Thermal conductivity is a property of a material that determines how well heat can flow through the material. Knowledge of the thermal conductivity of a given material is important when designing and engineering structures that contain the material. Several methods exist to measure this property in materials, each with their own advantages and disadvantages.
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Guarded Hotplate Method
This method relies on an experimental apparatus that is completely thermally insulated. The apparatus itself consists of several layers, with the heat source placed completely surrounding the material. The cooling source is placed at one end of the material, and measurements are taken from different locations. These differences in temperature will help determine the thermal conductivity. In this method, it is important to obtain steady-state heating conditions (i.e. the temperature at different points within the apparatus does not change over time) as the experimental method does not account for temperature fluctuations over time.
Searle's Bar Method
In this method, a rod is placed within the experimental apparatus, which insulates the rod from outside heat sources. One end of the rod is then heated using steam while the other end is cooled using cold water. Four thermometers are set up along the length of the rod, and the rod is cooled for a specific amount of time. Using the collected temperatures, the elapsed period of time, the distance between the thermometers, and several physical constants, the thermal conductivity can be calculated directly. This method is generally used when the material is a good conductor of heat.
Lee's Disc Method
This experimental method involves placing the material whose conductivity you would like to measure between two specially designed discs. After ensuring the temperature across the discs is constant, one of the discs should be heated to a certain temperature, after which the heating source should be removed. Heat will move from one disc to the other but must go through the material first, which will lead to further heat loss. By measuring the difference in temperature of the two discs, it is possible to determine the thermal conductivity of the material between the discs. This method is best suited for materials that are poor conductors.
This method involves driving a hypodermic needle, employed as a probe, into the material to determine the conductivity of the material. The probe contains a heating source and a thermocouple (to measure the temperature). As the material is heated by the probe, the temperature is simultaneously measured by the probe. For a certain period of time, the temperature increase of the material will be constant, before the environment reaches a steady state. The thermal conductivity can be determined during the period when the temperature increase is constant.
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