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In-situ measurements by impedance spectroscopy of highly resistive alpha-alumina
KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap.
KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap.ORCID-id: 0000-0002-4431-0671
KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap.ORCID-id: 0000-0002-9453-1333
2006 (Engelska)Ingår i: Corrosion Science, ISSN 0010-938X, E-ISSN 1879-0496, Vol. 48, nr 1, s. 243-257Artikel i tidskrift (Refereegranskat) Published
Abstract [en]

In-situ impedance spectroscopy has been used for characterisation of oxides at elevated temperatures. However, for highly resistive oxides, the influence of electrode contact and leakage currents due to gas phase and surface conduction needs to be taken into account. In this study, IS measurements of pure and dense alpha-alumina (alpha-Al2O3) samples were performed in the temperature range 400-1000 degrees C with different types of electrode contact, in air and in nitrogen. The results show that above 700 degrees C the influence is negligible, whereas at lower temperatures the surface leakage current was substantial, and a so-called guard electrode recommendable.

Ort, förlag, år, upplaga, sidor
2006. Vol. 48, nr 1, s. 243-257
Nyckelord [en]
aluminium oxide; impedance spectroscopy; ionic conductivity; high temperature corrosion; HIGH-TEMPERATURE OXIDATION; SINGLE-CRYSTAL ALUMINA; ELECTRICAL CONDUCTIVITY; SELF-DIFFUSION; OXIDE-FILMS; POLARIZATION; ALPHA-AL2O3; STEEL
Nationell ämneskategori
Oorganisk kemi
Identifikatorer
URN: urn:nbn:se:kth:diva-6899DOI: 10.1016/j.corsci.2004.11.027ISI: 000234618800017Scopus ID: 2-s2.0-28044446986OAI: oai:DiVA.org:kth-6899DiVA, id: diva2:11743
Anmärkning
QC 20100825Tillgänglig från: 2007-03-15 Skapad: 2007-03-15 Senast uppdaterad: 2017-12-14Bibliografiskt granskad
Ingår i avhandling
1. Ionic Transport in Metal Oxides Studied in situ by Impedance Spectroscopy and Cyclic Voltammetry
Öppna denna publikation i ny flik eller fönster >>Ionic Transport in Metal Oxides Studied in situ by Impedance Spectroscopy and Cyclic Voltammetry
2007 (Engelska)Doktorsavhandling, sammanläggning (Övrigt vetenskapligt)
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.

Ort, förlag, år, upplaga, sidor
Stockholm: KTH, 2007. s. x, 61
Serie
Trita-CHE-Report, ISSN 1654-1081 ; 2007:3
Nyckelord
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.
Nationell ämneskategori
Oorganisk kemi
Identifikatorer
urn:nbn:se:kth:diva-4312 (URN)978-91-7178-587-9 (ISRN)978-91-7178-587-9 (ISBN)
Disputation
2007-03-30, F3, Lindstedtsvägen 26, Stockholm, 14:00
Opponent
Handledare
Anmärkning
QC 20100825Tillgänglig från: 2007-03-15 Skapad: 2007-03-15 Senast uppdaterad: 2010-08-26Bibliografiskt granskad

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