In case of amorphous systems( telluride glass) the exothermic curve has occurred after the Tg, as we call it as crystallization peak. exactly what does it mean?
If You notice an exothermic peak after an endothermic glass transition, this usually comes from crystallization.
Glassy state has a higher disorder (higher entropy - say, S1), therefore, when glassy matter takes order in the form of crystals (long range order, e.g. nanometers). Crystals lower the entropy of the system (glass + crystals) - S2
S1 > S2
The change in internal energy can be expressed in the form fundamental thermodynamic equation:
dU = T*dS - p*dV (T - temperature, dS - change of entropy, p - pressure, dV - change of volume)
We can neglect the p*dV term as the volume difference is usually small enough to permit it.
dU = T*dS (and T*dS = dH - the change in enthalpy)
dS < 0 --> T*dS < 0 --> dH < 0
Negative change in the enthalpy provides information on transition of the system to a lower energetic state. This reaction is exothermic - the excess energy is removed from the system in the form of heat.
The crystallization enthalpy is the area of the peak (with respect to the base). If the end of the peak is on the same level as the beginning You can use a linear base.
Sometimes, the level of the DSC signal will be different for the beginning and for the end of the peak (there will be a significant change in Cp before and after the crystallization). In that case a sigmoid base would be more appropriate.
I have the same question,
I have a crystalline organic compounds, which has an endothermic peak then an exothermic peak. First peak is usually the melting point ( Solid to Liquid) which it is in the range ( 60-130 C). But what about the second peak, the exothermic peak?????
Manal Alwahsh If the measurement is conducted in air, the exothermic peak usually corresponds to combustion. Exothermic decomposition is usually a good first guess, but one should consult a different method to make sure, e.g. perform a TGA measurement in the same temperature range or (that would be way better) a FT-IR spectroscopy of gas products during a DSC run.
i have found two Tg in my sample. what should i do? could there be possible to have three Tg? suggest me
elaborate crystallization temp(Tc) and peak crystallization temp (Tp).
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