Margaret M. Hyland

We have studied the evolution of surface basicity as a function of calcination temperature in a range of transition alumina‐rich analogues of metallurgical grade aluminas produced by calcination of Bayer gibbsite. Specific basicity, the number of basic sites per unit surface area, was determined by thermometric back‐titration and found to increase with calcination temperature up to ~700°C to 800°C, decreasing dramatically thereafter. The population of tetrahedrally coordinated Al ³⁺ , which exhibits a qualitatively similar evolution, was determined by Al K ‐edge near‐edge X‐ray absorption fine structure (NEXAFS) spectroscopy but found not to correlate with basicity changes. Interestingly, a shift to higher and then lower binding energy of Al 2 p photoelectrons by X‐ray photoelectron spectroscopy and changes in the intensity of O K ‐edge NEXAFS spectra do appear to correlate with surface basicity. Exploiting the differing surface sensitivities of NEXAFS spectra collected in partial electron and total fluorescence yield modes, we find O K ‐edge spectra intensities, Al 2 p X‐ray photoelectron spectrometer binding energies, and surface basicity all reflect the reorganisation of internal surfaces rather than changes in AlO ₄ :AlO ₆ occupation. Copyright © 2017 John Wiley & Sons, Ltd.

Droplet spreading behaviour is critical in influencing the spreading diameters, material splashing and interfacial bonding conditions with the substrate during plasma spraying. Numerical simulations were performed to understand the effects of droplet impact temperatures, velocities and contact angle on spreading dynamics during the deposition of plasma-sprayed nickel splats. It was found that increasing droplet superheat (droplet temperature of 3000 K) was beneficial for increasing the degree of substrate melting and the droplet spreading diameters due to constrained solidification. However, solidification occurring at the droplet expanding front pinned the liquid–solid contact line and induced finger splashing for the case of 2800 K droplet temperature. The solidified layer disturbed droplet spreading and induced material jetting at the early spreading stage under the circumstance of high droplet impact velocities. A new simplified equation relating the maximum spreading ratios and Reynolds number was proposed. The effect of contact angle influenced droplet spreading under the condition of a large interfacial thermal contact resistance.