Jing Guo

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.

Cold Sintering: A Paradigm Shift for Processing and Integration of Ceramics

This paper describes a sintering technique for ceramics and ceramic-based composites, using water as a transient solvent to effect densification (i.e. sintering) at temperatures between room temperature and 200u2009°C. To emphasize the incredible reduction in sintering temperature relative to conventional thermal sintering this new approach is named the Cold Sintering Process (CSP). Basically CSP uses a transient aqueous environment to effect densification by a mediated dissolution-precipitation process. CSP of NaCl, alkali molybdates and V2 O5 with small concentrations of water are described in detail, but the process is extended and demonstrated for a diverse range of chemistries (oxides, carbonates, bromides, fluorides, chlorides and phosphates), multiple crystal structures, and multimaterial applications. Furthermore, the properties of selected CSP samples are demonstrated to be essentially equivalent as samples made by conventional thermal sintering.

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 (>80u2006000 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 8u2006000 GHz–10u2006000 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 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).

Hydrothermal-Assisted Cold Sintering Process: A New Guidance for Low-Temperature Ceramic Sintering

Sintering is a thermal treatment process that is generally applied to achieve dense bulk solids from particulate materials below the melting temperature. Conventional sintering of polycrystalline ceramics is prevalently performed at quite high temperatures, normally up to 1000 to 1200 °C for most ceramic materials, typically 50% to 75% of the melting temperatures. Here we present a new sintering route to achieve dense ceramics at extraordinarily low temperatures. This method is basically modified from the cold sintering process (CSP) we developed very recently by specifically incorporating the hydrothermal precursor solutions into the particles. BaTiO3 nano polycrystalline ceramics are exemplified for demonstration due to their technological importance and normally high processing temperature under conventional sintering routes. The presented technique could also be extended to a much broader range of material systems than previously demonstrated via a hydrothermal synthesis using water or volatile solutions. Such a methodology is of significant importance, because it provides a chemical roadmap for cost-effective inorganic processing that can enable broad practical applications.

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.

Synthesis, structure, and characterization of new low-firing microwave dielectric ceramics: (Ca1−3xBi2xΦx)MoO4

A series of A-site deficient scheelite ceramics (Ca1−3xBi2xΦx)MoO4 (x = 0.005, 0.015, 0.025, 0.035, 0.05, 0.1, 0.15, and 0.2, and Φ: A-site vacancy) were synthesized via the solid state reaction route. The structures were analyzed using a combination of X-ray diffraction and X-ray absorption fine structure spectroscopy (Mo K-edge and Bi L3-edge) to determine average and local structures. A series of defective scheelite (Ca1−3xBi2xΦx)MoO4 compositions can be formed as a solid solution, and local structures of Mo and Bi indicate that a MoO4 tetrahedron and a BiO8 polyhedron become more distorted with the x value. The large change in the Bi–O1 (the first shell) and Bi–O2 (the second shell) distances is an important insight into the nature of the defective structures. The statistical disorder of a Bi–O bond is one order of magnitude larger than that of a Mo–O bond. The microstructures and microwave dielectric properties were investigated by scanning electron microscopy and through network analyzer resonance studies. All the compositions can be sintered well below 900 °C. With slight Bi substitutions (x = 0.005 and 0.015), the samples exhibit improved Q × f values. At x = 0.15, temperature stable (TCF = −1.2 ppm per °C) low-firing (ST = 700 °C) microwave dielectric materials were obtained with a permittivity of 21.2 and a Q × f value of 29u2006300 GHz. The factors affecting dielectric properties are associated with the local structures of Mo and Bi across the solid solution.

Microwave dielectric properties and low temperature firing of (1 − x)Li2Zn3Ti4O12–xLi2TiO3 (0.2 ≤ x ≤ 0.8) ceramics with B2O3–CuO addition

In the present work, the (1xa0−xa0x)Li2Zn3Ti4O12–xLi2TiO3 (0.2xa0≤xa0xxa0≤0.8) ceramics were prepared via the solid state reaction method. The 0.8Li2Zn3Ti4O12–0.2Li2TiO3 ceramic sample sintered at 1,160xa0°C for 2xa0h demonstrated high microwave dielectric property with a relative permittivity of 18.0, a high quality factor (Qxa0×xa0f)xa0~xa0100,000xa0GHz (at 7.2xa0GHz), and a temperature coefficient of resonant frequency about −47.8xa0ppm/°C. With 2.0xa0wt% 0.4B2O3–0.6CuO addition, a relative permittivity of 17.5, a Qxa0×xa0f value of 71,000xa0GHz and a temperature coefficient of resonant frequency of −44.4xa0ppm/oC can be obtained in 0.8Li2Zn3Ti4O12–0.2Li2TiO3 ceramic sintered at 925xa0°C for 5xa0h and the chemical compatibility with silver electrode indicates that the ceramics may be a suitable candidate for the low temperature co-fired ceramic technology application.