John R. LaGraff

Concentration-dependent oxygen diffusivity in YBa2Cu3O6+x: I. Argon annealing studies

Abstract Electrical resistance measurements were used to follow the isothermal in-diffusion characteristics in YBa 2 Cu 3 O 6+ x (YBCO) as a function of the initial oxygen content, x , at temperatures ranging from 650–708°C. Prior to the determination of oxygen in-diffusion rates, the initial oxygen contents in both polycrystalline and single crystal specimens were controlled by out-gassing in argon for different periods of time. An increasing dependence of the chemical diffusivity with decreasing initial oxygen content was observed which was typical of a vacancy diffusion mechanism. However, the magnitude of the dependence (and comparison with tracer diffusion studies) suggested that other factors (e.g., correlation effects or the thermodynamic factor) were responsible for the concentration dependence. The results are important for the development of atomic diffusion models for the oxygen mobility in YBCO.

Concentration-dependent oxygen diffusivity in YBa2Cu3O6+x: II. Oxygen partial pressure studies

Abstract Electrical resistance measurements were used to determine the equilibrium resistivity characteristics and the oxygen diffusion behavior in YBa 2 Cu 3 O 6+ x (YBCO) as a function of oxygen partial pressure at temperatures ranging from 450–850°C. The equilibrium resistivity values depended on the oxygen partial pressures as n = ϑ Log ϱ / ϑ Log[ P (O 2 )], where n ranged from approximately − 1 6 up to − 1 2 from low to high temperatures. A clear p-type to n-type transition was observed near 750°C for an oxygen content of 6.1 ( x = 0.1). Values of the oxygen exponent, n , for single crystal YBCO at 650°C and 708°C were similar to those for polycrystalline specimens. Measurements of the time-dependent resistivity behavior between different oxygen partial pressures yielded diffusion kinetics between well-established boundary values of the oxygen content, x . In single crystal specimens, the chemical diffusivity for oxygen in-diffusion was observed to increase with decreasing oxygen content. The kinetic data for the polycrystalline specimens between 450–850°C also depended on the oxygen concentration; however, the temperature dependence was non-Arrhenius.

In-situ observation of oxygen adlayer formation on Cu(110) electrode surfaces

Abstract In-situ atomic force microscopy (AFM) was used to image Cu(110) single crystals in aqueous solutions during the initial stages of oxidation. Images obtained in pH 2.5–2.7 HClO4 and H2SO4 solutions revealed the growth of oxygen adlayers consisting primarily of [001] oriented chains. A majority of these chains (ca. 70%) were arranged in (2 × 1) and (3 × 1) structures. These chain structures were observed in the thermodynamically forbidden region of the pH-potential phase diagram, which indicates that stable oxygen adlayers develop prior to bulk oxide formation.

Concentration-dependent oxygen diffusivity in YBa2Cu3O6+x: III. Diffusion mechanisms

Abstract In this paper, aspects of diffusion theory are discussed which could account for the reported concentration-dependent chemical diffusion coefficients previously reported [Physica C 212 (1993) 470, 478 (this issue)]. These factors include correlation effects, the thermodynamic factor, the type of mechanism (i.e., vacancy or interstitial), and the different types of fundamental atomic jumps. It is proposed that defect interactions are primarily responsible for a concentration-dependent D . Examination of jump pathways in orthorhombic YBCO indicate that diffusion along the copper-oxygen chain direction (along the b -axis) is more favorable if, (1) the oxygen starts at the end of a chain and, (2) the adjacent chain is empty of oxygen. Intrachain diffusion (along the a -axis) between two copper-oxygen chains is more slow and depends on the oxygen concentration.

Oxygen stoichiometry and mobility effects on domain wall motion in ferroelastic YBa2Cu3O7-δ

Abstract The purpose of this paper is to examine the interrelationship between oxygen stoichiometry and mobility with ferroelastic properties in YBa2Cu3O7-δ. Changes in oxygen stoichiometry, δ, affect the orthorhombic-tetragonal phase transformation behavior and consequently can be expected to directly affect the ferroelastic domain structure and switching properties. A free energy formulation is used to derive an expression for the energy γT of a {110} twin wall in YBa2Cu3O7-δ in terms of the spontaneous strain, coercive stress, and twin wall separation. An upper limit of 40 mJ/m2 was estimated. Ferroelastic viscosity, η, of YBa2Cu3O7-δ was assumed to be an activated process which depends on both the diffusivity and concentration of oxygen according to the relation η = η0(0.6-δ)exp(-E/kT) for δ < 0.6. Two types of domain reorientation mechanisms are proposed, a cooperative shear process of oxygen along a twin wall and a stress-assisted short-range oxygen hopping process in the bulk of the domain, both of...

Electrical measurements near the orthorhombic-tetragonal phase transformation in single crystal YBa2Cu3O7−δ

Abstract Normal-state electrical resistance measurements were made on single crystal YBa 2 Cu 3 O 7−δ over the temperature range 200–800°C in oxygen and argon atmospheres, and the twin structure was observed simultaneously by hot-stage optical microscopy. The resistance of the crystal maintained a linear metallic-type behavior to higher temperatures (500°C) than reported for bulk ceramic specimens. Anomalous resistance behavior was observed in the vicinity of the orthorhombic-to-tetragonal phase transformation under certain probe configurations and measuring conditions. Correlations between observed in situ twinning and resistance characteristics are discussed.

AFM Studies of Copper Solid-Liquid Interfaces

It is extremely important to characterize bare copper surfaces in situ before any subsequent corrosion or deposition chemistry on these materials can be understood. In this paper, in situ Atomic Force Microscopy (AFM) was used to image the low-index faces of Cu single crystals in H2SO4 and HC1O4 acidic solutions. Oxide monolayers were observed on Cu(100) and Cu(110) crystals in the thermodynamically forbidden region of the pH-potential phase diagram, which indicates that stable oxide adlayers develop prior to bulk oxide formation. The AFM was also used to follow the electrodeposition of Cu. The AFM is shown to locally enhance the electrochemical deposition of Cu on single crystal Cu surfaces. The tip-sample interaction increases the growth rate of Cu resulting in the localized formation of nanometer-scale epitaxial deposits. The results are consistent with a heterogeneous nucleation and growth mechanism in which the tip-sample interaction creates surface defect sites active towards the electrochemical adsorption of Cu species. Precise control of feature sizes allows this technique to be used for fabrication and constructive modification of solid-liquid interfaces.

In-Situ Observation of Oxide Monolayer Formation on Copper Solid-Liquid Interfaces

It is extremely important to characterize the various bare copper surfaces in situ before any subsequent corrosion or deposition chemistry can be understood. In this paper, in situ Atomic Force Microscopy (AFM) was used to image the low-index faces of Cu single crystals in H 2 SO 4 and HClO 4 acidic solutions. Cu(100) surfaces exhibited potential-dependent c(2x2) adlayers in pH=1 solutions which were attributed to oxide (or hydroxide) overlayers. Images of Cu(1 10) obtained in pH 2.5-2.7 solutions revealed the growth of primarily [001] oriented (nxl) adlayer chain structures, where n is an integer. Preliminary measurements on Cu(111) did not reveal any adlayer structures between pHs of 1-3. The oxide monolayers on Cu(100) and Cu(110) crystals were observed in the thermodynamically forbidden region of the pH-potential phase diagram, which indicates that stable oxide adlayers develop prior to bulk oxide formation.

Chemical Diffusion of Oxygen in YBa2Cu3O6+x

Chemical diffusion of oxygen has been measured in both single crystal and polycrystalline YBa2Cu3O6+x in order to distinguish between intrinsic diffusion behavior and extrinsic characteristics which depend on microstructure. Isothermal electrical resistance measurements were used to monitor the dynamics of oxygen diffusion from 350-780°C under both oxidizing and reducing conditions. Measured activation energies depended on sample morphology, temperature, and whether indiffusion or out-diffusion of oxygen was occurring. Below 600°C, the activation energies for out-diffusion in porous (~75%) and dense (~95%) polycrystalline material were found to be 0.5(1)eV and 0.6(1) eV, respectively. These low energies suggest a high diffusivity pathway such as a grain boundary. Polycrystalline material exhibited a change in functional form for out-diffusion near 600°C in both dense and porous samples which was attributed to a change from primarily grain boundary diffusion to significant bulk lattice diffusion. Above 600°C, oxygen outdiffusion in dense polycrystalline and single crystal YBa2Cu3O6+x was found to be surface-reaction controlled. The activation energies for outdiffusion in the dense material were 1.93(6)eV and 1.7(1)eV above and below ~700°C; and in the single crystal 1.6(1)eV and 1.00(4)eV above and below ~680°C. Oxygen in-diffusion was found to have activation energies of 0.4(1)eV from 400-780°C in dense polycrystalline material and 1.16(6)eV in single crystal material from 600-780°C. The lower activation energy in the polycrystalline material may be due to percolation effects in which the rapid formation of a highly oxygenated shell masks intrinsic diffusion behavior.

In-Situ Scanning Probe Microscopy of Solid-Liquid Interfaces: Role of Epitaxial Oxide Adlayers on Cu Electrodeposition

We discuss how the nanoscale structural and chemical properties of copper (Cu) single crystal surfaces immersed in acidic aqueous solutions affect local electrochemical function. In particular, perturbation of oxide adlayers with in-situ atomic force microscopy (AFM) is shown to locally enhance the electrochemical deposition of Cu on Cu electrode surfaces. The results are consistent with a heterogeneous nucleation and growth mechanism in which the tip-sample interaction creates surface defect sites in passivating oxide adlayers which are active towards the electrochemical adsorption of Cu species. This “protect-deprotect-react” scheme enables precise control of feature sizes and allows this technique to be used for fabrication and constructive modification of solid-liquid interfaces.