MoO4 Ceramics Research Articles

Microwave dielectric properties of a temperature-stable and ultra-low sintering Li3AlMo3O12/Li0.5Bi0.5MoO4 composite ceramics for ULTCC application

Temperature-stable ULTCC composite (1-x)Li3AlMo3O12-xLi0.5Bi0.5MoO4 ceramics were prepared by solid-state reaction method. X-ray and EDS results showed that Li3AlMo3O12 phase and Li0.5Bi0.5MoO4 phase exist independently, and no other phase is generated. Due to the low sintering temperature of both two phases, the composite ceramics can obtain high density at 560 ℃. Microwave dielectric properties mainly depend on the ceramic Li0.5Bi0.5MoO4 content x, the addition of Li0.5Bi0.5MoO4 effectively increases the τf of Li3AlMo3O12 from −65.05 ppm/℃ to 4.96 ppm/℃, but caused the dielectric loss increase. When x=0.45 and sintered at 560 ℃, microwave dielectric properties of 0.55Li3AlMo3O12-0.45Li0.5Bi0.5MoO4 ceramics with close zero τf: εr = 15.32, Q×f = 13,158 GHz, τf = 4.96 ppm/℃.

Cold sintered temperature stable xLi2MoO4-(1-x)(LiBi)0.5MoO4 microwave dielectric ceramics

Temperature stable xLi2MoO4-(1-x)(LiBi)0.5MoO4 (x = 0, 40, 50, 60, 70, 100 vol%) microwave dielectric ceramics obtained by cold sintering process (CSP) under a mild sintering condition (120 ℃/30 min/200 MPa) are introduced in this work. The XRD, SEM, and Raman analysis indicate the coexistence of Li2MoO4 and (LiBi)0.5MoO4 phases. Li2MoO4-(LiBi)0.5MoO4 ceramics are compatible with Ag and Al electrode materials under cold sintering condition. The specimens exhibit high relative densities and good microwave dielectric properties (relative permittivities: 31.5–5.5; Q×f values: 1900 - 18,500 GHz; TCF values: from +144 ppm/℃ to −106 ppm/℃), in particular, TCF = +0.7 ppm/℃ can be obtained in the case of x = 50 vol%. The extrapolated microwave dielectric properties obtained by the fitted infrared reflectivity spectrum are close to the measured data, revealing that the dielectric responses of cold sintered Li2MoO4-(LiBi)0.5MoO4 ceramics in the microwave range are dominated by the polar optical phonons.

Structure-property relationships in temperature stable low firing Ag2Mo2O7-Ag0.5Bi0.5MoO4 microwave dielectric ceramics

In this work, a series of temperature stable dielectric ceramics with an ultra-low sintering temperature (480 °C) have been prepared through mixing Ag0.5Bi0.5MoO4 with Ag2Mo2O7. The XRD and SEM-EDS analysis reveal that the two phases could coexist. The microwave dielectric properties of diphase ceramics are calculated that are in a good agreement with the experimental data. Raman and infrared spectroscopic techniques are used to study the structure-property relationships from the point of the polarization mechanism. When the amount of Ag0.5Bi0.5MoO4 is 0.65, the composite ceramics sintered at 480 °C show the best temperature stability with εr = 25.7, Q × f = 16 400 GHz, tanδ = 4.31 × 10−4, and τf = 0.3 ppm/°C. In addition, the ceramics exhibit good chemical compatibility with aluminum. These results indicate that (1-x) Ag2Mo2O7-xAg0.5Bi0.5MoO4 ceramics show the excellent temperature stability among those with a sintering temperature as low as 500 °C and could be good candidates for microwave device applications.

Novel scheelite‐type [Ca0.55(Nd1‐xBix)0.3]MoO4 (0.2 ≤ x ≤ 0.95) microwave dielectric ceramics with low sintering temperature

AbstractNovel scheelite‐type [Ca0.55(Nd1‐xBix)0.3]MoO4 (0.2 ≤ x ≤ 0.95) ceramics were prepared using the solid‐state reaction method. According to the X‐ray diffraction data, a solid solution was formed in 0.2 ≤ x ≤ 0.95 and all the samples belong to pure scheelite phase with the tetragonal structure. As revealed by Raman spectroscopy, the number of vibrational modes decreased with the increase in x value, which further indicated that Bi3+ ions occupied A‐site of scheelite structure. As the x value increased, the sintering temperature decreased from 740°C to 660°C; the permittivity increased from 12.6 to 20.3; the Qf value first decreased slightly and gradually remained stable. Based on the infrared reflectivity spectrum analysis, the calculated permittivity derived from the fitted data shared the same trend with the measured value. The [Ca0.55(Nd0.05Bi0.95)0.3]MoO4 ceramic sintered at 660 °C attained a near‐zero value temperature coefficient ~τf (−7.1 ppm/°C) and showed excellent microwave dielectric properties with a ɛr ~ 20.3 and a Qf ~ 33 860 GHz, making this system a promising candidate in the ultralow temperature cofired ceramic (ULTCC) technology.

Temperature stable 0.35Ag2MoO4-0.65Ag0.5Bi0.5MoO4 microwave dielectric ceramics with ultra-low sintering temperatures

In this study, the novel temperature-stable (1-x)Ag2MoO4-xAg0.5Bi0.5MoO4 microwave dielectric ceramics were prepared by a modified solid-state reaction method. The phase composition, microstructures and microwave dielectric properties of the (1-x)Ag2MoO4-xAg0.5Bi0.5MoO4 ceramics were investigated. All the compounds can be sintered well at ultra-low temperatures (<540 °C). The XRD and SEM analysis indicate that the Ag2MoO4 and the Ag0.5Bi0.5MoO4 can coexist with each other. When x = 0.65, the ceramics exhibit the best microwave dielectric properties with a relative permittivity of 23.9, a Q × f value of 16,200 GHz (at 7.3 GHz) and a near-zero TCF value of -2.4 ppm/°C at 520 °C. The results indicate that temperature-stable (1-x)Ag2MoO4-xAg0.5Bi0.5MoO4 ceramics are promising candidates for low temperature co-fired ceramics (LTCC) applications.

Ultra-low temperature sintering and microwave dielectric properties of a novel temperature stable Na2Mo2O7-Na0.5Bi0.5MoO4 ceramic

The novel ultra-low temperature sintering (1-x)Na2Mo2O7-xNa0.5Bi0.5MoO4 ceramics have been obtained via solid-state reaction method for passive integration use. The Na2Mo2O7 and Na0.5Bi0.5MoO4 crystal phases are found to be compatible with each other from the results of XRD and SEM-EDS. With the x value changing from 0.36 to 1.00, the εr increases from 16.0 to 32.0 and the τf value varies from −58 to 47ppm/°C. At x=0.75, the 0.25Na2Mo2O7-0.75Na0.5Bi0.5MoO4 ceramic sintered at an ultra-low sintering temperature of 580°C can be densified (>96%) and possesses good microwave dielectric properties of an εr of 24.0, a Q×f value of 13,000GHz (at 6.2GHz), and a τf value of 3ppm/°C. The theoretical εr and τf of the (1-x)Na2Mo2O7-xNa0.5Bi0.5MoO4 composites were calculated using the mixing law and in accordance with the measured values.

Influence of W substitution on crystal structure, phase evolution and microwave dielectric properties of (Na0.5Bi0.5)MoO4 ceramics with low sintering temperature

In this work, the (Na0.5Bi0.5)(Mo1−xWx)O4 (x = 0.0, 0.5 and 1.0) ceramics were prepared via solid state reaction method. All the samples can be well densified at sintering temperature about ~720 °C. Dense and homogeneous microstructure with grain size lying between 2~8 μm can be observed from scanning electron microscopy (SEM). Microwave dielectric permittivity of the (Na0.5Bi0.5)(Mo0.5W0.5)O4 ceramic was found to be temperature-independent in a wide range between 25~120 °C with a temperature coefficient of frequency (TCF) ~−6 ppm/°C, a permittivity ~28.9, and Qf values 12,000~14,000 GHz. Crystal structure was refined using Rietveld method and lattice parameters are a = b = 5.281 (5) Å and c = 11.550 (6) Å with a space group I 41/a (88). The (Na0.5Bi0.5)(Mo1−xWx)O4 ceramics might be good candidate for low temperature co-fired ceramics (LTCC) technology.

Cold sintering process: A new era for ceramic packaging and microwave device development

AbstractCold sintering process (CSP) is an extremely low‐temperature sintering process (room temperature to ~200°C) that uses aqueous‐based solutions as transient solvents to aid densification by a nonequilibrium dissolution‐precipitation process. In this work, CSP is introduced to fabricate microwave and packaging dielectric substrates, including ceramics (bulk monolithic substrates and multilayers) and ceramic‐polymer composites. Some dielectric materials, namely Li2MoO4, Na2Mo2O7, K2Mo2O7, and (LiBi)0.5MoO4 ceramics, and also (1−x)Li2MoO4−xPTFE and (1−x)(LiBi)0.5MoO4−xPTFE composites, are selected to demonstrate the feasibility of CSP in microwave and packaging substrate applications. Selected dielectric ceramics and composites with high densities (88%‐95%) and good microwave dielectric properties (permittivity, 5.6‐37.1; Q × f, 1700‐30 500 GHz) were obtained by CSP at 120°C. CSP can be also used to potentially develop a new co‐fired ceramic technology, namely CSCC. Li2MoO4−Ag multilayer co‐fired ceramic structures were successfully fabricated without obvious delamination, warping, or interdiffusion. Numerous materials with different dielectric properties can be densified by CSP, indicating that CSP provides a simple, effective, and energy‐saving strategy for the ceramic packaging and microwave device development.

Correlation between vibrational modes and dielectric properties in (Ca1−3xBi2xΦx)MoO4 ceramics

The Raman, infrared reflectivity and terahertz spectra of A-site deficient scheelite materials (Ca1−3xBi2xΦx)MoO4 (Φ: A-site vacancy) were studied to evaluate the correlation between the vibrational modes and the dielectric properties. The Raman spectra were fitted with the Lorentzian model and the changes of resonance frequencies and full width at half maximum of the resonance were discussed. With the substitution of Bi into the A site of CaMoO4, the associated ionic compensation produces an increase in disorder which can contribute to an increase in dielectric losses. The harmonic oscillator model was used to fit infrared reflectivity spectra and the obtained complex dielectric response was extrapolated down into the terahertz and microwave region. The contributions of each mode to the microwave dielectric properties were investigated. It was found that the modes at low frequencies which are related to AO8 polyhedra play the most important role in the microwave dielectric properties of (Ca1−3xBi2xΦx)MoO4.

Structural refinement, Raman spectroscopy, optical and electrical properties of (Ba1−xSrx)MoO4 ceramics

In this paper, structural refinement, Raman spectroscopy, optical and electrical properties of barium strontium molybdate [(Ba1−x Sr x )MoO4] ceramics with different (x) contents (x = 0; 0.1; 0.2; 0.3; 0.4; 0.5; 0.6; 0.7; 0.8; 0.9; and 1) were synthesized by the solid state reaction method. These ceramics were structurally characterized by X-ray diffraction (XRD), Rietveld refinement, and micro-Raman spectroscopy. The shape of the grains for these ceramics was observed by means of scanning electron microscopy (SEM) images. The optical properties were investigated using ultraviolet–visible (UV–Vis) absorption spectroscopy and photoluminescence (PL) measurements. The dielectric and ferroelectric properties were analyzed by permittivity (er), loss tangent (tan δ) and polarization versus electric field (P–E) hysteresis loop. XRD patterns, Rietveld refinement, and micro-Raman spectra showed that all ceramics are monophasic with a scheelite-type tetragonal structure. A decreased of lattice parameters and unit cell volume was observed with the increase of Sr2+ ions into BaMoO4 lattice. Rietveld data were employed to model the [BaO8], [SrO8] and [MoO4] clusters in the tetragonal lattices. The SEM images indicate that increased x content promotes a decrease in the grain size and modifications in the shape. UV–Vis spectra indicated a decrease in the optical band gap values with an increase in x content in the (Ba1−x Sr x )MoO4 ceramics. PL emissions exhibit a non-linear behavior to increase or decrease with the increase of Sr2+ ions in the tetragonal lattices, when excited by a wavelength of 350 nm. The P–E decreases along with slim hysteresis loop towards higher Sr2+ ions concentration. These effects are correlated with decrease in lattice parameters and c/a ratio in this tetragonal lattice. The microwave dielectric constant and quality factor were measured using the method proposed by Hakki–Coleman. Temperature coefficient and quality factor of these materials were measured by vector network analyzer.

Microwave dielectric properties of low firing (Na0.5Ln0.5)MoO4 (Ln=Nd and Ce) ceramics

A new series of sheelite structured (Na0.5Ln0.5)MoO4 (Ln=Nd and Ce) ceramics was prepared by using the traditional solid state reaction method. The (Na0.5Nd0.5)MoO4 ceramic could be well densified at 760°C with excellent microwave dielectric properties (a permittivity of ~10.5, a quality factor Q×f of ~19,605GHz (f=9.15GHz) and a temperature coefficient of frequency of (TCF or τf)~ −49ppm/oC). Meanwhile, the (Na0.5Ce0.5)MoO4 ceramic sintered at 780°C possessed the best microwave dielectric properties with a permittivity of ~11.2, a Q×f value of 19,365GHz (f=8.98GHz) and TCF of approximately −44ppm/°C. The (Na0.5Nd0.5)MoO4 ceramic was found to be chemically compatible with Ag electrode at its sintering temperature. It is a promising microwave dielectric material for low-temperature co-fired ceramic technology (LTCC). However, the (Na0.5Ce0.5)MoO4 ceramic was found to react with Ag after co-firing at 780°C for 2h and this limited its application in LTCC technology.

Microwave Dielectric Properties of (Li0.5Ln0.5)MoO4 (Ln = Nd, Er, Gd, Y, Yb, Sm, and Ce) Ceramics

In the present work, a series of scheelite‐related (Li0.5Ln0.5)MoO4 (Ln = Nd, Er, Gd, Y, Yb, Sm, and Ce) ceramics were prepared by using the solid‐state reaction method. Except for the (Li0.5Yb0.5)MoO4, the other compositions were found to have a scheelite structure. Among all the compositions, the (Li0.5Sm0.5)MoO4, (Li0.5Nd0.5)MoO4, and (Li0.5Ce0.5)MoO4 ceramics were found to possess low sintering temperatures below melting point of aluminum (660°C) and be chemically compatible with Al. The relation between microwave dielectric properties and structure was analyzed by using the Shannon additive rule and bond‐valence theory. This series of microwave dielectric ceramics might be candidate for low temperature co‐fired ceramics technology.

Structure-property relationships of novel microwave dielectric ceramics with low sintering temperatures: (Na(0.5x)Bi(0.5x)Ca(1-x))MoO(4).

A novel series of microwave dielectric ceramics (Na0.5xBi0.5xCa1-x)MoO4 (0 ≤ x ≤ 0.6) was synthesized by the solid state reaction method. The crystal structures, microstructures, dielectric responses, and vibrational properties were investigated using X-ray diffraction, scanning electron microscopy, a microwave network analyzer, and terahertz, Raman and infrared spectroscopies. All the samples could be sintered well below 850 °C and a scheelite solid solution could be formed without any secondary phase. At x = 0.5 and x = 0.6, low-firing (750-775 °C) high performance microwave dielectric materials were obtained with permittivities of 19.1-21.9, Q × f values of 20 660-22 700 GHz, and near-zero temperature coefficients. The factors affecting microwave dielectric properties were discussed based on the vibrational data. As revealed by Raman spectroscopy, the disorder degree grows with x rising, which might increase the permittivities and decrease the Q × f values. The infrared spectra were analyzed using the classical harmonic oscillator model, and the complex dielectric responses gained from the fits were extrapolated down to the microwave and THz range. It is believed that the external vibration modes located at low frequencies dominate the main dielectric polarization contributions, especially the Na-O/Bi-O translational mode. This result indicates that the microwave dielectric properties of (Na0.5xBi0.5xCa1-x)MoO4 ceramics mainly depend on the behavior of AO8 polyhedra.

Infrared spectroscopy and microwave dielectric properties of ultra-low temperature firing (K0.5La0.5)MoO4 ceramics

The (K0.5La0.5)MoO4 ceramics with intrinsic ultra-low sintering temperature were prepared via the solid state reaction method. Pure monoclinic (K0.5La0.5)MoO4 phase was formed at around 660°C. High performance of microwave dielectric properties were obtained in ceramic sample sintered at 680°C with a permittivity of ∼10.3, a temperature coefficient of resonant frequency about −81ppm/°C, and Qf value of 59,000GHz. Complex dielectric spectra gained from the infrared spectra were extrapolated down to microwave range, and they were in good agreement with the measured microwave permittivity and dielectric losses. The novel (K0.5La0.5)MoO4 ceramic might be a good candidate for microwave devices application.

Ferroelastic phase transition compositional dependence for solid-solution [(Li0.5Bi0.5)xBi1−x][MoxV1−x]O4 scheelite-structured microwave dielectric ceramics

A compositional dependence on the ferroelastic phase transition in scheelite solid-solution [(Li0.5Bi0.5)xBi1−x][MoxV1−x]O4 ceramics was characterized by the microwave dielectric data over the temperature range 10–420K. As x values increased from 0.0 to 0.125, a dielectric temperature-dependent anomaly consistent with a phase transition decreased linearly from 528K for the pure BiVO4 end member to 264K for the solid-solution composition at x=0.125. With further increasing x, the transition temperature decreased sharply to approximately 45K and became stable for x⩾0.52. The phase transition point for pure tetragonal (Li0.5Bi0.5)MoO4 ceramics is approximately 43K. As the x value increases, the Raman band of the [(Li0.5Bi0.5)xBi1−x][MoxV1−x]O4 ceramics at room temperature broadens and overlaps. Ferroelastic domain structures were observed as a function of composition with diffraction contrast imaging and high-resolution imaging with transmission electron microscopy. Structure–property relations are inferred from the microscopy observations and the Raman spectra.

Microwave Dielectric Ceramics in Li2O–Bi2O3–MoO3 System with Ultra‐Low Sintering Temperatures

A series of compounds in the Li2O–Bi2O3–MoO3 ternary system were investigated with regard to the preparation, phase composition, microwave dielectric properties, and chemical compatibility with silver (Ag) and aluminum (Al) electrodes. All the ceramics in this work have sintering temperatures lower than 750°C. The sintering behaviors and microwave dielectric properties of three single phases Li2MoO4, (Li0.5Bi0.5)MoO4, and Li8Bi2Mo7O28 bulk ceramics, were of particular focus in this investigation. The Li2MoO4 ceramic can be sintered to a high density at 540°C/2 h with a relative permittivity ∼5.5, a Q×f value of 46 000 GHz, and a temperature coefficient of resonant frequency (TCF) of ∼−160 ppm/°C. The (Li0.5Bi0.5)MoO4 ceramic has a scheelite structure and the largest relative permittivity of 44.4 among the ceramics studied in this work with a sintering temperature around 560°C, a Q×f value of 3200 GHz, and a large positive TCF of ∼+245 ppm/°C. The Li8Bi2Mo7O28 ceramic could be sintered at 540°C and has a relative permittivity of 13.6, a Q×f value of 8000 GHz, and a small negative TCF value of ∼−59 ppm/°C. From the X‐ray diffraction analysis of cofired ceramics, the Li2MoO4 ceramic does not react with either Ag or Al powders. The Li8Bi2Mo7O28 ceramic reacts with Ag but not with Al at its densification temperature. The (Li0.5Bi0.5)MoO4 ceramic was found to strongly react with Ag powder and to a limited extent with Al powders. From this study, the Li2O–Bi2O3–MoO3 ternary system has a number of attractive new materials with low sintering temperatures, high‐performing microwave dielectric properties, chemical compatibility with both Ag and Al metal electrodes, nontoxicity, and low‐cost constituents. All these materials can be included in the new field of ultra‐low‐temperature cofiring dielectrics for multilayer applications.