In this letter the authors report the observation of double hysteresis loops in Cu-doped K 0.5 Na 0.5 NbO 3 ͑KNN͒ ceramics. Unlike other ferroelectric titanates ͑e.g., BaTiO 3 ͒, aging is not required for the ceramic to exhibit the double-loop-like characteristics. Based on the symmetry-conforming principle of point defects, it is suggested that defect dipoles are formed by the acceptor dopant ions-Cu 2+ and O 2− vacancies along the polarization direction after the diffuse tetragonal-orthorhombic phase transition of the ceramic. Because of the low migration rates of defects, the defect dipoles remain in the original orientation during the P-E loop measurement, providing a restoring force to reverse the switched polarization. The defect dipoles also provide "pinning" effects in the normal piezoelectric activities. As a result, the ceramic becomes "hardened," exhibiting an extraordinarily high mechanical quality factor ͑2500͒, while the other piezoelectric properties remain reasonably good: electromechanical coupling coefficients k p = 39%, k t = 47%, and piezoelectric coefficient d 33 =82 pC/N.
Biodegradable poly(L-lactic acid) (PLLA) fibers were processed by a twostep melt-spinning method (melt extrusion and hot draw) from PLLA with three different viscosity-average molecular weights (494,600, 304,700, and 262,800). Before spinning, the polymer flakes were first milled into powders and dried under vacuum. Viscosity-average molecular weight of PLLA following the fabrication process was monitored. Tensile properties of as-spun and hot-drawn fibers were investigated. Morphology of the PLLA fibers was viewed under a scanning electron microscope. Crystallinity of these fibers was assessed by thermogram analysis of differential scanning calorimetry. Results showed that the extent of decrease in the viscosity-average molecular weight of PLLA dropped sharply by 13.1-19.5% during pulverization and by 39.0 -69.0% during melt-extrusion. The hot-draw process in this study had a little effect on the viscosity-average molecular weight of PLLA. Smoother fibers could be obtained for the die temperature at least 230°C for raw materials with higher crystallinity (more than 75%) and at least 220°C for raw materials with lower crystallinity (about 60%). The as-spun fibers showed crystallinity of 16.5-22.8% and the value increased to 50.3-63.7% after hot draw. Tensile moduli of the as-spun fibers were in the range of 1.2-2.4 GPa, which were raised to 3.6 -5.4 GPa after hot draw. The final PLLA fibers with 110 -160 m diameters showed tensile strengths of 300 -600 MPa.
CuO-doped K0.5Na0.5NbO3 (K0.5Na0.5NbO3 + x mol% CuO) lead-free ceramics have been prepared by an ordinary sintering technique. The results of x-ray diffraction patterns show that all the ceramics possess a pure pervoskite structure with orthorhombic symmetry. Based on the symmetry-conforming principle of point defects, defect dipoles are formed by the acceptor dopant ions Cu2+ and O2− vacancies along the polarization direction and remain in the original orientation, providing a restoring force to reverse the switched polarization, which results in double polarization-electric hysteresis loops for the ceramics with x ≥ 0.50 at 100 Hz. That is, the ferroelectric dipoles are back to their original orientation on the removal of electric field due to the strong restoring force generated by defect dipoles. As a result, the ceramics (x ≥ 0.5) become considerably ‘hardened’ and possess an extraordinarily high Qm (825–2523) and simultaneously the other piezoelectric properties remain reasonably good (kp = 39%, kt = 47% and d33 = 82 pC N−1).
The use of lead-free materials has recently become a very important issue in environmental protection of the earth. Two groups of lead-free ceramics, ͑K 0.5 ,Na 0.5 ͒NbO 3 based ͑KNN͒ and Bi 1−y ͑Na x K 1−x ͒ y TiO 3 based ͑BNKT͒, were studied for their thermal, dielectric, and pyroelectric properties as candidates for pyroelectric sensor applications. The BNKT-based ceramic, ͓Bi 0.5 ͑Na 0.94 K 0.05 Li 0.016 ͒ 0.5 ͔ 0.95 Ba 0.05 TiO 3 ͑BNKLBT͒, shows excellent pyroelectric properties when compared with KNN-based ceramic and lead zirconate titanate. Its properties were measured as follows: pyroelectric coefficient p = 360 C / m 2 K, pyroelectric figure of merit of current, voltage, and detectivity F i = 221 pm/ V, F v = 0.030 m 2 / C, and F d = 14.8 Pa −1/2 . With these outstanding pyroelectric properties, the BNKLBT ceramic can be a promising material for pyroelectric sensor applications. The BNKLBT ceramic with different thicknesses ͑i.e., 0.3, 0.5, and 0.7 mm͒ have been used as the sensing element for fabricating infrared detectors. The current responsivity of the sensors was evaluated as functions of frequency.
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