Simultaneous measurements of the thermal diffusivity and conductivity of thermal insulators using lock-in infrared thermography

Optically excited lock-in infrared thermography provides fully non-contact methods to measure the thermal diffusivity of solids. However, measuring the thermal conductivity requires precise knowledge of the energy absorbed by the sample and of the absolute temperature rise. We have found that the temperature profile of poor thermal conductors illuminated by a modulated and focused laser beam is greatly affected by heat conduction to the surrounding air. In this work, we take advantage of the thermal coupling between the sample and the adjacent air to retrieve simultaneously the in-plane thermal diffusivity and conductivity of the sample. A sensitivity analysis demonstrates that the accuracy in the value of the thermal conductivity dramatically increases as it approaches the thermal conductivity of the air and/or when experiments are performed at very low modulation frequencies (<0.1 Hz). This means that this method is specially suited to determine the thermal transport properties of thermal insulators. Experiments performed on homogeneous polymers, on paper sheets and on extruded polystyrene foams confirm the validity of the method.