Thermal Analysis: TGA

Professor Maria L. AuadE-mail: auad@auburn.edu

Phone: 334 844-5459Office: 103 Textile Building

2TG or TGA: thermal gravimetric analysis or thermogravimetry. Measures the mass change of the sample with a thermo-balance. A variation on this is DTG, or derivative thermogravimetry, which measures the slope or derivative of the mass change with temperature, dm/dt.

In this technique (TG or TGA), changes in the mass of a sample are studied while the sample is subjected to a controlled temperature change.

The temperature programmed is most often a linear increase in temperature, but isothermal studies can also be carried out, when the changes in sample mass with time are followed.

3The main processes amenable to study are:

Process Weight gain

Weight loss

Ad- or absorption *Desorption *Dehydration/desolvation *Sublimation *Vaporization *Decomposition *Solid-solid reactions *

Solid-gas reactions * *

TG is inherently quantitative, and therefore an extremely powerful thermal technique, but gives no direct chemical information. The ability to analyze the volatile products during a weight loss is of great value (FTIR techniques).

4Instrumentation

The essential components of the equipment used, called a thermobalance, are a recording balance, furnace, temperature programmer, sample holder, an enclosure for establishing the required atmosphere, and a means of recording and displaying the data.

Balance sensitivity is usually around one microgram, with a total capacity of a few hundred milligrams. A typical operating range for the furnace is ambient to 1000C, with heating rates up to 100C/min. The quality of the furnace atmosphere deserves careful attention, particularly the ability to establish an inert (oxygen-free) atmosphere, and it is useful to be able to quickly change the nature of the atmosphere. Compatibility between the materials of construction and the sample and its decomposition products, and the gaseous atmosphere, must be considered. Sample holder materials commonly available include aluminium, platinum, silica, and alumina.

Indication of the sample temperature is by a thermocouple close to the sample. Careful calibration for temperature is important, especially for kinetic studies. Various means are available for temperature calibration, which is not a trivial matter, though reproducibility is often more important than absolute accuracy. Weight calibration is readily achieved using standard weights.

5Factors affecting the TG curve

-The primary factors are heating rate and sample size, an increase in either of which tends to increase the temperature at which sample decomposition occurs, and to decrease the resolution between successive mass losses.

-The particle size of the sample material, the way in which it is packed, the crucible shape, and the gas flow rate can also affect the progress of the reaction.

Careful attention to consistency in experimental details normally results in good repeatability.

Calcium Oxalate Decomposition

1st Step CaC2O4H2O (s) CaC2O4 (s) + H2O (g)Calcium Oxalate Monohydrate Calcium Oxalate

2nd Step CaC2O4 (s) CaCO3 (s) + CO (g) Calcium Oxalate Calcium Carbonate

3rd Step CaCO3 (s) CaO (s) + CO2 (g)Calcium Carbonate Calcium Oxide

7The measured losses above agree well with the theoretical losses, according to the usual scheme

CaC2O4.H2O CaC2O4 CaCO3

calcium oxalate monohydrate

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Calcium Oxalate RepeatabilityOverlay of 8 runs, same conditions

9The plot also shows the derivative of the TG curve, or the DTG curve, which is often useful in revealing extra detail, such as the small event around 400C, which would not have been seen on the TG curve itself. The DTG curve is sometimes used to determine inflection points on the TG curve, to provide reference points for weight change measurements in systems where the weight losses are not completely resolved.

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Applications

Thermal Stability: related materials can be compared at elevated temperatures under the required atmosphere. The TG curve can help to elucidate decomposition mechanisms.

Kinetic Studies: a variety of methods exist for analyzing the kinetic features of all types of weight loss or gain, either with a view to predictive studies, or to understanding the controlling chemistry.

Material characterization: TG and DTG curves can be used to "fingerprint" materials for identification or quality control.

Corrosion studies: TG provides an excellent means of studying oxidation, or reaction with other reactive gases or vapours.

Simulation of industrial processes: the thermobalance furnace may be thought of as a mini-reactor, with the ability to mimic the conditions in some types of industrial reactor.

Compositional analysis: by careful choice of temperature programming and gaseous environment, many complex materials or mixtures may be analyzed by selectively decomposing or removing their components. This approach is regularly used to analyze e.g. filler content in polymers; carbon black in oils; ash and carbon in coals, and the moisture content of many substances.

The ability of TG to generate fundamental quantitative data from almost any class of materials, has led to its widespread use in every field of science and technology. Key application areas are listed below:

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The next curve was obtained using a variable heating rate technique. Although there is still incomplete separation of the degradation of the two components, measurement of the mass change between the plateau either side of the saddle in the derivative mass loss curves allows quantitative analysis of each of the components which agrees well with theoretical calculations based upon the thickness of each layer. Auxiliary experiments on the pure components show that the nylon-6 decomposes before the LDPE.

nylon-6 LDPE blend

nylon-6

LDPE

Test for Oxygen Contamination of N2 Purge Gas

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Copper Oxalate

Effect of Oxygen on Copper Oxalate Decomposition

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Calcium Oxalate in Air Calcium Oxalate in Nitrogen

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Inert vs. Oxidative Purge Gas

Simultaneous DSC-TGA

Calcium Oxalate Decomposition as a Function of Atmosphere Carbon Monoxide Oxidizes in the Presence of Oxygen