Kovar temperature

Maximum use temperatures exceed 1000°C except for Si02 (>800° C) melting points range from 1450°C (Kovar) to 3660°C (tungsten). 

Crystalline films of TiB2 obtained by CVD are used at lower temperatures as protective coatings. The interaction of these films with various substrates is studied" and desirable properties of the substrate in cases when the coating is deposited according to Eq. (a) are formulated. Substances meeting the requirements in this respect are W, Ta, Mo, WC, TiC, graphite, Fe-Ni-Co-Mn alloy (Kovar) and some high-chrome steels. 

The reactor assembly was heated by electric heaters. The maximum operating temperature Is determined by the window construction. Sapphire windows (from EIMAC), brazed into Kovar sleeves, were used the sleeves were then welded directly into the stainless steel reactor housing. We found that the cell so constructed was capable of trouble-free, continuous operation at 450°C operations at somewhat higher temperatures are probably still possible but were not explored. Sapphire was chosen as a window material because it is insensitive to water vapor and is transparent in tljie wave number range of our interest (about 2400 cm to 2000 cm in these experiments). Moreover, the thermal expansion characteristics of the reactor were found to match well with those of the window fixture. 

Certain alloys of iron, nickel, and cobalt (Kovar, Fernico, etc.) have thermal expansion curves which nearly match those of borosilicate glasses, and a good bond may be formed between the two. Kovar is similar to carbon steel in its chemical properties. For example, it oxidizes when heated in air and is not wet by mercury. It may be machined, welded, copper brazed, and soft soldered. Silver solders should not be used with Kovar since they may cause embrittlement. At low temperatures Kovar undergoes a phase transformation, and the change in expansion coefficient below this temperature may be sufficient to cause failure of a glass-to-Kovar seal. The transformation temperature usually is below... 

C. The exact transformation temperature varies from one batch of the metal to the next and some samples of Kovar have transformation temperatures below — 196°C. Another significant property of Kovar is its low thermal conductivity. 

Wang, J., Taleff E.M.. and Kovar, D., High-temperature deformation of A1203/Y-TZP particulate composites , Acta Mater., 2003, 51, 3571-83. 

Cerny and Kovar (241) determined the heat of interaction with vacuum-evaporated films of dysprosium and yttrium at room temperature. These metals yielded high heats of adsorption of 960 and 1060 kj mol", respectively. It was suggested that the oxygen molecules undergo dissociative adsorption on the surface and penetrate into the subsurface, where they form strong bonds to the metal. 

Kovar et al. (81AP532) treated l,2-dihydro-4-isopropyl-5-methyl-2-phenyl-3/7-pyrazol-3-one with fuming nitric acid at room temperature that resulted in at least 27 products of which eight main nitro products were isolated. One of the eight products identified was 2,4-dihydro-4-isopropyl-5-methyl-4-nitro-2-(2,4-dinitrophenyl)-37/-pyrazol-3-one. 

Electrical-stability testing is essential for conductive adhesives used for electrical connections. Electrical conductivity can degrade at elevated temperatures, on aging with or without power, and on exposiue to humidity and temperature. The specific test method used depends on the application. One test used for die-attach adhesives specified in NASA MSFC-SPEC-592 (now inactive) involves a series of gold-plated Kovar tabs attached with conductive epoxy to metal pads on an interconnect substrate. In the test vehicle, a bias of 5 V and cmrent density of 139 3.9 A7cm (900 A/in ) are applied to a series of wire-connected tabs, and the resistance change is measured after exposure to 150 °C periodically up to 1,000 horns. The maximum allowable resistance change is 5%. 

Fytche, L.M. Hupe, M. Kovar, J.B. Pilon, R, Ion mobility spectrometry of drugs of abuse in customs scenarios concentration and temperature study, J. Forensic Sci. 1992, 37, 1550-1566. 

Kovar A trade name for an alloy of iron, cobalt, and nickel with an expansivity similar to that of glass. It is therefore used in making glass-to-metal seals, especially in circumstances in which a temperature variation can be expected. 

The stability of the electrical parameters after ageing at high temperature under power is of prime importance in the selection of die attach adhesives. The test requires a current density of 139.5 A cm applied to five gold-plated Kovar tabs, each tab being adhesively bonded to thin film gold conductors and series bonded with gold wires. The biased test specimens are heated at 150°C in nitrogen for 1000 h, and the electrical resistance of the bonds is measured at 25°C at 200 h intervals. The specification requirement states that the resistance of the five tabs does not increase more than 5%. 

Another such alloy, with the trade name of Kovar, has been designed to have expansion characteristics close to those of borosilicate (or Pyrex) glass when joined to Pyrex and subjected to temperature vari-... 

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