Thermogravimetric analysis

Thermogravimetric analysis, often called TGA, is a vital method of thermal analysis. It allows scientists to measure the mass of a sample as the temperature changes over time. This process reveals how materials react to heat through physical or chemical changes. Physical phenomena include phase transitions, absorption, adsorption, and desorption. Chemical phenomena include chemisorptions, thermal decomposition, and solid-gas reactions like oxidation or reduction. By tracking weight changes, researchers can understand the fundamental nature of a substance.

The mechanism of TGA relies on a specialized instrument called a thermogravimetric analyzer. This device contains a precision balance and a sample pan. The pan is placed inside a furnace that has programmable temperature controls. As the furnace heats the sample, the balance continuously measures its mass. The temperature usually increases at a constant rate to trigger a thermal reaction. Scientists can also control the atmosphere surrounding the sample during this process. They might use ambient air, a vacuum, or inert gases. Other options include oxidizing, reducing, corrosive, or carburizing gases. They may even use liquid vapors or a self-generated atmosphere.

There are three distinct types of thermogravimetry used depending on the research goal. The first is isothermal or static thermogravimetry. In this method, the sample weight is recorded at a constant temperature over time. The second type is quasistatic thermogravimetry. This technique raises the temperature in sequential steps. These steps are separated by isothermal intervals where the mass reaches stability. The third type is dynamic thermogravimetry. In this approach, the sample is heated in an environment where the temperature changes in a linear manner.

Data from these tests is compiled into a TGA curve. This is a plot where the y-axis shows mass or the percentage of initial mass. The x-axis represents either temperature or time. Scientists often use the first derivative of this curve, known as the DTG curve. The DTG curve helps determine inflection points for deeper interpretations. This data is essential for materials characterization through decomposition patterns. It allows researchers to see exactly how and when a material loses its structure.

One major application of TGA is evaluating thermal stability. If a species is thermally stable, there will be no observed mass change in a specific temperature range. A lack of slope in the TGA trace indicates little or no mass loss. TGA also identifies the upper use temperature of a material. Beyond this point, the material will begin to degrade. This is especially useful for studying polymers like thermoplastics, thermosets, and elastomers. Most polymers melt or degrade before reaching 200 °C. However, some thermally stable polymers can withstand 300 °C in air. In inert gases, these special polymers can reach 500 °C without losing strength.

NASA uses TGA for important research on advanced copper alloys. They are studying these alloys for possible use in combustion engines. Scientists must check for oxidative degradation, which occurs when copper oxides form in oxygen-rich atmospheres. Understanding resistance to oxidation is vital because NASA wants to reuse shuttle materials. TGA can also detect inconsistencies in materials like unpurified carbon nanotubes. If these nanotubes contain metal catalysts, they may undergo combustion. This causes a rapid temperature change and a dramatic slope change in the DTG plot. Such deviations can help diagnose if a sample is anisotropic. For example, testing the top and bottom of a sample can reveal if particles have undergone sedimentation.

Finally, TGA is often used in combination with other analytical methods. It can be coupled with Fourier-transform infrared spectroscopy (FTIR) and mass spectrometry. This allows for gas analysis as the sample decomposes. The analyzer can heat samples to temperatures as high as 1000 °C during these tests. This helps measure the weight percentage of each resulting mass change. While TGA measures weight, another method called Differential Thermal Analysis (DTA) measures temperature differences. DTA can reach higher temperatures, sometimes as high as 1600 °C. Together, these tools provide a complete picture of how matter behaves under heat.