University of Cambridge

Department of Materials Science and Metallurgy.

Doctor of Philosophy (PhD)

Thesis

The field-ion microscope has the ability to resolve single atoms but there is no unambiguous correlation between the appearance of a spot in the field-ion image and the chemical nature of the chemical identification of an individual atom led to the invention of the atom-probe which is a time-of-flight mass spectrometer attached to a field-ion microscope. In general the atom-probe is not used to analyse single atoms but instead to obtain an estimate of the composition of a small region. To facilitate the collection of a large number of ions in a reasonable time an electronic timer and semi-automatic control system has been built. With the larger number of results available it has been possible to plot "mass spectra" and the factors affecting the form of a mass spectrum have been investigated. In some cases the high spatial resolution of the field-ion microscope is only required in a direction perpendicular to the specimen surface, and to obtain an even larger sample from such specimens a modified atom probe has been constructed in a prototype form. The range of applications of the atom-probe is also discussed.