The shape of the enthalpy change vs temperature or

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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 energy. Cs 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 . dq /dt 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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