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About thermal
analysis and calorimetry
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.
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