Powder Technology, 62 zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA (1990) 147 - 154 zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDC Electrical Conductivity of Ceramic Powders and Pigments K. KENDALL ICI, P.O. Box 11, The Heath, Runcorn (U.K.) (Received August 28, 1989; in revised form January 2, 1990) 147 ABSTRACT Electrical conductivity of powder compacts has been predicted theoretically and verified experimentally using carbon black particles. Conductivity depends on the contact diam- eter between grains, and this can be calculated from the van der W aals attractive forces, knowing the size and elastic modulus of the particles. There is also a strong dependence on particle packing volume fraction @ I and this has been modelled by a #4 relation. The resulting analysis allows interpretation of experiments with various mechanical loads, with different particle sizes, in media which reduce the van der W aals interaction, and with ceramic parti- cles which are covered with thin insulating layers. INTRODUCTION Electrically conducting powders are used as antistatic agents in polymer fibres, for exam- ple in carpets. They also serve in paints for electrical contacts or for corrosion protection, in plastics used for electromagnetic shielding of business machines, and in electrorheolog- ical fluids for valves and clutches [ 11. Although metal powders such as silver, and ceramic powders like carbon and doped tin oxide have traditionally been used in these applications, there is a need for new con- ducting pigments and powders, particularly to replace carbon black, which is too dark to be used in decorative materials [2] and which is too readily oxidised, for example by chlorine, in electrochemical use. Several ceramic powders offer advantages over carbon. One candidate is reduced titan- ium dioxide (EbonexR obtained from Ebonex Technologies Inc.) which can attain conduc- tivities from lo-l2 to lo5 S/m depending on 0032-5910/90/$3.50 oxygen deficiency. Titanium diboride and titanium nitride are even better electrical conductors (10’ S/m). These powders give better chemical resistance than carbon because oxidation is limited by a thin surface layer of impervious oxide. The purpose of this paper is to examine several ceramic powders in compacted form, in polymer composites and in sintered con- dition, comparing the powders with carbon black, in order to understand their electrical behaviour as a function of particle size, packing density, applied load, material prop- erties, surface films, and degree of floccula- tion. MAJOR PROBLEMS OF CONDUCTING POWDERS The first major problem of powder con- ductivity was illustrated by mixing carbon black (Philblack N774) into two different resins. In the first resin (20% volume fraction in ICI acrylic polymer, trade name Modar) the carbon was flocculated and formed con- ducting pathways. Therefore the mixture was conducting, with a conductivity of 1 S/m, sufficient to leak electrostatic charges. How- ever, in the second resin (20% volume fraction in polyvinyl alcohol (Nippon Gohsei, Gohsenol KH17s)), the carbon particles remained relatively well dispersed and separated by thin layers of polymer which prevented good conduction. The conductivity in this case was much reduced to 4.0 X 1O-3 S/m. Clearly, the colloidal forces acting between the carbon particles are important in determining conductivity. These forces must be accounted for theoretically, The second major problem was revealed when dry powders were compacted together to form an interconnecting pellet of material. @ Elsevier Sequoia/Printed in The Netherlands