Kovar
Kovar is an alloy with an especially low thermal expansion coefficient of appr. 5 ppm/K. Kovar is composed of iron, nickel and cobalt. Since the thermal expansion coefficient is in the same range as that of glass and ceramic, Kovar is frequently used for glass seals and ceramic-metal composites. The danger of developing cracks due to differences in thermal expansion when exposed to soldering temperatures is vastly reduced.