Arc synthesis and magnetic properties of graphite encapsulated nanocrystals

Nanocrystals have unique physical properties as a result of their small dimensions and large proportion of surface atoms. This large proportion of reactive surface atoms has previously hampered the study and utilization of nanocrystals by allowing rapid oxidation. The recently discovered encapsulation of nanocrystals using graphite layers prevents oxidation, protecting the nanocrystals and forming true nanocomposites. However, there is significant confusion in the field over both how encapsulated nanocrystals form and what their properties are. This work clarifies both points by examining the formation mechanism and magnetic properties of graphite encapsulated nanocrystals.;The formation mechanism leading to graphite encapsulated nanocrystals is examined by experimental analysis and literature review. By varying four of the parameters used in the arc production of graphite encapsulated nickel nanocrystals and studying the effect of these changes on the encapsulated nanocrystals produced, a speculative formation mechanism is developed. This formation mechanism (the two step mechanism) is used to predict the encapsulation response of over 20 other elements. This response is tested against the experimental attempts to encapsulate these elements in the literature, and significant agreement is found. Additionally, the two step mechanism is used to predict the encapsulation response for elements that have not been tested yet.;The magnetic properties resulting from the small particle morphology of these samples is examined as well. The magnetic properties are selected because many applications require knowledge of the magnetic properties and the magnetic properties are strongly affected by the particle size. Most of the previous work on the magnetic properties of small particles was affected by oxygen contamination, which leads to an antiferromagnetic layer on the particles, altering the magnetic properties by exchange coupling. The encapsulated cobalt nanocrystals are found to be ferromagnetic at all temperatures, and to grow during annealing. This particle size growth appears to be the result of diffusion of metal through the graphite layers, resulting in growth by a mechanism similar to Ostwald ripening. The particle size growth causes successive changes in the magnetic properties during annealing. The magnetic coercivity may increase slightly before decreasing as the particles become large enough to contain multiple magnetic domains. The saturation magnetization increases as the particle size increases due to the possible presence of a weakly magnetic surface layer.