Kovar (trademark of CRS Holdings, inc., Delaware[1]) is a nickel–cobaltferrousalloy compositionally identical to Fernico 1, designed to have substantially the same thermal expansion characteristics as borosilicate glass (~5 × 10−6 /K between 30 and 200 °C, to ~10 × 10−6 /K at 800 °C) to allow a tight mechanical joint between the two materials over a range of temperatures. It finds application in glass-to-metal seals in scientific apparatus, and conductors entering glass envelopes of electronic parts such as vacuum tubes (valves), X-ray and microwave tubes and some lightbulbs.[citation needed]
Assortment of Kovar metal shapes from an advertisement in a 1950 electronics magazine
Kovar was invented to meet the need for a reliable glass-to-metal seal, which 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 cannot seal to glass because their coefficient of thermal expansion is not the same as glass; as the joint cools after fabrication the stresses due to the different expansion rates of the glass and metal cause the joint to crack.[citation needed]
Kovar not only has thermal expansion similar to glass, but its nonlinear 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(II) oxide and cobalt(II) 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.[4][6] The presence of cobalt makes the oxide layer easier to melt and dissolve in the molten glass. A grey, grey-blue or grey-brown color indicates a good seal. A metallic color indicates lack of oxide, while black color indicates overly oxidized metal, in both cases leading to a weak joint.[2][citation needed]
The name Kovar is often used as a general term for Fe–Ni alloys with these particular thermal expansion properties. The related particular Fe–Ni alloy Invar exhibits minimum thermal expansion.
^USPTO United States Patent and Trademark Office (1993). "Trademark Assignment Abstract". Retrieved 18 June 2014.
External links
Kovar specification page
March 04, 2023
kovar, other, uses, disambiguation, trademark, holdings, delaware, nickel, cobalt, ferrous, alloy, compositionally, identical, fernico, designed, have, substantially, same, thermal, expansion, characteristics, borosilicate, glass, between, allow, tight, mechan. For other uses see Kovar disambiguation Kovar trademark of CRS Holdings inc Delaware 1 is a nickel cobalt ferrous alloy compositionally identical to Fernico 1 designed to have substantially the same thermal expansion characteristics as borosilicate glass 5 10 6 K between 30 and 200 C to 10 10 6 K at 800 C to allow a tight mechanical joint between the two materials over a range of temperatures It finds application in glass to metal seals in scientific apparatus and conductors entering glass envelopes of electronic parts such as vacuum tubes valves X ray and microwave tubes and some lightbulbs citation needed Assortment of Kovar metal shapes from an advertisement in a 1950 electronics magazine Kovar was invented to meet the need for a reliable glass to metal seal which 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 cannot seal to glass because their coefficient of thermal expansion is not the same as glass as the joint cools after fabrication the stresses due to the different expansion rates of the glass and metal cause the joint to crack citation needed Kovar not only has thermal expansion similar to glass but its nonlinear 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 II oxide and cobalt II 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 4 6 The presence of cobalt makes the oxide layer easier to melt and dissolve in the molten glass A grey grey blue or grey brown color indicates a good seal A metallic color indicates lack of oxide while black color indicates overly oxidized metal in both cases leading to a weak joint 2 citation needed The name Kovar is often used as a general term for Fe Ni alloys with these particular thermal expansion properties The related particular Fe Ni alloy Invar exhibits minimum thermal expansion Contents 1 Typical composition 2 Properties 3 References 4 External linksTypical composition EditGiven in percentages of weight Fe Ni Co C Si Mnbalance 29 17 lt 0 01 0 2 0 3 Properties EditProperty sintered HIPedDensity g cm3 8 0 8 35Hardness HV1 160 150Youngs Modulus GPa 138 138reduction of area at fracture 30 30yield strength MPa 270 270thermal conductivity W K m 17Curie Temperature C 435electrical resistivity W mm2 m 0 49specific heat J g K 0 46thermal expansion coefficient 10 6 K 1 25 200 C 5 5 25 300 C 5 1 25 400 C 4 9 25 450 C 5 3 25 500 C 6 2References Edit USPTO United States Patent and Trademark Office 1993 Trademark Assignment Abstract Retrieved 18 June 2014 External links EditKovar specification page Retrieved from https en wikipedia org w index php title Kovar amp oldid 990899503, wikipedia, wiki, book, books, library,
