MAE438-538 notes

MAE438-538 notes - University at Buffalo Department of...

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University at Buffalo Department of Mechanical and Aerospace Engineering MAE 438/538 Prof. D.D.L. Chung Smart Materials 1. 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. For 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 transition . The boiling of water is an example of a liquid-vapor phase transition . In some 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 a structure. 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 µ r is much higher for a ferromagnetic phase than a paramagnetic phase, so the magnetic induction B decreases abruptly upon heating past the Curie temperature. Due to the thermal agitation overshadowing the exchange interaction more and more as the temperature increases, B starts to decrease even below the Curie temperature, as shown in Fig. 1. At the Curie temperature, the exchange interaction vanishes totally. If the
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This note was uploaded on 07/09/2011 for the course MAE 438 taught by Professor Chung during the Spring '09 term at SUNY Buffalo.

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MAE438-538 notes - University at Buffalo Department of...

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