2002 Volume 49 Issue 2 Pages 135-140
We propose two methods for the size selection at nanometer level based on manipulation of the surface interaction between particles. The control over the particle interaction has been achieved by modulating the interparticle distances through: (a) coating the nanometer size particles with anionic surfactants having different chain lengths, and (b) introducing polar solvents to suspensions in non-polar solvents dispersing nanometer size particles coated with sodium oleate. An effective size selection of nanometer size magnetite particles has been achieved using both of these methods and the results are presented here. Depending on the conditions, stable suspensions of nearly monodispersed magnetite nanoparticles less than 10 nm in diameter with a standard deviation below 0.2 were obtained from polydispersed powder less than 40nm in diameter and 0.6 in standard deviation. The size selection was confirmed by using the TEM measurements of separated fractions., The magnetic measurements of these size classified samples were carried out in zero field cooled (ZFC) and field cooled (FC) conditions. The relation between particle size and blocking temperature conformed to theoretical predictions and also confirmed the success of the selection methods. Furthermore, the field-cooled measurements of separated fractions suggested that the influence of surface spins to magnetic properties of the particle become considerable when the particle diameter is below 6nm.
