Glossary of Terms for Phase Equilibria Diagrams
- Components (of a System)
The smallest number of independently variable chemical constituents necessary and sufficient to
express the composition of each phase present in any state of equilibrium.
Zero and negative quantities of the components are permissible in expressing the composition of a phase.
Reference: Levin, E.M., McMurdie, H.F., and Hall, F.P., Phase Diagrams for Ceramists: Volume 1,
The American Ceramic Society, Columbus, Ohio, p. 5, 1956.
- Composition Tetrahedron
In the phase diagram of a condensed quaternary system, the four triangular planes connecting the compositions
of four solid phases which can coexist in equilibrium with liquid.
The composition of the liquid is represented by a quaternary invariant point, which may lie within the
composition tetrahedron (eutectic point) or outside the tetrahedron (peritectic
or reaction point).
Reference: Levin, E.M., McMurdie, H.F., and Hall, F.P., Phase Diagrams
for Ceramists: Volume 1, The American Ceramic Society, Columbus, Ohio, p. 5, 1956.
Examples of Composition Tetrahedrons
- Composition (or Compatibility) Triangle
In the phase diagram of a condensed ternary system the three joins connecting the composition points
of the three primary phases whose liquidus surfaces meet at a point.
If in the diagram of a ternary system, all of the Alkemade lines be constructed, the ternary
diagram will be divided into a number of composition triangles. If the three substances designating
the vertices of any of these triangles are not miscible in the solid state, they represent the final
equilibrium products of crystallization at the solidus temperature for compositions within
the triangle. When crystalline solid solutions exist between any of the three substances, the final
products of crystallization may be reduced in number by one or two.
Reference: Levin, E.M., McMurdie, H.F., and Hall, F.P., Phase Diagrams
for Ceramists: Volume 1, The American Ceramic Society, Columbus, Ohio, p. 5, 1956.
Examples of Composition Triangles
- Condensed System
One in which the vapor pressures of the solid and liquid phases present are negligible or
small in comparison to the atmospheric pressure. For such systems, e.g., the refractory
oxide ones, as the pressure may be considered constant, one degree of freedom is lost,
and the phase rule may be modified accordingly: The sum of the number of phases plus the
number of degrees of freedom equals the sum of the number of components plus one (instead of two).
Reference: Levin, E.M., McMurdie, H.F., and Hall, F.P., Phase Diagrams
for Ceramists: Volume 1, The American Ceramic Society, Columbus, Ohio, p. 5, 1956.
Example of Condensed System
- Congruent Melting Point
At a specified pressure, the temperature at which a solid substance changes to a liquid of
identical chemical composition.
Reference: Levin, E.M., McMurdie, H.F., and Hall, F.P., Phase Diagrams
for Ceramists: Volume 1, The American Ceramic Society, Columbus, Ohio, p. 5, 1956.
Example of Congruent Melting Point
- Conjugate Phase
One of two phases in equilibrium with each other defining a conode.
Reference: Levin, E.M., McMurdie, H.F., and Hall, F.P., Phase Diagrams
for Ceramists: Volume 1, The American Ceramic Society, Columbus, Ohio, p. 5, 1956.
Example of Conjugate Phase
- Conode (or Tie Line)
For a particular temperature, the straight line connecting the composition of
two equilibrium with each other.
Reference: Levin, E.M., McMurdie, H.F., and Hall, F.P., Phase Diagrams
for Ceramists: Volume 1, The American Ceramic Society, Columbus, Ohio, p. 5, 1956.
Examples of Conodes (Tie Lines)
- Critical Pressure
In a one component system, the unique pressure at which
the liquid and vapor phases become identical.
Reference: Levin, E.M., McMurdie, H.F., and Hall, F.P., Phase Diagrams
for Ceramists: Volume 1, The American Ceramic Society, Columbus, Ohio, p. 5, 1956.
Example of Critical Pressure
- Critical Temperature
In a one component system, the unique temperature at
which the liquid and vapor phases become identical. At the critical
temperature the system passes from a heterogeneous state to a homogeneous
phase. Above the critical temperature no liquid phase can exist however great the pressure.
Reference: Levin, E.M., McMurdie, H.F., and Hall, F.P., Phase Diagrams
for Ceramists: Volume 1, The American Ceramic Society, Columbus, Ohio, p. 5, 1956.
Example of Critical Temperature
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