Asbjørn Slagtern Fjellvåg

We report on the structural and magnetic properties of Li_5+xFe_1−xCo_xO₄ (0≤x≤1).< The system assumes two distinct crystal structures at room temperature (α and β), and shows a transition in preference from α to β between x=0.18 and x=0.25. This tendency is supported by DFT‐calculations. Upon heating, Li₅FeO₄ undergoes a reconstructive transition from α→β at ∼1060 K, while Li₆CoO₄ shows an order‐disorder transition from β→ϵ at ∼885 K, where ϵ is a cubic cation‐disordered antifluorite structure. Li_5.50Fe_0.50Co_0.50O₄ is stabilised towards the ordered β‐phase until at least 1170 K. XANES‐measurements confirm that the oxidation states of Fe and Co remain unchanged throughout the series with charge compensation from additional Li⁺ on increasing x. Magnetic measurements show onset of low‐temperature antiferromagnetic ordering around 3 K to 12 K. The magnetic structure of Li₅FeO₄ is described on the basis of powder neutron diffraction at 1.5 K, while the magnetic structures of Li_5.50Fe_0.50Co_0.50O₄ and Li₆CoO₄ are found to be slightly incommensurate below the ordering temperature.

Platinum-rhodium gauzes are frequently used to catalyse the high temperature ammonia oxidation step for production of synthetic nitrogen-based fertilisers. The gauzes suffer from Pt loss in the form of platinum dioxide (PtO₂), due to the highly exothermic nature of the oxidation reaction. Industrially this is mitigated by installing one or more palladium-nickel catchment gauzes directly downstream of the combustion gauzes, to capture the lost Pt. The Pd-Ni catchment gauzes undergo severe structural modification during operation. In this study, we undertake a systematic study in a laboratory-scale furnace system to determine the role of each of the constituent gases O₂, H₂O and PtO₂ on the structural changes of the Pd-Ni gauzes. In addition, some samples are exposed to real industrial conditions in an ammonia combustion pilot plant reactor. Fresh and spent catchment gauzes are analysed by means of scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX), thermogravimetric analysis (TGA) and inductively coupled plasma mass spectroscopy/optical emission spectroscopy (ICP-MS/OES). By combining analysis of samples from furnace and pilot scale experiments, the main findings are that Pd-Ni gauzes undergo internal oxidation to nickel(II) oxide (NiO); which in the presence of steam results in Ni depletion and that PtO₂ vapour causes severe grain reconstruction. Furthermore, in laboratory-scale experiments no significant Pd loss is observed, which is in contrast to observations from the pilot plant where the samples are exposed to real post-ammonia oxidation conditions. Pd loss is likely attributed to some gas species contained in the real post-ammonia oxidation gas stream.

A method for calculating stored magnetic energy in a thin superconducting film based on quantitative magneto-optical imaging is developed. Energy and magnetic moment are determined with these calculations for full hysteresis loops in a thin film of the superconductor NbN. Huge losses in energy are observed when dendritic avalanches occur. Magnetic energy, magnetic moment, sheet current and magnetic flux distributions, all extracted from the same calibrated magneto-optical images, are analyzed and discussed. Dissipated energy and the loss in moment when dendritic avalanches occur are related to each other. Calculating these losses for specific spatially-resolved flux avalanches is a great advantage, because of their unpredictable and non-reproducible nature. The relative losses in energy are much higher than the relative losses in moment.