Wenzhong Lu

Phase transition behaviour and electromechanical properties of (Na1/2Bi1/2)TiO3–KNbO3 lead-free piezoelectric ceramics

Lead-free piezoelectric ceramics (1 − x)(Na1/2Bi1/2)TiO3–xKNbO3 (abbreviated as NBT–KN100x, x = 0–0.12) were synthesized by the conventional ceramic sintering technique. The phase transition behaviour in NBT–KN100x ceramics with respect to changes in composition and temperature was investigated using x-ray diffraction analysis, dielectric and ferroelectric characterizations. At x = 0.04–0.06, the rhombohedral and pseudo-cubic morphotropic phase boundary (MPB) occurs in NBT–KN100x ceramics. Near MPB, the NBT–KN5 ceramics exhibit the optimum piezoelectric properties: kp = 0.22 and d33 = 125 pC N−1. The curves of permittivity versus temperature show that NBT–KN100x ceramics exhibit the diffuse phase transition and are characterized by a strong frequency dispersion of the permittivity. The hysteresis loops at different temperatures indicate that there is no existence of antiferroelectrics above the depolarization temperature Td. The depolarization reason is that the tetragonal nonpolar phase occurs and leads to the macro–micro domain transformation at about Td.

Morphotropic phase boundary and piezoelectric properties of (Bi1∕2Na1∕2)TiO3–(Bi1∕2K1∕2)TiO3–KNbO3 lead-free piezoelectric ceramics

The crystal structure and piezoelectric properties of ternary ceramics x(Bi1∕2Na1∕2)TiO3–y(Bi1∕2K1∕2)TiO3–zKNbO3 (abbreviated as BNT-BKT-KN100x∕100y∕100z) system have been investigated. Effects of BKT and KN on the crystalline structure were examined. The morphotropic phase boundary (MPB) phase diagram of BNT-BKT-KN ternary system was determined by x-ray diffraction patterns from sintered specimens. Near the MPB, BNT-BKT-KN82∕16∕2 ceramics had good performances with piezoelectric constant d33=215pC∕N which reached a maximum value in this ternary system and planar electromechanical coupling factor kp=35%. Besides, BNT-BKT-KN87∕10∕3 ceramics had good performances with piezoelectric constant d33=138pC∕N and electromechanical coupling factor kp=38% which attained a maximum value in this ternary system.

Structural and electrical properties of Ba(Sn,Sb)O3 electroceramics materials

Abstract The conductivity of BaSnO 3 ceramics have been measured at elevated temperature as a function of oxygen partial pressure. The experimental data allows us to propose that the dominant defect in BaSnO 3 is an oxygen vacancy. The effects of doping with Sb 2 O 3 on the electrical properties of BaSnO 3 have been investigated. The dependence of the resistivity value of Ba(Sn,Sb)O 3 materials on the doping content has a pronounced minimum at the molar concentration of Sb 2 O 3 about 2%. At first the introduction of Sb 2 O 3 results in a significant decrease of the resistivity of BaSnO 3 , the successive increase of Sb 2 O 3 results in the appearance of the insulator phases. This leads to the increase of the resistivity value of Ba(Sn,Sb)O 3 materials. No variation of the crystal cell parameter is observed under the doping procedure. The resistance characteristics of Ba(Sn,Sb)O 3 materials as a function of temperature show that Ba(Sn,Sb)O 3 is a kind of electroceramics with low activation energy.

Influence of TiO2/Sb2O3 ratio on ZnO varistor ceramics

Abstract The influence of TiO2/Sb2O3 (Ti/Sb) ratio on the microstructure development and electrical properties of varistor ceramics in ZnO–(Bi2O3–TiO2–Sb2O3)–CoO–MnO2 system was investigated. Characterization of varistors was performed by XRD, SEM, d.c. degradation behavior, and electrical measurements. It is found that the Ti/Sb ratio greatly influences the microstructure and electrical properties of the ZnO varistor. The specimen with Ti/Sb ratio of 5 has the lowest breakdown field (20V/mm) and improved d.c. degradation behavior.

Study of piezoelectric ceramic materials for high-temperature and high-frequency applications

Abstract In this paper, some methods to get piezoelectric ceramic materials for high-temperature and high-frequency applications based on modified PbTiO3 piezoelectric ceramics have been introduced. A solid solution system (1 − X)PbTiO3 + XPb(Cd1/3Nb2/3)O3 has been studied for its piezoelectric properties as the B-position complex substitution of ABO3 perovskite structure. The system with X=14 mol% has good piezoelectric properties by adding some suitably modified additives, such as WO3, MnO2 and NiO. The Curie temperature is over 450 °C and the dielectric constant, es, is about 200 at room temperature. The piezoelectric coupling factor Kt is 0.45 and the mechanical quality factor Qmcan be changed from 500 to 2400 according to different situations. The high stability of these materials makes it possible for them to be used in high-temperature and high-frequency situations.

Dielectric performance of polymer-based composites containing core-shell Ag@TiO2 nanoparticle fillers

This paper reports composites prepared by embedding core-shell Ag@TiO2 fillers into polytetrafluoroethylene. Ag nanoparticles were homogeneously coated with TiO2, to give a shell thickness of approximately ∼8–10 nm. The composite containing Ag@TiO2 nanoparticles with rutile shells exhibited better dielectric properties than the composite containing Ag@TiO2 nanoparticles with anatase shells. The relative permittivity (er) of the composite containing 70 vol. % filler was approximately 240 at 100 Hz, which was more than 100 times higher than that of pure polytetrafluoroethylene (er = 2.1). An effective medium percolation theory model is used to account for the dielectric constant of the composite.

A Compact Dual-Band Filter with Close Passbands using Asymmetric λ/4 Resonator Pairs with Shared Via-Hole Ground

A novel asymmetric λ/4 resonator pairs with shared via-hole ground is proposed, which is composed of two different λ/4 resonators that share the same via. Compared with traditional λ/2 stepped impedance resonators (SIRs) and λ/4 SIRs, the asymmetric λ/4 resonator pairs can realize flexible passband allocation and size reduction when it is utilized to design multi-band filters, especially for the multi-band filters with close dual passbands. To verify the presented concept, one experiment example of filter with a dual-band Chebyshev response operating at 2.4, 3.6 GHz has been designed and fabricated. The measured results are in good agreement with the full-wave simulation results.

Novel ZnAl2O4-Based Microwave Dielectric Ceramics with Machinable Property and its Application for GPS Antenna

(1−x)(0.79ZnAl 2 O 4 -0.21Mg 2 TiO 4 )-xSrTiO 3 microwave dielectric ceramics with proper machinable properties were prepared by the solid-state process. The effects of SrTiO 3 on the microstructures and microwave dielectric properties of the 0.79ZnAl 2 O 4 -0.21Mg 2 TiO 4 ceramics were investigated. A typical machinable microwave dielectric ceramics was synthesized with ϵ r = 10.6, Q • f of 55000 GHz and τ f of +9.3 ppm/°C when x is about 0.05. The mechanical properties and thermal properties of these ceramics were also presented including flexural strength, compressive strength and thermal expansion coefficient. Afterwards, a microstrip antenna for GPS with several via holes was designed and measured. Experimental results showed that the antenna operated at 1.57 GHz with an impedance bandwidth of 3% (VSWR ≤ 2), and satisfied axial ratio was obtained.

Calcining temperature dependence of microwave dielectric properties of (1 -x)ZnAl2O4-xTiO2 (x = 0.21) ceramics

(1-x)ZnAl2O4–xTiO2 (x = 0.21) ceramics were synthesized at 1500 °C using the solid-state reaction after the starting powders were calcined at 1050–1250 °C. The effects of calcining temperature on the microstructures, phase compositions, and microwave dielectric properties of the ceramics had been investigated. In the calcined (1-x)ZnAl2O4–xTiO2 (x = 0.21) powders, the amount of the secondary phase Zn2Ti3O8 decreases gradually with increasing calcining temperature and it disappears at more than 1150 °C. The er and Qf values increase slowly with increasing calcining temperature and then decrease after reaching the maximum value at 1150 °C, whereas the τf values are nearly independent of the calcining temperature and approach zero. When the calcining temperature reaches 1150 °C, the optimal microwave dielectric properties of the (1-x)ZnAl2O4–xTiO2 (x = 0.21) ceramics can be exhibited with a er of 11.6, a Qf of 74,000 GHz (at about 6.5 GHz), and a τf of -0.4 ppm/°C.

Dielectric and ferroelectric behavior of an incipient ferroelectric Sr(1−3x/2)CexTiO3 novel solid solution

Phase formation, chemical structure, microwave (MW) dielectric properties, and relaxor-to-ferroelectric phase transition behavior of a novel solid solution with Ce-doped, A-site SrTiO3 (Sr(1−3x/2)CexTiO3) ceramic sintered in nitrogen have been investigated. X-ray diffraction (XRD) showed that the samples with x ≤ 0.3 appeared cubic but exhibited splitting and superstructure reflections consistent with tetragonal (0.4 ≤ x ≤ 0.45) and orthorhombic (0.5 ≤ x ≤ 0.6) structures. Chemical structure analysis revealed that the addition of small amounts of ceria (x ≤ 0.3) promoted Ti3+ cation and oxygen vacancy . By contrast, the addition of large amounts of ceria (x ≥ 0.4) inhibited this process. Sr(1−3x/2)CexTiO3 ceramics displayed frequency-dependent relaxor dielectric anomalies and frequency-independent normal ferroelectric behaviors. The relaxor/dielectric properties were strongly dependent on structure-chemical factors. The value of er decreased several times when Sr was subsequently substituted with Ce, and induced a relaxor-to-ferroelectric phase transition. A good combination of the following microwave dielectric properties was obtained for the Sr0.1Ce0.6TiO3 orthorhombic solid solution at an optimum temperature of 1300 °C for 3 h: er = 50, Q × f = 11 311 GHz, and τf = +3 ppm per °C.