Oxidation behavior matrix

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A considerable amount of work has been conducted on the oxidation behavior of NiAl because it is a single-phase, primary-AI2O3 former which is often considered a model alloy for oxidation studies. Whereas in many ways NiAl is a model material, a review of results also shows a few unique characteristics, especially with regard to its transient oxidation behavior. NiAl has also been an excellent matrix material for studying the reactive element (RE) effect discussed in Section 6.2.2. 

In the preceding discussion on oxidation of composites, each matrix phase reacted with oxygen to form its corresponding oxide. No interactions between matrix or oxide phases were reported. In this section, a few examples are discussed in which the composite scale contains oxide compounds formed from more than one component of the matrix phases, e.g. mullite, zircon, or aluminum borates. This is not intended to be a complete review, but an illustration of the kinds of effects compound formation can have on oxidation behavior. For example, formation of the compound oxide scales may result in higher oxidation rates if the compound oxide is more permeable to oxygen, or likewise, lower oxidation rates if the compound oxide is less oxygen permeable. The volatility of scale components can also be reduced by the formation of compound oxides as will be discussed below for aluminum borates. 

While few examples of this phenomenon exist and little work has been done in this area, it is possible to affect the oxidation behavior of materials which form a single phase oxide solid solution. For example, upon oxidation, a HfC/ZrC composite would form a solution of Hf02/Zr02. Use of this type of composite, and choosing the HfC to ZrC ratio might be useful to control the monoclinic to tetragonal transformation temperature of the resulting scale or other properties of the matrix or scale. 

These CMCs include a wide range of multiphase ceramics with the additional phase added to impart improved fracture toughness. Examples of this include whiskers, particulates, and continuous fibers in a ceramic matrix (Evans, 1990). In many cases, the additional phase and/or coating around that phase has substantially different oxidation behavior than the matrix. This leads to complex oxidation and corrosion behavior for the entire composite. 

Multi-component ceramics allow the optimization of various physical properties. These include ceramics which form multi-component oxides as well as fiher-rein-forced ceramic matrix composites. However, the oxidation behavior of these materials is complex compared with the pure materials. The leading fiber-reinforced composites are silicon-based and contain continuous SiC fibers with coatings of graphitic carbon or hexagonal boron nitride. The oxidation of the fiber coating at intermediate temperatures is a major issue and models of this process are discussed for both carbon and boron nitride coatings. 

Similar experiments with copper dispersed on AI2O3 did not show any unusual behavior of the Al(ls) or Cu(2p) photolines. In this case, the copper could be easily cycled between CuO under oxidative conditions, to Cu metal during reducing conditions. We observed only a slight shift (<0.4 eV) of the aluminum (Is) line upon initial heating, which was attributed to the loss of water in the alumina matrix. 

In the direct ammoxidation of propane over Fe-zeolite catalysts the product mixture consisted of propene, acrylonitrile (AN), acetonitrile (AcN), and carbon oxides. Traces of methane, ethane, ethene and HCN were also detected with selectivity not exceeding 3%. The catalytic performances of the investigated catalysts are summarized in the Table 1. It must be noted that catalytic activity of MTW and silicalite matrix without iron (Fe concentration is lower than 50 ppm) was negligible. The propane conversion was below 1.5 % and no nitriles were detected. It is clearly seen from the Table 1 that the activity and selectivity of catalysts are influenced not only by the content of iron, but also by the zeolite framework structure. Typically, the Fe-MTW zeolites exhibit higher selectivity to propene (even at higher propane conversion than in the case of Fe-silicalite) and substantially lower selectivity to nitriles (both acrylonitrile and acetonitrile). The Fe-silicalite catalyst exhibits acrylonitrile selectivity 31.5 %, whereas the Fe-MTW catalysts with Fe concentration 1400 and 18900 ppm exhibit, at similar propane conversion, the AN selectivity 19.2 and 15.2 %, respectively. On the other hand, Fe-MTW zeolites exhibit higher AN/AcN ratio in comparison with Fe-silicalite catalyst (see Table 1). Fe-MTW-11500 catalyst reveals rather rare behavior. The concentration of Fe ions in the sample is comparable to Fe-sil-12900 catalyst, as well as... 

A similar example of curing inhibition In an acidic medium has been observed when moderate amounts of salicylic acid were added to epoxy-amine matrix systems. (26) Such behavior may beresponsible for the poor bond strengths observed with the NTMP-containing oxide-primer specimens. In environments containing an aggressive species (e.g.. Cl ), the anion may interact with and become incorporated into the NTMP-oxide matrix, whereby it can attack the metal surface. 

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