DSC

General definition

According to the ASTM standard E473, Differential Scanning Calorimetry (DSC)[1]is a technique where the heat flow rate difference into a substance and a reference is measured as a function of temperature, while the sample is subjected to a controlled temperature program. This way, quantitative calorimetric information of the sample can be obtained, such as melting and crystallization temperatures; heat of fusion; heat capacity; heat of reaction and others. DSC is considered an accurate, simple and user-friendly technique and, consequently, it is probably the most popular thermal analysis method. It is widely used for the characterization of materials in the field of thermal energy storage [1,2].

[1] Note that the acronym DSC can refer either to the technique (differential scanning calorimetry) or to the measuring device itself (differential scanning calorimeter)

Main advantages and limitations

Nowadays, DSC is a routine technique and the instruments are relatively inexpensive; therefore, it can be found in numerous characterization laboratories. It allows a cost-effective determination of certain relevant thermal properties of materials with a moderate effort. Unfortunately, this implies some drawbacks. It is a popular misconception that a DSC experiment can be successfully completed just by recording a DSC peak [2]. This is not true, especially for research purposes, where reproducibility of the results is essential. Literature [3] demonstrates that significant differences can arise between DSC results for the same material when they measured in different laboratories. Standard procedures are yet to be developed in order to avoid these uncertainties. Besides, DSC shows other limitations, like the reduced sample size to be measured. The dynamic nature of the technique can be also considered a drawback for certain applications, because it implies a “lack of equilibrium” conditions whereas the properties to be determined they are inherently equilibrium properties.

Main DSC types

Two main types of DSC exist nowadays: power compensation DSC and heat flux DSC. Power compensation DSC was created by Gray and O’ Neil at the Perkin-Elmer Corporation [4] in 1963. Alternatively, heat flux DSC grew out of Differential Thermal Analysis (DTA).

A heat flux DSC usually consist of a cell containing the reference and sample holders surrounded by a heating block that acts as a homogeneous-temperature body. The two holders are heated (or cooled) by means of the heating block and therefore they are submitted to the same temperature program. The measured signal is the temperature difference, which is computed by temperature sensors located below (or surrounding[1]) the sample and reference holders. If during the temperature program there is thermal symmetry between the two holders, no heat exchange will occur between them. Conversely, if any thermal asymmetry occurs, owed for instance to a phase transition within the sample or to the different thermal capacity of the sample and the reference, then a temperature difference will take place. This temperature difference is used to calculate the heat exchanged between the holders and, eventually, to determine the thermal properties of the sample.

The power compensation DSC[2] consists of two microfurnaces of the same type, each of which contains a temperature sensor and a heating resistor. Upon heating, the same heating power is supplied to both microfurnaces via a control circuit. If there is thermal symmetry, the temperature of both microfurnaces is always the same. When an asymmetry occurs, for example as a result of a phase change in the sample, a temperature difference results between the microfurnace accommodating the sample and that containing the reference sample. That temperature difference is used as the input signal of a second control circuit, which tries to reduce it by increasing or decreasing an additional heating power. This compensating heating is proportional to the heat consumed or released in the sample during the event that led to the asymmetry and, therefore, can be used to determine the thermal properties of the sample.

Calvet-type DSC is a heat flux DSC developed by the company Setaram Inc [5]. It is based on the Calvet detector construction. In contrast to the traditional plate-shaped heat flow sensors, the Calvet type is based on a three dimensional heat flow meter sensor, formed by an array of thermocouples surround the tubular sample and reference holders. This radial arrangement of the thermopiles guarantees an almost complete integration of the heat, which leads to an increased accuracy. Besides, the sensitivity of the DSC is no longer significantly affected by the type of crucible, type of purge gas and flow rate. In addition, 3D Calvet sensors can be calibrated by the Joule Effect method, avoiding the need of certified reference materials and allowing for a calibration at almost any temperature, with the consequent advantages. This arrangement also allows the use of larger sample sizes and higher pressures compared to traditional DSC devices.