Highest ZT Value Research Articles

Investigation of Thermal Properties of Half-Heusler Alloy FeVZ (Z= As, P, Sb) within Density Functional Theory

The figure of merit (ZT) holds an important role in thermoelectric material that indicates the efficiency of thermoelectric (TE) devices. The higher ZT value, the higher efficiency would be obtained of TE devices. One of the sub-class materials that has high potential application for thermoelectric material is half-Heusler (HH) alloy. In order to investigate the ZT value of HH conduct simulation using to investigate the ZT value of HH alloy FeVZ (Z= As, P, Sb) by using density-functional theory (DFT). Our research results show that FeVAs has higher ZT values than FeVP and FeVSb in the range temperature of 150-900 K. All of those compounds still have good ZT value around ~0.7 even in the temperature regime of 900 K, and shows a good trend for p-type thermoelectric materials.

A COMBINATION OF POINT DEFECTS AND NANOSIZED GRAINS TO MINIMIZE LATTICE THERMAL CONDUCTIVITY OF Sn AND Se CO-DOPED CoSb3 VIA MIXED BALL MILLING AND SPARK PLASMA SINTERING

The efficient strategies to minimize thermal conductivity in skutterudite materials are creating point defects along with nanosized grains. In this report, Sn and Se co-doped CoSb3 materials were synthesized through mixed-ball milling and spark plasma sintering techniques to utilize this strategy. Their phases, microstructure and thermoelectric properties were investigated under the content variation of Sn and Se in CoSb3 samples. The experimental results revealed that the Sn and Se were substituted at Sb sites in CoSb3 crystal structure and grain sizes were restricted to a hundred nanometer. The lattice thermal conductivity was reduced to 2.4[Formula: see text]W/mK at 298K. Interestingly, increasing Sn and Se doped content could further minimize the lattice thermal conductivity. The lowest value at room temperature is 1.79[Formula: see text]W/mK for CoSb[Formula: see text]Sn[Formula: see text]Se[Formula: see text] which was dramatically lower than pure CoSb3. Moreover, the increment of Sn and Se content also increased the electrical conductivity of doped samples, while the negative Seebeck coefficient sign tended to decrease. As expected, low electrical conductivity and substantial reduction in the Seebeck coefficient of doped samples at high measurement temperature, resulting in low power factor and low ZT values. It was clearly seen that the highest power factor of 880[Formula: see text][Formula: see text]W/mK2 was found at 516[Formula: see text]K in CoSb[Formula: see text]Sn[Formula: see text]Se[Formula: see text]. Furthermore, it also dominated the highest ZT value of 0.29 at 565 K, compared to the other Sn and Se co-doped samples. From these results, ball milling under dry conditions followed by wet conditions not only allowed a longer milling process but also generated a small fraction of pore which was a part of the reduction in thermal conductivity. Especially, the advantage of the existence of Sn and Se point defects and nanosized grains from this work will be escalated when it was applied to prepare materials that have high power factor values.

Exceptionally high open circuit thermoelectric figure of merit in two-dimensional tin sulphide

Thermoelectric materials with high values of power factor and thermoelectric figure of merit (ZT) are in great demand to make efficient thermoelectric devices. In this work, we explore the thermoelectric transport properties of layered tin sulphide (SnS) using first-principles method combined with Boltzmann transport theory. Our calculations show that the two-dimensional (2D) SnS materials have exceptionally high charge carrier mobilities and low lattice thermal conductivities as compared to other 2D materials such as graphene, phosphorene, MoS2, etc. Consequently, these 2D SnS materials have high power factor and ZT values.

Critical role of tellurium self-compensation in enhancing the thermoelectric performance of p-Type Bi0.4Sb1.6Te3 alloy

Traditionally, preparing p-type (Bi,Sb)2Te3 based materials by the combination of ball milling and hot pressing is regarded effective to reduce the lattice thermal conductivity and obtain high zT value. However, the deteriorated electrical transport properties and poor reproducibility restrict the further promotion of such technology. In this work, we prepare Bi0.4Sb1.6Te3 materials with Te self-compensation method, which leads to a high average zT value of 1.39 from 30 to 200 ℃ with significantly enhanced reproducibility. The Te self-compensation is found to be effective in manipulating the Bi(Sb)Te- and VTe2+ native defects in Bi0.4Sb1.6Te3 and thus suppress the “donor-like” effect caused by ball milling. The resulting enhanced carrier concentration further brings about a convergence of multi-valley bands, which leads to an increase in the density of states (DOS) near Fermi level and delays the onset of bipolar effect, in favor of improving electrical properties meanwhile reducing the bipolar thermal conductivity. Moreover, in the melting process, the excess Te facilitates the formation of Te-rich precipitates which remain in the ball-milled powders and then volatilize during the spark plasma sintering (SPS) process, thus leaving dispersedly distributed submicron pores and concequently reduced the lattice thermal conductivity. Taken together, the peak zT value reaches 1.55 at 100 ℃ and shows excellent reproducibility in three repeated measurements, which can promote the widespread application of the ball-milled (Bi,Sb)2Te3 based materials in solid-state refrigeration.

SrTiO3 Nanokubus: Sintesis, Kontrol Morfologi dan Sifat Termoelektrik (Tinjauan)

Thermoelectrics (TE), enabling the conversion of heat energy into electricity directly, provide the opportunity as alternative energy for conventional power generator with high efficiency. To date, bulk SrTiO3 nanocubes is the most promising oxide-based TE materials candidate. The nanoarchitecture of 3-dimensional SrTiO3 nanocubes (3D-STO) is the main target in realizing high performance TE materials with high ZT value comparable to commercially available telluride-based compound. The proper synthesis method is needed to form SrTiO3 with uniform nano-sized cubic particles. Liquid synthesis technique namely hydrothermal and solvothermal, succeeded in forming 20 nm of SrTiO3 nanocubes with high crystallinity and homogeneous morphology

Microstructural analysis and thermoelectric properties of skutterudite CoSb3 materials produced by melt spinning and spark plasma sintering

Skutterudite CoSb3 thermoelectric materials were synthesized by a combination of melt spinning (MS) and consolidation by spark plasma sintering (SPS). Microstructural analysis was carried out on the MS-SPS samples using various analytical techniques such as XRD, SEM, EBSD and DSC/TGA. In the melt-spun ribbons, the initial microstructure consisted of a multiphase mixture of Sb and CoSbx (x = 1, 2, or 3) where CoSb3 is the only thermoelectric phase. Subsequent SPS processing caused conversion of other phases present (Sb, CoSb and CoSb2) into single-phase CoSb3 and recrystallization concurrently. The phase conversion mechanism has been explained in the present work using a thermodynamic interpretation. Thermoelectric transport measurement results show that the thermal conductivity of MS-SPS sample was significantly decreased as compared with that of the sample prepared by traditional methods combined with SPS (TM-SPS) due to the reduction of its grain size. Compared with TM-SPS sample, MS-SPS sample has a high ZT value at room temprerature because of its high electrical conductivity and low lattice thermal conductivity.

The efficiency of short sintering time on thermoelectric properties of delafossite CuCr0:85Mg0:15O2 ceramics

Introduction: Harvesting the waste heat emitted from the activities of humanity based on thermoelectric devices is an appropriate way to reduce the overconsumption of fossil fuel nowadays.
 Methods: In this work, CuCr0:85Mg0:15O2 compounds prepared by conventional solid-state reaction method were investigated to find out that the short sintering time is enough for thermoelectric applications, directly low the cost of the devices.
 Results and Conclusion: We find out that there is a significant change in the crystal structure, the chemical state, and thermoelectric properties along with the increase of the sintering time, but eventually, the dimensionless figure of merit ZT is almost constant regardless of the long or short sintering time which means that the increase of electrical conductivity will compromise the increase of thermal conductivity. The highest ZT value is 0.03 measured at 500 oC for both samples prepared at the sintering time of 3 and 12 hours.

Properties of HPT-Processed Large Bulks of p-Type Skutterudite DD0.7Fe3CoSb12 with ZT > 1.3

The influence of shear strain on the microstructural, physical, and mechanical properties was studied on large bulk samples (diameter: 30 mm, thickness: 1 or 8 mm), which were consolidated by high-pressure torsion (HPT) from a commercial powder DD0.7Fe3CoSb12. Particularly, the thick sample (mass �53 g) allowed measuring the thermoelectric (TE) properties with respect to various orientations of the specimen in the sample. All data were compared with those of a hot-pressed (HP) reference sample, prepared with the same powder. Transmission electron microscopy, as well as X-ray powder diffraction profile analyses, Hall measurements, and positron annihilation spectroscopy, supported these investigations. Furthermore, synchrotron data for the temperature range from 300 to 825 K were used to evaluate the changes in the grain size and dislocation density as well as the thermal expansion coefficient via the change in the lattice parameter during heating. In addition, hardness and direct thermal expansion measurements of the HPT samples were performed and compared with the HP reference sample's values. With the increase of the shear strain from the center to the rim of the sample, the electrical resistivity becomes higher, whereas the thermal conductivity becomes lower, but the Seebeck coefficient remained almost unchanged. For the thin as well as thick samples, the enhanced electrical resistivity was balanced out by a decreased thermal conductivity such that the maximum ZT values (ZT = 1.3-1.35 at 856 K) do not vary much as a function of the shear strain throughout the sample, however, all ZTs are higher than that of the HP sample. The thermal-electric conversion efficiencies are in the range of 14-15 (for 423-823 K). With similar high ZT values for the n-type skutterudites, fabricated in the same fast and sustainable way, these p- and n-type skutterudites may serve as legs for TE generators, directly cut from the big HPT bulks. ©

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

Summary Discovering novel materials and attaining higher performance are the eternal pursuit of thermoelectric materials research. Here, we report a material series, (Cu1−xAgx)2(Te1−ySy) (0.16 ≤ x ≤ 0.24, 0.16 ≤ y ≤ 0.24), which adopts a complex orthorhombic structure differing from any known crystal structure of (Cu/Ag)2(S/Te). This material series is featured by the crystal-amorphicity duality induced by the large anionic size mismatch: a crystalline sublattice of highly size-mismatched anions Te/S coexists with an amorphous-like sublattice of cations Cu/Ag. In the context of structure-property correlation, the crystal-amorphicity duality gave rise to not only interesting electrical properties but also exceptionally low lattice thermal conductivities from 300 to 1,000 K. A state-of-the-art figure of merit zT of 2.0 is obtained in the x = y = 0.22 sample at 1,000 K. These results give insights into crystal-amorphicity duality as a paradigm-shifting materials design approach to develop high-performance thermoelectric materials.

Enhanced Thermoelectric Performance in High Entropy Alloys Sn0.25Pb0.25Mn0.25Ge0.25Te.

Entropy is a physical quantity gauging the degree of chaos in the system. High entropy alloying is thus an effective strategy to reduce the lattice thermal conductivity of the thermoelectric materials. In this paper, PbTe, GeTe, and MnTe are coalloyed with SnTe to form a single-phase solid solution. Because of the inclusion of various elements at the cationic (Sn2+) site, the configurational entropy increases, and the phonon scattering is strongly enhanced, leading to a reduced lattice thermal conductivity. In addition, the Seebeck coefficient is improved because of the band modification via this coalloying. Ga is then further doped to optimize the carrier concentration to ∼5.7 × 1020 cm-3 and reduce the room-temperature lattice thermal conductivity to ∼0.6 W m-1 K-1. Finally, a high peak ZT value of ∼1.52 at 823 K and an average ZT value ∼1.0 from 323 to 823 K were obtained in Ga0.025(Sn0.25Pb0.25Mn0.25Ge0.25)0.975Te.

Structural stability and thermoelectric properties of new discovered half-Heusler KLaX (X = C, Si, Ge, and Sn) compounds

Structural, elastic, electronic, and thermoelectric properties of KLaX (X = C, Si, Ge and Sn) half Heusler compounds have been studied using the full potential linearized augmented plane wave (FP-LAPW) method within density functional theory (DFT). It is shown that the KLaX half Heusler compounds are energetically and mechanically stable. The calculated band structure estimates with modified Becke-Johnson (mBJ) potential, indicates a semiconducting nature with direct band gaps of 0.76 eV, 0.79 eV, and 0.81 eV for KLaC, KLaSn, and KLaX (X = Si and Ge) respectively. The valence band of KLaX is more influenced by spin-orbit coupling than in the conduction band. The Seebeck coefficient (S), electronic conductivity (σ), electronic thermal conductivity (κe), figure of merit (ZT) and the power factor (PF) have been investigated using the semi-classical Boltzmann transport theory with rigid band theory. The KLaX half Heusler compounds exhibit high thermopower values and p-type charge carriers for their thermoelectric performance than electron doping. Including the spin-orbit coupling in our calculations increases the flatness of the valence band from X = Si to Sn. The high values of ZT and PF of KLaX half-Heusler compounds make it promising materials for thermoelectric applications.

Thermodynamic, mechanical stabilities and thermoelectric behavior of the XVSi (X = Co, Rh) half-Heuslers

Utilizing Boltzmann transport equations, structural, electronic, elastic, and thermoelectric properties, as well as thermodynamic stability of RhVSi and CoVSi half-Heusler compounds were investigated using the first-principle calculations in the framework of the density functional theory. The results of structural and elastic calculations and the thermodynamic phase diagrams indicate that these compounds are stable in the MgAgAs-type cubic structure with the F $$\overline{4}$$ 3m space group. The electronic structure was calculated using the Generalized Gradient Approximation with Tran–Blaha potential. It was found that both RhVSi and CoVSi are non-magnetic semiconductors with indirect gaps along X–W direction with values of 1.20 eV and 0.92 eV, respectively. High ZT values at 1000 K of 0.79 and 0.78 were obtained for RhVSi and CoVSi. The maximum value of the Seebeck coefficient (S) value for RhVSi and CoVSi is about $$244\,\frac{{\upmu {\text{V}}}}{{\text{K}}}$$ and $$226\,\frac{{\upmu {\text{V}}}}{{\text{K}}}$$ , respectively. The phonon dispersions reveal that the structures of these compounds are stable.

Pressure induced electronic, optical and thermoelectric properties of cubic SrZrO3: DFT investigation

FP-LAPW method is used to determine the electronic, optical, and thermoelectric characteristics of the SrZrO3. The band structure (BS) plot reveals indirect (Γ-M) energy gap for 0 GPa and 10 GPa. However, at 30 GPA, it converted to direct band gap. The total and partial density of states (PDOS) described the contribution of each atom in the material. The optical characteristics such as complex dielectric function, extinction and absorption coefficients, refractive index, and reflectivity are studied under pressure. The highest absorption peaks are originated due to the direct optical transition O-2p states to the Zr-4d orbitals. Moreover, thermoelectric properties are also investigated with respect to temperature. The Seebeck coefficient and electrical conductivity rises with the upsurge in pressure upto 40 GPa and highest value of ZT is 0.73. The simulated outcomes disclose that SrZrO3is a promising applicant for thermoelectric and optelctronic devices.

Point defects control in non-stoichiometric CuInTe2 compounds and its corresponding effects on the microstructure and thermoelectric properties

Thermoelectric properties of CuInTe2 compounds are constrained by characteristic low carrier concentration (~1017/cm3) and high thermal conductivity (~6 W/mK) at room temperature. These limitations need to be enhanced in order to make these compounds better thermoelectric materials. To resolve the aforementioned issues, we opted to introduce diverse point defects in pristine CuInTe2 through elemental composition control by either removing cation or adding anion, leading to non-stoichiometric compounds. Compounds with the general chemical composition of Cu1−xIn1−yTe2+z (x, y, z = 0, 0.05), representing Cu poor, In poor and Te rich, were studied in this work. The introduced charged point defects alter both the carrier concentration and tunes phonon scattering. It was found that the carrier concentration of the Cu poor sample increased by about 50 times to ~ 1019/cm3 compared to the pristine sample. As a result, the highest power factor of ~16.1 μW/cmK2 at 723 K was obtained in the Cu poor compound. In addition, a superlattice structure formed by partially ordered Cu vacancies was observed through field-emission transmission electron microscopy, which contributed enhanced phonon scattering. A high ZT value of ~0.98 was obtained at 823 K resulting from decreased lattice thermal conductivity and a record high power factor in the Cu poor condition.

Thermoelectric Performance Enhancement in BiSbTe Alloy by Microstructure Modulation via Cyclic Spark Plasma Sintering with Liquid Phase

AbstractThe widespread application of thermoelectric (TE) technology demands high‐performance materials, which has stimulated unceasing efforts devoted to the performance enhancement of Bi2Te3‐based commercialized thermoelectric materials. This study highlights the importance of the synthesis process for high‐performance achievement and demonstrates that the enhancement of the thermoelectric performance of (Bi,Sb)2Te3 can be achieved by applying cyclic spark plasma sintering to BixSb2–xTe3‐Te above its eutectic temperature. This facile process results in a unique microstructure characterized by the growth of grains and plentiful nanostructures. The enlarged grains lead to high charge carrier mobility that boosts the power factor. The abundant dislocations originating from the plastic deformation during cyclic liquid phase sintering and the pinning effect by the Sb‐rich nano‐precipitates result in low lattice thermal conductivity. Therefore, a high ZT value of over 1.46 is achieved, which is 50% higher than conventionally spark‐plasma‐sintered (Bi,Sb)2Te3. The proposed cyclic spark plasma liquid phase sintering process for TE performance enhancement is validated by the representative (Bi,Sb)2Te3 thermoelectric alloy and is applicable for other telluride‐based materials.

Effects of NbCl5-doping on the thermoelectric properties of polycrystalline Bi2S3

Bi2S3 is considered a potential thermoelectric material owing to its non-toxic and earth-abundant components and intrinsically low thermal conductivity. However, owing to its low electrical conductivity and the coupling relationship between the electric transport properties and thermal transport properties, achieving a high ZT value for polycrystalline Bi2S3 is a major challenge. In this work, a high-valence halogen compound, NbCl5, was employed to enhance the thermoelectric properties of polycrystalline Bi2S3 fabricated by the melting method combined with the spark plasma sintering technique. The enhanced electrical conductivity of polycrystalline Bi2S3 is derived from the increased carrier concentration, which is caused by the substitution of Nb and Cl ions into Bi and S sites, respectively. A peak power factor of 363 μWm−1K‒2 was obtained for Bi2S3 with 1.0 ​wt% NbCl5 doping at 623 ​K. Finally, a maximum ZT of ~0.28, which is approximately four times higher than that of the pristine Bi2S3 sample, was obtained. This work proves that NbCl5 is an effective n-type dopant for the Bi2S3 system, and indicates that high thermoelectric performance can be realized for Bi2S3 with environmentally friendly and low-cost elements.

Defective perovskites Cs2SeCl6 and Cs2TeCl6 as novel high temperature potential thermoelectric materials

On basis of our first principle calculations, we suggest lead free defective perovskites Cs2SeCl6 and Cs2TeCl6 as efficient high temperature thermoelectric materials. Our calculations predict high stability of these materials. The calculated flat valence bands in these compounds results high values of Seebeck coefficient. The density of states predicts the p-type conductivity in both compounds which was confirmed from positive value of Seebeck coefficient. The calculated thermal conductivity was found as low, 1.068 and 0.697 W/mK for Cs2SeCl6 and Cs2TeCl6, respectively at 800 K. This results high ZT values in comparison of lead halide perovskites. The combination of high electrical conductivity with Seebeck coefficient give rise to reasonable values of power factor 0.81 × 10−3 W/mK2 for Cs2SeCl6 and 0.83 × 10−3 W/mK2 for Cs2TeCl6 at 800 K. Furthermore, our calculated mechanical properties predicted the studied compounds as ductile. The ductile nature along with high values of thermoelectric parameters makes Cs2SeCl6 and Cs2TeCl6 compounds more promising for thermoelectric power generation at high temperatures.

First-Principles study of thermoelectric properties of Half-Metallic Non-Magnetic Half-Heusler alloy Mn2Ge

Research about Heusler alloys have been massively conducted through experimental and theoretical studies due to their fascinating properties. Most of Heusler alloy compounds exhibit half-metallic behavior, in which one of the spin channels of such a material behaves as a metal and the other behaves as an insulator because there is an open energy gap at the Fermi level. On the other hand, the thermoelectric properties of the materials usually work well in the insulator or semiconductor phase rather than in the metal phase. Motivated by such interesting phenomena, we conduct first-principles study to explore the thermoelectric properties of Mn2Ge, one of Heusler compounds that exhibits half-metallic behavior. Our calculation results show that Mn2Ge of the spin-down channel has a high Seebeck coefficient and ZT value (electron part) at room temperature around 892 MV/K and 0.97, respectively. The positive sign of the Seebeck coefficient on spin-up and spin down indicate that the majority charge carrier in Mn2Ge are positive charges (holes).

Excellent thermoelectric performance of boron-doped n-type Mg3Sb2-based materials via the manipulation of grain boundary scattering and control of Mg content

Thermoelectric devices require thermoelectric materials with high figure-of-merit (ZT) values in the operating temperature range. In recent years, the Zintl phase compound, n-Mg3Sb2, has received much attention owing to its rich chemistry and structural complexity. However, it hardly achieves high ZT values throughout the medium temperature range. Herein, by increasing the sintering temperature as much as possible, we successfully increased the average grain size of the compound by 15 times, and the grain boundary scattering was manipulated to obtain high carrier mobility of up to 180 cm2 V−1 s−1. Simultaneously, we optimized the Mg content for ultralow lattice thermal conductivity. We first doped the Mg3Sb2-based materials with boron for higher sintering temperature, good thermal stability, and higher hardness. The synergistic optimization of electrical and thermal transport resulted in excellent ZT values (0.62 at 300 K, 1.81 at 773 K) and an average ZT of 1.4 (from 300 to 773 K), which are higher than the state-of-the-art values for n-type thermoelectric materials, demonstrating a high potential in device applications.

A High Thermoelectric Performance <i>ZT</i> in p-Type LaSe<sub>2</sub> Crystal

In this work, we use first principles DFT calculations, anharmonic phonon scatter theory and Boltzmann transport method, to predict a comprehensive study on the thermoelectric properties as electronic and phonon transport of layered LaSe2 crystal. The flat-and-dispersive type band structure of LaSe2 crystal offers a high power factor. In the other hand, low lattice thermal conductivity is revealed in LaSe2 semiconductor, combined with its high power factor, the LaSe2 crystal is considered a promising thermoelectric material. It is demonstrated that p-type LaSe2 could be optimized to exhibit outstanding thermoelectric performance with a maximum ZT value of 1.41 at 1100K. Explored by density functional theory calculations, the high ZT value is due to its high Seebeck coefficient S, high electrical conductivity, and low lattice thermal conductivity .