Structure and Types
The most recent types of optical dilatometer use a direct-beam system so as to avoid contact-related problems by measuring the image that the specimen projects on an image sensor, when irradiated by a light beam. By using a single beam of light with short wavelength and a very high resolution image sensor it is possible to achieve good resolutions, although not comparable to those of the standard dilatometers. For example, using a blue light with a wavelength below 1500 micrometers, it is possible to achieve images with an actual resolution of nearly 1500 micrometers/pixel of the camera (not an interpolated resolution but an actual resolution). By using two light beams, which illuminate two small portions of material at the extremities of the sample, set perpendicularly to the beams direction, it is then possible to achieve an absolute measurement of longitudinal variation while heat-treating the specimen.
There are two major types of optical dilatometers which are currently in use:
- Horizontal Optical Dilatometer: the rod-specimen is laid horizontally on the sample holder and, during the thermal treatment in the oven, it is completely free to expand and contract. The displacement of the sample holder is not relevant for the result of the measurement because the optical system follows the horizontal movements of small portions of the specimen at its extremities.
- Vertical Optical Dilatometer: the rod-specimen is set vertically on the sample holder (alumina thin plate) and while one camera “watches” the top of the plate the other follows the vertical displacements of the sample’s top edge. This system has been used so far to find the characteristic temperatures (phase transitions, maximum sintering speed, swelling, etc.) of ceramic materials. In fact, phase transitions are always associated to dimension variation of the material: sintering is accompanied by high shrinkages, related to the decrease of porosity -swelling usually appears at high temperatures. After sintering occurs and the viscosity of the ceramics formulation strongly decreases and trapped gas is then free to exit.
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