Differential thermal analysis (DTA) occurs when two substances are exposed to the same temperatures and thermal changes over time. The test typically includes a reference substance, of which its behavior is known under the applied temperature conditions. Another substance is subjected to the same temperatures and rates of change as the reference. The sample can either absorb heat, generally meaning it is cooler than the reference, or emit heat when it is hotter than the first material. With the data that is plotted, researchers can determine how specific materials react at certain temperatures, as well as over time.
Reference materials typically do not melt or freeze; they need to remain at a steady state for the experiment to work. A differential thermal analysis technique is often done by placing each material in a separate container. Each container is usually in a separate cavity in the same space. Instruments called thermocouples are generally used to record temperature differences between both materials. The thermocouples can detect a change in phase, such as melting or vaporizing, as a signal.
Another method of differential thermal analysis can be the use of conductive pans, with thermocouples attached to them, inside a furnace. Both materials are more often heated equally with time when DTA is carried out like this. The results of the test are typically recorded by using a DTA curve. Differences in temperature between the materials, or a change in temperature over time, can be plotted. The graph often visualizes latent heat of transition that occurs while a substance changes from one state to another; this usually causes the sample’s temperature to temporarily be less than the reference.
Differential thermal analysis often involves testing samples at higher temperatures than other forms of thermal analysis. It can be conducted with materials such as metal, ceramic, glass, and ceramic. Reference materials that are often used include silicon carbide and aluminum. Liquid reference standards sometimes include silicon oil.
Technology in the 21st century has combined differential thermal analysis with the ability to detect loss in material mass as well as change in temperature. Software programs can automatically monitor the process and record the measurements. Before a test, instruments for differential thermal analysis generally have to be calibrated. A separate calibration procedure runs materials known to respond a particular way over given temperature ranges. Local and regional regulations can guide the process of calibration for differential thermal analysis in pharmaceutical, food, or environmental applications.