Fazni dijagram
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U fizikalnoj hemiji, mineralogiji i nauci o materijalima, fazni dijagram je vrsta grafik koji pokazuje jednačinu uvijeta između termodinamički različitih faza. U matematici i fizici, fazni dijagram ima drugu značenje kao sinonim za fazni prostor.
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[uredi] Dijagrami pritisak-temperatura
Najjednostavniji fazni dijagrami su dijagrami pritisak-temperatura jedne jednostavne substance, kao što je voda. Apscisa i ordinata odgovaraju pritisku i temperaturi. Fazni dijagram pokazuje, u prostoru pritisak-temperatura, linije ravnoteže ili fazne granice između tri faze: čvrste, tečne i gasovite.
Oznake na faznom dijagramu pokazuju gdje je slobodnu energiju nemoguće odrediti. Otvoreni prostori, gdje je slobodnu enrgiju moguće izračunati, odgovaraju fazama. Faze su odvojene linijama neodređene energije gdje se fazni prijelazi odvijaju, koje se zovu granice faza.
In the diagram, the phase boundary between liquid and gas does not continue indefinitely. Instead, it terminates at a point on the phase diagram called the critical point. This reflects the fact that, at extremely high temperatures and pressures, the liquid and gaseous phases become indistinguishable, in what is known as a supercritical fluid. In water, the critical point occurs at around 647 K (374 °C or 705 °F) and 22.064 MPa.
The existence of the liquid-gas critical point reveals a slight ambiguity in our above definitions. When going from the liquid to the gaseous phase, one usually crosses the phase boundary, but it is possible to choose a path that never crosses the boundary by going to the right of the critical point. Thus, the liquid and gaseous phases can blend continuously into each other. However, it is impossible for the solid-liquid phase boundary to end in a critical point in the same way as the liquid-gas boundary, because the solid and liquid phases have different symmetry.
An interesting thing to note is that the solid-liquid phase boundary in the phase diagram of most substances, such as the one shown above, has a positive slope. This is due to the solid phase having a higher density than the liquid, so that increasing the pressure increases the melting temperature. However, in the phase diagram for water the solid-liquid phase boundary has a negative slope. This reflects the fact that ice has a lower density than water, which is an unusual property for a material.
[uredi] Binarni fazni dijagrami
Ostale, mongo komplikovanije vrste faznih dijagrama, mogu se konstruisati, pogotovo kada je prisutno više od jedne čiste komponente. U ovom slučaju, koncentracija postaje veoma bitna varijabla. Fazni dijagrami, sa više od dvije dimenzije, mogu se konstruisati kako bi se pokazao uticaj više od dvije varijable na fazu substance. Fazni dijagrami mogu koristit i druge varijable kao nadopunu ili kao zamijenu za temperaturu, pritisak i sastav (npr. jačina eletričnog ili magnetnog polja), kao i substance koje uzimju više od tri stanja materije.
One type of phase diagram plots temperature against the relative concentrations of two substances in a binary mixture called a binary phase diagram. Such a mixture can be either a solid solution, eutectic or peritectic, among others. These two types of mixtures result in very different graphs. A textbook example of a eutectic phase diagram is that of the olivine (forsterite and fayalite) system.
In addition to the above mentioned types of phase diagrams, there are thousands of other possible combinations. Some of the major features of phase diagrams include congruent points, where a solid phase transforms directly into a liquid. There is also the peritectoid, a point where two solid phases combine into one solid phase during heating. The inverse of this, when one solid phase transforms into two solid phases during heating, is called the eutectoid.
A complex phase diagram of great technological importance is that of the iron-carbon system for less than 7% carbon (see steel).
The x-axis of such a diagram represents the concentration variable of the mixture. As the mixtures are typically far from dilute and their density as a function of temperature usually unknown the preferred concentration measure is mole fraction. A volume based measure like molarity would be unadvisable.
[uredi] Uobičajne komponente faznog dijgrama
Linije ravnoteže ili fazne granice odnose se na linije koje pokazuju gdje se odvijaju fazni prijelazi.
Trostruka tačka je, u faznom dijagramu pritisak-temperatura, jednistveno prosjecište linija ravnoteže između tri stanja materije, najčeće čvrste, tečne i gaovite faze.
Za fazne dijagrame sa temperaturom na ordinati, solidus linija je linija ispod koje je substanca stabilna u čvrstom stanju. Likvidus linija je linija iznad koje je substanca stabilna u tečnom stanju. Između solidus i likvidus linija može se nalaziti praznina; unutar praznine substanca se sastoji od čvrstih kristala i tečnosti.
[uredi] Fazni dijagram tečnih kristala
U fizici tečnih kristala, fazni dijagrami se koriste u slučaju miješanja nematogeničnih komponenti, kako bi se uspostavila razlika između izotropne i nematiče tečne faze.
[uredi] Također pogledajte
- Gibbsovo pravilo faze
- Hamiltonska mehanika
[uredi] Vanjski linkovi