Internal oxide morphology

The enormous amount of research at the interface between physical and structural chemistry has been expertly reviewed recently by Schmalzried in a book about chemical kinetics of solids (Schmalzried 1995), dealing with matters such as morphology and reactions at evolving interfaces, oxidation specifically, internal reactions (such as internal oxidation), reactions under irradiation, etc. 

X-ray difl raaion (structure grain size preferred orientation stress) Scanning laser microscopy Optical microscopy Oocnl thickness topography nucleation general morphology internal oxidation) l.R. spectroscopy (specialised analysis and applications)... 

Systematically speaking, so-called internal oxidation reactions of alloys (A,B) are extreme cases of morphological instabilities in oxidation. Internal oxidation occurs if oxygen dissolves in the alloy crystal and the (diffusional) transport of atomic oxygen from the gas/crystal surface into the interior of the alloy is faster than the countertransport of the base metal component (B) from the interior towards the surface. In this case, the oxidation product BO does not form a stable oxide layer on the alloy surface. Rather, BO is internally precipitated in the form of small oxide particles. The internal reaction front moves parabolically ( Vo into the alloy. Examples of internal reactions are discussed quantitatively in Chapter 9. 

Many of the effects of internal oxidation, both on the overall corrosion process for an alloy and on influencing the mechanical, magnetic, etc., properties of the alloy, are intimately related to the morphology of the oxide precipitates. The following is a rather qualitative discussion of the factors, which influence the size, shape, and distribution of internal oxides. 

These predictions have been born out in a number of studies of particle morphology available in the literature.However, the internal oxidation process is quite complex and many systems show deviations from the simple analysis provided above. Douglass has described these phenomena which include the following. 

Fra] Fratzl, P., Paris, O., Internal Oxidation of Cu-Fe-II. The Morphology of Oxide Inclusions from the Minimization of Elastic Misfit Energy , Acta Metall. Mat., 42(6), 2027—2033 (1994) (Theory, 15)... 

The morphologies shown in Figirre 9.20 are somewhat idealized. In fact, the oxide scales resulting from simrrltaneous oxidation of alloys often have a more complicated structure several stacked layers, presence of precipitates in the adjacent metal zone due to internal oxidation, presence of metal islands in the oxide films, formation of pores and of cavities, etc. In addition, the oxidation of an alloy often exhibits non steady state behavior involving an initial phase during which the structure and the composition of the scales change transient oxidation). 

The morphology of the internal oxidation zone and stabihty of transformation front were studied in [29]. Solid-state reactions with formation of two-phase zones were analyzed in [3, 30, 31]. 

Oxidation morphology is globally the same for other Cr-rich high-temperature alloys like Alloy 230 and AUoy X exposed to oxidizing helium but for the extent of processes. Fig. 3.13 plots evolution of the above-listed microstructural features with time. All oxidation-induced phenomena visibly follow parabolic laws with specific rate constants. It was shown that surface oxidation is the first contributor to the mass gain, with a measurable part issuing from internal oxidation for AUoy 617. 

The system Ni-Al203 was chosen for study primarily because it is of interest as a practical composite system ( ), but also because single-crystal aluminum oxide, sapphire, is transparent in the visible region. The latter feature allows direct observation of internal interface morphologies without any disturbance to the system. 

Figure 3.6 shows the surface and cross-section morphologies of the Ni-6Cr alloy after oxidation at 900°C under an oxygen partial pressure of 10 Pa for 2 min (Fig. 3.6a), 10" Pa for 40 hr (b) and 10 Pa for 40 hr (c). It can be seen that after both 2 min and 40 hr oxidation, oxide scales with small particles and big metal nodules appeared on the surfaces (Figs 3.6a and 3.6b) an internal oxidation zone can be seen in the cross-sectional image as well (Fig. 3.6c).

Goethite crystals produced by oxidation of Fe solutions at ambient temperature in neutral solution (Fig. 4.7 right) - a process likely to occur in nature - are usually much less developed and the crystals are smaller (MCLb 10 nm) than those obtained in alkaline Fe " solutions. If Al is taken up in the structure, these crystals become extremely small (MCL 5 nm) and show almost no particular habit. At higher pH (-12) the crystals are again acicular (MCL -30 nm) despite containing structural Al (Al/(Al-i-Fe) -0.3) they show internal disorder, however, and stars are frequent. This morphology is also observed for soil goethites (see Chap. 16). 

Figure 11-10. a) Reduction of an oxide crystal, (A,B)0, resulting in internal precipitation. of A (schematic), b) Cross section of a (Ni,Mg)0 single crystal, reduced in H2/H20. Typical morphology of the reaction product if ANi0> 10%. Pores connect the reaction front with the external reducing gas. 

Specific spectroscopic techniques are used for the analysis of polymer surface (or more correctly of a thin layer at the surface of the polymer). They are applied for the study of surface coatings, surface oxidation, surface morphology, etc. These techniques are typically done by irradiating the polymer surface with photons, electrons or ions that penetrate only a thin layer of the polymer surface. This irradiation is followed by the absorption of a part of the incident radiation or by the emission of specific radiation, which is subsequently analyzed providing information about the polymer surface. One of the most common techniques used for the study of polymer surfaces is attenuated total reflectance in IR (ATR), also known as internal reflection spectroscopy. Other techniques include scanning electron microscopy, photoacoustic spectroscopy, electron spectroscopy for chemical analysis (ESCA), Auger electron spectroscopy, secondary ion mass spectroscopy (SIMS), etc. 

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