4J29, K94610, 1.3981 Kovar expansion alloy

Kovar alloy is an iron-nickel-cobalt alloy with a density of 8.36g/cm3; between 20 and 500°C, its linear expansion coefficient is equal to that of glass and ceramics.

Alloy principle

Most metals cannot be sealed to glass because they have a different coefficient of thermal expansion than glass, so after the joint cools, the stresses caused by the different rates of expansion of the glass and metal crack the joint. The nonlinear thermal expansion curve of KOVAR alloys can often be matched to that of glass, allowing joints to withstand a wide temperature range.

Chemically, KOVAR alloys bond to the glass through an intermediate oxide layer of nickel oxide and cobalt oxide; the bond strength is highly dependent on the thickness of the oxide layer. The proportion of iron oxide is lower due to the reduction of cobalt, and the presence of cobalt makes it easier for the oxide layer to melt and dissolve in the molten glass. Judging by the color, gray, gray-blue or taupe indicate a good seal; metallic colors indicate lack of oxides, and black indicates excessive oxidation of the alloy, both of which will result in a weak joint.

Applications

KOVAR alloy achieves a tight mechanical connection between two materials over a certain temperature range and is suitable for glass/ceramic-metal seals in scientific equipment, as well as conductors entering glass envelopes in electronic components, such as vacuum tubes (valves), X-ray and Microwave tubes, transistors, diode integrated circuits, etc.

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