Background

Thermal Analysis is the measurement and interpretation of the relationship between the physical and/or chemical properties of a sample and its temperature. Several methods are commonly used - these are distinguished from one another by the property which is measured:

 

 * Differential Thermal Analysis (DTA) : temperature difference

 * Differential Scanning Calorimetry (DSC) : heat difference

* Thermogravimetric Analysis (TGA) : mass

* Thermomechanical Analysis (TMA) : dimension

* Dynamic Mechanical Analysis (DMA) : mechanical stiffness & damping

* Thermally Stimulated Current (TSC) : dipole alignment & relaxation

* Dielectric Analysis (DEA) : dielectric permittivity & loss factor

* Evolved Gas Analysis (EGA) : gaseous decomposition products

* Thermo-optical Analysis (TOA) : optical properties

 

Sometimes different properties may be measured at the same time e.g. TGA-DSC or TGA-EGA.

 

Other, less-common, methods measure the sound or light emission from a sample, or the electrical discharge from a dielectric material, or the mechanical relaxation in a stressed specimen. The essence of all these techniques is that the sample's response is recorded as a function of temperature (and time).

 

It is usual to control the temperature in a predetermined way - either by a continuous increase or decrease in temperature at a constant rate (linear heating/cooling) or by carrying out a series of determinations at different temperatures (stepwise isothermal measurements). More advanced temperature profiles have been developed which use an oscillating (usually sine or square wave) heating rate (Modulated Temperature Thermal Analysis) or modify the heating rate in response to changes in the system's properties (Sample Controlled Thermal Analysis).

 

In addition to controlling the temperature of the sample, it is also important to control its environment (e.g. atmosphere). Measurements may be carried out in air or under an inert gas (e.g. nitrogen or helium). Reducing or reactive atmospheres have also been used and measurements are even carried out with the sample surrounded by water or other liquids. Inverse Gas Chromatography is a technique which studies the interaction of gases and vapours with a surface - measurements are often made at different temperatures so that these experiments can be considered to come under the auspices of Thermal Analysis.

 

Atomic force microscopy uses a fine stylus to map the topography and mechanical properties of surfaces to high spatial resolution. By controlling the temperature of the heated tip and/or the sample a form of spatially resolved thermal analysis can be carried out.

 

Calorimetry is the measurement of the flow of heat or work energy arising from chemical or physical changes in a material. Calorimeters can be broadly classified in three categories: Isothermal, Adiabatic and Temperature Scanning.

 

 * Isothermal calorimeters: Heat energy is exchanged from a heat sink maintaining the reaction environment at a constant temperature. Modern isothermal calorimeters have a very high degree of sensitivity and can detect enthalpy changes in the order of 5 nJ.

 * Adiabatic calorimeters: These calorimeters allow a rise in temperature of the reaction system for exothermic reactions or a decrease in temperature for endothermic reactions. A reaction is followed by measurement of a temperature change as a function of time.

* Temperature scanning calorimeters provide a constant change in heat energy to pass between the sample. Two types of temperature scanning instrument are used: heat flux and power compensation.

 

Thermal Analysis is also often used for the study of heat transfer through structures. Many of the basic engineering data for modelling such systems comes from measurements of heat capacity and thermal conductivity.

Thermal Methods Group

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