Oxygen transfer properties of ion-implanted yttria-stabilized zirconia

The influence of surface modification by ion implantation on oxygen transfer in yttria-stabilized zirconia (YSZ) has been studied. Implantation of 15 keV 56Fe in YSZ with a maximum dose of 8×1016 atoms cm−2 yields reproducible surface layers of approximately 20 nm deep with a maximum Fe cation fraction of 0.5 at the surface. After annealing these layers are stable up to 700–800°C. The exchange current densities for the Fe-implanted layers, measured using porous gold electrodes, are a factor of 10–50 larger than observed for not-implanted YSZ. 18O isotope exchange experiments show that for Fe-implanted samples the surface oxygen exchange rate is at least a factor 30 larger than for normal YSZ. The electrode kinetics has been studied for normal and implanted YSZ using current-overvoltage measurements and impedance measurements under bias. An electrode reaction model for the transfer of oxygen has been developed. This model is able to explain the low frequency inductive loop in the impedance diagram which is observed at high cathodic and anodic polarizations for implanted as well as normal YSZ.\u

Electrode polarization at the Au, O2(g)/yttria stabilized zirconia interface. Part II: electrochemical measurements and analysis

The impedance of the Au, O2 (g) / yttria stabilized zirconia interface has been measured as function of the overpotential, temperature and oxygen partial pressure. At large cathodic overpotentials (η < −0.1 V) and large anodic overpotentials (η > +0.1 V) inductive effects are observed in the impedance diagram at low frequencies. Those inductive effects result from a charge transfer mechanism where a stepwise transfer of electrons towards adsorbed oxygen species occurs. A model analysis shows that the inductive effects at cathodic overpotentials appear when the fraction of coverage of one of the intermediates increases with more negative cathodic overpotentials. The steady state current-voltage characteristics can be analyzed with a Butler-Volmer type of equation. The apparent cathodic charge transfer coefficient is close to c=0.5 and the apparent anodic charge transfer coefficient varies between 1.7< a<2.8. The logarithm of the equilibrium exchange current density (Io) shows a positive dependence on the logarithm of the oxygen partial pressure with a slope of m= (0.60 ± 0.02). Both the apparent cathodic charge transfer coefficient and the oxygen partial pressure dependence of Io are in accordance with a reaction model where a competition exists between charge transfer and mass transport of molecular adsorbed oxygen species along the electrode/solid electrolyte interface. The apparent anodic charge transfer coefficients deviate from the model prediction.\u

Electrode polarization at the Au, O2(g)/Fe implanted yttria-stabilized zirconia interface

Ion implantation has been applied to modify the surface properties of yttria-stabilized zirconia (YSZ). A three-electrode cell was used for measuring steady state current-overpotential curves and for determining the electrode impedance. An increase of the equilibrium exchange current density at the Au, O2(g)/yttria stabilized zirconia interface with a factor 10–50 has been observed after the implantation of 15 kV 56Fe up to a dose of 8 × 1016 at.cm−2. This increase results from a broadening of the active surface area due to an increase in the electronic conductivity of the Fe implanted YSZ surface and from an increase of the fraction of coverage of the adsorbed oxygen molecules. The double layer capacitance of the Au, O2,g/YSZ interface increases with a factor 10–100 after the Fe implantation. This is most likely due to the variable oxidation state of the implanted Fe ions, thus providing an additional way for charge accumulation. In comparison with the not implanted Au, O2,g/YSZ interface no changes in the rate-determining steps of the oxygen exchange mechanism occur after Fe implantation. Similar apparent charge-transfer coefficients have been determined. A slight decrease in the oxygen partial pressure dependence of Io is observed. The experimental results can still be explained with a reaction model where the charge-transfer process is in competition with the surface diffusion of molecular adsorbed oxygen species along the noble metal-solid electrolyte interface. At cathodic and anodic overpotentials inductive effects appear at low frequencies in the impedance diagram. The inductive effects result from a charge-transfer mechanism where a step-wise transfer of electrons to adsorbed oxygen species takes place.\u

Electrode polarization at the Au, O2(g)/yttria stabilized zirconia interface. Part I: Theoretical considerations of reaction model

Three different reaction models are discussed which describe the oxygen exchange reaction at the Au, O2(g)/yytria stabilized zirconia interface. The first model assumes the charge transfer process to be rate determining. If the electron transfer to the adsorbed oxygen species occurs in a stepwise fashion low frequency inductive effects can be simulated in the frequency dispersion of the electrode impedance. If the charge transfer process is in competition with mass transport of oxygen along the Au, O2(g)/stabilized zirconia interface the second model can predict “apparent” Tafel behaviour of the current-overpotential curve. The real charge transfer coefficients change from c = a = 1 to apparent values of c = 0.5 and c = 1.5. Due to a gradient in the fraction of coverage of the molecular adsorbed oxygen species along the Au, O2(g)/stabilized zirconia interface, the oxygen partial pressure dependence of the equilibrium exchange current density changes from I0 ∝ PO21/4 to I0 ∝ PO25/8. Depending on the basic charge transfer mechanism inductive effects at the electrode remain possible. The electrode impedance derived from this model under equilibrium conditions thus far revealed only capacitive effects. This makes this reaction model difficult to distinguish from the electrode impedance of a pure charge transfer process with an adsorbed intermediate. In case the mass transport process is rate determining limiting currents are predicted at moderate values of the applied overpotential. The electrode impedance then consists of a finite-length Warbung diffusion element and inductive effects cannot be predicted

Effect of ion implantation doping on electrical properties of yttria-stabilized zirconia thin films

The change in conductivity of Fe and Ti implanted rf-sputtered layers of yttria-stabilized zirconia (YSZ) was studied as a function of the temperature (400–800°C) and oxygen partial pressure. In an oxidized state and in the temperature range of 400–600°C, the conductivity of the Fe implanted YSZ film (15keV, 8×1016 at.cm−2) was dominated by the n-type electronic conductivity of a thin Fe2O3 layer with an estimated thickness of less than 2 nm on top of the YSZ thin film. Due to the incorporation of a part of the implanted Fe atoms in the yttria-stabilized zirconia lattice, the ionic conductivity was somewhat decreased. In a reducing atmosphere this electronic conduction was no longer observed. In an oxidized state, the conductivity of the YSZ film was not influenced by the implantation of Ti (15keV, 8×1016at.cm−2). After reduction in a H2 atmosphere, an increase in the conductivity of the sputtered film with 2–3 orders of magnitude was observed. This has been ascribed to the presence of nonstoichiometric TiO2−x, which is an semiconductor.\u

Characterization of Fe implanted yttria-stabilized zirconia by cyclic voltammetry

The technique of cyclic voltammetry has been applied to study reduction and oxidation phenomena which are observed at low oxygen partial pressures during steady state current-overpotential measurements of the Au, O2(g)/Fe implanted yttria-stabilized zirconia interface. The redox potential (EO) of the observed redox couple is in close agreement with the thermodynamic potential of coexistent Fe2O3 and Fe3O4 phases. Hence in the forward sweep of the cyclic voltammogram, defined for negatively swept potential, part of the Fe3+ is reduced to Fe2+. The peak currents in the voltammogram result from a redox reaction which is rate limited by the diffusion of electrons or electron holes in the Fe implanted YSZ surface to the implanted Fe ions rather than by the diffusion of the Fe ions themselves

Structural, electrical and catalytic properties of ion-implanted oxides

The potential application of ion implantation to modify the surfaces of ceramic materials is discussed. Changes in the chemical composition and microstructure result in important variations of the electrical and catalytic properties of oxides

Oxidation state of Fe and Ti ions implanted in yttria-stabilized zirconia studied by XPS

The oxidation state of Fe and Ti ions implanted in yttria stabilized zirconia (YSZ) was studied by XPS (X-ray photoelectron spectroscopy) in combination with depth profiling using Ar+ sputtering. In the “as-implanted” state of the sample Fe was found to be present as Fe3+, Fe2+ and as metallic Fe0. This is in agreement with earlier conversion electron Mössbauer Spectroscopy measurements. For Ti-implanted YSZ in the “as-implanted” state the majority of the Ti is present as Ti4+, Ti3+, and Ti2+ ions, while a part of the Zr cations is present in the divalent oxidation state (Zr2+). After oxidation in air, the Fe and Ti ions are present only in the valence three and four oxidation states, respectively

Ceramic nanostructure materials, membranes and composite layers

Synthesis methods to obtain nanoscale materials will be briefly discussed with a focus on sol-gel methods. Three types of nanoscale composites (powders, membranes and ion implanted layers) will be discussed and exemplified with recent original research results. Ceramic membranes with a thickness of 1–10 μm consist of a packing of elementary particles with a size of 3–7 nm. The mean pore size is about 2.5–3 nm. The preparation routes are based on sol and sol-gel technologies. The pores can be modified by liquid as well as by gas deposition techniques. This leads to modification of the chemical character and the effective pore size and gives rise to microstructure elements well below the size of the pores (3 nm). The modification of ceramic surface layers with a thickness of 0.05–0.5 μm by ion implantation and annealing procedures yields amorphous or strongly supersatured metastable solid solutions of e.g. Fe2O3 (or TiO2) in zirconia-yttria solid solutions or of very finely dispersed metal particles in the ceramic surface layers. Particle sizes are of the order of 2–4 nm. Both types of structures have interesting transport, catalytic and mechanical properties

The oxygen transfer process on solid oxide/noble metal electrodes, studies with impedance spectroscopy, polarization and isotope exchange

The electrochemical oxygen transfer process at the yttria stabilized zirconia (YSZ) and Fe-implanted YSZ, and at the erbia stabilized bismuth oxide (BE25) surface is studied with dc polarization and impedance spectroscopy using gold electrodes, and with 18O gas phase exchange. The surface modification by Fe-implantation increases the exchange current density up to a factor of 50, but analysis of the impedance spectra at different polarization levels indicates that the type of electrode reaction is not changed by the implantation. Inductive effects at cathodic polarizations are interpreted with a stepwise transfer of electrons. Isotope exchange experiments show an increase in adsorption/reaction sites at the surface after implantation. The high exchange current density, I0, for BE25 is independent of type of electrode but does depend on electrode morphology. I0 can be equated to the surface oxygen exchange rate, indicating that the entire electrolyte surface is active in the electrode exchange process. Qualitative interpretation of the impedance spectra measured at different levels of polarization results in a model where adsorbed oxygen species diffuse over the oxide surface, while charge transfer occurs across the surface