Thermal Analysis

a set of methods for determining the temperatures at which processes involving the liberation or absorption of heat occur. An example of the first type of process is crystallization from a liquid; examples of the second type include melting and thermal dissociation.

The visual method of thermal analysis consists in the measurement of the temperature at which an inhomogeneity first appears or disappears in a medium when the medium is cooled or heated. Typical examples of inhomogeneity are the precipitation of crystals and the disappearance of turbidity in a system of two immiscible liquids. This method is used only for transparent, low-melting substances.

A much more general method is that of plotting temperature-time curves. In this case, a specimen under study is heated or cooled, its temperature is measured at short time intervals, and the results of the measurements are represented graphically by plotting time along the axis of abscissas and temperature along the axis of ordinates. If no transformation occurs, the heating or cooling curve is smooth; transformations are indicated by the appearance of breaks or horizontal arrests in the curves.

Differential thermal analysis is the most accurate method. Here, the specimen under study is heated or cooled together with, and under the same conditions as, a standard specimen that does not undergo transformations under the conditions of the experiment. A temperature-time curve and a curve of the specimen-standard temperature difference as a function of time are plotted on the same graph. A temperature difference occurs whenever the specimen undergoes a transformation involving the absorption or liberation of heat. The nature of the transformation is deduced from the form of the simple heating or cooling curve, and the transformation temperature is precisely determined from the difference curve. Instruments used for recording heating and cooling curves include graphic recording instruments —such as Kurnakov’s automatic pyrometer—electronic automatic potentiometers, and optical pyrometers.

Thermal analysis is used to obtain such quantitative characteristics as phase composition and heats of reaction during the heating or cooling of test specimens. It is widely applied in the study of metal alloys, alloys containing nonmetallic elements, minerals, and other rocks of geological interest.