Thermal analysis

Thermal methods are techniques in which changes in physical and/or chemical properties of a substance are measured as a function of temperature. Several methods of analysis are used:

• Thermogravimetry (TG) is a technique in which a change in the weight of the substance under investigation is monitored with respect to temperature or time.
• Differential thermal analysis (DTA) is a technique for measuring the difference in temperature between the substance under investigation and an inert reference material with respect to temperature or time.
• Differential scanning colorimetry (DSC) is a technique in which the energy necessary to establish a zero temperature difference between the substance under investigation and a reference material is monitored with respect to temperature or time.

When carrying out a thermal analysis procedure it is important to consider and record the following details:

• Sample: a chemical description of the sample, plus its source and any pretreatment. Also, the purity, chemical composition and formula, if known. Other important items to note are: the particle size, whether the sample has been mixed with a 'binder' (and, if so, what it has been mixed with and in what ratio) and the 'history' of the sample.
• Crucible: the material and design of the sample holder is important. Obviously it is important that the crucible does not react with the sample during heating. In addition, the geometry of the crucible can influence the gas flow.
• Rate of heating: this is very important if you intend to repeat the experiment on a subsequent occasion. Obviously the rate of heating of the sample in the crucible is not instantaneous but depends upon conduction, convection and radiation within the system. Thermal lag is therefore likely to be observed.
• Atmosphere: The nature of the atmosphere surrounding the sample is important in relation to the transfer of heat and the chemistry of the sample reaction. Common sample atmospheres are shown in Table 36.1. In addition, the flow rate of the gas is important: a static system will not remove reaction products from the sample.
• Mass of sample: obviously the amount of sample will have an effect on the heating rate. Also, sample homogeneity may be an issue with very small samples.
  
  

  Table 36.1 Common sample atmospheres