THE MICROSTRUCTURE AND MORPHOLOGY OF CARBON-BLACK - A STUDY USING SMALL-ANGLE NEUTRON-SCATTERING AND CONTRAST VARIATION

This is a study of the form and structure of particles and particle ag gregates of an experimental high surface area carbon black (HSA) using small-angle neutron scattering, and the method of contrast variation. Contrast variation was effected by studying suspensions of the carbon black in cyclohexane containing different fractions of deuterocyclohe xane. We find that the approximately 28 nm diameter HSA particles are arranged as small, linear aggregates with an average aggregation numbe r between 4 and 6. The structure averaged over the particle population is best represented by a prolate ellipsoid of revolution with semiaxe s 14.5 and 76.4 nm. The surface of the aggregates appears smooth over length scales larger than 1 nm, which places an upper limit on the sur face roughness observed by other methods. The internal structure of th e aggregates is described by a shell-core model, with the shell densit y being consistent with a graphitic structure and the core being of lo wer density, more like amorphous carbon. Some fraction of the core vol ume (0.1 to 0.2) is taken up by voids that are not accessible to the s olvent. An estimate of the shell thickness gives 1 to 2 nm along the e llipsoid minor axis and 6 to 10 nm along the major axis. The particles of the aggregate appear to be fused so that the less dense amorphous core is continuous through the inner parts of the aggregate. The infor mation that can be obtained on the internal structure using contrast v ariation is limited by heterogeneity in the chemical composition of ca rbon black aggregates.