Thermogravimetry

Thermogravimetry (also knows by acronym "TG" and obsolete names thermo-gravimetry, thermogravimmetry) is a branch of physical chemistry, materials research, and thermal analysis. It is based on continuous recording of mass changes of a sample of material, as a function of a combination of temperature with time, and additionally of pressure and gas composition.

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Process

A sample of material (ranging from 1 mg to 100 mg, but sometimes as large as 100 g) is placed on an arm of a recording microbalance, also called thermobalance where that arm and the sample are placed in a furnace. The furnace temperature is controlled in a pre-programmed temperature/time profile (most commonly), or in the rate-controlled mode, where the pre-programmed value of the weight changes imposes the temperature change in the way necessary to achieve and maintain the desired weight-change rate. The most common temperature profiles are: jumping to isotherm and holding there for a specified time ("soak"), temperature ramping at constant rate (linear heating or cooling, and combination of ramp and soak segments. The profile "ORTA" ("Oscillation-Rate Thermal Analysis") is used in other methods of thermal analysis, but not in TG, due to unavoidable disturbance forces. The rate-controlled method is very time-efficient, but for some types of materials it produces incorrect results or "ghost effects"; since this method does not reveal the automatically-imposed temperature profile, the users may be mislead by their trust for the "sophisticated, computerized program, which saves the analysis time tremendously".

The gaseous environment of the sample can be: ambient air, vacuum, inert gas, oxidizing/reducing gases, corrosive gases, carburizing gases, vapors of liquids or "self-generating atmosphere". The pressure can range from high vacuum or controlled vacuum, through ambient, to elevated and high pressure; the latter is hardly practical due to strong disturbances.

The commonly investigated processes are: thermal stability and decomposition, dehydration, oxidation, determination of volatile content and other compositional analysis, binder-burnout, high-temperature gas corrosion etc. The kinetic data obtained by TG are reliable only for irreversible processes, whereas reversible ones are grossly affected by diffusion, and only special procedures can handle them. Although many industrial processes could benefit from thermogravimetric investigations, the industry is often discouraged by the natural discrepancies between the data produced by milligram-size samples, and those of the bulk processes. In this respect gram-size and larger TG samples are more suitable for optimization research of industrial processes.

The conventional TG focuses on various aspects of ANALYSIS of materials; the other facet of thermogravimetry is studying SYNTHESIS, e.g. using a thermobalance for monitoring of making of materials. The industrial processes of CVD, chemical vapor deposition, CVI, chemical vapor infiltration, metallurgical carburization, synthesis of carbon-carbon composites can greatly benefit from modeling them with large-sample TG instruments.