Viscosità
Quartz glass exhibits an inversely proportional, exponential
temperature dependence on viscosity. Up to 1,000°C, the material
can be regarded as a true solid. From that temperature, quartz
glass starts to flow slowly. At approximately 1,700 °C, quartz
glass is still highly viscous, comparable with the viscosity of
tar on frosty winter days, but can be deformed to an increasing
extent. Impurities have a great influence on viscosity. The
hydroxyl content as well as traces of alkali and halogen, such
as sodium and chloride, reduce the viscosity, whereas small
concentrations of aluminium or difficulty melting metals, such
as molybdenum, have the reverse effect. After extended heating,
devitrification**
occurs. By metered doping of
foreign matter or adjustment of the OH content, quartz glass can
be optimized for the intended application temperatures.
**
Devitrification
 The
thermodynamically preferential structure of quartz is
crystalline. Quartz glass is amorphous and exhibits a high
viscosity, preventing a structural re-organization of the atoms.
However, at increased temperatures and in the presence of
certain foreign matter, this process, referred to as
devitrification, is initiated. Particularly the alkali metals
sodium or potassium act as electron donators and destroy the
cross-linked silicon/ oxygen structure, triggering
devitrification. A high steam pressure or chloride addition
likewise accelerates the process. Devitrification usually starts
on the surface, from where it penetrates further into the
material. The penetration speed is an exponential function of
the temperature. In the devitrification process, a crystalline
material forms whose structure corresponds to cristobalite. As
the density is virtually identical, the cristobalite layer on
the quartz glass cannot be discerned visually. When cooled, the
crystal structure of cristobalite turns at 275°C from a cubical
to a tetragonal structure, which is accompanied by a strong
reduction in density. The material may then crack or break,
visible on the surface by white spots. The tetragonal
cristobalite structure exhibits a refractive index, which
greatly differs from that of quartz glass. |
Applicazioni
specifiche >>
Proprietà del vetro di quarzo
Proprietà meccaniche
Proprietà termiche
Viscosità
Proprietà elettriche
Proprietà ottiche
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