Electric breakdown and moisture adsorption in evaporated dielectric films

McLeod, James E.S (1965) Electric breakdown and moisture adsorption in evaporated dielectric films. PhD thesis, University of Glasgow.

Abstract

Previous work on evaporated films and on measurement of electric breakdown is reviewed and its relation to the present work is shown. Electric breakdown theory is also briefly reviewed. Results of measurements of the breakdown fields of dielectric films between evaporated metal electrodes are then presented. Studies have been made of freshly deposited films in a vacuum which remained unbroken between deposition and testing, and also of films exposed to air saturated with water vapour. The dielectrics investigated were sodium chloride, lithium fluoride, and cryolite (Na3AIF6). Sodium chloride was chosen since it has attracted the attention of numerous previous workers so that, its breakdown field is well known. Lithium fluoride has not been quite so widely studied, but it is unique among the alkali halides in being; only sparingly soluble. This is an important advantage in the work on moist specimens since soluble films tend to disintegrate on exposure to water vapour. Cryolite, another insoluble fluoride, is in common use as a thin film material for optical purposes since it forms films of good stability. If its electrical properties are also suitable it might well be promising as a dielectric for evaporated capacitors. Measurements of its when virtually all moisture was removed before the test, by drying; in vacuum. Three possible explanations of the rise in breakdown field could be broadly distinguished (a) adsorbed moisture is solely responsible, more being present on the films tested under moist conditions than on those dried in vacuum but some. water molecules still remaining on the latter films, or (b) the increase in breakdown field of the specimens dried in vacuum is due to recrystallisation and only the further increase in the specimens tested in the presence of moisture; is directly due to adsorbed layers, or (c) the increase is entirely due to a recrystallisation. Process modified in some way by the simultaneous presence of, the moisture and the electric field to give the further increase in the specimens; tested, under moist, conditions. Experiments were therefore performed in which the films were exposed to a field just below the breakdown value while still in their environment of water vapour but in which the actual test; took place after drying in vacuum. The, results were similar to those for the specimens tested while still in moist air. This is in general consistent only with explanation (c). Some indications were obtained that the breakdown field increases on exposure to moisture for cryolite and sodium, chloride as well as lithium fluoride.

• Item Type: Thesis (PhD)
• Qualification Level: Doctoral
• Additional Information: Adviser: J Irving
• Keywords: Materials science, Electrical engineering
• Date of Award: 1965
• Depositing User: Enlighten Team
• Unique ID: glathesis:1965-73676
• Copyright: Copyright of this thesis is held by the author.
• Date Deposited: 14 Jun 2019 08:56
• Last Modified: 14 Jun 2019 08:56
• URI: https://theses.gla.ac.uk/id/eprint/73676