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High-Entropy Oxides for Thermoelectric Application
Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.ORCID iD: 0000-0002-8305-9926
2025 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

High-entropy oxides (HEOs) are a new class of single-phase inorganic materials with a high specific capacity, high structural stability, and super-electronic conductivity and exhibit a wide range of useful properties. HEOs are better semiconductor materials compared to traditional ones due to their lattice distortion. Because parameters, such as crystal symmetry, different lattice parameters, etc., have a significant influence on the thermal conductivity of the material, lowering it via phonon-phonon or phonon-electron scattering. The entropy stabilization produces the high stability of the phase but also can result in interesting properties of the materials due to the contribution of different elements through four main effects: high-entropy effect, severe lattice distortion, sluggish diffusion effect, and cocktail effect.

This thesis identified potential HEOs with the chemical composition Co-Cr-Fe-Mn-Ni-O by doing a thorough literature review. During the research, we have focused on the synthesis process and electrical properties of the HEOs (Co0.33Cr0.22Fe0.22Mn0.11Ni0.11)3O4, (Co0.33Cr0.22Fe0.22Mn0.11Cu0.11)3O4, and (Co0.2Cr0.2Fe0.2Mn0.2Cu0.2)3O4.

Oxides were synthesized via Spark Plasma Sintering and Solid-State Reaction resulting in obtaining two or more phases with different crystal structures for the materials (Co0.33Cr0.22Fe0.22Mn0.11Ni0.11)3O4, and single-phased for the (Co0.33Cr0.22Fe0.22Mn0.11Cu0.11)3O4 and (Co0.2Cr0.2Fe0.2Mn0.2Cu0.2)3O4 at specific synthesis conditions. As expected, obtained single-phased materials exhibit higher values of electrical conductivity, which is probably due to the less electron-phonon scattering.

Two types of semiconductors are needed for thermoelectric applications: p- and n-type. Due to the different synthesis temperatures, materials with Ni were obtained in both types. This can lead to the production of the Peltier module with the same chemical composition inside.

With the Ni-Cu substitution, it became easier to produce single-phased materials, probably due to the melting point of the reagents. These materials also presented higher electrical properties, which the changes in carrier concentration can explain due to the differences in the electronic structures.

All obtained samples exhibit low values of the electronic part of thermal conductivity, which can lead to low values of total thermal conductivity. It shows that the main contributor to the thermal conductivity will be from the phonons (lattice thermal conductivity). Overall, the expected thermal conductivity for these materials should be lower compared to the traditional semiconductor materials due to the crystal distortion, which can lead to higher phonon-phonon and phonon-electron scattering.

Furthermore, this research shows that HEOs with unequal content of metals can be produced as single-phase materials and have even better or similar electrical properties compared to known compositions. Also, these oxides with impurities still exhibit promising electrical properties.

Place, publisher, year, edition, pages
Luleå: Luleå University of Technology, 2025.
Series
Doctoral thesis / Luleå University of Technology 1 jan 1997 → …, ISSN 1402-1544
Keywords [en]
High-Entropy Oxides, Thermoelectric materials, Semiconductors, Electrical properties, Seebeck coefficient
National Category
Other Physics Topics
Research subject
Experimental Physics
Identifiers
URN: urn:nbn:se:ltu:diva-111266ISBN: 978-91-8048-732-0 (print)ISBN: 978-91-8048-733-7 (electronic)OAI: oai:DiVA.org:ltu-111266DiVA, id: diva2:1926457
Public defence
2025-03-06, E632, Luleå University of Technology, Luleå, 09:00 (English)
Opponent
Supervisors
Available from: 2025-01-13 Created: 2025-01-12 Last updated: 2025-02-06Bibliographically approved
List of papers
1. Co-Cr-Fe-Mn-Ni Oxide as a Highly Efficient Thermoelectric High-Entropy Alloy
Open this publication in new window or tab >>Co-Cr-Fe-Mn-Ni Oxide as a Highly Efficient Thermoelectric High-Entropy Alloy
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2023 (English)In: ACS Omega, E-ISSN 2470-1343, Vol. 8, no 16, p. 14484-14489Article in journal (Refereed) Published
Abstract [en]

Among the existing materials for heat conversion, high-entropy alloys are of great interest due to the tunability of their functional properties. Here, we aim to produce single-phase high-entropy oxides composed of Co-Cr-Fe-Mn-Ni-O through spark plasma sintering (SPS), testing their thermoelectric (TE) properties. This material was successfully obtained before via a different technique, which requires a very long processing time. Hence, the main target of this work is to apply spark plasma sintering, a much faster and scalable process. The samples were sintered in the temperature range of 1200–1300 °C. Two main phases were formed: rock salt-structured Fm3̅m and spinel-structured Fd3̅m. Comparable transport properties were achieved via the new approach: the highest value of the Seebeck coefficient reached −112.6 μV/K at room temperature, compared to −150 μV/K reported before; electrical properties at high temperatures are close to the properties of the single-phase material (σ = 0.2148 S/cm, σ ≈ 0.2009 S/cm reported before). These results indicate that SPS can be successfully applied to produce highly efficient TE high-entropy alloys in a fast and scalable way. Further optimization is needed for the production of single-phase materials, which are expected to exhibit an even better TE functionality.

Place, publisher, year, edition, pages
American Chemical Society, 2023
National Category
Materials Chemistry
Research subject
Experimental Physics; Engineering Materials
Identifiers
urn:nbn:se:ltu:diva-97040 (URN)10.1021/acsomega.2c08278 (DOI)000974321200001 ()37125128 (PubMedID)2-s2.0-85154024927 (Scopus ID)
Funder
The Kempe FoundationsKnut and Alice Wallenberg FoundationLuleå University of Technology
Note

Validerad;2023;Nivå 2;2023-05-10 (joosat);

Funder: Swedish Foundations Consolidator Fellowship;

Licens fulltext: CC BY License

Available from: 2023-05-10 Created: 2023-05-10 Last updated: 2025-01-12Bibliographically approved
2. Influence of Ni-Cu substitution and uneven element content on electrical properties of High Entropy Oxides
Open this publication in new window or tab >>Influence of Ni-Cu substitution and uneven element content on electrical properties of High Entropy Oxides
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(English)Manuscript (preprint) (Other academic)
National Category
Other Physics Topics
Identifiers
urn:nbn:se:ltu:diva-111264 (URN)
Available from: 2025-01-11 Created: 2025-01-11 Last updated: 2025-01-13Bibliographically approved
3. Bifunctional Co-Cr-Fe-Mn-Ni High-Entropy Oxides: Exploring Thermoelectric Properties and Solar Water Desalination
Open this publication in new window or tab >>Bifunctional Co-Cr-Fe-Mn-Ni High-Entropy Oxides: Exploring Thermoelectric Properties and Solar Water Desalination
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(English)Manuscript (preprint) (Other academic)
National Category
Other Physics Topics
Identifiers
urn:nbn:se:ltu:diva-111265 (URN)
Available from: 2025-01-11 Created: 2025-01-11 Last updated: 2025-01-13Bibliographically approved
4. Influence of Copper Oxide on Electrical Properties of High-Entropy Oxide with the Chemical Composition Co-Cr-Fe-Mn-Cu-O
Open this publication in new window or tab >>Influence of Copper Oxide on Electrical Properties of High-Entropy Oxide with the Chemical Composition Co-Cr-Fe-Mn-Cu-O
(English)Manuscript (preprint) (Other academic)
National Category
Other Physics Topics
Research subject
Experimental Physics
Identifiers
urn:nbn:se:ltu:diva-111459 (URN)
Funder
Knut and Alice Wallenberg FoundationThe Kempe Foundations
Note

Funder: Swedish Foundations Consolidator Fellowship; Italian Ministry of University and Research;

Available from: 2025-01-29 Created: 2025-01-29 Last updated: 2025-02-06Bibliographically approved
5. Enhanced Thermoelectric Properties by Embedding Fe Nanoparticles into CrN Films for Energy Harvesting Applications
Open this publication in new window or tab >>Enhanced Thermoelectric Properties by Embedding Fe Nanoparticles into CrN Films for Energy Harvesting Applications
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2024 (English)In: ACS Applied Nano Materials, E-ISSN 2574-0970, Vol. 7, no 3, p. 3428-3435Article in journal (Refereed) Published
Abstract [en]

Nanostructured materials and nanocomposites have shown great promise for improving the efficiency of thermoelectric materials. Herein, Fe nanoparticles were imbedded into a CrN matrix by combining two physical vapor deposition approaches, namely, high-power impulse magnetron sputtering and a nanoparticle gun. The combination of these techniques allowed the formation of nanocomposites in which the Fe nanoparticles remained intact without intermixing with the matrix. The electrical and thermal transport properties of the nanocomposites were investigated and compared to those of a monolithic CrN film. The measured thermoelectric properties revealed an increase in the Seebeck coefficient, with a decrease of hall carrier concentration and an increase of the electron mobility, which could be explained by energy filtering by internal phases created at the NP/matrix interface. The thermal conductivity of the final nanocomposite was reduced from 4.8 W m-1 K-1 to a minimum of 3.0 W m-1 K-1. This study shows prospects for the nanocomposite synthesis process using nanoparticles and its use in improving the thermoelectric properties of coatings.

Place, publisher, year, edition, pages
American Chemical Society, 2024
Keywords
electrical transport, electron energy filtering, nanocomposites, nanoparticles, physical vapor deposition, thermal transport, thermoelectric thin films
National Category
Materials Chemistry
Research subject
Experimental Physics
Identifiers
urn:nbn:se:ltu:diva-104596 (URN)10.1021/acsanm.3c06054 (DOI)001159423900001 ()2-s2.0-85184907583 (Scopus ID)
Note

Validerad;2024;Nivå 2;2024-04-04 (joosat);

Funder: Swedish Research Council VR-RFI (2019-00191); Swedish Foundation for Strategic Research (RIF14-0053); Knut and Alice Wallenberg Foundation (KAW 2016.0346); VR (2021-03826);

Full text license: CC BY

Available from: 2024-03-14 Created: 2024-03-14 Last updated: 2025-02-06Bibliographically approved

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The full text will be freely available from 2025-02-13 09:00
Available from 2025-02-13 09:00

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