Papers by Author: Zhi Jun Xu

Abstract: Lead-free (Ba1-xCax)(Ti0.96Zr0.02Sn0.02)O3 (BCTZS) (x=0, 0.01, 0.02, 0.03) ceramics were prepared using conventional solid-state reaction technique. The effects of Ca content on the microstructure and electrical properties of the BCTZS ceramics were investigated. High piezoelctric coefficient was obtained for the sample at x=0.02.

Abstract: Lead-free piezoelectric ceramics Sr₂Bi_4-xGd_xTi₅O₁₈ were prepared by conventional solid-state reaction method. Pure bismuth layered structural ceramics with uniform gain size were obtained in all samples. The effect of Gd-doping on the dielectric, ferroelectric and piezoelectric properties of Sr₂Bi₄Ti₅O₁₈ ceramics were also investigated. It was found that that Gd³⁺ dopant gradually decreased the Curie temperature (T_c) with the lower dielectric loss (tand) of SBTi ceramics. In addition, Gd-doping with appropriate content improved the ferroelectric and piezoelectric properties of the SBTi ceramics. The piezoelectric constant (d₃₃) of the Sr₂Bi_3.9Gd_0.1Ti₅O₁₈ ceramic reached the maximum value, which is 22 pC/N. The results showed that the Sr₂Bi_4-xGd_xTi₅O₁₈ ceramic was a promising lead-free piezoelectric material.

Abstract: Lead-free piezoceramics (K_0.5Na_0.5)_0.94-3xLi_0.06La_xNbO₃ were synthesized by the conventional solid-state reaction process. The effect of La-doping on the phase structure and electrical properties of (K_0.5Na_0.5)_0.94Li_0.06NbO₃ ceramics was investigated. X-ray diffraction analysis showed that the crystal structure of the ceramics changed from coexistence of orthorhombic and tetragonal phases to pseudo-cubic phase with the increase of La-doping level, and the change of phase structure seriously weakened the dielectric, ferroelectric and piezoelectric properties of the ceramics. In addition, it was found that the La-doping improved the density of the ceramics and the ceramics became dense. Meanwhile, the ceramics with La-doping displayed a relative lower loss tangent (tanδ) from room temperature to approximately 350°C.

Abstract: La₂O₃-doped lead-free 0.82Bi_0.5Na_0.5TiO₃-0.18Bi_0.5K_0.5TiO₃ (abbreviated to 0.82BNT-0.18BKT) piezoelectric ceramics were synthesized by the conventional mixed-oxide method, and the effect of La₂O₃ addition on the dielectric and piezoelectric properties was investigated. X-ray diffraction (XRD) patterns show that La₂O₃ diffuses into the lattice of the 0.82BNT-0.18BKT ceramics to form a solid solution with a pure perovskite structure. SEM images indicate that the grain size of the 0.82BNT-0.18BKT ceramics increased with the addition of La₂O₃ doping. The electrical properties of 0.82BNT-0.18BKT ceramics have been greatly improved by certain amount of La₂O₃ substitutions. At room temperature, the 0.82BNT-0.18BKT ceramics doped with 0.25 wt. % La₂O₃ exhibited the optimum properties with high piezoelectric constant (d₃₃ = 142 pC/N) and high planar coupling factor (k_p = 0.23).

Abstract: Nb-doped and Nb-Mn-codoped (1-xmol%)BaTiO3-xmol%(Bi0.5Na0.5)TiO3 (BBNTx) lead-free positive temperature coefficient of resistivity (PTCR) ceramics were prepared by the conventional solid state reaction method. The XRD patterns indicated that all BBNTx samples formed a single perovskite structure with tetragonal phase. 0.25 mol% Nb doped BBNT1 ceramic, sintered at 1330°C for 1h in air, had low room-temperature resistivity (ρ25) of 80 Ω•cm and a high resistivity jump (maximum resistivity [ρmax]/minimum resistivity [ρmin]) of 4.2 orders of magnitude with Tc about 152°C. The Nb-doped BBNTx (10≤x≤60) ceramics also showed distinct PTC effect with Tc between 185 and 232°C by sintering in N2, which was shut off when samples were cooled to a low temperature. In addition, The Nb-Mn-codoped BBNT1 ceramics exhibited higher resistivity jump than the single Nb-doped ones, with increasing the room-temperature resistivity.

Abstract: BaTiO3 ceramics doped with 1, 2, 3 and 4 mol% La were prepared by sol-gel method. The effects of La doping on structure and dielectric characteristics of Ba1−xLaxTiO3 were investigated. The results show that: All the powders calcined at 800°C were found to be perovskite structure with cubic phase. Ba1−xLaxTiO3 were sintered at 1300°C, phase transition behavior appeared from tetragonal phase to cubic phase with increasing La-doping level. As the doping concentration increases, the grain growth of the Ba1−xLaxTiO3 ceramics was controlled efficiently and Curie point shifted to lower temperature. The grain size decreased from 20μm with 0 mol% La doped to 0.5μm with 4 mol% La doped. The Curie point shift rate was 22 °C/ mol% with increasing La content. The dielectric constant of La doped Ba1−xLaxTiO3 ceramics increased with increasing La content and a maximum value of 8900 was attained with 4 mol% La doped.

Abstract: Zr-doped Sr2Bi4Ti5O18 (SBTi) bismuth layer-structured ferroelectric ceramics were prepared and studied. XRD patterns revealed that all the ceramic samples were single phase compounds. SEM images showed that dense microstructures with uniform gain size were obtained in all samples. The effects of Zr4+ doping on the dielectric, ferroelectric and piezoelectric properties of SBTi ceramics were also investigated. It was found that Zr4+ dopant gradually decreased the Curie temperature (Tc), enhanced the remnant polarization (2Pr) and decreased the coercive field (Ec) of SBTi ceramics. Furthermore, the piezoelectric properties of the SBTi ceramics were improved by enlarging the content of Zr. The SBTi ceramics with 4 mol% of Zr4+ dopant exhibited good electrical properties: d33 = 21 pC/N, 2Pr = 14.3 μC/cm2, Tc = 251°C, εr = 376, tanδ = 1.8%.

Abstract: Sr2Bi (4-x/3) Ti (5-x) VxO18 (x = 0.0-0.06) (SBTV) ceramics were prepared by a solid-state reaction method, and the ferroelectric and dielectric properties of the ceramics were investigated with respect to the amount of V deficiency. XRD analysis indicated that the oxide compounds were Aurivillius phases. The Curie temperature of SBTV ceramics was improved by doping of certain amount of V. The V deficiency also led to a steady increase in the density together with a decrease in the coercive field. For the ceramics with x = 0.06, the properties become optimum: d33 = 19 pC/N, Tc = 306oC, Ec = 43.7 KV/cm.

Abstract: Rectangular cavity perturbation method was used to measure microwave dielectric loss (MDL) during the solid state reaction synthesis of SrFeCo0.5Oy. In the process of solid state reaction, the dielectric loss is investigated under different temperatures. The phases of the samples synthesized at different temperatures were characterized by XRD. The variation of MDL with temperature illustrates that the phase transformation occurs. The reasons why the pure perovskite phase can be obtained by using microwave processing were also discussed.

Abstract: Sr4Bi4Ti7O24 (SBT7) ceramics were prepared. X-ray powder diffraction indicated that single layered perovskite ferroelectrics were obtained. The ferroelectric and dielectric properties of SBT7 ceramics were investigated. The results showed that Sr4Bi4Ti7O24 is piezoelectric material. The dielectric properties were measured as a function of temperature and two peaks were observed in the curve.