Extension and optimization of a three-layer method for the estimation of thermal conductivity of super-insulating materials

The three-layer method is a characterization method dedicated to the measurement of the thermal conductivity of small samples made out of insulating or super-insulating materials. It is based on the transient recording of the mean temperatures of two brass plates placed, respectively, above and under the sample, one of them being heated. In case of a super insulating material, the rate of heat flow from the heated to the unheated plate through the surrounding air is not negligible with respect to the rate of heat flow through the sample. It is shown that introduction of a simple parallel thermal resistance allows taking this flow of heat into account. An analytical model, based on a new quadrupolar approach, was developed to estimate this resistance considered further as a known parameter in the estimation process. An experimental study was carried out to characterize samples whose thermal conductivities have been measured previously with another method. The deviation of the estimated thermal conductivities from these reference values with respect to the three-layer method results reaches 29% without considering parallel heat flow but is lower than 2% using the new model. (C) 2014 AIP Publishing LLC.