Huidong Xie

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.

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.

Microwave Dielectric Properties of Sol-Gel Processed Bi4Si3O12 Ceramics and Single Crystal

Microwave dielectric ceramics Bi4Si3O12 was made from powders prepared via sol-gel method as a potential candidate of low temperature co-fired ceramics in microwave applications. The microwave dielectric properties of sol-gel processed Bi4Si3O12 ceramics were compared with that of single crystal. The sintering temperature of the ceramics ranged from 920° to 1010°C. The best microwave dielectric properties were obtained when the ceramics was sintered at 980°C for 8 h with a permittivity of ∼8.8, a Q×f value of ∼41,898 GHz (at 11.5 GHz) and a temperature coefficient value of –72 ppm/°C. The permittivity, Q×f value, and temperature coefficient value of Bi4Si3O12 single crystal were ∼15.9, 45,326 GHz (at 7.0 GHz) and –92 ppm/°C, respectively.

Sol–gel preparation and luminescent properties of red-emitting phosphor Sr-Ba-Mo-W-O-(Eu3+,Sm3+)

Two series of red-emitting phosphors Sr-Ba-Mo-W-O:Eu,Sm and Sr-Ba-Mo-W-O:Eu have been synthesized by a sol-gel method. The effects of the chemical composition, concentrations of Sm(3+) and Eu(3+), the Sr(2+)/Ba(2+) ratio, and the W(6+)/Mo(6+) ratio on the luminescent properties were investigated. The as-prepared phosphors were characterized by X-ray diffraction and Raman spectra. Results showed that single phases of the two series were prepared. The compositions of Sr0.6 Ba0.13Mo0.8 W0.2O4:Eu0.10Sm0.08 and Sr0.75Ba0.1Mo0.8 W0.2O4:Eu0.10 had the strongest luminescent intensity. The excitation spectra of Sm(3+), Eu(3+) co-doped phosphors were broader and the strongest peak moved to 404 nm when compared with that of Eu(3+) single-doped phosphors. The luminescent intensity of the Sr0.6Ba0.13Mo0.8W0.2O4:Eu0.10 Sm0.08 at 618 nm were 2.8 times greater than that of Sr0.75Ba0.1Mo0.8 W0.2O4:Eu0.10. The luminescent intensity of Sr0.6Ba0.13Mo0.8 W0.2 O4:Eu0.10Sm0.08 and Sr0.75Ba0.1Mo0.8W0.2O4:Eu0.10 at 150 °C decreased to 56.8% and 50.3% of the initial value at room temperature, respectively.

Luminescent properties of sol–gel processed red‐emitting phosphor Ca0.6Sr0.4–1.5x‐0.5yMo0.4 W0.6O4:EuxLiy

A series of red-emitting phosphors Ca0.6Sr(0.4-1.5x-0.5y)Mo0.4W0.6O4:Eux Liy (x = 0.02-0.12, y = 0-0.12) has been synthesized by a sol-gel method. The effects of calcining temperature, concentrations of Li(+) and Eu(3+) , and compensation ions on the luminescent properties were investigated. X-ray diffraction and scanning electron microscopic results showed that as-prepared phosphors were of single phase with several microns. The Li(+) compensated compositions showed remarkably intense red emission at 619 nm. The emission intensity of the series reached maximum for compositions at x = 0.08 and y = 0.08 when the calcining temperature was 900 °C.