Biomedical Engineering Reference
In-Depth Information
12.6 CRYSTALLIZATION OF IONOMER GLASSES
Another interesting aspect in the characterisation of ionomer glasses is the study
of the crystallisation mechanism. Ionomer glasses have been studied for applica-
tions as bone substitutes in both orthopaedic and dental fi elds. The resulting glass
ceramics exhibit excellent mechanical properties and good osteo-conductivity
[45 - 50] .
The exact amount of fl uorite in an ionomer glass composition has a dramatic
effect on the nucleation and the crystallisation behaviour of the glasses. This
effect is not only a result of the stoichiometric considerations of crystal formation
but also of the network disrupting role of fl uorine within the glass network. Addi-
tion of fl uorite, whilst not altering the basicity of the glass, leads to increased dis-
ruption of the glass network and other effects as mentioned above. An interesting
composition is the apatite stoichiometric composition where Ca/P = 1.67 and par-
ticularly the resulting glass-ceramics after appropriate heat treatments, which
crystallise to two main phases: apatite and mullite. Apatite-mullite glass-ceramics
based on SiO 2 - Al 2 O 3 - P 2 O 5 - CaO - CaF 2 glasses were developed by Hill and co-
workers [10, 11, 51]. The glasses based on the general composition 4.5SiO 2 - 3Al 2 O 3 -
1.5P 2 O 5 - (5 - x)CaO - xCaF 2 where x is between zero and three, crystallised to
fl uorapatite (FAP) and mullite on appropriate heat treatments. Both the apatite
and mullite phases had a needle like habit and interlocked with each other giving
rise to high fracture toughness values. Differential scanning calorimetry (DSC)
and X-ray diffraction analysis [11] showed two crystallisation temperatures that
decreased with CaF 2 content and corresponded to Ca 5 (PO 4 ) 3 F (fl uorapatite,
FAP) and 3Al 2 O 3 - 2SiO 2 (mullite) phases, respectively, while the fl uorine free
glass exhibited two crystallisation temperatures corresponded to
-
calcium phosphate) and CaAl 2 Si 2 O 8 (anorthite), respectively. MAS-NMR was
used successfully in the past to characterise phosphorus containing alumino-
silicate glasses as well as their crystallisation process [51-57]. Stamboulis, Hill and
Law used multinuclear 27 Al, 29 Si, 31 P and 19 F MAS-NMR experiments in order to
characterise the crystallisation process of glass ceramics based on 4.5SiO 2 - 3Al 2 O 3 -
1.5P 2 O 5 - (5 - x)CaO - xCaF 2 .
The mechanism of apatite crystal nucleation in these glass-ceramics was
thought to occur as a result of prior amorphous phase separation (APS) or glass -
in - glass phase separation . The amorphous phase separation has been under
investigation for many years for different glass systems. Glass often appears
homogeneous but if one looks closer on the glass microstructure on a scale of a
few hundred atoms, glass is not as homogeneous as a perfect crystal or a liquid
solution. There are two processes leading to the development of inhomogeneous
glass microstructure. The fi rst one is the crystallisation or devitrifi cation, where
defi nite crystals nucleate and grow from a supercooled liquid mass and the second
one is based on the theory that crystallites are not microcrystals but they possess
distorted lattices and have defi nite chemical composition that is determined by
the phase equilibrium diagram of the glass composition. In the simplest situation,
the glass is considered to be a liquid that undergoes demixing as it cools. If the two
β
- Ca 3 (PO 4 ) 2 (
β
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