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In thermogravimetric analysis (TGA), a sample is continually weighted while heating, as an inert gas atmosphere is passed over it. Many solids undergo reactions that evolve gaseous byproducts. In TGA, these gaseous byproducts are removed and changes in the remaining mass of the sample are recorded. Three variations are commonly employed:

• Dynamic TGA – Temperature continues to increase over time as mass is recorded. This allows simulataneous identification of how much gas is removed and the temperature at which it occurs.
• Static TGA – Temperature is held constant as the mass is measured. This can be used to gain more information on a decomposition that happens at a certain temperature or to investigate a material’s ability to withstand a given temperature.
• Quasistatic TGA – Sample is heated in multiple temperature intervals, and held at those intervals for a time, often until the mass stabilizes. This is ideal for investigating substances that are known to decompose in various ways at different temperatures, and better characterizing the way in which they decompose.

How to interpret the data

Figure 1 shows a TGA curve in green. Figure from Physical Methods in Chemistry and Nanoscience by Pavan M.V. Raja and Andrew R. Barron (chemlibretext link).

Data from thermogravimetric analysis is often shown by a graph representing mass as a function of temperature for dynamic TGA. For static TGA, mass is instead plotted as a function of time at a given temperature. Quasistatic TGA produces multiple mass vs. time plots for various temperatures. The derivative of the mass change with temperature is often plotted on the same graph to improve ease of identifying the points at which different mass changes occur; especially helpful in cases where multiple decomposition reactions happen in close proximity to one another.

Examples of some characteristic TGA curves for dynamic TGA

Figure 1. Classification of the different observable TGA Curves. Image source

In general, mass fluctuations correspond to chemical reactions, with some exceptions. A common example is drying, which can easily be seen as a quick initial drop at the beginning of heating that isn’t known to correspond to any chemical reactions. Evaporation/sublimation may also appear on the plot depending on the material to be analyzed. Multistage decomposition is also common, and shows as a step-like pattern. In some cases, these steps may blend together during dynamic TGA, necessitating either far slower heating rates, or step-wise methods like quasistatic TGA. Note that TGA itself may not be sufficient to identify the decomposition products; chemical testing of the sample after TGA analysis is often required to ascertain the identities of suspected decomposition products. TGA itself does not identify substances; other methods such as chemical testing or differential calorimetry must performed alongside TGA to verify the identity of products. 

Good literature examples 

TGA is vital when designing materials that are intended to withstand high temperatures, as if there is even slight decomposition of the material at a temperature that the material would be expected to encounter, devices made of the material may fail over repeated use. The carefully controlled environment of the TGA analyzer also allows for measuring decomposition reaction kinetics. Differential scanning calorimetry can be incorporated into the TGA analyzer to allow for monitoring potential phase changes. Phase changes generally require addition of heat, yet do not increase the temperature of the sample undergoing a phase change. Furthermore, different phases of a material have different heat capacity, and the temperature change per joule of heat applied will vary with phase. By adding a reference pan to the TGA analyzer, changes in the heat capacity in addition to mass changes can be monitored. In this way, both phase changes and thermal decomposition reactions can be simultaneously measured by TGA.

TGA used for decomposition reaction:

Figure S6. Thermogravimetric analysis (TGA) under pure nitrogen flow at 100 mL/min to show a) clean decomposition of 3DP-HKUST-1gel and b) decomposition of 3DP-HKUST-1gelTEA showing that it has several side products during decomposition. (Lim et al. 2019)

Figure 2b. Thermogravimetric analysis (TGA) under simulated ambient conditions (SI section 5), showing desolvation followed by oxidation of 3DP-HKUST-1gel to CuO.

The authors are looking to use colloidal gels containing only ethanol and Cu3(BTC)2 (BTC = 1,3,5-benzenetricarboxylate) (HKUST-1) nanoparticles as ink for the direct ink writing (DIW) of pure densely packed and self-standing Metal-organic frameworks (MOF) monoliths. Traditionally they are synthesized in powder form. The authors are observing the decomposition behavior of the 3DP-HKUST-1gel (made using DIW) and 3DP-HKUST-1gel-TEA (made by triethylamine-induced HKUST-1 gels). It can be observed in the sudden change in weight over the 100-200 °C for Figure S6b that several side products have formed as oppsed to Figure S6a which shows a much cleaner decomposition. The Figure seen in the paper was Figure 2b, the authors attibute the first weight change (16.2 mg) to residual molecules such as H2O, acetate from the copper (II) acetate monohydrate precursor, and excess ethanolic solvent that is trapped inside the 3DP-HKUST-1gel structure. The second weight change (6.2 mg) was observed at 300 °C and is caused by the decomposition of the organic linkers and network. 

In-depth reading and works cited

1. Physical Methods in Chemistry and Nanoscience by Pavan M.V. Raja and Andrew R. Barron (chemlibretext link).
2. A Beginners Guide Thermogravimetric Analysis (TGA): A Beginners Guide to TGA that has FAQ and basic information.
3. Thermogravimetry Analysis (TGA) – online training course: Youtube training course that as good explanation about how the TGA works. Also has good examples of plots and how to understand them.
4. Thermogravimetric Analysis (TGA) & Differential Scanning Calorimetry (DSC): Slideshow with information about the TGA. Includes figures on classification of curves, how balance works, analyis of curves, effects of heat rate, shifts caused by heat rate, etc.
5. THERMAL ANALYSIS OF POLYMERS – Fundamentals and Applications: (WARNING!: This link will download a whole 388 pg. book!) Has a whole chapter dedicated to TGA.

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TGA is a powerful and robust technique to explore the thermal stability of a material. By accurately monitoring the weight of a sample while heating at a constant rate, we can measure changes in a sample’s weight and attribute this to a specific material response to a thermal stress (Figure 1). This is perfect for exploring, in detail, decomposition temperatures and ensuring a material performs adequately in a given temperature range.

During a test, a carrier gas flows over the sample and the weighing mechanism.  This carrier gas serves two purposes: protecting the internals from corrosion/oxidation above 500 °C, and to interact with the sample through gas-solid or gas-liquid reactions. By providing an inert atmosphere, we can test the thermal stability of a material. Reducing environments can explore gas-solid phase reduction reactions or protect specific samples from being oxidized. By the nature of the sensitivity of the TGA balance, we also can observe the absorption of gas onto a porous material at various temperatures. This is an ideal technique for exploring metal organic frameworks, or catalytic porous materials. Owing to the sensitivity of the balance on our SETARAM Labsys Evo instrument, we have ideal sensitivity for running thermokinetic experiments. This experiment has applications towards thermal stability of a given material at elevated temperatures.

The SETARAM Labsys offers the ability to perform TGA analysis up to 1600 °C and additionally under a variety of gas mixtures due to our gas-mixing option.

Differential Scanning Calorimetry (DSC)

DSC is a flexible technique to explore thermal transitions within a given sample. By heating a sample and measuring the heat flow as compared to a reference standard, we can access thermoanalytical information on a given material. DSC curves are generated by plotting heat flow (mW) vs sample temperature (°C), and an example plot is found in Figure 2, and demonstrates the melting and fusion of Indium Corp. Indalloy 80Au/20Sn solder. In this case, we observe a phase change (solid-liquid followed by liquid-solid); however, we can also observe other thermal transitions within a material, such as evaporation, thermal transitions between polymorphs, and determination of key thermal constants. One of these key thermal constants is heat capacity (Cp), which an be difficult to acquire due to the demanding experiment required to gain access to this information. Heat capacity requires precise and very specific sensitivity of the DSC sensor, as it is a very small and difficult thermal effect to capture effectively. Owing to our 3D Calvet type sensor on our µDSC 7, we have the capability to measure such small thermal effects (0.02 µW) requiring the upmost sensitivity and precision from 0 – 120 °C. For higher temperatures, our SETARAM Labsys Evo instrument has a specifically designed 3D-psuedo-Calvet sensor, which allows TAL to perform C_p testing with better than 2% accuracy from ambient temperatures to 1600 °C. This very sensitive and difficult to determine thermal effects also include the glass transition point (T_g), water state in materials and other thermodynamic and thermokinetic effects.

Differential Thermal Analysis (DTA)

DTA is a similar technique to DSC, however instead of measuring the heat flow between the furnace temperature and the sample, you measure the temperature difference between a sample and a standard reference using thermocouples. This is particularly useful for phase-change materials and the study of organic and polymeric materials using analytical precision. Owing to the more sensitive detector within typical DTA sensors, DTA testing is particularly useful for running thermokinetic experiments due to the lower thermal inertial barrier. TAL offers DTA testing from ambient to 1200 °C, allowing us to explore thermal effects at elevated temperature, capabilities which we have newly acquired.

While each of these techniques may be used to probe into a single physical characteristic of a material, the real power we provide is simultaneous thermal analysis techniques for niche applications on a single sample. With TAL’s Labsys Evo 1600 and DSC 131, we offer capabilities to include high temperature ranges with mixtures of gases. For example, TAL offers TGA, DSC and DTA experiments from ambient temperatures up to 1600 °C. Figure 3 shows a TGA-DSC decomposition experiment captured on our Setaram Labsys Evo apparatus, where CuSO₄•5H₂O decomposition can be measured by TGA (water, SO₂ and O₂) and DSC. In addition to these coupled thermal experiments, we offer the capacity to provide gas mixtures for high precision control over the exact atmospheric exposure during thermal analysis runs. TAL also offers the capacity to run samples under vacuum (10^-3 Bar) using our Labsys Evo system, which is useful for isolating gas-sample interactions. TAL also offers pressurized DSC experimentation, allowing for the study of various samples under 200 psi. This can provide insight into thermal transitions under pressure, such as those in the oil and fuel industry or in the case of high-pressure lubricants.