Special Issue "Materials Processing and Crystal Growth for Thermoelectrics"

A special issue of Crystals (ISSN 2073-4352). This special issue belongs to the section "Crystal Engineering".

Deadline for manuscript submissions: closed (31 July 2017)

Special Issue Editor

Guest Editor
Prof. George S. Nolas

Department of Physics, University of South Florida, Tampa, FL 33620, USA
Website | E-Mail
Interests: materials processing; thermoelectrics; condensed-matter physics; crystal growth and design; materials for energy conversion

Special Issue Information

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

Manuscript Submission Information

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Keywords

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

Published Papers (9 papers)

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Research

Open AccessArticle Solvent-Dependent Thermoelectric Properties of PTB7 and Effect of 1,8-Diiodooctane Additive
Crystals 2017, 7(10), 292; doi:10.3390/cryst7100292
Received: 19 August 2017 / Revised: 17 September 2017 / Accepted: 26 September 2017 / Published: 29 September 2017
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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
[...] Read more.
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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Open AccessArticle Microstructure Evolution of Ag-Alloyed PbTe-Based Compounds and Implications for Thermoelectric Performance
Crystals 2017, 7(9), 281; doi:10.3390/cryst7090281
Received: 17 August 2017 / Revised: 8 September 2017 / Accepted: 12 September 2017 / Published: 18 September 2017
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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
[...] Read more.
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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Open AccessArticle Spark Plasma Sintering of Tungsten Oxides WOx (2.50 ≤ x ≤ 3): Phase Analysis and Thermoelectric Properties
Crystals 2017, 7(9), 271; doi:10.3390/cryst7090271
Received: 26 July 2017 / Revised: 21 August 2017 / Accepted: 23 August 2017 / Published: 5 September 2017
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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 WOx (2.50 ≤ x ≤ 3) during spark plasma sintering (SPS). A new phase “WO2.82
[...] Read more.
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 WOx (2.50 ≤ x ≤ 3) during spark plasma sintering (SPS). A new phase “WO2.82” 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 κlat which is significantly reduced until 700 K for the sample with the composition x = 2.84, which contains the phases W18O49 and “WO2.82”. In single-phase WO2.90 and multi-phase WOx materials (2.90 ≤ x ≤ 3), which contain crystallographic shear plane phases, a similar reduced κlat 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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Open AccessArticle Enhanced Thermoelectric Performance of Te-Doped Bi2Se3−xTex Bulks by Self-Propagating High-Temperature Synthesis
Crystals 2017, 7(9), 257; doi:10.3390/cryst7090257
Received: 12 June 2017 / Revised: 19 August 2017 / Accepted: 21 August 2017 / Published: 28 August 2017
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Abstract
Polycrystalline Bi2Se3−xTex (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
[...] Read more.
Polycrystalline Bi2Se3−xTex (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 Bi2Se1.5Te1.5 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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Open AccessArticle High Temperature Transport Properties of Yb and In Double-Filled p-Type Skutterudites
Crystals 2017, 7(9), 256; doi:10.3390/cryst7090256
Received: 18 July 2017 / Revised: 9 August 2017 / Accepted: 18 August 2017 / Published: 23 August 2017
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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
[...] Read more.
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 Yb0.39In0.018Co2.4Fe1.6Sb12 specimen. Full article
(This article belongs to the Special Issue Materials Processing and Crystal Growth for Thermoelectrics)
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Open AccessArticle Microstructure Analysis and Thermoelectric Properties of Melt-Spun Bi-Sb-Te Compounds
Crystals 2017, 7(6), 180; doi:10.3390/cryst7060180
Received: 25 May 2017 / Revised: 19 June 2017 / Accepted: 19 June 2017 / Published: 20 June 2017
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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 Bi0.36Sb1.64Te3 thermoelectric materials and evaluate
[...] Read more.
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 Bi0.36Sb1.64Te3 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 Bi0.36Sb1.64Te3 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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Open AccessArticle Structural and Electrical Properties Characterization of Sb1.52Bi0.48Te3.0 Melt-Spun Ribbons
Crystals 2017, 7(6), 172; doi:10.3390/cryst7060172
Received: 28 April 2017 / Revised: 1 June 2017 / Accepted: 7 June 2017 / Published: 13 June 2017
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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 Bi0.48Sb1.52Te3 ribbons produced by melt-spinning. The
[...] Read more.
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 Bi0.48Sb1.52Te3 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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Open AccessArticle Synthesis and Thermoelectric Properties of Copper Sulfides via Solution Phase Methods and Spark Plasma Sintering
Crystals 2017, 7(5), 141; doi:10.3390/cryst7050141
Received: 27 April 2017 / Revised: 12 May 2017 / Accepted: 13 May 2017 / Published: 16 May 2017
Cited by 1 | PDF Full-text (4876 KB) | HTML Full-text | XML Full-text | Supplementary Files
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
[...] Read more.
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 Cu1.8S after sintering due to the instability of β-Cu2S during the sintering process. The Cu1.8S bulk sample with a Cu1.96S 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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Open AccessArticle Enhanced Thermoelectric Properties of Graphene/Cu2SnSe3 Composites
Crystals 2017, 7(3), 71; doi:10.3390/cryst7030071
Received: 28 January 2017 / Revised: 26 February 2017 / Accepted: 27 February 2017 / Published: 28 February 2017
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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/Cu
[...] Read more.
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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