Thermal Conductivity Decreases With Increasing Temperature

The ability of a material to transfer heat from one point to another is described by its thermal conductivity. It is a function of the properties of atoms and molecules within the crystal lattice of a solid or molecular structure in a fluid, and it depends on the temperature gradient that it is transferring heat across. This heat transfer is called conduction, and it differs from other forms of energy transport such as convection or molecular work because it does not involve macroscopic flows or internal stresses.

Understanding how moisture affects thermal conductivity is usually given by the equation Qt=kADTd where q is the heat flow, A is the cross sectional area of the material, DT is the difference in temperature between two points, and d is the thickness of the material. For solids, a more accurate expression is used that involves first-principles calculations and is given by k = (Qt/A)T and can be expressed as a second-rank tensor or a matrix.

For metals, the thermal conductivity decreases with increasing temperature because a higher temperature will cause phonons and free electrons to vibrate more vigorously. This vibration will reduce the mean free path of the electrons and thus the thermal conductivity. For non-metals, the relationship between temperature and conductivity is more complex. A higher temperature may increase the thermal conductivity if it causes a greater packing density of atoms or molecules in the crystal lattice or molecular structure, which will allow more electrons to have an easy time passing through them.

Moreover, the thermal conductivity of a material is affected by the amount of moisture present in the system. Moisture affects the temperature at which a surface reaches equilibrium with the surrounding air, and it also affects the energy required for a molecular transition. The presence of a large number of water molecules will reduce the thermal conductivity of a liquid, while the presence of a small number of water molecules will increase it.

A common misconception is that a dry specimen will have a lower thermal conductivity than a wet specimen of the same material, because moisture will slow down the phonon or electron transition. This is not always the case, however, and in many cases a wet specimen will have a similar or even lower thermal conductivity than a dry specimen of the same material.

The physical processes that determine a concrete’s thermal conductivity are complex and varied, so it is not possible to predict the exact value of a particular concrete’s thermal conductivity from its initial properties or its composition. However, it is possible to develop a simple laboratory test that can measure the impact of moisture on a concrete’s thermal conductivity. This method uses a special heat-flux transducer on each side of the specimen, which avoids the need to embed the temperature sensor into the concrete and allows for a quick measurement of the effect of moisture on a concrete’s thermal properties. A range of different concretes have been tested with this technique, including a standard concrete, a micro-concrete used for model structural analysis, a standard shotcrete, and a foamed light-weight cellular shotcrete.