Journal of Applied Polymer Science, Vol.91, No.5, 2756-2770, 2004
Novel V-shaped negative temperature coefficient of conductivity thermistors and electromagnetic interference shielding effectiveness from butyl rubber-loaded boron carbide ceramic composites
The presentation of a new conductive composite with good effective applications like negative and positive temperature coefficient of conductivity thermistors (i.e., V-shaped thermistors) and electromagnetic interference shielding effectiveness (EMI) was the aim of this study. The effect of boron carbide (B4C) contents on the vulcanization characteristics and network structure of butyl rubber (IIR) composites were analyzed in detail. The prediction of the type of crosslinks based on the affine and phantom network theory was also analyzed. The influence of the volume fraction of B4C on the dc conductivity and thermoelectric power was investigated. The electrical and dielectric properties of IIR composites were investigated. The results suggest that the conduction occurs by a tunneling mechanism and behaves as a p-type semiconductor. Isothermal resistance at different temperatures, as a function of B4C content, was displayed. The current-voltage characteristics showed properties of switches, which were explained by coulombic repulsive force. For practical applications, as self-electrical heating the temperature-time cycles were investigated under certain applied power. It was found that increasing the B4C content increases the thermal stability of the composite. However, the theoretical modeling of the current-voltage characteristic is very useful for planning groups in industrial applications of conducting polymer composites. Furthermore, the endurance test under applied power indicates that the proposed composites could be useful as temperature sensors with good reliability. Specific heat as a function of B4C contents was evaluated by experimental and various energy balance models. Furthermore, the temperature dependency of thermal conductivity and thermal diffusivity were investigated. Finally, the standing wave ratio, reflection coefficient, return loss, and attenuation of IIR/B4C composites were studied in the 1- to 4-GHz frequency range. The resulting values of electromagnetic interference shielding effectiveness were compared with theoretical models. (C) 2004 Wiley Periodicals, Inc.