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The structure of divalent and trivalent cation substituted β-tricalcium phosphate
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Grigg, Andrew T. (2013) The structure of divalent and trivalent cation substituted β-tricalcium phosphate. PhD thesis, University of Warwick.
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Official URL: http://webcat.warwick.ac.uk/record=b2691715~S1
Abstract
Current methods of nuclear waste disposal are not suitable for the immobilisation of
novel defence-based waste due to the high halogen content resulting from pyrochemical
reprocessing. The objective of this thesis was to investigate β-tricalcium phosphate (β-
TCP, Ca3(PO4)2) encapsulated in a sodium aluminoborophosphate glass (NABP) matrix
as a potential host for this waste using a variety of structural probes. Samples were
prepared to determine the structural changes in β-TCP as components of the simulated
waste streams were substituted into the material. Zn, Mg, Al, and Ga incorporation was
investigated.
A combination of X-ray and neutron diffraction was used to determine the changes
in long-range order as a function of Zn and Mg substitution up to 13% and 25% cation
substitution respectively. Both Zn and Mg substitution caused a contraction of the unit
cell up to complete substitution of the Ca(5) site, at which point the contraction ceased.
Under further substitution on the Ca(4) site, the a lattice parameter continued to decrease,
while the c lattice parameter increased, resulting in an unchanged unit cell
volume. Evidence of tricalcium trimagnesium phosphate second phase was observed
for the Mg-based compositions above Ca2:8Mg0:2(PO4)2, as has been previously documented,
however single phase samples were observed for all Zn-based compositions, in
contrast to previous studies. 31P NMR was used to confirm this Ca(5)-Ca(4) substitution
model for the Zn-based β-TCP compositions by tightly constrained simulations as
a function of composition.
A combination of solid-state NMR techniques were used to identify the substitution
mechanism of Al and Ga in β-TCP up to the composition Ca9M(PO4)7, where M
is Al or Ga respectively. The 31P and 43Ca NMR spectra were simulated as with the
divalent cations mentioned above to determine the origin of each resonance in the spectra.
Subsequently, 27Al-f31Pg and 71Ga-f31Pg R3-HMQC experiments were performed to
explicitly identify substitution on the Ca(5) site only.
Studies were also performed to model the NABP:β-TCP interface formed as a result
of the encapsulation process, for both pure β-TCP and Ga-substituted β-TCP. To simulate
the range of compositions expected at this interface, calcium phosphate and NABP
preparations were mixed in proportions from 10 wt:% to 80 wt:% (Ga-substituted) β-
TCP. 31P NMR and Raman spectroscopy showed a progressive depolymerisation of the
phosphorus network, consistent with the replacement of Al3+ and Na+ with Ca2+. The
Al3+ was shown to exist primarily in a 4 coordinated state, showing a tendency to exist
within the phosphorus network, whereas 11B NMR showed the B to move from a 4
coordinated site in NABP to a B-rich 3 coordinated environment.
Differential thermal analysis showed an increase in the temperature of the two recrystallisation
events as a function of both β-TCP and Ga-substituted β-TCP. Studies
of the phases present after recrystallisation of the pure β-TCP-based samples showed
calcium sodium phosphate and Na Al co-substituted β-TCP for the lower and higher
crystallisation temperatures with β-TCP incorporation. For the Ga-containing samples,
Na Al Ga co-substituted β-TCP was observed for both crystallisation temperatures.
Critically, Ga was shown to displace Al in the β-TCP phase.
Item Type: | Thesis (PhD) |
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Subjects: | Q Science > QC Physics Q Science > QD Chemistry |
Library of Congress Subject Headings (LCSH): | Calcium phosphate, Substitution reactions, X-rays -- Diffraction, Neutrons -- Diffraction, Magnetic resonance imaging |
Official Date: | October 2013 |
Institution: | University of Warwick |
Theses Department: | Department of Physics |
Thesis Type: | PhD |
Publication Status: | Unpublished |
Supervisor(s)/Advisor: | Holland, Diane; Dupree, Ray |
Extent: | 1 volume (various pagings) : charts |
Language: | eng |
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