The Optics Route to Thermal Analysis

Determining the coefficient of thermal expansion, glass and phase transitions, and investigating aging and curing.

Thermal analysis is the study of material properties as they change with temperature. This branch of material science plays a key role in virtually every industry – from investigations into the temperature behavior of sealants in the automotive industry to studying the melting conditions of cocoa butter in food science.

Why thermal analysis?

Thermal analysis offers a wide range of applications for investigating sample properties under different temperature conditions, for example:

  • the application temperature range of materials
  • the coefficient of thermal expansion (CTE)
  • glass and phase transitions
  • aging, polymerization, and curing processes
  • reaction kinetics, e.g. chemical conversion and volume shrinkage
  • vaporization of organic solutions

This information can be used to optimize products and processes.

Major thermal analysis instrument types

The thermal analysis market is currently dominated by three major branches.

DSC (differential scanning calorimetry) measures the rate of heat flow to a sample in comparison to a reference during temperature changes. This calorimetric method is used to detect any reaction or change inside the sample, which causes a change in the heat flow e.g. phase transition.

TMA (thermo-mechanical analysis) measures changes in the mechanical properties during temperature changes. One important parameter is the dimensional change called the coefficient of thermal expansion.

TGA (thermo-gravimetric analysis) measures the sample weight during temperature changes or during chemical reaction to determine e.g. vaporization or absorption.

A new optical approach to thermal analysis

A new approach to thermal analysis – TORC – uses a periodic thermal excitation and analyzes the optical response. Measurements provide data to determine glass and phase transitions, curing time, curing rate, and curing temperature as well as volume shrinkage and volume expansion. Integrated mathematical models allow determination of the coefficient of thermal expansion in a mechanical, perturbation-free way.

TORC is an abbreviation of Thermo-optical Oscillating Refraction Characterization. It works as follows:

The TORC device induces a thermal excitation and measures the optical response

The temperature is modulated in the order of 0.1 K. This leads to a corresponding response in the measured refractive index of the sample caused by the temperature-induced density change.

This provides the following results:

The thermal expansion coefficient is calculated

The amplitude of the refractive index oscillation is a measure of the coefficient of thermal expansion (CTE). CTE is calculated automatically using thermo-optical models (e.g. Beysens and Lorentz-Lorenz).

The temperature dependence of the refractive index is measured

The change of refractive index with temperature (dn/dT) is measured from the amplitude of the refractive index oscillation.

Structural changes in the sample are determined

Phase changes, glass transitions, or other structural changes in the sample can cause a delay between the temperature modulation and the refractive index response. The phase shift-based loss term is plotted and clearly indicates the time or temperature at which structural changes occur.

Reaction kinetics and aging are determined

Density changes caused by the reaction influence the mean refractive index. This provides information about reaction conversion, volume shrinkage, and sample aging.

When can TORC be used?

A wide range of samples can be measured, including fluids, gels, and strongly adhesive substances as well as samples undergoing phase transitions. Just a sample volume of a few µL is required.

For more information on the TORC technique for thermal analysis, see TORC 5000 product page.

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