The core/rim structures of compounds in the Ti(C,N)-xWC20Ni system have been investigated to determine the effect of WC and nitrogen content on the microstructure of the system. In addition, the relative dissolution rate of WC to Ti(C,N) in the system was studied by analyzing the rim compositions. Variations in the WC content had a much-lesser influence, as an additive, on the general microstructure than that of other carbides that have been used in previous studies. However, the nitrogen content in Ti(C 1؊X N X ) had a significant effect on its microstructure. The composition of the rim structure was determined by the ratio of solutes that were dissolved in the liquid binder, under the given processing conditions. The dissolution rate of WC was ϳ2 and 5 times faster than that of Ti(C,N) in the system at temperatures of 1300°and 1510°C, respectively. The results have been interpreted in terms of phase stability and precipitation phenomena.
The formation of vacancies and the structural stability of layered lithium nickel oxide (LNO)-based cathode materials are investigated. The thermodynamic stability of oxygen and lithium vacancies and their most stable configurations are examined by first-principles density functional theory calculations. The underlying chemical mechanism is analyzed by a molecular orbital method. The weaker ionic bonding between Ni and O than between Co and O is found to be the main cause for the imperfect structure of LNO crystals. On the basis of these calculations, phase diagrams of the Li−(Ni,Co)−O system were simulated. The crystals containing vacancies are included as independent phases in the simulation. This approach enabled investigation of the relationship between the processing conditions and vacancy formation. The O and Li vacancy pairs are simulated to appear with high temperature processing. On the basis of the calculation of energy barriers, we speculate that these vacancy pairs provide an alternative migration route for Ni ions, which causes the observed structural instability. The effect of oxygen partial pressure was also examined. The first-principles calculation results were compared with experimental results, which showed excellent agreement confirming the validity of the models and calculation methods used in this study.
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