Kovar is a precision alloy with a predetermined coefficient of linear thermal expansion , usually consisting of 29% nickel (Ni) , 17% cobalt (Co) and 54% iron (Fe) with impurities of silicon , carbon , and manganese .
It has a coefficient of thermal expansion close in a wide temperature range to the coefficient of thermal expansion of borosilicate glass , used for the manufacture of cylinders of incandescent lamps , fluorescent lamps , electro-vacuum devices , glass-metal insulators and ceramic-metal cases of microcircuits.
It is characterized by high adhesion to molten glass, which is why it is widely used for the manufacture of electrical leads of vacuum, gas-filled and sealed devices and various lamps passing through glass.
The name Kovar is a registered trademark of Carpenter Technology Corporation CRS Holdings, inc. , Delaware [1] . In the USSR and Russia it has the name "NK29 alloy" and "NK29-VI alloy" [2] .
Content
- 1 Properties
- 2 Ferromagnetic properties [5]
- 3 Temperature coefficient of linear expansion
- 4 Chemical composition
- 5 Production
- 6 Application
- 7 See also
- 8 Notes
- 9 References
Properties
| Property | After sintering | After hot pressing |
|---|---|---|
| Density , g / cm 3 | 8.0 | 8.35 |
| Melting point , ° C | 1450 | |
| Specific heat capacity J / (kg • K) | 460 | |
| Thermal conductivity , W / (K • m) | 17; (16.7; 17.3; 19) [3] | |
| Hardness according to vickers (load 1 kgf ) | 160 | 150 |
| Tensile strength tensile strength, MPa ( kgf ) | 650 (65) | |
| Relative extension at break,% | thirty | |
| Poisson's ratio | 0.32-0.42; 0.317 [4] | |
| Young's modulus , GPa | 138—196 | |
| Elastic Strength , MPa | 270 | |
| Specific electric resistance , Ohm • mm 2 / m | 0.49 | |
It is a soft, ductile metal of silver-white color.
In a humid environment, the alloy is susceptible to corrosion and requires a protective anti- corrosion coating. Usually, for this purpose, the leads made of alloy are nickel-plated .
The alloy is well tinned with tin - lead solders . When soldered to glass, it forms a reliable vacuum tight grip. Through the clear, colorless glass it can be seen that the wire made of the alloy in the junction has a copper-red color, so it is sometimes mistakenly believed that the wire is made of copper .
Young's modulus and Poisson's ratio depend on the heat treatment of the alloy and its deformation, after annealing or in a cured state: Young's modulus from 138 MPa to 196 MPa , Poisson's ratio from 0.317 to 0.42.
Ferromagnetic properties [5]
- The alloy is ferromagnetic . The Curie temperature is 420 ° C ; according to other sources, 435 ° C.
- Coercive force : 74 A / m .
- Initial magnetic permeability : 0.78 m GN / m .
- Maximum magnetic permeability: 6.36 mH / m .
- Residual magnetic field induction : 0.9750 T.
- Magnetic induction in the field of 8 oersted : 1,325 T.
Linear expansion temperature coefficient
The temperature coefficient of linear expansion of the alloy (TEC) is well matched with the TEC of some special grades of glass. For example, glass grades C49-2, C51-1, C51-2 have a TEC in the temperature range from 20 to 300 ° C 5.2 · 10 −6 1 / K [6] .
At a temperature at the Curie point, a phase transition occurs in the alloy — up to this temperature, the LTEC is about 5.5 · 10 −6 1 / K, and above the Curie point is about 9 · 10 −6 1 / K. This break in the temperature dependence of the thermal expansion coefficient is called the inflection point . The temperature value of the inflection point is normalized by alloy standards [5] . For the NK29 alloy, the inflection point should be 420 ° C.
The thermal expansion coefficient of 29KN and 29NK-VI alloys in the form of an annealed tape is given in the table.
| Temperature ° C | -one hundred | -80 | -60 | -40 | -twenty | one hundred | 200 | 300 | 400 | 500 | 600 | 700 | 800 |
| Linear expansion temperature coefficient alloys 29NK and 29NK-VI, × 10 6 1 / K | 7.6 | 7.5 | 7.4 | 7.4 | 7.1 | 6.3 | 5.9 | 5.2 | 5,0 | 6.4 | 7.7 | 9.0 | 9.8 |
Chemical Composition
In the USSR and Russia, the chemical composition of precision alloys is established by GOST 10994-74 “Precision alloys. Stamps. " For example, an alloy grade 29NK has the following composition in mass%:
| Iron | Nickel | Cobalt | Carbon | Silicon | Manganese | Phosphorus | Sulfur | Chromium | Copper | Aluminum | Titanium |
|---|---|---|---|---|---|---|---|---|---|---|---|
| rest | 29th | 17 | no more 0,03 | no more 0.3 | no more 0.4% | no more 0.015 | no more 0.015 | no more 0.1 | no more 0.2 | no more 0.2 | no more 0.1 |
Other chemical elements, except iron, nickel and cobalt are undesirable in the composition of the alloy, as they worsen its properties.
Production
The alloy is smelted in electric arc furnaces . Alloying components are added in the form of ferroalloys . In the smelting process, the chemical composition of the alloy is carefully controlled, so this alloy is referred to as precision alloys.
After smelting, the alloy ingots are rolled and drawn to produce wire, rods of different sections, tapes, pipes and other profiles.
Before application for soldering with glass or ceramics, alloy billets are annealed in a humid hydrogen atmosphere at a temperature of 800–900 ° C and then an oxide film of the required thickness is created on the surface by heating in air with a given duration to a controlled temperature. The oxide film consists of cobalt and nickel oxides with a slight admixture of iron oxide, since the iron oxide formed during oxidation is reduced by cobalt. The oxide film significantly improves the adhesion to molten glass [7] [4] .
Application
Now the main consumer of the alloy (after crowding out electrovacuum devices with semiconductor devices ) is the production of incandescent lamps and fluorescent lighting lamps , semiconductor devices in metal-glass and ceramic-metal cases, sealed electrical connectors with glass insulators, where a wire or tape made of alloy is used for sealed current leads passing through glass or ceramics.
To a lesser extent, the alloy is used for the manufacture of microcircuit terminals (even optional microcircuits in metal-glass or ceramic-metal cases, also in plastic). This application of the alloy for microcircuit terminals in plastic cases and other applications is due to the relative cheapness of the alloy, availability and good processability - the alloy is plastic in the cold state, it rolls well, extends , is stamped with deep drawing, it is welded and soldered with tin-lead and hard solders .
See also
Some alloys with standardized thermal expansion coefficient:
- Invar (alloy).
- Elinwar (group of alloys).
- Platinum (alloy).
Notes
- ↑ Trademark Assignment Abstract (1993). Date of treatment June 18, 2014.
- ↑ NK29 alloy
- ↑ According to other sources.
- ↑ 1 2 Espi Metels. Kovar.
- ↑ 1 2 3 GOST 14080-78. Precision alloy tape with a predetermined temperature coefficient of linear expansion. Technical conditions
- ↑ The temperature coefficient of linear expansion of some grades of glass at different temperatures.
- ↑ High Temp. Kovardata.
Links
- Coursework: Technological processes of sealing IC (integrated circuits) .
- Kovar on t-book.net.
- Kovar (yield strength, tensile strength) (inaccessible link) // RedMetSplav.ru.
- Characteristics of Kovar 29NK .