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Title: Trivalent cation substitution of β-tricalcium phosphate
Author: Mee, Martin
Awarding Body: University of Warwick
Current Institution: University of Warwick
Date of Award: 2011
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Abstract:
The mineral β-tricalcium phosphate (β-TCP), Ca3(PO4)2, is currently of interest in the immobilisation of nuclear wastes, along with various biomedical applications. Several substituted forms of β-tricalcium phosphate have been produced with the general formula Ca9M3+ x (PO4) 6+x, where M3+ is a trivalent metal ion, with Ga, Al and La being the main focus of study here, and x generally falling between 0 and 1. The aim of this study has been to investigate the substitution mechanisms of ions into β-tricalcium phosphate along with the effects which substitution has on its structure. This has mainly been carried out by nuclear magnetic resonance on many of the available nuclei, along with x-ray and neutron diffraction of the materials produced. It has been shown that substituted β-tricalcium phosphate can be readily produced, although with additional phases present in some samples, generally upon high levels of substitution, and especially in the Al case. Rietveld refinements of x-ray and neutron diffraction patterns show the β-TCP structure remains intact throughout the samples produced, with Ga thought to substitute onto the Ca(5) site, La onto the Ca(3) site and Al onto both the Ca(4) and Ca(5) sites. However, values determined for the Ca site occupancies have in some cases deviated from those expected and often have large errors associated with them. Part of this is due to problems with the peak shape used with the x-ray diffraction data, but it is also thought that disorder may be present in the substituted samples, causing unexpected values in the refinements. Total correlation functions have also been produced from the neutron diffraction data and again show the β-TCP structure to remain relatively unchanged in most cases with 6-coordinated Al and Ga present in samples containing those ions. The addition of Al does, however, change the structure considerably, possibly through distortions of the sites involved due to the smaller size of the Al ions. 31P NMR has been performed on all samples and appears extremely sensitive to changes in the Ca site occupancies. Fitting has been performed on spectra from the Ga system, with fits produced consistent with substitution onto the Ca(5) site. The method of substitution for the Al and La systems has proven to be too complex for fitting of the 31P spectra to be practical in these cases. Natural abundance 43Ca NMR has been performed on some of the samples and, although having poor signal to noise ratios, the spectra produced can be fitted according to the method of site substitution predicted. 27Al NMR on the Al containing samples has produced interesting spectra, with an additional feature being present in addition to the expected 6-coordinated resonance. This feature lies within the range associated with AlO6 or Al(OP)5 environments and is thought to be due to distortion of one of the Ca sites that the Al is substituting into, probably the smaller Ca(4) site.
Supervisor: Not available Sponsor: Atomic Weapons Establishment (Great Britain) (AWE)
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID: uk.bl.ethos.560283  DOI: Not available
Keywords: QC Physics
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