Dilatometry

Dilatometry (DIL) is a method that involves measuring the dimensional changes of a substance under minimal load, such as expansion or shrinkage, in relation to temperature. This is done while subjecting the substance to a controlled temperature program within a specified atmosphere.

Dilatometry can yield the following measurement results:

  1. Determination of the coefficient of thermal expansion (CTE)
  2. Measurement of linear thermal expansion (ΔL)
  3. Identification of sintering temperatures and sintering steps
  4. Determination of the glass transition temperature (Tg)
  5. Detection of phase changes
  6. Optimization of burning processes
  7. Assessment of volume changes
  8. Rate Controlled Sintering (RCS)
  9. Analysis of decomposition
  10. Assessment of changes in density

The coefficient of linear expansion – CTE

The coefficient of linear expansion is a key parameter that reflects changes in the length of a material due to temperature fluctuations. In dilatometry, the sample is positioned on a specialized holder within a furnace. A push rod, crafted from the same material as the sample holder (e.g., quartz glass or aluminum oxide), conveys the thermal expansion of the sample to a displacement transducer, which quantifies the displacement. This displacement measurement can be conducted either in an analog or digital manner.

Materials undergo geometric variations as temperature changes. This behavior can be characterized by the linear coefficient of thermal expansion (CTE_linear), which is defined as follows:

CTE_linear = ΔL·1/ΔT·1/L

Here, ΔL signifies the change in length due to a temperature change (ΔT) in the initial length (L) of the sample.

The displacement transducer is securely affixed to a reference point. Because both the reference and the push rod undergo comparable elongation during heating, the expansion of the sample is determined relative to the material of the reference. The actual thermal expansion of the sample is then equal to the value measured by the displacement transducer, augmented by the expansion of a piece of reference material with the same length as the sample.

Optical Dilatometry

Optical dilatometry is a technique that enables the examination of sintering processes and the linear thermal expansion, particularly for anisotropic and fragile materials, as well as samples with complex geometries. Additionally, it can be used for non-contact thermomechanical assessments to determine the softening point and melting temperatures of materials.

In an optical dilatometry measuring device, the process operates as follows:

A dilatometer equipped with a camera directs light onto the sample from one side, while shadows cast by the sample are captured on the opposite side. Using the data acquired, a computer calculates the change in length of the sample.

Optical dilatometry – monochrome image taken during a heating process

The following norms relate to this method:

Certainly, here are the rewritten descriptions for the ASTM standards:

  1. ASTM E831 – This standard test method, ASTM E831, is employed for assessing the linear thermal expansion of solid materials through thermomechanical analysis.
  2. ASTM D696 – ASTM D696 outlines a test method that pertains to the determination of the coefficient of linear thermal expansion for plastic materials characterized by coefficients of expansion exceeding 1 µm/(m·°C). This is achieved using a vitreous silica dilatometer.
  3. ASTM E228 – The ASTM E228 standard test method addresses the measurement of the linear thermal expansion of solid materials, and it utilizes a push-rod dilatometer for this purpose.
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