P age 4 2 t he temperature calibration of the dsc

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Unformatted text preview: sured melting points of the mercury ( M . P . gallium ( M . P . = = 2 3 4 . 6 K ) and 3 0 2 . 8 K ) w ere within 0 .5 K o f the accepted values when heated at 1 0 K /min. D ifferent heating rates were used with the 3 . 5 0 m g indium sample and empty reference pans in order to determine the dependence of the recorded transition temperature on heating rate (Fig. 2 0 ) . K /min. w ere used. H eating rates of 1 , 2 , 5 , 1 0 , a nd 2 0 T h e average increase in the recorded transition temperature was 0 . 3 K f or each factor of 2 i ncrease in the heating rate for heating rates above 2 K /min. T h e recorded transition temperature was the same for heating rates of 1 a nd 2 K /min. C ooling rates of 1 , 2 , 5 , a nd 1 0 K /min. w ere obtainable with the 3 . 5 0 m g . indium s a m p l e . T h e average decrease in the measured transition temperature was less than 0 . 2 K f or each factor of 2 d ecrease in the cooling rate. The difference between the transition temperatures measured for heating and cooling decreased as the h eating/cooling r ate decreased as expected. Heating or cooling the sample at slower rates reduces the possibility of superheating or supercooling the sample before the sample undergoes any first order transitions. An effect of using low h eating/cooling r ates is that the recorded transition peak becomes lower and broader thus making it difficult to accurately determine the starting point of the transition and thus the transition temperature. Page -43a- F igure 2 0 dh/dt vs. T EMPEKAI'URL: for d ifferent S CAN R ATES TEMPERATURE (K) Page 4-7 ( a ) -44- C ELL CALIBRATION COEFFICIENT ( E ) T h e cell calibration c o.efficient ( E) is a conversion factor to convert the differential thermocouple signal from the D SC c ell to a differential power. The coefficient ( E ) i s determined from data taken o n a sapphire disk and compared t o the known values of the specific heat of sapphire at various temperatures. E is given by (66). E = c ell calibration coefficient C = h eat capacity of the sample at the appropriate temperature H = p rogrammed h eat...
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