The difference in "diameter" from micron to nano is 10 3 which is 10 9 (a billion) times difference in volume or mass. This implies that the properties that we have intuitive feeling for at the micron scale are considerably modified at masses a billion times smaller. Indeed it is for this reason that "nano" materials have been exploited. For example, in a 3.5 nm Au aggregate/particle ("cluster") then about 40% of the total atoms are directly on the surface [T Shibita, H Tostmann, B Bunker, A Henglein, D Meisel, S Cheong, M Boyanov ( 2001), XAFS study of gold and silvergold nanoparticles in aqueous solutions J. Synchrotron Rad., 8, 545-547. [1]] introducing novel catalytic properties not present in the larger systems. As another example, gold clusters supported on a substrate will reach the melting point of solid gold only if they contain 1000 or more atoms [Michael A. Duncan and Dennis H. Rouvray, (1989), Microclusters, Scientific American, December 1989 issue, 110-115. [2]].
Measurement of particle size in such systems is a complex issue with each sizing technique describing different aspects of the particulate system.
We report a study of a ceramic material, barium ferrite, characterized by BET and laser diffraction where the term "nano" could be applied to the former measurements and micron to the latter technique. The reasons for the different interpretations of the result sets in the 2 cases are discussed. The ratio between the BET and diffraction results enables a measure of the agglomeration in the system to be defined. Thus "nanopowders" are collections of micron sized and larger agglomerates and aggregates and which simultaneously exhibit both nano and micron based properties.