Thermal analysis is defined as a group of methods by which the physical or chemical properties of a substance, a mixture and/or a reactant are measured as a function of temperature or time whilst the sample is subjected to a controlled temperature program. The program may involve heating or cooling (dynamic), or holding the temperature constant (isothermal), or any sequence of these.
Thermal methods are multi-component techniques and include thermogravimetry, differential thermal analysis and differential scanning calorimetry. These methods find widespread use in both quality control and research applications on industrial products such as polymers, pharmaceuticals, metals and alloys.
In a thermogravimetric (TG) analysis the mass of a sample in a controlled atmosphere is measured as a function of temperature or time. TG may be used to monitor any reaction that involves a gaseous phase, such as oxidation or dehydration. The sample size varies from a few mg to 10 g depending on the equipment used. Studies may be conducted at temperatures up to 1,550 oC. This method is useful for determining sample purity and water, carbonate and organic contents and for studying decomposition reactions.
The thermogram plots weight versus temperature or time thus generating information about thermal stability of the sample, reaction-rates and composition.
Thermogravimetric instruments consist of a sensitive analytical balance, a furnace, a purge gas system and a data handling system.
A number of different thermobalances are available that provide quantitative information about samples ranging in mass from 1 mg to 100 g. The most common type of balance has a range of 5 to 20 mg. Although the sample must be housed within the furnace, the rest of the balance must be thermally isolated from the furnace. The sample is placed in a sample container that is positioned in the furnace on a quartz beam. Any change in sample weight causes a deflection of the beam, which is sensed by one of the photodiodes.
The beam is then restored to the original null position by a feedback current sent from the photodiodes to the coil of the balance. The current is proportional to the change in weight of the sample.
Most furnaces have a temperature range from ambient to 1,500 oC in either inert or reactive atmospheres. Insulation and cooling of the exterior of the furnace are required to avoid heat transfer to the balance. Nitrogen (N2) or argon (Ar) is usually used to purge the furnace and prevent oxidation of the sample.
Instrumentation control/ Data handling
The temperature recorded in a thermogram is ideally the actual temperature of the sample. This temperature can be measured with a small thermocouple directly in the sample, as close as possible to the sample container. Modern thermobalances use a computerized temperature control routine that automatically compares the voltage output of the thermocouple with a voltage versus temperature table that is stored.