Thermal analysis

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Thermal analysis is a branch of materials science where the properties of materials are studied as they change with temperature. Techniques include:

• Differential scanning calorimetry
• Dynamic mechanical analysis
• Thermomechanical analysis
• Thermogravimetric analysis
• Differential thermal analysis
• Dilatometry
• Optical Dilatometry
• Dielectric Thermal Analysis
• Evolved Gas Analysis
• Thermo-optical Analysis
• Production Thermal Analysis of Metals
• Thermal Analysis of Foods

Production Thermal Analysis of Metals

Production of many metals (Base Iron, Gray Iron, Ductile Iron, Compacted Graphite Iron, 300 series Aluminum, Copper alloys, Silver, and complex steels) are aided by a production technique also referred to as Thermal Analysis. A sample of liquid metal is removed from the furnace and poured into a sample cup with a thermal couple embedded in it. The temperature is then monitored, and the phase diagram arrests (Liquidus, Eutectic, and Solidus) are noted. From this information chemical composition based on the phase diagram can be calculated, or the crystalline structure of the cast sample can be estimated.

Advanced techniques use differential curves to locate endothermic inflection points such as gas holes, and shrinkage, or exothermic phases such as carbides, beta crystals, inter crystalline copper, magnesium silicide, iron phosphide's and other phases as they solidify. Detection limits seem to be around 0.01% to 0.03% of volume.

In addition, integration of the area between the zero curve and the first derivative is a measure of the specific heat of that part of the solidification which can lead to rough estimates of the percent volume of a phase. (Something has to be either known or assumed about the specific heat of the phase verses the overall specific heat.) In spite of this limitation, this method is better than estimates from two dimensional micro analysis, and a lot faster than chemical dissolution.

Thermal Analysis of Foods

Most foods are subjected to variations in their temperature during production, transport, storage, preparation and consumption, e.g., pasteurization, sterilization, evaporation, cooking, freezing, chilling etc. Temperature changes cause alterations in the physical and chemical properties of food components which influence the overall properties of the final product, e.g., taste, appearance, texture and stability. Chemical reactions such as hydrolysis, oxidation or reduction may be promoted, or physical changes, such as evaporation, melting, crystallization, aggregation or gelation may occur. A better understanding of the influence of temperature on the properties of foods enables food manufacturers to optimize processing conditions and improve product quality. It is therefore important for food scientists to have analytical techniques to monitor the changes that occur in foods when their temperature varies. These techniques are often grouped under the general heading of thermal analysis. In principle, most analytical techniques can be used, or easily adapted, to monitor the temperature-dependent properties of foods, e.g., spectroscopic (NMR, UV-visible, IR spectroscopy, fluorescence), scattering (light, X-rays, neutrons), physical (mass, density, rheology, heat capacity) etc. Nevertheless, at present the term thermal analysis is usually reserved for a narrow range of techniques that measure changes in the physical properties of foods with temperature (TG/DTG, DTA,DSC and Transition temperature).

References

• M C Ramos-Sánchez, F J Rey, M L Rodríguez, F J Martín-Gil, J Martín-Gil, "DTG and DTA studies on typical sugars", Themochim Acta, 134: 55-60. 1988. Elsevier Science Publishers B.V., Amsterdam

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