El-Mashri, Saleh M. (1985) Structure of anodic-oxide and hydrated oxide films on pure aluminium. PhD thesis, University of Warwick.

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Abstract

The technique of electron yield-EXAFS has been used to derive information about the different structures of amorphous films, formed anodically on pure aluminium when various electrolyte solutions are used. It has been found that the uniform non-porous (barrier-type) oxide films which are formed in neutralised sodium tartrate or sodium borate electrolyte are amorphous and have an average Al-0 bond length of 0.190 nm. The amorphous oxide produced in neutralised sodium oxalate gives an average Al-0 separation of 0.185 nm, while the porous oxides formed in strong aggressive electrolytes, chromic acid and phosphoric acid, have an average Al-0 bond length of 0.183 nm and 0.180 nm respectively. Both the non-porous and the porous types of films have also been examined by high resolution scanning electron microscopy. The films formed in neutralised electrolytes show a structureless morphology, while the films prepared in strong acid are shown to have a porous morphology. All these oxide films become hydrated when exposed to hot water at 85°C. The time for complete hydration varies according to whether the film is porous or not. Electron yield-EXAFS analysis of these hydrated films yields two well defined Al-0 distances, 0.205 nm and 0.280 nm, which appear to be associated with the formation of an oxy-hydroxide similar in structure to boehmite. The SEM observation of these hydrated films shows a narked change in the micromorphology during hydration. A "cornflake" structure is developed which is related to the oxy-hydroxide structure (boehmite-like phase). These measured Al—O bond lengths derived from the EXAFS differ, depending on the nature of the anodising treatment, which suggests different states of aluminium-oxygen coordination. A possible model for the structure of amorphous alumina, based on this information, is proposed. These results are also discussed in relation to the structural chemistry of the hydration process.

Item Type:	Thesis or Dissertation (PhD)
Subjects:	Q Science > QC Physics
Library of Congress Subject Headings (LCSH):	Aluminum -- Anodic oxidation, Aluminum oxide, Extended X-ray absorption fine structure, Amorphous substances, Thin films -- Structure
Official Date:	January 1985
Dates:	Date Event January 1985 Submitted
Institution:	University of Warwick
Theses Department:	Department of Physics
Thesis Type:	PhD
Publication Status:	Unpublished
Supervisor(s)/Advisor:	Forty, A. J. (Arthur John)
Format of File:	pdf
Extent:	120 leaves : illustrations, charts
Language:	eng

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