Determination of the type of charge carriers in carbon fiber/polymer composite

The Hall effect method has been used to determine the charge carrier density in polycarbonate/carbon fibers composites. It was found that the measured Hall voltage varies almost linearly with Hall current. This behavior indicates that the majority of the charge carriers is of one type. The sign of the slope of this relation varies from positive to negative for composite specimens of about 18 wt% carbon fibers. This suggests that a composite of low fiber content is functioning as p-type material and then changes to n-type with increasing carbon fiber content. The observed anisotropic behavior of the charge carrier density makes it difficult to calculate the kinetic parameters of the carrier mobility of such composite materials from the Hall effect analysis.