Oxidation[2,3] (continued)Steam Oxidation
Iron has an equilibrium constant that favors complete Fe3O4formation below 1050°F (565°C) in a steam atmosphere (eq. 6). Above this temperature, a mixture of Fe3O4and FeO is obtained depending on the H2O content and the H2O/H2ratio. At temperatures below 1525°F (830°C), H2O is a stronger oxidizing medium than CO. The reactions are as follows:
(5) Fe + H2O = FeO + H2
(6) 3Fe + 4 H2O = Fe3O4+ 4 H2
Exothermic Gas Oxidation
Carbon dioxide produced in lean exothermic gas can be used to produce an acceptable glass-to-metal seal. Natural gas and air combusted in a specific air-to-gas ratio (typically 7:1 or 8:1) creates an atmosphere that is a mixture of N2, CO, CO2and H2. Below 1050°F (565°C) in a lean exothermic-gas atmosphere, iron has an equilibrium constant that favors complete Fe3O4formation. Above this temperature, FeO is common unless the dew point is sufficiently high – in the range of +40°F (+4.5°C) as introduced into the furnace chamber. On reheating FeO + Fe to temperatures of approximately 750°F (400°C) but less than 1040°F (560°C) – in the range of mask stabilizing and frit sealing – the oxide changes to Fe3O4. Above a temperature of 1525°F (830°C), CO2is a stronger oxidizing medium than H2O.
Nitrogen/Hydrogen (Wet/Dry) Oxidation
The most common approach used today involves gases such as nitrogen or argon, a portion of which can be bubbled through water and then combined with a small percentage of hydrogen for use as a furnace atmosphere. The amount of moisture picked up by a gas is a function of the water temperature and the dwell time within the saturator.
Oxidation typically takes place from 1470-1920°F (800-1050°C) in atmospheres that include a mixture of N2, H2O and H2. Water is an easily controlled oxidizing species and provides an adherent and uniform oxide on metal surfaces. Hydrogen is added to protect against air leaks by combining with the oxygen to form water. Some researchers have found that for Kovar, the atmosphere which performed best was N2–1% H2O–0.4% H2run at 1830°F (1000°C) for 10 minutes. This produced an oxide scale of about 1 µm (40 micro-inch) and an oxide intergranular penetration of about 4 µm (155 micro-inch).
Seal FormationParts consisting of preoxidized metal bodies and pins (or leads) are assembled onto fixtures designed to hold the parts in the proper orientation. Fixtures can be machined graphite blocks or ceramic. If graphite is used, attention must be paid to the furnace atmosphere with respect to deterioration (life) of the fixtures. If ceramic, the temperature profile must be considered with respect to thermal shock.
These assemblies are then placed in a furnace operating around 1800°F (1000°C) in a nitrogen/hydrogen or lean exothermic-gas atmosphere (N2-H2-H2O) setup to produce a slightly oxidizing atmosphere. This ensures a good glass seal while not over-oxidizing the surface (making the part difficult to clean). A slow cool from sealing temperature to around 1000-1200°F (540-650°C) is often used to prevent cracking of the glass due to thermal contraction.
Thermal-expansion differences between the metal alloy and the glass must be carefully matched to produce a good seal. Thermal expansion changes significantly as a function of the nickel content of the alloy. For example, a Kovar (Fe-29Ni-17Co) alloy has a coefficient of thermal expansion between 77-575°F (25-300°C) of 3.0 ppm/°F (5.4 ppm/°C) while Alloy 42 (Fe-42Ni) has a coefficient of thermal expansion between 77–575°F (25–300°C) of 4.4 ppm/°F (8 ppm/°C).
Ceramic-to-metal seals do not have as stringent a limitation on thermal expansion because yielding of the soft metal “solder” applied during the joining process results in a reduction of strain. Nevertheless, it is advisable to use comparable thermal-expansion values wherever possible.