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Materials Processing and Crystal Growth for Thermoelectrics

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A special issue of Crystals (ISSN 2073-4352). This special issue belongs to the section "Inorganic Crystalline Materials".

Deadline for manuscript submissions: closed (31 July 2017) | Viewed by 59633

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Prof. Dr. George S. Nolas

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Department of Physics, University of South Florida, Tampa, FL 33620, USA
Interests: structure–property relationships and thermal properties of materials; the effect of structural and chemical variations on the transport, optical, and magnetic properties of materials
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Dear Colleagues,

Due to formidable synthetic challenges, many materials of scientific and technological interest are not readily accessible. Often poor quality materials, due to the difficulty in preparation of such materials in high purity and high yield, results in erroneous interpretations of their observed physical, and sometimes structural properties. New or improved processing techniques are crucial in order to investigate the intrinsic properties of new and novel materials and elucidate their fundamental physical properties in order to apply them to technologically important applications. One of the important technological challenges today is energy sustainability. Increasing awareness and concern for energy resources and the environment has stimulated advances in materials for energy conversion and refrigeration in recent years. Thermoelectrics offer the possibility of an all-solid-state technology for power conversion, refrigeration, temperature stability and temperature control. In many cases, modifications of the processing conditions allow for modifications of the transport properties, thus providing an avenue for “tuning” of the thermoelectric properties. Cost effective scale-up is also of interest for eventual mass-production processing.

The Special Issue on “Materials Processing and Crystal Growth for Thermoelectrics” is intended to provide a unique international forum aimed at covering a broad description of results involved with the processing and crystal growth of thermoelectric materials, as well as physical properties characterization needed in order to understand the important influences materials processing has on the properties of materials. Scientists working in a wide range of disciplines are invited to contribute to this special issue.

The topics summarized by the keywords broadly cover some of the different examples of sub-topics.The volume is, however, open for contributions involving all aspects of the different processing techniques and new material developments for thermoelectric applications.

Prof. George S. Nolas
Guest Editor

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Keywords

materials processing for thermoelectrics
crystal growth and synthetic routes
structure-property and processing-property relationships
transport properties
mechanical properties

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Research

9 pages, 3944 KiB

Open AccessArticle

Simultaneous Enhancement of Electrical Conductivity and Seebeck Coefficient of [6,6]-Phenyl-C71 Butyric Acid Methyl Ester (PC₇₀BM) by Adding Co-Solvents

by Mina Rastegaralam, Changhee Lee and Urszula Dettlaff-Weglikowska

Crystals 2018, 8(6), 237; https://doi.org/10.3390/cryst8060237 - 26 May 2018

Cited by 3 | Viewed by 3835

Abstract

Chemical modification by co-solvents added to [6,6]-Phenyl-C71 butyric acid methyl ester, commonly known as an n-type semiconducting fullerene derivative PC₇₀BM, is reported to change the electrical and thermoelectric properties of this system. Power factor of the casted PC₇₀BM samples achieves values higher than that determined for a variety of organic compounds, including conducting polymers, such as PEDOT:PSS in the pristine form. After chemical functionalization by different solvents, namely N,N-Dimethylformamide (DMF), dimethyl sulfoxide (DMSO), N-Methyl-2-pyrrolidone (NMP), acetonitrile (AC), and 1,2-Dichloroethane (DCE), the four-probe in-plane electrical conductivity and Seebeck coefficient measurements indicate a simultaneous increase of the electrical conductivity and the Seebeck coefficient. The observed effect is more pronounced for solvents with a high boiling point, such as N,N-Dimethylformamide (DMF), dimethyl sulfoxide (DMSO), and N-Methyl-2-pyrrolidone (NMP), than in acetonitrile (AC) and 1,2-Dichloroethane (DCE). We identified the origin of these changes using Hall mobility measurements, which demonstrate enhancement of the PC₇₀BM charge carrier mobility upon addition of the corresponding solvents due to the improved packaging of the fullerene compound and chemical interaction with entrapped solvent molecules within the layers. Full article

(This article belongs to the Special Issue Materials Processing and Crystal Growth for Thermoelectrics)

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2020 KiB

Open AccessArticle

Solvent-Dependent Thermoelectric Properties of PTB7 and Effect of 1,8-Diiodooctane Additive

by Mina Rastegaralam, Changhee Lee and Urszula Dettlaff-Weglikowska

Crystals 2017, 7(10), 292; https://doi.org/10.3390/cryst7100292 - 29 Sep 2017

Cited by 9 | Viewed by 5438

Abstract

Conjugated polymers are considered for application in thermoelectric energy conversion due to their low thermal conductivity, low weight, non-toxicity, and ease of fabrication, which promises low manufacturing costs. Here, an investigation of the thermoelectric properties of poly({4,8-bis[(2-ethylhexyl)oxy]benzo [1,2-b:4,5-b′] dithiophene-2,6-diyl}{3-fluoro-2-[(2-ethylhexyl) carbonyl]thieno[3,4-b] thiophenediyl}), commonly known as PTB7 conjugated polymer, is reported. Samples were prepared from solutions of PTB7 in three different solvents: chlorobenzene, 1,2-dichlorobenzene, and 1,2,4-trichlorobenzene, with and without 1,8-diiodooctane (DIO) additive. In order to characterize their thermoelectric properties, the electrical conductivity and the Seebeck coefficient were measured. We found that, by increasing the boiling point of the solvent, both the electrical conductivity and the Seebeck coefficient of the PTB7 samples were simultaneously improved. We believe that the increase in mobility is responsible for solvent-dependent thermoelectric properties of the PTB7 samples. However, the addition of DIO changes the observed trend. Only the sample prepared from 1,2,4-trichlorobenzene showed a higher electrical conductivity and Seebeck coefficient and, as a consequence, improved power factor in comparison to the samples prepared from chlorobenzene and 1,2-dichlorobenzene. Full article

(This article belongs to the Special Issue Materials Processing and Crystal Growth for Thermoelectrics)

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2920 KiB

Open AccessArticle

Microstructure Evolution of Ag-Alloyed PbTe-Based Compounds and Implications for Thermoelectric Performance

by Tom Grossfeld, Ariel Sheskin, Yaniv Gelbstein and Yaron Amouyal

Crystals 2017, 7(9), 281; https://doi.org/10.3390/cryst7090281 - 18 Sep 2017

Cited by 13 | Viewed by 5608

Abstract

We investigate the microstructure evolution of Ag-alloyed PbTe compounds for thermoelectric (TE) applications with or without additions of 0.04 at. % Bi. We control the nucleation and temporal evolution of Ag₂Te-precipitates in the PbTe-matrix applying designated aging heat treatments, aiming to achieve homogeneous dispersion of precipitates with high number density values, hypothesizing that they act as phonon scattering centers, thereby reducing lattice thermal conductivity. We measure the temperature dependence of the Seebeck coefficient and electrical and thermal conductivities, and correlate them with the microstructure. It is found that lattice thermal conductivity of PbTe-based compounds is reduced by controlled nucleation of Ag₂Te-precipitates, exhibiting a number density value as high as 2.7 × 10²⁰ m⁻³ upon 6 h aging at 380 °C. This yields a TE figure of merit value of ca. 1.4 at 450 °C, which is one on the largest values reported for n-type PbTe compounds. Subsequent aging leads to precipitate coarsening and deterioration of TE performance. Interestingly, we find that Bi-alloying improves the alloys’ thermal stability by suppressing microstructure evolution, besides the role of Bi-atoms as electron donors, thereby maintaining high TE performance that is stable at elevated service temperatures. The latter has prime technological significance for TE energy conversion. Full article

(This article belongs to the Special Issue Materials Processing and Crystal Growth for Thermoelectrics)

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7889 KiB

Open AccessArticle

Spark Plasma Sintering of Tungsten Oxides WO_x (2.50 ≤ x ≤ 3): Phase Analysis and Thermoelectric Properties

by Felix Kaiser, Paul Simon, Ulrich Burkhardt, Bernd Kieback, Yuri Grin and Igor Veremchuk

Crystals 2017, 7(9), 271; https://doi.org/10.3390/cryst7090271 - 5 Sep 2017

Cited by 17 | Viewed by 8520

Abstract

The solid-state reaction of WO₃ with W was studied in order to clarify the phase formation in the binary system W-O around the composition WO_x (2.50 ≤ x ≤ 3) during spark plasma sintering (SPS). A new phase “WO_2.82” is observed in the range 2.72 ≤ x ≤ 2.90 which might have the composition W₁₂O₃₄. The influence of the composition on the thermoelectric properties was investigated for 2.72 ≤ x ≤ 3. The Seebeck coefficient, electrical conductivity and electronic thermal conductivity are continuously tunable with the oxygen-to-tungsten ratio. The phase formation mainly affects the lattice thermal conductivity κ_lat which is significantly reduced until 700 K for the sample with the composition x = 2.84, which contains the phases W₁₈O₄₉ and “WO_2.82”. In single-phase WO_2.90 and multi-phase WO_x materials (2.90 ≤ x ≤ 3), which contain crystallographic shear plane phases, a similar reduced κ_lat is observed only below 560 K and 550 K, respectively. Therefore, the composition range x < 2.90 in which the pentagonal column structural motif is formed might be more suitable for decreasing the lattice thermal conductivity at high temperatures. Full article

(This article belongs to the Special Issue Materials Processing and Crystal Growth for Thermoelectrics)

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2662 KiB

Open AccessArticle

Enhanced Thermoelectric Performance of Te-Doped Bi₂Se_3−xTe_x Bulks by Self-Propagating High-Temperature Synthesis

by Rui Liu, Xing Tan, Guangkun Ren, Yaochun Liu, Zhifang Zhou, Chan Liu, Yuanhua Lin and Cewen Nan

Crystals 2017, 7(9), 257; https://doi.org/10.3390/cryst7090257 - 28 Aug 2017

Cited by 56 | Viewed by 7084

Abstract

Polycrystalline Bi₂Se_3−xTe_x (x = 0~1.5) samples were prepared by self-propagating high-temperature synthesis (SHS) combined with spark plasma sintering (SPS) and their thermoelectric properties were investigated. The SHS-SPS process can shorten the time with few energy consumptions, and obtain almost pure Bi₂Se₃-based phases. Consequently, the Se vacancies and anti-site defects contribute to the converged carrier concentration of ~2 × 10¹⁹ cm⁻³ while the increased carrier effective mass enhances the Seebeck coefficient to more than −158 μV K⁻¹ over the entire temperature range. The lattice thermal conductivity is suppressed from 1.07 Wm⁻¹ K⁻¹ for the pristine specimen to ~0.6 Wm⁻¹ K⁻¹ for Te-substitution samples at 300 K because of point defects caused by the difference of mass and size between Te and Se atoms. Coupled with the enhanced power factor and reduced lattice thermal conductivity, a high ZT of 0.67 can be obtained at 473 K for the Bi₂Se_1.5Te_1.5 sample. Our results reveal that Te-substitution based on the SHS-SPS method is highly-efficient and can improve the thermoelectric properties of Bi₂Se₃-based materials largely. Full article

(This article belongs to the Special Issue Materials Processing and Crystal Growth for Thermoelectrics)

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1700 KiB

Open AccessArticle

High Temperature Transport Properties of Yb and In Double-Filled p-Type Skutterudites

by Dean Hobbis, Yamei Liu, Kaya Wei, Terry M. Tritt and George S. Nolas

Crystals 2017, 7(9), 256; https://doi.org/10.3390/cryst7090256 - 23 Aug 2017

Cited by 6 | Viewed by 3973

Abstract

Yb and In double-filled and Fe substituted polycrystalline p-type skutterudite antimonides were synthesized by direct reaction of high-purity elements, followed by solid-state annealing and densification by hot pressing. The stoichiometry and filling fraction were determined by both Rietveld refinement of the X-ray diffraction data and energy dispersive spectroscopic analyses. The transport properties were measured between 300 K and 830 K, and basically indicate that the resistivity and Seebeck coefficient both increase with increasing temperature. In both specimens, the thermal conductivity decreased with increasing temperature up to approximately 700 K, where the onset of bipolar conduction was observed. A maximum ZT value of 0.6 at 760 K was obtained for the Yb_0.39In_0.018Co_2.4Fe_1.6Sb₁₂ specimen. Full article

(This article belongs to the Special Issue Materials Processing and Crystal Growth for Thermoelectrics)

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5711 KiB

Open AccessArticle

Microstructure Analysis and Thermoelectric Properties of Melt-Spun Bi-Sb-Te Compounds

by Weon Ho Shin, Jeong Seop Yoon, Mahn Jeong, Jae Min Song, Seyun Kim, Jong Wook Roh, Soonil Lee, Won Seon Seo, Sung Wng Kim and Kyu Hyoung Lee

Crystals 2017, 7(6), 180; https://doi.org/10.3390/cryst7060180 - 20 Jun 2017

Cited by 8 | Viewed by 4858

Abstract

In order to realize high-performance thermoelectric materials, a way to obtain small grain size is necessary for intensification of the phonon scattering. Here, we use a melt-spinning-spark plasma sintering process for making p-type Bi_0.36Sb_1.64Te₃ thermoelectric materials and evaluate the relation between the process conditions and thermoelectric performance. We vary the Cu wheel rotation speed from 1000 rpm (~13 ms⁻¹) to 4000 rpm (~52 ms⁻¹) during the melt spinning process to change the cooling rate, allowing us to control the characteristic size of nanostructure in melt-spun Bi_0.36Sb_1.64Te₃ ribbons. The higher wheel rotation speed decreases the size of nanostructure, but the grain sizes of sintered pellets are inversely proportional to the nanostructure size after the same sintering condition. As a result, the ZT values of the bulks fabricated from 1000–3000 rpm melt-spun ribbons are comparable each other, while the ZT value of the bulk from the 4000 rpm melt-spun ribbons is rather lower due to reduction of grain boundary phonon scattering. In this work, we can conclude that the smaller nanostructure in the melt spinning process does not always guarantee high-performance thermoelectric bulks, and an adequate following sintering process must be included. Full article

(This article belongs to the Special Issue Materials Processing and Crystal Growth for Thermoelectrics)

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15469 KiB

Open AccessArticle

Structural and Electrical Properties Characterization of Sb_1.52Bi_0.48Te_3.0 Melt-Spun Ribbons

by Viktoriia Ohorodniichuk, Anne Dauscher, Elsa Branco Lopes, Sylvie Migot, Christophe Candolfi and Bertrand Lenoir

Crystals 2017, 7(6), 172; https://doi.org/10.3390/cryst7060172 - 13 Jun 2017

Cited by 6 | Viewed by 4614

Abstract

Melt-spinning (MS) has been reported as a promising tool to tailor the microstructure of bulk thermoelectric materials leading to enhanced thermoelectric performances. Here, we report on a detailed characterization of p-type Bi_0.48Sb_1.52Te₃ ribbons produced by melt-spinning. The microstructure of the melt-spun ribbons has been studied by means of X-ray diffraction, scanning and transmission electron microscopy (TEM). The analyses indicate that the ribbons are highly-textured with a very good chemical homogeneity. TEM reveals clear differences in the microstructure at large and short-range scales between the surface that was in contact with the copper wheel and the free surface. These analyses further evidence the absence of amorphous regions in the melt-spun ribbons and the precipitation of elemental Te at the grain boundaries. Low-temperature electrical resistivity and thermopower measurements (20–300 K) carried out on several randomly-selected ribbons confirm the excellent reproducibility of the MS process. However, the comparison of the transport properties of the ribbons with those of bulk polycrystalline samples of the same initial composition shows that MS leads to a more pronounced metallic character. This difference is likely tied to changes in deviations from stoichiometry due to the out-of-equilibrium conditions imposed by MS. Full article

(This article belongs to the Special Issue Materials Processing and Crystal Growth for Thermoelectrics)

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4876 KiB

Open AccessArticle

Synthesis and Thermoelectric Properties of Copper Sulfides via Solution Phase Methods and Spark Plasma Sintering

by Yun-Qiao Tang, Zhen-Hua Ge and Jing Feng

Crystals 2017, 7(5), 141; https://doi.org/10.3390/cryst7050141 - 16 May 2017

Cited by 30 | Viewed by 8318

Abstract

Large-scale Cu₂S tetradecahedrons microcrystals and sheet-like Cu₂S nanocrystals were synthesized by employing a hydrothermal synthesis (HS) method and wet chemistry method (WCM), respectively. The morphology of α-Cu₂S powders prepared by the HS method is a tetradecahedron with the size of 1–7 μm. The morphology of β-Cu₂S is a hexagonal sheet-like structure with a thickness of 5–20 nm. The results indicate that the morphologies and phase structures of Cu₂S are highly dependent on the reaction temperature and time, even though the precursors are the exact same. The polycrystalline copper sulfides bulk materials were obtained by densifying the as-prepared powders using the spark plasma sintering (SPS) technique. The electrical and thermal transport properties of all bulk samples were measured from 323 K to 773 K. The pure Cu₂S bulk samples sintered by using the powders prepared via HS reached the highest thermoelectric figure of merit (ZT) value of 0.38 at 573 K. The main phase of the bulk sample sintered by using the powder prepared via WCM changed from β-Cu₂S to Cu_1.8S after sintering due to the instability of β-Cu₂S during the sintering process. The Cu_1.8S bulk sample with a Cu_1.96S impurity achieved the highest ZT value of 0.62 at 773 K. Full article

(This article belongs to the Special Issue Materials Processing and Crystal Growth for Thermoelectrics)

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8604 KiB

Open AccessArticle

Enhanced Thermoelectric Properties of Graphene/Cu₂SnSe₃ Composites

by Degang Zhao, Xuezhen Wang and Di Wu

Crystals 2017, 7(3), 71; https://doi.org/10.3390/cryst7030071 - 28 Feb 2017

Cited by 31 | Viewed by 6471

Abstract

Cu₂SnSe₃ material is regarded as a potential thermoelectric material due to its relatively high carrier mobility and low thermal conductivity. In this study, graphene was introduced into the Cu₂SnSe₃ powder by ball milling, and the bulk graphene/Cu₂SnSe₃ thermoelectric composites were prepared by spark plasma sintering. The graphene nanosheets distributed uniformly in the Cu₂SnSe₃ matrix. Meanwhile, some graphene nanosheets tended to form thick aggregations, and the average length of these aggregations was about 3 μm. With the fraction of graphene increasing, the electrical conductivity of graphene/Cu₂SnSe₃ samples increased greatly while the Seebeck coefficient was decreased. The introduction of graphene nanosheets can reduce the thermal conductivity effectively resulting from the phonon scattering by the graphene interface. When the content of graphene exceeds a certain value, the thermal conductivity of graphene/Cu₂SnSe₃ composites starts to increase. The achieved highest figure of merit (ZT) for 0.25 vol % graphene/Cu₂SnSe₃ composite was 0.44 at 700 K. Full article

(This article belongs to the Special Issue Materials Processing and Crystal Growth for Thermoelectrics)

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