In situ network structure, electrical and thermal properties of conductive epoxy resin-carbon black composites for electrical heater applications

Epoxy composites at high carbon black (CB) concentration exhibited good electrical and thermal stability, which can make them attractive for consideration in heating devices and conducting composite applications. The effect of CB on the network structure of epoxy composites, like volume fraction of epoxy network (V-r), the extent of CB reinforcing (gamma), interparticle distance between conductive particles (IPD), epoxy-solvent interaction parameter (Z), average number molecular weight between cross-links (M-c), number of elastically effective chains (NEC), cross-linking density (CLD) and scanning electron microscope (SEM), was investigated in details. The conductivity of an insulating epoxy matrix increases continuously with CB content and is well explained by percolation theory. The effects of different concentration of CB and sintering on the electrical conductivity of epoxy composite as a function of temperature during heating and cooling cycles were discussed. The conduction mechanism and negative temperature coefficient of conductivity (NTCC) of epoxy-CB composites were analyzed. The current-voltage-temperature and working power-temperature characteristics of these composites as a function of CB content were investigated. We attempt to contribute to a better understanding of the negative resistance phenomena in epoxy-CB composites. The nonlinear coefficient (alpha), the size of conductive particles (SCP) and hopping distance (a(h)) of charge carriers in epoxy-CB composites were evaluated. The thermal stability was tested by means of temperature-time curve at certain applied power, on and off, for one cycle. The characteristic thermal properties, like characteristic time growth tau(g), decay tau(d) and current t(i) constants of epoxy as a function of CB content, were estimated. The specific heat (C-p) and amount of heat transferred by radiation and convection (h(r)) were calculated based on energy balance concept as a function of CB content. The thermoelectric power (TEP) and thermal conductivity (lambda) as a function of temperature of epoxy-CB composite were discussed. In conclusion, the epoxy-CB composites show good thermal stability and they can be used as heating devices for consumer products. (C) 2002 Elsevier Science B.V. All rights reserved.