Intermetallic particles

Table 26. Electrode Potentials of Aluminum Solid Solutions and Intermetallic Particles...

This chapter aims to present some recent results on the characterization and properties of small metallic and intermetallic particles (a few nanometres (nm) in diameter) supported on ceramic oxide substrates or on carbon. Due to the rapidly expanding development in materials sciences and technology and,... 

Pits initiate at flaws adjacent to intermetallic particles due to the breakdown of passivity intermetallics act as cathode and Mg matrix as the anode80... 

In the case of the Pt/Ceo.8TbojjQ2-x catalyst, the formation of particles of a LnPts (Ln = Ce, Tb) phase, isostructural with CePts, has been confirmed (155). Figure 4.26(c) shows a HREM in which a particle of this intermetallic is present. The details of the DDP, Figure 4.26(d), can be interpreted as due to a [011] orientation of the alloy phase. HREM thus provides evidence about the incorporation of the lanthanides present in the support to the metal particles but, in the case of the catalysts based on the mixed Ce/Tb oxide, it fails to reveal the extent to which each of them come into the alloyed state. From the analysis of the contrasts in the HREM images of the intermetallic particles it is not possible to precise this point. 

Secondly, according to both HR M images and diffraction patterns, the intermetallic particles grow under particular orientation relationships with respect to the support. The following equations describe the relationships we have found ... 

A detailed comparison of the structure of the intennetallic and of the metal allows to establish a correlation between them (391). Taking into account this correlation it can be proved that the orientation relationships observed with the intermetallic particles are directly derived fiom those observed in the metal/support systems (391). 

These features in the HREM images and in the DDPs suggest the coexistence of metallic platinum and CeOi in the polycristalltne aggreagates resulting from the reoxidation at 773 K of the intermetallic particles. Moreover, these aggregates seem to consist of a core of platinum, comprised of one or at least only a few f.c.c. units, covered by a large number of nanometer-sized, randomly oriented, ceria surface patches. 

Figure 4.29. Experimental images of a (5%)Pt/Ceo.gTbo.20i, catalyst reduced at 1173 K registered in profile view a) and planar view c). Simulated images obtained using n els considering well faceted (beryl type morphology) CePts particles supported on a mixed oxide crystal b) and d). Model of a supported intermetallic particle used to obtain the simulated images e) (155).

Atmospheric corrosion is electrochemical in nature and depends on the flow of current between anodic and cathodic areas. The resulting attack is generally localized to particular features of the metallurgical stmcture. Features that contribute to differences in potential include the intermetallic particles and the electrode potentials of the matrix. The electrode potentials of some soHd solutions and intermetallic particles are shown in Table 26. Iron and sihcon impurities in commercially pure aluminum form intermetaUic constituent particles that are cathodic to aluminum. Because the oxide film over these constituents may be weak, they can promote electrochemical attack of the surrounding aluminum matrix. The superior resistance to corrosion of high purity aluminum is attributed to the small number of these constituents. 

Instrumentation. The experimental procedure for an AFM equipped with a suitably coated tip has been outlined above. In a study of an aluminum alloy AA2024-T3, intermetallic particles and the matrix phase could be separated clearly [98]. The different surface films on these phases could be associated with their corrosion behavior. Inclusions and their corrosive behavior have been studied with a combination of SKPFM and AFM [101]. The effect of chloride-containing solution on corrosion at the matrix and the intermetallic particles was studied with SKPFM, in addition, light scratching with the AFM in the contact mode was applied to study the effect of the mechanical destabilization [102]. The intermetallic particles dissolved immediately after the film on their surface had been destabilized by mechanical abrasion. 

The DiSalvo group at Cornell Uifiversity has intensely studied intermetallics for formic acid electrooxidation and observed significant enhancements in turnover efficiencies [16, 46, 72-79]. Table 4.3 compares the activity of several extended intermetallic surfaces in comparison to a Pt baseline [16]. The onset potential relevant to enhanced reactivity through the direct dehydrogenation pathway was most impacted by the addition of Sb. The introduction of both Sn and Sb into the Pt unit cell negatively impacted the anodic peak current. While Bi increased the peak current, it had an adverse impact on the onset potential. It increased the onset potential by 0.06 V and nearly doubled the peak current. The key challenges related to intermetallics for DFAFCs are surfactant-free synthesis methods and reduced nanoparticle sizes (>10 nm) to improve mass activity of the catalyst [74, 75, 80]. Mastumoto et al. compared the mass activity of PtPb 10-20 nm intermetallic particles to a commercial nanocatalyst [79], During a 9 h hold at 0.197 V vs. RHE, the PtPb intermetallic catalyst demonstrated over a twofold sustained mass activity over that of Pd. 

Nisancioglu et al. (1990) found that all iron and manganese in the alloy were incorporated in intermetallic particles in the AlMnFe series and found that no elemental manganese or iron could be detected in the matrix by X-ray EDS. Lunder et al. (1987a), however, detected neither iron particles nor... 

Figure 9-17. Schematic showing the corrosion of aluminum around an aluminum-copper intermetallic particle in an aluminum copper alloy with a copper content of 0.5-2%. The aluminum-copper particle, in the presence of pure aluminum, promotes the reduction of water (shown) or oxygen (not shown). Simultaneously, the reduction reaction causes the pure aluminum to oxidize and then dissolve. This localized corrosion (Al dissolution) results in the formation of pits.

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