Heat of segregation

Heats of Segregation of Carbon Adatoms on Ni(lOO) with Different Sulfur Coverages... 

Most calculations of defects at interfaces have been carried out for surfaces, where one of the fundamental issues is that of segregation. Here Mackrodt and Tasker (1989) have shown that it is important not to confuse the experimental heat of segregation (the Arrhenius slope of the coverage plot) with the calculated energy of segregation of an ion to the surface. This identification is true in the case of very low coverage when the Langmuir isotherm applies. 

However, it is possible to obtain Langmuir-like plots even at high coverages. In this case, the heat of segregation is given by... 

Complete treatment of the equilibrium surface composition must involve the minimization of the total free energy of the multicomponent regular solution [23]. To this end, the contributions of atom interactions and surface and solute strain energies must all be included in the calculation. To the first approximation, all these effects can be combined into a unified formalism by setting the heat of segregation AH gr equal to the exponent of the right-hand side of Eqs. 3.55 and 3.57 ... 

Table 13.1. Approximate ranges of contribution to the heat of segregation of solutes in metal oxides (kj/moi) ...

Fig. 12. Illustration of the correspondence between the surface 4f level shift, A g, and the surface heat of segregation of the final state impurity.

The heat of surface segregation is the difference between the heat of chemisorption of I/2N2 and the heat of segregation... 

Table 5 Heat of Sulfur Adsorption A/7 (Referred to A S2), Heat of Sulfide Formation AT/f (Referred to Hy M S ), Heat of Dissolution from the Gas Phase Vi S2), and Heats of Segregation...

N1 surface concentrations determined from ESCA are plotted as a function of bulk N1 content in Figures 1 and 2. In the case of homogeneous alloys the points should fall on the 45 diagonal line. It can be seen that In both (N1 SI ) and (N1 Th ) series the surfaces of the alloys are nickel-poor, Ss compared to tHe bulk. Similar observations have been made In the case of N1 A1 (11,12) and Co Th (13) alloys. Surface enrichment In Si or tS i2 to be expected be cause of the higher heats of formation of S10 and ThO, compared to NiO (-210, -292, and -58.4 kcal/mol, respectively). This would lead to a higher chemical affinity of SI and Th toward the ambient gas and consequently an Increased driving force of SI and Th for segregation. 

Table 5.1. Adsorption properties of metal monolayers on metal substrates. The clean substrate properties are also given for comparison. Substrates are ordered by lattice type (fee, bcc, hep, cubic, diamond and rhombic). The structures, nearest neighbor distances and heats of vaporization refer to the bulk material of the substrate or the adsorbate. VD, ID and S stand for vapor deposition, ion beam deposition and surface segregation, respectively. TD, WF and TED stand for thermal desorption, work function measurements and transmission electron diffraction, respectively...

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