Two switching phenomena of poly (vinylidene fluoride) (PVDF) containing carbon black (CB) with different crystallization histories are studied. Both the percolation threshold (Φpc) and the critical concentration of the positive temperature coefficient (PTC) materials (Φc) increase as the crystallization temperature declines. PTC and the negative temperature coefficient (NTC) effect become obvious when the cooling speed is reduced. The morphologies of PVDF crystals play an important role in modifying the shape of the resistivity-temperature (ρ-T) curves. The PTC and NTC intensities of these systems, whose CB contents lie between Φpc and Φc, increase with increasing crystallinity, which are realized by altering the crystallization histories of the matrices.
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Zhang et al. (1996) studied this question.