Determination of mesopore size of aerogels from thermal conductivity measurements

Abstract

A novel method to determine the average mesopore size of aerogels was developed. This method is based on the findings that the heat transfer through the gas phase in porous materials is strongly affected by the size of mesopores. Polyisocyanurate acrogels were synthesized and the thermal conductivity of these materials was investigated from vacuum to ambient pressure using the transient hot-wire method. Thermal conductivity of porous materials can be recognized as the sum of gaseous, solid, radiative, and convective thermal conductivity. Extraction of gas phase thermal conductivity from total thermal conductivity was performed by considering different heat transfer mechanisms. Gas phase thermal conductivity increases with pressure in 'S' shape due to collisions of gas molecules with pore walls. Average mesopore size could be obtained by fitting gas phase thermal conductivity data with existing equations. Investigation of polyisocyanurate aerogels by SEM revealed that this method gives a reasonable estimate for the average mesopore size. It is expected that pore structure of fragile mesoporous materials can be analyzed by further refinements of this novel method. (C) 2002 Elsevier Science B.V. All rights reserved.