Sealing Element Samples in Ampoules
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Why seal elements in ampoules? Many elements will tarnish and corrode when exposed to air and/or moisture. Examples are lanthanum, cerium, barium, and many other metals. Sealing samples of these metals in glass ampoules in argon and vacuum is the best way to preserve the original shine and appearance of these elements indefinitely. Also some elements are toxic and are best presented in this form to protect against exposure to humans.
How is the ampoule made? First one end of the ampoule is sealed under normal atmospheric pressure. It is then filled with Argon gas (an inert, completely non-toxic gas) and the metal piece is inserted. A deep vacuum is then applied and the other end is heat-sealed, permanently encasing the metal piece in the ampoule and protecting it from the corrosive effects of air and moisture. What's left in the ampoule is the metal piece surrounded by a very small amount of argon gas in a vacuum. The final step is to apply an identifying label to the ampoule.
Why argon and vacuum? Most commercial argon gas including that used for these ampoules is very pure, 99.999%. The small amount of impurities in the argon can include air and moisture. Sealing reactive metals like this in argon alone provides a measure of protection against tarnishing and corrosion of the metal by air and moisture. However applying a vacuum in addition to the argon-filled ampoule pulls out a large percentage of the argon and its impurities, resulting in virtually no air or moisture being left behind in the ampoule when it is sealed.
How can you tell if there is a vacuum inside the ampoule? Look at the photos below. The image to the left shows the first end that is sealed under normal pressure. The image on the right shows the other end that was heat-sealed under vacuum. Note that in the right photo the hot molten glass was partially drawn into the ampoule by the vacuum inside it. That is the sign that a vacuum exists within the ampoule.
How do you know the ampoule will preserve the shine and appearance for a long time? The metal piece is cleaned and polished before being sealed in the ampoule. The glass used for making the ampoules is heat-treated prior to sealing. Several of the more reactive metal ampoules have undergone periodic testing after sealing whereby the sealed ampoule was heated at high temperature for a few hours. This test is done to gain confidence that the metal piece will stay shiny even under adverse heat conditions.
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