Thermochemical boundaries

On the other hand, the re-oxidation step required to convert the reduced form of the catalyst into the active species also sets some thermochemical boundaries, because only stoichiometric oxidation can be expected, if D([M] - O) is too low. With regard to the most conceivable terminal oxidants, O2, N2O, or H2O2, a bond strength in the order of D([M] -O) 250 kJ/mol... 

The relationship between heat transfer and the boundary layer species distribution should be emphasized. As vaporization occurs, chemical species are transported to the boundary layer and act to cool by transpiration. These gaseous products may undergo additional thermochemical reactions with the boundary-layer gas, further impacting heat transfer. Thus species concentrations are needed for accurate calculation of transport properties, as well as for calculations of convective heating and radiative transport. 

Since detailed chemical kinetic mechanisms involve the participation of a large number of species in a large number of elementary reactions, sensitivity and reaction path analyses are also essential elements of DCKM. Sensitivity analysis provides a means to assess the limits of confidence we must put on our model predictions in view of uncertainties that exist in reaction rate parameters and thermochemical and thermophysical data utilized, as well as the initial and boundary conditions used in the modeling work. Through... 

The FACTSage thermochemical database was used to identify the thermodynamically stable phases that could exist in a system comprised of a pure metal, oxygen, HC1 and Cl2 at 500°C (Suppiah, 2008). The predominant Fe, Ni, Cu and Cr phases in an 02/HCl/Cl2 environment were determined. The equilibrium reaction boundary was plotted as a function of the partial pressures of 02 and HC1, for a constant Cl2 partial pressure. The resulting predominance diagrams were plotted over an 02 and HC1 partial pressure range of 10-20 to 1 atm for Cl2 partial pressures between 10-6 and 1 atm. The predominant Ni and Cr species are solids, suggesting that a corrosion resistant protective layer could be formed on the metal. 

The experimental boundary of the field of synthesis of goethite and hematite was compared with the theoretical data. Constants of a-FeOOH obtained from thermochemical measurements = 14.43 0.15... 

Zirconia also undergoes high pressure phase transitions from the monoclinic to two orthorhombic structures. A thermochemical study of these phases, combined with phase equilibrium observations (Ohtaka et al. 1991) suggests that surface energy changes the apparent position of phase boundaries, as well as enhances the kinetics of transformation. However Ohtaka et al. (1991) were unable to quantify these effects. 

A more sophisticated approach for determination of the grain boundary segregation is similar to the determination of the surface tension of silicon melt. The novel approach of surface tension simulation has been successfully implemented in the thermochemical database. Hence, the assessment of the parameters for impurity segregation in solid silicon phase may greatly extend the application of the thermochemical database. The calculation results for C and O segregation are shown as dashed lines in Fig. 13.28. The McLean segregation isotherm can be reproduced using the approach similar to the surface tension simulation. 

Internally equilibrated subsystems, which act as free energy reservoirs, are already as random as possible given their boundary conditions, even if they are not in equilibrium with one another because of some bottleneck. Tlius, the only kinds of perturbation that can arise and be stabilized when they are coupled are those that make the joint system less constrained than the subsystems originally were. (This is Boltzmann s H-theorem [9] only a less constrained joint system has a liigher maximal entropy than die sum of entropies from the subsystems independently and can stably adopt a different form.) The flows that relax reservoir constraints are thermochemical relaxation processes toward the equilibrium state for tlte joint ensemble. The processes by wliich such equilibration takes place are by assumption not reachable within the equilibrium distribution of either subsystem. As the nature of the relaxation phenomenon often depends on aspects of the crosssystem coupling that are much more specific than the constraints that define either reservoir, they are often correspondingly more complex than the typical processes... 

Thermochemical properties of numerous different condensed phases were determined from the gas phase data as shown in the preceding section. In addition phase boundaries were also evaluated thereby improving our knowledge of phase diagrams. The liquidus line determined by Timberg and Toguri... 

Grain boundary engineering of nanostructured materials including thermochemical treatment based on concept of useful additives. 

Figure 3. Eh-pH relations between solids and predominant ionic species in the system Fe-H20-02. The position of the Fe2+-ferric oxyhydroxide boundary is shown for pJCs of 37.1 and 44. The position of the Fe3 -FeOH2+ boundary is based on Milburn (12). Other thermochemical data used to construct the figure are from Langmuir (8).

Calculations of the oxygen partial pressure P 0 for the boundary condition both by simply taking the values from the JANAF-tables [12] or by using a modern thermochemical program [13] yield similar results with an equilibrium P Oi) as low as wlO bar at 1000°C. The prediction from the equilibrium would thus be, that at conditions with P 02) > 10 bar we should find the formation of a silica layer on a bare Si surface. Experimental observation shows that this only takes place at F(02)-levels more then 20 orders of magnitude higher. 

This success story demonstrates that equilibrium calculations disregarding physical boundary conditions can be misleading, and that thermochemical calculation is capable of giving good results if those boundaries are taken into account. 

Other approaches to active corrosion prediction utilizing thermochemical calculations [8,17,18] require the experimental determination of effective parameters. They show both the importance of physical boundary conditions and the extremely low level of partial pressures at which active corrosion is potentially dangerous. 

---