Positron annihilation lifetime in ordered porous silica SBA-3

To clarify the correlation between the positron lifetime and the pore size in mesoporous materials, the positron lifetime profiles of SBA-3 were measured. SBA-3 has ordered cylindrical pores, and the pore size can be controlled using surfactant molecules of different length as templates. Thus, SBA-3 is appropriate as a model material in order to establish the correlation. The lifetime of the longest component was attributed to annihilations in the intergrain space. The lifetime of the second longest component increased for larger pores, and this component is attributed to the annihilation of o-Ps in the primary cylindrical pores. The lifetime component corresponding to the primary pores was obtained because the sizes of the grains were greater than or comparable to the diffusion length of o-Ps; this is in contrast to the case of MCM-41 where the grain size is very small. The effect of the escape of o-Ps from the primary pores was also observed in the intensities of the longest and the second longest components. The relation between the positron lifetime and the pore size was compared to those in several theoretical models, and the cylindrical or channel models agreed rather well with the experimental results, although significant deviations were found for the SBA-3-12 and SBA-3-18 samples. These deviations can be attributed to the possible irregularities in the sample and the escape of o-Ps from the primary pores. These results clearly show that the escape of o-Ps from the primary pores is an important process in analysis of the pore structure by using positron lifetime measurements.