Nonlinear electrical conductivity of tin-filled urea-formaldehyde-cellulose composites

This work reports on the elaboration and characterization of composite materials prepared by compression molding of mixtures of tin powder and a commercial grade thermosetting resin of urea-formaldehyde filled with alpha-cellulose in powder form. The morphology of constituents and composites has been characterized by optical microscopy. The porosity rate of the composites has been determined from density measurements. These results show that the composites are homogeneous. Furthermore, it has been shown that the hardness of samples remains almost constant with the increase of metal concentration. The electrical conductivity of the composites is < 11(-11), S/cm unless the metal content reaches the percolation threshold at a volume fraction of 18.6%, beyond which the conductivity increases markedly by as much as 11 orders of magnitude. The results have been well interpreted in the statistical percolation theory frame. (c) 2005 Society of Plastics Engineers.