A Higher Thermal Conductivity Is Better Than A Lower One

A material’s thermal conductivity is its ability to transfer heat from one point to another. Unlike convection or radiation, which are other common methods of heat transfer, Understanding how moisture affects thermal conductivity relies on molecular agitation to move energy between points. The temperature gradient that is present and the specific characteristics of the material determine its conductivity. A material with more atoms or molecules will be more thermally conductive than one with fewer atoms or molecules, and the length of the path that energy must travel to reach its destination influences its conductivity as well.

In general, metals are good conductors of electricity and thermal energy, while non-metals such as wood and plastic are poor conductors. The reason is that metals have more ordered atomic or molecular structures and free valence electrons, which are easier to transfer. Non-metals, on the other hand, tend to have more atoms or molecules and bound valence electrons, which are more difficult to transfer. The temperature of a material also affects its conductivity, since it is easier to transfer kinetic energy from hotter to colder areas.

Generally speaking, a higher thermal conductivity is better than a lower one, as it means that heat can pass through the material faster. However, this is not always the case and it depends on the application and the desired behavior of the material. For example, in applications like heat exchangers, high thermal conductivity is ideal, as it improves the rate at which heat moves to and from the heat transfer fluid. Conversely, in applications where the goal is to keep heat from moving to surrounding components, a low thermal conductivity is preferable.

Moisture in Thermal Conductivity

The moisture content of a material can significantly impact its thermal conductivity. For this reason, it is important to take moisture into account when testing a sample’s thermal conductivity. This is especially important when testing concrete, as a lack of water can make the concrete appear to be more insulating than it really is.

When measuring a sample’s thermal conductivity, there are two broad categories of measurement techniques: steady-state and transient. Steady-state techniques infer a material’s thermal conductivity by measuring the state of a material once it has reached thermal equilibrium. In contrast, transient methods measure the instantaneous state of a system during its approach to thermal equilibrium.

Both types of measurements can be performed using a single-needle KD2 thermal properties meter (Decagon Devices, Pullman, WA) by heating the sample to its test temperature and monitoring its temperature rise vs. time curve. By utilizing this method, the need to embed temperature sensors in the specimen is eliminated, enabling testing of samples with varying amounts of moisture without having to modify the test setup.

The exact mathematical expression for a given material’s thermal conductivity is complex, as it depends on the microscopic structure and molecular interactions of the material. For this reason, high-accuracy first-principles calculations are often impractical. Instead, empirical or theoretical expressions are often fit to existing thermal conductivity data over a wide range of temperatures and pressures. These expressions are called reference correlations and can be found in many thermophysical modeling software programs.