Tellurite And Fluorotellurite Glasses For Active And Passive

8. Fibre drawing; MDO 362
undercooled glass), and allowed to grow, certain directions will grow preferentially. Fig.
(8.18) illustrates this [27].
3
2
1
0
{100}
{111}
0 1 2 3
Fig. (8.18): Growth of a sphere cut out of a cubic crystal in a saturated solution [27].
The cubic {001} and octahedral {111} planes will grow primarily, but not uniformly (as
illustrated by steps 0 to 3 in fig. (8.18)). This is due to different rates of plane growth.
Initially the cubic {001} plane grows most rapidly, growing smaller until it merges, and
is enveloped by the octahedral {111} plane. Less material is deposited at the octahedral
{111} plane, therefore it moves at a slower rate. As a consequence, the corners and edges
of crystals can be defined as the planes with the highest rates of growth [27]. In a glass
melt or undercooled glass, due to the high viscosity, diffusion is slow. Crystallisation will
occur in preferred directions which are typically those with the highest growth rates, and
a dendritic / skeleton structure will result. These directions of preferred growth can be