Stella Pedrazzini

Nanoscale Stoichiometric Analysis of a High-Temperature Superconductor by Atom Probe Tomography

The functional properties of the high-temperature superconductor Y1Ba2Cu3O7-δ (Y-123) are closely correlated to the exact stoichiometry and oxygen content. Exceeding the critical value of 1 oxygen vacancy for every five unit cells (δ>0.2, which translates to a 1.5 at% deviation from the nominal oxygen stoichiometry of Y7.7Ba15.3Cu23O54-δ ) is sufficient to alter the superconducting properties. Stoichiometry at the nanometer scale, particularly of oxygen and other lighter elements, is extremely difficult to quantify in complex functional ceramics by most currently available analytical techniques. The present study is an analysis and optimization of the experimental conditions required to quantify the local nanoscale stoichiometry of single crystal yttrium barium copper oxide (YBCO) samples in three dimensions by atom probe tomography (APT). APT analysis required systematic exploration of a wide range of data acquisition and processing conditions to calibrate the measurements. Laser pulse energy, ion identification, and the choice of range widths were all found to influence composition measurements. The final composition obtained from melt-grown crystals with optimized superconducting properties was Y7.9Ba10.4Cu24.4O57.2.

In-Service Oxidation and Microstructural Evolution of a Nickel Superalloy in a Formula 1 Car Exhaust

The oxidation response and microstructural evolution of an Inconel 625 alloy exhaust manifold exposed to an automobile racing environment has been examined using a range of advanced electron microscopy-based techniques, atom probe tomography and high-sensitivity laser ablation mass spectrometry. The dynamic, corrosive gas conditions result in accelerated oxidation, with the inner exhaust surface also heavily contaminated by multiple species including Zn, P, K and Na. Nb carbides and Ti nitrides identified in stock control samples evolve into mixed (Ti, Nb)N species during exposure, decorated by smaller Mo, Si-rich precipitates. The exposed alloy component therefore reveals unique surface and subsurface features following in-service use.

On the Effect of Environmental Exposure on Dwell Fatigue Performance of a Fine-Grained Nickel-Based Superalloy

The influence of sulfur contamination on the corrosion-fatigue behavior of a polycrystalline superalloy used in aero-engines is considered. Samples tested under a variety of environmental conditions (including exposures to air, SOx gas, and salt) are characterized through a suite of high-resolution characterization methods, including transmission electron microscopy (TEM), secondary ion mass spectroscopy (nanoSIMS), and atom probe tomography (APT). The primary effect of sulfur contamination is to accelerate the crack growth rate by altering the failure mechanism. The SIMS and TEM analyses indicate Cr-Ti sulfide particle formation at grain boundaries ahead of and around oxidized cracks. The APT analysis suggests that these particles then oxidize as the crack propagates and are enveloped in chromia. The chromia is surrounded by a continuous layer of alumina within the cracks. All of the sulfur detected was confined within the particles, with no elemental segregation found at grain boundaries.

Characterisation of ordering in the A-site deficient perovskite Ca1-xLa2x/3TiO3 using STEM / EELS

Perovskite structures based on the formulation Ca1-xLa2x/3TiO3 have been extensively studied across a wide range of possible applications, such as anodes for solid oxide fuel cells [1], dielectric resonators [2], high density memory storage devices [4], and as host matrices for inert matrix nuclear fuels and as containment media for high-level nuclear waste forms [5–6]. Understanding the crystallographic ordering at the atomic scale and the nature of present defects is essential in order to successfully utilize this class of perovskites across the multitude of applications. We have studied the vacancy ordering behaviour of the A-site deficient perovskite system, Ca1-xLa2x/3TiO3, using atomic resolution scanning transmission electron microscopy (STEM) in conjunction with electron energy-loss spectroscopy (EELS), with the aim of determining the role of A-site composition changes. At low La content (x = 0.2), this system adopts Pbnm symmetry, with no indication of long-range ordering. Atomic resolution high-angle annular dark-field (HAADF) STEM image, acquired along [010]p pseudo-cubic zone axis, Figure 1(a), shows varying intensities indicating changes in La3+ / Ca2+ ratio across the field of view. Elemental intensity maps from characteristic core-loss edges, shown in Figure 1(b), demonstrate anti-correlated Ca versus La intensities. Domains, with clear boundaries, were observed in bright-field (BF) imaging, but were not immediately visible in the corresponding high-angle annular dark-field (HAADF) image. These boundaries, with the aid of polarisation maps from A-site cations in the HAADF signal, are shown to be tilt boundaries. At the La-rich end of the composition (x = 0.9), adopting Cmmm symmetry, long-range ordering of vacancies and La3+ ions was observed, with alternating La-rich and La-poor layers on (001)p planes, creating a double perovskite lattice along the c axis. One such ordered region is imaged in Figure 2(a) along the [100]p zone axis, in conjunction with EELS elemental maps shown in panel (b), showing the alternating La-rich and La-poor atomic planes. These highly-ordered domains can be found isolated within a random distribution of vacancies / La3+, or within a large population, encompassing a large volume. In regions with a high number density of double perovskite domains, e.g. the area imaged in Figure 3, these highly-ordered domains were separated by twin boundaries, with 90° or 180° lattice rotations across boundaries, as shown in panels (a) and (b), respectively. The occurrence and characteristics of these ordered structures will be discussed and compared with similar perovskite systems. Keywords: perovskite; STEM; EELS; ordering

TEM Characterization of ion radiation damage in Ca (1-X) La 2X/3 TiO 3 Perovskites

Perovskite phasesgeneric formula ABO3, with both A and B denoting cation speciescan find diverse applications both in fission and fusion nuclear energy production. Perovskites have been considered as potential matrix candidates for inert matrix fuel designs [1] and also, among other ceramics, as a more suitable encapsulation media for high-level nuclear waste disposal [1, 2]. A number of high-temperature superconducting Perovskites have also been proposed in the design of the magnetic containment of plasma in fusion energy reactors [3]. In all of the applications above, the Perovskite phase would need to withstand high doses of radiation exposure and potential incorporation of inert gases, e.g. Xe and Kr, produced during various nuclear reactions. Upon accumulation of a critical dose of radiation damage, the structure would inevitably undergo a crystalline-to-amorphous phase transition. Understanding the characteristics of this phase transformation is the key to the successful engineering application of Perovskites, as amorphisation is normally accompanied by volume change, cracking, and reduced thermodynamic stability.