Thermal Analysis Notes

Thermal Analysis

1.0 Introduction to Thermal Analysis

• Thermal analysis techniques study changes in physical properties with temperature.

• Used to characterize materials and their behavior over a range of temperatures.

• Helps determine if materials are suitable for specific uses.

• Determines the temperature range materials can withstand without changing.

• Includes techniques like:

  • Thermogravimetric analysis (TGA)

  • Differential thermal analysis (DTA)

  • Differential scanning calorimetry (DSC)

  • Thermometric titration (TT)

  • Direct injection enthalpimetry

  • Dynamic mechanical analysis (DMA)

  • Thermomechanical analysis (TMA)

Table 16.1 Applications of Thermal Analysis Methods

| Applications | TGA | DSC/DTA | TMA | DMA |
| :--------------------- | :-: | :-------: | :-: | :-: |
| Compositional analysis | X | | | |
| Curing studies | X | X | | |
| Glass transition | | X | X | X |
| Heat of reaction | | X | | |
| Oxidative stability | X | X | | |
| Corrosion | | X | | |
| Creep | | | X | |
| Stress relaxation | | | X | |
| Thermal stability | X | | | |
| Viscoelastic properties| | | | X |
| Protein denaturation | | X | | |
| Shrinkage | | | X | |

Table 16.2 Sample Types and Properties Examined by Thermal Analysis

2.0 Thermogravimetry

• Thermogravimetry (TGA) measures the mass (weight) of a sample in a specified atmosphere as the temperature is programmed.

• Common temperature program: linear increase in temperature with time, while constantly recording the sample's mass.

• Isothermal and stepped temperature programs can also be used.

• Output: plot of mass (or mass %) vs. temperature, called a thermal curve.

TGA Thermal Curve Example: Decomposition of Calcium Carbonate

• The thermal curve shows the decomposition of calcium carbonate $CaCO_3$.

• Loss of mass is due to the release of $CO_2$ gas.

• The remaining compound is $CaO$.

Information from Weight Changes

• Determines the temperature at which a material loses or gains weight.

• Weight loss indicates decomposition or evaporation.

• Weight gain indicates adsorption or chemical reaction with the atmosphere.

• Identifies temperatures at which no weight change occurs, indicating temperature stability.

• Weight changes at specific temperatures are physical properties under experimental conditions.

• Weight lost helps determine the composition of a compound.

• Aids in identifying unknown crystals or determining the percentage of a compound in a mixture.

Example Calculation

• If pure calcium carbonate ($CaCO_3$) is heated to $850^\circ C$, it loses 44% of its weight.

• Evolved gas can be collected and identified as $CO_2$.

• If starting with 50.0 mg of $CaCO<em>3$, the mass loss due to $CO</em>2$ can be calculated.

Thermal Curve: Hydrated Calcium Oxalate

• Curve of hydrated calcium oxalate $(CaC<em>2O</em>4 \cdot nH_2O)$.

• Shows loss of adsorbed water starting at $90 ^\circ C$ and loss of bound water at around $150 ^\circ C$.

• The stable compound above $225 ^\circ C$ is anhydrous calcium oxalate $(CaC<em>2O</em>4)$.

• $Ca(COO)<em>2$ loses CO at about $450 ^\circ C$ to form $CaCO</em>3$.

• Calcium carbonate is stable until approximately $600 ^\circ C$, when it loses $CO_2$ to form $CaO$.

Analysis of Thermal Curve of Hydrated Calcium Oxalate

• Calcium oxalate $CaC<em>2O</em>4 \cdot nH_2O$ loses weight from $100 ^\circ C$ to $225 ^\circ C$ due to water evaporation.

• Driven off water includes adsorbed water and water of crystallization (hydration).

• Heating to $110 ^\circ C$ drives off absorbed water, but some bound water is also lost.

• Similarly, heating to $125 ^\circ C$ drives off absorbed water, but a different amount of bound water is lost.

• Reproducible TGA results require a constant drying temperature.

• Drying above $225 ^\circ C$ but below $400 ^\circ C$ results in a stable form of $CaC<em>2O</em>4$, suitable for gravimetric analysis.

2.1 TGA Instrumentation

• Consists of a gas-tight enclosure, sample heater, temperature programmer, balance controller, and furnace.

• Includes connections for gas input, temperature measurement (furnace and sample), and weight measurement.

Modern TGA Equipment

• Sensitive microbalance for continuous weight measurement.

• Furnace surrounding the sample holder.

• Purge gas system for inert or reactive atmospheres.

• Computer control of furnace and data collection (weight vs. temperature).

• Intelligent autosamplers available for unattended analysis.

Balance Specifications

• Balances available for sample masses from 1 to 1000 mg.

• Usual sample weight between 5 and 20 mg.

• Specialized systems for samples up to 100 g or microgram quantities.

• Small sample size requires a homogeneous or representative sample.

• Balance must be thermally isolated from the furnace; the sample holder and sample must be in the furnace.

• Vertical balance/furnace configurations are affected by buoyancy due to gas density changes with temperature.

• Horizontal configurations designed to minimize buoyancy effects.

Furnace Specifications

• Surrounds the sample and holder.

• Programmable for linear heating rates (up to $1000 ^\circ C$/min available).

• Commercial instruments heat up to $200 ^\circ C$/min from room temperature to about $1200 ^\circ C$; forced air cooling at $50 ^\circ C$/min.

• Furnaces available with upper temperatures of $1500 ^\circ C$, $1700 ^\circ C$, or $2400 ^\circ C$ for refractory and engineering materials.

• Must be purged with a desired gas to provide the correct atmosphere and remove gaseous products.

  • Argon or nitrogen for inert atmospheres.

  • Air for oxidation and combustion studies.

  • Hydrogen (with precautions) for reducing atmospheres.

• Modern instruments allow automatic switching of purge gases.

Sample Holder and Thermocouple

• Must withstand high temperatures and be inert.

• Quartz, platinum, and ceramics are used for the sample holder and other parts.

• The sample is placed in a small pan or crucible made of Pt, quartz, or ceramic.

• The thermocouple is small and placed close to the sample holder (sometimes in contact with the bottom of the sample pan).

• Thermocouples are made of chromel/alumel or Pt alloy.

• The thermocouple is never inserted directly into the sample to avoid contamination, catalytic reactions, particle size effects, sample packing effects, and weighing errors.

Temperature Calibration

• Uses the Curie temperature of ferromagnetic standard materials.

• Materials undergo specific and reversible changes in magnetic behavior at their Curie temperature.

• Standards are available for the temperature range of $242 \text{--} 771 ^\circ C$.

2.2 Analytical Applications of Thermogravimetry

• Determination of correct drying temperatures for precipitates used in gravimetric analysis.

• Identification of gases given off during temperature increase.

• Determination of residue composition using XRD, XRF, and other techniques.

Data Analysis

• Thermogram: graph of mass versus temperature (sometimes as % of original mass).

• Draw tangents of the curve to find the onset and offset points.

• $m<em>i$, $m</em>f$, and $\Delta m$ are fundamental properties of the sample.

• $T<em>i$ and $T</em>f$ depend on operating variables.

Typical TG Curves

• Various types of TG curves:

  1. No change.

  2. Desorption/drying (rerun).

  3. Single-stage decomposition.

  4. Multi-stage decomposition.

  5. As 4, but no intermediates or heating rate is too fast.

  6. Atmospheric reaction.

  7. As 6, but the product decomposes at a higher temperature.

Study of Chemical Decomposition Processes

• Identification of the formulas of starting materials.

Determination of Water of Crystallization

• If the initial sample mass was 20.00 mg, the mass loss at the first step would be 2.46 mg if there is 1 mole of water of crystallization.

• If the mass loss is 4.92 mg, there are 2 moles of water of crystallization and the formula would be $Ca(COO)<em>2 \cdot 2H</em>2O(s)$, a dihydrate.

Identification of Compounds in Mixtures

• Using TGA thermal curves for pure substances A and B, and a mixture of A and B, the composition of the mixture may be determined because A and B have unique temperatures at which mass is lost.

Organic Compound Decomposition

• Useful for studying polymers.

• Example: vinyl acetate copolymers; weight loss at $340 ^\circ C$ is due to loss of acetic acid, giving a quantitative measure of the amount of vinyl acetate in the polymer.

Decomposition Temperatures of Common Polymers

• TGA thermal curves showing decomposition temperatures of common polymers:

  • PVC (polyvinyl chloride)

  • PMMA (polymethylmethacrylate)

  • HDPE (high-density polyethylene)

  • PTFE (polytetrafluoroethylene)

  • PI (polyimide)

2.3 Sources of Error in Thermogravimetry

A) Mass

• Classical buoyancy.

• Effect of temperature on balance.

• Convection and/or turbulence.

• Viscous drag on suspension.

• These are lumped together as the