High Pressure Phase Diagrams

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High Pressure Phase Diagrams

• Sam Schoenberg
• Cer. 251
• 11/20/00

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High Pressure Diagrams

• Normally phase diagrams are shown at standard
  pressure (1 atm)
• High pressures create many new polymorphic phase
  transitions
• High pressures can allow lower temperatures to be
  used in order to reach a certain phase
• There are very few high pressure multi-component
  diagrams

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Examples

• Bridgman Ice Diagram
• Ice has several polymorphs
• Many new triple points are shown

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• High pressure carbon system
• Important for synthetic diamond
• Ceramic industry uses about one third of all the
  industrial diamonds produced

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• High pressure silica system
• Additional polymorphs of silica

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Uses of high pressures in ceramics

• Uniaxial hot pressing
• Hot isostatic pressing
• Others

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Uniaxial hot pressing

• Involves the addition of pressure during
  sintering
• Reduces sintering time and temperature
• Can only densify simple geometric shapes

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Hot isostatic pressing

• Involves densifying a green body with a high gas
  pressure.
• Argon is usually used, as well as oxygen and
  nitrogen
• The green body must have closed porosity
• Can reduce processing temperatures
• Produces materials with finer grain size and
  isotropic properties

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Others

• Pressure calcination
• Ceramic materials undergo decomposition during
  the densification process
• Reactive hot pressing
• Components react under pressure to form a high
  density ceramic body
• Hot forging
• Pressure is applied to a partially densified
  compact in order to eliminate pores

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How high pressure phase diagrams help these
processes

• Its important to know how a ceramic is going to
  react to high pressures
• It may go through an unwanted polymorphic phase
  transition if not careful
• In some rare cases high pressures can cause
  unwanted melting
• More cost effective if the temperature can be
  minimized

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Conclusions

• High pressure phase diagrams are important to
  ceramics
• They aid in our full understanding of materials
  and their properties