Wen-Bo Li

Novel temperature stable high-εr microwave dielectrics in the Bi2O3–TiO2–V2O5 system

In the present work, a series of low temperature firing (1 − x)BiVO4–xTiO2 (x = 0.4, 0.50, 0.55 and 0.60) microwave dielectric ceramics was prepared using traditional solid state reaction method. From back-scattered electron images (BEI), X-ray diffraction (XRD) and energy dispersive analysis (EDS), there was negligible reaction between BiVO4 and TiO2 at the optimal sintering temperature ∼900 °C. As x increased from 0.4 to 0.60, permittivity (er) increased from 81.8 to 87.7, quality factor value (Qf) decreased from 12 290 to 8240 GHz and temperature coefficient (TCF) shifted from −121 to +46 ppm per °C. Temperature stable microwave dielectric ceramic was obtained in 0.45BiVO4–0.55TiO2 composition sintered at 900 °C with a er ∼ 86, a Qf ∼ 9500 GHz and a TCF ∼ −8 ppm per °C. Far-infrared reflectivity fitting indicated that stretching of Bi–O and Ti–O bonds in this system dominated dielectric polarization. This series of ceramics are promising not only for low temperature co-fired ceramic (LTCC) technology but also as substrates for physically and electrically small dielectrically loaded micro-strip patch antennas.

Phase composition, crystal structure, infrared reflectivity and microwave dielectric properties of temperature stable composite ceramics (scheelite and zircon-type) in BiVO4–YVO4 system

(1 − x)BiVO4–xYVO4 (x ≤ 0.65) ceramics were prepared using the solid state reaction method. X-ray diffraction, Raman spectra and scanning electron microscopy techniques were employed to study the phase composition and crystal structure. The ceramic samples were composed of both monoclinic scheelite and tetragonal zircon-type phases. The best microwave dielectric properties, with a permittivity ∼45, a Qf value 14 000 GHz and a temperature coefficient of resonant frequency (TCF) +10 ppm °C−1, were obtained in the 0.81BiVO4–0.19YVO4 ceramic sintered at 870 °C for 2 h. Far-infrared spectra study showed that Bi–O oscillations dominate microwave dielectric polarizations in the (1 − x)BiVO4–xYVO4 ceramics. The (1 − x)BiVO4–xYVO4 ceramics might be potential candidates for microwave devices application and low temperature co-fired ceramic technology (LTCC).

Phase evolution and microwave dielectric properties of xBi2/3MoO4–(1 − x)BiVO4 (0.0 ≤ x ≤ 1.0) low temperature firing ceramics

In the present work, a full range of compositions of xBi(2/3)MoO4-(1 -x)BiVO4 (0.0 ≤ x ≤ 1.0) was prepared by the solid state reaction method. All the ceramic compositions could be readily densified to below 850 °C. As the x value increased, the monoclinic scheelite structure continuously changed to a tetragonal structure at x = 0.10, which means the ferroelastic phase transition temperature was lowered to near room temperature. In the compositional range 0.50 ≤ x < 0.70, a novel ordered scheelite phase was formed, most likely through A-site vacancy ordering. For compositions x ≥ 0.70, a composite two-phase region consisting of the ordered scheelite and Bi(2/3)MoO4 phases was formed. High microwave permittivity around 75 and Qf values around 8000 GHz could be obtained in the compositions near the phase boundaries between monoclinic and tetragonal scheelite phases. The intrinsic microwave dielectric properties were extrapolated from the far infrared reflectivity spectra, and it was found that the polarization was dominated by the Bi-O stretches when x ≤ 0.10.

Structure, phase evolution, and microwave dielectric properties of (Ag0.5Bi0.5)(Mo0.5W0.5)O4 ceramic with ultralow sintering temperature.

In the present work, the microwave dielectric ceramic (Ag0.5Bi0.5)(Mo0.5W0.5)O4 was prepared by using the solid-state reaction method. (Ag0.5Bi0.5)(Mo0.5W0.5)O4 was found to crystallize in the scheelite structure, in which Ag(+) and Bi(3+) occupy the A site randomly with 8-coordination while Mo(6+) and W(6+) occupy the B site with 4-coordination, at a sintering temperature above 500 °C, with lattice parameters a = b = 5.29469(2) Å and c = 11.62114(0) Å, space group I4(1)/a (No. 88), and acceptable Rp = 9.38, Rwp = 11.2, and Rexp = 5.86. High-performance microwave dielectric properties, with permittivity ∼26.3, Qf value ∼10,000 GHz, and temperature coefficient ∼+20 ppm/°C, were obtained in the sample sintered at 580 °C. Its chemical compatibility with aluminum at its sintering temperature was revealed and confirmed by both X-ray and energy dispersive spectrometer analysis. This ceramic could be a good candidate for ultralow-temperature cofired ceramics.

Novel barium titanate based capacitors with high energy density and fast discharge performance

Recently, dielectric capacitors have attracted much attention due to their high power density based on fast charge–discharge capability. However, their energy storage applications are limited by their low discharge energy densities. In this work, we designed novel lead-free relaxor-ferroelectric 0.88BaTiO3–0.12Bi(Li0.5Nb0.5)O3 (0.88BT–0.12BLN) ceramics with high breakdown strength and high discharge energy density. The 0.88BT–0.12BLN ceramics were prepared by a conventional solid state reaction method. Optimal energy storage properties were obtained in 0.88BT–0.12BLN ceramics sintered at 1220 °C with an impressive discharge energy density of 2.032 J cm−3 and a charge–discharge efficiency of beyond 88% at 270 kV cm−1. The energy storage properties of the 0.88BT–0.12BLN also displayed good thermal stability from 20 to 120 °C at an electric field of 150 kV cm−1. Moreover, the discharge speed behavior was investigated by using pulsed current. The pulsed discharge current waveforms showed that all the samples have fast discharge times (less than 0.5 μs) under different electric fields. This work significantly increases the intrinsic breakdown strength and discharge energy density of BaTiO3-based materials with high charge–discharge efficiency for high power energy storage devices.

Abnormal dielectric properties and phase transition in Bi0.783(Mo0.65V0.35)O4 scheelite-related structured ceramic

In the present work, an interesting phase transition at 150 °C in ordered scheelite structured Bi0.783(Mo0.65V0.35)O4 ceramic was observed for the first time. This phase transition was believed to be related to the ordered arrangement of A site defects (or A site cations Bi3+). X-ray diffraction (XRD) and Raman spectra were employed to study the structure. Thermal expansion data showed that a sudden decrease in cell volume was observed during the phase transition, which resulted in a sharp increase in dielectric permittivity and loss over a wide frequency range (100 Hz to 7.5 GHz). Far-infrared reflectivity and THz spectra were used to study the intrinsic dielectric properties. It was found that the Bi–O stretches contributed mainly to the polarization. This study extended the design of A site ordered scheelite structured materials and the knowledge of their phase transitions.

Phase Evolution, Crystal Structure, and Microwave Dielectric Properties of Water-Insoluble (1 – x)LaNbO4–xLaVO4 (0 ≤ x ≤ 0.9) Ceramics

In the present work, a series of low-temperature firing scheelite structured microwave dielectric in water-insoluble La2O3-Nb2O5-V2O5 system was prepared via the traditional solid-state reaction method. Backscattering electron diffraction, X-ray diffraction (XRD), energy-dispersive analysis, and Rietveld refinements were performed to study the phase evolution and crystal structure. In the full composition range of (1 - x)LaNbO4-xLaVO4 (0 ≤ x ≤ 0.9) ceramics, at least four typical phase regions including monoclinic fergusonite, tetragonal sheelite, B-site ordered sheelite, and composite of monoclinic LaVO4 and tetragonal sheelite phases can be detected according to XRD analysis. The variations of relative dielectric constant εr, quality factor Q × f, and resonant frequency τf could be attributed to Nb/V-O bond ionicity, lattice energy, and the coefficient of thermal expansion. Infrared reflectivity spectra analysis revealed that ion polarization contributed mainly to the permittivity in microwave frequencies ranges. Furthermore, the 0.7LaNbO4-0.3LaVO4 ceramic sintered at 1160 °C possessed excellent microwave dielectric properties with an εr of ∼17.78, a Q × f of ∼75 940 GHz, and a τf of ca. -36.8 ppm/°C. This series of materials might be good candidate for microwave devices.