9.2 Thermogravimetric analysis (TGA)

Thermogravimetric analysis (TGA) measures how a sample's mass changes with temperature. It's a key tool for understanding material properties, thermal stability, and decomposition processes. TGA data helps scientists figure out what's in a material and how it behaves when heated.

TGA has wide-ranging applications in materials science, chemistry, and engineering. By analyzing TGA curves, researchers can determine things like moisture content, decomposition temperatures, and oxidation behavior. This info is crucial for developing new materials and improving existing ones.

Thermogravimetric Analysis Principles

TGA Instrumentation and Components

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• TGA is a thermal analysis technique that measures the change in mass of a sample as a function of temperature or time under a controlled atmosphere
• The basic components of a TGA instrument include
  • High-precision balance
  • Programmable furnace
  • Sample pan (usually made of platinum, alumina, or aluminium)
  • Thermocouple
  • Gas flow system
• The sample is placed in a refractory pan, which is suspended from the balance and heated in the furnace
• The atmosphere in the furnace can be controlled by purging with
  • Inert gases (nitrogen or argon)
  • Reactive gases (air or oxygen)

TGA Measurement and Thermal Events

• The mass change is continuously monitored and recorded as the sample is subjected to a controlled temperature program, typically a linear ramp at a constant heating rate
• TGA can be used to study various thermal events
  • Decomposition
  • Oxidation
  • Reduction
  • Dehydration
  • Adsorption/desorption

Analyzing TGA Data for Material Properties

Thermograms and Composition Analysis

• TGA data is presented as a thermogram, which plots the mass change (in percent or absolute units) against temperature or time
• The composition of a material can be determined by analyzing the mass loss steps in the TGA curve, which correspond to the loss of specific components or functional groups
• Quantitative analysis can be performed by
  • Measuring the mass loss associated with each step
  • Comparing it with the theoretical mass loss expected for the proposed composition

Thermal Stability Assessment

• The thermal stability of a material can be assessed by evaluating
  • Onset temperature of decomposition
  • Temperature range over which mass loss occurs
  • Residual mass at the end of the experiment
• Derivative thermogravimetry (DTG) curves, obtained by plotting the first derivative of the mass change against temperature, can be used to
  • Identify overlapping mass loss events
  • Determine the temperature at which the rate of mass loss is maximum

Interpreting TGA Curves

Key Features and Their Significance

• The onset temperature is the temperature at which a significant mass loss begins, indicating the start of a thermal event such as decomposition or dehydration
• Mass loss is the decrease in sample mass observed during a specific temperature range, which can be expressed as a percentage of the initial mass or in absolute units
• The residual mass is the mass remaining at the end of the TGA experiment, which may represent the non-volatile components or the thermally stable residue

Curve Characteristics and Thermal Events

• Plateaus in the TGA curve indicate temperature ranges where no significant mass change occurs, suggesting thermal stability or the completion of a thermal event
• Abrupt changes in the slope of the TGA curve signify the onset or completion of a mass loss event, while gradual changes may indicate a slow or multi-step process

TGA Applications for Thermal Processes

Decomposition and Kinetics Studies

• Thermal decomposition can be studied by analyzing the mass loss steps in the TGA curve, which correspond to the breaking of chemical bonds and the formation of volatile products
• The decomposition mechanism and kinetics can be investigated by
  • Varying the heating rate, the sample mass, and the atmosphere (inert or reactive)
  • Applying appropriate kinetic models

Oxidation and Dehydration Processes

• Oxidation reactions can be examined by performing TGA experiments in the presence of oxygen or air, where the mass gain due to the formation of oxidation products is observed
• Dehydration processes can be characterized by the mass loss associated with the removal of water or other solvents, which can provide information about
  • Hydration state
  • Strength of the solvent-sample interactions

Coupling with Other Analytical Techniques

• TGA can be coupled with other analytical techniques to gain further insights
  • Mass spectrometry (MS) to identify the evolved gases
  • Fourier-transform infrared spectroscopy (FTIR) to analyze the decomposition products and mechanisms