Evaluation by tunneling effect for the temperature-dependent electric conductivity of polymer-carbon fiber composites with visco-elastic properties

Abstract

The conductivity of composites with a carbon fiber (CF) content slightly higher than the percolation threshold was measured at increasing temperatures up to −10 °C. The tunneling effect was theoretically calculated using a rigorous nonparabolic potential barrier. The tunneling barrier width D and the surface area A were determined. For a polyethylene (PE) matrix, good agreement between theoretical and experimental results was obtained using D=1.00 nm and A=1.35 to 1.68 nm2 at a 30-vol% CF content and using D=1.30 nm and A=1.25 to 1.28 nm2 at a 25-vol% CF content. That is, almost perfect agreement between experiment and theory was obtained by adjusting the parameters except over the temperature ranges in which the β and γ relaxation peaks appeared. Dynamic loss modulus and positron annihilation measurements were also conducted. However, a theoretical analysis that was derived using a parabolic potential barrier produced inconsistent results. That is, the tunneling barrier width D was less than the c-axis length of a PE crystal unit, and the surface area A was considerably less than the a-b plane area of a PE crystal unit.