HITEGLA BLOG
Sealing quartz glass in high-temperature environments – methods and problems
Quartz glass is widely used in systems operating in extreme temperature conditions, such as vacuum furnaces, chemical reactors, high-pressure lamps and optical systems. A key aspect in these applications is ensuring the tightness of connections, especially at the interface between quartz glass and other materials such as metal, ceramics or other types of glass.
The seals must withstand not only high temperatures (often above 800°C), but also differences in thermal expansion, vacuum, aggressive chemicals and heating and cooling cycles.
1. Methods of sealing quartz glass
A. Hard (permanent) seals
1. Active brazing
• Used to join quartz glass with metals (e.g. molybdenum, niobium, titanium).
• The solder contains active elements (e.g. Ti, Zr) that form permanent bonds with the glass surface.
• The process takes place in a vacuum or protective atmosphere at a temperature of 850–1000°C.
• Good for vacuum or gas-tight applications.
Advantages: durability, tightness
Disadvantages: complicated process, matching of expansion coefficients required
2. Sintering and welding
• Quartz glass can be bonded to other quartz elements by local melting.
• Requires a very precise thermal process.
• Only possible with homogeneous materials.
B. Soft seals (flexible, removable)
1. Metal seals (e.g. CF, ConFlat type)
• Copper or silver seals that adapt to the surface are used in vacuum systems.
• They are used together with metal flanges attached to the glass (by soldering or other techniques).
2. Ceramic or high-temperature fibre seals
• Used in furnaces and thermal systems.
• They can be made of aluminium silicate or aluminium oxide fibres.
• Used for insulation rather than hermetic sealing.
3. Adhesives and sealants (ceramic, sol-gel, glazes)
• Used where detachable joints are not required.
• Examples: adhesives based on SiO₂, aluminium oxides or boron-silicon oxides.
• They withstand temperatures of 1000–1200°C well, but may be brittle after curing.
2. Problems and challenges in sealing quartz glass
A. Differences in thermal expansion
• Quartz glass has a very low expansion coefficient (~5.5×10⁻⁷/K).
• Metals and other types of glass can have a coefficient that is 10–20 times higher.
• This causes high stresses in the joint during heating and cooling → which can lead to microcracks or loss of tightness.
B. Ageing and brittleness of joints
• Some adhesives and sealants become brittle over time, especially in the presence of oxygen or water vapour.
• The thermal cycle (heating-cooling) leads to microdamage.
C. Chemical compatibility issues
• Reactions between materials (e.g. metal–glass, metal–adhesive) can lead to seal degradation.
• In vacuum or corrosive environments, passivation or barrier layers are necessary.
D. Difficulty in quality control
• High-temperature joints are difficult to test non-destructively (NDT).
• Microcracks or leaks may be invisible at room temperature but only become apparent during operation.
Summary
Sealing quartz glass in high-temperature environments is a complex task requiring precise selection of materials and technologies. The key criteria are thermal expansion compatibility, chemical and mechanical resistance, and long-term sealing durability. Despite the challenges, the right technique – whether active soldering, metal gaskets or high-temperature adhesives – allows for the reliable use of quartz in the most demanding applications.
