The diagrams are now becoming really difficult! Figure 8.21 illustrates an Si–Al–N–O tetrahedron that clearly has difficulty in showing temperature effects. If we fix T and the total pressure (or any of the partial pressures), there is no remaining variable.
Two phase region ternary diagram plus#
If we precipitate out carbon, then we have two phases (gas plus carbon) and F = 2. We can fix T and the total pressure and still vary pCO, pCO 2, pO 2, or a ratio of any two. In the gas phase we have two components ( C = 2) but only one phase (the gas), so F = 3. The gray area in the C–O system is instructive-we cannot reach this in equilibrium because carbon would then form and the solid curve would represent equilibrium. The curves are for the situation in which the total pressure is fixed at 1 atm, so we do not need a vacuum system. In each case the temperature is plotted against a gas partial-pressure ratio with the curves showing the contours for constant pO 2. These diagrams can be very useful in the laboratory because this is how we try to reach the pressures indicated in Figure 8.20a: notice that neither system easily takes us below 10 −23 atm. If we look back at Figure 8.20a, the areas there represented conditions in which one or two phases are in equilibrium with a gas phase.įigure 8.20c and d shows the related diagrams for the H–O and C–O systems. Present and all the lines represent two-phase regions. Vacuum systems whenever possible because they greatlyĨ.20b, the areas show situations in which only one phase isįIGURE 8.20 (a, b) The FeO–Fe 2O 3 phase diagram (c) the H–O system (d) the C–O system. Stant, the oxidation state of the Fe ion decreases. Not try to change or control the total pressure-we avoid If we increase the temperature while keeping the pO 2 con. Pressing is an established commercial method for processingĨ.20a (because it does not show the composition of theĬondensed phase), it does emphasize one special feature: Geologists are interested in much higher pressures, and hotĭiagram does not show as much information as Figure In ceramics, we usually run experiments at 1 atm, butĮach combination of temperature and pO 2. This is the special feature for ceramics, Is solid and kinetics become the controlling factor. This case, it reaches T E, at which point the whole sample Lar slope is that they must connect the appropriate isobars Path follows the steepest descent on the liquidus until, in Nents ( C = 2 Fe and O), so we have two degrees ofĬomponent systems as shown in Figure 8.19. The basic ideas are the same as for the two. In the wüstite phase (region W) there is one condensedĮnvision what occurs as we lower the temperature of the So the oxygen isobars (lines ofĬonstant pO 2) on the phase diagram must be horizontal. = 2 Fe and O), so we have only one degree of freedom: Wüstite plus magnetite) there are two condensed phases In the two condensed phases region (region W + M: Because of this interest, the diagram has Phase field contains a single phase (wüstite or magnetite).įormation. PO 2 are horizontal whereas in two-phase fields they areįIGURE 8.19 Illustration of a cooling path in a ternary system. In Figure 8.20 is the fact that usually the lines of constant We can call themįe and O or FeO and Fe 2O 3 as we wish. Shown in Figure 8.20 here gas is important. We will spend some time discussing the Fe–O diagram Then essentially the Ni/NiO equilibrium sets the pO 2. Have just two variables (since we have two phases, X = 2), If there is a solid present (e.g., graphite or Ni) then we So weĬan vary T, P total, and the composition (CO 2/CO ratio or Namely, the gas, and X + F = C + 2 gives F = 3. If there is no solid present then we just have one phase,