Modem thermal analysis includes various techniques that measure some physical property of a material (such as weight) as the material is heated. The measurements are often carried out on very small samples. They can be used to give an idea of how substances will change over extended periods under normal conditions of temperature.
records the loss in weight of a substance as its temperature is gradually increased. A counterbalanced sample of the substance is suspended in a furnace and heated. A thermocouple detects the rise in temperature. As the sample loses weight, the other end of the balance beam falls. This moves a shutter and allows light to fall onto a photomultiplier. The signal from the photomultiplier is amplified and then passed through a coil fixed to the pivot of the balance beam. The coil is also situated in a magnetic field. Thus, the current flowing in it causes it to rotate slightly, which twists the suspension, restoring the balance and making the shutter interrupt the light beam. The current in the coil is a measure of the weight lost by the sample. The weight loss is plotted by a chart recorder as a thermal gravimetric graph (see example on the opposite page).
Thermogravimetry
Thermogravimetry, the best-known form of thermal analysis, consists of measuring the weight change of a sample as it is steadily heated. The resulting weight/temperature graph is characteristic for the material being studied. Sharp changes in weight usually occur at specific temperatures, corresponding to the breaking of chemical or physical bonds. Such changes are often associated with the loss of volatile substances such as water, carbon dioxide, or oxygen from the molecules of the sample.
On a thermal gravimetric graph, information is provided by the sizes and shapes of changes in the line. A straight line on the graph indicates no weight change and hence no decomposition. Slopes and curves up or down show that a weight change has taken place due to some material loss. The curve is also quantitative—weight losses between one level and another correspond to losses of definite parts of the molecular structure.
Thermogravimetric measurements are made on a thermal balance—a special type of balance that can operate in furnace temperatures as high as 3000° F. (1650° C). The accuracy of the results depends on using small samples ground down to a uniform particle size. Thermogravimetry is used to study the decomposition of pure materials. It may also be used to determine the percentage composition of mixtures of two known substances. The individual weight loss at each step on the curve can usually be related to one or the other of the two components.
Differential analysis
Two other forms of thermal analysis include differential thermal analysis (DTA) and differential scanning calorimetry (DSC). In DTA, the material being examined is compared with a corresponding amount of reference material (usually alumina, a naturally occurring oxide from which aluminum is obtained). The two materials are heated together to produce a steady rise in temperature. As heating progresses, any change in the sample results in the release or absorption of energy. This is measured as a temperature difference between the sample and the reference. When graphed, this difference, which DTA measures, produces a series of peaks (exotherms) and troughs (endotherms), the former corresponding mainly to chemical changes in compounds, the latter indicating physical changes in the crystalline structure or fusion of molecules. The widths of the peaks and troughs also convey data on changes in the sample.
By contrast, differential scanning calorimetry (DSC) measures the difference in energy required to keep both the sample and reference substances at the same temperature. For example, when an endotherm occurs on DTA, that heat difference in DSC is made up by the instrument adding more heat to the sample. In this case, the instrument measures the amount of extra energy that has to be added to either the sample or reference container in order to maintain both at the same temperature. In all cases, the shapes of the peaks obtained are affected by the rate of heating, the sample size, and even the shape of the sample containers.
The equipment for DTA and DSC consists of an electrically heated metal block containing two identical recesses for the sample and reference capsules. The temperatures of the two substances are measured as the block is heated. The whole system is operated according to a preset temperature program, often controlled by a microcomputer.
Applications of thermal methods of analysis are expanding rapidly. They have been used to determine deterioration in types of cement that contain alumina, as well as for establishing optimum temperatures for making epoxy resins and for fusing mixtures of inorganic compounds.
