Dongmei Zhu

An approach to further improve piezoelectric properties of (K0.5Na0.5)NbO3-based lead-free ceramics

An approach to further improve piezoelectric properties of (1−x)(K0.5Na0.5)NbO3–xLiNbO3 (KNLN) ceramics, i.e., the optimum poling temperatures (Tp) of KNLN ceramics should be chosen near the polymorphic phase transition temperatures, is proposed. This is contrary to conventional wisdom associated with electrical poling of ceramics. Piezoelectric constant d33 of KNLN (x=0.07) ceramics increases from 210to274pC∕N by selecting optimum Tp. This result indicates that this approach is very effective in improving piezoelectric properties of KNLN ceramics. Moreover, this approach can also be applied to other (K0.5Na0.5)NbO3 based ceramics.

Phase structure, dielectric properties, and relaxor behavior of (K0.5Na0.5)NbO3–(Ba0.5Sr0.5)TiO3 lead-free solid solution for high temperature applications

The (1−x)(K0.5Na0.5)NbO3–x(Ba0.5Sr0.5)TiO3 (KNN-BST) solid solution has been synthesized by conventional solid-state sintering in order to search for the new lead-free relaxor ferroelectrics for high temperature applications. The phase structure, dielectric properties, and relaxor behavior of the (1−x)KNN-xBST solid solution are systematically investigated. The phase structure of the (1−x)KNN-xBST solid solution gradually changes from pure perovskite phase with an orthorhombic symmetry to the tetragonal symmetry, then to the pseudocubic phase, and to the cubic phase with increasing addition of BST. The 0.90KNN-0.10BST solid solution shows a broad dielectric peak with permittivity maximum near 2500 and low dielectric loss (<4%) in the temperature range of 100–250 °C. The result indicates that this material may have great potential for a variety of high temperature applications. The diffuse phase transition and the temperature of the maximum dielectric permittivity shifting toward higher temperature with in...

Design and electrical properties' investigation of (K0.5Na0.5)NbO3-BiMeO3 lead-free piezoelectric ceramics

Based on the discussion on the “origin” of the high piezoelectric properties of Pb-based piezoelectric ceramics, it was predicted that (K0.5Na0.5)NbO3–BiMeO3 solid solutions (where Me3+=Sc, Al, Ga, Y, In, etc.) should possess high piezoelectric properties because of the formation of the morphotropic phase boundary and the hybridization between the Bi 6p and O 2p orbits. (1−x)(K0.5Na0.5)NbO3–xBiScO3 [(1−x)KNN-xBS] ceramics were selected as an example to verify this prediction. (1−x)KNN-xBS ceramics were synthesized by conventional solid-state sintering. The phase structure, microstructure, and dielectric and piezoelectric properties of (1−x)KNN-xBS ceramics were investigated. At room temperature, the polymorphic phase transition (PPT) (from the orthorhombic to the tetragonal phase) in (1−x)KNN-xBS ceramics is identified at x=0.0175 by the analysis of x-ray diffraction patterns and dielectric spectroscopy. The ceramics (x=0.0175) with PPT near room temperature exhibit excellent electrical properties (d33=∼2...

Phase structure, microstructure, and electrical properties of bismuth modified potassium-sodium niobium lead-free ceramics

(K0.5Na0.5)1−3xBixNbO3 (KNBN) ceramics were prepared by conventional solid-state sintering without cold-isostatic pressing. The phase structure, microstructure, and electrical properties of KNBN ceramics are studied. The phase structure of KNBN ceramics (x=0.01) is pure perovskite phase with orthorhombic symmetry at room temperature. The addition of Bi3+ significantly improves electrical properties of (K0.5Na0.5)NbO3 ceramics while keeping the tetragonal-orthorhombic phase transition temperature above 170 °C. The KNBN (x=0.01) ceramics show the optimum electrical properties (d33=164 pC∕N, kp=0.47, Qm=120, TC=403 °C, TO-T=174 °C, Pr=30.1 μC∕cm2, and Ec=6.18 kV∕cm). Take into account electrical properties and the polymorphic phase transition temperature (TO-T), it can be concluded that the KNBN (x=0.01) ceramic is a promising lead-free piezoelectric candidate material for practical application. The related mechanisms for high piezoelectric properties in KNBN (x=0.01) ceramics are also discussed.

Perovskite lithium and bismuth modified potassium-sodium niobium lead-free ceramics for high temperature applications

[(K(1−x)∕2Na(1−x)∕2Lix)1−3yBiy]NbO3 (abbreviated as KNLBN) ceramics were prepared by conventional solid-state sintering. The addition of Li+ and Bi3+ makes the orthorhombic-tetragonal phase transition temperature (TO-T) decrease from 200°C for pure (K0.5Na0.5)NbO3 ceramics to −10°C for KNLBN ceramics (x=0.06, y=0.005), while maintaining high Curie temperature (TC=455°C) and piezoelectric properties (d33=185pC∕N, kp=0.43, kt=0.45, e33T∕e0=1020, and tanδ=0.023) at room temperature. In addition, good temperature stability of electrical properties is obtained in KNLBN ceramics (x=0.06, y=0.005) owing to the decrease of TO-T. These results indicate that KNLBN (x=0.06, y=0.005) ceramic is a promising lead-free piezoelectric candidate material for high temperature applications.

High Tm lead-free relaxor ferroelectrics with broad temperature usage range: 0.04BiScO3−0.96(K0.5Na0.5)NbO3

In order to develop high temperature lead-free relaxors, xBiScO3−(1−x)(K0.5Na0.5)NbO3[xBS−(1−x)KNN] ceramics were proposed using a tolerance factor approach. To verify this proposal, xBS−(1−x)KNN ceramics were synthesized by conventional solid-state sintering. A stable perovskite phase was obtained when KNN content was greater than 96 mol %. The diffuse phase transition and frequency dispersion of the dielectric constant, which are two typical characteristics of relaxor ferroelectrics, were observed in xBS−(1−x)KNN ceramics. The dielectric relaxor behavior follows a modified Curie–Weiss law relationship. In addition, 0.04BS-0.96KNN ceramics show a broad and stable permittivity maximum near 2500 from 100 to 300 °C and lower dielectric loss (<5%) at broad temperature usage range (100–300 °C). The results indicate that this material may have great potential for high temperature capacitors in automobile applications.

Structure and electrical properties' investigation of (K0.5Na0.5)NbO3?(Bi0.5Na0.5)TiO3 lead-free piezoelectric ceramics

Lead-free piezoelectric ceramics (1 − x)(K0.5Na0.5)NbO3–x(Bi0.5Na0.5)TiO3 [(1 − x)KNN–xBNT] were synthesized by conventional solid-state sintering. The phase structure and electrical properties of (1 − x)KNN–xBNT ceramics are investigated. The polymorphic phase transition (from the orthorhombic to the tetragonal phase) (PPT) plays a very important role in (1 −x)KNN–xBNT ceramics. A PPT at room temperature in (1 − x)KNN–xBNT ceramics is identified for 0.02 < x ≤ 0.03. The (1 − x)KNN–xBNT (x = 0.025) ceramics show optimum electrical properties (d33 256 pC N−1, d31 = 85 pC N−1, kp = 0.48, kt = 0.52 and TC = 373 °C). The related mechanisms for high piezoelectric properties in (1 − x)KNN–xBNT (x = 0.025) ceramics are also discussed. These results indicate that the (1 − x)KNN–xBNT (x = 0.025) ceramic is a promising lead-free piezoelectric candidate material.

Polymorphic phase transition dependence of piezoelectric properties in (K0.5Na0.5)NbO3–(Bi0.5K0.5)TiO3 lead-free ceramics

Lead-free ceramics (1 − x)(K0.5Na0.5)NbO3–x(Bi0.5K0.5)TiO3 [(1 − x)KNN–xBKT] were synthesized by conventional solid-state sintering. The phase structure, microstructure and electrical properties of (1 − x)KNN–xBKT ceramics were investigated. At room temperature, the polymorphic phase transition (from the orthorhombic to the tetragonal phase) (PPT) was identified at x = 0.02 by the analysis of x-ray diffraction patterns and dielectric spectroscopy. Enhanced electrical properties (d33 = 251 pC N−1, kp = 0.49, kt = 0.50, , tan δ = 0.03 and TC = 376 °C) were obtained in the ceramics with x = 0.02 owing to the formation of the PPT at 70 °C and the selection of an optimum poling temperature. The related mechanisms for high piezoelectric properties in (1 − x)KNN–xBKT (x = 0.02) ceramics were discussed. In addition, the results confirmed that the selection of the optimum poling temperature was an effective way to further improve the piezoelectric properties of KNN-based ceramics. The enhanced properties were comparable to those of hard Pb(Zr, Ti)O3 ceramics and indicated that the (1 − x)KNN–xBKT (x = 0.02) ceramic was a promising lead-free piezoelectric candidate material for actuator and transducer applications.

Effect of magnetic fillers on the electromagnetic properties of CaCu3Ti4O12–epoxy composites within the 2–18 GHz range

The electromagnetic properties of the CaCu3Ti4O12 (CCTO)-magnetic particles–epoxy composite in the frequency range of 2–18 GHz are reported in this paper. The results showed that both the dielectric loss and the magnetic loss of the CCTO–epoxy composite were lower, and the dielectric constant was nearly independent of frequency over the measured frequency range. In order to improve the microwave absorption of the CCTO–epoxy composite, soft magnetic particles (barium ferrite, Fe–Si–Al and carbonyl iron flakes) were introduced. The effect of the magnetic fillers on the electromagnetic properties of the CCTO–epoxy composites within the 2–18 GHz range was also investigated. The experimental results showed that the complex permittivity and permeability of the composites were dependent on the type of filler and the frequency band, and the increasing rates of the real and imaginary parts with respect to the types of fillers were all different. These different rates can have a great effect on the thickness and microwave absorption in the design of single-layer microwave absorbers.

Influence of pyrolytic carbon coatings on complex permittivity and microwave absorbing properties of Al2O3 fiber woven fabrics

The pyrolytic carbon (PyC) coatings were fabricated on Al2O3 fiber fabrics by the method of chemical vapor deposition (CVD). The microstructures of Al2O3 fibers with and without PyC coatings were characterized by SEM and Raman spectroscopy. The influence of deposition time of PyC on the DC conductivity (σd) of Al2O3 filaments and complex permittivity of fabrics at X band (8.2-12.4 GHz) were investigated. The values of σd and complex permittivity increase with increasing deposition time of PyC. The electron relaxation polarization and conductance loss were supposed to be contributed to the increase of ɛ′ and ɛ″, respectively. In addition, the reflection loss (RL) of fabrics was calculated. The results show that the microwave absorbing properties of Al2O3 fiber fabrics can be improved by PyC coatings. The best RL results are for 60 min-deposition sample, of which the minimum value is about −40.4 dB at about 9.5 GHz and the absorbing frequency band (AFB) is about 4 GHz.