KOVAR alloy is an iron-nickel-cobalt alloy with a density of 8.36 g / cm3 and a coefficient of linear expansion equal to that of glass and ceramics with a temperature of 20-500 °C. Most metals cannot be hermetically sealed with glass because their coefficient of thermal expansion is different from that of glass and the joints are easily damaged during cooling. The nonlinear thermal expansion curve of KOVAR alloy can usually be matched with glass. It is bonded to glass through an intermediate oxide layer of nickel oxide and cobalt oxide, and the bond strength depends on the thickness of the oxide layer. The reduction of cobalt reduces the proportion of iron oxide, makes the oxide layer more soluble in the molten glass and allows the joint to withstand a wide range of temperatures.
KOVAR alloys provide a tight mechanical connection between hard glass (such as borosilicate glass) and ceramics within a specified temperature range. Judging from the color of the interface, gray, gray blue, or gray brown means a good seal and black indicates a weak bond due to the alloy being overoxidized. Kovar alloys are suitable for glass/ceramic metal seals of scientific instruments such as vacuum tubes (bulbs), microwave tubes, X-ray and microwave tubes, transistors and diode integrated circuits.
Invar alloy commercial brand
- Carpenter
- Special metal
- Kovar®/Rodar®
- Niro® K
