Analysis of the hot-disk technique applied to low-density insulating materials

The hot-disk technique is a very practicable transient method of measurement of the thermal properties of solid materials. It has been applied successfully to a wide variety of materials. However, it is based on several approximations regarding the nature of the heat transfer. Notably, the probe is considered thermally neutral, and the heat transfer is assumed purely conductive. These two assumptions are questionable when dealing with low-density thermal insulators. In order to evaluate the accuracy of the method, we have generated numerically noised thermograms reproducing the thermal response that would be recorded when measurements are applied to those type of materials. Thereafter, the best-fitting procedure of the classical hot-disk technique was applied to these thermograms. The analysis of the identification results show that the presence of a radiative contribution do not affect the accuracy of the thermal properties identified. The conductivity measured actually corresponds to the equivalent conductivity. On the other hand, when the method is applied to materials with thermal inertia strongly different from the probe (≈2 order of magnitude lower or more), the accuracy of the method becomes questionable. This is notably the case for common insulators used in the building industry like polymer foam or mineral wools. The preceding conclusions have been validated by experimental measurements on a standard low-density XPS foam sample and a superinsulating silica areogel.