the Thermoelectric Figure of Merit in Blended Cu2Sn1–xZnxS3 Nanobulk Materials

Among the promising waste heat harvesting technologies, thermoelectric materials and devices have attracted considerable interest in recent decades because they offer a promising route to convert waste heat into electrical power. The Cu2Sn1–xZnxS3 system consists of green and abundant elements and exhibits modest thermoelectric performance at a relatively low temperature range. However, the thermoelectric figure of merit, ZT, of Cu2Sn1–xZnxS3 is still too low for practical applications. This study encompasses the bottom-up wet chemical synthesis of Cu2Sn1–xZnxS3 nanoparticles with varying Zn compositions. By blending Cu2Sn0.85Zn0.15S3 and Cu2Sn0.9Zn0.1S3 nanoparticles in a weight fraction of 9:1 to make a nanobulk material, we could achieve a maximum ZT value of 0.64 at 670 K, which is 1.7 and 1.9 times higher than the ZT values of the pristine Cu2Sn0.85Zn0.15S3 and Cu2Sn0.9Zn0.1S3 nanobulk materials, respectively.

Impact of Ni Content on the Thermoelectric Properties of Half-Heusler Thermoelectrics

Motivated by high peak zT values reported 13 years ago for the (Ti,Zr,Hf)NiSn Half-Heusler system, Half-Heusler materials are identified as a promising materials class for thermoelectric applications. We investigate the TiNiSn sub-system, which does not rely on expensive Hf. From phase boundary mapping, we find that the thermoelectric TiNiSn Half-Heusler phase shows a narrow solubility range on the Ti-Ni-Sn phase diagram primarily in the range of excess Ni that can be approximated as TiNi1+xSn, where x is temperature dependent with 0 ≤ x ≤ 0.06 at 1223 K. Four phase boundary compositions with different Ni contents associated with four three-phase regions are identified. We characterize the thermoelectric properties of these stable compositions and find significant difference between Ni-rich and Ni-poor phase boundary compositions of TiNiSn, which amounts up to 41%, 58%, and 25% difference in Seebeck coefficient, lattice thermal conductivity, and thermoelectric figure of merit respectively. This explains the large discrepancy of literature data on the thermoelectric properties of TiNiSn within a unified phase diagram framework. We demonstrate that Ni-rich TiNiSn results in a narrower band gap using the Goldsmid formula, which we interpret to be due to the formation of an impurity band from interstitial Ni in the forbidden gap as previously suggested. Interstitial Ni atoms scatter both electrons and phonons, with the latter effect being much stronger, thus a lower lattice thermal conductivity compensates the decrease in electron mobility leading to a high zT value of 0.6 at 850 K for intrinsic Ni-rich TiNiSn. With Sb doping, the carrier concentration in these stable boundary compositions can be tuned but the distinct features in their transport properties remain unchanged. A maximum zT value of 0.6 was also achieved at 850 K for intrinsic Ni-poor TiNiSn upon Sb doping. We further present results on the integration of n-type TiNiSn Half-Heusler material into 36-legs modules in conjunction with p-type FeTiNb Half-Heusler material including their multi-physics modelling-assisted matching and integration into a heat exchanger mounted and tested in the exhaust gas system of a compact van. Y. Tang, X. Li, L.H.J. Martin, E. Cuervo Reyes, T. Ivas, C. Leinenbach, S. Anand, M. Peters, G.J. Snyder, C. Battaglia, Energy & Environmental Science, 2018, 11, 311 D. Landmann, Y. Tang, D. Widner, R. Huber, P. Rickhaus, R. Widmer, C. Battaglia, in preparation

Thickness and temperature dependent thermoelectric properties of Bi87Sb13 nanofilms measured with a novel measurement platform

The temperature and thickness dependent thermoelectric properties of Bi87Sb13 nano-films with a thickness from 84 nm to 282 nm have been studied in a temperature range from 110 K up to 450 K. The films have been prepared by thermal evaporation of the raw material from an Al2O3 coated tungsten boat under vacuum conditions of at least 10−6 mbar. The measurements have been performed using a novel measurement platform, which allows the nearly simultaneous characterization of the thermal conductivity, electrical conductivity, the Seebeck coefficient and the Hall coefficient. All properties are measured in the in-plane direction at the same sample within one measurement run, avoiding many sources of uncertainties and allowing the calculation of the direction dependent, in-plane thermoelectric figure of Merit ZT with high precision. The maximum ZT value of 0.28 has been obtained for the thickest sample at a temperature of 265 K. All comparative measurements have been performed after an initial thermal annealing step, as the heat treatment shows a strong impact on the thermoelectric performance of the films.