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Despite the change in the physical properties of a material through its glass transition, the transition is not considered a [[phase transition]]; rather it is a phenomenon extending over a range of temperature and defined by one of several conventions.<ref name="Debenedetti">{{cite journal|doi=10.1038/35065704|last=Debenedetti|first=P. G.|author2=Stillinger|title=Supercooled liquids and the glass transition|journal=Nature|year=2001|volume=410|issue=6825|pages=259–267|pmid=11258381|bibcode = 2001Natur.410..259D }}</ref><ref name="Angell">{{cite journal|doi=10.1063/1.1286035|last=Angell|first=C. A.|author2=Ngai, K. L. |author3=McKenna, G. B. |author4=McMillan, P. F. |author5= Martin, S. W. |title=Relaxation in glassforming liquids and amorphous solids|journal=Appl. Phys. Rev.|year=2000|volume=88|issue=6|pages=3113–3157|bibcode = 2000JAP....88.3113A |url=http://lib.dr.iastate.edu/mse_pubs/70}}</ref> Such conventions include a constant cooling rate ({{Convert|20|K/min|F/min}})<ref name=iso11357-2/> and a viscosity threshold of 10<sup>12</sup> [[Pa·s]], among others. Upon cooling or heating through this glass-transition range, the material also exhibits a smooth step in the [[thermal expansion|thermal-expansion coefficient]] and in the [[specific heat]], with the ___location of these effects again being dependent on the history of the material.<ref name=z1/> The question of whether some phase transition underlies the glass transition is a matter of continuing research.<ref name="Debenedetti"/><ref name="Angell"/><ref>{{cite journal|doi=10.1134/1.1790021|title=Glass formation in amorphous SiO<sub>2</sub> as a percolation phase transition in a system of network defects|year=2004|last1=Ojovan|first1=M. I.|journal=Journal of Experimental and Theoretical Physics Letters|volume=79|issue=12|pages=632–634|bibcode = 2004JETPL..79..632O }}</ref> In a more recent model of glass transition, the glass transition temperature corresponds to the temperature at which the largest openings between the vibrating elements in the liquid matrix become smaller than the smallest cross-sections of the elements or parts of them when the temperature is decreasing.
As a result of the fluctuating input of thermal energy into the liquid matrix, the harmonics of the oscillations are constantly disturbed and temporary cavities ("free volume") are created between the elements, the number and size of which depend on the temperature. The glass transition temperature T<sub>g0</sub> defined in this way is a fixed material constant of the disordered (non-crystalline) state that is dependent only on the pressure. As a result of the increasing inertia of the molecular matrix when approaching
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