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A phase transition is the transformation of a thermodynamic system from one phase or state of matter to another one by heat transfer. The term is most commonly used to describe transitions between solid, liquid and gaseous states of matter, and, in rare cases, plasma. A phase of a thermodynamic system and the states of matter have uniform physical properties. During a phase transition of a given medium certain properties of the medium change, often discontinuously, as a result of the change of some external condition, such as temperature, pressure, or others. For example, a liquid may become gas upon heating to the boiling point, resulting in an abrupt change in volume. The measurement of the external conditions at which the transformation occurs is termed the phase transition. Phase transitions are common in nature and used today in many technologies. ==Types of phase transition== Examples of phase transitions include: *The transitions between the solid, liquid, and gaseous phases of a single component, due to the effects of temperature and/or pressure: : * (see also vapor pressure and phase diagram) * A eutectic transformation, in which a two component single phase liquid is cooled and transforms into two solid phases. The same process, but beginning with a solid instead of a liquid is called a eutectoid transformation. * A peritectic transformation, in which a two component single phase solid is heated and transforms into a solid phase and a liquid phase. * A spinodal decomposition, in which a single phase is cooled and separates into two different compositions of that same phase. * Transition to a mesophase between solid and liquid, such as one of the "liquid crystal" phases. * The transition between the ferromagnetic and paramagnetic phases of magnetic materials at the Curie point. * The transition between differently ordered, commensurate or incommensurate, magnetic structures, such as in cerium antimonide. * The martensitic transformation which occurs as one of the many phase transformations in carbon steel and stands as a model for displacive phase transformations. * Changes in the crystallographic structure such as between ferrite and austenite of iron. * Order-disorder transitions such as in alpha-titanium aluminides. * The dependence of the adsorption geometry on coverage and temperature, such as for hydrogen on iron (110). * The emergence of superconductivity in certain metals and ceramics when cooled below a critical temperature. * The transition between different molecular structures (polymorphs, allotropes or polyamorphs), especially of solids, such as between an amorphous structure and a crystal structure, between two different crystal structures, or between two amorphous structures. * Quantum condensation of bosonic fluids (Bose–Einstein condensation). The superfluid transition in liquid helium is an example of this. * The breaking of symmetries in the laws of physics during the early history of the universe as its temperature cooled. * Isotope fractionation occurs during a phase transition, the ratio of light to heavy isotopes in the involved molecules changes. When water vapor condenses (an equilibrium fractionation), the heavier water isotopes (18O and 2H) become enriched in the liquid phase while the lighter isotopes (16O and 1H) tend toward the vapor phase. Phase transitions occur when the thermodynamic free energy of a system is non-analytic for some choice of thermodynamic variables (cf. phases). This condition generally stems from the interactions of a large number of particles in a system, and does not appear in systems that are too small. It is important to note that phase transitions can occur and are defined for non-thermodynamic systems, where temperature is not a parameter. Examples include: quantum phase transitions, dynamic phase transitions, and topological (structural) phase transitions. In these types of systems other parameters take the place of temperature. For instance, connection probability replaces temperature for percolating networks. At the phase transition point (for instance, boiling point) the two phases of a substance, liquid and vapor, have identical free energies and therefore are equally likely to exist. Below the boiling point, the liquid is the more stable state of the two, whereas above the gaseous form is preferred. It is sometimes possible to change the state of a system diabatically (as opposed to adiabatically) in such a way that it can be brought past a phase transition point without undergoing a phase transition. The resulting state is metastable, i.e., less stable than the phase to which the transition would have occurred, but not unstable either. This occurs in superheating, supercooling, and supersaturation, for example. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「phase transition」の詳細全文を読む スポンサード リンク
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