Why in differential scanning calorimetry thermograms the heat capacity(cp)in crese and after that it decrease. I attach the overall photo. I want to know it generally not for a particular case there might ne a reason for it
Hello, i think the answer is that the heat capacity can increase as a material starts to degrade or decompose, and then decrease as the material fully degrades or decomposes. Also, it maybe due to some phase transitions or reactions that are locallized at certain temperatures, so when the temperature exceeds the phase transition or the reaction's temperature the heat capacity decreases.
Hi Negin Hadisadegh
How did you measure the mentioned Cp ?
Did you used some DSC instruments that tell you directly Cp or you have calculated?
in case of using calculation, did you used ASTM e1269 method or not?
please explain I can help.
If the specific heat decreases with increasing temperature, and there are no endothermic processes running in the material, it is often related to the lack of repeatability of the DSC baseline used in the measurement. Using ASTM method, the DSC baseline signal should be repeated until the required repeatability is achieved. It's painful but often allows you to solve this problem.
In polymers it may be due to enthalpy relaxation. If the glassy polymer is stored below the Tg but not too far from it, there is a densification of of the polymer structure causing an "overshoot" in the Cp curve - during the first heating run. If you repeat the same measurement with freshly cooled glass, it will disappear. See e.g. Article Long-Term Physical Aging Tracked by Advanced Thermal Analysi...
Theoretically, when there is a change in the state/phase change (the peak that you observed in the thermogram), there will be changes in heat capacity.
Simply put and make it easy to understand, the heat capacity of water is 4.187 kJ/kgK, for ice 2.108 kJ/kgK, and for water vapor (steam) 1.996 kJ/kgK. heat capacity is defined as the heat needed to increase 1C For the material.
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