Thermal Analysis Methods


Overview of Thermal Analysis

  • Thermal analysis encompasses a collection of techniques measuring changes in a property as a function of temperature.
      - Thermogravimetric Analysis (TGA)
      - Differential Thermal Analysis (DTA)
      - Differential Scanning Calorimetry (DSC)

Thermogravimetric Analysis (TGA)

  • Definition: TGA measures the mass and rate of change in mass of a material as a function of temperature or time in a controlled atmosphere.

  • Instrument Components:
      - Thermobalance (very sensitive)
      - Furnace
      - Gas-purging system
      - Computer

Key Learnings from TGA

  • Thermal Stability of Materials

  • Oxidative Stability of Materials

  • Composition of Multi-Component Systems

  • Estimated Lifetime of a Product

  • Decomposition Kinetics of Materials

  • Effect of Reactive or Corrosive Atmospheres on Materials

  • Moisture and Volatiles Content of Materials

Sample & Sample Pans

  • Common Materials:
      - Platinum:
        - Pros: Easy to clean, nonporous, reusable.
        - Cons: Alloys with most metals.
      - Alumina (Ceramic):
        - Suitable for corrosives/inorganics, reusable, allows for large samples.
      - Aluminum (TGA):
        - Cost-effective but limited to low temperatures (<600°C).

  • Typical sample sizes are 5-20 mg, with 50-100 mg for volatile measurements.

  • TGA instruments generally have a baseline drift of ±0.025 mg (equivalent to 0.25% of a 10 mg sample).

TGA Curves (Thermograms)

  • Defined as plots of weight/weight% vs. time/temperature.

  • Mass losses/gains correspond to chemical changes at different temperatures.

  • Example Observations (coal sample):
      - Loss of H2O and volatile contaminants under nitrogen.
      - Combustion of carbon under oxygen.

Combined Thermal Instruments

  • Some instruments can simultaneously measure mass change and heat flow, referred to as simultaneous differential thermal (SDT) analysis.

  • Advantages of SDT:
      - Provides transition temperatures, heats of fusion and reactions, melting and boiling points that TGA alone cannot.

Temperature Measurement in TGA

  • Important for analyzing material characteristics and contamination levels.

  • Calibration Method:
      - Curie Point Transition Method:
        - The Curie Point Temperature is where a paramagnetic material loses its magnetic susceptibility.
        - Utilizes magnets and well-characterized transition materials.
        

TGA Applications

  • Widely used in:
      - Pharmaceutical industry
      - Polymer/plastics industry
      - Research labs

High-Resolution TGA

  • Complex materials can exhibit broad weight losses. Slower heating rates yield better resolution but are time-intensive.

  • Method:
      - Heat quickly until weight loss indicates changes, then proceed with slower heating rates for detailed measurements.

    derivate plotcalcium oxalate
Evolved Gas Analysis

  • While TGA determines mass loss, it does not identify lost species. Analysis of gases in the effluent stream can help identify these species.

  • Common analyzers include FTIR and MS (Mass Spectrometry).

Application of Evolved Gases Analysis

  • Applicable across several domains:
      - Polymers: Study composition and hazard evaluation.
      - Natural Products: Contamination assessment and material selection.
      - Catalysts: Analyze product/by-product conversions.
      - Inorganics: Elucidate reactions and stoichiometry.
      - Pharmaceuticals: Stability studies and residual solvent analysis.

Specific Techniques - TGA-FTIR and TGA-MS

  • TGA-FTIR:
      - Advantages: Online measurement, easy identification of hydrocarbons.
      - Disadvantages: Cannot detect inert gases (no dipole moment) or limited detection of inorganic gases.

  • TGA-MS:
      - Effluent gas is fed into the mass spectrometer, requiring specialized "atmospheric sampling" setups.

Differential Thermal Analysis (DTA)

  • DTA involves two sample pans subjected to a controlled temperature program, with one containing the material of interest and another a reference material.

  • Key Measurement: Temperature difference (ΔT) as a function of the sample temperature.

DTA Observations

  • ΔT occurs during processes where ΔH ≠ 0, such as:
      - Phase transitions
      - Chemical reactions

  • For polymers, the glass transition temperature (Tg) is indicated through a subtle baseline shift rather than a ΔH change.

DTA Applications

  • Qualitative analysis provides temperatures of processes but does not quantify energy. Widely used for studying polymers, ceramics, and metals, with temperatures reaching up to 2400 °C.

Differential Scanning Calorimetry (DSC)

  • Similar to DTA, but DSC quantifies energy associated with temperature changes. Measures heat flow differences to prevent a ΔT between the sample and reference.

  • Thermograms: Typically plotted as heat flow vs. temperature.

Types of DSC Instruments

  • Power-Compensated DSC: Measures power difference needed to maintain both sample and reference at the same temperature.

  • Heat-Flux DSC: Utilizes a single heater to monitor heat flow differences.

  • Modulated DSC: Offers additional variation in heat flow.

DSC Applications

  • Widely utilized for polymer studies, including:
      - Tg determination
      - Melting point measurements
      - Crystallinity assessments
      - Reaction kinetics, particularly monitoring polymerization.

Conclusion: Key Applications of DSC

  • Purity Analysis: Evaluates the purity of compounds, such as dimethyl terephthalate (DMT).

  • Analysis for Lithium Batteries: Investigates thermal runaway processes and component decomposition.

  • Estimation of Tg values from normalized heat flow data provides important material characteristics in polymer science.