Introduction

Differential thermal analysis (DTA) is a thermo-analytical technique for collecting sample’s information like thermodynamic properties, purity, stability etc. by measuring the temperature difference between studied sample and a reference material. In DTA measurement, the difference in temperature between the sample and a reference material is monitored against time or temperature while the temperature of the sample is programmed in specified conditions.^[1]The determined temperature difference is caused by the changes and the properties of the analyzed sample. The DTA measurement generally generates a plot of the difference in temperature or heat flow as the ordinate against the temperature or time as abscissa. The peaks position, orientation, and the area give analyst the information about the studied sample. The position gives the point of event start and stop. The upward peak means exothermic phenomena, and the downward peak means endothermic phenomena.

Apparatus

The major parts of the DTA system are the furnace and its temperature programmer, the sample holder, and a recording system (shows in Figure 1). The sample holder consists a thermocouple each for the sample and reference, surrounded by a ceramic or metallic block for ensuring an even heat distribution. The metallic blocks are less prone to baseline drift, compared to ceramics. Although, high thermal conductivity of metal causes smaller DTA peaks.^[2]The sample and the reference are placed in a small crucible or a pan. The crucible and pan could be made by different material depending on the test conditions like the studied sample and temperature. Most low-temperature instruments use aluminum pans and lids. For more aggressive environments, platinum or ceramic crucibles may be used. The conductivity of the crucibles, pans and their contact with the sensors affect the thermal analysis curves. For reference, an inert material is needed. Al₂O₃ is used in many applications.

In DTA measurement, the obtained curves will depend on the samples and instrument conditions used as listed in Table 1.^{[1, p.78]}

Table 1. The factors affecting the final DTA curve.

The conventional DTA does not always give sufficient information to allow the analyst to be sure what is occurring. The peaks in a DTA experiment means that an endothermic or exothermic event occurs over this temperature range. It could not tell analyst whether this is a chemical reaction or a physical change, or whether any gases are evolved. Since that, the DTA now always be included in an integrated instrument which involves Thermogravimetric analysis (TGA), Differential scanning calorimetry (DSC). This integrated instrument could provide a comprehensive information and be more convenient and faster.^[3]

Applications

There are a large number and variety of application of DTA. The applications may be divided roughly into two categories.^{[1, p.78]}

1. Physical changes and measurements: such as melting point, crystalline phase changes, phase diagrams, heat capacity, and glass transitions, thermal conductivity etc.
2. Chemical reactions such as dehydrations, decompositions, polymer curing, glass formation and oxidative attack.

Consider to detail application, DTA has been widely used in the pharmaceuticals research and food industrial.

Example of Analysis

The Figure 2 shows the DTA curve for Al-alloy and the mold used in the study of Ol'Khovik.^[4]

The study developed a simple techique based on the differential thermal analysis and proposed a method for determining of the heat storage capasity of the mold materials.

References