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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- Fall '09
- Diffraction, Sulfur, Intercalation, lattice parameters