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Special Issue "Advances in Thermoelectric Materials"

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A special issue of Materials (ISSN 1996-1944). This special issue belongs to the section "Advanced Composites".

Deadline for manuscript submissions: closed (31 January 2017)

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Special Issue Editor

Guest Editor

Dr. Paz Vaqueiro

Department of Chemistry, School of Chemistry, Food and Pharmacy, University of Reading, Whiteknights, Reading, RG6 6AD, UK
Website | E-Mail
Interests: solid state chemistry; materials chemistry; materials for energy conversion technologies; solvothermal synthesis; sulfides; selenides; tellurides; crystallography; neutron diffraction; thermoelectric materials

Special Issue Information

Dear Colleagues,

Ensuring a sustainable energy supply is one of the grand challenges for science and technology in the 21st century. There is an urgent need for improved ways of generating power, without heavy reliance on fossil fuels. Thermoelectric devices, which exploit the Seebeck effect to provide direct conversion of thermal energy into electrical energy, offer considerable attractions for a more efficient use of existing energy resources. In particular, thermoelectric power generation enables useful electrical power to be extracted from waste heat. However, the performance, cost and availability of thermoelectric materials are significant barriers to the broad implementation of thermoelectric technology. Commercial thermoelectric devices are still largely based on bismuth telluride alloys, and their thermoelectric figure of merit, ZT ≈ 1, combined with the scarcity of tellurium, limit these devices to niche applications.

For these reasons, research in thermoelectric materials is very active worldwide, with the field rapidly advancing into entirely new classes of materials. This encompasses not only a wide range of inorganic materials, but also organic molecules and polymers. This Special Issue will focus on recent advances in thermoelectric materials. Potential topics include, but are not limited to:

New thermoelectric materials, as well as optimisation by doping of existing materials
Bulk inorganic thermoelectric materials
Organic and polymer thermoelectric materials
Nanoscale thermoelectric materials, including composites
Advances in thermoelectric materials synthesis and processing

It is my pleasure to invite you to submit a manuscript for this Special Issue. Full papers, communications, and reviews are all welcome.

Prof. Dr. Paz Vaqueiro
Guest Editor

Manuscript Submission Information

Manuscripts should be submitted online at www.mdpi.com by registering and logging in to this website. Once you are registered, click here to go to the submission form. Manuscripts can be submitted until the deadline. All papers will be peer-reviewed. Accepted papers will be published continuously in the journal (as soon as accepted) and will be listed together on the special issue website. Research articles, review articles as well as short communications are invited. For planned papers, a title and short abstract (about 100 words) can be sent to the Editorial Office for announcement on this website.

Submitted manuscripts should not have been published previously, nor be under consideration for publication elsewhere (except conference proceedings papers). All manuscripts are thoroughly refereed through a single-blind peer-review process. A guide for authors and other relevant information for submission of manuscripts is available on the Instructions for Authors page. Materials is an international peer-reviewed open access monthly journal published by MDPI.

Please visit the Instructions for Authors page before submitting a manuscript. The Article Processing Charge (APC) for publication in this open access journal is 1500 CHF (Swiss Francs). Submitted papers should be well formatted and use good English. Authors may use MDPI's English editing service prior to publication or during author revisions.

Keywords

thermoelectric materials
inorganic thermoelectric materials
organic thermoelectric materials
nanostructured thermoelectric materials

Published Papers (11 papers)

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Research

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Open AccessFeature PaperArticle Thermostability of Hybrid Thermoelectric Materials Consisting of Poly(Ni-ethenetetrathiolate), Polyimide and Carbon Nanotubes

by Keisuke Oshima, Shifumi Sadakata, Hitoshi Asano, Yukihide Shiraishi and Naoki Toshima

Materials 2017, 10(7), 824; doi:10.3390/ma10070824

Received: 31 May 2017 / Revised: 4 July 2017 / Accepted: 14 July 2017 / Published: 18 July 2017

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Three-component organic/inorganic hybrid films were fabricated by drop-casting the mixed dispersion of nanodispersed-poly(nickel 1,1,2,2-ethenetetrathiolate) (nano-PETT), polyimide (PI) and super growth carbon nanotubes (SG-CNTs) in N-methylpyrrolidone (NMP) at the designed ratio on a substrate. The dried nano-PETT/PI/SG-CNT hybrid films were prepared by the stepwise cleaning of NMP and methanol, and were dried once more. The thermoelectric properties of Seebeck coefficient S and electrical conductivity σ were measured by a thin-film thermoelectric measurement system ADVANCE RIKO ZEM-3M8 at 330–380 K. The electrical conductivity of nano-PETT/PI/SG-CNT hybrid films increased by 1.9 times for solvent treatment by clearing insulated of polymer. In addition, the density of nano-PETT/PI/SG-CNT hybrid films decreased 1.31 to 0.85 g·cm⁻³ with a decrease in thermal conductivity from 0.18 to 0.12 W·m⁻¹·K⁻¹. To evaluate the thermostability of nano-PETT/PI/SG-CNT hybrid films, the samples were kept at high temperature and the temporal change of thermoelectric properties was measured. The nano-PETT/PI/SG-CNT hybrid films were rather stable at 353 K and kept their power factor even after 4 weeks. Full article

(This article belongs to the Special Issue Advances in Thermoelectric Materials)

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Open AccessFeature PaperArticle Effect of Substitutional Pb Doping on Bipolar and Lattice Thermal Conductivity in p-Type Bi_0.48Sb_1.52Te₃

by Hyun-sik Kim, Kyu Hyoung Lee, Joonyeon Yoo, Jehun Youn, Jong Wook Roh, Sang-il Kim and Sung Wng Kim

Materials 2017, 10(7), 763; doi:10.3390/ma10070763

Received: 11 May 2017 / Revised: 29 June 2017 / Accepted: 4 July 2017 / Published: 6 July 2017

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Cation substitutional doping is an effective approach to modifying the electronic and thermal transports in Bi₂Te₃-based thermoelectric alloys. Here we present a comprehensive analysis of the electrical and thermal conductivities of polycrystalline Pb-doped p-type bulk Bi_0.48Sb_1.52Te₃. Pb doping significantly increased the electrical conductivity up to ~2700 S/cm at x = 0.02 in Bi_0.48-xPb_xSb_1.52Te₃ due to the increase in hole carrier concentration. Even though the total thermal conductivity increased as Pb was added, due to the increased hole carrier concentration, the thermal conductivity was reduced by 14–22% if the contribution of the increased hole carrier concentration was excluded. To further understand the origin of reduction in the thermal conductivity, we first estimated the contribution of bipolar conduction to thermal conductivity from a two-parabolic band model, which is an extension of the single parabolic band model. Thereafter, the contribution of additional point defect scattering caused by Pb substitution (Pb in the cation site) was analyzed using the Debye–Callaway model. We found that Pb doping significantly suppressed both the bipolar thermal conduction and lattice thermal conductivity simultaneously, while the bipolar contribution to the total thermal conductivity reduction increased at high temperatures. At Pb doping of x = 0.02, the bipolar thermal conductivity decreased by ~30% from 0.47 W/mK to 0.33 W/mK at 480 K, which accounts for 70% of the total reduction. Full article

(This article belongs to the Special Issue Advances in Thermoelectric Materials)

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Open AccessArticle Structure and Thermoelectric Properties of Bi_2−xSb_xTe₃ Nanowires Grown in Flexible Nanoporous Polycarbonate Templates

by Anuja Datta, Abhijeet Sangle, Nick Hardingham, Charles Cooper, Max Kraan, David Ritchie, Vijay Narayan and Sohini Kar-Narayan

Materials 2017, 10(5), 553; doi:10.3390/ma10050553

Received: 1 April 2017 / Revised: 10 May 2017 / Accepted: 13 May 2017 / Published: 19 May 2017

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We report the room-temperature growth of vertically aligned ternary Bi_2−xSb_xTe₃ nanowires of diameter ~200 nm and length ~12 µm, within flexible track-etched nanoporous polycarbonate (PC) templates via a one-step electrodeposition process. Bi_2−xSb_xTe₃ nanowires with compositions spanning the entire range from pure Bi₂Te₃ (x = 0) to pure Sb₂Te₃ (x = 2) were systematically grown within the nanoporous channels of PC templates from a tartaric–nitric acid based electrolyte, at the end of which highly crystalline nanowires of uniform composition were obtained. Compositional analysis showed that the Sb concentration could be tuned by simply varying the electrolyte composition without any need for further annealing of the samples. Thermoelectric properties of the Bi_2−xSb_xTe₃ nanowires were measured using a standardized bespoke setup while they were still embedded within the flexible PC templates. Full article

(This article belongs to the Special Issue Advances in Thermoelectric Materials)

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Open AccessArticle Thermoelectric Properties of Poly(3-Hexylthiophene) Nanofiber Mat with a Large Void Fraction

by Shogo Hiura, Naoki Okada, Junma Wakui, Hikari Narita, Shinji Kanehashi and Takeshi Shimomura

Materials 2017, 10(5), 468; doi:10.3390/ma10050468

Received: 20 February 2017 / Revised: 23 April 2017 / Accepted: 25 April 2017 / Published: 28 April 2017

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The thermoelectric properties of a poly(3-hexylthiophene) (P3HT) nanofiber mat which has higher crystallinity—and thus exhibits larger carrier mobility—than a non-fibrous P3HT film, were investigated. No significant difference was observed in the maximum values of the power factor between the P3HT nanofiber mat and the P3HT film. However, the thermal conductivity of the nanofiber mat was less than half that of the film despite having almost the same electrical conductivity. This higher thermoelectric property of the nanofiber mat than the film is attributed to the existence of highly effective conducting pathways and a large void fraction, and the result means that the nanofiber mat was a good candidate for use as a thermoelectric material. Full article

(This article belongs to the Special Issue Advances in Thermoelectric Materials)

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Open AccessArticle Thermoelectric Properties of Highly-Crystallized Ge-Te-Se Glasses Doped with Cu/Bi

by Bhuvanesh Srinivasan, Catherine Boussard-Pledel, Vincent Dorcet, Manisha Samanta, Kanishka Biswas, Robin Lefèvre, Franck Gascoin, François Cheviré, Sylvain Tricot, Michael Reece and Bruno Bureau

Materials 2017, 10(4), 328; doi:10.3390/ma10040328

Received: 10 January 2017 / Revised: 17 March 2017 / Accepted: 20 March 2017 / Published: 23 March 2017

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Chalcogenide semiconducting systems are of growing interest for mid-temperature range (~500 K) thermoelectric applications. In this work, Ge₂₀Te₇₇Se₃ glasses were intentionally crystallized by doping with Cu and Bi. These effectively-crystallized materials of composition (Ge₂₀Te₇₇Se₃)_100−xM_x (M = Cu or Bi; x = 5, 10, 15), obtained by vacuum-melting and quenching techniques, were found to have multiple crystalline phases and exhibit increased electrical conductivity due to excess hole concentration. These materials also have ultra-low thermal conductivity, especially the heavily-doped (Ge₂₀Te₇₇Se₃)_100−xBi_x (x = 10, 15) samples, which possess lattice thermal conductivity of ~0.7 Wm⁻¹ K⁻¹ at 525 K due to the assumable formation of nano-precipitates rich in Bi, which are effective phonon scatterers. Owing to their high metallic behavior, Cu-doped samples did not manifest as low thermal conductivity as Bi-doped samples. The exceptionally low thermal conductivity of the Bi-doped materials did not, alone, significantly enhance the thermoelectric figure of merit, zT. The attempt to improve the thermoelectric properties by crystallizing the chalcogenide glass compositions by excess doping did not yield power factors comparable with the state of the art thermoelectric materials, as these highly electrically conductive crystallized materials could not retain the characteristic high Seebeck coefficient values of semiconducting telluride glasses. Full article

(This article belongs to the Special Issue Advances in Thermoelectric Materials)

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Open AccessArticle ZT Optimization: An Application Focus

by Richard Tuley and Kevin Simpson

Materials 2017, 10(3), 309; doi:10.3390/ma10030309

Received: 3 February 2017 / Revised: 5 March 2017 / Accepted: 14 March 2017 / Published: 17 March 2017

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Significant research has been performed on the challenge of improving thermoelectric materials, with maximum peak figure of merit, ZT, the most common target. We use an approximate thermoelectric material model, matched to real materials, to demonstrate that when an application is known, average ZT is a significantly better optimization target. We quantify this difference with some examples, with one scenario showing that changing the doping to increase peak ZT by 19% can lead to a performance drop of 16%. The importance of average ZT means that the temperature at which the ZT peak occurs should be given similar weight to the value of the peak. An ideal material for an application operates across the maximum peak ZT, otherwise maximum performance occurs when the peak value is reduced in order to improve the peak position. Full article

(This article belongs to the Special Issue Advances in Thermoelectric Materials)

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Open AccessArticle Roles of Cu in the Enhanced Thermoelectric Properties in Bi_0.5Sb_1.5Te₃

by Feng Hao, Pengfei Qiu, Qingfeng Song, Hongyi Chen, Ping Lu, Dudi Ren, Xun Shi and Lidong Chen

Materials 2017, 10(3), 251; doi:10.3390/ma10030251

Received: 19 January 2017 / Revised: 18 February 2017 / Accepted: 24 February 2017 / Published: 1 March 2017

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Recently, Cu-containing p-type Bi_0.5Sb_1.5Te₃ materials have shown high thermoelectric performances and promising prospects for practical application in low-grade waste heat recovery. However, the position of Cu in Bi_0.5Sb_1.5Te₃ is controversial, and the roles of Cu in the enhancement of thermoelectric performance are still not clear. In this study, via defects analysis and stability test, the possibility of Cu intercalation in p-type Bi_0.5Sb_1.5Te₃ materials has been excluded, and the position of Cu is identified as doping at the Sb sites. Additionally, the effects of Cu dopants on the electrical and thermal transport properties have been systematically investigated. Besides introducing additional holes, Cu dopants can also significantly enhance the carrier mobility by decreasing the Debye screen length and weakening the interaction between carriers and phonons. Meanwhile, the Cu dopants interrupt the periodicity of lattice vibration and bring stronger anharmonicity, leading to extremely low lattice thermal conductivity. Combining the suppression on the intrinsic excitation, a high thermoelectric performance—with a maximum thermoelectric figure of merit of around 1.4 at 430 K—has been achieved in Cu_0.005Bi_0.5Sb_1.495Te₃, which is 70% higher than the Bi_0.5Sb_1.5Te₃ matrix. Full article

(This article belongs to the Special Issue Advances in Thermoelectric Materials)

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Open AccessArticle Large-Scale Surfactant-Free Synthesis of p-Type SnTe Nanoparticles for Thermoelectric Applications

by Guang Han, Ruizhi Zhang, Srinivas R. Popuri, Heather F. Greer, Michael J. Reece, Jan-Willem G. Bos, Wuzong Zhou, Andrew R. Knox and Duncan H. Gregory

Materials 2017, 10(3), 233; doi:10.3390/ma10030233

Received: 29 January 2017 / Revised: 16 February 2017 / Accepted: 21 February 2017 / Published: 26 February 2017

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A facile one-pot aqueous solution method has been developed for the fast and straightforward synthesis of SnTe nanoparticles in more than ten gram quantities per batch. The synthesis involves boiling an alkaline Na₂SnO₂ solution and a NaHTe solution for short time scales, in which the NaOH concentration and reaction duration play vital roles in controlling the phase purity and particle size, respectively. Spark plasma sintering of the SnTe nanoparticles produces nanostructured compacts that have a comparable thermoelectric performance to bulk counterparts synthesised by more time- and energy-intensive methods. This approach, combining an energy-efficient, surfactant-free solution synthesis with spark plasma sintering, provides a simple, rapid, and inexpensive route to p-type SnTe nanostructured materials. Full article

(This article belongs to the Special Issue Advances in Thermoelectric Materials)

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Open AccessArticle Long-Term High-Temperature Stability of Directionally Grown [Bi₂Ba₂O₄]_p[CoO₂] Rods

by Juan C. Diez, María A. Madre, Miguel A. Torres, Shahed Rasekh and Andrés Sotelo

Materials 2017, 10(2), 146; doi:10.3390/ma10020146

Received: 28 November 2016 / Revised: 18 January 2017 / Accepted: 26 January 2017 / Published: 8 February 2017

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[Bi₂Ba₂O₄]_p[CoO₂] thermoelectric ceramics have been successfully grown from the melt using the laser floating zone method, followed by a thermal treatment at 700 °C under air between 0 and 1532 h. The microstructural, thermoelectric, and mechanical properties were evaluated as a function of the thermal treatment length. Microstructure has shown that as-grown samples are composed of thermoelectric grains, together with a relatively high amount of secondary phases. Thermal treatment decreased the number and amount of secondary phases, producing nearly single-phase samples after 384 h. Consequently, the thermoelectric properties evaluated through the power factor showed a slight increase with the thermal treatment length, mainly due to the decrease of electrical resistivity, while the Seebeck coefficient was nearly unchanged. On the other hand, flexural strength was practically constant after 24 h thermal treatment. Full article

(This article belongs to the Special Issue Advances in Thermoelectric Materials)

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Review

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Open AccessReview Thermoelectric Transport in Nanocomposites

by Bin Liu, Jizhu Hu, Jun Zhou and Ronggui Yang

Materials 2017, 10(4), 418; doi:10.3390/ma10040418

Received: 23 November 2016 / Revised: 13 March 2017 / Accepted: 12 April 2017 / Published: 15 April 2017

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Thermoelectric materials which can convert energies directly between heat and electricity are used for solid state cooling and power generation. There is a big challenge to improve the efficiency of energy conversion which can be characterized by the figure of merit (ZT). In the past two decades, the introduction of nanostructures into bulk materials was believed to possibly enhance ZT. Nanocomposites is one kind of nanostructured material system which includes nanoconstituents in a matrix material or is a mixture of different nanoconstituents. Recently, nanocomposites have been theoretically proposed and experimentally synthesized to be high efficiency thermoelectric materials by reducing the lattice thermal conductivity due to phonon-interface scattering and enhancing the electronic performance due to manipulation of electron scattering and band structures. In this review, we summarize the latest progress in both theoretical and experimental works in the field of nanocomposite thermoelectric materials. In particular, we present various models of both phonon transport and electron transport in various nanocomposites established in the last few years. The phonon-interface scattering, low-energy electrical carrier filtering effect, and miniband formation, etc., in nanocomposites are discussed. Full article

(This article belongs to the Special Issue Advances in Thermoelectric Materials)

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Open AccessReview BiCuSeO Thermoelectrics: An Update on Recent Progress and Perspective

by Xiaoxuan Zhang, Cheng Chang, Yiming Zhou and Li-Dong Zhao

Materials 2017, 10(2), 198; doi:10.3390/ma10020198

Received: 26 December 2016 / Revised: 7 February 2017 / Accepted: 14 February 2017 / Published: 17 February 2017

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A BiCuSeO system has been reported as a promising thermoelectric material and has attracted great attention in the thermoelectric community since 2010. Recently, several remarkable studies have been reported and the ZT of BiCuSeO was pushed to a higher level. It motivates us to systematically summarize the recent reports on the BiCuSeO system. In this short review, we start with several attempts to optimize thermoelectric properties of BiCuSeO. Then, we introduce several opinions to explore the origins of low thermal conductivity for BiCuSeO. Several approaches to enhance thermoelectric performance are also summarized, including modulation doping, introducing dual-vacancies, and dual-doping, etc. At last, we propose some possible strategies for enhancing thermoelectric performance of BiCuSeO in future research. Full article

(This article belongs to the Special Issue Advances in Thermoelectric Materials)

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Planned Papers

The below list represents only planned manuscripts. Some of these manuscripts have not been received by the Editorial Office yet. Papers submitted to MDPI journals are subject to peer-review.

Title: Thermoelectric performances and long-term high-temperature stability of directionally grown Bi2Ba2Co2Ox rods
Author: J. Carlos Diez
Abstract: Since 1997, with the discovery of large thermoelectric (TE) properties in NaxCoO2 great efforts have been carried out to explore new CoO families with high TE performances. Following this intense research work, some layered cobaltites, such as Ca3Co4O9 and Bi2AE2Co2Ox (AE = alkaline earth) were also found to exhibit promising thermoelectric properties. As layered CoO oxides are materials with strong crystallographic, electrical and thermal anisotropy, a proper alignment of the grains is necessary to attain interesting TE properties in bulk samples. On the other hand, in order to be adequate for practical applications these materials should maintain their high TE properties, as well as their mechanical ones, at high temperatures for long time.
Taking into account these previously discussed effects, Bi2Ba2Co2Ox samples have been directionally grown from the melt using the laser floating zone (LFZ) technique. In order to study the samples behaviour at working temperatures, annealed samples have been maintained at 700ºC under air for different time lengths (0, 12, 24, 48, 96, 192, 384, 768 and 1536 hours). The evolution of microstructure, TE and mechanical properties has been studied as a function of the thermal treatment length.

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