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see also:
Desmearing of an alloy melting peak
For correct kinetic
analysis and for correct purity determination it is
necessary to correct the DSC/DTA measurement regarding
thermal resistance and regarding time constant of the
measuring system.
The Indium melting peak,
measured with DSC 204, shows the typical shape of a melting
peak of a pure metal:
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The endothermal
peak increases nearly linearly up to the
maximum.
This behavior is caused by the so-called thermal
resistance.
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After the maximum
the melting peak decreases nearly
exponentially.
A more precise analysis indicates that the decay is
the sum of more then one exponential
functions.
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According to all
standards, the temperature during DSC measurements is
measured on the reference side. Therefore, the given
temperature is not the temperature inside the
sample.
In order to receive the true sample temperature the
measurement is to correct regarding to thermal
resistance.
After correction regarding
thermal resistance, the left front of melting peak increases
nearly perpendicularly, indicating that the sample
temperature is constant.
After correction regarding
time constant the DSC curve is decreasing approximately
perpendicularly to the baseline without
overshooting.
After the complete
correction, the shape of the DSC melting peak is nearly a
delta-pulse, an indication that the sample temperature now
is constant during the melting process and that the DSC peak
ends very sharply. This special shape is only given in
temperature scaling.
In time scaling it is
visible that after both corrections the sample temperature
is constant during the melting process. Now, the corrected
DSC curve decreases perpendicularly to the baseline and
partial area exhibits a clear end point of melting
process.
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