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14 pages, 7908 KiB

Open AccessArticle

Microstructure Evolution and Mechanical Properties of Melt Spun Skutterudite-based Thermoelectric Materials

by Huiyuan Geng, Jialun Zhang, Tianhong He, Lixia Zhang and Jicai Feng

Materials 2020, 13(4), 984; https://doi.org/10.3390/ma13040984 - 22 Feb 2020

Cited by 14 | Viewed by 2791

Abstract

The rapid solidification of melt spinning has been widely used in the fabrication of high-performance skutterudite thermoelectric materials. However, the microstructure formation mechanism of the spun ribbon and its effects on the mechanical properties are still unclear. Here, we report the microstructure evolution [...] Read more.

The rapid solidification of melt spinning has been widely used in the fabrication of high-performance skutterudite thermoelectric materials. However, the microstructure formation mechanism of the spun ribbon and its effects on the mechanical properties are still unclear. Here, we report the microstructure evolution and mechanical properties of La–Fe–Co–Sb skutterudite alloys fabricated by both long-term annealing and melt-spinning, followed by sintering approaches. It was found that the skutterudite phase nucleated directly from the under-cooled melt and grew into submicron dendrites during the melt-spinning process. Upon heating, the spun ribbons started to form nanoscale La-rich and La-poor skutterudite phases through spinodal decomposition at temperatures as low as 473 K. The coexistence of the micron-scale grain size, the submicron-scale dendrite segregation and the nanoscale spinodal decomposition leads to high thermoelectric performance and mechanical strength. The maximum three-point bending strength of the melt spinning sample was about 195 MPa, which was 70% higher than that of the annealed sample. Full article

(This article belongs to the Special Issue Advanced Thermoelectric Materials)

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6 pages, 1024 KiB

Open AccessArticle

Low Temperature Joining and High Temperature Application of Segmented Half Heusler/Skutterudite Thermoelectric Joints

by Weian Wang, Xiaoya Li, Ming Gu, Yunfei Xing and Yefeng Bao

Materials 2020, 13(1), 155; https://doi.org/10.3390/ma13010155 - 31 Dec 2019

Cited by 7 | Viewed by 2322

Abstract

A low temperature joining process has been developed to fabricate segmented half Heusler/skutterudite thermoelectric joints, and high temperature service behavior of the joints has been studied. The microstructure and electrical resistance across the joint before and after aging were investigated. The joint is [...] Read more.

A low temperature joining process has been developed to fabricate segmented half Heusler/skutterudite thermoelectric joints, and high temperature service behavior of the joints has been studied. The microstructure and electrical resistance across the joint before and after aging were investigated. The joint is well bonded and no cracks appear at the interfaces of the joint before and after aging, which can attribute to the formation of high melting point intermetallic compounds. The electrical resistance crosses the bonding layer smoothly and the contact resistance is low. These results show the process is effective, and promising for preparation of segmented thermoelectric devices. Full article

(This article belongs to the Special Issue Advanced Thermoelectric Materials)

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12 pages, 4896 KiB

Open AccessArticle

Electrical Transport and Thermoelectric Properties of SnSe–SnTe Solid Solution

by Jun-Young Cho, Muhammad Siyar, Woo Chan Jin, Euyheon Hwang, Seung-Hwan Bae, Seong-Hyeon Hong, Miyoung Kim and Chan Park

Materials 2019, 12(23), 3854; https://doi.org/10.3390/ma12233854 - 22 Nov 2019

Cited by 23 | Viewed by 5014

Abstract

SnSe is considered as a promising thermoelectric (TE) material since the discovery of the record figure of merit (ZT) of 2.6 at 926 K in single crystal SnSe. It is, however, difficult to use single crystal SnSe for practical applications due to the [...] Read more.

SnSe is considered as a promising thermoelectric (TE) material since the discovery of the record figure of merit (ZT) of 2.6 at 926 K in single crystal SnSe. It is, however, difficult to use single crystal SnSe for practical applications due to the poor mechanical properties and the difficulty and cost of fabricating a single crystal. It is highly desirable to improve the properties of polycrystalline SnSe whose TE properties are still not near to that of single crystal SnSe. In this study, in order to control the TE properties of polycrystalline SnSe, polycrystalline SnSe–SnTe solid solutions were fabricated, and the effect of the solid solution on the electrical transport and TE properties was investigated. The SnSe_1−xTe_x samples were fabricated using mechanical alloying and spark plasma sintering. X-ray diffraction (XRD) analyses revealed that the solubility limit of Te in SnSe_1−xTe_x is somewhere between x = 0.3 and 0.5. With increasing Te content, the electrical conductivity was increased due to the increase of carrier concentration, while the lattice thermal conductivity was suppressed by the increased amount of phonon scattering. The change of carrier concentration and electrical conductivity is explained using the measured band gap energy and the calculated band structure. The change of thermal conductivity is explained using the change of lattice thermal conductivity from the increased amount of phonon scattering at the point defect sites. A ZT of ~0.78 was obtained at 823 K from SnSe_0.7Te_0.3, which is an ~11% improvement compared to that of SnSe. Full article

(This article belongs to the Special Issue Advanced Thermoelectric Materials)

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13 pages, 4061 KiB

Open AccessArticle

Enhanced Thermoelectric Cooling through Introduction of Material Anisotropy in Transverse Thermoelectric Composites

by Bosen Qian, Fei Ren, Yao Zhao, Fan Wu and Tiantian Wang

Materials 2019, 12(13), 2049; https://doi.org/10.3390/ma12132049 - 26 Jun 2019

Cited by 1 | Viewed by 2668

Abstract

Transverse thermoelectric materials can achieve appreciable cooling power with minimal space requirement. Among all types of material candidates for transverse thermoelectric applications, composite materials have the best cooling performance. In this study, anisotropic material properties were applied to the component phase of transverse [...] Read more.

Transverse thermoelectric materials can achieve appreciable cooling power with minimal space requirement. Among all types of material candidates for transverse thermoelectric applications, composite materials have the best cooling performance. In this study, anisotropic material properties were applied to the component phase of transverse thermoelectric composites. A mathematical model was established for predicting the performance of fibrous transverse thermoelectric composites with anisotropic components. The mathematical model was then validated by finite element analysis. The thermoelectric performance of three types of composites are presented, each with the same set of component materials. For each type of component, both anisotropic single-crystal and isotropic polycrystal material properties were applied. The results showed that the cooling capacity of the system was improved by introducing material anisotropy in the component phase of composite. The results also indicated that the orientation of the anisotropic component’s property axis, the anisotropic characteristic of a material, will significantly influence the thermoelectric performance of the composite. For a composite material consisting of Copper fiber and Bi₂Te₃ matrix, the maximum cooling capacity can vary as much as 50% at 300 K depending on the property axis alignment of Bi₂Te₃ in the composite. The composite with Copper and anisotropic SnSe single crystal had a 51% improvement in the maximum cooling capacity compared to the composite made of Copper and isotropic SnSe polycrystals. Full article

(This article belongs to the Special Issue Advanced Thermoelectric Materials)

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10 pages, 4545 KiB

Open AccessArticle

p-n Control of AlMgB₁₄-Based Thermoelectric Materials by Metal Site Occupancy

by Takuya Fujima, Natsuki Shimizu and Hideki Arimatsu

Materials 2019, 12(4), 632; https://doi.org/10.3390/ma12040632 - 20 Feb 2019

Cited by 6 | Viewed by 3378

Abstract

The mechanism for the p-n control of AlMgB₁₄-based thermoelectric material was investigated using Rietveld refinement and the first principle calculation. The p- and n-type AlMgB₁₄-based thermoelectric materials were prepared by spark plasma sintering (SPS) with changing raw powder mixture [...] Read more.

The mechanism for the p-n control of AlMgB₁₄-based thermoelectric material was investigated using Rietveld refinement and the first principle calculation. The p- and n-type AlMgB₁₄-based thermoelectric materials were prepared by spark plasma sintering (SPS) with changing raw powder mixture ratio. Temperature dependence of Seebeck coefficient and electrical conductivity were different between the two types of samples. Seebeck coefficient shifted from positive to negative with increasing the number of valence electrons in the metal sites calculated by the metal site occupancy. The density of states and electron density distribution indicated that the electrons transfer from metal atoms to the B atoms. Full article

(This article belongs to the Special Issue Advanced Thermoelectric Materials)

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12 pages, 2520 KiB

Open AccessArticle

Cu-Doped ZnO Electronic Structure and Optical Properties Studied by First-Principles Calculations and Experiments

by Zhanhong Ma, Fengzhang Ren, Xiaoli Ming, Yongqiang Long and Alex A. Volinsky

Materials 2019, 12(1), 196; https://doi.org/10.3390/ma12010196 - 8 Jan 2019

Cited by 128 | Viewed by 10226

Abstract

The band structure, the density of states and optical absorption properties of Cu-doped ZnO were studied by the first-principles generalized gradient approximation plane-wave pseudopotential method based on density functional theory. For the Zn_1-xCu_xO (x = 0, x [...] Read more.

The band structure, the density of states and optical absorption properties of Cu-doped ZnO were studied by the first-principles generalized gradient approximation plane-wave pseudopotential method based on density functional theory. For the Zn_1-xCu_xO (x = 0, x = 0.0278, x = 0.0417) original structure, geometric optimization and energy calculations were performed and compared with experimental results. With increasing Cu concentration, the band gap of the Zn₁-_xCu_xO decreased due to the shift of the conduction band. Since the impurity level was introduced after Cu doping, the conduction band was moved downwards. Additionally, it was shown that the insertion of a Cu atom leads to a red shift of the optical absorption edge, which was consistent with the experimental results. Full article

(This article belongs to the Special Issue Advanced Thermoelectric Materials)

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8 pages, 1847 KiB

Open AccessFeature PaperArticle

Effect of Oxygen Nonstoichiometry on Electrical Conductivity and Thermopower of Gd_0.2Sr_0.8FeO_3−δ Ferrite Samples

by Vyacheslav Dudnikov, Yury Orlov, Aleksandr Fedorov, Leonid Solovyov, Sergey Vereshchagin, Alexander Burkov, Sergey Novikov and Sergey Ovchinnikov

Materials 2019, 12(1), 74; https://doi.org/10.3390/ma12010074 - 26 Dec 2018

Cited by 1 | Viewed by 3289

Abstract

The behavior of the resistivity and thermopower of the Gd_0.2Sr_0.8FeO_3−δ ferrite samples with a perovskite structure and the sample stability in an inert gas atmosphere in the temperature range of 300–800 K have been examined. It has been [...] Read more.

The behavior of the resistivity and thermopower of the Gd_0.2Sr_0.8FeO_3−δ ferrite samples with a perovskite structure and the sample stability in an inert gas atmosphere in the temperature range of 300–800 K have been examined. It has been established that, in the investigated temperature range, the thermoelectric properties in the heating‒cooling mode are stabilized at δ ≥ 0.21. It is shown that the temperature dependencies of the resistivity obtained at different δ values obey the activation law up to the temperatures corresponding to the intense oxygen removal from a sample. The semiconductor‒semiconductor electronic transitions accompanied by a decrease in the activation energy have been observed with increasing temperature. It is demonstrated that the maximum thermoelectric power factor of 0.1 µW/(cm·K²) corresponds to a temperature of T = 800 K. Full article

(This article belongs to the Special Issue Advanced Thermoelectric Materials)

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10 pages, 2703 KiB

Open AccessArticle

Enhanced Thermoelectric Properties of Ca_3−xAg_xCo₄O₉ by the Sol–Gel Method with Spontaneous Combustion and Cold Isostatic Pressing

by Youyu Fan, Xiaoling Qi and Dechang Zeng

Materials 2018, 11(12), 2573; https://doi.org/10.3390/ma11122573 - 17 Dec 2018

Cited by 3 | Viewed by 3198

Abstract

In this study, Ca_3−xAg_xCo₄O₉ ceramics were synthesized by the sol–gel method combined with spontaneous combustion and cold isostatic pressing. The Ca_3−xAg_xCo₄O₉ ceramics were characterized via X-ray diffraction and scanning [...] Read more.

In this study, Ca_3−xAg_xCo₄O₉ ceramics were synthesized by the sol–gel method combined with spontaneous combustion and cold isostatic pressing. The Ca_3−xAg_xCo₄O₉ ceramics were characterized via X-ray diffraction and scanning electron microscopy. Thermoelectric properties of the ceramics were measured from 323 to 673 K. The results indicated that Ag doping significantly affected the microstructure and thermoelectric properties. With the increase in Ag content and gradual increase in electrical conductivity, the Seebeck coefficient first increased and then decreased, whereas the thermal conductivity exhibited the opposite case. The figure of merit, ZT, was 0.17 at 673 K for the Ca_2.8Ag_0.2Co₄O₉ sample. These results indicated that the thermoelectric properties could be optimized remarkably with the substitution of Ag. Full article

(This article belongs to the Special Issue Advanced Thermoelectric Materials)

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11 pages, 2595 KiB

Open AccessArticle

Effect of Oxygen Partial Pressure on the Phase Stability of Copper–Iron Delafossites at Elevated Temperatures

by Thomas Stöcker and Ralf Moos

Materials 2018, 11(10), 1888; https://doi.org/10.3390/ma11101888 - 2 Oct 2018

Cited by 15 | Viewed by 4660

Abstract

Oxide-based materials are promising candidates for use in high temperature thermoelectric generators. While their thermoelectric performance is inferior to commonly used thermoelectrics, oxides are environmentally friendly and cost-effective. In this study, Cu-based delafossites (CuFeO₂), a material class with promising thermoelectric properties [...] Read more.

Oxide-based materials are promising candidates for use in high temperature thermoelectric generators. While their thermoelectric performance is inferior to commonly used thermoelectrics, oxides are environmentally friendly and cost-effective. In this study, Cu-based delafossites (CuFeO₂), a material class with promising thermoelectric properties at high temperatures, were investigated. This work focuses on the phase stability of CuFeO₂ with respect to the temperature and the oxygen partial pressure. For this reason, classical material characterization methods, such as scanning electron microscopy, energy dispersive X-ray spectroscopy, and X-ray diffraction, were combined in order to elucidate the phase composition of delafossites at 900 °C at various oxygen partial pressures. The experimentally obtained results are supported by the theoretical calculation of the Ellingham diagram of the copper–oxygen system. In addition, hot-stage X-ray diffraction and long-term annealing tests of CuFeO₂ were performed in order to obtain a holistic review of the phase stability of delafossites at high temperatures and varying oxygen partial pressure. The results support the thermoelectric measurements in previous publications and provide a process window for the use of CuFeO₂ in thermoelectric generators. Full article

(This article belongs to the Special Issue Advanced Thermoelectric Materials)

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Review

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31 pages, 4893 KiB

Open AccessReview

TiO₂–SrTiO₃ Biphase Nanoceramics as Advanced Thermoelectric Materials

by Alexey Zavjalov, Sergey Tikhonov and Denis Kosyanov

Materials 2019, 12(18), 2895; https://doi.org/10.3390/ma12182895 - 7 Sep 2019

Cited by 13 | Viewed by 6348

Abstract

The review embraces a number of research papers concerning the fabrication of oxide thermoelectric systems, with TiO₂−SrTiO₃ biphase ceramics being emphasized. The ceramics is particularly known for a two-dimensional electron gas (2DEG) forming spontaneously on the TiO₂/SrTiO₃ [...] Read more.

The review embraces a number of research papers concerning the fabrication of oxide thermoelectric systems, with TiO₂−SrTiO₃ biphase ceramics being emphasized. The ceramics is particularly known for a two-dimensional electron gas (2DEG) forming spontaneously on the TiO₂/SrTiO₃ heterointerface (modulation doping), unlike ordinary 2DEG occurrence on specially fabricated thin film. Such effect is provided by the SrTiO₃ conduction band edge being 0.40 and 0.20 eV higher than that for anatase and rutile TiO₂, respectively. That is why, in the case of a checkered arrangement of TiO₂ and SrTiO₃ grains, the united 2D net is probably formed along the grain boundaries with 2DEG occurring there. To reach such conditions, there should be applied novelties in the field of ceramics materials science, because it is important to obtain highly dense material preserving small (nanoscale) grain size and thin interface boundary. The review also discusses some aspects of reactive spark plasma sintering as a promising method of preparing perovskite-oxide TiO₂−SrTiO₃ thermoelectric materials for high-temperature applications. Full article

(This article belongs to the Special Issue Advanced Thermoelectric Materials)

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