Civil Engineering Reference
In-Depth Information
reference material isothermal to each other is then recorded as a function of
temperature. Moreover, in power-compensation DSC, an endothermic
transition, which corresponds to an increase in enthalpy, is indicated as a peak
in the upward direction (since power is supplied to the sample), while an
exothermic transformation, a decrease in enthalpy, is shown as a negative
peak. This, therefore, differs from the DTA curve since the peaks are in
opposite direction and the information obtained is heat flow, rather than
T,
as a function of temperature (see Fig. 2). Also, as will be shown later, the
integration of a DSC curve is directly proportional to the enthalpy change.
The heat-flux DSC instrument is very often based on the Tian-Calvet
calorimeter. The original calorimeter, built in the early 1920s by Tian,
[29]
consisted of a single compensation vessel and the measurement was via a
thermopile. Calvet modified this setup about twenty-five years later by
making it a twin calorimeter, i.e., applying the differential technique.
[29]
The energy measuring device is a thermopile consisting of approximately
500 Pt-Pt/10%-Rh thermocouples which are equally spaced and connected
in series. This arrangement enables the electromotive force (emf ) to be
directly proportional to the amount of heat lost by the sample and reference
holders. Essentially, this type of calorimeter measures the difference in
temperature between the sample and reference as a function of time, and
since the temperature varies linearly with time, as a function of temperature
as well. The heat-flux is actually derived from a combination of the
∆
T
(
t
)
curve and the
d
∆
T
(
t
)
/dt,
both of these are transparent to the user since the
electronics used yield a direct heat flux value from these terms. If tempera-
ture compensation is required, then it is done by Joule heating (for an
endothermic process) or by Peltier effect (for an exothermic process). As
in the DTA case, an endothermic signal is in the negative direction, while
an exothermic signal is the upward direction (see Fig. 2).
Both the heat-flux calorimeters and power-compensation calorim-
eters have their advantages and disadvantages, but, the end result is the
same, the two will yield the same information. The advantage of the heat-
flux type is that it can accommodate larger sample volumes, has a very high
sensitivity, and can go above 1100 K. The disadvantage is that it cannot be
scanned at rates faster than 10
K min
-1
at
high temperatures and not faster
than 3
K min
-1
at sub-ambient temperatures. The main advantage of the
power-compensation calorimeter is that it does not require a calibration in
that the heat is obtained directly from the electrical energy supplied to the
sample or reference compartment (a calibration is still necessary, however,
to convert this energy into meaningful units) and that very fast scanning
rates can be obtained. The disadvantage of this system is that the electronic
∆
