Page 21 - Handbook of Thermal Analysis of Construction Materials

Section 2.0 - Classical Techniques 5

2.2 DSC

The DTA calorimeter, sometimes called DSC, was developed by
David in 1964. [24][25] The term DTA calorimeter is more appropriate since
this system actually measures ∆T directly from the experiment. Unlike
conventional DTA however, the experiment is performed at quasi-equilib-
rium conditions, i.e., sample mass is less than 10 mg, slow cooling/heating
rate, and only one calibration coefficient needs to be measured for the entire
temperature range. This, therefore, yields quantitative data but by defini-
tion remains a DTA instrument. The other two categories of DSC apparatus
are true calorimetric instruments in that the calorimetric information is
obtained directly from the measurement, i.e., no conversion factor is
required to convert ∆T into readily used energy units as the thermometric
data is obtained directly. A constant is still required to convert the energy
term into more suitable units. The main goal of any enthalpic experiment,
which is to determine the enthalpy of a sample as a function of temperature,
is attained by measuring the energy obtained from a sample heated at a
constant rate with a linear temperature or time programming. These two
DSC instruments are based on the method developed by Sykes in the mid-
1930s. [26][27] Sykes’ apparatus was designed so that the temperature of the
metal block, which contained the sample, was slightly lower than the
temperature of the sample itself. To maintain the sample at the same
temperature as the block, power was supplied to the sample. The main
disadvantage of this apparatus was that a correction factor had to be applied
to account for the heat transfer between the surrounding medium and the
block. Both the heat flux and power-compensation DSC instruments over-
come this drawback because, as the name suggests, they are differential
instruments. The heat-flux instruments measure the flux across a thermal
resistance, whereas the power compensating differential scanning calorim-
eters measure the energy applied to the sample (or the reference) by an
electrical heater in order to maintain a zero-temperature differential.
The first commercial DSC instrument was introduced by Watson
and his co-workers at Perkin-Elmer (Model DSC-1) in 1964. [28] Watson,
et al., also appear to be the first to have used the nomenclature differential
scanning calorimetry. Their instrument, a power-compensating DSC,
maintained a zero temperature difference between the sample and the
reference by supplying electrical energy (hence, the term power-compen-
sation) either to the sample or to the reference, as the case may be,
depending on whether the sample was heated or cooled at a linear rate. The
amount of heat required to maintain the sample temperature and that of the