Atmosphere ferrites

Hydrogen Atmospheres Austenitic stainless steels, by virtue of their structure and high chromium contents, are usually resistant to hydrogen atmospheres. Ferritic alloys are more susceptible to hydrogen embrittlement. 

Minchener el al. report that the bubble phase of atmospheric fluidised bed combustion has a pOi in the range 2 x 10 to 2 x 10 Combustion in the dense phase is sub-stoichiometric, with the /Oj as low as 10 and SO2 and SO3 present in the range 500-5 000ppm. Low Cr-Mo steels show heavy scaling in these conditions, whereas 9-12% Cr steels show good resistance to sulphidation up to 650°C. Roberts et however, report that for pressurised fluidised-bed combustion, ferritic steels at or below 9% Cr show heavy general corrosion above 540-560°C. 

Oxides play many roles in modem electronic technology from insulators which can be used as capacitors, such as the perovskite BaTiOs, to the superconductors, of which the prototype was also a perovskite, Lao.sSro CutT A, where the value of x is a function of the temperature cycle and oxygen pressure which were used in the preparation of the material. Clearly the chemical difference between these two materials is that the capacitor production does not require oxygen partial pressure control as is the case in the superconductor. Intermediate between these extremes of electrical conduction are many semiconducting materials which are used as magnetic ferrites or fuel cell electrodes. The electrical properties of the semiconductors depend on the presence of transition metal ions which can be in two valence states, and the conduction mechanism involves the transfer of electrons or positive holes from one ion to another of the same species. The production problem associated with this behaviour arises from the fact that the relative concentration of each valence state depends on both the temperature and the oxygen partial pressure of the atmosphere. 

For the practical use of ferrite it is necessary to get high-density material. In the firing process, the starting compound mixture has to be ferritized and densified by solid -solid and solid-gas reactions. In this process, the value of (5 and the grain size also have to be controlled. For this purpose, a heat-treatment, which was called atmosphere controlled two step firing by Tanaka, was devised, as shown schematically in Fig. 3.12 and in detail in Table 3.1. For reference, the usual heat-treatment for preparation of ferrites is shown in Fig. 3.12(b). The new heat treatment is composed of the following six processes. In process (1), the starting mixed compounds are heated to... 

Fig. 3.12 Manufacturing processes for the ferrites (a) atmosphere controlled two step firing (b) usual heat-treatment (in Nj gas). In the hatched regions (1) and (6) in (a), the samples are heated or cooled under flowing N2 gas. In the other regions, Po. has to be controlled to obtain the required oxygen content.

The best-known eutectoid reaction is that which occurs in steel where the austenite phase, stable at high temperatures, transforms into (he eutectoid structure known as pcarlitc In this transformation, the austenite phase, containing 0.8% carbon in solid solution, transforms to a mixture of ferrite (nearly pure body-centered cubic irom anti iron-carbide (Fe-.Ct. Al atmospheric pressure, the equilibrium temperature for this reaction is 723 C. This temperature is the eutectoid temperature... 

For high temperature SOFCs, a conventional commercial alloy cannot be used as the interconnector because they are easily oxidized under a high temperature oxidizing atmosphere. Therefore presently, special rare metals or doped-LaCr03 are used as the interconnector. For low temperature SOFCs, a commercial alloy can be used as the interconnector and thus, various alloys are considered as the interconnector. Austenite stainless steels have a high TEC in comparison to YSZ, and only ferritic stainless steels are considered as a candidate for the interconnector. 

The rates of reaction of the clinker phases are greatly influenced by the RH of the atmosphere in which curing occurs. For a typical Portland cement paste of w/c ratio 0.59 cured at 20°C and 100% RH, Patel el al. (P28) found the fractions of the alite, belite, aluminate and ferrite phases hydrated after 90 days to be respectively 0.94, 0.85, 1.00 and 0.51. If the RH was lowered to 80%, the corresponding values were 0.77, 0.19, 0.83 and 0.32. The hydration rate of the belite thus appears to be especially sensitive to RH. On the basis of earlier data from the literature, Parrott and Killoh (P30) concluded that the effect of RH on the hydration rate (da/d/) of each of the phases could be represented by a factor (RH — 0.55)/0.45. ... 

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