page_title: Glass-to-metal seal
section_title: Matched seal
hierarchical_section_title: Glass-to-metal seal / Matched seal
caption_attribution_description: Čeština: Přizpůsobené zátavy kovu a skla
context_page_description: Glass-to-metal seals are a very important element of the construction of vacuum tubes, electric discharge tubes, incandescent light bulbs, glass encapsulated semiconductor diodes, reed switches, pressure tight glass windows in metal cases, and metal or ceramic packages of electronic components.
  Properly done, such a seal is hermetic. To achieve such a seal, two properties must hold:
  The molten glass must be capable of wetting the metal, in order to form a tight bond, and
  The thermal expansion of the glass and metal must be closely matched so that the seal remains solid as the assembly cools.
  Thinking for example about a metal wire in a glass bulb sealing, the metal glass contact can break if the CTEs are not well aligned. For the case that the CTE of the metal is larger than the CTE of the glass, the sealing shows a high probability to break upon cooling. By lowering the temperature, the metal wire shrinks more than the glass does, leading to a strong tensile force on the glass, which finally lead to breakage. On the other hand, if the CTE of the glass is larger than the CTE of the metal wire, the seal will tighten upon cooling since compression force is applied on the glass.
context_section_description: In a matched seal the thermal expansion of metal and glass is matched. Copper-plated tungsten wire can be used to seal through borosilicate glass with a low coefficient of thermal expansion which is matched by tungsten. The tungsten is electrolytically copper plated and heated in hydrogen atmosphere to fill cracks in the tungsten and to get a proper surface to easily seal to glass. The borosilicate glass of usual laboratory glassware has a lower coefficient of thermal expansion than tungsten, thus it is necessary to use an intermediate sealing glass to get a stress-free seal.
  There are combinations of glass and iron-nickel-cobalt alloys (Kovar) where even the non-linearity of the thermal expansion is matched. These alloys can be directly sealed to glass, but then the oxidation is critical. Also, their low electrical conductivity is a disadvantage. Thus, they are often gold plated. It is also possible to use silver plating, but then an additional gold layer is necessary as an oxygen diffusion barrier to prevent the formation of iron oxide.
  While there are Fe-Ni alloys which match the thermal expansion of tungsten at room temperature, they are not useful to seal to glass because of a too strong increase of their thermal expansion at higher temperatures.
  Reed switches use a matched seal between an iron-nickel alloy (NiFe 52) and a matched glass. The glass of reed switches is usually green due to its iron content because the sealing of reed switches is done by heating with infrared radiation and this glass shows a high absorption in the near infrared.
  The electrical connections of high-pressure sodium vapour lamps, the light yellow lamps for street lighting, are made of niobium alloyed with 1% of zirconium.
  Historically, some television cathode ray tubes were made by using ferric steel for the funnel and glass matched in expansion to ferric steel. The steel plate used had a diffusion layer enriched with chromium at the surface made by heating the steel together with chromium oxide in a HCl-containing atmosphere. In contrast to copper, pure iron does not bond strongly to silicate glass. Also, technical iron contains some carbon which forms bubbles of CO when it is sealed to glass under oxidizing conditions. Both are a major source of problems for the technical enamel coating of steel and make direct seals between iron and glass unsuitable for high vacuum applications. The oxide layer formed on chromium-containing steel can seal vacuum tight to glass and the chromium strongly reacts with carbon. Silver-plated iron was used in early microwave tubes.
  It is possible to make matched seals between copper or austenitic steel and glass, but silicate glass with that high thermal expansion is especially fragile and has a low chemical durability.
caption: Matched glass-to-metal seals
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