Using two hexagonal H{sub}2O ices, polycrystalline and single-crystalline ices, the electrical treeing has been studied under an applied impulse voltage. The crystallinity and temperature of ices play an important role in the events of the treeing. In the single-crystalline ice, the tree in samples at -25°C always progressed along the basal plane of the ice crystal, but at -196°C, it progressed along the C-axis of the crystal. These distinctive patterns on the tree progression were attributed to the crystal-axis dependence of the relative permittivity at each temperature. Intermittent current pulses accompanying the trees were visible in the wave tail of the applied voltage. These current pulses appears to generate by the movement of accumulated charges after the formation of tree channels.
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