Differential Thermal Analysis (DTA)

Differential thermal analysis and differential scanning calorimetry are very similar techniques but will be discussed separately. Differential thermal analysis is carried out by heating the sample together with a reference standard under identical thermal conditions in the same oven and measuring the temperature difference between the sample and reference substance during the period of heating. As soon as the sample changes state, crystal form, melts, etc. the latent heat of phase transition will be absorbed and the temperature of the sample will lag behind that of the reference material. In this way any change in state will be shown and the temperature at which it occurs will be recorded. The established oven system is depicted in figure 6.

Figure 6. The Basic Differential Thermal Analysis Apparatus

The sample and the reference standard are placed in containers situated in two cavities contained in a high thermal capacity block. Thermocouples are situated in the sample and the reference materials and are connected in opposition so only a temperature difference between the two will be recorded. The reference standard must not suffer any change in state over the temperature range employed to examine the sample. Alumina and silicon carbide are satisfactory reference standards for solids whereas dinonyl phthalate and certain silicone oils have been used as standards for liquids. As the temperature is increased any change in phase by the sample will result in a negative or poitive temperature signal from the thermocouples.

An alternative and probably a more popular system is shown in figure 7. The sample and reference standard are placed in the furnace on flat, highly thermally conductive pans and the thermocouples are physically attached to the pans directly under the sample. This procedure avoids or reduces any thermal lag resulting from the time required for the heat to transfer to the sample and reference materials and then to the thermocouples. The thermocouples are connected in opposition in the same manner as the previous system.

Figure 7. An Alternative Differential Thermal Analysis Apparatus

In a similar manner any change in state that involves a latent heat of transition will cause the temperature of the sample to lag or lead that of the reference standard and identify the change of state and the temperature at which it occurred. The type of output that usually obtained for a single change of state is depicted in figure 8.

Figure 8. A Typical Differential Thermogram Showing a Single Change in State