K. W. Kwok

Double hysteresis loop in Cu-doped K0.5Na0.5NbO3 lead-free piezoelectric ceramics

In this letter the authors report the observation of double hysteresis loops in Cu-doped K0.5Na0.5NbO3 (KNN) ceramics. Unlike other ferroelectric titanates (e.g., BaTiO3), 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-Cu2+ and O2− 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 (2500), while the other piezoelectric properties remain reasonably good: electromechanical couplin...

Structure and electrical properties of K0.5Na0.5NbO3–LiSbO3 lead-free piezoelectric ceramics

Lead-free piezoelectricceramics ( 1 − x ) K 0.5 Na 0.5 Nb O 3 – x Li Sb O 3 have been fabricated by a conventional ceramicsintering technique. The results of x-ray diffraction suggest that Li + and Sb 5 + diffuse into the K 0.5 Na 0.5 Nb O 3 lattices to form a solid solution with a perovskite structure. The ceramics can be well sintered at 1070 – 1110 ° C . The introduction of Li Sb O 3 into the Na 0.5 K 0.5 Nb O 3 solid solution decreases slightly the paraelectric cubic-ferroelectric tetragonal phase transition temperature ( T c ) , but greatly shifts the ferroelectric tetragonal-ferroelectric orthorhombic phase transition ( T O – F ) to room temperature. Coexistence of the orthorhombic and tetragonal phases is formed at 0.05 < x < 0.07 at room temperature, leading to a significant enhancement of the piezoelectric properties. For the ceramic with x = 0.06 , the piezoelectric properties become optimum: piezoelectric constant d 33 = 212 pC ∕ N , planar and thickness electromechanical coupling factors k P = 46 % and k t = 47 % , respectively, remanent polarization P r = 15.0 μ C ∕ cm 2 , coercive field E c = 1.74 kV ∕ mm , and Curie temperature T C = 358 ° C .

Evaluation of the material parameters of piezoelectric materials by various methods

The elastic, dielectric and piezoelectric constants of four piezoelectric materials, including polyvinylidene fluoride, vinylidene fluoride-trifluoroethylene copolymer, PZT/epoxy 1-3 composite, and lead metaniobate ceramic, have been evaluated from the impedance data using five different methods. A method described in ANSI/IEEE Std. 176-1987, though based on formulae derived for loss-less materials, is found to be applicable to materials with moderate loss. However, for high-loss materials such as polyvinylidene fluoride, the electromechanical coupling constant (/spl kappa//sub t/) obtained by the method of Std. 176 is substantially higher than the actual value. Calculations based on a piezoelectric resonance analysis program (PRAP) combine the best features of two earlier methods. In addition to the impedance at the parallel resonance frequency, impedances at two other frequencies are required for calculation. The PRAP method gives quite accurate material parameters regardless of the magnitude of the loss, but the parameters (including /spl kappa//sub t/) vary by as much as 15% depending on the choice of data. In the nonlinear regression method described in the present work, all the impedance data points around the resonance are least-squares fitted to the theoretical expression for the impedance. Besides the advantage of requiring no arbitrary choice of data, the nonlinear regression method can readily take account of the frequency dependence of the dielectric constant.

Microstructure, phase transition, and electrical properties of (K0.5Na0.5)1−xLix(Nb1−yTay)O3 lead-free piezoelectric ceramics

Lead-free ceramics (K0.5Na0.5)1−xLix(Nb1−yTay)O3 have been prepared by an ordinary sintering technique. Our results reveal that Li+ and Ta5+ diffuse into the K0.5Na0.5NbO3 lattices to form a solid solution with a perovskite structure. The substitution of Li+ induces an increase in the Curie temperature (TC) and a decrease in the ferroelectric tetragonal–ferroelectric orthorhombic phase transition temperature (TO-T). On the other hand, both TC and TO-T decrease after the substitution of Ta5+. A coexistence of the orthorhombic and tetragonal phases is formed at 0.03<x<0.06 and 0.10<y<0.25 near room temperature, leading to significant enhancements of the piezoelectric properties. For the ceramic with x=0.04 and y=0.225, the piezoelectric properties become optimum, giving a piezoelectric coefficient d33=208pC∕N, electromechanical coupling factors kP=48% and kt=49%, remanent polarization Pr=14.2μC∕cm2, coercive field Ec=1.21kV∕mm, and Curie temperature TC=320°C.

Lead-free ceramics for pyroelectric applications

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, (K0.5,Na0.5)NbO3 based (KNN) and Bi1−y(NaxK1−x)yTiO3 based (BNKT), were studied for their thermal, dielectric, and pyroelectric properties as candidates for pyroelectric sensor applications. The BNKT-based ceramic, [Bi0.5(Na0.94K0.05Li0.016)0.5]0.95Ba0.05TiO3 (BNKLBT), shows excellent pyroelectric properties when compared with KNN-based ceramic and lead zirconate titanate. Its properties were measured as follows: pyroelectric coefficient p=360μC∕m2K, pyroelectric figure of merit of current, voltage, and detectivity Fi=221pm∕V, Fv=0.030m2∕C, and Fd=14.8μPa−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 0.7mm) have been used as the sensing element for fabricating infrared detectors. The current responsivi...

Energy harvesting with piezoelectric drum transducer

Piezoelectric materials can convert ambient vibrations into electrical energy. In this letter, the capability of harvesting the electrical energy from mechanical vibrations in a dynamic environment through a piezoelectric drum transducer has been investigated. Under a prestress of 0.15N and a cyclic stress of 0.7N, a power of 11mW was generated at the resonance frequency of the transducer (590Hz) across an 18kΩ resistor. It is found that the energy from the transducer increases while the resonance frequency of the transducer decreases when the prestress increases. The results demonstrate the potential of the drum transducer in energy harvesting.

Piezoresponse and ferroelectric properties of lead-free [Bi0.5(Na0.7K0.2Li0.1)0.5]TiO3 thin films by pulsed laser deposition

Polycrystalline lead-free piezoelectric [Bi0.5(Na0.7K0.2Li0.1)0.5]TiO3 (BNKLT) thin films were grown on Pt∕Ti∕SiO2∕Si substrates using pulsed laser deposition (PLD). In this letter, we report the ferroelectric properties and piezoresponse of the PLD-produced BNKLT thin films. X-ray diffraction characterization revealed a good crystallinity and a pure perovskite structure in the films. The films exhibited a well-defined polarization hysteresis loop with a remnant polarization Pr of 13.9μC∕cm2 and a coercive field Ec of 10.2MV∕m. The domain structure and its thermal-driven evolution from the ferroelectric to nonferroelectric phase were observed by piezoresponse force microscopy. The results were consistent with the phase transition profile of BNKLT bulk ceramics. Typical butterfly-shaped piezoresponse loop was obtained and the effective piezoelectric coefficient d33,f of the BNKLT thin films was about 64pm∕V.

Dielectric and piezoelectric properties of (K0.5Na0.5)NbO3–Ba(Zr0.05Ti0.95)O3 lead-free ceramics

Lead-free ceramics (1−x)(K0.5Na0.5)NbO3–xBa(Zr0.05Ti0.95)O3 doped with 1mol% MnO2 have been fabricated by pressureless sintering. With the MnO2 doping, all the ceramics can be well sintered at 1100–1160°C and exhibit a dense and pure perovskite structure. After the addition of Ba(Zr0.05Ti0.95)O3, a relax behavior is induced and both the cubic-tetragonal and tetragonal-orthorhombic phase transitions shift to lower temperatures. Coexistence of the orthorhombic and tetragonal phases is hence formed in the ceramics with 0.04<x<0.07 at room temperature. It is suggested that owing to the more possible polarization states resulting from the coexistence of the two phases, the piezoelectric and dielectric properties of the ceramics are enhanced significantly. The ceramic with x=0.06 exhibits the following optimum properties: d33=234pC∕N, kp=0.49, kt=0.48, er=1191, tanδ=1.20%, and TC=318°C.

Microstructure and electrical properties of La-modified K0.5Na0.5NbO3 lead-free piezoelectric ceramics

Lead-free ceramics (K0.5Na0.5)1−3xLaxNbO3 (0 ≤ x ≤ 0.0175) have been fabricated by a conventional sintering technique. The results of XRD show that the ceramics possess a perovskite structure with orthorhombic symmetry. Moreover, doping inhibits the grain growth, decreases the ferroelectric–paraelectric phase transition temperature and induces a diffuse phase transition. At low doping levels (x ≤ 0.0075), the observed remanent polarization (Pr) and coercive field (Ec) remain almost unchanged. As x increases from 0.0075 to 0.0175, Pr starts to decrease while Ec increases. Nevertheless, due to the increase in relative permittivity, the ceramic with x = 0.0125 exhibits the optimum piezoelectric properties, giving a large piezoelectric coefficient (d33 = 135 pC N−1) and a high planar electromechanical coupling coefficient (kP = 0.40).

Structure, piezoelectric and ferroelectric properties of Li- and Sb-modified K0.5Na0.5NbO3 lead-free ceramics

Lead-free ceramics (K0.5Na0.5)1−xLix(Nb1−ySby)O3 have been prepared by an ordinary sintering technique. Li+ and Sb5+ diffuse into the K0.5Na0.5NbO3 lattices to form a solid solution with the perovskite structure. The substitution of Li+ increases the Curie temperature Tc and shifts the ferroelectric tetragonal–ferroelectric orthorhombic phase transition (TO–T) to low temperatures. On the other hand, the substitution of Sb5+ decreases Tc and has a minor effect on TO–T. The coexistence of the orthorhombic and tetragonal phases at x = 0.06 and y = 0.06 near room temperature and the strong covalence of Sb lead to significant improvements in the piezoelectric properties. For the (K0.5Na0.5)0.94Li0.06(Na0.94Sb0.06)O3 ceramic, piezoelectric constant d33 = 212 pC N−1, planar and thickness electromechanical coupling factors kP = 46% and kt = 47% and Tc = 358 °C. The donor-type doping of BaCO3, SrCO3 or Bi2O3 leads to further enhancements of the piezoelectric and ferroelectric properties.