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Low thermal expansion alloys
are often used in microwave packaging, particularly when hermeticity
is required. For over 100 years engineers have been playing around
with alloys to control their temperature expansion.
Low expansion alloys are often
used in microwave housings
and glass feedthroughs.
"Invar" usually refers
to the alloy Invar 36. Invar is the original low-expansion iron
alloy, and dates to 1896. The word Invar was derived from "invariant"
by its inventor, Swiss Charles Édouard Guillaume. For his
work he received the Nobel Prize in Physics in 1920. And he now
appears in the Microwave
Hall of Fame! The applications of low expansion alloys are many,
particularly when you consider glass materials have naturally low
thermal expansion, and glass and metal are often joined to create
things that we need. Observatory telescope mirrors are mounted on
Invar supports so that they don't distort with the ambient temperature.
Read more about the history
of Invar on Carpenter's web site.
Invar's composition is denoted
Fe36Ni (36% nickel, balance is iron). Iron nickel alloys have a
minimum thermal expansion at 36% nickel, thus the term "Invar
36" refers to this minimum.
Kovar is the most popular iron
alloy used in packaging. Kovar is very similar to Invar, but with
a touch of cobalt: Fe29Ni17Co, with a trace of some other elements
thrown in. Kovar is a trademark of Carpenter Technology Corporation,
but the trademark has been diluted over the years so that you almost
never see the little symbol next to it. Shame on us all for
our lack of respect!