Iron films

It has been shown by several authors that fee iron films exhibit a rich magnetic structure, depending very sensitively on the atomic volume. In particular in Fe... 

Figure 12. Absorption spectra of (A) Fe and Fe304 and (B) iron films after treatment in chromate and nitrite solutions. (From Ref. 71, with permission.)...

Figure 15. Derivatives of the near-edge region of spectra for nitrite- (A) and chromate- (B) passivated iron films under in situ ( 4-) and ex situ ( ) conditions. (From Ref. 72, with permission.)...

Hoffman and Kordesch74,75 have presented a series of studies on the passive films on iron with particular attention to cell design. They have employed a so-called bag cell that allows for the in situ passivation and/or cathodic protection of the iron films. These were deposited onto gold films deposited on Melinex. 

Figure 16. Fluorescence detected X-ray absorption spectra for a 4-nm iron film in emersion cell. Spectra are for (A) dry film (B) cathodi-cally protected film (C) passivated film (D) background electrolyte. (From Ref. 74, with permission.)...

Thermal decomposition of [Fe(CO)5] can produce at relatively low temperatures, when compared to oxide supports, clean iron deposits when the support is copper or silver [78, 79], Thus, at 80 K under UHV, [Fe(CO)5] has been adsorbed on a Cu(lll) substrate. Decomposition of the iron precursor begins at 233 K and produces mainly adsorbed Fe(CO)4 species. A moderate heating to 323 K allows an iron film to be formed through decarbonylation of [Fe(CO)4]ads. No carbon contamination of the deposit has been detected [78]. A similar effect has been observed... 

A few measurements have been carried out on iron, and Fig. 10 shows an ascending and descending isobar at 0.1 mm. pressure (analogous to that shown for nickel in Fig. 4) for an iron film sintered at 200°. The behavior is clearly very similar to that of nickel. 

Fig. 16. Heats of adsorption of hydrogen on evaporated iron films at 23°C. as a...

Fig. 17. Heats of adsorption of hydrogen on evaporated iron film at — 183°C. (points) in comparison with the heats obtained at 23°C. (curve) as a function of surface covered.

Iron films behave in every way similar to nickel films except that upon admitting oxygen, about seven oxygen atoms are sorbed instantaneously for every crystallographic site, forming an iron oxide film seven atom layers deep, and except that the heat of adsorption of hydrogen on such an oxide covered film is almost identical with that on the clean iron surface even immediately after the oxidation has taken place. 

A point which has not been examined is the nature of the surface during exchange reactions carried out at high temperatures such as those required for the exchange of methane. Surface carbides may be formed under these conditions. The inactivity of iron films and the comparatively small activity of cobalt films at 300° for the exchange of ethane 19) may possibly be due to the tendency of these metals to form not only surface but also bulk carbides. 

Fig. 36. Conversion electron spectrum of an iron film after treatment in oxygen at 620 K for 5 min. Figure according to Simmons et al. 235).

It is, therefore, understandable that Bagg and Tompkins explain the constancy of the heat of sorption that they found for oxygen on iron films at room temperature in terms of immobility. In view of what we learned in Sec. VII,6, however, a solution of oxygen in the surface layers is more probable. 

Fig. 38. Heats of chemisorption of nitrogen on an iron film as a function of 0 (S63).

Figure 38 gives the heats of chemisorption of nitrogen on iron films, recently published by Bagg and Tompkins (363), and we may compare the decrease of this curve with the increase shown by the curve of Fig. 36. 

As we have seen in Sec. IX,9, it is not necessary to introduce active and nonactive parts. In the case of iron films, as investigated by Porter and Tompkins, both phenomena apparently occur on the surface which is freely available. Fast chemisorption may quite normally be followed by slow chemisorption associated with an activation energy when the potential curves follow the picture laid down in Fig. 37. The activation energy is a normal consequence of the decrease in the heat of chemisorption. 

The heat of chemisorption of hydrogen, adsorbed on iron that has previously been covered with nitrogen up to 0 = 0.18, is, indeed, lower than the heat of chemisorption of hydrogen adsorbed on a clean surface 395). The heat of chemisorption of CO on an iron film partly covered with nitrogen is also lower than on a clean film, but Bagg and Tompkins 395) found that hydrogen when adsorbed on a film partly covered with CO shows a higher heat of adsorption than when adsorbed on a clean film. 

The gas reactants and products of the reaction were studied with a mass spectrometer and the solid reaction of the oxidation of iron was studied with Mossbauer spectrometry with electron diffraction. The Mossbauer study of the oxidized iron powder was carried out in a constant acceleration equipment [121], The electron diffraction study of the oxidized iron film, evaporated over a carbon covered transmission electron microscopy sample holder and introduced into the 5L spherical Pyrex glass container, where the Fe evaporation takes place, was carried out with the help of an Hitachi 100C transmission electron microscope [119],... 

D. Sander, R. Skomski, C. Schmidthals, A. Enders, and J. Kirschner, Film Stress and Domain Wall Pinning in Sesquilayer Iron Films on W(110) , Phys. Rev. Lett. 77, 2566(1996). 

Homann K, Kuhlenbeck H, Freund HJ. N2 adsorption and discussion on thin iron films on W(110). Surf Sci. 1995 327 216-24. 

Addition of a small quantity of iron to the alloy or to the solution improves the erosion-corrosion resistance of the alloy because of the tenacity of the iron film on the metal surface. This protection function of iron has led to the successful use of iron sacrificial pieces in preference to the familiar zinc sacrificial anodes in water boxes of condensers and heat exchangers. 

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