T hus for a constant heating rate d t dt t he rate of

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Unformatted text preview: eline indicate exothermic transitions in t h e case of DUPONT D S C scans and endothermic transitions in the case of P E R K I N - E L M E R D SC scans. D u r i n g the first section of the curve ( F i g . 1 8), t he s ample undergoes no Page -38a- F igure 18 T ypical DSC c urve during h e a t i n g . T ypical D S C C urve Page - 3 9 t ransitions ( dh/dt = 0 , T S-Tr = c o n s t a n t , d /dt(qs-qr) = c onstant) s o that the baseline displacement from z e r o is due solely to mismatches i n t h e specific heats of the sample and reference ( C - C ) m ultiplied by t h e heating r a t e d T / dt. Sr P D u r i n g a t ransition the height of the curve a s measured from t h e baseline is a measure of the rate of t h e t ransition k i n e t i c s ( t h e rate that the sample generates or gives off heat energy during the transition first order transitions. = d h/dt) i n t h e c a s e o f T h e slope of the transition part of t h e curve is a measure of a thermal time constant ( R*CS) f or the equipment used multiplied by the difference in the r a t e that differential energy i s being applied to the sample and 2 2 r eference d /dt (q - q ) i n the case of PERKIN-ELMER Sr equipment. Similarly in the case of Dupont equipment the slope of the transition part of the curve is a measure o f a thermal time constant ( R*C ) m ultiplied by the r a t e that the s t emperatures between the sample and reference a r e changing divided by the thermal resistance ( 1/R 4 -5 * d /dt(TS-Tr)). M odifications T h e D u p o n t S e r i e s 99 T hermal Analyzer was used to control a Dupont Differential Scanning Calorimeter (DSC) cell base with a modified standard D S C cell and modified liquid nitrogen cooling accessory. T h e specified o p e r a t i n g temperature r a n g e of t h e D S C cell i s between -180 C a nd 725 C w hen equipped with the optional liquid nitrogen cooling P age - 4 0 - - a ccessory supplied b y D upont. The liquid nitrogen cooling a ccessory w as f ound to b e i nadequate f or e xtended operation below ambient temperature d u e to t he r apid b o i l...
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