Measuring the Thermal Expansion of Ultra-Thin Samples
Measuring the thermomechanical behavior of ultra-thin materials has always been a challenge due to the low mechanical strength of very thin sheets. Even when reducing the force applied by the push rod of a standard electronic dilatometer to a minimum, it is nearly impossible to get a reliable measurement on sheets with a thickness below 100 microns because they bend too easily.
Recently, a new instrument* has been introduced specifically for measuring the thermal expansion of ultra-thin materials. The instrument can carry out thermomechanical measurements without making any contact with the sample, providing extremely accurate results. Additionally, the test procedure is designed to be easy and hassle-free, with no need to run calibration curves.
Test MethodThe new instrument uses a double-beam optical dilatometer based on a new concept. Two microscopes with a very long focal length use blue light to illuminate both ends of a specimen placed inside the dilatometer’s furnace (see Figure 1). Since the specimen is very thin and unable to stand alone, the sample is placed horizontally on a special sample holder (see Figure 2).
coupled device (CCD) of the cameras during the heat treatment.
One potential problem with this measuring method is that the sample is free to expand in both directions. In some cases, a negligible amount of friction between one side of the sample and the sample holder might cause the actual expansion to occur only on the other side of the sample. As a result, the image of the edge of the sample that is expanding can go out of the field of view of one camera due to the extremely high degree of magnification. However, this problem can easily be corrected by moving the optical path of the camera with a linear motor to bring the edge of the specimen back into the camera’s field of view. The displacement is registered electronically, and the final data plot is completely seamless.
Because there is no need for multiple calibration curves and the sample is measured without making contact, the heating rate can be changed during the test to more closely replicate actual heat treatment curves.
Thermal Expansion of Thin SheetsTo test the new instrument, researchers used it to measure the thermal expansion of several different samples:
• A 10-micron-thick foil of rolled aluminum
• A 50-micron-thick sheet of rolled brass
• A 100-micron-thick green ceramic tape, cast with a doctor blade
None of the samples were able to stand alone under their own weight, so all were placed inside the special sample holder to keep them in place during the measurement.
The optical dilatometer can also be used to measure the thermal expansion of glass samples. Since the measurement is carried out without making contact, it is possible to overcome the softening temperature of the glass with no danger to the measuring system.