Although
electrostriction is ubiquitous for dielectric polymers,
giant electrostriction has not been observed until relaxor ferroelectric
(RFE) poly(vinylidene fluoride) (PVDF)-based polymers are achieved.
However, the exact origin for giant electrostriction in these polymers
has not been fully understood. By studying the electrostriction in
the uniaxially stretched films of a ferroelectric poly(VDF-co-trifluoroethylene) [P(VDF-TrFE)] random copolymer and
an RFE poly(VDF-co-TrFE-co-chlorotrifluoroethylene)
[P(VDF-TrFE-CTFE)] random terpolymer in this work, we confirmed that
ferroelectric switching with large hysteresis, such as in the case
of P(VDF-TrFE), was not genuine electrostriction. By decreasing large
ferroelectric domains to the nanometer scale (i.e., 2–3 nm),
such as in the case of the P(VDF-TrFE-CTFE) terpolymer, electrostriction
with low hysteresis could be achieved. Two origins of the large electrostriction
in these polymers were identified. The first was the mechano-electrostriction
due to the poling field-induced conformation transformation of oriented
polymer chains. The second was the electric repulsion of electrically
aligned nanodomains. These effects could occur in both crystals and
the oriented amorphous fraction, which links between the nanocrystals
and the isotropic amorphous fraction. When the poling field was relatively
low (e.g., <40 MV/m), the mechano-electrostriction was the major
contribution and the electric repulsion effect was a minor contribution
to electrostriction. Meanwhile, a strong temperature dependence of
the low-field electrostriction coefficient was observed. Finally,
we found an empirical inverse relationship between the electrostriction
coefficient and the product of Young’s modulus and dielectric
constant. The knowledge obtained from this study provides an insightful
understanding of the electrostriction mechanism in PVDF-based electroactive
polymers, which will find potential applications in sensors and actuators
for wearable electronics and soft robotics.
Unipoled piezoelectric transformers with different input and output area ratios were fabricated. The electrical properties in terms of voltage step-up ratio, output power, efficiency, and temperature rise were measured at various load resistance. The voltage step-up ratio increased proportionally with load resistance. The relative efficiency showed a maximum value of more than 98% in the transformer, which is 23.6 mm in diameter and 1.0 mm in thickness with input/out electrode area ratio of 0.62. With the driving voltage of 35 V rms , the 8.1 W output power was obtained in the transformer with an input/output electrodes area ratio of 6.47. In such conditions, the temperature rise was only 15 C from room temperature. This transformer, which has approximately ten times higher power density than a rectangular Rosen-type piezoelectric transformer, was successfully applied to operate an 8 W compact fluorescent lamp.
A solid solution of lead zirconate–lead nickel niobate ceramics, Pb[Zr1−x(Ni1/3Nb2/3)x]O3 (PZNN) with x = 0.0–0.5, was synthesized via the columbite precursor method. The crystal structures as well as the thermal and dielectric properties were investigated in terms of the lead nickel niobate (PNN) concentration. X-ray diffraction indicated that all samples exhibited a single-phase perovskite structure. At room temperature, Pb[Zr1−x(Ni1/3Nb2/3)x]O3 is orthorhombic for a composition where x = 0, rhombohedral for the compositions where x = 0.1, 0.2 and 0.3 and pseudo-cubic for compositions where x = 0.4 and 0.5. The results of the addition of lead nickel niobate to the lead zirconate ceramic showed enhancement of the room-temperature dielectric permittivity. Lead nickel niobate substitution also led to lower transition temperatures. Furthermore, this transition from normal to relaxor FE ceramics was typified by a quasi-linear relationship between the diffuseness parameter δγ and the PNN mole fraction x.
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