Application of heat by raising temperature of a material results in physico-chemical changes such as:
- Expansion in size or volume
- Change of phase,ie, melting, evaporation or structural transitions
- Loss of trapped water and water of hydration
- Excitation resulting in emission of specific wavelengths of light
- Self ignition or explosion
- Chemical reactions with environmental gases
- Chemical transformations resulting in heat liberation or absorption
In this article you shall be introduced to some thermal methods of analysis which exploit such behaviour on heating of materials for their characterization. Temperature change or rate of change in temperature is recorded as an independent variable whereas in some methods it is recorded as a variable against time. The recorded curves or thermograms are used to study the thermal behaviour of the materials.
Thermometric Enthalpy Titrations
Thermometric titrations or enthalpy titrations are one of the earliest techniques falling under thermal methods of analysis which make use sudden change in temperature on stoichiometric completion of reaction under controlled environmental conditions.
Thermometric titrations have a number of applications for evaluation of concentration, heat of reaction, equilibrium and other thermodynamic constants for reactions involving acid– base, redox, precipitation reactions and complexometric titrations in aqueous as or non-aqueous media and even molten salts at high temperatures.
Differential Scanning Calorimetry (DSC)
In DSC technique the sample and reference material are subjected to a precise temperature program under identical conditions in a furnace. Temperature changes due to thermal transitions in the sample are compensated by application of current to the reference. The amount of current applied to compensate the change in temperature is measure of the exothermic or endothermic change in the sample (measured as area under the peak)
DSC has been applied to study several thermal transitions such as heat of fusion, crystallization, glass transition temperature, melting range and purity of materials
Thermogravimetric Analysis (TGA)
Thermo gravimetric analysis measures change in mass of a material as its temperature is increased in a linear mode. Change in mass often takes place due to liberation of gas on heating. TGA has also been used to monitor multistage decomposition at different temperatures such as liberation of water of crystallization or decomposition of mixtures at different temperatures, pyrolysis behaviour of coal and petroleum, determination of moisture,volatile matter or ash content. Other areas of study are sublimation, absorption or desorption of gases under controlled environments.
Differential Thermal Analysis (DTA)
Differential Thermal analysis measures difference in temperature between a sample and reference measured as a function of temperature ∆T versus ∆T plots.
Fusion, crystalline state transitions and solid-state reactions may not show changes in mass as no gas is liberated but DTA can be used for such evaluations
Thermal methods have been used for predicting change in mechanical properties with change in temperature such as Thermo Mechanical analyser (dimensional changes versus temperature) and Dynamic Mechanical analyser (DMA) which measures modulus under varying conditions of stress and temperature
In summary thermal methods of analysis have found several applications in material characterization in several areas such as :
- Textiles and fibres
- Polymers and plastics
- Minerals, soils and clays
- Glass and ceramics
- Building materials, concrete and steel
- Metal and alloys
- Liquid crystals
The scope of thermal methods is ever expanding and more and more materials are being characterized using these techniques.