Optical glass-ceramics are polycrystalline materials with very small crystals evenly distributed over the volume,
which are connected by interlayers of residual glass. Single crystals in glass-ceramic have a size of less than 0.4 microns,
which is less than the wavelength of light, so the material remains optically transparent. The refractive indices of crystals
and the residual glass phase are close, so the loss of light due to light scattering remains within acceptable limits.
The content of the crystalline phase in optical glass-ceramics is 75%.
According to their composition, optical glass-ceramics contain oxides of silicon, aluminum and lithium with the addition of
titanium dioxide. After a special heat treatment, crystals of lithium aluminosilicates - spodumene, eucryptite or petalite -
are released, which have a very low or even negative coefficient of thermal expansion. In combination with a positive coefficient
of expansion of the residual glass phase, this makes it possible in a given temperature range to have a negative, zero, weakly
positive (5x10-7 оС-1) and positive (57x10-7 оС-1) expansion
coefficient of glass-ceramic glass.
Optical glass-ceramics are an ideal material for astronomical mirrors and other optical parts, in which the absence of changes in linear dimensions and surface shape with significant temperature changes is important. The thermal stability of glass-ceramic allows the manufacture of mirrors of the highest degree of accuracy in much shorter processing times, since measurements can be made without waiting for thermal equilibrium to be reached.
THERMAL PROPERTIES
Mean temperature coefficient of linear expansion
(within temperature range -60 до +60 oС)
10-7 oC-1
± 1,5
CTE homogeneity
10-7 oC-1
≤ 0,2
Specific heat capacity
kcal kg-1 oC-1
0,22
Heat conductivity
kcal hr-1 m-1 oC-1
1,02
Thermal resistance
oC
550
MECHANICAL PROPERTIES
Density
g cm-3
2,46
Micro hardness
107 Па
843
Lapping relative hardness
(with reference to glass, K-8 type)
-
1,78
Modulus of elasticity (Young’s)
GPa
90,2
Bending strength
107 Па
16,3
Poisson’s coefficient
-
0,28
OPTICAL PROPERTIES
Refractive index ne
-
1,538
Refractive index nD
-
1,536
Average variance nF' - nC
-
0,0104
Microbubbles and striaes
-
up to ZERO EXTRA class***
Stress birefringence
nm/cm
up to 9-14
*** - ZERO EXTRA class: no bubbles, striaes and inclusions with linear sizes more than 0.2 mm for one kg of material.