通过测热分析的热膨胀分析

The thermal expansion of a material is extremely important when considering its use in a system with fluctuating temperature. Dilatometry is a technique used to measure the area, shape, length, or volume changes of a material as it experiences fluctuations in temperature. Thereby enabling the calculation of thermal expansion. In this video we will introduce the dilatometer, and demonstrate how to measure the thermal expansion of a metal sample in the laboratory.

Dilatometry is first performed by measuring the initial length of the sample using calipers. Then the sample is placed in a furnace, and in the case of this experiment, connected to a vertical push bar. A purge gas flows through the furnace to provide consistent conditions and prevent oxidation of the sample during heating. The sample then is heated to a predetermined temperature at a specified rate. The thermal expansion of the sample is transferred to the push bar, which is then transferred to the displacement sensor. Most materials expand with increased temperature and then contract upon cooling. Since the rod is also exposed to the high temperature in the furnace, it too experiences thermal expansion and contraction. Thus the measurement must be corrected to account for this.

The thermal expansion experienced by the sample is calculated by dividing the change in length by the initial length of the sample. This yields the average linear thermal expansion of the material. We can calculate the linear thermal expansion coefficient, αL, by dividing the average linear expansion by the change in temperature experienced. The volumetric expansion coefficient, αV, is then 3-times the linear expansion coefficient for isotropic materials. Some anisotropic materials, meaning a materials whose properties are direction dependent, may exhibit different linear expansion coefficients in different directions. Now that you’ve learned the basics of thermal expansion using a dilatometer, let’s take a look at the technique in the laboratory.

To begin, power up the dilatometer operating system and allow the sample to sit at room temperature to equilibrate. Make sure that the cooling system for the instrument is running, and that nitrogen gas is connected to the furnace. Do not turn the gas flow on yet, the gas will be turned on when the furnace turned is turned on. Now check that the calibration run has been performed on the system prior to testing your sample and select the most recent calibration that meets or exceeds your maximum temperature range and preferably is run at the same temperature ramp rate. Here we will use a previously conducted calibration run of the standard crystal locks. Next, accurately measure the length of the sample using high quality calibers.

Take several measurements along the length in order to establish the measurement error. Ensure that the sample is long enough to allow the push rod to exert some force on top of the sample. If it is not tall enough, use a spacer of a material with known thermal expansion and measure its height so that the spacer can be subtracted from the results. If a spacer is used, it must be parallel to the sample within the 1º. Then power the system on and ensure that the furnace is close to room temperature. Now raise the tube-chamber out of the furnace by pulling the knob on the side to release the tube. Raise up the tube and clean the bottom surface of the chamber with isopropanol and a wipe to ensure that the sample has a flat place to stand. Then, place the sample in the furnace with the flat surfaces towards the bottom of the chamber and the push rod and lower the push rod until it contacts the top of the sample. Lower the tube chamber containing the sample back into the furnace and ensure that the sample did not shift by checking the displacement gauge. Now, input the heating parameters into the dilatometer operating system.

Here the metal sample will be heated 20º-1000ºc at a constant rate of 5º/minute. To cool the furnace, just allow the temperature to equilibrate with room temperature. Before starting the test, double check that all systems are all on and functioning. Turn on the nitrogen purge gas and ensure that it is flowing to the system. Then initiate the test and check back periodically to make sure that it is running appropriately. When the run is complete and the system has cooled back to room temperature, export and save the data. Then repeat the scan another 2 times to account for any exaggerated expansion on the first run. After all runs have been completed and all data saved, ensure that the furnace is cool. Then raise the tube out of the furnace and remove the sample. To raise the tube out of the furnace, pull the black knob on the side of the furnace to release the tube. Finally, shut down the furnace, cooling system, and purge gas.

Now let’s take a look at the results. The program returns the values for: 1. Time, 2. Sample Temperature, 3. Gauge Reading, 4. Corrected Expansion, 5. Time in seconds, 6. Dimensionless Gauge Reading, 7. System Correction. First, calculate the change in length of the sample for each temperature point using a spreadsheet program, and then divide each value by the original length to obtain values of ΔL/L. Then plot ΔL/L vs Temperature. As you can see from the plot here, 3 metals were heated to a preset temperature and then cooled back down to room temperature. Though it was heated to a lower temperature, aluminum exhibited a more significant thermal expansion than stainless steel or cold worked steel.

In the case of aluminum and stainless steel, thermal expansion and contraction both follow a linear slope; meaning that thermal expansion was linear. And the linear expansion coefficient was constant. However, the thermal expansion is not always linear, meaning that the linear expansion is not always constant, as we see for cold worked steel. The cold worked steel sample exhibited a non-linear change between 700º and 900º, which can be attributed to defects in the lattice structure of the material called dislocations.

It is important to understand the thermal expansion and contraction of materials for a wide range of applications. For example, it is essential to account for the thermal expansion for materials when designing structures such as railroads and bridges. The thermal expansion of sections of railroad tracks is the main cause of rail buckling, which caused almost 200 train derailments in the US over a period of just 10 years. The measurement of thermal expansion using dilatometry can also be used to examine defects in crystals. Dislocations are defects in a materials lattice structure, and can take many different forms such as a point dislocation where one atom is missing, or an edge dislocation where an extra half plane of atoms is introduced in the lattice. Since dislocations occupy volume, density changes in response to heat treatment. Thus, high resolution dilatometry has extended the technique to study rearrangement of dislocations. Essential to understanding strength and possible areas of failure.

You have just watched Joves introduction to the analysis of thermal expansion via dilatometry. You should now understand the fundamentals of thermal expansion, the dilatometry technique, and some areas where thermal expansion is analyzed in structural and materials engineering. Thanks for watching.