J. Nowotny

Surface segregation of defects in oxide ceramic materials

Abstract It is documented that composition and crystal ordering of oxide surfaces and grain boundaries differ from the bulk as a result of segregation of lattice defects. Theoretical approaches on segregation in metals and limitations of their application to ionic solids are discussed. Several experimental approaches to evaluate segregation in ionic solids are considered. The approaches involve mainly the studies of surface (near-surface) versus bulk studies of several structural and composition-sensitive parameters as diffusion coefficient, Fermi energy level and chemical composition data. The oxygen-pressure exponent of defect-related parameters such as electrical conductivity, Seebeck coefficient, work function, weight change is also a sensitive probe in studies of segregation. Finally when the grain size decreases below a certain critical value, which is comparable to dimensions of the near-surface or grain boundary layer, then most of physical and chemical parameters are partially or entirely sensitive to properties of the layer. Examples are given to illustrate segregation in nonstoichiometric oxides and their solid solutions. The results are discussed in term of different bulk versus near-surface miscibility ranges and resulting impact on interpretation of phase diagrams for ionic solids. It is shown that segregation leads to formation of surface or grain boundary bidimensional phases which exhibit extraordinary properties. Segregation and thus formed strong electric fields lead to the formation of the near-surface diffusive resistance for transport of charged defects across interfaces. The resistance may have a significant effect on the rate of heterogeneous processes even at high temperatures. Practical aspects of segregation on chemical processes and materials properties are discussed.

Surface relaxation of Y2O3-stabilized ZrO2

Abstract Surface phenomena accompanying oxidation and reduction process of yttria-stabilized zirconia (Zr 0.82 Y 0.18 O 2−δ ) were studied using work function (WF) measurements. Relaxation experiments were performed at 780°C in the oxygen partial pressure range from 1.74×10 2 to 4.3×10 4 Pa. It is observed that the WF value after both the education and oxidation runs change continuously within about 150 h and remains constant at longer times. The WF of the individual oxidation and reduction runs depends on the partial pressure with a power which changes from 1 4 within initial 10 h to about 1 2 at 120 h. The oxygen partial pressure dependence of quantities related to the bulk defect structure remain constant at the same time. Oxidation and reduction experiments are interpreted in terms of the thermodynamic equilibrium between the gas phase and oxygen vacancies which are assumed to be predominant lattice defects both in the bulk and in the near-surface layer of zirconia as long as the power of the oxygen partial pressure dependence (determined from WF) is 1 4 . The long term drift of both absolute WF values and the oxygen exponent is caused by surface segregation of impurities involving predominantly Al and Ca. It is concluded that the equilibration kinetics for the near-surface layer, determined by segregation, is much slower than the bulk kinetics controlled by diffusion of oxygen vacancies. Based on WF and SIMS data, the sandwich-type model for the near-surface layer of yttria-doped ZrO 2 is developed. The sandwich involves the outermost layer enriched predominantly by Al and Ca, the sublayer enriched in Y and the crystalline bulk.

Near-Surface Defect Structure of CoO in the Vicinity of the CoO/Co3O4 Phase Boundary

The near-surface defect structure of CoO was studied by “in situ” work function measurements in the temperature range 790 – 905°C at oxygen partial pressure near the equilibrium with Co3O4. The studies indicate that the near-surface oxide layer is enriched in cobalt interstitials which are not observed in the bulk. While CoO is oxidized and its non-stoichiometry increases, the observed phenomena in the outer surface layer may be considered within several consecutive regimes: 1 The formation of cobalt vacancies, 2 The formation of both cobalt vacancies and cobalt interstitials, 3 The formation of the Co3O4 superficial layer over CoO grains, 4 The phase transition CoO → Co3O4 in the bulk phase.

Surface reactivity of yttria-doped zirconia with oxygen

Abstract The interaction between gaseous oxygen and the surface of yttria-stabilized zirconia (9 mol%) was studied by in situ surface potential measurements at 780°C under p O 2 ranging between 1.74×10 2 and 4.3×10 4 Pa. Different surface treatments of both single- and poly-crystalline specimens were applied. The studies involved both oxidation and reduction experiments. The exponent of oxygen pressure determined from the surface potential measurements is discussed in terms of the defect chemistry of the near-surface region. The oxygen exponents is 1 4 for polycrystalline samples annealed at 1300°C and then quenched to 780°C. This corresponds to the formation of doubly ionized oxygen vacancies. Subsequent annealing at 780°C for 150 h results in an increase of the oxygen exponent to 1 2 as a result of segregation of impurities leading to the formation of a sandwich-type low-dimensional near-surface layer. The layer is composed of a top layer enriched predominantly in Ca and Al and a sublayer enriched in Y. Polishing of single crystals with 0.1 μm diamond powder leads to the formation of a surface phase which exhibits a decrease of work function in oxygen. The surface phase disappears after about 80 h of annealing at 780°C. Then the sample behaves similarly as the polycrystalline specimen.

Thermoelectric power of YBa2Cu3Ox in equilibrium with gas phases

AbstractThermoelectric power (TP) and electrical conductivity (EC) measurements were performed for YEa2Cu3Ox at 1128 K under controlled oxygen partial pressure varying between 50 and 105 Pa. Three regimes are observed for the electrical properties. At low

Nickel monoxide-oxygen interaction between 20 and 1000 °c and its impact on the nickel monoxide reduction mechanism

p_{{\text{O}}_{\text{2}} } (< 1.6{\text{ }} \times {\text{ 10}}^{\text{2}} {\text{ }}{\text{Pa}})

Defect Structure of Cobalt Monoxide: I, The Ideal Defect Model

both TP and EC remain constant with