Jin-Feng Wang

Piezoelectric properties in perovskite 0.948(K0.5Na0.5)NbO3–0.052LiSbO3 lead-free ceramics

Lead-free piezoelectric ceramics, with the nominal composition of 0.948(K0.5Na0.5)NbO3–0.052LiSbO3 (KNN-LS5.2), were synthesized by conventional solid-state sintering, and the piezoelectric and electromechanical properties were characterized as a function of temperature. The Curie temperature of the KNN based perovskite material was found to be 368°C with an orthorhombic-tetragonal polymorphic phase transition (TO-T) temperature at approximately ∼35°C. The room temperature dielectric permittivity (e33T∕e0) and loss were found to be 1380 and 2%, respectively, with piezoelectric properties of k33∼62% and d33∼265pC∕N and k31∼30% and d31∼−116pC∕N. The temperature dependence of the properties mimicked the compositional variation seen in the proximity of a morphotropic phase boundary [e.g., lead zirconate titanate (PZT)], with a maxima in the dielectric and piezoelectric properties and a corresponding “softening” of the elastic properties. Unlike that found for PZT-type materials, the modified KNN material exhi...

Perovskite (Na0.5K0.5)1−x(LiSb)xNb1−xO3 lead-free piezoceramics

Lead-free potassium sodium niobate piezoelectric ceramics substituted with lithium and antimony (Na0.5K0.5)1−x(LiSb)xNb1−xO3 have been synthesized by conventional solid state sintering method. Compositionally engineered around the orthorhombic-tetragonal polymorphic phase transition, the dielectric and piezoelectric properties were further enhanced with the addition of lithium and antimony substituted into the perovskite structure. The combined effects of lithium and antimony additions resulted in a downward shift in the orthorhombic-tetragonal (TO-T) without significantly reducing TC. The dielectric, piezoelectric, and electromechanical properties were found to be e∕e0>1300, d33>260pC∕N, and kp>50%, while maintaining low dielectric loss. The enhanced polarizability associated with the polymorphic TO-T transition and high TC transition (∼390°C) should provide a wide range of temperature operation.

Piezoelectric properties of (Li, Sb, Ta) modified (Na,K)NbO3 lead-free ceramics

Lead-free alkaline niobate based (Na0.52K0.48−xLix)Nb1−x−ySbxTayO3 piezoceramics have been prepared by the conventional mixed oxide method without using other techniques. An experimental formula for producing a set of ceramics with high piezoelectric properties is obtained while cutting down the Ta content and maintaining a high Curie temperature. The highest piezoelectric constant d33 is 308pC∕N, with a dielectric loss tanδ of about 2.0% and a Curie temperature of 339°C. The samples also possess outstanding high-field piezoelectric strain effects. The high-field piezoelectric strain coefficient d33* is as high as 490pm∕V. (Li, Sb, Ta) modified (Na,K)NbO3 shifts the orthorhombic to tetragonal phase transition to near room temperature, which plays an important role in the improvement of the piezoelectric properties.

Grain boundary effect on the dielectric properties of CaCu3Ti4O12 ceramics

The electrical properties of CaCu3Ti4O12 ceramic materials, showing an enormously large dielectric constant, were investigated. It was found that the grain boundary plays an important role in the giant dielectric behaviour of these ceramics. Measurement of the electrical current density (J) versus the electrical field (E) was carried out. A good linear relationship between lnJ and E1/2 was found, which demonstrates that the Schottky barrier should exist at the grain boundary. A double Schottky barrier model composed of a depletion layer and a negative charge sheet was proposed, analogous to the barrier model for ZnO varistors. An activation energy value of about 0.6 eV was obtained from the data of the characteristic frequency corresponding to the peak of the imaginary part of the dielectric permittivity versus temperature, which may be attributed to the activation of to in the depletion layer.

Electromechanical properties of A-site (LiCe)-modified sodium bismuth titanate (Na0.5Bi4.5Ti4O15) piezoelectric ceramics at elevated temperature

The Aurivillius-type bismuth layer-structured (NaBi)0.46(LiCe)0.04Bi4Ti4O15 (NBT-LiCe) piezoelectric ceramics were synthesized using conventional solid-state processing. Phase analysis was performed by x-ray diffraction and microstructural morphology was assessed by scanning electron microscopy. The dielectric, piezoelectric, ferroelectric, and electromechanical properties of NBT-LiCe ceramics were investigated. The piezoelectric activities were found to be significantly enhanced compared to NBT ceramics, which can be attributed to the lattice distortion and the presence of bismuth vacancies. The dielectric and electromechanical properties of NBT-LiCe ceramics at elevated temperature were investigated in detail. The excellent piezoelectric, dielectric, and electromechanical properties, coupled with high Curie temperature (Tc=660 °C), demonstrated that the NBT-LiCe ceramics are the promising candidates for high temperature applications.

High performance Aurivillius phase sodium-potassium bismuth titanate lead-free piezoelectric ceramics with lithium and cerium modification

The piezoelectric properties of the lithium and cerium modified A-site vacancies sodium-potassium bismuth titanate (NKBT) lead-free piezoceramics are investigated. The piezoelectric activity of NKBT ceramics is significantly improved by the modification of lithium and cerium. The Curie temperature TC, piezoelectric coefficient d33, and mechanical quality factor Qm for the NKBT ceramics modified with 0.10mol% (LiCe) are found to be 660°C, 25pC∕N, and 3135, respectively. The Curie temperature gradually decreases from 675to650°C with the increase of (LiCe) modification. The dielectric spectroscopy shows that all the samples possess stable piezoelectric properties, demonstrating that the (LiCe) modified NKBT-based ceramics are the promising candidates for high temperature applications.

High temperature (NaBi)0.48◻0.04Bi2Nb2O9-based piezoelectric ceramics

The effect of (LiCe) substitution for A site on the properties of (NaBi)0.48◻0.04Bi2Nb2O9 (NB◻N)-based ceramics was investigated. The coercive fields (EC) of NB◻N)-based ceramics were significantly decreased from 61.0to32.5kV∕cm and the Curie temperature (TC) gradually decreases from 820to803°C with increasing the (LiCe) modification. The piezoelectric coefficient d33, planar coupling factor kp, and mechanical quality factor Q of (NaBi)0.38(LiCe)0.05◻0.14Bi2Nb2O9 ceramic were found to be 27pC∕N, 11.2%, and 2600, respectively, together with the high TC (∼809°C) and stable piezoelectric properties, demonstrating that the (LiCe) modified NB◻N-based material a promising candidate for high temperature applications.

Nonlinear electrical properties of TiO2–Y2O3–Nb2O5 capacitor-varistor ceramics

Abstract The nonlinear electrical properties of TiO2–Y2O3–Nb2O5 ceramics were investigated as a new varistor material. It was found that an optimal doping composition of 99.75%TiO2–0.60%Y2O5–0.10% Nb2O5 was obtained with low breakdown voltage of 8.8 V mm−1, high nonlinear constant of 7.0 and ultrahigh relative dielectric constant of 7.6×104, which is consistent with the highest and narrowest grain boundary barriers in the composition. Samples doped with 0.10 mol.% Nb2O5 exhibit the highest permittivitty and resistivity at low frequencies and comparatively lower values at high frequencies in comparison with other samples studied. In view of these electrical characteristics, the ceramics of 99.75%TiO2–0.60%Y2O3–0.10%Nb2O5 is a viable candidate for capacitor-varistor functional devices. The performance of the ceramics as a function of Nb-doping depends primarily on the extent of substitution of Ti4+ with Nb5+. In order to illustrate the role of grain boundary barriers for high Nb-doping co-concentrations in TiO2–Y2O3–Nb2O5 varistors, a grain-boundary defect barrier model was introduced.

Effect of (Li,Ce) doping in Aurivillius phase material Na0.25K0.25Bi2.5Nb2O9

The effect of (Li,Ce) substitution for A site on the properties of Na0.25K0.25Bi2.5Nb2O9-based ceramics was investigated. The piezoelectric activity of Na0.25K0.25Bi2.5Nb2O9-based ceramics is significantly improved by the modification of lithium and cerium. The Curie temperature (TC) gradually increases from 668to684°C with increasing the (Li,Ce) modification. The piezoelectric coefficient d33 of the [(Na0.5K0.5)Bi]0.44(LiCe)0.03[ ]0.03Bi2Nb2O9 ceramic was found to be 28pC∕N, the highest value among the Na0.25K0.25Bi2.5Nb2O9-based ceramics and also almost 50% higher than the reported d33 values of other bismuth layer-structured ferroelectric systems (∼5–19pC∕N). The planar coupling factors kp and kt were found to be 8.0% and 23.0%, together with the high TC (∼670°C) and stable piezoelectric properties, demonstrating that the (Li,Ce) modified Na0.25K0.25Bi2.5Nb2O9-based material a promising candidate for high temperature applications.

Effect of Mn2+ on the electrical nonlinearity of (Ni, Nb)-doped SnO2 varistors

The reason that the (Ni, Nb)-doped SnO2 varistors exhibit poorer densification and electrical nonlinearity than the (Co, Nb)-doped SnO2 varistors is explained. The effect of Mn2+ on the electrical nonlinear properties of SnO2 based ceramics were investigated. The sample doped with 0.10 mol% MnCO3 exhibits the highest reference electrical field of 686.89 V/mm, the highest electrical nonlinear coefficient of 12.9, which is consistent with the highest grain-boundary defect barriers. It can be explained by the effect of the substitution of Sn4+ for Mn2+, which facilitate the formation of the defect barriers, and the maximum of the substitution. The shrinkage rates increase with the doping of MnCO3, although the sample doped with 0.5 mol% MnCO3 appears the highest density (ρ=6.87 g/cm3). In order to illustrate the grain boundary barriers formation in SnO2.Ni2O3.Nb2O5.MnCO3 varistors, a grain-boundary defect barrier model was introduced.