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Nanostructured Bulk Thermoelectrics: Scalable Fabrication Routes, Processing and Evaluation
KTH, School of Information and Communication Technology (ICT). (Material & NanoPhysics)ORCID iD: 0000-0003-0855-5265
2016 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Current fossil fuel based energy sources have a huge shortcoming when one discusses their efficiency. The conversion efficiency of fossil fuel-based technologies is less than 40% in best cases. Therefore, until the renewable energy section is mature enough to handle all the energy demand one has to research and develop the technologies available to harvest the energy from the waste heat generated in fossil fuel-based supply sources. One of these emerging technologies is the use of thermoelectric (TE) devices to achieve this goal, which are solid-state devices capable of directly interconverting between heat and electrical energy. In the past decade there has been a significant scientific and financial investment within the field to enhance their properties and result in time/energy efficient fabrication processes of TE materials and devices for a more sustainable environment.

In this thesis with use of chemical synthesis routes for nanostructured bulk thermoelectric materials iron antimonide (FeSb2), skutterudites (based on general formula of RzMxCo1-xSb3-yNy) and copper selenide (Cu2Se) are developed. These materials are promising candidates for use in thermoelectric generators (TEG) or for sensing applications. Using chemical synthesis routes such as chemical co-precipitation, salt melting in marginal solvents and thermolysis, fabrication of these TE materials with good performance can be performed with high degree of reproducibility, in a much shorter time, and easily scalable manner for industrial processes. The TE figure of merit ZT of these materials is comparable to, or better than their conventional method counterparts to ensure the applicability of these processes in industrial scale.

Finally, through thorough investigation, optimized consolidation parameters were generated for compaction of each family of materials using Spark Plasma Sintering technique (SPS). As each family of TE nanomaterial investigated in this thesis had little to no prior consolidation literature available, specific parameters had to be studied and generated. The aim of studies on compaction parameters were to focus on preservation of the nanostructured features of the powder while reaching a high compaction density to have positive effects on the materials TE figure of merit.

Abstract [sv]

Dagens fossilbränslebaserade energikällor har en enorm brist gällande effektivitet. Effektiviteten av fossilbränslebaserade teknologiers omvandling är mindre än 40 % i bästa fall. Därför tills förnybar energi är mogen nog att hantera alla energibehov, måste man forska och utveckla teknik för att skörda energi från spillvärme i fossilbränslebaserade försörjningskällor. En av dessa nya tekniker är tillämpning av termoelektriska (TE) material för att uppnå målet. Nämnde material är Soldi-State materialer som kan transformera mellan värme och elektrisk energi. Under det senaste decenniet har det pågått en stor vetenskaplig och ekonomisk investering inom området för att förbättra termoelektriska materials egenskaper. Dessutom ville man ta fram tid/energieffektiva TE material och komponenter för en mer hållbar miljö.

I denna avhandling utvecklades och producerades termoelektriska material såsom järn antimonid (FeSb2), skutterudit (baserat på allmänna formeln RzMxCo1-xSb3-YNY) och koppar selenid (Cu2Se) med hjälp av kemiska syntesmetoder. Genom att Använda kemiska syntesmetoder som kemisk samutfällning, salt smältning i marginella lösningsmedel och termolys, kan material med hög grad av reproducerbarhet och ställbar för industriella processer tillverkas.   Termoelektrisk omvandling effektivitet hos uppnådde material är betydligt högre än resultat av andra studier. I och med detta kan man säga att materialet kan användas inom industri.

Slutligen, genom en grundlig undersökning optimerades packningsparametrar som genererades för packning av varje materialgrupp med hjälp av Spark Plasma Sintring teknik (SPS). Eftersom ingen relevant studie finns för varje grupp av termoelektriska nanomaterial som undersökts i denna avhandling, studerades och genererades dessa specifika parametrar. Syftet med studien är att fokusera på bevarande av nanostrukturerade egenskaperna hos pulvret och att samtidigt nå en hög packningstäthet för att ha positiva effekter på materialens termoelektriska omvandlingseffektivitet.

Place, publisher, year, edition, pages
Stockholm, Sweden: KTH Royal Institute of Technology, 2016. , x, 34 p.
Series
TRITA-ICT, 2016:10
Keyword [en]
Thermoelectric, Iron antimonide (FeSb2) Skutterudite, Copper Selenide (Cu2Se), Spark Plasma Sintering (SPS), nanomaterial
National Category
Condensed Matter Physics
Research subject
Materials Science and Engineering; Energy Technology
Identifiers
URN: urn:nbn:se:kth:diva-186124ISBN: 978-91-7595-945-0 (print)OAI: oai:DiVA.org:kth-186124DiVA: diva2:925565
Public defence
2016-05-27, Sal C, Isafjordsgatan 22, Kista, 10:00 (English)
Opponent
Supervisors
Projects
NEXTECSCALTEG
Funder
Swedish Foundation for Strategic Research , EM11‐0002EU, FP7, Seventh Framework Programme, 263167
Note

QC 20160503

Available from: 2016-05-03 Created: 2016-05-02 Last updated: 2016-05-10Bibliographically approved
List of papers
1. Fabrication and characterization of nanostructured bulk skutterudites
Open this publication in new window or tab >>Fabrication and characterization of nanostructured bulk skutterudites
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2013 (English)In: 2013 MRS Spring Meeting - Symposium H/I/V – Nanoscale Thermoelectric Materials, Thermal and Electrical Transport, and Applications to Solid-State Cooling and Power Generation, Materials Research Society, 2013, 105-110 p.Conference paper, Published paper (Refereed)
Abstract [en]

Latest nanotechnology concepts applied in thermoelectric (TE) research have opened many new avenues to improve the ZT value. Low dimensional structures can improve the ZT value as compared to bulk materials by substantial reduction in the lattice thermal conductivity, κL. However, the materials were not feasible for the industrial scale production of macroscopic devices because of complicated and costly manufacturing processes involved. Bulk nanostructured (NS) TEs are normally fabricated using a bulk process rather than a nano- fabrication process, which has the important advantage of producing in large quantities and in a form that is compatible with commercially available TE devices. We developed fabrication strategies for bulk nanostructured skutterudite materials based on FexCo1-xSb3. The process is based on precipitation of a precursor material with the desired metal atom composition, which is then exposed to thermochemical processing of calcination followed by reduction. The resultant material thus formed maintains nanostructured particles which are then compacted using Spark Plasma Sintering (SPS) by utilizing previously optimized process parameters. Microstructure, crystallinity, phase composition, thermal stability and temperature dependent transport property evaluation has been performed for compacted NS Fe xCo1-xSb3. Evaluation results are presented in detail, suggesting the feasibility of devised strategy for bulk quantities of doped TE nanopowder fabrication.

Place, publisher, year, edition, pages
Materials Research Society, 2013
Series
Materials Research Society Symposium Proceedings, ISSN 0272-9172 ; 1543
Keyword
Fabrication and characterizations, Fabrication strategies, Industrial-scale production, Lattice thermal conductivity, Low dimensional structure, Nanostructured particles, Skutterudite materials, Thermochemical processing
National Category
Engineering and Technology
Identifiers
urn:nbn:se:kth:diva-143308 (URN)10.1557/opl.2013.947 (DOI)2-s2.0-84893404121 (Scopus ID)978-160511520-7 (ISBN)
Conference
2013 MRS Spring Meeting; San Francisco, CA; United States; 1 April 2013 through 5 April 2013
Note

QC 20140320

Available from: 2014-03-20 Created: 2014-03-19 Last updated: 2016-05-03Bibliographically approved
2. Fabrication and characterization of nanostructured thermoelectric FexCo1-xSb3
Open this publication in new window or tab >>Fabrication and characterization of nanostructured thermoelectric FexCo1-xSb3
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2015 (English)In: Open Chemistry, ISSN 2391-5420, Vol. 13, no 1, 629-635 p.Article in journal (Refereed) Published
Abstract [en]

A novel synthesis route for the fabrication of p-type nanostructured skutterudite, FexCo1-xSb3 in large quantity is reported. This scalable synthesis route provides nano-engineered material with less impact on the environment compared to conventional synthesis procedures. Several Fe substituted compositions have been synthesized to confirm the feasibility of the process. The process consists of a nano-sized precursor fabrication of iron and cobalt oxalate, and antimony oxides by chemical co-precipitation. Further thermochemical processes result in the formation of iron substituted skutterudites. The nanopowders are compacted by Spark Plasma Sintering (SPS) technique in order to maintain nanostructure. Detailed physicochemical as well as thermoelectric transport properties are evaluated. Results reveal strongly reduced thermal conductivity values compared to conventionally prepared counterparts, due to nanostructuring. P-type characteristic was observed from the Seebeck measurements while electrical conductivity is high and shows metallic behavior. The highest TE figure of merit of 0.25 at 800 K has been achieved, which is strongly enhanced with respect to the mother compound CoSb3. This suggests the promise of the utilized method of fabrication and processing for TE applications with improved performance.

Keyword
Skutterudite (CoSb3), thermoelectric, iron substituted skutterudite, bottom-up synthesis, SPS
National Category
Materials Chemistry
Research subject
Materials Science and Engineering
Identifiers
urn:nbn:se:kth:diva-160646 (URN)10.1515/chem-2015-0074 (DOI)000355403100076 ()
Funder
Swedish Foundation for Strategic Research , EM11-0002EU, FP7, Seventh Framework Programme, 263167
Note

QC 20150312

Available from: 2015-02-25 Created: 2015-02-25 Last updated: 2016-05-03Bibliographically approved
3. Temperature Dependent Structure Stability Studies on Thermoelectric Yb0.025Fe0.3Co0.7Sb3
Open this publication in new window or tab >>Temperature Dependent Structure Stability Studies on Thermoelectric Yb0.025Fe0.3Co0.7Sb3
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2015 (English)In: Materials Research Society Proceeding / [ed] S.R. Bishop , D. Cahen , R. Chen , E. Fabbri , F.C. Fonseca and D. Ginley, Materials Research Society , 2015, Vol. 1735Conference paper, Published paper (Refereed)
Abstract [en]

Depending on their application temperature thermoelectric (TE) materials are classified in three main categories; as low (up to 250°C), intermediate (up to 550°C) and high (above 600°C) temperature. Currently, Skutterudites (CoSb3) based materials have shown promising results in the intermediate temperature range (300-500°C). This family of material is highly suitable for automotive, marine transportation and industrial power generation applications to recover the waste heat from the exhaust and generate electricity. Conventional TE modules need p- and n-type semiconductor materials and for the skutterudite family, iron (Fe) has proven to be among the best candidates for the substitution of cobalt sites. Additionally, rare earths are introduced as rattlers in the crystal cages of the skutterudite to decrease the thermal conductivity, thus improving the figure of merit ZT of the TE material. For practical application for device fabrication, stability of these materials is of great importance. Compositional stability is being addressed as the material decomposes above certain temperature. Temperature dependent x-ray diffraction study was performed on Fe substituted, Yb-filled skutterudites, using Beam Line I711 at MAX LAB, to observe the crystal structure as a function of temperature. Diffraction patterns were collected from room temperature up to 500°C by utilizing Huber furnace. The results show success in filling process showing almost 80% reduction of the thermal conductivity from bulk. Additionally the thermal expansion coefficient value was within the average value for skutterudites which proves practical application of this powder for industrial applications.

Place, publisher, year, edition, pages
Materials Research Society, 2015
Keyword
thermoelectric, nanostructure, thermal conductivity
National Category
Materials Engineering
Research subject
Energy Technology
Identifiers
urn:nbn:se:kth:diva-176849 (URN)10.1557/opl.2015.308 (DOI)
Conference
2014 MRS Fall Meeting
Funder
Swedish Research Council, VR-SRL 2013-6780Swedish Foundation for Strategic Research , EM11-0002
Note

QC 20151201

Available from: 2015-11-10 Created: 2015-11-10 Last updated: 2016-05-03Bibliographically approved
4. On the chemical synthesis route to bulk-scale skutterudite materials
Open this publication in new window or tab >>On the chemical synthesis route to bulk-scale skutterudite materials
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2016 (English)In: Ceramics International, ISSN 0272-8842, E-ISSN 1873-3956, Vol. 42, no 4, 5312-5318 p.Article in journal (Refereed) Published
Abstract [en]

In this article an alternative high yield route for the synthesis of CoSb3-based unfilled skutterudites is presented. Using low-melting temperature salts of the constituents, melting and mixing them homogeneously in a hydrophobic liquid with postprocessing of the powders we achieve a more intimately mixed alloy compared to the conventional melting and metallurgical processes. The proposed method consists of a fast and low-temperature processing step followed by a thermochemical post-processing step, compared to the conventional methods of fabricating skutterudites, which require high temperatures and long processing times. Several structural characterization techniques were used to assess the mechanism of synthesis, verify the purity of the material as well as the reproducibility of the process. Detailed analysis and results are presented in support of the proposed process. Additionally, compaction of the powders with SPS technique provided a safe route to maintaining the nanopowder size and achieving low thermal conductivity (3 W/mK). The proposed method can easily be scaled up and adopted by the industry.

Place, publisher, year, edition, pages
Elsevier, 2016
Keyword
Thermal conductivity, Thermoelectric, Skutterudite, TGA-FTIR
National Category
Ceramics
Identifiers
urn:nbn:se:kth:diva-183303 (URN)10.1016/j.ceramint.2015.12.061 (DOI)000369460500088 ()2-s2.0-84955655812 (Scopus ID)
Note

QC 20160309

Available from: 2016-03-09 Created: 2016-03-07 Last updated: 2017-11-30Bibliographically approved
5. Chemical Synthesis of Iron Antimonide (FeSb2) and Its Thermoelectric Properties
Open this publication in new window or tab >>Chemical Synthesis of Iron Antimonide (FeSb2) and Its Thermoelectric Properties
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2016 (English)In: Inorganic Chemistry, ISSN 0020-1669, E-ISSN 1520-510X, Vol. 55, no 4, 1831-1836 p.Article in journal (Refereed) Published
Abstract [en]

Low temperature thermoelectric (TE) materials are in demand for more efficient cooling and power generation applications. Iron antimonide (FeSb2) draws great attention over the past few years because of its enhanced power factor values. Polycrystalline bulk FeSb2 nanopowder was prepared via a low-temperature molten salts approach followed by subsequent thermal treatment in synthetic air and hydrogen gas for calcination and reduction reactions, respectively. Structural analysis confirms the desired final phase with submicrometer grain size and high compaction density after consolidation using spark plasma sintering (SPS). TE transport properties revealed that the material is n-type below 150 K and p-type above this temperature; this suggests antimony vacancies in FeSb2. The electrical conductivity increased significantly, and the highest conductivity achieved was 6000 S/cm at 100 K. The maximum figure-of-merit, ZT, of 0.04 is achieved at 500 K, which is about 6 times higher than the earlier reported state-of-the art ZT value for the same material.

Place, publisher, year, edition, pages
American Chemical Society (ACS), 2016
National Category
Chemical Sciences
Identifiers
urn:nbn:se:kth:diva-184039 (URN)10.1021/acs.inorgchem.5b02658 (DOI)000370395000055 ()26836130 (PubMedID)2-s2.0-84958818272 (Scopus ID)
Note

QC 20160323

Available from: 2016-03-23 Created: 2016-03-22 Last updated: 2017-11-30Bibliographically approved
6. Microwave Assisted Organometallic Synthesis,Structural Characterization and ThermoelectricEvaluation of Cu2Se
Open this publication in new window or tab >>Microwave Assisted Organometallic Synthesis,Structural Characterization and ThermoelectricEvaluation of Cu2Se
(English)Manuscript (preprint) (Other academic)
Abstract [en]

In this article we report a highly efficient synthesis route for fabrication of nanostructured bulk thermoelectric (TE) copper selenide (Cu2Se) nanopowders. Using Microwave (MW) assisted synthesis and through bottom-up synthesis approach, copper selenide nanopowders were fabricated. The nanopowders then underwent an optimized SPS compaction generated with the aim of high compaction density as well as the overall thermoelectric figure of merit ZT. The structural and thermoelectric transport property evaluation was performed and a ZT of 2.1 at 900 K was achieved. The high ZT value was attributed to the dramatically reduced thermal conductivity which reached values of as low as 0.38 W/mK. Detailed process, structural evaluation and TE transport property measurements of the prepared samples are presented.IntroductionThermoelectric

Keyword
Thermoelectric, Microwave Assisted Synthesis, Copper Selenide
National Category
Condensed Matter Physics
Research subject
Materials Science and Engineering
Identifiers
urn:nbn:se:kth:diva-186171 (URN)
Funder
Swedish Foundation for Strategic Research , EM11-0002Swedish Research Council, VR-SRL 2013-6780
Note

QC 20160503

Available from: 2016-05-03 Created: 2016-05-03 Last updated: 2016-05-03Bibliographically approved

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