material that meets such stringent requirements is Schott’s patented BOROFLOAT glass, a high-quality borosilicate
glass, which was further machined and
Added strength through the fifth
element — Boron
Borosilicate’s namesake is the element
boron, which serves — similar to silica
— as a glass network former and adds
to the glass’s high chemical durability
and superior mechanical strength. During
melting, boron and alumina are mixed
with traditional silica and other components commonly used in glass production
to create a stronger, more rugged glass.
The high amount of boron oxide in the
glass composition strengthens the chemical bonds within its network and is
responsible for the exceptionally low
thermal expansion behavior and low
refractive index (nd = 1.471). Together
with the material’s superior transmission,
these characteristics are key requirements
for precise spectrometer measurement
results required for the HETDEX project.
The chemically resistant glass is
traditionally utilized wherever excellent
resistance to high or changing temperatures is required, such as sight glasses
for the chemical and petroleum industry.
However, due to its outstanding transmission it is also used as a substrate in
many optical applications, such as optical filters, hot and cold mirrors, or the
VIRUS specialty spectroscope mirrors.
As its thermal expansion coefficient of
3. 25 × 10− 6 K−1 is perfectly matched to
that of silicon, it is also the glass of choice
for anodic bonding or as a carrier wafer
for temporary wafer processing. The low,
nonbridging oxygen holes in its glass
network are responsible for a high radiation resistance (low glass darkening)
making it a good candidate as a component in x-ray radiation technology.
Light transmittance is significantly
impacted by the impurity levels of the
raw materials used and depends on the
thickness of the optical glass panel.
BOROFLOAT glass is offered as thin
as 0.7 mm and up to 1 in. thick and is
the industrial flat glass with the lowest
iron impurity level allowing for transmis-
1) and reduced solarization tendency in
the UV range (Figure 2). The glass was
machined and coated by Precision Glass
Solarization is a “glass-darkening
phenomenon,” which typically occurs
in multicomponent glasses and depends
on the glass structure, the emitted light
spectrum, the radiation source and the
radiation exposure dose. High energy
doses can generate structural micro- and
nano-electronic defects, which are then
absorbed at short wavelengths resulting
in a UV-cutoff shift in the light trans-
mission curve. Such radiation-induced
defects tend to solarize depending on
impurity levels of polyvalent ions, such as
iron (Fe2+/Fe3+) in the glass composition.
Technical glasses usually have iron impurity levels of several hundred ppm. Pure
raw materials were used in manufacturing
the glass for the VIRUS mirrors, resulting
in extremely low iron impurity levels
(approximately 85 ppm Fe2O3), which tend
to solarize much less intensely than other
flat glasses (Figure 3).
The optical mirrors used in VIRUS
capitalize on the glass’ high transmission,
allowing VIRUS sensors to read and record the incoming light more accurately,
adding to the mission’s precise data collection.
Figure 1. Low iron impurity levels result in BOROFLOAT glass’ excellent optical transmission.
Figure 2. Exceptionally high UV transmission is a key benefit for low wavelength applications.