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  5. δ-Bi 2 O 3 has long been touted as a potential material for use in solid oxide fuel cells (SOFC) due to its intrinsically high ionic conductivity. However, its limited operational temperature has led to stabilising the phase from >725 °C to room temperature either by doping, albeit with a compromise in conductivity, or by growing the phase confined within superlattice thin films. Superlattice architectures are challenging to implement in functional μSOFC devices owing to their ionic conducting channels being in the plane of the film. Vertically aligned nanocomposites (VANs) have the potential to overcome these limitations, as their nanocolumnarmore »structures are perpendicular to the plane of the film, hence connecting the electrodes at top and bottom. Here, we demonstrate for the first time the growth of epitaxially stabilised δ-Bi 2 O 3 in VAN films, stabilised independently of substrate strain. The phase is doped with Dy and is formed in a VAN film which incorporates DyMnO 3 as a vertically epitaxially stabilising matrix phase. Our VAN films exhibit very high ionic conductivity, reaching 10 −3 S cm −1 at 500 °C. This work opens up the possibility to incorporate thin film δ-Bi 2 O 3 based VANs into functional μSOFC devices, either as cathodes (by pairing δ-Bi 2 O 3 with a catalytically active electronic conductor) and/or electrolytes (by incorporating δ-Bi 2 O 3 with an insulator).« less
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  7. Observation of high-speed reactive flows using laser-absorption-based imaging techniques is of interest for its potential to quantitatively reveal both gas-dynamic and thermochemical processes. In the current study, an ultraviolet (UV) laser-absorption imaging method based on nitric oxide (NO) is demonstrated to capture transient flows in a shock tube. A tunable laser was used to generate a continuous-wave UV beam at 226.1019 nm to coincide with a strong NO absorption feature. The UV beam was expanded to a 20-mm diameter and routed through the shock tube to image the flow adjacent to the end wall. Time-resolved imaging was realized using amore »Lambert HiCATT high-speed UV intensifier coupled to a Phantom v2012 high-speed camera. Static absorbance measurements of 1.97% NO/Ar mixtures were first performed to validate the proposed imaging concept, showing good agreement with values predicted by a spectroscopic model. UV laser-absorption images of incident and reflected shock waves captured at 90 kHz temporal resolution are then reported. Translational temperature profiles across the incident and reflected shocks calculated from absorbance images show reasonable agreement with calculated values. After the passage of the reflected shock wave, the flow near the end wall was monitored to probe the development of the end-wall thermal boundary layer. Thermometry measurements across the thermal boundary layer show good agreement with analytical solutions. This study demonstrates the potential of UV laser-absorption imaging in high-speed flow fields, to be applied to more complex applications in the future.« less
    Free, publicly-accessible full text available January 1, 2023
  8. The dynamics of flame propagation at high unburned-gas temperatures are of critical importance to the performance and operability of modern engine systems but have long existed beyond the temperature regimes accessible to controlled laboratory study. The shock-tube flame speed method has been demonstrated to enable the study of premixed, freely propagating flames over a wide range of previously unachievable engine-relevant unburned-gas temperature conditions. This study reports the first systematic investigation of end-wall-induced effects on the propagation and stability of flames subject to asymmetric flow confinement in a shock tube. Through the flexibility afforded by newly available optical access, the axialmore »position of flame ignition was varied over a range spanning from 3.3 to 15.5 cm from the driven end wall. Experiments performed under static conditions isolated the effect of asymmetric end-wall confinement and provided an opportunity to measure the flow velocity induced by the confinement effect; results show the expected functional scaling exists between flame radius, distance from the end wall, and flow velocity, but the velocity scaling deviates from that predicted. Experiments performed behind reflected shock waves are then used to probe the interplay between the confinement and gas-dynamic effects in the post-reflected-shock environment. In a break with intuition, the post-shock results show a non-monotonic relationship between position and flame stability, with one particular distance (6.4 cm) producing significantly more severe distortion than flames ignited either nearer or farther from the end wall. Finally, experiments demonstrating the generation of hemispherically expanding flames in the shock tube are reported, providing a baseline to inform the consideration of such flames as an alternative basis for flame speed measurements. The experimental measurements reported in this work provide valuable new validation targets against which detailed modeling of confinement and gas-dynamic effects can be compared, while the side-wall observations reaffirm that spherically expanding flames suitable for use in reliable laminar flame speed measurements can be generated in a post-reflected-shock environment.« less
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