Phonons in potassium doped graphene: the effects of electron-phonon interactions, dimensionality and ad-atom ordering

Graphene phonons are measured as a function of electron doping via the addition of potassium adatoms. In the low doping regime, the in-plane carbon G-peak hardens and narrows with increasing doping, analogous to the trend seen in graphene doped via the field-effect. At high dopings, beyond those accessible by the field-effect, the G-peak strongly softens and broadens. This is interpreted as a dynamic, non-adiabatic renormalization of the phonon self-energy. At dopings between the light and heavily doped regimes, we find a robust inhomogeneous phase where the potassium coverage is segregated into regions of high and low density. The phonon energies, linewidths and tunability are remarkably similar for 1-4 layer graphene, but significantly different to doped bulk graphite.Comment: Accepted in Phys. Rev. B as a Rapid Communication. 5 pages, 3 figures, revised text with additional dat

Microwave Synthesis Methods for Lithium-Ion Battery Cathodes

With the rapidly expanding market for mobile devices such as mobile phones, laptops and cameras, as well as the push towards greener, more renewable energy sources, rechargeable batteries have been brought to the fore. With the many uses of lithium-ion batteries from high drain appliances to power storage, to vehicle propulsion, varying cathodes are required to provide these differing functionalities. This thesis contains a comprehensive literature review, outlining the history of secondary batteries, their uses, current technologies and ongoing research topics within the field of lithium-ion battery cathodes. Bronze-phase vanadium dioxide, VO2(B), is a very promising cathode material, with higher theoretical capacity than the current commercial lithium cobalt oxide, LiCoO2. Current synthesis techniques for VO2(B) utilise solvothermal synthesis in a Teflon-lined Parr bomb, which is then placed in an oven for 48 hours. Microwave-assisted synthesis had never before been used for VO2(B), but through its use the reaction time for formation has been significantly reduced. VO2(B) was also characterised through the use of X-ray absorption spectroscopy whilst undergoing a discharge cycle at the Diamond Synchrotron near Oxford, the first experiment of this kind on VO2(B) using a relatively new battery cell design. Olivine phosphate structures of iron, manganese and cobalt were also prepared through microwave-assisted synthesis, with lithium iron phosphate being developed as a future electric vehicle battery cathode. These were successfully characterised and cells containing them charge-discharged - the results of these are presented

Magnetic Levitation for Long-Life Space Mechanisms: Technology Assessment and Remaining Challenges

Spacecraft mechanisms and mechanical systems must operate reliably and without failure to enable successful, long-term space missions. Such requirements place demands upon the tribological elements, especially bearings, which are frequently difficult or impossible to satisfy. Several recent, high-profile bearing failures in coolant fluid pumps and attitude control system (ACS) momentum wheels provided the impetus to assess the state-of-the-art non-contacting magnetic levitation-based, rotor support technologies.Magnetic levitation technology continues to gain acceptance for terrestrial applications and has been spaceflight demonstrated in mechanical systems such as reaction wheels (RWs) but is not in widespread use. The specific reasons inhibiting this new technology are not readily clear but include cost, weight, performance, and perceived risk. These reasons arise from a variety of real and perceived technical limitations in areas like materials, controls, sensors, thermal management and others. This white paper seeks to determine, define, and quantify the technical hurdles and gaps that must be overcome to enable the broad adoption of non-contacting bearings for long-life space mechanisms. It is anticipated that a better understanding of this complex topic may guide resource investments and clear the path to improved performance mechanical systems for spacecraft

Gas Foil Bearings for Space Propulsion Nuclear Electric Power Generation

The choice of power conversion technology is critical in directing the design of a space vehicle for the future NASA mission to Mars. One candidate design consists of a foil bearing supported turbo alternator driven by a helium-xenon gas mixture heated by a nuclear reactor. The system is a closed-loop, meaning there is a constant volume of process fluid that is sealed from the environment. Therefore, foil bearings are proposed due to their ability to use the process gas as a lubricant. As such, the rotor dynamics of a foil bearing supported rotor is an important factor in the eventual design. The current work describes a rotor dynamic analysis to assess the viability of such a system. A brief technology background, assumptions, analyses, and conclusions are discussed in this report. The results indicate that a foil bearing supported turbo alternator is possible, although more work will be needed to gain knowledge about foil bearing behavior in helium-xenon gas