IONOMER GLASSES: DESIGN AND CHARACTERIZATION - Advanced Biomaterials: Fundamentals, Processing, and Applications - page 418

with the two glass phases present. One phase was calcium, phosphate and fl uoride
rich and crystallised to FAP and the other phase was aluminium and silicon rich
and crystallised to mullite. Rafferty et al. [64] reported that the ionomer glasses
studied showed the presence of sparse droplets (20-100 nm) dispersed in a matrix.
The second phase droplets were spherical with clear interfaces, distributed ran-
domly with no obvious connectivity, suggesting a phase separation mechanism
involving nucleation and growth. The matrix, on the other hand, looked speckled.
According to James [65], for some glass systems, the effective spinodal boundary
may be depressed at low temperatures and the samples have to traverse a nucle-
ation region before reaching the spinodal. The speckled matrix might occur
because of the fi ne APS within the spinodal. Evidence of an APS mechanism that
involved both prior nucleation and spinodal decomposition came later in time
when Hill et al. [66] reported that a FAP glass-ceramic studied by a real-time
small angle neutron scattering exhibited two characteristic scales of phase separa-
tion and underwent APS. Isothermal small angle neutron scattering (SANS)
studies at 740 and 750 °C showed the same phenomena but without any change in
the scattering after 30 and 12 minutes, respectively. This observation lead to the
suggestion that further phase separation and crystal growth were restricted by the
high glass transition temperature of the second glass phase. Figure 12.10 shows
the schematic representation of the above suggestion on sub- micro - scale, where
the size of FAP crystals was correlated with the size of the droplet phase suggest-
ing, that FAP crystals did not grow beyond the boundaries of the droplet phase
until the higher glass transition temperature of the second phase was reached.
The above is important as similar ideas could be applied for glasses that could
undergo APS on a nano-meter scale.
Real time neutron diffraction studies showed that crystallisation of FAP in
ionomer glasses occurred fi rst, followed by crystallisation of both FAP and mullite.
Dissolution at higher temperatures and re-crystallisation during cooling occurred
(Figure 12.10). The results showed that the volume fraction of FAP decreased
during holding at 1200 °C and then FAP re-crystallised on cooling, suggesting that
FAP Crystals
Ca, F + P
Rich Glass
Phase
Al+Si Rich
Glass Phase
Figure 12.10. Schematic of FAP crystal growth being inhibited by the droplet size. The size
of FAP crystals is correlated with the size of the droplet phase, suggesting that FAP crystals do
not grow beyond the boundaries of the droplet phase until the glass transition temperature
of the second phase is reached [66].