Hans Juergen Seifert

Protective coatings on zirconium-based alloys as accident-Tolerant fuel (ATF) claddings

Abstract Surface-modified zirconium (Zr)-based alloys, mainly by fabricating protective coatings, are being developed and evaluated as accident-tolerant fuel (ATF) claddings, aiming to improve fuel reliability and safety during normal operations, anticipated operational occurrences, and accident scenarios in water-cooled reactors. In this overview, the performance of Zr alloy claddings under normal and accident conditions is first briefly summarized. In evaluating previous studies, various coating concepts are highlighted based on coating materials, focusing on their performance in autoclave hydrothermal corrosion tests and high-temperature steam oxidation tests. The challenges for the utilization of coatings, including materials selection, deposition technology, and stability under various situations, are discussed to provide some valuable guidance to future research activities.

Microwave Crystallization of Lithium Aluminum Germanium Phosphate Solid-State Electrolyte

Lithium aluminum germanium phosphate (LAGP) glass-ceramics are considered as promising solid-state electrolytes for Li-ion batteries. LAGP glass was prepared via the regular conventional melt-quenching method. Thermal, chemical analyses and X-ray diffraction (XRD) were performed to characterize the prepared glass. The crystallization of the prepared LAGP glass was done using conventional heating and high frequency microwave (MW) processing. Thirty GHz microwave (MW) processing setup were used to convert the prepared LAGP glass into glass-ceramics and compared with the conventionally crystallized LAGP glass-ceramics that were heat-treated in an electric conventional furnace. The ionic conductivities of the LAGP samples obtained from the two different routes were measured using impedance spectroscopy. These samples were also characterized using XRD and scanning electron microscopy (SEM). Microwave processing was successfully used to crystallize LAGP glass into glass-ceramic without the aid of susceptors. The MW treated sample showed higher total, grains and grain boundary ionic conductivities values, lower activation energy and relatively larger-grained microstructure with less porosity compared to the corresponding conventionally treated sample at the same optimized heat-treatment conditions. The enhanced total, grains and grain boundary ionic conductivities values along with the reduced activation energy that were observed in the MW treated sample was considered as an experimental evidence for the existence of the microwave effect in LAGP crystallization process. MW processing is a promising candidate technology for the production of solid-state electrolytes for Li-ion battery.

Thin film passivation of laser generated 3D micro patterns in lithium manganese oxide cathodes

The increasing need for long-life lithium-ion batteries requires the further development of electrode materials. Especially on the cathode side new materials or material composites are needed to increase the cycle lifetime. On the one hand, spinel-type lithium manganese oxide is a promising candidate to be used as cathode material due to its non-toxicity, low cost and good thermal stability. On the other hand, the spinel structure suffers from change in the oxidation state of manganese during cycling which is also accompanied by loss of active material into the liquid electrolyte. The general trend is to enhance the active surface area of the cathode in order to increase lithium-ion mobility through the electrode/electrolyte interface, while an enhanced surface area will also promote chemical degradation. In this work, laser microstructuring of lithium manganese oxide thin films was applied in a first step to increase the active surface area. This was done by using 248 nm excimer laser radiation and chromium/quartz mask imaging techniques. In a second step, high power diode laser-annealing operating at a wavelength of 940 nm was used for forming a cubic spinel-like battery phase. This was verified by means of Raman spectroscopy and cyclic voltammetric measurements. In a last step, the laser patterned thin films were coated with indium tin oxide (ITO) layers with a thickness of 10 nm to 50 nm. The influence of the 3D surface topography as well as the ITO thickness on the electrochemical performance was studied by cyclic voltammetry. Post-mortem studies were carried out by using scanning electron microscopy and focused ion beam analysis.

Influence of composition and heating schedules on compatibility of FeCrAl alloys with high-temperature steam

Abstract FeCrAl alloys are proposed and being intensively investigated as alternative accident tolerant fuel (ATF) cladding for nuclear fission application. Herein, the influence of major alloy elements (Cr and Al), reactive element effect and heating schedules on the oxidation behavior of FeCrAl alloys in steam up to 1500 °C was examined. In case of transient ramp tests, catastrophic oxidation, i.e. rapid and complete consumption of the alloy, occurred during temperature ramp up to above 1200 °C for specific alloys. The maximum compatible temperature of FeCrAl alloys in steam increases with raising Cr and Al content, decreasing heating rates during ramp period and doping of yttrium. Isothermal oxidation resulted in catastrophic oxidation at 1400 °C for all examined alloys. However, formation of a protective alumina scale at 1500 °C was ascertained despite partial melting. The occurrence of catastrophic oxidation seems to be controlled by dynamic competitive mechanisms between mass transfer of Al from the substrate and transport of oxidizing gas through the scale both toward the metal/oxide scale interface.

High-temperature oxidation resistance and self-healing behavior of Cr2AlC MAX phase coating on Zircaloy-4

Zirconium-based alloys are currently utilized as fuel cladding and structural components in commercial light water reactors due to their low thermal neutron absorption cross section, good mechanical properties and reasonable corrosion resistance during operation conditions. One undesirable feature of zirconium-based alloy cladding is their extremely fast oxidation kinetics with high-temperature steam during loss of cooling accidents (LOCA). A considerable amount of heat and hydrogen gas is produced by the reaction of zirconium and steam. The claddings undergo severe degradation and hydrogen explosion can occur, followed by subsequent release of highly-radioactive fission products to the environment like during the nuclear accidents at the Fukushima Daiichi Nuclear Power Plant in 2011. One strategy to improve the accident tolerance of the state-of-the-art zirconium-based alloy fuel claddings is to coat the outer surface with an oxidation resistant coating. This solution promises the elimination of corrosion degradation during normal operation, as well as significant reduced oxidation kinetics with steam during off-normal conditions. Mn+1AXn(MAX) phases represent a family of ternary layered carbides or nitrides which possess a unique combination of the merits of both metals and ceramics. Alumina-forming MAX phase materials, like Ti2AlC and Cr2AlC, are being considered as protective coatings with respect to their excellent oxidation resistance up to 1400°C. In this study, Cr2AlC coatings have been deposited on Zircaloy-4 substrates by magnetron sputtering using elemental nano-multilayer thin films, and subsequent thermal annealing in argon. The total thickness of the coatings is around 6.5 μm and both coatings have a 500 nm Cr layer as bonding layer and diffusion barrier. One design of coatings also deposited a 1.5μm thick Cr capping layer to migrate potential fast dissolve of Al during normal operation. Crystallization of Cr2AlC MAX phase starts from 480°C by annealing in Ar and formation of phase-pure Cr2AlC MAX phase but with surface microcracks at 550°C is confirmed. Both coatings demonstrated high adherence, excellent oxidation resistance up to at least 1200°C and self-healing capability with growth of protective Al2O3 scale or of protective Al2O3 scale beneath Cr2O3 during high-temperature oxidation.

Laser-induced self-organizing surface structures on cathode materials for lithium-ion batteries

Rechargeable lithium-ion batteries have emerged as an attractive power source for a wide variety of applications, in particular for portable electronics. The development and modification of electrode materials is a major issue for the improvement of energy density and power density of lithium-ion batteries. For this purpose, laser-induced self-organizing surface structures were generated using UV-excimer laser radiation with a wavelength of 248 nm and a pulse width of 4-6 ns. The self-organization process was applied for thin films made of lithium cobalt oxide with a thickness of about 3 μm. In order to identify the chemical changes due to laser processing time-of-flight secondary ion mass spectroscopy measurements were performed. It was found that this process can be linked to a decomposition of the electrode material forming a lithium oxide surface layer. Similar self-organized surface structures could also be obtained for thick film electrode materials consisting of LiCoO2 powder mixed with binder and carbon black which were tape-casted onto aluminium foils. The thickness of these films was in the range of 50 - 100 μm.