Bulk alloy formation

In principle, the formation of a 2D Me-S surface alloy phase and/or a 3D Me-S bulk alloy phase in the underpotential range must be taken into account additionally if the solubility of Me in S is non-zero, which is the case in most Me UPD systems. However, these processes are usually kinetically strongly hindered at room temperature. Therefore, it is experimentally difficult to study 2D and 3D Me-S alloy formation processes. Usually, experiments at elevated temperature or with long-time polarization at room temperature are necessary to investigate these phenomena. Nevertheless, there is clear experimental evidence for 2D Me-S surface alloy and 3D Me-S bulk alloy formation in some Me UPD systems. 

The kinetics of 2D Me-S surface alloy and 3D Me-S bulk alloy formation also strongly depend on surface inhomogeneities of S such as atomic disorder, kink sites, monatomic steps, dislocations, grain boundaries, etc. (cf. Chapter 1). Therefore, the... 

In many systems, 3D Me-S bulk alloy formation is connected with considerable changes of the lattice type, unit cell, and/or interatomic distances producing significant internal mechanical stress. This is illustrated, for example, for the electrochemical formation of the p phase of the Li-Al alloy in the system A1 (polycrystalline)/molten Li, IC, cr at AEfs 260 mV [3.345]. As a result, acoustic emission is generated during the 3D Me-S bulk alloy formation as illustrated in Fig. 3.62 [3.345]. 

D Me-S surface alloy and/or 3D Me-S bulk alloy formation and dissolution (eq. (3.83)) is considered as either a heterogeneous chemical reaction (site exchange) or a mass transport process (solid state mutual diffusion of Me and S). In site exchange models, the usual rate equations for the kinetics of heterogeneous reactions of first order (with respect to the species Me in Meads and Me t-S>>) are applied. In solid state diffusion models, Pick s second law and defined boundary conditions must be solved using Laplace transformation. 

It is plausible that the formation of 2D Me-S surface alloy as well as the initial stage of 3D Me-S bulk alloy formation is better described by a site exchange model than by a diffusion model. More advanced stages of 3D Me-S bulk alloy formation are usually characterized by a parabolic rate law and well described by a diffusion model. 

The results strongly depend on the crystallographic orientation of the substrate and on the crystal imperfection density. The time-dependence of 3D Me-S bulk alloy formation obeys a parabolic rate law (Fig. 3.65) as found for many other systems. The results were discussed in terms of a semiinfinite-linear diffusion model assuming mutual diffusion of Me and S and reversible 2D Meads overlayer formation. The following time-dependence of q(,AE,i) was derived... 

Figure 3.64 Kinetics of 3D Me-S bulk alloy formation in the system Ag(lll)/5 x 10 M CdS04 + 5 X 10 M Na2S04 + 5 x 10 M H2SO4 at T = 298 K [3.102J. Cyclic voltammogram without extended polarization and anodic stripping curves depending on extended polarization at AE = 2 mV Polarization time tp/s = 0 (1) 60 (2) 300 (3) 600 (4) 1200 (5).

The kinetics of the initial stage of 3D Me-S bulk alloy formation process can be affected by nucleation and growth phenomena. A typical example is the formation of the fi phase of 3D Li-Al bulk alloy in the systems Al(polycrystalline)/molten Li, Cf and A1 (polycrystalline)/LP, 0104, propylene carbonate [3.345, 3.346]. In both systems, non-monotonous current transients were observed in the initial stage of alloy formation as shown in Fig. 3.66 [3.345]. 

Figure 3.66 Kinetics of 3D Me-S bulk alloy formation in the system A1 (poly)/molten LiCl-KCl at T = 698 K [3.345]. Current density transients for the formation of p phase of Li-Al alloy from potentiostatlc pulse experiments. Initial and final underpotentials hEJrsN = 305, A f/mV= 296 (1) 292 (2) 288 (3).

Formation of 2D Me surface alloy and 3D Me bulk alloy have to be taken into account besides 2D Meads overlayer formation for Me UPD systems with nonvanishing Me solubility in S. Influences of crystallographic orientation and crystal imperfection density of S on the rate of 2D and 3D Me-S alloy formation are observed. 2D Me-S surface alloy formation processes are pronounced on S(lll). The mechanisms of 2D Me surface alloy and 3D Me bulk alloy formation processes are still not well understood. More realistic models are necessary to describe these processes... 

Figure 3. Bulk glass formation range in Pd-Cu-P alloys. Filled symbols are the eompositions where amorphous rods with diameter of 7 mm ean be formed, open eireles represent the formation of erystalline phases.

Provided the mole fraction of A does not fall below N, then the oxide AO will be formed exclusively. The important criterion is the ratio of the oxidation parabolic rate constant to that of the diffusion coefficient of For A1 in Fe, the parabolic rate constant is very low, whilst the diffusion coefficient is relatively high, whereas the diffusion coefficient of Cr is much lower. Hence, the bulk alloy composition of A1 in iron required for the exclusive formation of AI2O3 at any given temperature is lower than the Cr concentration required for the exclusive formation of CrjOj. 

By using a thermodynamic model based on the formation of self-associates, as proposed by Singh and Sommer (1992), Akinlade and Awe (2006) studied the composition dependence of the bulk and surface properties of some liquid alloys (Tl-Ga at 700°C, Cd-Zn at 627°C). Positive deviations of the mixing properties from ideal solution behaviour were explained and the degree of phase separation was computed both for bulk alloys and for the surface. 

Further heating under hydrogen at 500 °C of the above sample results in an increase of the number of platinum atoms surrounding tin up to circa 5. This can be explained by a migration of the tin atom into the platinum particle with formation of a surface or in a bulk alloy (Scheme 2.41 and Figure 2.16). 

Formation of several successive layers of bulk intermetallic compounds has been shown. Also, Lee et al. [480] have detected, during Al UPD, the formation of two alloys on polycrystalline Au electrodes from acidic l-ethyl-3-methylimidazolium chloroaluminate that melt at room temperature. Moreover, in the Al UPD region, fast phase transition between these two intermetallic compounds has been evidenced. Later, the same group of researchers [481] has performed EQCM studies on Al deposition and alloy formation on Au(lll) in ambient temperature molten salts/benzene mixtures. 

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