Thermal analysis can also be described as the measurement of heat transfer through structured mediums. Each subcategory of an analyzer measures different properties of a sample as its temperature is either heated or cooled. The temperature of the environment that the sample is within is controlled; how that temperature is controlled varies based on the device’s make and model.
A simple control method is increasing or decreasing the temperature at a constant rate, referred to as linear heating/cooling. The analysis is done by recording multiple measurements of the same material at different temperatures, also known as a stepwise isothermal measurement.
Having the rate of change in temperature oscillate or having the heating or cooling rate change in response to alteration in the sample would be two examples of other, more complicated, control techniques. These can also be referred to as modulated temperature thermal analysis and sample controlled thermal analysis, respectively.
Thermal analyzers are used in material science, pharmaceutical processes, polymer analysis, medical research, and quality assurance. Their ability to ascertain the physical qualities of a material in relation to changes in temperature makes them indispensable to these fields.
The analysis can also look at a dielectric material’s electrical discharge, a stressed sample’s mechanical relaxation, or any light or sound emissions, all in relation to changes in temperature.
Additionally, many of the thermal analysis methods covered below can be coupled with other analytical techniques such as gas chromatography, mass spectrometry, or microscopy in order to provide more accurate results.
Various analytical instruments and methods are used to monitor such thermal endothermic and exothermic processes as melting, boiling, sublimation, phase transition, crystallization, oxidation, desolvation, and more.
Furthermore, many TA instruments now offer automation features which help streamline workflows and allow for researchers to focus on other priorities while analyses are performed.
Conducted by thermogravimetric analyzers, thermogravimetric analysis (TG) continuously measures the mass of a sample’s temperature changes over time. The result of such an analysis is a plot of mass as a function of either time or temperature. These devices consist of a lab balance with the desired sample on it contained within a temperature-controlled furnace. These analyzers are also known as thermobalances.
They are used in determining materials thermal stability, oxidation, and combustion mass losses, in addition to the thermal decomposition present in pyrolysis and computation of materials.
Identifying gases released directly from a sample during thermal treatment cannot happen without coupling a spectroscopic method like Fourier-Transform-Infrared (FT-IR) spectroscopy to a TG analyzer. This integration, referred to as TG-FTIR, pairs the quantification capabilities of TG with the identification abilities of FTIR spectroscopy.
Observing dimensional changes in materials as a function of temperature or time is done using a thermomechanical analyzer. Thermomechanical analysis is considered a sub-discipline of thermomechanometry.
The sample is placed inside a furnace and a force generator is attached to it. The generator is used to deform the sample either by compression, tension, flexure, or torsion, and those changes are measured and recorded. The constant application of force or non-oscillating stress deforms the sample over time.
Expansion/compression, penetration, or tension probes are used depending on what type of measurement is needed.
A dielectric thermal analyzer, or more simply put a dielectric analyzer (DEA), measures a material’s capacitance, conductance, and phase change by applying an oscillating electrical field to it and observing it as a function of time and temperature.
DETAs are also used to observe the curing behavior of thermosetting resin systems, composite materials, and other polymers. This is achieved by placing two electrodes on the material and applying a sinusoidal voltage to one of them. The response measured from the second electrode is recorded and is used to determine other material properties.
A thermoanalytical technique that measures the temperature of a substance as it is heated or cooled at a specific rate. Simultaneously, a known reference inert material undergoes an identical thermal cycle and its temperature is also recorded.
The difference between the two temperatures is plotted against either time or temperature and this plot then is used to determine other properties of the substance. This curve can then be used to plot exothermic and/or endothermic changes in the substance to identify transformation temperatures or when it melts, sublimates, and crystalizes.
Differential thermal analyzers’ ability to identify these transition points makes it extremely useful for material identification. DTA is found in pharmaceutical, mineralogical, and environmental fields.
Furthermore, it can be coupled with thermogravimetric analysis (TG-DTA) to characterize multiple thermal properties of a sample in a single experiment. Fundamentally, TG involved in TG-DTA is very similar to the standard thermogravimetric analysis in that it can provide measurements concerning temperatures changes as a result of decomposition, reduction, or oxidation.
Similar to thermomechanical analyzers, dynamic mechanical analyzers look at a material’s dimensional changes as a function of time, temperature, and frequency of stress. As the frequency of the strain that is applied varies, the resulting stress on the material is measured. This sinusoidal force is applied by a probe that deforms the material, and the relationship between the force applied and the deformation is then measured.
When testing polymer materials, different deformation techniques are used depending on the force that needs to be measured. Tension, compression, dual cantilever bending, three-point bending, and shear modes are examples of a few techniques.
Dilatometry is the technique used to measure dimensional changes in a sample’s volume as changes in temperature occur. A simple and well-known dilatometer is a mercury-in-glass thermometer. As the mercury heats up, its volume expands at a measurably consistent rate. DILs can be subdivided into several types:
Known as both a calorimetric and thermal analysis technique, differential scanning calorimetry (DSC) is used to provide test data for a wide range of materials. This includes polymers, rubber, petroleum, chemicals, plastics, adhesives, composites, coatings, organic materials, pharmaceuticals, and much more.
DSC devices measure how change’s in a material’s temperature alter its heat capacity. It is used to evaluate material properties such as glass transition temperature, melting, crystallization, specific heat capacity, thermal stability, and oxidation behavior, among others.
Additionally, DSC can be coupled with TG (TGA-DSC) in order to perform microplastic characterization.
Simultaneous thermal analysis—or STA—is a combination of DSC and TG methods. By coupling thermogravimetric effects with the recording of a DSC heat flow signal, both measuring methods can be compared. This excludes the influence changed sample atmospheres can have on the reaction equilibrium, which can often occur with individual measurements.
Simultaneous thermal analyzers are able to make up for the uncertainties that TG or DSC measurements can create due to such inaccuracies as sample inhomogeneities, sample geometry, and temperature fluctuations.
Combining these two methods ultimately results in an almost complete sample characterization, especially for complex reactions. The complimentary pairing allows researchers to observe and understand differentiation between endothermic and exothermic events, which have no associated weight change and those that do involve weight change.
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