Tellurite And Fluorotellurite Glasses For Active And Passive

2. Literature review; MDO 19
cooling, reaching its peak at the apex of the nose. Below this temperature, crystallisation
is suppressed due to the rising viscosity of the supercooled liquid [2].
The critical cooling rate, [dT/dt]c to avoid 0.0001 % volume crystallisation can be
approximately obtained from the TTT diagram, at any temperature on the nose, Tn, and
corresponding time, tn.
⎡d
T ⎤ Tl
− T
⎢ ≈
⎣ dt
⎥
⎦ t
c
where Tl is the liquidus temperature [2].
n
n
(2.3)
Heterogeneous nucleation has a strong influence on this critical cooling rate,
compared to homogeneous nucleation, resulting in a [dT/dt]c value 5 to 10 times slower
for homogeneous nucleation. As the contact angle of the heterogeneous nuclei decreases,
so does [dT/dt]c. Complex compositions, and those close to eutectics can hinder
nucleation processes (see below). Ideally a glass melt should have a high viscosity at Tl,
or rapidly increasing below Tl on cooling [2], as seen with ‘typical’ glass formers in
section 2.1.1.
However, many low viscosity melts, such fluorozirconates (ZBLAN – ZrF4-BaF2-
LaF3-AlF3-NaF), can be quenched to form a glass relatively easily. These ‘novel’ glass
systems have viscosities of the order of water (10 -2 Pa.s) around their melting
temperatures. Seddon et al. [8] showed that the most stable ZBLAN compositions
occurred at the interface between two crystalline phases on the phase diagram (LaF3-
2ZrF4 and NaF-BaF2-2ZrF2). Therefore, it is believed that in the undercooled melt,