Thermogravimetric analysis (TGA) is a quantitative analytical technique that monitors the mass of a sample from 1 mg to several g as a furnace ramps temperature to as high as 1600°C under a stable or changing gas flow. The first gravimetric test was in 27 BC when Vitruvius measured limestone's change of mass as it calcined to lime. In modern chemical engineering, researchers apply the technique to derive conversions, kinetics, and mechanisms for any process with a change of mass by isothermal, non‐isothermal, and quasi‐isothermal methods. The mass drops as the sample decomposes, volatile compounds evaporate, or the oxidation state decreases, while in reactive environments (with O2, for example), the mass of transition metals may increase. TGA is incapable of detecting phase transitions, polymorphic transformations, or reactions for which mass is invariant. DSC or DTA couple with TGA to help deconvolute a DSC plot by separating physical changes from chemical changes. Evolved gas analysis techniques monitor the gaseous products exiting the TGA furnace on‐line as the temperature ramps. A bibliometric map of keywords from articles citing TGA indexed by Web of Science in 2016 and 2017 identified five research clusters: nanoparticles, performance, and films; crystal structures, acid, and oxidation; composites, nanocomposites, and mechanical properties; kinetics, pyrolysis, and temperature; and adsorption, water and wastewater, and aqueous solutions. This review provides an overview of the basic principles of modern TGA.
Differential calorimetry assesses energy flow between a sample and its environment. The sample may be heated at a known heating rate (either constant or temperature modulated), or held in an isothermal environment or adiabatic environment depending on instrument and experimental design. The subset of differential calorimetry that deals with known heating or cooling rates is termed differential scanning calorimetry (DSC) and is a foundational technique to modern thermodynamics. It reports the heat flow versus temperature or time from which we calculate specific heat capacity at constant pressure, c P , enthalpy of fusion, and the heat of reaction. Moreover, it identifies how microstuctural properties evolve and thermal arrests-a characteristic of phase transitions. Heat-flux DSCs measure the temperature difference between a reference and a sample that sit on a thin two-dimensional plate. Power compensated DSCs heat reference material and the sample in independent furnaces while maintaining each at the same temperature. The Tian-Calvet DSC is similar to the heat-flux DSC, but minimizes error induced at high temperature with ring shaped thermopiles that surround the reference and the sample and in most designs incorporate the independent furnaces characteristic of heat flux DSC (three-dimensional heat flow probe). Convection and radiation energy leaks compromise accuracy above 600 • C, particularly for pan-style heat flux and power-compensated DSC, which are sensitive to heat transfer by conduction only. The Tian-Calvet DSC maximizes the signal-to-noise ratio by enveloping the sample and reference in the thermopile. Web of Science indexed 11 800 articles in 2016 and 2017 that mentioned DSC and assigned 789 to chemical engineering, which ranks it 5th after polymer science, material science, physical chemistry, and multi-disciplinary chemistry. A bibliometric analysis recognizes four research clusters: polymers and nano-composites, alloys and kinetics, nano-particles and drug delivery, and fibres.
An exact transient analytical solution of the temperature fields and heat fluxes for the modified transient plane source (MTPS) method is presented. The MTPS sensor is made of a thin disc with a heater/sensor and a heated guard ring on its surface, providing a one-dimensional heat flow to a semi-infinite sample in contact with its surface. The MTPS sensor measures the thermal effusivity of the sample. Two different solutions are derived, one is in the form of an infinite series with no thermal resistance, the other is in integral form and includes thermal resistance between the MTPS sensor surface and the measured sample. The theoretical model is supplemented by experimental results.
The paper will review the feasibility of adapting the Modified Transient Plane Source (MTPS) method as a screening tool for early-detection of explosives and hazardous materials. Materials can be distinguished from others based on their inherent thermal properties (e.g. thermal effusivity) in testing through different types of barrier materials. A complimentary advantage to this technique relative to other traditional detection technologies is that it can penetrate reflective barrier materials, such as aluminum, easily. A strong proof-of-principle is presented on application of the MTPS transient thermal property measuring in the early-screening of liquid explosives. The work demonstrates a significant sensitivity to distinguishing a wide range of fluids based on their thermal properties through a barrier material. The work covers various complicating factors to the longer-term adoption of such a method including the impact of carbonization and viscosity. While some technical challenges remain, the technique offers significant advantages in complimenting existing detection methods in being able to penetrate reflective metal containers (e.g. aluminum soft drinkscans) with ease.
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