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Unformatted text preview: Two different a p p r o a c h e s . h a v e b een used to determine the difference in enthalpic changes between a sample and
reference material with respect to time or temperature. One approach (used b y t he Perkin-Elmer Company) was to apply
differential heating to the sample and reference in order to
maintain a constant temperature difference between the sample
and reference material. The difference in power supplied to each heater is related to the difference in enthalpic changes
between the sample and reference. The s e c o n d - a p p r o a c h (used by the Dupont Company) was to apply heat to the sample and
reference at a uniform rate while measuring the temperature
difference of identical c hrome1 w afers (of known mass m a nd
specific heat C - F ig. 16) s ituated close to the sample and
reference and thus indirectly measuring the difference in
heat flow ( dqr/dt - d qg/dt) b etween the sample and reference
where d q/dt = m *C*dT/dt f or the sample and reference respectively. 4 -3 T heory of Operation
The two approaches to D SC m ay be better understood through an understanding of a simplified theoretical
treatment of the basic principles of differential scanning
calorimetry. The D SC c ell is shown in Fig. 16. T he principles of D SC a re derived from the rearrangement of the . Page -34a- Figure 16 DSC sample c hamber, D SC S AMPLE C HAMBER P age - 3 5 f ollowing two equations, which apply to the simplified model
( F i g . 17) o f the DSC apparatus used in the experiments
described later. Conservation o f e n e r z y d h/dt = C S*dTS/dt - d qS/dt N e w t o n ' s law where:
d h/dt - i s the rate at which the sample generates
d T / dt
S - i s the specific heat of the sample.
- C StdTS/dt i s the rate of temperature increase of the sample.
- i s the rate that energy is being used to increase the temperature of the s a m p l e .
S - i s the rate of energy flow f rom/to t he
s ample through the thermal resistance R . - i s the temperature of the heat s ource/sink. - i...
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- Fall '09