What is the scope of application of differential thermal analysis?

Differential thermal analysis involves comparing an unknown substance with an equal amount of a stable substance (a reference material) that does not undergo any chemical or physical changes at a certain experimental temperature. Under conditions of uniform temperature change in the same environment, any chemical or physical changes that occur in the unknown substance will result in a temporary increase or decrease in its temperature compared to that of the standard material located in the same environment. A decrease is manifested as an endothermic reaction, while an increase is manifested as an exothermic reaction. When the same amount of heat is applied to the substance under test and the reference substance, their temperature rises will necessarily differ due to their different thermal properties; the temperature difference between the two is measured to achieve the purpose of analysis. The curve obtained with the temperature difference between the reference and the sample on the vertical axis and temperature on the horizontal axis is called a DTA curve. In differential thermal analysis, thermocouples with a temperature difference are used to detect such slight variations in temperature. It is made of two different types of metal wires. Typically, an appropriate section of nickel-chromium alloy or platinum-rhodium alloy is used, with its two ends respectively welded by arc welding to two sections of platinum wire of equal thickness, thus forming a thermocouple based on temperature differences. In differential thermal analysis, an equal amount of powdered sample with the same particle size as the reference material is placed in two crucibles; the bottoms of these crucibles are in contact with the two welding points of the temperature difference thermocouple. At a distance and at the same height as these two crucibles, there is a thermocouple used to measure the temperature of the heating furnace, with both ends of this thermocouple connected to the circuit of the recorder. During the process of steady-rate heating, there is a linear relationship between temperature and time, meaning that the rate of temperature increase remains relatively constant, which facilitates accurate determination of the temperature at which the sample undergoes changes. The sample shows no change in any temperature increase zone; it neither absorbs nor releases heat. No temperature difference occurs at the two welding points of the thermocouple, and on the differential thermal analysis graph, it appears as a straight line, which is known as the baseline. If a thermal effect occurs in the sample within a certain temperature range, a temperature difference is created at the two welding points of the thermocouple. This results in a thermoelectric potential difference at both ends of the thermocouple. The signal generated as a result of this difference is amplified and sent to the recorder, causing the recording device to deviate from its baseline; once the reaction is complete, it returns to the baseline. The directions of the thermoelectromotive forces generated by endothermic and exothermic effects are opposite to each other; therefore, they appear on the differential thermal curve on opposite sides of the baseline. The magnitude of these thermoelectromotive forces is proportional not only to the amount of sample but also to the properties of the substance itself. The magnitude of the thermoelectric potential generated by different substances varies with temperature; therefore, differential thermal analysis can not only be used to study the properties of substances but also to identify unknown substances based on these properties. Thermal analysis is a technique that measures the functional relationship between the physicochemical properties of a substance and temperature under programmably controlled temperature conditions. Programmable temperature control can use linear, logarithmic, or inverse programs. Thermal analysis methods vary depending on the physical properties of the sample being tested, including differential thermal analysis, differential scanning calorimetry, thermogravimetric analysis, thermal expansion analysis, and thermomechanical analysis. Among these, the first three techniques are widely used in pharmaceutical research.