VYCOR
On the one hand, VYCOR describes a process for the production of porous glass and quartz glass at relatively low temperatures and, on the other hand, the resulting glass.
history
The fact that many glasses tend to segregate and the resulting micro phases have different levels of resistance to water , acids and bases and, in extreme cases, can completely remove a phase, gave rise to the VYCOR process developed by Corning Glass Works in the early 1930s . VYCOR glasses were first registered for patent by ME Nordberg and HP Hood in 1934.
The VYCOR process
The advantage of this process is the production of quartz glass at a low temperature. Usually temperatures of 2000 ° C are required for the production of quartz glass, the VYCOR process allows a reduction to 1000 to 1300 ° C.
The VYCOR process can be described by the following steps.
- Melting of the base glass (composition in the ternary system M 2 O-B 2 O 3 -SiO 2 in the area of the VYCOR glasses)
- Shaping the base glass by pressing, drawing, blowing
- Thermal treatment (time and temperature dependent)
- Etching of the surface of the molded body ( hydrofluoric acid , caustic soda , mechanical)
- Washing process (alcohol, water, diluted soda solution)
- Extraction process (acids, inorganic salt solutions, 90 to 100 ° C)
- Washing process, drying → porous VYCOR glass (see below)
- Sintering process (1000 to 1300 ° C) → quartz glass
In the last step, the porous glass sinters with a 30 percent volume contraction to form a clear and almost pure silica glass.
In the VYCOR basic glass, the sodium ions can also be replaced by other ions such as lithium or potassium . This also changes the optimal VYCOR base glass composition, since the boric acid anomaly in the binary Li 2 O-B 2 O 3 and K 2 O-B 2 O 3 glass systems is at different alkali concentrations than in the binary Na 2 O-B 2 O 3 glass system. Sometimes a small amount of aluminum oxide (Al 2 O 3 ) is added to the VYCOR glass melt , which means that the phase separation and leaching process can be better controlled. A certain residual Al 2 O 3 content can then be determined in the VYCOR quartz glass .
Properties of VYCOR glass
VYCOR glasses have the following composition and properties:
Properties of VYCOR glass
- linear thermal expansion: α = 7 ... 8 × 10 −7 K −1
- maximum thermal load at around 1100 ° C
- permanent thermal load: 800 ... 900 ° C
- Density : ρ = 2.18 g cm −3
- chemical resistance (acid, alkali and water resistance ): class 1
- Softening point: 1500 ... 1530 ° C
Composition of the starting glass and the VYCOR glass
| Sodium borosilicate starting glass | VYCOR glass |
|---|---|
| 55-75 mass% SiO 2 | 95-98 mass% SiO 2 |
| 20-35 mass% B 3 O 3 | 2.5-3.5 mass% B 2 O 3 |
| 5-10 mass% Na 2 O | 0.3-0.6 mass% Na 2 O |
The porous VYCOR glass obtained after step 7) is an opaque and highly porous glass , which has a specific surface area of 100 to 300 m² / g, a porosity of 25 to 40% by volume and a pore diameter of 2 to 5 nm possesses.
Applications of VYCOR glass
In the manufacture of halogen lamps that are subject to high thermal loads, VYCOR glass is primarily used in the form of tubes. Furthermore, it has found importance in the production of simple laboratory devices, whereby the volume contraction during production must be taken into account.
Applications of porous VYCOR glass
Porous VYCOR glass can be used in chromatography, as a carrier for catalysts and in membrane technology. Due to its wide range of applications, it quickly lost its intermediate product character. However, since it is only produced in a very narrow range of pore sizes, numerous investigations have been carried out to change this through subsequent processes.
literature
- Werner Vogel: Glass chemistry . Springer-Verlag, Berlin 1992, ISBN 3-540-55171-9 .
- F. Janowski, W. Heyer: Porous glasses - production, properties and applications . VEB German publishing house for basic industry, Leipzig 1982.
- ME Nordberg: Properties of Some Vycor-Brand Glasses . In: J. Americ. Ceram. Soc. tape 27 , 1944, pp. 299-305 , doi : 10.1111 / j.1151-2916.1944.tb14473.x .
- F. Janowski: The application of porous glasses in process engineering offers great potential . In: machine market . tape 99 , 1993, pp. 28-33 .