Harmonics in SiC/Alkyd resin composites with different SiC particle sizes under different temperatures

In order to investigate the AC response and the harmonic characteristics of stress grading material, samples of SiC/alkyd resin with 20 and 40 μm SiC particles were prepared and experiments were carried out from 30 to 100 °C. At the higher temperature, current in the composite with 20 μm SiC particles increased whereas the weak effect of temperature on current was observed with 40 μm SiC particles. In addition, the threshold voltage of volt-ampere curve decreases with increased SiC particle size. The current spectrum shows that the third harmonic current accounts for the majority of the harmonics. As the SiC particle size increases, the harmonic ratio increases and the nonlinearity of the composites becomes larger, whereas higher temperature leads to lower harmonic ratio and lower nonlinearity of composites. A power function is proposed to describe the correlation between the harmonic ratio and the nonlinearity of volt-ampere curve. These may be associated with charge hopping at low voltage and tunneling at high voltage. The lower number of barriers per unit distance in the composite with larger SiC particle size promotes the tunneling process, which leads to the decrease of the threshold voltage. Tunneling becomes the primary reason for the current after the threshold voltage and it is not influenced by temperature. Thus the composite with larger SiC particle size has current not influenced greatly by temperature after the threshold voltage. Tunneling leads to a greater increase in current and more waveform distortion, thus the composite with larger SiC particle size shows larger nonlinearity and higher harmonic ratio. Furthermore, hopping is associated with temperature while tunneling is not. The higher temperature decreases the ratio of tunneling to hopping which leads to the relative decrease in tunneling process. Thus the harmonic ratio decreases and the nonlinearity of composites becomes smaller with increased temperature.