Making glass-to-metal seals for homemade vacuum tubes
16 hours ago
- #vacuum tubes
- #thermal expansion
- #glass-to-metal seals
- Premade tube stock makes glasswork easy for vacuum tubes; heating and sealing with atmospheric pressure creates a vacuum that lasts.
- Copper oxide bonds strongly with glass but thermal expansion differences (copper: 17 μm/m·K, glass: ~3 μm/m·K) cause cracking upon cooling.
- Tungsten and molybdenum have compatible thermal expansion with borosilicate glass, but tungsten wire (e.g., 10 μm) is hard to handle and flammable.
- Steel wire (CTE ~11 μm/m·K) improves on copper but reacts with hot glass; electroplating with copper helps, yet thermal mismatch still breaks the seal.
- Soda-lime glass (CTE ~10 μm/m·K) works with plated steel but requires annealing to prevent cracking during cooling.
- Copper disc or foil seals (e.g., Houskeeper seal) allow stress relief by stretching, working with any glass type as thermal expansion becomes irrelevant.
- Knife-edge seals with tapered metal edges reduce stress but may still fail at the edges under contraction forces.
- Mercury or gallium alloys can form vacuum-tight seals at low temperatures but risk evaporation or leakage into the tube.
- Glue seals are not vacuum-tight due to gas permeation through polymer chains, similar to helium balloon deflation.
- Treating copper with borax or boric acid before bonding improves adhesion and may reduce stress, though bubbles can form if not fully dehydrated.
- Leak testing via capacitive glow discharge with fluorinated refrigerant detects leaks by changing glow color, requiring caution due to flammability.