Sealing Glass Ampules or Samples under Vacuum
By Joe Walas, East Carolina University
Department of Chemistry Glassblowing Facility

Removing Residual Thermal Stress

Vacuum sealing of ampules in borosilicate glass tubing is best accomplished using a hand torch which uses natural gas (or propane) and oxygen. Hydrogen/oxygen flames are not recommended for borosilicate glass seals. When a hydrogen flame is used to flame anneal the finished ampule, the flame temperature is too hot and will result in residual thermal stress in the ampule. As air mixes with the hydrogen flame, temperatures above the strain and annealing point will result. Tests have shown that using a hydrogen flame will result in the thermal stress line being relocated into the ampule body rather than removing the strain. Flame annealing of the finished borosilicate ampule should be accomplished by using the following technique. Immediately after sealing, remove the oxygen from the torch flame and begin heating uniformly around the seal area using a gas only flame. This flame annealing process should also include the area 10 to 20 mm from the sealed end. During sealing operations, the most severe thermal stress accumulates in this 10 to 20 mm region. Continue annealing the ampule until a thin carbon black deposit appears on the surface of the glass. Do not set the hot seal on a cold bench top, etc., instead, prop the ampoule on a heat resistant material in a draft free location to cool. (See Glass Strain and Annealing)

Hydrogen/oxygen flames are generally required for quartz glass seals. Because of the low coefficient of expansion of the quartz, residual strain in the quartz seal is not as severe and flame annealing is not generally required.

The finished tube will look similar to this. Ideally, the mass of glass where the seal has been made will not be overly thick.

Prepare the quartz sample tube by making a small indent in the tube body. Insert a solid quartz plug (rests on indent) after loading sample. Evacuate. Heat and collapse the quartz sample tube onto the solid plug. With care a vacuum tight seal can be obtained. Remove the entire sample tube from the vacuum system. The sample bulb can be removed from the tube top by using a wet saw.

In some cases, it will be necessary to freeze the material within the ampoule to reduce the vapor pressure during sealing operations. Care must be taken to eliminate the possibility of condensing liquid oxygen into the ampoule. (See Cryogenics and the Scientific Glassblower). Additionally, frozen aqueous solutions will expand and crack ampoules.

Almost any material sealed into a closed ampule will expand when warmed to room temperature or heated in an oven. With a known quantity of material, calculate the vapor pressure vs.temperature of the material inside during subsequent processes. Do not exceed the Maximum Allowable Internal Pressure (MAIP) rating of the glass ampule. Remember that the MAIP ratings are given for clean, abrasion and stress free glass tubes. The vacuum seal will become the weakest part of the completed ampule. The seal shape, uniformity, and your ability to remove thermal stress from the seal is important when considering the integrity of the finished ampule.

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