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Self-diffusion activation energies in alpha-Al2O3 below 1000 degrees C: measurements and molecular dynamics calculation
KTH, Tidigare Institutioner                               , Kemi.ORCID-id: 0000-0002-4431-0671
KTH, Tidigare Institutioner                               , Kemi.
KTH, Tidigare Institutioner                               , Fysik.ORCID-id: 0000-0001-7531-3210
KTH, Tidigare Institutioner                               , Fysik.ORCID-id: 0000-0002-2076-5911
Vise andre og tillknytning
2004 (engelsk)Inngår i: Philosophical Magazine Letters, ISSN 0950-0839, E-ISSN 1362-3036, Vol. 84, nr 12, s. 781-789Artikkel i tidsskrift (Fagfellevurdert) Published
Abstract [en]

Results from impedance spectroscopy measurements at temperatures between 400 and 1000° C, for single crystal and highly pure and dense polycrystalline α-Al2O3 samples with well-defined grain size, are compared with that from molecular dynamics calculation. Between 650 and 1000° C, the measured activation energy for conductivity is 1.5 eV for the single crystal, and increases from 1.6 to 2.4 eV as the grain size decreases from 15 to 0.5 μ m. The molecular dynamics calculation leads to the conclusion that the self-diffusion activation energy is about 1.5 eV for O and 1.0 eV for Al in single crystal α-Al2O3. The much higher mobility of O ions makes the O ions responsible for the conductivity of the single crystal oxide. It seems that the grain boundary leads to an increase in the activation energy. However, the quantitative influence of grain boundary still needs to be explained. Between 400 and 650° C, the measured activation energy is about 1.0 eV and independent of the grain size.

sted, utgiver, år, opplag, sider
2004. Vol. 84, nr 12, s. 781-789
Emneord [en]
Activation energy; Crystal growth from melt; Diffusion; Electric conductivity; Grain boundaries; Grain size and shape; Molecular dynamics; Polycrystalline materials; Single crystals; Spectroscopic analysis; Thermal effects; Impedance spectroscopy; Molecular dynamics calculation; Self-diffusion activation energy; Single crystal oxides; Alumina
HSV kategori
Identifikatorer
URN: urn:nbn:se:kth:diva-6901DOI: 10.1080/09500830500071051ISI: 000228882900004Scopus ID: 2-s2.0-19944413992OAI: oai:DiVA.org:kth-6901DiVA, id: diva2:11745
Merknad
QC 20100826Tilgjengelig fra: 2007-03-15 Laget: 2007-03-15 Sist oppdatert: 2017-12-14bibliografisk kontrollert
Inngår i avhandling
1. Ionic Transport in Metal Oxides Studied in situ by Impedance Spectroscopy and Cyclic Voltammetry
Åpne denne publikasjonen i ny fane eller vindu >>Ionic Transport in Metal Oxides Studied in situ by Impedance Spectroscopy and Cyclic Voltammetry
2007 (engelsk)Doktoravhandling, med artikler (Annet vitenskapelig)
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.

sted, utgiver, år, opplag, sider
Stockholm: KTH, 2007. s. x, 61
Serie
Trita-CHE-Report, ISSN 1654-1081 ; 2007:3
Emneord
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.
HSV kategori
Identifikatorer
urn:nbn:se:kth:diva-4312 (URN)978-91-7178-587-9 (ISRN)978-91-7178-587-9 (ISBN)
Disputas
2007-03-30, F3, Lindstedtsvägen 26, Stockholm, 14:00
Opponent
Veileder
Merknad
QC 20100825Tilgjengelig fra: 2007-03-15 Laget: 2007-03-15 Sist oppdatert: 2010-08-26bibliografisk kontrollert

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