16 t he principles of d sc a re derived from the

Info iconThis preview shows page 1. Sign up to view the full content.

View Full Document Right Arrow Icon
This is the end of the preview. Sign up to access the rest of the document.

Unformatted text preview: s the temperature of the sample. - i s the thermal resistance between the Page - 35a- F igure 17 S implified model of t he DSC sample chamber. R - t hermal resistance Ts - s ample t e m ~ e r a t u r e Tp - t hermal en;rgy s ource/sink C s - specific heat of sample d h/dt - e nergy g enerated/absorbed b y t he sample d q/dt - r ate of energy flow t o/from t he sample P age -36s ample/source a nd the heat s ource/sink, a nd between the r eference/source a nd the heat s ourcelsink. ( A l l e nergy exchanges occur through the thermal resistance R ) . E quations 4-1) and 4 - 2 ) written for the sample (subscript s ) and the reference (subscript r) a r e : d h/dt = C S*dTS/dt-dqS/dt 0 = C r * dTr/dt-dqr/dt s ample r eference sample reference Combining equations 4-(3a) and 4-(3b) for the sample and the reference: Combining equations 4-4a and 4-4b for the sample and reference: P age - 37- a nd differentiating equation 4 - 6 y ields Substitution of equation 4 - 7 A i nto equation 4 - 5 g ives r earrangement of equation 4 - 8 y ields It is assumed that the reference material generates no energy (undergoes no phase transistions) and thus dh /dt r = 0. T hus for a constant heating rate ( d T / dt) t he rate of energy P flow to or from the reference is constant ( dqr/dt = 2 c onstant) and therefore d q / dt r 2 = 0 a nd d T / dt P = d Tr/dt. R e-writing equation 4-9 w ith the latter restriction: . P a g e - 38- ( CS-C ) r s pecific heat difference between the - sample and reference material. d T / dt P r ate of change of the temperature of - the heat s ource/sink. i s a thermal time constant which is - a m easure of the thermal response of the s a m p l e . d qg/dt - r ate of change of heat flow between the sample and the surroundings through the thermal resistance R . d h/dt - r ate that energy is generated by the sample while t h e s a m p l e is undergoing a transition. 4 -4 T ypical D S C C urves A t ypical D S C curve is shown in Fig. 1 8 . P ositive excursions from the bas...
View Full Document

This note was uploaded on 10/08/2009 for the course CME MAT E 630 taught by Professor Dr. during the Fall '09 term at University of Alberta.

Ask a homework question - tutors are online