Thermal Analysis Techniques and Applications

Thermal Methods

41.1 Introduction to Thermal Methods of Analysis

• Heating matter causes physical and chemical changes across a wide temperature range.

• Physical Changes: Melting, boiling, occur at varying temperatures depending on material.

• Chemical Changes: Decomposition/reactions also occur at different temperatures.

• Characteristic changes in temperature and heat during reactions help to identify materials and their compounds present.

• Information from thermal analysis is useful for predicting the service lifetime of industrial materials like varnishes and paints.

• Definition of Thermal Analysis: Techniques that determine and/or record physical parameters as functions of temperature.

• Major techniques include:

  1. Thermogravimetry (TG)

  2. Differential Thermal Analysis (DTA)

  3. Thermometric Titrations (TT)

  4. Differential Scanning Calorimetry (DSC)

  5. Pyrolysis-Gas Chromatography (Pyrolysis-GC)

  6. Thermomechanical Analysis (TMA)

Table 41.1: Some Thermal Analysis Techniques

• Thermal methods study thermal events by recording changes in thermal properties, resulting in a thermal analysis curve (thermogram).

• Curves characterize a sample qualitatively and quantitatively but may be complex to interpret.

• Combination of multiple thermal techniques (e.g., TG with DSC or DTA) can yield comprehensive results.

Table 41.2: Thermal Events

• Examples include phase transitions (e.g., melting, sublimation), thermal decomposition, oxidation, combustion, etc.

Expanding Definitions and Techniques

• According to Wendlandt, any analytical technique measuring physical parameters as functions of temperature can be a thermal analysis method including techniques like NMR and mass spectrometry.

41.2 Thermogravimetry (TG)

Introduction

• Definition: Technique where the weight of a substance is measured in a controlled heating/cooling environment as a function of time or temperature.

• Types of thermogravimetry:

  1. Isothermal (Static): Weight recorded over time at constant temperature.

  2. Quasistatic: Sample heated to constant weight at series of increasing temperatures.

  3. Dynamic: Sample heated at a predetermined rate (usually linear).

Recording of Results

• Thermobalance: Precision balance programmed for linear temperature rises, plotting weight change vs. temperature/time, resulting in a TG curve.

• Weight plotted on the ordinate (y-axis) decreases downward, while temperature/time is on the abscissa (x-axis), increasing from left to right.

Information from a TG Curve

1. Plateaus (Horizontal Portions): Indicate thermal stability.

2. Curved Portions: Indicate weight losses corresponding to the decomposition processes.

3. Procedural Decomposition Temperature (pdt): Minimum temperature indicating detectable cumulative mass change.

4. Final Temperature (TF): Indicates maximum cumulative weight change corresponding to complete reaction.

5. Reaction Interval (TI-TF): Difference between initial and final temperatures defines the area of interest in thermal analysis.

• Examples from TG data:

  • Calcium carbonate (CaCO3) decomposition shows a 44% weight loss at 850°C. Reaction: $ext{CaCO}_3 <br>ightarrow ext{CaO} + ext{CO}_2$

Factors Affecting Thermogravimetric Curves

• Influences on TG curves can be categorized into

  1. Instrumental Effects (e.g., heating rate, atmosphere, sample holder geometry).

  2. Sample Characteristics (e.g., weight, particle size, heat of reaction, compactness, previous history).

Instrumentation for Thermogravimetry

• Modern thermobalances include components like recording balances, furnaces, temperature controllers, and data recorders. Each component's specifics depend on application.

Applications of Thermogravimetry

1. Automatic Thermogravimetric Analysis: Fast procedures for determining the composition of precipitates and mixtures.

2. Evaluation of Gravimetric Precipitates: Determine drying temperatures, purity tests, and standard evaluations in analytical chemistry.

3. Curie Point Determination: Evaluate changing ferromagnetic properties relating to temperature.

• Organic compounds demonstrated by TG techniques include decomposition studies which demonstrate stability and weight loss characteristics crucial for identification.

41.3 Differential Thermal Analysis (DTA)

Introduction

• In differential thermal analysis, the temperature differences between a sample and reference material are recorded as they undergo controlled thermal conditions, revealing endothermic and exothermic peak indicators.

Theoretical Basis of DTA

• Mathematical analysis of the DTA curve relates peak areas to sample characteristics, influenced by assumptions regarding uniform temperature and physical constants between samples and references.