Differential Scanning Calorimeter
Differential Scanning Calorimeter (model: DSC Q100 from TA Instruments)
Heat capacity and transition enthalpies are the basic quantities available through calorimetry. They are needed for the design of a chemical facility, hence, the measuring and documentation of these thermodynamic properties is an important duty.
Since 2005 a differential scanning calorimeter from TA Instruments (model: DSC Q100) is available for measuring the caloric quantities. A picture and a scheme of the measuring cell are shown in the following figure
Fig. 1: The measuring cell of the DSC Q100 (for detail, see text below)
Differential scanning calorimetry is a thermal analysis technique that detects the temperatures and heat flows caused by changes in heat capacity or by endothermic and exothermic processes of materials as a function of time and temperature. The DSC Q100 operates in a temperature range from 93 K to 823 K because it is equipped with a liquid nitrogen cooling system. The sample material is enclosed in a non-recyclable hermetic pan which consists of aluminum, has an internal volume of 10 mm3 and withstands an internal pressure up to 3 bar (Fig. 2).
Fig. 2: Sample pan for the DSC Q100before and after encapsulating
The sample mass is in the range of 5 to 10 mg. The sample pan and an empty reference pan are placed on separate raised platforms of a constantan body surrounded by a furnace. When the temperature of the furnace is changed, heat is transported to both the sample and the reference through the constantan sensor. The resulting temperature difference between sample and reference, which is needed to define the differential heat flow, is measured by area thermocouples on the underside of each platform. To achieve an equal heat transfer in the DSC cell, dry helium with a flow rate of 25 mL∙min-1 is used as purge gas.
In a conventional DSC experiment, the furnace temperature varies with time by a linear factor. In order to determine heat capacity of a material, the "three-step" method is necessary, i.e. the same temperature program must be applied to the sample, a reference sample (e.g. sapphire) and an empty sample pan.
With the DSC Q100 modulated-temperature differential scanning calorimetry is possible too. Basically, the furnace temperature is no longer changed at a linear rate. Instead, the conventional temperature program is superimposed by a periodic, normally a sinusoidal, temperature modulation. A mathematical treatment of the total heat flow for a MDSC™ experiment leads to two expressions called "reversing" and "non-reversing" heat flows. Heat capacity of a material is determined from the reversing heat flow. Therefore, with MDSC™ only one experiment is required to measure the heat capacity of a substance.
With the software from TA Instruments it is very easy and fast to analyze the heat flow curves. For example, Fig. 3 shows the solid-solid-phase transition and the melting for cyclohexane.
Fig. 3: The main window of the analysis program