Kovar Controlled Expansion Alloy Iron-nickel-cobalt Alloy for Hard
Glass Sealing
Kovar is one of the most popular of the controlled expansion alloys
for hermetic sealing applications. And it is one of A-ONE ALLOY's
featured products. We stocks Kovar sheet, Kovar plate, Kovar rod,
Kovar strip, Kovar wire, Kovar round bar as well as ingot. We make
fast delivery to meet customer requirements.
And we also delivery Kovar in form of pipe, tube, capillary and
part per customer spec&drawing.
Kovar was invented to meet the requirement for a reliable
glass-to-metal seal. It is required in electronic devices such as
light bulbs, vacuum tubes, cathode ray tubes, and in vacuum systems
in chemistry and other scientific research. Most metals could not
seal to glass because their coefficient of thermal expansion is not
the same as glass, so as the joint cools after fabrication the
stresses due to the differential expansion rates of the glass and
metal cause the joint to crack.
Kovar’s non-linear thermal expansion curve can often be made to
match a glass, thus allowing the joint to tolerate a wide
temperature range. Chemically, it bonds to glass via the
intermediate oxide layer of nickel oxide and cobalt oxide; the
proportion of iron oxide is low due to its reduction with cobalt.
The bond strength is highly dependent on the oxide layer thickness
and character the presence of cobalt makes the oxide layer easier
to melt and dissolve in the molten glass. A grey, grey-blue or
grey-brown colour indicates a good seal. A metallic colour
indicates lack of oxide, while black colour indicates overly
oxidized metal, in both cases leading to a weak joint.
Chemical composition
29% Nickel, 17% Cobalt
Density: 8.24g/cm3
Coefficient of Thermal Expansion (Nominal):
30 oC - 400 oC 4.90(µm/m- oC)
25 oC - 300 oC 5.13(µm/m- oC)
30 oC - 450 oC 5.30(µm/m- oC)
25 oC - 100 oC 5.86(µm/m- oC)
25 oC - 500 oC 6.15(µm/m- oC)
25 oC - 900 oC 11.26(µm/m- oC)
