Development of the Asymmetric Microstructure of Carbon Molecular Sieve Membranes as Probed by Positron Annihilation Spectroscopy

摘要

The development of the microstructure of carbon molecular sieve membranes (CMSMs) was examined using a variable monoenergy slow positron beam ( VMSPB). To vary the structure of the CMSMs fabricated by pyrolyzing a Kapton precursor, diiferent pyrolysis temperatures at a fixed heating rate under vacuum conditions and various periods of holding time at a given pyrolysis temperature were applied. The VMSPB was integrated with the platinum (Pt) capping technique, in which a layer of Pt was sputtered on the membrane surface to eliminate the back diffusion effect of positrons. On the basis of the depth profile obtained, the membranes carbonized at 800 and 900 °C were demonstrated to exhibit asymmetric microstructures at the top layer. The analysis of positron annihilation spectroscopic data of the membranes using a VEPFIT program revealed three structural layers at the most: a dense top layer, a transition layer, and an underlying layer. At prolonged holding time, it was found that the decrease in the gas permeation was correlated not only to the reduction in the pore volume but also to the increase in the dense layer thickness.