Thermal analysis is a
branch of materials science where the properties of materials are studied as
they change with 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
Dilatometry (DIL): volume
Dynamic mechanical analysis (DMA) : mechanical stiffness &
damping
Dielectric thermal
analysis (DEA): dielectric permittivity & loss factor
Evolved gas analysis (EGA) : gaseous decomposition products
Thermo-optical analysis(TOA) : optical properties
Simultaneous Thermal Analysis (STA) generally refers to the
simultaneous application of Thermogravimetry (TGA) and Differential scanning
calorimetry (DSC) to one and the same sample in a single instrument. The test
conditions are perfectly identical for the TGA and DSC signals (same atmosphere,
gas flow rate, vapor pressure of the sample, heating rate, thermal contact to
the sample crucible and sensor, radiation effect, etc.). The information
gathered can even be enhanced by coupling the STA instrument to an Evolved Gas
Analyzer (EGA) like Fourier transform infrared spectroscopy (FTIR) or Mass
Spectometry (MS).[1]
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.
Thermal Analysis is also often used as a term 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.
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