Chemical stability domains

Armed with these important relationships, it is now possible to tackle the next important topic, namely, the delineation of the chemical stability domains of ceramic compounds. 

The chemical stability domain of a compound represents the range of activity or gaseous partial pressure over which that compound is stable. For example, experience has shown that under sufficiently reducing conditions, all oxides are unstable and are reducible to their parent metal(s). Conversely, all metals, with the notable exception of the noble ones, are unstable in air — their oxides are more stable. From a practical point of view, it is important to be able to predict the stability or lack thereof of a ceramic in a given environment. A related question, whose answer is critical for the successful reduction of ores, is this at what oxygen partial pressure will an oxide no longer be stable  

To illustrate, it is instructive to consider an oxide MO, for which a higher oxide MO also exists (that is, y z) and to calculate its stability domain. The equilibrium partial pressure of the oxide that is in equilibrium with the parent metal is determined by applying Eq. (5.29) to the following reaction  

As noted above, the standard state of a gas is chosen to be the state of 1 mol of pure gas at 1 atm (0.1 MPa) pressure and the temperature of interest. One should thus realize that whenever a partial pressure P, appears in an expression such as Eq. (5.28), it is implicit that one is dealing with the dimensionless ratio, Pj/ atm. 

Calculate the chemical stability domains for the phases in the Fe-O system at 1000 K, given the following standard free energies of formation  

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Based on the foregoing analysis, stoichiometry (defined as the point at which the numbers of anions and cations equal a simple ratio based on the chemistry of the crystal) is a singular point that occurs at a very specific oxygen partial pressure. This immediately begs the question if stoichiometry is a singular point in a partial pressure domain, then why are some oxides labeled stoichiometric and others nonstoichiometric To answer the question, examine Table 6.1 in which a range of stoichiometries and chemical stability domains for a number of oxides are listed. The deviation from stoichiometry, defined by Ax, where Ax is the difference between the... 

Using the relevant thermodynamic data, calculate the chemical stability domain (in terms of oxygen partial pressure) of FeO and NiO. Plot to scale a figure such as Fig. 6.7 for each compound, using the data given in Table 6.1. Which of these two oxides would you consider the more stoichiometric Why ... 

Solid materials are chemically stable when their activities are within certain ranges of values. For example, refractories possess activities such that they are stable in oxidizing conditions. The range of activity values in which a material is chemically stable is the chemical stability domain for that material. With the help of thermodynamics, we can calculate the stability domains. 

In this equation, AGMOy° is the standard free energy change for the formation of MOy. Thus, the chemical stability domain for MO at a temperature T is between the partial pressures of oxygen corresponding to AGi° and AG2°. 

Calculate chemical stability domains for these oxides. 

Phosphorus oxynitride, PON, is a useful starting product, as a phosphorus and nitrogen source, to prepare various nitridooxophos-phates, in particular phosphorus oxynitride glass compositions (211). Moreover, it shows as a material excellent chemical stability with potential applications in several domains. In microelectronics, for example, PON has been used to form by evaporation insulating films for the passivation of III-V InP substrates and the elaboration of MIS (metal-insulator-semiconductor) structures (190, 212-215). PON could have also valuable properties in flame retardancy (176,191,216). 

Concluding this section, we may mention a paper by Daams and Villars (1993) concerning an atomic environment classification of the chemical elements. Critically evaluated crystallographic data for all element modifications (and recommended atomic volumes) have been reported. Special structural stability diagrams were used to separate AET stability domains and to predict the structure (in terms of environment types) of hitherto unknown high-pressure and high-temperature modifications. Reference to the use of AET in thermodynamic (CALPHAD) modelling and calculation has been made by Ferro and Cacciamani (2002). 

The primary tool for representing the phase behavior of a chemical system is the phase diagram, a graphical roadmap of phase stability domains. For a pure substance, with... 

The interplay of the two phases on the PtaSnCl 11) surface has been object of an extensive study carried out by Ceelen et al. [18] who used mainly a combination of LEIS and SPA-LEED, also carrying the sample at higher temperatures than those attained in the previous studies. A wealth of temperature dependent phenomena was observed in this study concerning bulk-surface chemical equilibrium, domain size variation and phase transitions. The main conclusions that can be drawn from these combined structural and compositional studies is that the ( /3 x /3) R30° reconstruction is stabilized by the depletion of tin in the subsurface layers and that this depletion is caused by a the combination of sputtering and high temperature annealing (Fig. 3). 

Microemulsions form spontaneously and exhibit nano-disperse structures. In contrast to emulsions there is no additional energy input necessary for the production of a microemulsion. The formation is thermodynamically favoured due to the ultra-low interfacial tension between the oil and water domains. The microemulsified fuels are in principle thermodynamically stable for an unlimited period of time only the chemical stability of the single components could be a limiting factor. A further advantage of microemulsions in contrast to emulsions is the fact that the water content can be adjusted over a broad range. Therefore, the combustion process can be customised to specific needs. An important criterion for a microemulsion to be used as fuel is that the one-phase region extends over a wide temperature range (Fig. 11.4). Mixtures of ionic and non-ionic surfactants, which exhibit almost temperature-invariant phase behaviour by optimal composition, are suitable to meet these standards. 

Figure 6.7 Distinction between a stoichiometric and a nonstoichiometric MO/, o / oxide, where the functional dependence of the changes in stoichiometry on the oxygen partial pressure for two hypothetical compounds having the same range of chemical stability is compared. From the foregoing discussion, it follows that the oxide for which A.v varies widely over the stability domain will be labeled nonstoichiometric. and vice versa.

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