Alloy compositions, characterization

Alloy Compositions and Product Forms. The nominal compositions of various cobalt-base wear-resistant alloys are Hsted in Table 5. The six most popular cobalt-base wear alloys are Hsted first. SteUite alloys 1, 6, and 12, derivatives of the original cobalt—chromium—tungsten alloys, are characterized by their carbon and tungsten contents. SteUite aUoy 1 is the hardest, most abrasion resistant, and least ductile. 

The ability to measure the energy of electronic transitions and their line widths accurately, in a convenient manner, is one of the most important aspects of semiconductor characterization. The former can be used to evaluate alloy compositions... 

Our view of the catalyst surface is schematically depicted in Figure 4. The indirect and direct characterization data for Pt indicates that it is present in a zero valence state. The Pt will therefore be distributed among Pt atoms, Pt clusters that are larger than one atom and Pt present as a Pt/Sn alloy. Thus, a description of the state of Pt in the Pt-Sn-alumina catalyst involves determining the fraction present in each of the three states. Furthermore, both of the direct methods for determining the Pt/Sn alloy composition, XRD and TEM, indicates that only the PtSn =1 1 alloy is formed. Thus,... 

The analysis of metal artifacts has been used extensively to differentiate materials by sources. X-ray fluorescence and neutron activation analysis have both proved valuable in determining elemental concentrations. Native metals, such as gold, contained impurities that could, in some cases, be used to characterize their sources. However, the smelting of ores to recover the metals often changed the concentrations of impurities. Later, as alloys (e.g., bronze and brass) were produced, the compositions were intentionally altered and controlled. In some cases, the re-use of materials or the lack of quality control made the alloy composition quite variable (especially in terms of the trace components). 

Clearly, size-selected cluster catalysts will play a key role in the future of model catalysis and will be an important tool in developing a detailed understanding of size effects in catalysis. Improvements in characterization under reaction conditions are needed to study the stability of these systems. In addition, exciting new possibilities for examining the effects of surface loading (number density) and alloy composition exist which will drive the field forward in the next decade. 

A preparation and characterization of new PtRu alloy colloids that are suitable as precursors for fuel-cell catalysts have been reported [43cj. This new method uses an organometallic compound both for reduction and as colloid stabilizer leading to a Pt/Ru colloid with lipophilic surfactant stabilizers that can easily be modified to demonstrate hydrophilic properties. The surfactant shell is removed prior to electrochemical measurements by reactive annealing in O2 and H2. This colloid was found to have nearly identical electrocatalytic activity to several other recently developed Pt/Ru colloids as well as commercially available Pt/Ru catalysts. This demonstrates the potential for the development of colloid precursors for bimetallic catalysts especially when considering the ease of manipulating the alloy composition when using these methods. 

The nature of copper dissolution from CuAu alloys has also been studied. CuAu alloys have been shown to have a surface Au enrichment that actually forms a protective Au layer on the surface. The anodic polarization curve for CuAu alloys is characterized by a critical potential, E, above which extensive Cu dissolution is observed [10]. Below E, a smaller dissolution current arises that is approximately potential-independent. This critical potential depends not only on the alloy composition, but also on the solution composition. STM was used to investigate the mechanism by which copper is selectively dissoluted from a CuAu electrode in solution [11], both above and below the critical potential. At potentials below E, it was found that, as copper dissolutes, vacancies agglomerate on the surface to form voids one atom deep. These voids grow two-dimensionally with increasing Cu dissolution while the second atomic layer remains undisturbed. The fact that the second atomic layer is unchanged suggests that Au atoms from the first layer are filling... 

S. E. Nam, K.H. Lee, Preparation and characterization of palladium alloy composite membranes with a diffusion barrier for hydrogen separation, Ind. Eng. Chem. Res. 2005, 44, 100-105. 

In a Curie-point pyrolyzer, an oscillating current is induced into the pyrolysis filament by means of a high-frequency coil. It is essential that this induction coil be powerful enough to permit heating the wire to its specific Curie-point temperature quickly. In such systems, the filament temperature is said to be self-limiting, since the final or pyrolysis temperature is selected by the composition of the wire itself, and not by some selection made in the electronics of the instrument. Properly powered, a Curie-point system can heat a filament to pyrolysis temperature in milliseconds. Providing that wires of the same alloy composition are used each time, the final temperature is well characterized and reproducible. 

H. B. Zhao, G. X. Xiong, N. Stroh and H. Brunner, Preparation and characterization of Pd-Ag alloy composite membrane with magnetron sputtering, Sci. China, Ser. B Chem., 1999, 42, 581-588. 

The goal of the addition of a secondary metal is to enhance activity and/or stability. Adatoms are adsorbed onto preformed catalyst surfaces. While for both alloys and intermetallics, the composition of a base metal (typically R or Pd) is altered by the addition of a secondary metal as part of the preparation procedure. The key difference between the two is that alloys are characterized by a random mixture of at least two metallic solid solution phases, while intermetalUcs are defined as ordered solid solution phases with fixed stoichiometry and identical atomic unit cells. The resulting intermetallics have uniform geometries, resulting in control of the electronic environment [16]. 

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