Isothermal stability diagrams

When a metal reacts with a gas containing more than one oxidant, a number of different phases may form depending on both thermodynamic and kinetic considerations. Isothermal stability diagrams, usually constructed with the logarithmic values of the activities or partial pressures of the two non-metallic components as... 

Figure 2-3. Schematic isothermal stability diagram for a gas consisting of two compounds X2 and Y2 A is the metal, AX and AY are the respective reaction products.

These isothermal diagrams can be used to consider the phase stability areas for more than one metal in contact with a common atmosphere and thus to assess the condensed phases which can be stable under the prevailing conditions. Figure 7.75 shows a stability diagram having phase areas for Co-S-O solid lines) and for Cu-S-O system broken lines). From this diagram it can be seen clearly that at 950 K at certain gas mixtures, pure metals Co and... 

Figure 16. Calculated isothermal Li-Cd-Sn ternary phase stability diagram at ambient temperature [48],...

Conditions for the thermod3mamic stability of an acid salt, MHXg, are usefully systematized in the isothermal phase-diagram for the ternary system HgO—HA—MX. When there is no stable acid salt, the triangular diagram takes on the form shown in Fig. 1 (a). The point A represents the solubility of the acid in water, and the curve AB the effect on this solubility of adding the salt MX to the solution C represents the solubility of this salt in water and CB the effect of added acid on this solubility. 

Vasy-p isothermal phase diagram (Sastry et al. 1995) for 775 C, up to po = 60 MPa, is shown in fig. 11. Highly oxidized phases are stabilized imder such pressures of oxygen ... 

It was shown some time ago that one can also use a similar thermodynamic approach to explain and/or predict the composition dependence of the potential of electrodes in ternary systems [22-25], This followed from the development of the analysis methodology for the determination of the stability windows of electrolyte phases in ternary systems [26]. In these cases, one uses isothermal sections of ternary phase diagrams, the so-called Gibbs triangles, upon which to plot compositions. In ternary systems, the Gibbs Phase Rule tells us... 

The diagram on the left shows a series of isotherms and the P — V conditions that determine phase stabilities at each temperature. At the highest... 

The isothermal solubility curve of mixtures of potassium sulphate and sulphuric acid expresses the composition of the soln. at 25° in equilibrium with the solid phase or phases, when the mol. ratio of K2SO4 and SO3 per 1000 grms. of soln. are plotted as co-ordinates. The ranges of stability in the ternary system K20—S03—H20, are diagrammed in Fig. 51, where the conditions have been studied in the vicinity of the SOs-apex, as far as the formation of KHS207, hut not as far as the well-known potassium pyrosulphate. The meaning of the diagram... 

The previous two chapters have considered the stationary-state behaviour of reactions in continuous-flow well-stirred reactions. It was seen in chapters 2-5 that stationary states are not always stable. We now address the question of the local stability in a CSTR. For this we return to the isothermal model with cubic autocatalysis. Again we can take the model in two stages (i) systems with no catalyst decay, k2 = 0 and (ii) systems in which the catalyst is not indefinitely stable, so the concentrations of A and B are decoupled. In the former case, it was found from a qualitative analysis of the flow diagram in 6.2.5 that unique states are stable and that when there are multiple solutions they alternate between stable and unstable. In this chapter we become more quantitative and reveal conditions where the simplest exponential decay of perturbations is replaced by more complex time dependences. 

Fig. 19. Schematic T-Xi -X2 diagram of the Cu-Mo-S system. Temperature stability and solid solution widths of the X-phase ( CuMo2S3) in the Cu-Mo-S system are displayed. To simplify the perspective drawing a few isotherms are indicated with a projection onto the base, demonstrating the whole compositional X-phase solid solution...

Roos (1995) has used a combined sorption isotherm and state diagram to obtain critical water activity and water content values that result in depressing Tg to below ambient temperature (Figure 1-25). This type of plot can be used to evaluate the stability of low-moisture foods under different storage conditions. When the Tg is decreased to below ambient temperature, molecules are mobilized because of plasticization and reaction rates increase because of increased diffusion, which in turn may lead to deterioration. Roos and Himberg (1994) and Roos et al. (1996) have described how glass transition temperatures influence nonenzymatic browning in model systems. This deteriorative reaction... 

Fig. 76. Diagrams of mineral equilibria in silicate iron-formations in the absence of carbon dioxide (isothermal sections) / = actual boundaries of stability fields of minerals 2 = boundaries unrealistic under the given conditions S = isobars of fluid pressure (P, = jO + kbar) 4 = isobars of log...

Fig. 79. Diagrams of phase equilibria in the system Fe-Si-C-02 (isothermal sections in coordinates of log/co)- f = stable boundaries 7 = metastable boundaries 7 = line of graphite stability 4 = isobars of fluid pressure (F, = Pqq + Pqq, in kbar). Field of metastable states is hatched.

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