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Influence of Grain Size on Ionic Conductivity of Pure and Dense alpha-Al2O3 in the Temperature Range 400-1000 degrees C
KTH, Superseded Departments, Materials Science and Engineering.
KTH, Superseded Departments, Materials Science and Engineering.ORCID iD: 0000-0002-4431-0671
Kanthal AB, Hallstahammar.
2004 (English)In: High Temperature Corrosion And Protection Of Materials 6, Prt 1 And 2, Proceedings, 2004, Vol. 461-464, no II, 865-872 p.Conference paper, Published paper (Refereed)
Abstract [en]

Ionic transport in alpha-Al2O3 at high temperatures is important for oxidation/corrosion resistance of Al2O3-forming heat resistant alloys. Whereas most of previous studies were performed at temperatures above 1000degreesC, this work concerns the temperature range 400 to 1000degreesC. Electrical proper-ties of polycrystalline and single crystal alpha-Al2O3 were measured in air by using impedance spectroscopy in order to study the ionic transport in Al2O3. The influence of grain size was investigated by measurements on a series of highly pure and dense samples with well-defined grain size ranging from 0.5 to 15 mum, produced by the spark plasma sintering technique. The results show that the grain size has a certain influence on the conductivity in the temperature range between 650 and 1000degreesC, where the activation energy for conductivity decrease from 2.4 to 1.6 eV with increasing grain size. At lower temperatures the activation energy is about 1.0 eV and independent of the grain size, indicating a different conduction mechanism.

Place, publisher, year, edition, pages
2004. Vol. 461-464, no II, 865-872 p.
Series
Materials Science Forum, ISSN 0255-5476 ; 461-464
Keyword [en]
Alumina; Electrical conductivity; Grain size; Impedance spectroscopy; Charge transport; Impedance spectroscopy; Ionic transport; Low conductivity; Activation energy; Corrosion resistance; Diffusion; Electric conductivity; Grain size and shape; Ionic conduction; Leakage currents; Oxidation; Single crystals; Sintering; Superalloys; Thermal effects; Transport properties; alumina
National Category
Inorganic Chemistry
Identifiers
URN: urn:nbn:se:kth:diva-6900ISI: 000223339700103Scopus ID: 2-s2.0-8744239371ISBN: 0-87849-945-8 (print)OAI: oai:DiVA.org:kth-6900DiVA: diva2:11744
Conference
6th International Symposium on High Temperature Corrosion and Protection of Materials Les Embiez, FRANCE, MAY 16-21, 2004
Note

QC 20141208

Available from: 2007-03-15 Created: 2007-03-15 Last updated: 2014-12-08Bibliographically approved
In thesis
1. Ionic Transport in Metal Oxides Studied in situ by Impedance Spectroscopy and Cyclic Voltammetry
Open this publication in new window or tab >>Ionic Transport in Metal Oxides Studied in situ by Impedance Spectroscopy and Cyclic Voltammetry
2007 (English)Doctoral thesis, comprehensive summary (Other scientific)
Abstract [en]

Ionic transport in metal oxides is crucial for the functioning of a broad range of different components, such as heat resistant alloys designed for high temperature applications and oxide electrolytes in solid oxide fuel cells. This thesis presents results from in situ electrochemical studies of properties related to ionic transport in metal oxides that are important for their applications as protective oxides and ionic conductors.

Heat resistant alloys of alumina-former type are known to form an adherent, slowly growing and protective aluminium oxide (Al2O3) scale that protects metals from chemical degradation at high temperature. In situ impedance spectroscopy was used to study highly pure and dense samples of a-alumina in the temperature range 400 – 1000 °C. It was shown that surface conduction on the sample could severely distort the measurement below 700 °C. The magnitude of the distortions appeared to be sensitive to the type of electrodes used. The use of a so-called guard electrode was shown to effectively block the surface conduction in the measurements. By varying the grain size of the sintered alpha-alumina samples, the influence of grain size on the overall conductivity of the a-alumina was studied. It was shown that the activation energy for conductivity increased as the grain size decreased. Molecular dynamics calculations were performed in order to elucidate whether Al- or O ions are dominant in the ionic conductivity of the alpha-alumina. Comparing the calculation and experimental results, the dominating charge carrier was suggested to be oxygen ions.

Moreover, the ionic transport in thermally grown alumina-like oxide scales formed on a FeCrAl alloy was studied in situ by impedance spectroscopy between 600 and 1000 °C. It was shown that the properties of these scales differ largely from those of pure and dense alpha-alumina. Furthermore, the conductivity is mainly electronic, due to the multiphase/multilayer microstructure and substantial incorporation of species from the base metal. However, the diffusivity obtained from the ionic conductivity was in line with diffusion data in literature obtained by other methods such as thermogravimetry. Besides, the initial stage of oxidation of a number of Fe-, Ni- and Co-based alloys at temperatures between 500 and 800 °C was studied in situ by high temperature cyclic voltammetry, in which the oxygen activity was changed over a wide range. From the resulting voltammograms the redox reactions occurring on the alloy surface could be identified. It was concluded that the base metal oxidized readily on these alloys before a protective chromia- or alumina-like scale is formed. The base metal oxide is most likely incorporated into the more protective oxide.

Further, the oxygen ionic conductivity of highly pure and fully dense yttria-stabilized zirconia produced by spark plasma sintering was studied by impedance spectroscopy. The aim was to evaluate intrinsic blocking effects on the ionic conduction associated with the space charge layer in the grain boundary region. It was observed that the ionic conductivity of the spark plasma sintered oxides is equal or slightly higher than what has been achieved by conventional sintering methods. In addition, it was shown that the specific grain boundary conductivity increases with decreasing grain size, which can be explained by a decreasing Schottky barrier height (i.e., decreasing blocking effect). The quantitative results from this work verify the space charge model describing the influence of grain size on the ionic conductivity of yttria-stabilized zirconia through dopant segregation and oxygen vacancy depletion along the grain boundaries.

Place, publisher, year, edition, pages
Stockholm: KTH, 2007. x, 61 p.
Series
Trita-CHE-Report, ISSN 1654-1081 ; 2007:3
Keyword
ionic transport, alumina, zirconia, in situ impedance spectroscopy, molecular dynamics, high temperature cyclic voltammetry, spark plasma sintering, initial oxidation, FeCrAl alloy, grain size, space charge model.
National Category
Inorganic Chemistry
Identifiers
urn:nbn:se:kth:diva-4312 (URN)978-91-7178-587-9 (ISRN)978-91-7178-587-9 (ISBN)
Public defence
2007-03-30, F3, Lindstedtsvägen 26, Stockholm, 14:00
Opponent
Supervisors
Note
QC 20100825Available from: 2007-03-15 Created: 2007-03-15 Last updated: 2010-08-26Bibliographically approved

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