N. Nicoloso

Optical absorption studies of tetragonal and cubic thin-film yttria-stabilized zirconia

Optical absorption studies of oxidized and H2-reduced tetragonal and cubic zirconia films (75 ⩽ d ⩽ 2100 nm), YSZ single crystals and TZP ceramics (50 ⩽ d ⩽ 200 μm) have been performed in order (i) to provide reliable band gap values of these commonly used fast oxygen ion conductors and (ii) to investigate the type and extent of structural disorder introduced by the aliovalent dopant Y2O3 and the electronic charge carriers. At r.t. the optical band gaps of TZP (3 mol% Y2O3) and YSZ (8 mol% Y2O3) differ only slightly: 5.7 eV ± 0.1 eV and 5.8 eV ± 0.05 eV, respectively. Their temperature coefficients are ≈ −4 × 10−4 eV/K. As indicated by the pronounced Urbach-type band edge absorption, fairly wide band tail states exist in both materials as a consequence of structural disorder. The continuous change of the Urbach slope with the yttria content or oxygen-vacancy-defect concentration, respectively, demonstrates that these sub-gap states mainly arise from oxygen vacancy defects, VO. Upon reduction with H2, localized electron states are formed at the expense of these tail states.

Conduction mechanisms in RuO2-glass composites

Abstract The electrical transport and the microstructure of RuO 2 -glass composites with metal oxide volume fractions 0.01 ≤ f RuO 2 ≤ 0.4 have been investigated by SEM, HREM and frequency and electric field dependent conductivity studies at 4–;300 K. Different transport mechanisms controlled by the microstructure of the composite have been identified. In the limiting cases of high and low RuO 2 contents metallic or ionic transport prevails. For intermediate concentrations, 0.05 ≤ f RuO 2 ≤0.2, hopping and tunneling transport are superposed. Hopping transport has been identified by frequency dependent conductivity studies; evidence for tunneling transport stems from the electric field dependence of the conductivity at temperatures 2 clusters with a mean size of 250 nm and comparatively short intercluster distances form within the glass matrix. As importantly, each of these clusters consists of a large number of ultrafine RuO 2 particles ( n > 10 3 ) separated by a uniform glass layer of ≤ 2nm thickness, i.e., the metal oxide particles remain suspended in the matrix. Based on the statistical evaluation of the SEM- and HREM-data a quantitative description of the electrical transport behaviour of RuO 2 -glass composites is presented.

Optical absorption relaxation applied to SrTiO3 and ZrO2: An in-situ method to study trapping effects on chemical diffusion

Abstract Chemical diffusion coefficients have been reliably measured in the case of the electronic conductor SrTiO 3 (Fe-doped) and the ionic conductor yttria-stabilized ZrO 2 (Ni-doped) by using a contactless optical relaxation technique. A quantitative concept (proposed by one of us earlier) which allows the inclusion of internal trapping effects leads to a consistent analysis of D in terms of temperature ( T ), oxygen partial pressure ( P ) and doping level ( m ). In both materials, SrTiO 3 and ZrO 2 , the argument between experiment and theory is very good. In the first case, due to the availability of all the necessary parameters, no adjusting parameter has been used. In both cases, the analysis also leads to a reevaluation of literature data and is able to reconcile the apparent “scatter” of published results.

The ionic conductivity profile of thin evaporated AgCl films on a planar sapphire substrate

Abstract The ionic conductivity profile of AgCl-films stepwise evaporated onto a sapphire-substrate has been measured. After each step the films have been annealed. A bombardment of the uncompleted films by argon-ions (just before the successive evaporation processes) enhances the ionic conductivity. The first sheet (about 350 nm thick) of these films adjacent to the interface shows extremely high conductivity values up to about 10 −4 S/cm at 25°C. But also the remaining part of each film between about 350 and 800 nm away from the interface still shows an unexpectedly good conductivity of the same magnitude as the highest conductivity value (of 6×10 −6 S/cm at 25°C) found so far for disperse systems AgCl/Al-oxide. The measureds activation energies (between 0.34 and 0.47 eV) of these films are markedly higher than the migration enthalpy of Ag-vacancies (0.28 eV). The discussion suggests that the high conductivity and its profile are essentially determined by the microscopical structure of the films.

Opto-electrochemical study of minority charge carries in YSZ

Abstract Minority charge carrier trapping in single crystal YSZ has been investigated at high temperature ( T ⩽900° C ) by simultaneous electrochemical and optical studies. The results are supported by magnetic measurements at RT. Electron injection into the Pt/ YSZ interface is accompanied by valence changes of impurity ions, especially rare earth ions, and the formation of an intrinsic, colour centre, or polaron-like state. The concentration and mobility of these defects have been determined at different stages of reduction of YSZ. Their relevance for the exchange reaction between O 2 and Pt/YSZ is discussed.

SEM and TEM/EDX analysis of model interfaces in multicomponent electroceramics

Abstract The interfaces between (Ni,Zn)Fe 2 O 4 and MgTiO 3 as well as between MgTiO 3 and RuO 2 glass—models for the interfaces in multicomponent electroceramics—have been investigated by SEM and combined TEM/EDX analysis. Under the preparation conditions involved (1400 K, 2 h), the local properties of both interfaces are mainly thermodynamically controlled. In the case of the ferrite-titanate system, a μ m thick interface is formed, consisting of a compositionally modified, cation-exchanged spinel phase, Mg 1+ x Fe 2− 2x Ti x O 4 , with x = 0·66, and a pseudobrookite phase, Mg x Fe 2− 2x Ti 1+ x O 5 , with x = 0·75. Concerning the interface properties of MgTiO 3 -RuO 2 glass, interdiffusion of Ti and Mg appears to be the dominant process below 1100 K, whereas at higher temperature several new phases can be clearly identified in the 3 and PbTiO 3 .

Electrical response of oxygen sensing TiO2 surfaces and fractal Pt/YSZ interfaces

Abstract Titania and YSZ single crystals with 9.4 mol% Y 2 O 3 have been investigated by impedance, voltage relaxation and current-voltage studies. In the case of TiO 2 separation of the surface boundary layer conductivity from the bulk conductivity yields a first indication of rather different P O 2 dependencies in the two domains. For high current densities and CO 2 /O 2 gas mixtures the apparent exchange current density of YSZ is enhance by up to a factor of 40 in the low temperature regime. At T ⩾ 750° C the CO 2 effect vanishes and the exchange gets independent of gas composition and temperature. A similar continuous change is observed in the impedance of Pt/O 2 /YSZ under blocking conditions. The fractal description of the systems proved to be unsatisfactory Especially, there is a disagreement with the Nyikos and Pajkossy model of the impedance of fractal blocking electrodes. It appears that the morphology of the platinum is not the unique parameter determining the interface properties.