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Poly(vinylidene) fluoride (PVDF) is a prototypical piezoelectric polymer, which exhibits a strong piezoelectricity after a poling treatment. In this work, we investigate the polarization behaviors of PVDF film under cyclic electric fields by measuring polarized currents. A compensation method is used to distinguish the polarized current from the capacitive and resistive currents. The PVDF samples with a thickness of 20 or 30 μm and an electrode area of 3 × 3 or 4 × 4 mm2 are employed, respectively. From the observations, we find that the electrode area does not affect the current density, but a thicker film has a higher residual polarization due to its larger fraction of effective switchable dipoles. The amplitude of the applied field primarily determines the residual polarization. Impressively, an α‐ to β‐phase transformation occurs above 225 MV · m−1 and the polarized current reaches the maximum and becomes converges at 350 MV · m−1. Moreover, a high degree and reproducible residual polarization is obtained through gradually increasing applied field from 50 to 350 MV · m−1, and the relative standard deviation is 3.74% for 20 available samples. Furthermore, a dipole switching model is used to describe the polarization behaviors of PVDF with a form of f(E) = Ae−B|E + C| and the fitting parameters give a further illustration of polarization behaviors in PVDF. POLYM. ENG. SCI., 60:645–656, 2020. © 2020 Society of Plastics Engineers

The tensile properties and the corresponding deformation mechanism of the graded 304 stainless steel (ss) at both room and cryogenic temperatures were investigated and compared with those of the coarse-grained (CGed) 304 ss. Gradient structures were found to have excellent synergy of strength and ductility at room temperature, and both the yield strength and the uniform elongation were found to be simultaneously improved at cryogenic temperature in the gradient structures, as compared to those for the CG sample. The hetero-deformation-induced (HDI) hardening was found to play a more important role in the gradient structures as compared to the CG sample and be more obvious at cryogenic temperature as compared to that at room temperature. The central layer in the gradient structures provides stronger strain hardening during tensile deformation at both temperatures, due to more volume fraction of martensitic transformation. The volume fraction of martensitic transformation in the gradient structures was found to be much higher at cryogenic temperature, resulting in a much stronger strain hardening at cryogenic temperature. The amount of martensitic transformation at the central layer of the gradient structures is observed to be even higher than that for the CG sample at cryogenic temperature, which is one of the origins for the simultaneous improvement of strength and ductility by the gradient structures at cryogenic temperature.

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