Role of the interface in the temperature dependence of the resistivity of conductive composite thin films

The role of the interface properties in the surface resistivity-temperature (ρs-T) behavior in air and nitrogen was studied in conductive composite thin films consisting of epoxy resin and electrically conductive filler. To examine the role of the interface, we investigated the ρs-T behavior at various heating rates. The physicochemical changes in the properties of epoxy resin with temperature were characterized using thermodilatometry, thermogravimetry, dynamic mechanical spectroscopy, and Fourier transform infrared spectroscopy. The thin films exhibited an anomalous positive-temperature-coefficient-of-resistivity (PTCR) transition around 130°C. However, they showed negative-temperature-coefficient-of-resistivity (NTCR) behavior in air, but not in nitrogen, after the PTCR transition. The thermal and chemical analyses suggest that the number of OH functional groups in the epoxy resin influences the occurrence of the NTCR transition. The heating rate dependence of the ρs-T behavior showed that the anomalous PTCR effect in composite thin films occurs with a rapid thermal strain rate above the glass transition temperature of the matrix. This result suggests that composite thin films can be used in temperature sensors that will selectively detect a rapid environmental temperature increase. These results can be interpreted satisfactorily by considering the interaction between the components, which is strongly influenced by OH functional groups and the matrix thermal strain rate. We constructed a qualitative model of the anomalous resistivity-temperature behavior, focusing on the microscopic interaction between the components. The results show that the interface property plays a decisive role in controlling the resistivity-temperature behavior in conductive thermoset-matrix composites.