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  • 1.
    Pan, Jinshan
    et al.
    KTH, Tidigare Institutioner                               , Kemi.
    Öijerholm, Johan
    KTH, Tidigare Institutioner                               , Kemi.
    Belonoshko, Anatoly
    KTH, Tidigare Institutioner                               , Fysik.
    Rosengren, Anders
    KTH, Tidigare Institutioner                               , Fysik.
    Leygraf, Christofer
    KTH, Tidigare Institutioner                               , Kemi.
    Self-diffusion activation energies in alpha-Al2O3 below 1000 degrees C: measurements and molecular dynamics calculation2004Ingår i: Philosophical Magazine Letters, ISSN 0950-0839, E-ISSN 1362-3036, Vol. 84, nr 12, s. 781-789Artikel i tidskrift (Refereegranskat)
    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.

  • 2.
    Zhao, Zhe
    et al.
    KTH, Skolan för kemivetenskap (CHE), Kemi.
    Öijerholm, Johan
    KTH, Skolan för kemivetenskap (CHE), Kemi.
    Pan, Jinshan
    KTH, Skolan för kemivetenskap (CHE), Kemi.
    Microstructure and electrical conductivity of yttria stabilized zirconia ceramics sintered by spark plasma sintering: Part I. 3Y-TZPIngår i: Solid State Ionics, ISSN 0167-2738, E-ISSN 1872-7689Artikel i tidskrift (Övrigt vetenskapligt)
  • 3.
    Öijerholm, Johan
    KTH, Skolan för kemivetenskap (CHE), Kemi.
    Ionic Transport in Metal Oxides Studied in situ by Impedance Spectroscopy and Cyclic Voltammetry2007Doktorsavhandling, 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.

  • 4.
    Öijerholm, Johan
    KTH, Tidigare Institutioner, Materialvetenskap.
    Ionic transport of α-alumina below 1000°C: an in-situ impedance spectrosocpy study2004Licentiatavhandling, sammanläggning (Övrigt vetenskapligt)
    Abstract [en]

    Ionic conductivity of metal oxides is critical for the function of a broad range of different components, such as electrolytes in solid oxide fuel cells and alloys designed for high temperature applications. In both cases the ionic conductivity can be studied by in situ impedance spectroscopy, which is also able to reveal information on the dielectric properties of the metal oxides, and in some cases the influence of their microstructure. The focus of this thesis is on impedance spectroscopy measurements of α-alumina in the temperature range 400-1000 °C. This metal oxide has found extensive use as the protective scale on heat resistant alloys. Some unpublished work on oxygen ion conductivity of yttria-stabilized zirconia is also included.

    The low electrical conductivity of α-alumina can be a source for errors and misinterpretations during impedance spectroscopy measurements. A major disturbance originates from leakage currents that appear in the experimental setup. These leakage currents are due to conduction through the gas phase around the sample, conduction on the sample surface, or poor insulation in the sample holder. It was shown that below 700 °C, conduction on the sample surface could severely distort the measurement. 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.

    Conductivity of metal oxides is known to be dependent on their microstructure. Generally it is believed that ionic conductivity is favoured along grain boundaries and dislocations. The influence of microstructure on conductivity was studied for α-alumina in the temperature range 400-1000 °C. The conductivity of a series of highly pure and dense samples with narrow grain size distributions was measured by impedance spectroscopy. It appeared that the activation energy for conduction increased with decreasing grain size.

    Results based purely on impendence spectroscopy have some inherently weaknesses. For instance no information on the nature of the charge carrier can be found. Therefore the charge transport in single crystalline α-alumina was simulated by the molecular dynamics method. The results from the simulation were then compared to results from impedance measurements on single crystalline α-alumina. From the simulation it turned out that diffusion of aluminium ions had lower activation energy than diffusion of oxygen. The activation energy of oxygen was close to the measured activation energy, and the mobility of oxygen was higher than for aluminium. Therefore the dominating charge carrier was suggested to be oxygen ions.

  • 5.
    Öijerholm, Johan
    et al.
    KTH, Skolan för kemivetenskap (CHE), Kemi.
    Fafilek, Günter
    Pan, Jinshan
    KTH, Skolan för kemivetenskap (CHE), Kemi.
    Cyclic Voltammetry Study of the Early Stage of High Temperature Oxidation of Fe-, Co- and Ni-based alloysIngår i: Journal of the Electrochemical Society, ISSN 0013-4651, E-ISSN 1945-7111Artikel i tidskrift (Övrigt vetenskapligt)
  • 6.
    Öijerholm, Johan
    et al.
    KTH, Skolan för kemivetenskap (CHE), Kemi, Korrosionslära.
    Fafilek, Günter
    Institute of Chemical Technology and Analytics, University of Technology of Vienna.
    Pan, Jinshan
    KTH, Skolan för kemivetenskap (CHE), Kemi, Korrosionslära.
    Study of Initial Oxidation of Engineering Alloys by High Temperature Cyclic Voltammetry2007Ingår i: Electrochemical and solid-state letters, ISSN 1099-0062, E-ISSN 1944-8775, Vol. 10, nr 6, s. 47-50Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The initial oxidation of Fe-, Co-, and Ni-based engineering alloys was studied in situ by high-temperature cyclic voltammetry. Measurements were performed at constant temperatures between 500 and 800 degrees C in an electrochemical cell utilizing oxygen ion-conducting yttria-stabilized zirconia as electrolyte and a reference electrode whose potential is determined by the oxygen activity in the ambient gas. The recorded voltammograms show clear current peaks that are associated with the oxidation of the base metal in the investigated alloys. For the Fe- based alloy, multiple peaks are observed, showing oxidation of Fe into different valence oxides. The results indicate that the base metal oxide is formed in the initial oxidation process of these engineering alloys, which may influence long-term high-temperature oxidation resistance of the alloys.

  • 7.
    Öijerholm, Johan
    et al.
    KTH, Tidigare Institutioner, Materialvetenskap.
    Pan, Jinshan
    KTH, Tidigare Institutioner, Materialvetenskap.
    Jönsson, Bo
    Kanthal AB, Hallstahammar.
    Influence of Grain Size on Ionic Conductivity of Pure and Dense alpha-Al2O3 in the Temperature Range 400-1000 degrees C2004Ingår i: High Temperature Corrosion And Protection Of Materials 6, Prt 1 And 2, Proceedings, 2004, Vol. 461-464, nr II, s. 865-872Konferensbidrag (Refereegranskat)
    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.

  • 8.
    Öijerholm, Johan
    et al.
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap.
    Pan, Jinshan
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap.
    Leygraf, Christofer
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap.
    In-situ measurements by impedance spectroscopy of highly resistive alpha-alumina2006Ingår i: Corrosion Science, ISSN 0010-938X, E-ISSN 1879-0496, Vol. 48, nr 1, s. 243-257Artikel i tidskrift (Refereegranskat)
    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.

  • 9.
    Öijerholm, Johan
    et al.
    KTH, Skolan för kemivetenskap (CHE), Kemi, Korrosionslära (stängd 20081231).
    Pan, Jinshan
    KTH, Skolan för kemivetenskap (CHE), Kemi, Korrosionslära (stängd 20081231).
    Lu, Q.
    Kanthal AB, Hallstahammar.
    Leygraf, Christofer
    KTH, Skolan för kemivetenskap (CHE), Kemi, Korrosionslära (stängd 20081231).
    In-situ impedence spectroscopy study of electrical conductivity and ionic transport in thermally grown oxide scales on a commercial FeCrA1 alloy2007Ingår i: Oxidation of Metals, ISSN 0030-770X, E-ISSN 1573-4889, Vol. 68, nr 5-6, s. 253-269Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    In-situ impedance-spectroscopy measurements were performed at temperatures between 600 and 1,000 degrees C to investigate ionic transport in oxide scales formed on Kanthal AF alloy. The samples were pre-oxidized at 800, 900 and 1,000 degrees C in air. The impedance spectra of the oxide formed at 1,000 degrees C exhibited essentially one semicircle, whereas samples oxidized at lower temperatures showed an additional semicircle at high frequencies suggesting a more heterogeneous oxide. The ionic-transference number, derived by measuring the voltage across the oxide scale, indicates that the oxide is a predominant electronic conductor. Ionic diffusivity in the oxide scales formed at different pre-oxidizing temperatures was calculated, using the ionic-transference number. The ionic diffusivities obtained in this way are in reasonable agreement with literature data acquired by other methods. The oxide-formation temperature has a significant influence on the conductivity and ionic-transport properties of the oxide scale.

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