Come friends, today we will know about Thermogravimetric Analysis. Thermogravimetric analysis or thermal gravimetric analysis ( TGA ) is a method of thermal analysis in which the mass of a sample is measured as temperature changes over time . This measurement provides information about physical phenomena, such as phase transition , absorption , adsorption and desorption ; As well as chemical phenomena including chemisorption , thermal decomposition , and solid-gas reactions (eg, oxidation or reduction ).
thermogravimetric analyzer
Thermogravimetric analysis (TGA) is conducted on an instrument called a thermogravimetric analyzer. A thermogravimetric analyzer measures mass continuously while the temperature of a sample varies over time. In thermogravimetric analysis, mass, temperature and time are considered as base measurements, while many additional measures can be derived from these three basis measurements.
A typical thermogravimetric analyzer consists of a programmable control temperature with a precise balance with the sample pan located inside a furnace. The temperature is typically raised at a constant rate (or for some applications the temperature is controlled for a constant mass loss) to produce a thermal response. Thermal reactions can occur in a variety of environments, including: ambient air , vacuum , inert gases, oxidizing/reducing gases, corrosive gases, carburizing gases, vapors of liquids or “self-formed environments”; As well as different types of pressure including: a high vacuum, high pressure, constant pressure, or a controlled pressure.
Thermogravimetric data collected from the thermal reaction is compiled into a plot of the percentage of mass or initial mass on the y axis versus temperature or time on the x-axis. This plot, often smoothed , is called the TGA curve . The first derivative of the TGA curve (DTG curve) can be plotted to determine useful inflection points for in-depth interpretations as well as differential thermal analysis .
A TGA can be used for material characterization through analysis of characteristic decomposition patterns. It is a particularly useful technique for the study of polymer materials, including thermoplastics , thermosets , elastomers , composites , plastic films , fibers , coatings , paints , and fuels .
Types of TGA
There are three types of thermogravimetry:
- Isothermal or static thermogravimetry: In this technique, the sample weight is recorded as a function of time at a constant temperature.
- Quasistatic thermogravimetry: In this technique, the sample temperature is raised in sequential steps separated by isothermal intervals, during which the sample mass reaches stability before the start of the next temperature ramp.
- Dynamic thermogravimetry: In this technique the sample is heated in an environment whose temperature changes in a linear manner.
Application
thermal stability
TGA can be used to evaluate the thermal stability of a material. In a desired temperature range, if a species is thermally stable, there will be no mass change. Negligible mass loss corresponds to little or no slope in the TGA trace. TGA also gives the upper use temperature of the material. Beyond this temperature the material will begin to deform.
TGA is used in the analysis of polymers. Polymers are usually melted before being decomposed, thus TGA is mainly used to investigate the thermal stability of polymers. Most polymers melt or degrade before 200 °C. However, there is a class of thermally stable polymers capable of withstanding temperatures of at least 300 °C in air and 500 °C in inert gases without structural change or strength loss, which can be analyzed by TGA. Is. [2] [3] [4]
oxidation and combustion
The simplest material characterization is the residue remaining after a reaction. For example, a combustion reaction can be tested by loading a sample into a thermogravimetric analyzer under normal conditions. The thermogravimetric analyzer will cause the combustion of ions in the sample by heating it beyond its ignition temperature. The resulting TGA curve plotted along the y axis as a percentage of the initial mass will show the residue at the end point of the curve.
Oxidative mass loss is the most common observable loss in TGA.
It is very important to study the resistance to oxidation in copper alloys. For example, NASA (National Aeronautics and Space Administration) is researching advanced copper alloys for their potential use in combustion engines. However, oxidative degradation can occur in these alloys as copper oxides form in an oxygen-rich environment. Resistance to oxidation is very important because NASA wants to be able to reuse shuttle materials. TGA can be used to study the stable oxidation of such materials for practical use.
The TGA produced by combustion during TG analysis can be identified by the distinct markings made in the thermogram. An interesting example occurs with samples of impure carbon nanotubes produced in which large amounts of metal catalysts are present. Due to combustion, a TGA trace may deviate from the normal of a well-behaved function. This phenomenon results from rapid temperature changes. When weight and temperature are plotted in comparison to time, a dramatic slope change in the first derivative plot is concurrent with the mass loss of the sample and the sudden increase in temperature observed by the thermocouple. The mass loss may be the result of smoke particles released from the burn due to discrepancies in the material, beyond the oxidation of carbon due to poorly controlled weight loss.
Different weight loss on the same sample at different points can also be used as a diagnosis for the anisotropy of the sample. For example, sampling with dispersed particles on the top and bottom sides of a sample can be useful for detecting sedimentation, as the thermograms will not overlap, but will show a difference between them if the particle distribution differs from one side. . [6] [7]
thermogravimetric kinetics
Thermogravimetric kinetics can be explored for insight into the reaction mechanism of thermal (catalytic or non-catalytic) decomposition involved in the pyrolysis and combustion processes of various materials.
The activation energy of the decomposition process can be calculated using the Kissinger method.
Although a constant heating rate is more common, a constant mass loss rate can illuminate specific reaction kinetics. For example, the kinetic parameters of the carbonization of polyvinyl butyral were found using a constant mass loss rate of 0.2 wt%/min.
Operation in combination with other equipment
Thermogravimetric analysis is often combined with other procedures or used in conjunction with other analytical methods.
For example, the TGA instrument continuously weighs a sample as it is heated to temperatures of up to 2000 °C for coupling with Fourier-transform infrared spectroscopy (FTIR) and mass spectrometry gas analysis. As the temperature increases, the various components of the sample disintegrate and the weight percentage of each resulting mass change can be measured.
| No. | Thermal Gravimetric Analysis (TGA) | Differential Thermal Analysis (DTA) |
|---|---|---|
| 1 | Weight gain or loss in TGA is measured as a function of temperature or time. | In DTA the temperature difference between a sample and a reference is measured as a function of temperature. |
| 2 | The TGA curve appears as steps involving horizontal and curved parts. | The DTA curve shows the top and bottom peaks. |
| 3 | The device used in TGA is Thermobalance. | The device used in DTA is a DTA device. |
| 4 | TGA gives information only for substances that show a change in mass upon heating or cooling. | The DTA does not require a change in the mass of the sample to obtain meaningful information.DTA can be used to study any process in which heat is absorbed or released. |
| 5 | The upper temperature used for TGA is normally 1000 °C. | The upper temperature used for DTA is often higher than TGA (as high as 1600 °C). |
| 6 | Quantitative analysis is done from the thermal curve by measuring the loss in mass m. | Quantitative analysis is performed by measuring peak areas and peak heights. |
| 7 | The data obtained in TGA is useful in knowing the purity and composition of the material, the drying and ignition temperature of the material, and the stability temperature of the compounds. | The data obtained in DTA is used to determine the temperature of transitions, reactions and melting point of substances. |
