Effect of Hot Isostatic Pressing Treatment on the Thermoelectric Power Factors of Zinc Oxides

Effect of Hot Isostatic Pressing Treatment on the Thermoelectric Power Factors of Zinc Oxides | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Article Effect of Hot Isostatic Pressing Treatment on the Thermoelectric Power Factors of Zinc Oxides Hidenobu Mori, Haruhiko Yoshida This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-4844832/v1 This work is licensed under a CC BY 4.0 License Status: Published Journal Publication published 28 Dec, 2024 Read the published version in Scientific Reports → Version 1 posted 10 You are reading this latest preprint version Abstract The effect of hot isostatic pressing (HIP) on the thermoelectric power factor of zinc oxide (ZnO) has been examined. ZnO is expected to be a potential n-type oxide thermoelectric material that could enhance the thermoelectric conversion efficiency. The HIP treatment is useful for densifying the material and controlling crystal defects in the material by applying high temperatures and pressures simultaneously. Furthermore, the atmosphere during HIP treatment can be controlled to enable the application of this technique to both metallic and oxide materials. The thermoelectric power factor of ZnO increased due to a notable increase in electrical conductivity, although the Seebeck coefficient decreased by approximately 50% following HIP treatment under argon gas. The increase in the thermoelectric power factor is attributed to the oxygen vacancies introduced into ZnO subsequent to the HIP treatment. Consequently, HIP treatment represents a promising approach for enhancing the thermoelectric power factor of ZnO. Physical sciences/Energy science and technology Physical sciences/Engineering Physical sciences/Materials science Zinc Oxide Hot Isostatic Pressing Treatment Thermoelectric Power Factor Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Introduction A thermoelectric power generation system is expected to make efficient use of waste heat by directly converting thermal energy into electrical energy. 1 – 4 The performance of this system is determined by the efficiency of the thermoelectric materials, which is evaluated by the dimensionless figure of merit defined as ZT = (S 2 σ/κ)Τ, where S, σ, κ and T are the Seebeck coefficient, the electrical conductivity, the thermal conductivity and the absolute temperature, respectively. The figure of merit can also be transformed to Z = P/κ, where P is the thermoelectric power factor (P = S 2 σ). Therefore, for a thermoelectric material with high power, a large thermoelectric power factor and a small thermal conductivity are needed. Currently, nonoxides are employed in the fabrication of practical thermoelectric materials. However, from the standpoint of heat resistance, the reduction of environmental burdens, and production methods, metal oxide materials have attracted attention as the next generation of thermoelectric materials. 4 – 7 Recently, zinc oxide (ZnO) has emerged as a potential n-type thermoelectric material because of its low toxicity potential, high electron mobility, thermal resistance, and corrosion resistance. 8 – 15 To enhance the thermoelectric properties of ZnO, the effects of the introduction of additives such as Al 2 O 3 and Ga 2 O 3 8–12 , as well as the effects of nanostructures 13 – 15 , have been investigated. Nevertheless, enhancing the thermoelectric properties of ZnO from a practical application standpoint is essential. For other oxide materials, it has been reported that crystalline control contributes to the progressive advancement of thermoelectric properties and that a high relative density is an important parameter for the improvement of thermoelectric properties. 7 Conversely, HIP treatment is well known to be an effective method for controlling density and crystal defects in materials by simultaneously applying high temperature and pressure. 16 Furthermore, the atmosphere during HIP treatment can be controlled to enable the application of this technique to both metallic and oxide materials. It is therefore anticipated that HIP treatment will result in enhanced thermoelectric properties. This paper presents an investigation into the effect of HIP treatment on the thermoelectric power factor of ZnO, with the objective of enhancing the thermoelectric conversion efficiency. Results and Discussion The effects of HIP treatment on the thermoelectric properties, including the Seebeck coefficient, electrical conductivity, and thermoelectric power factor, of the samples were examined. Figure 1 shows the temperature dependence of the thermoelectric properties of the samples following HIP treatment. The absolute value of the Seebeck coefficient is shown in Fig. 1 (a). Although the Seebeck coefficient was reduced by approximately 50% before and after HIP treatment, its value remained independent of the HIP treatment temperature and exhibited nearly constant behavior, as shown in Fig. 1 (a). In contrast, the electrical conductivity substantially increased with increasing HIP treatment temperature, as shown in Fig. 1 (b). The power factor, which is the ratio of the Seebeck coefficient to the electrical conductivity, increased with increasing temperature during HIP, as shown in Fig. 1 (c). The improvement in the thermoelectric properties was attributed to the HIP treatment, despite the decrease in the Seebeck coefficient. In a previous study, it was reported that the generation of oxygen vacancies occurred in a sample following HIP treatment, resulting in a change in color from white to light gray. 17 In this study, the significant increase in conductivity suggested that oxygen vacancies were generated within the samples. To confirm the presence of oxygen vacancies in the HIP-treated samples, the phase identification of the samples before and after HIP treatment was analyzed via X-ray diffraction at room temperature. However, the X-ray diffraction results indicated that the sample after HIP treatment exhibited the same crystal structure as the sample before HIP treatment. Therefore, the oxygen vacancies introduced by HIP treatment may not be detectable by X-ray diffraction. We investigated the effects of the processing atmosphere, processing pressure, and heaters during HIP treatment to identify the factors contributing to the oxygen vacancies due to HIP treatment. The influence of gaseous species in the HIP treatment was verified. Figure 2 shows the thermoelectric power factor of the ZnO samples after HIP treatment with a mixture of argon and oxygen or argon gas. As shown in Fig. 2 , there are variations in the Seebeck coefficient, electrical conductivity and power factor of the samples after HIP treatment under both gas conditions. In particular, the electrical conductivity and the thermoelectric power factor of the sample after HIP treatment under argon gas improved. To clarify this factor, the relationship between the effect of HIP treatment and the density of ZnO was investigated. The density of the samples was measured via Archimedes’ method. Figures 3 (a), 3(b) and 3(c) show the volume, mass and density of the samples before and after HIP treatment at 1273 K under argon gas or under a mixed gas of argon and oxygen for 2 h, respectively. As shown in Fig. 3 , the sample after HIP treatment under argon gas was denser than the sample after HIP treatment under mixed gas. This occurred because the degree of reduction in the volume of the sample was large regardless of the decrease in mass during the HIP treatment under argon gas. Therefore, the atmosphere of argon gas was considered one of the factors leading to an increase in the number of oxygen vacancies, confirming that HIP treatment was effective in densifying ZnO. We compared the thermoelectric power factor between HIP treatment under high pressure and heat treatment under atmospheric pressure to verify the effect of pressure under argon gas. Figure 4 shows the thermoelectric power factor of the ZnO samples after HIP treatment or heat treatment. As shown in Fig. 4 , the electrical conductivity and thermoelectric power factor improved after both HIP treatment and heat treatment, indicating that HIP treatment is more effective than heat treatment. Therefore, the high treatment pressure was one of the factors leading to an increase in the number of oxygen vacancies. Since the reduction effect of graphite heaters in HIP equipment has been reported, we investigated the effect of the heater material to determine the generation factor of oxygen vacancies. Figure 5 shows the thermoelectric properties of the samples before and after HIP treatment with a graphite heater or a molybdenum (Mo) heater. However, the results when the graphite heater was used were almost the same as those when the Mo heater was used, as shown in Fig. 5 . This finding indicates that the generation of oxygen vacancies is independent of the type of heater material. We performed temperature annealing tests in an oxygen atmosphere to evaluate the thermal reliability of the HIP-treated ZnO samples. Oxygen annealing temperatures were carried out in the range of 773 K to 1273 K at atmospheric pressure. Figure 6 shows the temperature dependence of the effect of oxygen annealing on the thermoelectric properties of the HIP-treated ZnO samples. As shown in Fig. 6 , no change in the thermoelectric power factor was observed up to 873 K, but above this temperature, the thermoelectric power factor decreased with increasing temperature due to the decrease in electrical conductivity. In other words, it was confirmed that the HIP-treated ZnO samples can stably maintain a current value of up to 873 K in the atmosphere. Conclusion We investigated the effect of HIP treatment on the thermoelectric power factor of ZnO to improve the thermoelectric conversion efficiency. As a result, the thermoelectric power factor of ZnO reached a maximum value after HIP treatment under argon gas for 2 hours. The HIP treatment generated oxygen vacancies, and the generation factors of the oxygen vacancies were both the atmosphere and pressure during the HIP treatment. Moreover, it was confirmed that the HIP-treated ZnO samples can stably maintain a current value of up to 873 K in the atmosphere. Therefore, HIP treatment is a promising technique for improving the TE properties of ZnO samples. Experimental details The samples of the ZnO thermoelectric materials were prepared from powders of ZnO (purity 99.9%, particle size 1 µm; Kojundo Chemical Laboratory Co., Ltd.). The powders were compressed into pellets with a diameter of 10 mm for one minute at a pressure of 30 MPa. The pellets were then sintered at 1273 K for one hour in air. The HIP treatment was subsequently conducted at temperatures between 973 and 1273 K under an argon atmosphere or a mixed argon‒oxygen atmosphere at a pressure of 120 MPa. The samples reached the desired temperature and pressure within two hours and were then held under these conditions for an additional two hours. The HIP treatment was carried out with O 2 -Dr. HIP (Kobe Steel, Ltd.). The Seebeck coefficient was determined by the relationship between the Seebeck voltage of a sample and the temperature difference between a sample with a hot plate and a water-cooled heat sink. The Seebeck voltage was measured via a Keithley 2182 nanovoltmeter. The temperatures were measured via two K-type thermocouples connected to the hot plate and the heat sink side of the sample. The electrical conductivity was quantified via the van der Pauw method. The thermoelectric power factor of the sample was determined by evaluating the Seebeck coefficient and the electrical conductivity at room temperature in air. Declarations Author contributions H.M. prepared and tested the samples. H.Y. designed, formulated and supervised the study. H.M. and H.Y. analyzed the results and prepared the manuscript. Data availability Data sets generated during the current study are available from the corresponding author on reasonable request. Competing interests The authors declare that they have no competing interests. Additional information Correspondence and requests for materials should be addressed to H.M. References Zhang, X. & Zhao, L. Thermoelectric materials: Energy conversion between heat and electricity. J. Materiomics . 1 , 92-105 (2015). Jaziri, N. et al . A comprehensive review of Thermoelectric Generators: Technologies and common applications. Energy Reports . 6 , 264-287 (2020). Du, Y., Xu, J., Paul, B. & Eklund, P. Flexible thermoelectric materials and devices. Appl. Mater. Today . 12 , 366-388 (2018). Ohtaki, M. Recent aspects of oxide thermoelectric materials for power generation from mid-to-high temperature heat source. J. Ceram. Soc. Japan . 119 , 770-775 (2011). Terasaki, I., Sasago, Y. & Uchinokura, K. Large thermoelectric power in NaCo 2 O 4 single crystals. Phys. Rev. B . 56 , R12685-R12687 (1997). Terasaki, I. Cobalt Oxides and Kondo Semiconductors: A Pseudogap System as a Thermoelectric Material, Mater. Trans . 42 , 951-955 (2001). Funahashi, R., Matsubara, I., Ikuta, H., Takeuchi, T., Mizutani, U. & Sodeoka, S. An Oxide Single Crystal with High Thermoelectric Performance in Air. Jpn. J. Appl. Phys. 39 , L1127-L1129 (2000). Ohtaki, M., Tsubota, T., Eguchi, K. & Arai, H. High-temperature thermoelectric properties of (Zn 1-x Al x ) O. J. Appl. Phys. 79 , 1816-1818 (1996). Tsubota, T., Ohtaki, M., Eguchi, K. & Arai, H. Thermoelectric properties of Al-doped ZnO as a promising oxide material for hightemperature thermoelectric conversion. J. Mater. Chem. 7 , 85-90 (1997). Ohtaki, M., Shige, S. & Maehara, S. Enhanced Thermoelectric Performance of ZnO-based Oxide Materials. Trans. Mater. Res. Soc. Japan. 29 , 2727-2730 (2004). Jung, K., Lee, K. H., Seo, W. & Choi, S. An enhancement of a thermoelectric power factor in a Ga-doped ZnO system: A chemical compression by enlarged Ga solubility. Appl. Phys. Lett. 100 , 253902 (2012). Ohtaki, M., Araki, K. & Yamamoto, K. High Thermoelectric Performance of Dually Doped ZnO Ceramics. J. Electro. Mater. 38 , 1234-1238 (2009). Ohtaki, M. & Araki, K. Thermoelectric properties and thermopower enhancement of Al-doped ZnO with nanosized pore structure. J. Ceram. Soc. Japan . 119 , 813-816 (2011). Kinemuchi, Y., Mikami, M., Kobayashi, K., Watari, K. & Hotta, Y. Thermoelectric Properties of Nanograined ZnO. J. Electro. Mater. 39 , 2059-2063 (2010). Choi, M. et al . High figure-of-merit for ZnO nanostructures by interfacing lowly oxidized graphene quantum dots. Nat. Commun . 15 , 1996 (2024). Bocanegra, M. H. Hot Isostatic Pressing (HIP) technology and its applications to metals and ceramics. J. Mater. Sci. 39 , 6399-6420 (2004). Barinov, S. M., Ponomarev, V. F. & Shevchenko, V. Ya. Effect of hot isostatic pressing on the mechanical properties of aluminum oxide ceramics. Refract. Ind. Ceram. 38 , 9-12 (1997). Additional Declarations No competing interests reported. Cite Share Download PDF Status: Published Journal Publication published 28 Dec, 2024 Read the published version in Scientific Reports → Version 1 posted Editorial decision: Revision requested 25 Aug, 2024 Reviews received at journal 23 Aug, 2024 Reviews received at journal 18 Aug, 2024 Reviewers agreed at journal 13 Aug, 2024 Reviewers agreed at journal 13 Aug, 2024 Reviewers invited by journal 13 Aug, 2024 Editor assigned by journal 13 Aug, 2024 Editor invited by journal 13 Aug, 2024 Submission checks completed at journal 13 Aug, 2024 First submitted to journal 01 Aug, 2024 You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. As a division of Research Square Company, we’re committed to making research communication faster, fairer, and more useful. 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Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {"props":{"pageProps":{"initialData":{"identity":"rs-4844832","acceptedTermsAndConditions":true,"allowDirectSubmit":false,"archivedVersions":[],"articleType":"Article","associatedPublications":[],"authors":[{"id":344952380,"identity":"df5ee2c3-2d0d-4cb6-8c2b-b4e0ea728468","order_by":0,"name":"Hidenobu Mori","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAAA6klEQVRIiWNgGAWjYJCCAx8gNBtCSIKAloMzSNbCzIOhBR/Qbe99eNim4o5dg0QC22Oeilo5BvbDDxgsd+DWYnbmuMHhnDPPkoFa2I15zhw3ZuBJM2CQPINHy400hsO5bYeTGSTyv0nzth1LbGDIYWCQbCOgxfIfSEsCG0QL/xsitDA2HLaDaqlJbJAgZMuZYwwHe44dTmDjecAmOefMAWM2iWcGB/D65Xgb84cfNYft+dkT2CTeVNTJ8fMnP3wsiSfEYCAR5BImHobD4Ng5LNlAWIs9iGD8wVAH5jF+JELLKBgFo2AUjBgAABMwTR+9oOYFAAAAAElFTkSuQmCC","orcid":"","institution":"University of Hyogo","correspondingAuthor":true,"prefix":"","firstName":"Hidenobu","middleName":"","lastName":"Mori","suffix":""},{"id":344952381,"identity":"ca07231b-5804-462b-9407-4f9c05c72f39","order_by":1,"name":"Haruhiko Yoshida","email":"","orcid":"","institution":"University of Hyogo","correspondingAuthor":false,"prefix":"","firstName":"Haruhiko","middleName":"","lastName":"Yoshida","suffix":""}],"badges":[],"createdAt":"2024-08-02 00:40:39","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-4844832/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-4844832/v1","draftVersion":[],"editorialEvents":[{"content":"https://doi.org/10.1038/s41598-024-82880-z","type":"published","date":"2024-12-28T15:57:38+00:00"}],"editorialNote":"","failedWorkflow":false,"files":[{"id":64182190,"identity":"43277d04-c1af-4998-8812-9eaaab081a7a","added_by":"auto","created_at":"2024-09-09 15:10:02","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":20626,"visible":true,"origin":"","legend":"\u003cp\u003eHIP treatment temperature dependence of the (a) Seebeck coefficient, (b) electrical conductivity and (c) thermoelectric power factor of the ZnO samples.\u003c/p\u003e","description":"","filename":"1.png","url":"https://assets-eu.researchsquare.com/files/rs-4844832/v1/70d77cd77ef5f83a82b63ffe.png"},{"id":64182194,"identity":"833e0250-a5c8-4f3e-9d03-71ed1620e96e","added_by":"auto","created_at":"2024-09-09 15:10:03","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":43003,"visible":true,"origin":"","legend":"\u003cp\u003eChanges in the (a) Seebeck coefficient, (b) electrical conductivity and (c) thermoelectric power factor of the ZnO samples before and after HIP treatment in a mixture of argon and oxygen or argon gas.\u003c/p\u003e","description":"","filename":"2.png","url":"https://assets-eu.researchsquare.com/files/rs-4844832/v1/8b42daaadf52134b880477cd.png"},{"id":64182748,"identity":"4340af5f-bca8-49b6-a3ee-bab444348ed4","added_by":"auto","created_at":"2024-09-09 15:18:02","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":19159,"visible":true,"origin":"","legend":"\u003cp\u003eRelationships between the (a) volume, (b) mass and (c) density of the ZnO samples before and after HIP treatment with a mixture of argon and oxygen or argon gas.\u003c/p\u003e","description":"","filename":"3.png","url":"https://assets-eu.researchsquare.com/files/rs-4844832/v1/2f64bed7bbecceee2bf8d30b.png"},{"id":64182195,"identity":"f715e39b-6a76-4efa-8cce-2e1d1431c489","added_by":"auto","created_at":"2024-09-09 15:10:03","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":18614,"visible":true,"origin":"","legend":"\u003cp\u003eChanges in the (a) Seebeck coefficient, (b) electrical conductivity and (c) thermoelectric power factor of the ZnO samples in argon gas under atmospheric pressure during heat treatment or HIP treatment.\u003c/p\u003e","description":"","filename":"4.png","url":"https://assets-eu.researchsquare.com/files/rs-4844832/v1/02204e416c0a1dbdad622ee2.png"},{"id":64182192,"identity":"f0157395-0905-4284-a226-82315e17704a","added_by":"auto","created_at":"2024-09-09 15:10:03","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":18720,"visible":true,"origin":"","legend":"\u003cp\u003eDifferences in the (a) Seebeck coefficient, (b) electrical conductivity and (c) thermoelectric power factor of ZnO samples in argon gas with a graphite heater or molybdenum heater before and after HIP treatment.\u003c/p\u003e","description":"","filename":"5.png","url":"https://assets-eu.researchsquare.com/files/rs-4844832/v1/e777817214b72e90de639aa8.png"},{"id":64182193,"identity":"cbb644d2-52ed-4117-a11f-a0452ce13948","added_by":"auto","created_at":"2024-09-09 15:10:03","extension":"png","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":25084,"visible":true,"origin":"","legend":"\u003cp\u003eEffect of oxygen annealing on the (a) Seebeck coefficient, (b) electrical conductivity and (c) thermoelectric power factor of the HIP-treated ZnO samples.\u003c/p\u003e","description":"","filename":"6.png","url":"https://assets-eu.researchsquare.com/files/rs-4844832/v1/b486b2c489e4650c7cea166c.png"},{"id":72640651,"identity":"3ffe1453-1be9-4b3b-ba3b-809e7e925ed8","added_by":"auto","created_at":"2024-12-30 16:08:04","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":339420,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-4844832/v1/2911b459-320b-409d-9540-26e6f6553dbd.pdf"}],"financialInterests":"No competing interests reported.","formattedTitle":"Effect of Hot Isostatic Pressing Treatment on the Thermoelectric Power Factors of Zinc Oxides","fulltext":[{"header":"Introduction","content":"\u003cp\u003eA thermoelectric power generation system is expected to make efficient use of waste heat by directly converting thermal energy into electrical energy.\u003csup\u003e\u003cspan additionalcitationids=\"CR2 CR3\" citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e\u003c/sup\u003e The performance of this system is determined by the efficiency of the thermoelectric materials, which is evaluated by the dimensionless figure of merit defined as ZT = (S\u003csup\u003e2\u003c/sup\u003e σ/κ)Τ, where S, σ, κ and T are the Seebeck coefficient, the electrical conductivity, the thermal conductivity and the absolute temperature, respectively. The figure of merit can also be transformed to Z\u0026thinsp;=\u0026thinsp;P/κ, where P is the thermoelectric power factor (P\u0026thinsp;=\u0026thinsp;S\u003csup\u003e2\u003c/sup\u003e σ). Therefore, for a thermoelectric material with high power, a large thermoelectric power factor and a small thermal conductivity are needed.\u003c/p\u003e \u003cp\u003eCurrently, nonoxides are employed in the fabrication of practical thermoelectric materials. However, from the standpoint of heat resistance, the reduction of environmental burdens, and production methods, metal oxide materials have attracted attention as the next generation of thermoelectric materials.\u003csup\u003e\u003cspan additionalcitationids=\"CR5 CR6\" citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e\u003c/sup\u003e Recently, zinc oxide (ZnO) has emerged as a potential n-type thermoelectric material because of its low toxicity potential, high electron mobility, thermal resistance, and corrosion resistance.\u003csup\u003e\u003cspan additionalcitationids=\"CR9 CR10 CR11 CR12 CR13 CR14\" citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e\u003c/sup\u003e To enhance the thermoelectric properties of ZnO, the effects of the introduction of additives such as Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e and Ga\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e8\u0026ndash;12\u003c/sup\u003e, as well as the effects of nanostructures\u003csup\u003e\u003cspan additionalcitationids=\"CR14\" citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e\u003c/sup\u003e, have been investigated. Nevertheless, enhancing the thermoelectric properties of ZnO from a practical application standpoint is essential. For other oxide materials, it has been reported that crystalline control contributes to the progressive advancement of thermoelectric properties and that a high relative density is an important parameter for the improvement of thermoelectric properties.\u003csup\u003e\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e\u003c/sup\u003e Conversely, HIP treatment is well known to be an effective method for controlling density and crystal defects in materials by simultaneously applying high temperature and pressure.\u003csup\u003e\u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e\u003c/sup\u003e Furthermore, the atmosphere during HIP treatment can be controlled to enable the application of this technique to both metallic and oxide materials. It is therefore anticipated that HIP treatment will result in enhanced thermoelectric properties.\u003c/p\u003e \u003cp\u003eThis paper presents an investigation into the effect of HIP treatment on the thermoelectric power factor of ZnO, with the objective of enhancing the thermoelectric conversion efficiency.\u003c/p\u003e"},{"header":"Results and Discussion","content":"\u003cp\u003eThe effects of HIP treatment on the thermoelectric properties, including the Seebeck coefficient, electrical conductivity, and thermoelectric power factor, of the samples were examined. Figure\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e1\u003c/span\u003e shows the temperature dependence of the thermoelectric properties of the samples following HIP treatment. The absolute value of the Seebeck coefficient is shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e1\u003c/span\u003e(a). Although the Seebeck coefficient was reduced by approximately 50% before and after HIP treatment, its value remained independent of the HIP treatment temperature and exhibited nearly constant behavior, as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e1\u003c/span\u003e(a). In contrast, the electrical conductivity substantially increased with increasing HIP treatment temperature, as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e1\u003c/span\u003e(b). The power factor, which is the ratio of the Seebeck coefficient to the electrical conductivity, increased with increasing temperature during HIP, as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e1\u003c/span\u003e(c). The improvement in the thermoelectric properties was attributed to the HIP treatment, despite the decrease in the Seebeck coefficient. In a previous study, it was reported that the generation of oxygen vacancies occurred in a sample following HIP treatment, resulting in a change in color from white to light gray.\u003csup\u003e\u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e\u003c/sup\u003e In this study, the significant increase in conductivity suggested that oxygen vacancies were generated within the samples. To confirm the presence of oxygen vacancies in the HIP-treated samples, the phase identification of the samples before and after HIP treatment was analyzed via X-ray diffraction at room temperature. However, the X-ray diffraction results indicated that the sample after HIP treatment exhibited the same crystal structure as the sample before HIP treatment. Therefore, the oxygen vacancies introduced by HIP treatment may not be detectable by X-ray diffraction. We investigated the effects of the processing atmosphere, processing pressure, and heaters during HIP treatment to identify the factors contributing to the oxygen vacancies due to HIP treatment.\u003c/p\u003e \u003cp\u003eThe influence of gaseous species in the HIP treatment was verified. Figure\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e2\u003c/span\u003e shows the thermoelectric power factor of the ZnO samples after HIP treatment with a mixture of argon and oxygen or argon gas. As shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e2\u003c/span\u003e, there are variations in the Seebeck coefficient, electrical conductivity and power factor of the samples after HIP treatment under both gas conditions. In particular, the electrical conductivity and the thermoelectric power factor of the sample after HIP treatment under argon gas improved. To clarify this factor, the relationship between the effect of HIP treatment and the density of ZnO was investigated. The density of the samples was measured via Archimedes\u0026rsquo; method. Figures\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e3\u003c/span\u003e(a), 3(b) and 3(c) show the volume, mass and density of the samples before and after HIP treatment at 1273 K under argon gas or under a mixed gas of argon and oxygen for 2 h, respectively. As shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e3\u003c/span\u003e, the sample after HIP treatment under argon gas was denser than the sample after HIP treatment under mixed gas. This occurred because the degree of reduction in the volume of the sample was large regardless of the decrease in mass during the HIP treatment under argon gas. Therefore, the atmosphere of argon gas was considered one of the factors leading to an increase in the number of oxygen vacancies, confirming that HIP treatment was effective in densifying ZnO.\u003c/p\u003e \u003cp\u003eWe compared the thermoelectric power factor between HIP treatment under high pressure and heat treatment under atmospheric pressure to verify the effect of pressure under argon gas. Figure\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e4\u003c/span\u003e shows the thermoelectric power factor of the ZnO samples after HIP treatment or heat treatment. As shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e4\u003c/span\u003e, the electrical conductivity and thermoelectric power factor improved after both HIP treatment and heat treatment, indicating that HIP treatment is more effective than heat treatment. Therefore, the high treatment pressure was one of the factors leading to an increase in the number of oxygen vacancies.\u003c/p\u003e \u003cp\u003eSince the reduction effect of graphite heaters in HIP equipment has been reported, we investigated the effect of the heater material to determine the generation factor of oxygen vacancies. Figure\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e5\u003c/span\u003e shows the thermoelectric properties of the samples before and after HIP treatment with a graphite heater or a molybdenum (Mo) heater. However, the results when the graphite heater was used were almost the same as those when the Mo heater was used, as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e5\u003c/span\u003e. This finding indicates that the generation of oxygen vacancies is independent of the type of heater material.\u003c/p\u003e \u003cp\u003eWe performed temperature annealing tests in an oxygen atmosphere to evaluate the thermal reliability of the HIP-treated ZnO samples. Oxygen annealing temperatures were carried out in the range of 773 K to 1273 K at atmospheric pressure. Figure\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e6\u003c/span\u003e shows the temperature dependence of the effect of oxygen annealing on the thermoelectric properties of the HIP-treated ZnO samples. As shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e6\u003c/span\u003e, no change in the thermoelectric power factor was observed up to 873 K, but above this temperature, the thermoelectric power factor decreased with increasing temperature due to the decrease in electrical conductivity. In other words, it was confirmed that the HIP-treated ZnO samples can stably maintain a current value of up to 873 K in the atmosphere.\u003c/p\u003e"},{"header":"Conclusion","content":"\u003cp\u003eWe investigated the effect of HIP treatment on the thermoelectric power factor of ZnO to improve the thermoelectric conversion efficiency. As a result, the thermoelectric power factor of ZnO reached a maximum value after HIP treatment under argon gas for 2 hours. The HIP treatment generated oxygen vacancies, and the generation factors of the oxygen vacancies were both the atmosphere and pressure during the HIP treatment. Moreover, it was confirmed that the HIP-treated ZnO samples can stably maintain a current value of up to 873 K in the atmosphere. Therefore, HIP treatment is a promising technique for improving the TE properties of ZnO samples.\u003c/p\u003e"},{"header":"Experimental details","content":"\u003cp\u003eThe samples of the ZnO thermoelectric materials were prepared from powders of ZnO (purity 99.9%, particle size 1 \u0026micro;m;\u0026nbsp;Kojundo Chemical Laboratory Co., Ltd.). The powders were compressed into pellets with a diameter of 10 mm for one minute at a pressure of 30 MPa. The pellets were then sintered at 1273 K for one hour in air. The HIP treatment was subsequently conducted at temperatures between 973 and 1273 K under an argon atmosphere or a mixed argon‒oxygen atmosphere at a pressure of 120 MPa. The samples reached the desired temperature and pressure within two hours and were then held under these conditions for an additional two hours. The HIP treatment was carried out with O\u003csub\u003e2\u003c/sub\u003e-Dr. HIP (Kobe Steel, Ltd.).\u003c/p\u003e\n\u003cp\u003eThe Seebeck coefficient was determined by the relationship between the Seebeck voltage of a sample and the temperature difference between a sample with a hot plate and a water-cooled heat sink. The Seebeck voltage was measured via a Keithley 2182 nanovoltmeter. The temperatures were measured via two K-type thermocouples connected to the hot plate and the heat sink side of the sample. The electrical conductivity was quantified via the van der Pauw method. The thermoelectric power factor of the sample was determined by evaluating the Seebeck coefficient and the electrical conductivity at room temperature in air.\u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003eAuthor contributions\u003c/p\u003e\n\u003cp\u003eH.M. prepared and tested the samples. H.Y. designed, formulated and supervised the study. H.M. and H.Y. analyzed the results and prepared the manuscript.\u003c/p\u003e\n\u003cp\u003eData availability\u003c/p\u003e\n\u003cp\u003eData sets generated during the current study are available from the corresponding author on reasonable request.\u003c/p\u003e\n\u003cp\u003eCompeting interests\u003c/p\u003e\n\u003cp\u003eThe authors declare that they have no competing interests.\u003c/p\u003e\n\u003cp\u003eAdditional information\u003c/p\u003e\n\u003cp\u003eCorrespondence and requests for materials should be addressed to H.M.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\n\u003cli\u003eZhang, X. \u0026amp; Zhao, L. Thermoelectric materials: Energy conversion between heat and electricity. \u003cem\u003eJ. Materiomics\u003c/em\u003e. \u003cstrong\u003e1\u003c/strong\u003e, 92-105 (2015).\u003c/li\u003e\n\u003cli\u003eJaziri, N. \u003cem\u003eet al\u003c/em\u003e. A comprehensive review of Thermoelectric Generators: Technologies and common applications. \u003cem\u003eEnergy Reports\u003c/em\u003e. \u003cstrong\u003e6\u003c/strong\u003e, 264-287 (2020).\u003c/li\u003e\n\u003cli\u003eDu, Y., Xu, J., Paul, B. \u0026amp; Eklund, P. Flexible thermoelectric materials and devices. \u003cem\u003eAppl. \u003c/em\u003e\u003cem\u003eMater. Today\u003c/em\u003e. \u003cstrong\u003e12\u003c/strong\u003e, 366-388 (2018).\u003c/li\u003e\n\u003cli\u003eOhtaki, M. Recent aspects of oxide thermoelectric materials for power generation from mid-to-high temperature heat source. \u003cem\u003eJ. Ceram. Soc. Japan\u003c/em\u003e. \u003cstrong\u003e119\u003c/strong\u003e, 770-775 (2011).\u003c/li\u003e\n\u003cli\u003eTerasaki, I., Sasago, Y. \u0026amp; Uchinokura, K. Large thermoelectric power in NaCo\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e single crystals. \u003cem\u003ePhys. Rev. B\u003c/em\u003e. \u003cstrong\u003e56\u003c/strong\u003e, R12685-R12687 (1997).\u003c/li\u003e\n\u003cli\u003eTerasaki, I. Cobalt Oxides and Kondo Semiconductors: A Pseudogap System as a Thermoelectric Material, \u003cem\u003eMater. Trans\u003c/em\u003e. \u003cstrong\u003e42\u003c/strong\u003e, 951-955 (2001).\u003c/li\u003e\n\u003cli\u003eFunahashi, R., Matsubara, I., Ikuta, H., Takeuchi, T., Mizutani, U. \u0026amp; Sodeoka, S. An Oxide Single Crystal with High Thermoelectric Performance in Air. \u003cem\u003eJpn. J. Appl. Phys.\u003c/em\u003e \u003cstrong\u003e39\u003c/strong\u003e, L1127-L1129 (2000).\u003c/li\u003e\n\u003cli\u003eOhtaki, M., Tsubota, T., Eguchi, K. \u0026amp; Arai, H. High-temperature thermoelectric properties of (Zn\u003csub\u003e1-x\u003c/sub\u003eAl\u003csub\u003ex\u003c/sub\u003e) O. \u003cem\u003eJ. Appl. Phys.\u003c/em\u003e \u003cstrong\u003e79\u003c/strong\u003e, 1816-1818 (1996).\u003c/li\u003e\n\u003cli\u003eTsubota, T., Ohtaki, M., Eguchi, K. \u0026amp; Arai, H. Thermoelectric properties of Al-doped ZnO as a promising oxide material for hightemperature thermoelectric conversion. \u003cem\u003eJ. Mater. Chem.\u003c/em\u003e \u003cstrong\u003e7\u003c/strong\u003e, 85-90 (1997).\u003c/li\u003e\n\u003cli\u003eOhtaki, M., Shige, S. \u0026amp; Maehara, S. Enhanced Thermoelectric Performance of ZnO-based Oxide Materials. \u003cem\u003eTrans. Mater. Res. Soc. Japan.\u003c/em\u003e \u003cstrong\u003e29\u003c/strong\u003e, 2727-2730 (2004).\u003c/li\u003e\n\u003cli\u003eJung, K., Lee, K. H., Seo, W. \u0026amp; Choi, S. An enhancement of a thermoelectric power factor in a Ga-doped ZnO system: A chemical compression by enlarged Ga solubility. \u003cem\u003eAppl. Phys. Lett.\u003c/em\u003e \u003cstrong\u003e100\u003c/strong\u003e, 253902 (2012).\u003c/li\u003e\n\u003cli\u003eOhtaki, M., Araki, K. \u0026amp; Yamamoto, K. High Thermoelectric Performance of Dually Doped ZnO Ceramics. \u003cem\u003eJ. Electro. Mater.\u003c/em\u003e \u003cstrong\u003e38\u003c/strong\u003e, 1234-1238 (2009).\u003c/li\u003e\n\u003cli\u003eOhtaki, M. \u0026amp; Araki, K. Thermoelectric properties and thermopower enhancement of Al-doped ZnO with nanosized pore structure. \u003cem\u003eJ. Ceram. Soc. Japan\u003c/em\u003e. \u003cstrong\u003e119\u003c/strong\u003e, 813-816 (2011).\u003c/li\u003e\n\u003cli\u003eKinemuchi, Y., Mikami, M., Kobayashi, K., Watari, K. \u0026amp; Hotta, Y. Thermoelectric Properties of Nanograined ZnO. \u003cem\u003eJ. Electro. Mater.\u003c/em\u003e \u003cstrong\u003e39\u003c/strong\u003e, 2059-2063 (2010).\u003c/li\u003e\n\u003cli\u003eChoi, M. \u003cem\u003eet al\u003c/em\u003e. High figure-of-merit for ZnO nanostructures by interfacing lowly oxidized graphene quantum dots. \u003cem\u003eNat. Commun\u003c/em\u003e. \u003cstrong\u003e15\u003c/strong\u003e, 1996 (2024).\u003c/li\u003e\n\u003cli\u003eBocanegra, M. H. Hot Isostatic Pressing (HIP) technology and its applications to metals and ceramics. \u003cem\u003eJ. Mater. Sci.\u003c/em\u003e \u003cstrong\u003e39\u003c/strong\u003e, 6399-6420 (2004).\u003c/li\u003e\n\u003cli\u003eBarinov, S. M., Ponomarev, V. F. \u0026amp; Shevchenko, V. Ya. Effect of hot isostatic pressing on the mechanical properties of aluminum oxide ceramics. \u003cem\u003eRefract. Ind. Ceram.\u003c/em\u003e\u003cstrong\u003e38\u003c/strong\u003e, 9-12 (1997).\u003c/li\u003e\n \u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":false,"highlight":"","institution":"","isAcceptedByJournal":true,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"scientific-reports","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"scirep","sideBox":"Learn more about [Scientific Reports](http://www.nature.com/srep/)","snPcode":"","submissionUrl":"","title":"Scientific Reports","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"stoa","reportingPortfolio":"Scientific Reports","inReviewEnabled":true,"inReviewRevisionsEnabled":true},"keywords":"Zinc Oxide, Hot Isostatic Pressing Treatment, Thermoelectric Power Factor","lastPublishedDoi":"10.21203/rs.3.rs-4844832/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-4844832/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eThe effect of hot isostatic pressing (HIP) on the thermoelectric power factor of zinc oxide (ZnO) has been examined. ZnO is expected to be a potential n-type oxide thermoelectric material that could enhance the thermoelectric conversion efficiency. The HIP treatment is useful for densifying the material and controlling crystal defects in the material by applying high temperatures and pressures simultaneously. Furthermore, the atmosphere during HIP treatment can be controlled to enable the application of this technique to both metallic and oxide materials.\u003c/p\u003e \u003cp\u003eThe thermoelectric power factor of ZnO increased due to a notable increase in electrical conductivity, although the Seebeck coefficient decreased by approximately 50% following HIP treatment under argon gas. The increase in the thermoelectric power factor is attributed to the oxygen vacancies introduced into ZnO subsequent to the HIP treatment. Consequently, HIP treatment represents a promising approach for enhancing the thermoelectric power factor of ZnO.\u003c/p\u003e","manuscriptTitle":"Effect of Hot Isostatic Pressing Treatment on the Thermoelectric Power Factors of Zinc Oxides","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2024-09-09 15:09:58","doi":"10.21203/rs.3.rs-4844832/v1","editorialEvents":[{"type":"communityComments","content":0},{"type":"decision","content":"Revision requested","date":"2024-08-26T02:33:03+00:00","index":"","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2024-08-23T13:19:42+00:00","index":"hide","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2024-08-18T09:10:42+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"190772240629201708374688955119447416818","date":"2024-08-13T11:12:01+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"227105388494058405976032008163878964757","date":"2024-08-13T08:27:58+00:00","index":"hide","fulltext":""},{"type":"reviewersInvited","content":"","date":"2024-08-13T08:13:51+00:00","index":"","fulltext":""},{"type":"editorAssigned","content":"","date":"2024-08-13T08:02:04+00:00","index":"","fulltext":""},{"type":"editorInvited","content":"","date":"2024-08-13T07:54:39+00:00","index":"","fulltext":""},{"type":"checksComplete","content":"","date":"2024-08-13T07:51:48+00:00","index":"","fulltext":""},{"type":"submitted","content":"Scientific Reports","date":"2024-08-02T00:39:14+00:00","index":"","fulltext":""}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"scientific-reports","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"scirep","sideBox":"Learn more about [Scientific Reports](http://www.nature.com/srep/)","snPcode":"","submissionUrl":"","title":"Scientific Reports","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"stoa","reportingPortfolio":"Scientific Reports","inReviewEnabled":true,"inReviewRevisionsEnabled":true}}],"origin":"","ownerIdentity":"3fc6619e-9968-425d-a492-5263e6875ae0","owner":[],"postedDate":"September 9th, 2024","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"published-in-journal","subjectAreas":[{"id":36555645,"name":"Physical sciences/Energy science and technology"},{"id":36555646,"name":"Physical sciences/Engineering"},{"id":36555647,"name":"Physical sciences/Materials science"}],"tags":[],"updatedAt":"2024-12-30T16:02:07+00:00","versionOfRecord":{"articleIdentity":"rs-4844832","link":"https://doi.org/10.1038/s41598-024-82880-z","journal":{"identity":"scientific-reports","isVorOnly":false,"title":"Scientific Reports"},"publishedOn":"2024-12-28 15:57:38","publishedOnDateReadable":"December 28th, 2024"},"versionCreatedAt":"2024-09-09 15:09:58","video":"","vorDoi":"10.1038/s41598-024-82880-z","vorDoiUrl":"https://doi.org/10.1038/s41598-024-82880-z","workflowStages":[]},"version":"v1","identity":"rs-4844832","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-4844832","identity":"rs-4844832","version":["v1"]},"buildId":"qtupq5eGEP_6zYnWcrvyt","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}