Dimensionless Figure Of Merit Value Research Articles

Improved Thermoelectric Performance in Ga‐ and Te‐Co‐introduced Tetrahedrite Cu12Sb4S13

As an earth‐abundant, environment‐friendly, and cost‐effective material, tetrahedrite Cu12Sb4S13 has attracted much attention in thermoelectrics (TEs). However, the TE performance of the pristine Cu12Sb4S13 (CSS) is mediocre, thereby much improvement is required. In this work, both the electronic and phonon transports of the CSS are engineered using Ga2Te3 as a dopant. In the results, it is indicated that dual substitutions of Ga for Cu and Te for Sb not only cause the enhancement of power factor via electronic band structure tuning, but also leads to a drop in both the electronic and lattice thermal conductivities. As a result, the total thermal conductivity (κ) decreases to 0.98 W m−1 K−1 at 770 K, which is about 58% that of the pristine CSS, thus improving the TE performance with the highest dimensionless figure of merit value increasing to 0.95 for (Cu12Sb4S13)0.9(Ga2Te3)0.1 at 773 K, 1.9 times that of the pristine CSS (≈0.5 at 770 K).

Extraordinary thermoelectric performance, thermal stability and mechanical properties of n-type Mg3Sb1.5Bi0.5 through multi-dopants at interstitial site

Mg3Sb2-based thermoelectric materials have attracted much interest since the discovery of their excellent n-type thermoelectric performance with excess Mg content and proper dopant. Herein, we proposed a doping diagram for Mg3Sb2-based material in a binary parameters space of electronegativity difference and atomic radius difference of extrinsic elements with Mg atom. Based on this doping diagram, we designed a multiple interstitial doped TMx:Mg3Sb1.5Bi0.5 (TM = CrMnFeCoCu) material, and achieved a high dimensionless figure of merit value of 0.76 at room temperature and 1.83 at 500 °C, respectively. The multiple interstitial dopants significantly suppress the formation of Mg vacancy, which is of great importance in protecting the electrical transport channel and improving the thermal stability. Besides, the mechanical properties are also enhanced due to the random distribution of elements in the matrix, impeding the migration of dislocations. Our work verifies the benefits of maximized sub-lattice disordering through the multi-elements doping strategy in the Mg3Sb1.5Bi0.5 material.

High-performance copper selenide thermoelectric thin films for flexible thermoelectric application

Flexible thermoelectric (TE) materials have wide applications in fabricating portable/wearable devices owing to the advantages of being bendable, small size, and lightweight. Inorganic material–based flexible thin films have attracted much attention owing to their high TE performance. So far, developing high TE properties and environment friendly flexible thin films for practical applications is still a considerable challenge. Copper selenide (Cu2Se) is a non-toxic and low-cost material, providing relatively safe TE modules for thin film devices. In this work, Cu2Se-based thin films with single crystal phase have been fabricated successfully at a flexible substrate by an efficient layer-by-layer combination reaction method. By optimizing the composition, a high power factor of 5.3 μWcm−1K−2 and dimensionless figure of merit value of 0.35 with low thermal conductivity are achieved at room temperature. Through bending tests, it has been shown that the prepared thin films possess good flexibility and the designed flexible device displays stable output power, which demonstrates the potential of inorganic TE materials to be mountable on flexible/wearable substrates for energy harvesting and management devices.

Thermoelectric properties of paracostibite fabricated using chemically synthesized Co–Sb–S nanoparticles as building blocks

Paracostibite (CoSbS) is a promising candidate for n-type thermoelectric materials. In this study, a nanostructured CoSbS pellet was fabricated using chemically synthesized Co–Sb–S nanoparticles as building blocks. The CoSbS pellet showed the highest electrical conductivity (σ = 141 S/cm at 661 K) and lowest thermal conductivity [κ ≅ 2 W/(m K)] among the reported CoSbS. Detailed analysis of the electrical transport process in a wide temperature range (3 K–661 K) revealed the existence of a donor level. At a temperature less than 170 K, the resistivity showed Mott variable-range hopping behavior, while the band conduction became dominant as the temperature increased. Nanograins in the pellet significantly enhanced phonon scattering, resulting in suppression of κ. The maximum dimensionless figure of merit value was 0.05 at 661 K, which is comparable with previously reported values for CoSbS TE materials.

Synergistic enhancement effect of SiC whisker and nano-particle on mechanical properties of Co4Sb11.5Te0.5 skutterudite

Co4Sb11.5Te0.5 hybrid composites with dual-nano inclusions (1 vol %) of SiC whiskers (SiCw) and SiC particles (SiCp) are prepared by spark plasma sintering. It is found that the hybrid composites exhibit further improved flexural strength and fracture toughness compared with those reinforced by SiCw or SiCp alone, indicating a striking synergistic enhancement effect. And the dimensionless figure of merit values of the hybrid composites remain nearly unchanged compared with the sample reinforced by individual inclusion. Our study provides an effective route to further improve the mechanical prosperities of thermoelectric materials.

Exploring electronic, optoelectronic, and thermoelectric properties of ternary compound MgSrSe2 from first-principles study

The electronic, optoelectronic, and thermoelectric properties of a ternary structure MgSrSe2 are investigated using density functional theory. MgSrSe2 is found to be a direct bandgap semiconductor with a bandgap of 2.04 eV. The photon energy calculated results show that the absorption spectra are in UV-A light, and MgSrSe2 could be applied for a photodetector. Optoelectronic properties, such as the dielectric function, absorption coefficient, reflectivity, refractive index, extinction coefficient, and energy-loss of MgSrSe2, are systematically discussed. The effective mass of the band edge curvature analysis indicates that the p-type MgSrSe2 is suitable for the thermoelectric material, and the maximum dimensionless figure of merit value can be up to 1.33 at 800 K. The results show that MgSrSe2 is a potential optoelectronic and thermoelectric material, and encourage further experimental works for its synthesis.

Synthesis of Tin-Doped FeVSb Half-Heusler System by Mechanical Alloying and Evaluation of Thermoelectric Performance

In the current work, we tried to investigate and compare the thermoelectric performance of the FeVSb1−xSnx (x = 0.0–0.05) half-Heusler system. The elemental mixture was formulated and subjected to mechanical alloying, and subsequently consolidated using vacuum hot pressing. X-ray diffractometry analysis was used to observe phase transition during the process, and scanning electron microscopy was utilized for microstructure analysis. After vacuum hot pressing, a near single phase of FeVSb1−xSnx (x = 0.0–0.05) was observed with small portions of second phases. The vacuum hot-pressed samples were analyzed to study the thermoelectric properties as a function of temperature up to 980 K. The n-type conduction was confirmed from Seebeck and Hall coefficients in the test range. In addition, the electrical conductivity showed combined conduction behavior in the test range, while the thermal conductivities showed abnormal behavior. Mass fluctuation scattering and changing in carrier concentration might have taken place during doping. The Seebeck coefficient and electrical conductivity of the system also decreased after doping. Dimensionless figure of merit values were calculated and compared with the results of analogue studies. The maximum ZT value was obtained for x = 0.01 at 553 K.

Microstructure and thermoelectric properties of hot-extruded Bi–Te–Se bulk materials

Highly dense n-type Bi2Te2.85Se0.15 bulk thermoelectric materials were fabricated by a mechanical alloying (MA) and hot-extrusion technique. The extrusion behavior, microstructure, texture, thermoelectric and mechanical properties have been investigated. The extruded samples exhibited fine-grained microstructure and preferred grain orientation. The basal planes in the extrudates were preferentially oriented parallel to the extrusion direction. As the extrusion temperature increased, the absolute value of the Seebeck coefficient and electrical resistivity were found to decrease, while the thermal conductivity increased. A largest dimensionless figure of merit value of 0.47 was achieved at room temperature for the sample extruded at 400 °C. Moreover, the extruded samples showed high hardness values.

High thermoelectric performance of all-oxide heterostructures with carrier double-barrier filtering effect

Thermoelectric materials can realize significant energy savings by generating electricity from untapped waste heat; however, the coupling of the thermoelectric parameters unfortunately limits their efficiency and practical applications. Herein, rational all-oxide TiC1-xOx@TiOy-TiO2 (x<1, 1<y<2) heterostructures with significantly enhanced thermoelectric properties have been designed, and a high dimensionless figure of merit (ZT) value of up to 0.84 at 973 K was achieved in the all-oxide TiC0.1O0.9@TiOy-TiO2 heterostructures, which is one of the highest values in n-type oxide bulk thermoelectric materials to date. The TiC1-xOx@TiOy heterostructures, which include a thin film of approximately 5–10 nm on the surface of TiC1-xOx compounds prepared by a facile anodization process, exhibit an obvious improvement of the thermoelectric power factor. Furthermore, an excellent dimensionless figure of merit value was obtained in the TiC1-xOx@TiOy-TiO2 heterostructures prepared by the anodization process assisted by the sol-gel chemical route, which can be attributed to the decrease in the carrier concentration via the carrier double-barrier filtering effect. This work develops a facile strategy for synthesizing core-shell heterostructures and demonstrates their superior ability to optimize thermoelectric energy harvesting. All-oxide heterostructures exhibiting a high thermoelectric performance have been fabricated by scientists in China and Japan. Thermoelectric materials are attractive for generating electricity from waste heat, but their performance is limited by interdependencies between their thermoelectric parameters. Now, Hongmin Zhu at University of Science and Technology Beijing and co-workers have used heterostructure engineering to produce all-oxide composites with a dimensionless figure of merit (ZT) of up to 0.84 at 973 kelvin. They ascribe the excellent thermoelectric properties of these heterostructures to an enhanced Seebeck coefficient resulting from a carrier double-barrier filtering effect. The potential barrier of the heterostructure interface can be engineered by varying the anodization voltage, the TiO2 coating content and the solid-solution level of TiC1−xOx. The composites were produced by a straightforward anodization process in combination with a sol-gel chemical route. The rational all-oxide TiC1-xOx@TiOy-TiO2 heterostructures with the significantly enhanced thermoelectric properties had been designed, and a high dimensionless figure of merit (ZT) value of up to 0.84 at 973 K was achieved in all-oxide TiC0.1O0.9@TiOy-TiO2 heterostructures.

Characterization and thermoelectric properties of Bi0.4Sb1.6Te3 nanostructured bulk prepared by mechanical alloying and microwave activated hot pressing

The p-type Bi0.4Sb1.6Te3 nanostructured bulk thermoelectric materials were obtained by mechanical alloying combined with a new consolidation technique of microwave activated hot pressing (MAHP). The effect of sintering temperature and holding time on the microstructure and thermoelectric properties of p-type Bi0.4Sb1.6Te3 bulks were investigated in detail. Compared with the traditional microwave pressureless sintering, the MAHP technique can be used to produce relatively compact bulks, caused by a uniaxial pressure. Due to the microwave arcing effect, the irregular shape nanograins are formed in situ in the MAHP samples. The coexistence of nano-sized grains and micro-sized grains can promote the effective scattering of phonons of a wide spectrum of wavelength and decrease the lattice thermal conductivity. A minimum lattice thermal conductivity of 0.41W/(mK) and a maximum dimensionless figure of merit value of 1.13 were obtained for the MAHP sample sintered at 375°C and 15min. The MAHP technique introduced in this work can be extended to other ceramics to obtain fine microstructures at low sintering temperatures in a very short period of time.

Extrusion Behavior and Thermoelectric Properties of Bi2Te2.85Se0.15 Thermoelectric Materials

Highly dense n-type Bi2Te2.85Se0.15 bulk thermoelectric materials were fabricated by mechanical alloying (MA) and hot-extrusion techniques. The extrusion behavior, microstructure, texture, thermoelectric and mechanical properties have been investigated. The extruded samples exhibited fine-grained microstructure and preferred grain orientation. The basal planes in the extrudates were preferentially oriented parallel to the extrusion direction. As the extrusion temperature increased, the electrical resistivity was found to decrease, while the Seebeck coefficient and thermal conductivity increased. A largest dimensionless figure of merit value of 0.6 was achieved at room temperature for the sample extruded at 360°C. Moreover, the extruded samples showed high hardness values.

Thermal Cycling Effects on the Thermoelectric Properties of n-Type In,Ce-Based Skutterudite Compounds

n-Type In-filled CoSb3 is a known skutterudite compound that has shown promising thermoelectric (TE) properties resulting in high dimensionless figure of merit values at elevated temperatures. Use in various waste heat recovery applications will require survival and operation after exposure to harsh thermal cycling environments. This research focused on uncovering the thermal cycling effects on TE properties of n-type In0.2Co4Sb12 and In0.2Ce0.15Co4Sb12 skutterudite compositions as well as quantifying their temperature-dependent structural properties (elastic modulus, shear modulus, and Poisson’s ratio). It was observed that the Seebeck coefficient and resistivity increased only slightly in the double-filled In,Ce skutterudite materials upon thermal cycling. In the In-filled skutterudites the Seebeck coefficient remained approximately the same on thermal cycling, while the electrical resistivity increased significantly after thermal cycling. Results also show that the thermal conductivity marginally decreases in the case of In-filled skutterudites, whereas the reduction is more pronounced in In,Ce-based skutterudite compounds. The possible reason for this kind of reduction can be attributed to grain pinning effects due to formation of nanoinclusions. High-temperature structural property measurements (i.e., Young’s modulus and shear modulus) are also reported. The results show that these structural properties decrease slowly as temperature increases and that the compounds are structurally stable after numerous (up to 200) thermal cycles.

Synthesis of In2−xGexO3 nanopowders for thermoelectric applications

Abstract

The effect of the precursor nanopowder size on the thermoelectric properties of nanostructured Bi–Sb–Te bulk materials

This paper presents the effect of precursor powder size on the thermoelectric properties of sintered nanostructured bulk materials. The transport properties of the nanostructured bulk show a dramatic size effect. There are a lower thermal and electrical conductivity for the bulk with smaller nanopowders. The dimensionless figure-of merit values ( ZT) of almost all the samples are much lower than those of the list reported data in the paper because the decrease in the thermal conductivity is counteracted by the reduction in the electrical conductivity and the Seebeck coefficient. The combination route of hydro/solvothermal synthesis and spark-plasma-sintering method provide a well controlled way to significantly reduce the thermal conductivity.

Thermoelectricity of CaIrO3 ceramics prepared by spark plasma sintering

Polycrystalline bulk CaIrO3 as a main phase with minor phases of IrO2 and/or Ca2IrO4 was prepared by spark plasma sintering (SPS) at 1273 K. Metastable CaIrO3 with a perovskite-type structure (pv-CaIrO3) formed by heat-treatment at 1273 K for 43.2 ks and transformed into a post-perovskite structure (ppv-CaIrO3) by heat-treatment at 1273 K for 259.2 ks. The electrical conductivity (σ) of the pv-CaIrO3 specimen decreased from 1.74 × 104 to 1.45 × 104 Sm-1, while the σ of the ppv-CaIrO3 specimen increased from 5.0 × 101 to 9.8 × 102 Sm-1 with increasing temperature from 298 to 1023 K. The Seebeck coefficient (S) of the pv-CaIrO3 specimen was around -44 μVK-1 at room temperature and increase to the highest value of 40 μVK-1 at 1023 K. The S of the ppv-CaIrO3 specimen decreased with increasing temperature and had an almost constant value of about 60 μVK-1 above 600 K. The thermal conductivity (κ) of the ppv-CaIrO3 specimen decreased from 2.7 to 1.2 Wm-1K-1 and that of the pv-CaIrO3 specimen decreased from 1.5 to 1.2 Wm-1K-1 with increasing temperature from 298 to 1023 K. The highest dimensionless figure of merit values (ZT) of the pv-CaIrO3 and the ppv-CaIrO3 specimens were 0.02 and 0.003 at 1023 K, respectively.

Synthesis and thermoelectric properties of CoSb 3 compounds by spark plasma sintering

Single-phase polycrystalline CoSb 3 skutterudite thermoelectric compounds have been synthesized by spark plasma sintering (SPS) and the thermoelectric properties including Seebeck coefficient, electrical resistivity and thermal conductivity are measured from room temperature to 800 K. The CoSb 3 compound with relative density of 97.4% and grain size of about 100 nm is achieved under the sintering condition of 873 K for 5 min, and a maximum thermoelectric dimensionless figure of merit value of 0.095 is obtained at about 673 K.