University at Buffalo
Department of Mechanical and Aerospace Engineering
Prof. D.D.L. Chung
Background on phase transition
A phase transition
(also called a phase transformation
) is a change in phase that
occurs, usually upon a change in temperature. For example, the melting of ice is a phase
transition, as solid ice is transformed to liquid water and this occurs upon heating past the
melting temperature (0ºC at 1 atm).
A phase transition is usually reversible.
example, the melting of ice is reversible, as freezing of water occurs upon cooling below
the freezing temperature.
The melting of ice is an example of a solid-liquid phase
The boiling of water is an example of a liquid-vapor phase transition
materials, there is a solid-solid phase transition
, in which a solid phase is transformed into
another solid phase.
An example of a solid-solid phase transition is the transformation of
BCC iron to FCC iron upon heating past 910ºC (at 1 atm).
In general, accompanying the change in crystal structure are changes in density,
specific heat (amount of heat needed to raise the temperature by 1ºC), thermal
conductivity, thermal expansion coefficient, elastic modulus (stiffness), vibration
damping capacity, optical reflectivity, electrical resistivity, etc.
These changes may be
advantageously used to provide behavior that is activated by the temperature reaching the
phase transition temperature.
For example, a change in optical reflectivity at a phase
transition may be used for an optical component (e.g., a lens) that is temperature
sensitive, a change in electrical resistivity at a phase transition may be used for an
electrical device (e.g., a fuse) that is temperature sensitive; a change in specific heat at a
phase transition may be used for a temperature-sensitive heat retention device; a change
in thermal conductivity at a phase transition may be used for a temperature-sensitive heat
conduction device; a change in thermal expansion coefficient at a phase transition may be
used for temperature-sensitive actuation (strain); changes in elastic modulus and vibration
damping capacity at a phase transition may be used for temperature-sensitive damping of
In addition to solid-solid phase transitions that involve a change in crystal
structure, there are those that involve a change in magnetic structure (i.e., arrangement of
the magnetic dipoles), such as the change of a ferromagnetic phase to a paramagnetic
phase upon heating past the magnetic Curie temperature.
The relative permeability
much higher for a ferromagnetic phase than a paramagnetic phase, so the magnetic
decreases abruptly upon heating past the Curie temperature.
Due to the
thermal agitation overshadowing the exchange interaction more and more as the
starts to decrease even below the Curie temperature, as shown in
At the Curie temperature, the exchange interaction vanishes totally.