Vapor species diagrams

At low temperature, diffusion of oxygen and metal species through a compact oxide film 

At moderate and high temperatures, a combination of oxide film formation and oxide volatility 

At moderate and high temperatures, the formation of volatile metal and oxide species at the metal-oxide interface and transport through the oxide lattice and mechanically formed cracks in the oxide layer 

At high temperature, the gaseous diffusion of oxygen through a barrier layer of volatilized oxides 

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Even simple two-component mixtures are not ideal, as suggested by the comment about Amix V for solutions. Molecules in a liquid interact with each other, and molecules interact differently with liquid molecules of another species. These interactions cause deviations from Raoult s law. If the individual vapor pressures are higher than expected, the solution shows a positive deviation from Raoulfs law. If the individual vapor pressures are lower than expected, then the solution shows a negative deviation from Raoult s law. The liquid-vapor phase diagrams for each case show some interesting behavior. 

Figure 7-26. Volatility diagrams for (a) Pure Si at 1800 K, showing all vapor species, (Kohl et al., 1977.) (b) SiC showing only the most abundant vapor species, (Heuer and Lou, 1990. Reprinted with permission of the American Ceramic Society.) (c) Si3N4 showing only the most abundant vapor species. The dashed lines represent the equimolar conditions for P(02) = A P (SiO). (Heuer and Lou, 1990. Reprinted with permission of the American Ceramic Society.)...

The formation of vapor species at the ox-ide/ceramic interface can be displayed with a predominance diagram and overlaid equilibrium vapor species. These are illustrated in Figs 7-27 a and 7-27b for the Si-C-0 and Si-N-0 systems, respectively. At temperatures above 1800 K, substantial pressures can be generated at the SiC/Si02 interface (Jacobson et al., 1992). This is shown in a plot of P(Total) vs. temperature in Fig. 7-28. Note that the pressure is quite significant for carbon-saturated SiC/Si02 and can be reduced with silicon-saturated SiC/Si02. 

This figure shows schematically the P-T-composition surfaces Which represi equilibrium states of saturated vapor and saturated liquid for a binary syst The under surface represents saturated-vapor states it is the PTy surface, upper surface represents saturated-liquid states it is the PTx surface, surfaces intersect along the lines UBHCt and KAClt which represent the v pressure-vs.-T curves for pure species 1 and 2. Moreover, the under and u surfaces form a continuous rounded surface across the top of the diagram betw C and C2 the critical points of pure species 1 and 2 the critical points of... 

As discussed in Sec. 3.5 with respect to cubic equations of state for pu species, a subcritical isotherm on a PV diagram exhibits a smooth transit) from the liquid to the vapor region, shown by the curve labeled T2 < Tc on F 3.10. We tacitly assumed in that discussion independent knowledge of the va pressure at this temperature. In fact, this value is implicit in the equation of sta We reproduce in Fig. 14.1 the subcritical isotherm of Fig. 3.10, without... 

The third plane identified in Fig. 12.1 is the vertical one perpendicular to the composition axis and indicated by MNQRSLM. When projected on a parallel plane, the lines from several such planes present a diagram such as that shown by Fig. 12.4. This is the PT diagram lines t/C, and KC2 are vapor-pressure curves for the pure species, identified by the same letters as in Fig. 12.1. Each interior loop represents the PT behavior of saturated liquid and of saturated vapor for a mixture of fixed composition the different loops are for different compositions. Clearly, the PT relation for saturated liquid is different from that for saturated vapor of the same composition. This is in contrast with the behavior of a pure species, for which the bubble line and the dew line coincide. At points A and B in Fig. 12.4 saturated-liquid and saturated-vapor lines intersect. At such points a saturated liquid of one composition and a saturated vapor of another composition have the same T and P, and the two phases are therefore in equilibrium. The tie lines connecting the coinciding points at A and at B are perpendicular to the PT plane, as illustrated by the tie line VX in Fig. 12.1. 

A PT diagram for the ethane/heptane system is shown in Fig. 12.6, and a yx diagram for several pressures for the same system appears in Fig. 12.7. According to convention, one plots as y and x the mole fractions of the more volatile species in the mixture. The maximum and minimum concentrations of the more volatile species obtainable by distillation at a given pressure are indicated by the points of intersection of the appropriate yx curve with the diagonal, for at these points the vapor and liquid have the same composition. They are in fact mixture critical points, unless y = x = 0 or y = x = 1. Point A in Fig. 12.7... 

There are substantial numbers of small molecular species that are important species at high temperatures that are not normally discussed in standard inorganic courses. For example, the vapor over solid NaCl contains diatomic NaCl molecules as well as NaCl dimers. Does the stoichiometry NaCl fit with the simple ideas of valence derived from molecules like SiCU Explain. Construct a qualitative MO diagram with orbital drawings for the NaCl diatomic molecule and compare it with that for a homonuclear, isoelectronic diatomic molecule. Does this help in answering the first question ... 

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