Onset temperatures for solid to solid phase transition and melting are very close to each others for one polymorph. Enthalpy of phase transitions (both solid-solid and solid-liquid) decreases with increasing the heating rate in DSC. How can I explain??
Usually the heating rate does not effect the onset temperature in particular. The heating rate could change he shape of the heat flow and the corresponding temperature of other parameters. However, as to the onset temperature it does not vary much with the change of heating rate. you may look at this paper for more details about the effect of heating rate on the DSC analysis
http://www.ijert.org/view-pdf/14630/uncertainty-of-thermal-characterization-of-phase-change-material-by-differential-scanning-calorimetry-analysis
Thank you very much Rami Saeed for your valuable suggestions and congratulation for the publication.
Actually for my case solid to solid phase transition is very close to melting and for higher heating rate(5-10 degree/min) delta H is not comparable with that for lower heating rate(1-2 degree/min) although the onset temperatures are same for all the cases. What will be the possible reason for this?
You can find the answers for all of that in the paper. However, A possible explanation for the slight change in melting enthalpy at higher heating rate is that at
higher heating rates the transition occurs over a wider range of temperature due
to the internal thermal gradient in the DSC crucible "Sample Pan". In another words, temperature gradient is developed through the sample. Therefore liquid PCM may exist at higher temperatures than the melting temperature, resulting in some sensible heat being included in the latent heat measurement.
Moreover, the instrument sensitivity and resolution is important. this means that for lower heating rate, the instrument’s resolution is improved (transitions and thermal events become very close to each. but this increase in resolution is at the cost of its sensitivity.
all of this is mentioned in the paper i mentioned in the first comment and has been discussed for all of these conditions.
There may be some kinetic influence too: If the transition rates are different and/or comparable with the heating rates, the transition of the lower phase transition is not completed when the second transition starts. This effect may cause the effect, that the measured heat of transition changes with heating rate.
In DSC at lower heating rate the sample (organic material is poor in thermal conductivity ) temperature is very close to that shown by instrument as heat is well transferred. When the rate increases, the sample can not take up heat so fast and get close to the instrument temperature. the change occurs later. By the time temperature shown is higher.This amounts to sample showing phase transition at higher temperature.
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