Characteristics of Breakdown Strength of Hydrocarbon Thermosetting Resin in Low Temperature Environment

Dicyclopentadiene (DCP) resin is a hydrocarbon-based thermosetting resin (HTR) with better crack resistance than epoxy (EP) resin, as an organic insulating material used in low-temperature regions such as liquid helium (LHe) and liquid nitrogen (LN2). This paper presents dielectric breakdown strength, Eb, of DCP under AC and standard lightning impulse (Imp) voltage application at room temperature and LN2 temperature while comparing with results of EP resin. It was found that Eb of DCP at LN2 temperature exceeded that at room temperature, indicating ∂Eb∂/T < 0, and larger than that of EP. Experimental results showed that Eb of EP and DCP at room temperature decreased with increasing the sample thickness d, indicating ∂Eb∂/d < 0. Since EP exhibited ∂Eb∂/T < 0 and ∂Eb/∂d < 0, an attempt is made to discuss breakdown mechanism in terms of the electron avalanche breakdown theory as a dominant one in EP. As a result, the ionization coefficient H of EP was evaluated as 1670 kV/mm, the mean free path λ as 195 nm, and the mobility μ as 65.4×10-4 m2/Vs. On the other hand, since DCP shows a negative temperature dependence (∂Eb/∂T < 0) in the low temperature region, the breakdown mechanism is discussed with the electronic thermal breakdown mechanism proposed by Frohlich based on the balance between the energy gain due to the interaction between electrons in the excited impurity level (ΔV) and electrons in the conduction band, and the energy loss to the lattice system. As a result, ΔV was evaluated to be 5.7 meV in DCP.