Li-Xia Pang

Influence of Ce Substitution for Bi in BiVO4 and the Impact on the Phase Evolution and Microwave Dielectric Properties

In the present work, the (Bi1-xCex)VO4 (x ≤ 0.6) ceramics were prepared via a solid-state reaction method and all the ceramic samples could be densified below 900 °C. From the X-ray diffraction analysis, it is found that a monoclinic scheelite solid solution can be formed in the range x ≤ 0.10. In the range 0.20 ≤ x ≤ 0.60, a composite region with both monoclinic scheelite and tetragonal zircon solid solutions was formed and the content of the zircon phase increased with the calcined or sintering temperature. The refined lattice parameters of (Bi0.9Ce0.1)VO4 are a = 5.1801(0) Å, b = 5.0992(1) Å, c = 11.6997(8) Å, and γ = 90.346(0)° with the space group I112/b(15). The VO4 tetrahedron contracts with the substitution of Ce for Bi at the A site, and this helps to keep the specific tetrahedron chain stable in the monoclinic structure. The microwave dielectric permittivity was found to decrease linearly from 68 to about 26.6; meanwhile, the quality factor (Qf) value increased from 8000 GHz to around 23900 GHz as the x value increased from 0 to 0.60. The best microwave dielectric properties were obtained in a (Bi0.75Ce0.25)VO4 ceramic with a permittivity of ∼47.9, a Qf value of ∼18000 GHz, and a near-zero temperature coefficient of ∼+15 ppm/°C at a resonant frequency of around 7.6 GHz at room temperature. Infrared spectral analysis supported that the dielectric contribution for this system at microwave region could be attributed to the absorptions of structural phonon oscillations. This work presents a novel method to modify the temperature coefficient of BiVO4-type materials. This system of microwave dielectric ceramic might be an interesting candidate for microwave dielectric resonator and low-temperature cofired ceramic technology applications.

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 transition, Raman spectra, infrared spectra, band gap and microwave dielectric properties of low temperature firing (Na0.5xBi1−0.5x)(MoxV1−x)O4 solid solution ceramics with scheelite structures

A scheelite based structure that could host the solid solution (Na0.5xBi1−0.5x)(MoxV1−x)O4 (0.0 ≤ x ≤ 1.0) was prepared via the solid state reaction method. All the compositions can be sintered well below a temperature of 800 °C. A structural phase transition occurs from the monoclinic scheelite structure to a tetragonal scheelite structure at x = 0.10 at room temperature. This structural transition is related to a displacive ferroelastic–paraelastic phase transition. This phase transition was also confirmed by in situhigh temperature XRD and Raman studies, and a room temperature infrared spectra study. The compositions near the phase boundary possessed high dielectric permittivities (>70), and large Qf values (>80 000 GHz) with variable temperature coefficients of frequency and capacitance. For example, a temperature stable dielectric made as a composite with compositions of x = 0.05 and x = 0.10 was designed and co-sintered at 720 °C for 2 h to produce a dielectric with a permittivity of ∼77.3, a Qf value between 8 000 GHz–10 000 GHz, and a temperature coefficient of <±20 ppm/°C at 3.8 GHz over a temperature range of 25–110 °C. This material is a candidate for dielectric resonators and low temperature co-fired ceramics technologies. Near the phase boundary at x = 0.10 in the monoclinic phase region, the samples show strong absorption in the visible light region and we determine a band gap energy of about 2.1 eV, which means that it might also be useful as a visible light irradiation photocatalyst.

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).

Structure-property relationships of low sintering temperature scheelite-structured (1 -x)BiVO4-xLaNbO4 microwave dielectric ceramics

A series of (1 − x)BiVO4–xLaNbO4 (0.0 ≤ x ≤ 1.0) ceramics were prepared via a solid state reaction method. A scheelite-structured solid solution was formed for x ≤ 0.5 but for x > 0.5, tetragonal scheelite, monoclinic LaNbO4-type and La1/3NbO3 phases co-existed. As x increased from 0 to 0.1, the room temperature crystal structure gradually changed from monoclinic to tetragonal scheelite, associated with a decrease in the ferroelastic phase transition temperature from 255 °C (BiVO4) to room temperature or even below. High sintering temperatures were also found to accelerate this phase transition for compositions with x ≤ 0.08. Temperature independent high quality factor Qf >10000 GHz in a wide temperature range 25–140 °C and high microwave permittivity er ∼76.3 ± 0.5 was obtained for the x = 0.06 ceramic sintered at 800 °C. However, small changes in composition resulted in a change in the sign and magnitude of the temperature coefficient of resonant frequency (TCF) due to the proximity of the ferroelastic transition to room temperature. If TCF can be controlled and tuned through zero, then (1 − x)BiVO4–xLaNbO4 (0.0 ≤ x ≤ 1.0) is a strong candidate for microwave device applications.

Phase evolution, phase transition, and microwave dielectric properties of scheelite structured xBi(Fe1/3Mo2/3)O4–(1−x)BiVO4 (0.0 ≤ x ≤ 1.0) low temperature firing ceramics

In the present work, the xBi(Fe1/3Mo2/3)O4–(1−x)BiVO4 (0.0 ≤ x ≤ 1.0) ceramics were prepared via the solid state reaction method. All the ceramics can be densified at low sintering temperatures around 820 °C. At room temperature, the BiVO4 type scheelite monoclinic solid solution was formed in ceramic samples with a composition of x ≤ 0.10. When x lies between 0.1 and 0.7, a BiVO4 scheelite tetragonal phase is formed at room temperature. In the range 0.7 ≤ x < 0.9, the ceramic samples were found to be composites consisting of BiVO4 type tetragonal and Bi(Fe1/3Mo2/3)O4 type monoclinic scheelite phases, and when x ≥ 0.9, the Bi(Fe1/3Mo2/3)O4 type monoclinic scheelite solid solution was formed. In the BiVO4 type monoclinic solid solution region, the phase transition to tetragonal phase was studied by in situ Raman and Far-Infrared spectroscopies and by thermal expansion analysis. All of these methods indicated that the phase transition temperature almost linearly decreased from 255 °C for pure BiVO4 to about −9 °C for x = 0.1 sample. High performance microwave dielectric properties with a high permittivity of about 74.8, high Qf values above 11500 GHz, and a small temperature coefficient of resonant frequency within +20 ppm per °C in a wide temperature range of 20–140 °C can be obtained in the composite ceramic sample with 60 mol% x = 0.10 composition and 40 mol% x = 0.02 composition. The xBi(Fe1/3Mo2/3)O4–(1−x)BiVO4 (0.0 ≤ x ≤ 1.0) ceramics might provide useful candidate materials for microwave integrated capacitive devices, such as filters, antennas, etc.

Phase Evolution, Phase Transition, Raman Spectra, Infrared Spectra, and Microwave Dielectric Properties of Low Temperature Firing (K 0.5x Bi 1−0.5x )(Mo x V 1−x )O 4 Ceramics with Scheelite Related Structure

In the present work, the (K(0.5x)Bi(1-0.5x))(Mo(x)V(1-x))O(4) ceramics (0≤x ≤ 1.00) were prepared via the solid state reaction method and sintered at temperatures below 830 °C. At room temperature, the BiVO(4) scheelite monoclinic solid solution was formed in ceramic samples with x < 0.10. When x lies between 0.1-0.19, a BiVO(4) scheelite tetragonal phase was formed. The phase transition from scheelite monoclinic to scheelite tetragonal phase is a continuous, second order ferroelastic transition. High temperature X-ray diffraction results showed that this phase transition can also be induced at high temperatures about 62 °C for x = 0.09 sample, and has a monoclinic phase at room temperature. Two scheelite tetragonal phases, one being a BiVO(4) type and the other phase is a (K,Bi)(1/2)MoO(4) type, coexist in the compositional range 0.19 < x < 0.82. A pure (K,Bi)(1/2)MoO(4) tetragonal type solid solution can be obtained in the range 0.82 ≤ x ≤ 0.85. Between 0.88 ≤ x ≤ 1.0, a (K,Bi)(1/2)MoO(4) monoclinic solid solution region was observed. Excellent microwave dielectric performance with a relative dielectric permittivity around 78 and Qf value above 7800 GHz were achieved in ceramic samples near the ferroelastic phase boundary (at x = 0.09 and 0.10).

Phase transformation in BiNbO4 ceramics

Phase transformation from β-BiNbO4 to α-BiNbO4 in BiNbO4 bulk samples was studied. From x-ray diffraction patterns, the transformation from β to α phase of BiNbO4 could be observed by heating the bulk samples of β-BiNbO4 from low temperatures to 700–1030°C. Such a transformation did not occur in powder samples and in the cooling course. This phenomenon might be related with associated activation of stress and heat energy in the heating course. Differential thermal analysis, shrinkage, and dielectric properties as a function of temperature were carried out, and all the results confirmed the transformation from β to α phase of BiNbO4.

High permittivity and low loss microwave dielectrics suitable for 5G resonators and low temperature co-fired ceramic architecture

Bi2(Li0.5Ta1.5)O7 + xBi2O3 (x = 0, 0.01 and 0.02) ceramics were prepared using a solid state reaction method. All compositions were crystallized in a single Bi2(Li0.5Ta1.5)O7 phase without secondary peaks in X-ray diffraction patterns. Bi2(Li0.5Ta1.5)O7 ceramics were densified at 1025 °C with a permittivity (er) of ∼ 65.1, Qf ∼ 15 500 GHz (Q ∼ microwave quality factor; f ∼ resonant frequency; 16 780 GHz when annealed in O2) and the temperature coefficient of resonant frequency (TCF) was ∼ −17.5 ppm °C−1. The sintering temperature was lowered to ∼920 °C by the addition of 2 mol% excess Bi2O3 (er ∼ 64.1, a Qf ∼ 11 200 GHz/11 650 GHz when annealed in O2 and at a TCF of ∼ −19 ppm °C−1) with compositions chemically compatible with Ag electrodes. Bi2(Li0.5Ta1.5)O7 + xBi2O3 are ideal for application as dielectric resonators in 5G mobile base station technology for which ceramics with 60 < er < 70, high Qf and close to zero TCF are commercially unavailable. They may additionally prove to be useful as high er and high Qf materials in low temperature co-fired ceramic (LTCC) technology.

Enhanced energy storage density by inducing defect dipoles in lead free relaxor ferroelectric BaTiO3-based ceramics

In this work, Mn-doped 0.9BaTiO3-0.1Bi(Mg2/3Nb1/3)O3 ceramics were prepared by the conventional solid state reaction method, and the effect of defect dipoles on energy storage properties of lead free relaxor ferroelectric BaTiO3-based ceramics was studied. The crystal structure, dielectric properties, and energy storage properties were explored in detail. It was found that polarization hysteresis (P-E) loops of 0.9BaTiO3-0.1Bi(Mg2/3Nb1/3)O3-x wt. % MnCO3 (0.2–0.5) ceramics took on high maximum polarization (Pmax) and low remanent polarization (Pr). Meanwhile, recoverable energy density (Wrec) and energy conversion efficiency (η) were obviously enhanced by inducing defect dipoles into BaTiO3-Bi(Mg2/3Nb1/3)O3 relaxor ferroelectrics. The 0.9BaTiO3-0.1Bi(Mg2/3Nb1/3)O3-0.3 wt. % MnCO3 ceramic was found to exhibit good energy storage properties with a Wrec of about 1.70 J/cm3 and a η ∼ 90% under an electric field of 210 kV/cm. The breakdown electric field and Wrec of BaTiO3-based materials were significantly in...