Eutectic interface

As A and B diffuse across the eutectic interface into the P and a phases, respectively, the interface will move (just as we discussed previously in the context of the Kirkendal effect). This will occur even if the difftisivities of A and B in a and P are equal, because the local concentration gradients on the two sides of the interface will likely be different. (Remember, the flux depends on the diffusivity and the concentration gradient.)... 

Fig. 9. Comparison of calculated and observed eutectic interface shapes. Carbon tetrabromide-hexachloroethane eutectic growing as in Fig. 3>a (1000 x). (Phase contrast.)...

Fig. 10. Dendrites growing ahead of an eutectic interface in a carbon tetrabromide-rich alloy of carbon tetrabromide-hexachloroethane. (Phase contrast, 150 x.)...

Eutectoid structures are like eutectic structures, but much finer in scale. The original solid decomposes into two others, both with compositions which differ from the original, and in the form (usually) of fine, parallel plates. To allow this, atoms of B must diffuse away from the A-rich plates and A atoms must diffuse in the opposite direction, as shown in Fig. A1.40. Taking the eutectoid decomposition of iron as an example, carbon must diffuse to the carbon-rich FejC plates, and away from the (carbon-poor) a-plates, just ahead of the interface. The colony of plates then grows to the right, consuming the austenite (y). The eutectoid structure in iron has a special name it is called pearlite (because it has a pearly look). The micrograph (Fig. A1.41) shows pearlite. 

At temperatures above or near the eutectic temperature of the polymer phase, CSEi values are typically in the range of 0.1-2 pFcm-2 [5], However, for stiff CPEs or below this temperature, CSEI can be as low as 0.001 pFcm 2 (Fig. 16). When a CPE is cooled from 100 °C to 50 °C, the CSE1 falls by a factor of 2-3, and on reheating to 100 °C it returns to its previous value. This is an indication of void formation at the Li/CPE interface. As a result, the apparent energy of activation for ionic conduction in the SEI cannot be calculated from Arrhenius plots of 1// sei but rather from Arrhenius plots of 7SE)... 

Aircraft turbines in jet engines are usually fabricated from nickel-based alloys, and these are subject to combustion products containing compounds of sulphur, such as S02, and oxides of vanadium. Early studies of the corrosion of pure nickel by a 1 1 mixture of S02 and 02 showed that the rate of attack increased substantially between 922 K and 961 K. The nickel-sulphur phase diagram shows that a eutectic is formed at 910 K, and hence a liquid phase could play a significant role in the process. Microscopic observation of corroded samples showed islands of a separate phase in the nickel oxide formed by oxidation, which were concentrated near the nickel/oxide interface. The islands were shown by electron microprobe analysis to contain between 30 and 40 atom per cent of sulphur, hence suggesting the composition Ni3S2 when the composition of the corroding gas was varied between S02 02 equal to 12 1 to 1 9. The rate of corrosion decreased at temperatures above 922 K. 

Both solid-solid and solid-gas types of reactions lead from solid reactants to a solid product without the use of solvents. Solvent-less processes, however, are not necessarily solid-state processes. Indeed, it has been argued [8d,e] that many solid-state syntheses cannot be regarded as bona fide solid-solid reactions because they occur with the intermediary of a liquid phase, such as a eutectic phase or a melt, or may require destruction of the crystals prior to reaction. This latter situation is often observed, for instance, in the case of reactions activated by co-grinding, since the heat generated in the course of the mechanochemical process can induce local melting at the interface between the different crystals, or when kneading, i.e. grinding in the presence of small amounts of solvent, takes place (vide infra). 

Figure 4-34 is a phase diagram for the system titanite-anorthite. Suppose a crystal of titanite is initially in contact with a crystal of anorthite. The two are heated to 1350°C. Either phase by itself would not melt. But because the temperature is higher than the eutectic point of the two phases, at the interface there is melting. As melting proceeds, a thin melt layer would form between the two crystals. The melting of the two phases continues and the rate may be controlled by different factors. The rate would depend on the controls, as outlined below. 

Uozumi, lizuka, and coauthors have published several studies on the electrochemical behavior of Pu at liquid cadmium cathodes in LiCl—KCl eutectic melts [128-130]. In one account [130] the authors studied the reduction of Pu " " to Pu° at the LiCl— KCl melt and liquid Cd interface and compared the results to those obtained at a solid Mo cathode surface. The electrode reaction at liquid Cd was found to be close to fully reversible with rapid. 

Figure I. Transverse section of sample from flange of joint between halves of the Horse Rhyton (private collection). Illustrates presence of silver-copper eutectic at interfaces between grains of silver-rich solid solution (X225).

Now that the top-down internal state variable theory was established, the bottom-up simulations and experiments were required. At the atomic scale (nanometers), simulations were performed using Modified Embedded Atom Method, (MEAM) Baskes [176], potentials based upon interfacial atomistics of Baskes et al. [177] to determine the conditions when silicon fracture would occur versus silicon-interface debonding [156]. Atomistic simulations showed that a material with a pristine interface would incur interface debonding before silicon fracture. However, if a sufficient number of defects were present within the silicon, it would fracture before the interface would debond. Microstructural analysis of larger scale interrupted strain tests under tension revealed that both silicon fracture and debonding of the silicon-aluminum interface in the eutectic region would occur [290, 291]. 

Table 3.3. Standard enthalphies (heats) of formation of nickel aluminides and their effective heats of formation calculated for the effective concentration at the interface corresponding to the composition (3.5 at.% Ni, 96.5 at.% Al) of the eutectic with the lowest melting point in the Ni-Al binary system.261 For all the intermetallic compounds, the limiting element is nickel...

A significant number of studies have characterized the physical properties of eutectic-based ionic liquids but these have tended to focus on bulk properties such as viscosity, conductivity, density and phase behavior. These are all covered in Chapter 2.3. Some data are now emerging on speciation but little information is available on local properties such as double layer structure or adsorption. Deposition mechanisms are also relatively rare as are studies on diffusion. Hence the differences between metal deposition in aqueous and ionic liquids are difficult to analyse because of our lack of understanding about processes occurring close to the electrode/liquid interface. 

MLiquid phase formed at the sodium disilicate-quartz interface above the eutectic melting temperature and worked its way outward. Perforation refers to when the outer sodium disilicate shells were no longer continuous. 

Figure 7.8. Solidified drop of eutectic Ag-Si alloy on a-SiC after heating at 1200°C in a purified He atmosphere. In this non-reactive system, wetting is good and the interface is strong after cooling...

Additions of 3.1 at.% Ti to an Ag-Cu alloy with a composition close to the eutectic allow nearly perfect wetting of SiC to be achieved at 900°C in a few minutes. In this case, the interfacial reaction leads to the formation of two layers a layer of TiC about 1 pm thick on the SiC side and a layer a few microns thick on the metal side with a composition close to Ti2Si (Ljungberg 1992). The reaction mechanisms at a (Ag-Cu-Ti/monocrystalline a-SiC) interface were studied by in... 

When a drop of pure Ni, or Fe or Co, is placed on a graphite substrate, melting starts at the Ni/graphite interface at a temperature corresponding to the eutectic transformation. Dissolution of carbon in Ni can produce at least three effects ... 

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