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Understanding the Relationship Between Thermal Conductivity and Temperature

Mar 22

When designing products that require thermal management, it is important to understand the relationship between a material’s thermal conductivity and its temperature. This is because Understanding how moisture affects thermal conductivity measures how quickly heat is transferred through a sample, and is therefore an essential metric for determining what kind of performance a product will have under certain operating conditions. Generally speaking, higher temperatures will result in higher thermal conductivities, but there are other factors that can influence this measurement as well, including the density of the material and its moisture content.

The Temperature Effect

As a measure of heat transfer, thermal conductivity measures how quickly and efficiently a material can transfer energy. It is a function of a material’s temperature, its length and width, its mass, and its composition.

There are two main ways that a material can transfer energy: through elastic vibrations of its lattice, or through free electrons moving within the material. Since the latter is a more direct form of energy transfer, pure metals tend to have much higher thermal conductivities than non-metals. This is because the movement of electrons within a metallic crystal structure is a more efficient way of transporting energy than the agitation of molecules.

In liquids and gases, heat moves through a combination of particle agitation and the direct motion of the atoms or molecules themselves. This is why gases typically have lower thermal conductivities than solids. The motion of the atoms or molecules in gases is also impeded by their greater distance between one another, and this contributes to the slower rate of conduction as compared to solids.

A number of variables can affect a materials thermal conductivity, and the relationship between those variables and its temperature dependence is what gives rise to the different measurement techniques that exist. For example, steady state techniques rely on the measurement of the slope and intercept of a material’s temperature rise vs time curve, and use these to calculate a thermal conductivity value. On the other hand, transient methods rely on the measurement of a materials response to a change in its temperature, and use this to calculate a thermal conductivity value as a function of the time it takes for a material to reach its desired steady-state temperature profile.

The Moisture Effect

The presence of water in a material can significantly impact its thermal conductivity. For example, dry sand and clay have low thermal conductivities whereas moist soil exhibits a higher thermal conductivity than both sand and clay. This is due to the fact that moist soil is able to carry the heat from the sun through the soil pores more effectively than either sand or clay would be able to do on its own.

In general, denser materials will have a higher thermal conductivity than softer, more porous materials. However, the exact relationship is dependent on the specific microscopic structure of a material and its molecular interactions. It is for this reason that it is difficult to develop first principle theories predicting a material’s thermal conductivity.