Optical Dilatometer - Applications and Fields of Research

Applications and Fields of Research

In order to measure thermal expansion up to high temperatures, the material to be tested has to be set on a sample holder inside an oven which has its own thermal expansion. In order to achieve a good accuracy it is necessary to measure the expansion of the sample holder and to subtract it from the actual expansion of the specimen. The best approach is to split the laser beam into two beams of light, which are reflected by the top-edge of the sample and by the top-edge of the sample holder, or by both longitudinal edges of the specimen. By detecting the longitudinal variations at both the extremities of the specimen the measurement achieved is absolute and there is no need for further corrections. This is the most accurate way of measuring thermal expansion and it may attain nanometric resolution. This is the type of instrument used by the suppliers of certified standard materials. For example, the National Institute for Standards and Technology uses a Fizeau double-beam interferometer to certify the thermal expansion of their certified reference materials. This method has proved to be very accurate, with a resolution of a fraction of the wavelength of the incident light, but it is limited by the reflectivity of the surface of the specimen. If the specimen is not reflective, or it becomes non reflective during the test, it is then necessary to use a mirror, which is set in contact with the specimen by using a refractory push rod. By doing so such method loses the advantage of being non contact and becomes substantially similar to the electronic dilatometer.

Optical dilatometers are used along traditional dilatometers in ceramics for non-destructive testing of a sample's behavior during a thermal treatment. Optical dilatometers are used for thermal analysis of various types of materials, such as incoherent materials (expansion and contraction of an incoherent granular frit, as applied, for example, on raw tiles) and polymers (behaviour above the glass transition temperature, where the surface tension starts pulling the edges and making the sample shrink), as well as for analysis of various processes in materials manufacturing, for example, sintering kinetics, thermal expansion and sintering behaviour of thin glaze layer or polymer thin films.