Alloys surface energy

Metals A and B form an alloy or solid solution. To take a hypothetical case, suppose that the structure is simple cubic, so that each interior atom has six nearest neighbors and each surface atom has five. A particular alloy has a bulk mole fraction XA = 0.50, the side of the unit cell is 4.0 A, and the energies of vaporization Ea and Eb are 30 and 35 kcal/mol for the respective pure metals. The A—A bond energy is aa and the B—B bond energy is bb assume that ab = j( aa + bb)- Calculate the surface energy as a function of surface composition. What should the surface composition be at 0 K In what direction should it change on heaf)pg, and why ... 

In any brazing/soldering process, a molten alloy comes in contact with a surface of solid, which may be an alloy, a ceramic, or a composite material (see Ceramics Composite materials). For a molten alloy to advance over the soHd surface a special relationship has to exist between surface energies of the hquid—gas, soHd—gas, and Hquid—soHd interfaces. The same relationships should, in principle, hold in joining processes where a molten alloy has to fill the gaps existing between surfaces of the parts to be joined. In general, the molten alloy should have a lower surface tension than that of the base material. 

TABLE 9.2 Perdew-Wang (PW91) Binding Energies of O and O2 on the Pt and Pt3Co Alloy Surfaces. Reprinted with Permission from Xu et al. [2004]... 

Recent studies suggest that small NPs are far more fluid than initially thought. Lower melting points [9] and the dominance of surface energies [10] in small NPs suggest that these systems may be quite dynamic, which can be used to make new nanomaterials. By exploiting the differences in adsorbate bond enthalpies to different metals in small bimetallic NPs [8, 11], randomly mixed alloys can be... 

Recently Franken and Ponec (44) have published photoelectric work functions of Ni-Al alloy films. Since the phase diagram contains many inter-metallic compounds, the surface composition will now be determined by that compound which yields the lower surface energy, in this case the compound having the largest amount of aluminum. Indeed after an initial steep decrease in work function over a limited concentration range, a plateau of constant work function is found. 

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