Image Credits: Norenko Andrey/shutterstock.com
Differential thermal analysis (DTA) has become a popular thermal analysis (thermoanalytical) technique and is often used to measure the temperature of a material, which in turn is used to measure the endothermic and exothermic phase transitions of material. It is a technique that has found a lot of use across the pharmaceutical, organic chemical, inorganic materials, food, cement, mineralogical and archaeological sectors.
In principle, differential thermal analysis is a technique which is similar to differential scanning calorimetry (DSC), and the material being studied in DTA undergoes various thermal cycles (heating and cooling cycles), using an inert reference material, where the temperature difference between the reference and the material under analysis is determined. Both the reference and sample materials are kept at the same temperatures throughout the heating cycles to ensure that the testing environment is uniform.
Components in Differential Thermal Analysis
Differential thermal analysis is usually performed in a furnace, as this is the most efficient way to obtain a uniform temperature in the surrounding environment—especially with modern-day furnaces. The temperature itself is recorded using two thermocouples, which are specialist (and versatile) types of temperature sensors that use metal wires to form hot and cold junctions. The hot junction measures the temperature of the material while the cold junction provides a reference to compare the analysis temperature against. This is what happens inside every thermocouple to determine the temperature of a material. The reference, in this case, is not the reference temperature of the DTA analysis, rather, it is the reference inside each thermocouple device. So, two thermocouples are needed because one thermocouple measures the temperature of the sample, and the other measures the reference.
Aside from the thermocouples and the furnace, voltmeters are also employed to measure the voltages across the thermocouples (which is how they determine the temperature), as well as crucibles that are often used to hold the material—especially when small samples are under analysis. Inside the furnace, inert gases such as argon or helium are also used, as they don’t react with the sample or the reference materials, and this ensures that there is no interference during the measurements. In most cases, the analysis environment is air-tight to prevent any contaminants from affecting the results. Most furnaces used in modern-day DTA approaches can also provide a temperature environment from -150 °C and 2400 °C. Additionally, many different crucibles can be used, and the combination of these two factors enable a wide range of materials to be analyzed—and this is why differential thermal analysis spans many different industrial sectors.
To perform the analysis itself, both the sample material and the reference material are placed symmetrically in the furnace. The two materials then undergo a controlled program of heating and cooling, where both temperatures are kept as constant as possible (within a reasonable error) during each cycle. There is usually a slight delay in the recording of data due to the furnace heating up (and the length of delay typically depends on the heat capacity of the material).
During the analysis, the temperature difference is plotted on a graph against time. In some cases, it can also be plotted against temperature. From here (and how the curve manifests itself), the endothermic and exothermic transition temperatures of a material can be determined, and these larger classification categories include the glass transition temperature of the material, the crystallization temperature of a material, the melting temperature of a material, and the sublimation temperature of the material. These are often deduced because the changes in the temperature against the reference material can determine whether the material is absorbing heat (endothermic) or is giving out heat (exothermic). The presence of the thermocouples also helps to easily identify if a phase transition has occurred, because the voltmeter attached to the reference thermocouple will jump slightly when the phase change happens. This is due to the latent heat which arises from the material phase change causing the temperature of the inert gas to raise slightly (which in turn affects the voltage of the reference thermocouple).
Aside from conventional temperature phase transitions, differential thermal analysis can also be used to measure two inert samples when their responses to the heat cycle are not identical. In these specific cases, DTA can also be used to identify any phase changes that are not based around a change in enthalpy. These are often identified by discontinuities in the curve on a DTA graph.
While differential thermal analysis is formally defined as a method of determining the temperature difference between sample and reference materials, in practice, it can tell the user a lot about the phase properties of a material at different temperatures. The amount of information obtainable is of great benefit to many industries, hence it’s widespread use.
Sources and Further Reading