Thermal resistance technique for measuring the thermal conductivity of thin microporous membranes

The thermal resistance technique for measurement of the thermal conductivity of microporous thin membranes is described. A modified and enhanced Lees' disc apparatus was used. Several samples of membranes were held between a hot copper plate and a cold copper base, and the temperature differences between them were measured using thermocouples under steady-state conditions. The accuracy and reliability of the results were checked by means of a comparative test on a standard bad conductor. A dependence of the thermal resistance on the thickness of the samples was observed. The thermal conductivity of the membrane was deduced from a linear fit of the thermal resistance versus the number of membranes. Better results were obtained when the air layer effect was considered in the linear fit. Several models found in the literature and based on empirical correlations or on theoretical structure models (such as Maxwell's, Fricke's or Misra's models) were tested in order to calculate the effective thermal conductivity of the membrane. These values were compared with the experimental ones and they showed a better agreement than the parallel model commonly used in the literature for the membranes studied in this work.