Pyrochlore microdomain formation in fluorite oxides

The pyrochlore microdomain formation in the fluorite lattice was investigated by means of electron microscope techniques, especially for the system (TbxGd1−x)2 Zr2O7+y (0 x 1; 0 y < 0.25). Specimens with x> 0.2 or specimens with x 0.2 which were quenched from temperatures above the order-disorder transition temperature show diffuse scattering in the electron diffraction images, remarkably similar to diffuse scattering observed for other defect fluorite oxides. The diffuse scattering is discussed in terms of the formation of a small basic cluster in the fluorite lattice. Annealed specimens with x 0.2 show pyrochlore diffraction spots and microdomain formation. The domains have an average diameter ranging from a value smaller than 10 nm for x = 0.2 up to 100 nm for Gd2Zr2O7. The domains grow in a disordered (fluorite) matrix and finally form antiphase boundaries. The transition from the basic cluster to a well-developed microdomain is not yet clear. The observed microdomain structure is used to explain the results of oxygen ion conductivity experiments

Study of the oxygen electrode reaction using mixed conducting oxide surface layers. Part I: Experimental methods and current-overvoltage experiments

The oxygen gas electrode has been studied for a number of mixed conducting oxide surface layers on top of Gd2Zr2O7 (TGZO) solid electrolytes. In part II of this paper we present the results of frequency dispersion measurements for the electrode reaction, supplying additional information to the results of current-overvoltage experiments presented in part I. For both kinds of experiments the same trends were observed for the electrode polarization. Best results are obtained for a surface layer of TGZO, while p-type mixed conducting oxides give less decreased values of the electrode polarization. High electrode capacitances were found in the case of mixed conducting surface layers (about 700 F/m2). The electrode reactions follow a Butler-Volmer type of equation. Most probably a diffusion process is rate controlling the overall charge transfer process

Oxygen ion and mixed conductivity in compounds with the fluorite and pyrochlore structure

The effect of pyrochlore order in solid solutions with the defect fluorite structure was studied. The amount of order was changed by various heat treatments or by varying the cation composition. Examples of short range order and microdomain-formation have been observed in Tb2Zr2O7+x and Gd2Zr2O7. Pyrochlore order results in a decrease of the activation enthalpy and pre-exponential factor for oxygen ion conduction. The results are explained by a model based on the occurrence of a preferential diffusion path with a low strain energy component in ΔH. Optimal conductivity is achieved in materials which are not completely ordered. Mixed conductivity in Ln2Zr2O7 with P structure can be realized by partial replacement of Ln by Tb. This results in good ionic as well as electronic conductivities

Study of the oxygen electrode reaction using mixed conducting oxide surface layers. Part II: Small signal analysis

The oxygen gas electrode has been studied for a number of mixed conducting oxide surface layers on top of Gd2Zr2O7 (TGZO) solid electrolytes. In part II of this paper we present the results of frequency dispersion measurements for the electrode reaction, supplying additional information to the results of current-overvoltage experiments presented in part I. For both kinds of experiments the same trends were observed for the electrode polarization. Best results are obtained for a surface layer of TGZO, while p-type mixed conducting oxides give less decreased values of the electrode polarization. High electrode capacitances were found in the case of mixed conducting surface layers (about 700 F/m2). The electrode reactions follow a Butler-Volmer type of equation. Most probably a diffusion process is rate controlling the overall charge transfer process

Defect structures and migration mechanisms in oxide pyrochlores

Using computer simulation techniques the defect structure and oxygen ion migration mechanism of oxide pyrochlores (eg. Gd2Zr2O7) was investigated in order to explain the decreased activation enthalpy for oxygen ion conductivity as a function of order. Shell model potentials were found to be necessary in order to obtain sufficiently accurate physical properties for the pyrochlore compound. The oxygen Frenkel defect consisting of ‘a split 48f vacancy’ and 8b interstitial appeared to be the most stable instrinsic defect, but vacancies related to extended defect structures may play an important role in the diffusion mechanism too. The migration mechanism of oxygen ions is mainly based on 48f-48f jumps and involve 0.9 eV barrier energy, comparable with the experimental activation enthalpies of 70–85 kJ/mol

Proportionele aansprakelijkheid, omkeringsregel, bewijslastverlichting en eigen schuld: een inventarisatie van de stand van zaken

In het arrest Fortis/Bourgonje van 24 december 2010 over de aansprakelijkheid van de bank als vermogensbeheerder bevestigt de Hoge Raad dat aan de proportionele benadering uit het arrest Nefalit/Karamus een breder toepassingsbereik toekomt dan alleen de werkgeversaansprakelijkheid. Anderzijds wordt benadrukt dat het bij deze benadering om een uitzondering gaat, en wordt de toepassing door het hof op het voorliggende geval afgewezen. Dat roept de vraag op wanneer proportionele aansprakelijkheid wel, en wanneer niet een toelaatbare oplossing is. Uit de rechtspraak over dit onderwerp blijkt de nauwe verwevenheid met andere leerstukken zoals het bewijs van causaal verband, de omkeringsregel en de ‘eigen schuld’ van artikel 6:101 BW. Wie zoekt naar algemene lijnen voor de toepasselijkheid van het ene dan wel het andere leerstuk, moet concluderen dat de rechtspraak van de Hoge Raad de nodige vragen open laat. In dit artikel wordt de stand van zaken geïnventariseerd en worden enkele suggesties gedaan voor een nadere systematisering van dit terrein

Structure and conductivity of pyrochlore and fluorite type solid solutions

High oxygen conductivities can be achieved in cation ordered LnxZrl-x=xO2-l/2x (Ln=Gd, Nd) solid solutions with pyrochlore (P) structure. High values of the pre-exponential factor σo are correlated with the degree of anion disorder in the 8b oxygen sublattice (neutron diffraction). The activation energy ΔH is lowered by cation ordering (F-P transition; effect of ) due to the occurence of a preferential diffusion path. Maximum oxygen conductivity is achieved in (1−x)Bi2O3−x Ln2O3 solid solutions with fluorite related δ-Bi2O3 structure for Ln=Er and x=0.20. Neutron diffraction measurements indicate the occurence of short range ordering in “Er-O units” with relative small interatomic distances at T < 820 K. This leads to an increased activation energy

Determination of the relative concentrations of rare earth ions by x-ray absorption spectroscopy: Application to terbium mixed oxides

A method, based on X-ray absorption spectroscopy (XAS) in the range 0.8–1.5 keV, to determine the relative amounts of rare earth ions in different valencies is explained and tested for the case of terbium mixed oxides. The results are in agreement with those obtained by existing analytical methods. The XAS method is advantageous in that it can be applied where other, conventional, methods break down

Electric and electrochemical properties of catalytically active oxygen electrode materials

The electrical conductivity has been investigated of some oxygen ion and mixed conducting materials. Electrodes are prepared from thin sputtered layers of these oxides combined with a small Au or Pt strip. The kinetics of the oxygen reaction has been studied for temperatures of 820–1020 K and PO2 values of 10−4 - 1 atm. respectively.\ud Highest oxygen ion conductivities are found in solid solutions of 0.7 Bi2O3−0.3 Tb2O3.5 followed by 0,7 CeO2 - 0.3Tb2O3.5 (CT-30) and (TbxGd1−x) with pyrochlore structure and x=0 (TGZ-0). Highest, p-type, electronic conductivities are found in CT-30 and TGZ with x=1. TGZ-0 was used in all experiments as the solid electrolyte.\ud Combined current-overvoltage (ν) and impedance measurements show that with Au strips the nature of the oxidic materials has a pronounced effect on especially the cathodic polarization, the effect being larger with larger ν. Introduction of p-type conductivity does not decrease the electrode resistance. The Butler-Volmer equation is obeyed with effective cathodic and anodic transfer coefficients close to 0.5 and 1.5 respectively while the effective exchange current varies as the 0.5 power of PO2. These results can be interpreted by a mechanism where charge transfer in the metal-oxide region is rate controlled by surface diffusion on the oxide