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-xAg_0.5Bi_0.5MoO₄ 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-xAg_0.5Bi_0.5MoO₄ 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 Ag_0.5Bi_0.5MoO₄ 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-xAg_0.5Bi_0.5MoO₄ ceramics are promising candidates for low temperature co-fired ceramics (LTCC) applications.     

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

Cold 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.     

Low temperature firing microwave dielectric ceramics (K0.5Ln0.5)MoO4 (Ln = Nd and Sm) with low dielectric loss

In the present work, novel low temperature firing microwave dielectric ceramics (K_0.5Ln_0.5)MoO₄ (Ln = Nd and Sm) were prepared via the traditional solid state reaction method. A pure monoclinic phase can be formed at a low sintering temperature around 680 °C for both (K_0.5Nd_0.5)MoO₄ and (K_0.5Sm_0.5)MoO₄ ceramics. The densification temperature for the (K_0.5Nd_0.5)MoO₄ and (K_0.5Sm_0.5)MoO₄ ceramics are 700 °C and 800 °C for 2 h, respectively. The best microwave dielectric properties for (K_0.5Nd_0.5)MoO₄ was obtained in ceramic sample sintered at 760 °C for 2 h, with a dielectric permittivity of 9.8, a Qf about 69,000 GHz and a temperature coefficient of frequency about −62 ppm/°C. The best microwave dielectric properties for (K_0.5Sm_0.5)MoO₄ was obtained in ceramic sample sintered at 800 °C for 2 h, with a dielectric permittivity of 9.7, a Qf about 20,000 GHz and a temperature coefficient of frequency about −65 ppm/°C.     

Structure,Infrared Reflectivity and Microwave Dielectric Properties of (Na0.5La0.5)MoO4–(Na0.5Bi0.5)MoO4 Ceramics

A series of temperature‐stable microwave dielectric ceramics, (1?x)(Na_0.5La_0.5)MoO₄–x(Na_0.5Bi_0.5)MoO₄ (0.0 ≤ x ≤ 1.0) were prepared by using solid‐state reaction. All specimens can be well sintered at temperature of 580°C–680°C. Sintering behavior, phase composition, microstructures, and microwave dielectric properties of the ceramics were investigated. X‐ray diffraction results indicated that tetragonal scheelite solid solution was formed. Microwave dielectric properties showed that permittivity (ε_r) and temperature coefficient of resonant frequency (τ_f) were increased gradually, while quality factor (Q × f) values were decreased, at the x value was increased. The 0.45(Na_0.5La_0.5)MoO₄–0.55(Na_0.5Bi_0.5)MoO₄ ceramic sintered at 640°C with a relative permittivity of 23.1, a Q × f values of 17 500 GHz (at 9 GHz) and a near zero τ_f value of 0.28 ppm/°C. Far‐infrared spectra (50–1000 cm^?1) study showed that complex dielectric spectra were in good agreement with the measured microwave permittivity and dielectric losses.     

Microwave Dielectric Properties of LiKSm2(MoO4)4 Ceramics with Ultralow Sintering Temperatures

In this work, a novel low‐temperature firing microwave dielectric ceramic LiKSm₂(MoO₄)₄ was prepared via solid‐state reaction method. Ceramic samples with relative densities about 94.6% were obtained at sintering temperature 640°C–680°C. The best microwave dielectric properties was obtained in ceramic sample sintered at 620°C with a permittivity about 11.5, a Q × f value about 39 000 GHz and a temperature coefficient of frequency about ?15.9 ppm/°C. According to XRD patterns and backscattered electron micrograph, combined with Energy Dispersive Spectra analysis, of cofired samples with 30 wt% aluminum sintered at 620°C/4 h, the LiKSm₂(MoO₄)₄ ceramic was found to be chemically compatible with Al but react seriously with Ag, forming AgSmMo₂O₈ phase, at its sintering temperature.     

Microwave dielectric properties of Pb2MoO5 ceramic with ultra-low sintering temperature

Ultra-low firing microwave dielectric ceramic Pb₂MoO₅ with monoclinic structure was prepared via a conventional solid state reaction method. The sintering temperature ranged from 530 °C to 650 °C. The relative densities of the ceramic samples were about 97% when the sintering temperature was greater than 570 °C. The best microwave dielectric properties were obtained in the ceramic sintered at 610 °C for 2 h with a permittivity ∼19.1, a Q × f value about 21,960 GHz (at 7.461 GHz) and a temperature coefficient value of −60 ppm/°C. From the X-ray diffraction, backscattered electron image results of the co-fired samples with 30 wt% silver and aluminum additive, the Pb₂MoO₅ ceramics were found not to react with Ag and Al at 610 °C for 4 h. The microwave dielectric properties and ultra-low sintering temperature of Pb₂MoO₅ ceramic make it a promising candidate for low temperature co-fired ceramic applications.     

Microwave Dielectric Ceramics Li2MO4−TiO2 (M = Mo,W) with Low Sintering Temperatures

In this work, novel series of (1 ? x)Li₂MO₄–xTiO₂ (M = Mo, W; x = 0.3, 0.4, 0.45, 0.5, 0.6) ceramics were developed for microwave dielectric application. They were prepared via the mixed‐oxide process and the phase composition, microstructures, sintering behaviors, and microwave dielectric properties were investigated. The X‐ray diffraction (XRD) pattern and scanning electron microscope analysis indicated that the Li₂MO₄ (M = Mo, W) did not react with rutile TiO₂ and a stable two‐phase composite system Li₂MO₄–TiO₂ (M = Mo, W) was formed. The XRD pattern of cofired ceramics revealed that some parts of Li₂MoO₄ phase and very small part of Li₂WO₄ phase react with Ag to form Ag₂MoO₄ phase and Ag₂WO₄ phase, respectively. At x = 0.45–0.5, temperature stable microwave dielectric materials with low sintering temperature (700°C–730°C) were obtained: ε_r = 10.6–11.0, Qf = 30 060–32 800 GHz, and temperature coefficient of resonant frequency ~0 ppm/°C.     

Low-loss and ultra-low temperature sintering microwave dielectrics of pure and Ag-modified K2Mg2(MoO4)3 ceramics

Novel K_2–2xAg_2xMg₂(MoO₄)₃ (x = 0–0.09) ceramics were synthesized by conventional solid-state sintering method. Based on the X-ray diffraction (XRD) patterns, all samples were identified to belong to an orthorhombic structure with a space group of P212121(19). The pure phase K₂Mg₂(MoO₄)₃ specimen when sintered at 590 °C revealed the favorable microwave dielectric properties: ε_r of 6.91, Q×f of 21,900 GHz and τ_f of ?164 ppm/°C. The substitution of Ag⁺ for K⁺ in K_2–2xAg_2xMg₂(MoO₄)₃ (x = 0.01–0.09) ceramics led to the more stable structure and dramatically enhanced the Q×f to a value of 54,900 GHz at 500 °C. The microwave dielectric properties were related to the relative density, microstructure, ionic polarization, lattice energy, packing fraction, and bond valence of the ceramics. It was suggested that for ultra-low temperature co-fired ceramic (ULTCC) applications, K_1.86Ag_0.14Mg₂(MoO₄)₃ ceramic could be sintered at 500 °C, which revealed an excellent combination of microwave dielectric properties (ε_r =7.34, Q×f =54,900 GHz and τ_f =–156 ppm/°C) and good chemical compatibility with aluminum electrodes.     

Effects of (Cr0.5Ta0.5)4+ on the microstructure,chemical bond characteristics,and dielectric properties of molybdate-based ceramics at microwave and terahertz frequency

In this study, Ce₂ [Zr_1−x (Cr_0.5Ta_0.5)_x]₃(MoO₄)₉ (x = 0.02–0.10) ceramics were synthesized using the solid-state reaction technique, and the crystalline parameters, sintering behaviors, chemical bond characteristics, infrared reflection spectrum, and dielectric response at microwave and terahertz frequency were examined. X-ray diffraction results demonstrated the crystallization of all ceramics in the trigonal structure (R-3c space group), and additional peaks were not detected. The densification point of ceramic was 875 °C. The addition of (Cr_0.5Ta_0.5)⁴⁺ significantly reduced the dielectric loss in the host ceramic. For Ce₂ [Zr_0.96(Cr_0.5Ta_0.5)_0.04]₃(MoO₄)₉, outstanding microwave properties of ε_r = 10.66, Qf = 79,436 GHz, and τ_f = −19.07 ppm/°C were obtained at 875 °C. The chemical bond characteristics were also parameterized to explore the relationship between Zr(CrTa)–O and microwave properties. Infrared spectral results further indicate that phonon vibrations lower than 400 cm⁻¹ contribute to 80% of the polarization. In our comparison between the infrared spectrum and terahertz time-domain spectrum, we found that the permittivity extracted by the latter is closer to the observed value.     

Preparation and Microwave Dielectric Properties of Ultra‐low Temperature Sintering Ceramics in K2O–MoO3 Binary System

A series of microwave dielectric ceramics in the compositions of K₂Mo₂O₇, K₂Mo₃O₁₀, and K₂Mo₄O₁₃ in K₂O–MoO₃ binary system with ultra low sintering temperatures were prepared using the solid‐state reaction method. Their synthesis, phase composition, compatibility with metal electrodes, microstructures, and microwave dielectric properties were investigated. The K₂Mo₂O₇ ceramic sintered at 460°C with a triclinic structure has a relative permittivity of 7.5, a Q × f value of 22 000 GHz, and a τ_f value of ?63 ppm/°C. The X‐ray diffraction patterns indicate that K₂Mo₂O₇ does not react with Ag and Al electrodes at the co‐fired temperatures. The K₂Mo₃O₁₀ ceramic can be sintered well at 520°C with a relative permittivity of 5.6, a Q × f value of 35 830 GHz, and a τ_f value of ?92 ppm/°C. It has compatibility with Ag electrode. The K₂Mo₄O₁₃ ceramic sintered at 540°C possesses good microwave dielectric properties with a relative permittivity of 6.8, a Q × f value of 39 290 GHz and a τ_f value of ?67 ppm/°C and it is compatible with Al electrode. For K₂Mo₂O₇ and K₂Mo₄O₁₃, it is found that the grain sizes and the number of grain boundaries play an important role in the dielectric loss. From this study, it can be seen that the three ceramics in K₂O–MoO₃ system have good microwave dielectric properties, ultra‐low sintering temperatures, non‐toxic, and low‐cost characteristics. So they can be potentially applied to ultra‐LTCC devices.     

Microwave dielectric properties of tetragonal scheelite-structured (NaY)1/2MoO4 ceramics

(NaY)_1/2MoO₄ was fabricated via the solid-state reaction method of Na₂CO₃, Y₂O₃, and MoO₃. Scanning electron microscopy results demonstrated that all the (NaY)_1/2MoO₄ ceramics could be densified well in the sintering temperature range of 900–960°C. Results of X-ray diffraction analysis demonstrated that the (NaY)_1/2MoO₄ ceramics crystallized into tetragonal scheelite structure. Sintering (NaY)_1/2MoO₄ at 940°C for 2 h optimized the microwave dielectric properties of the ceramics. The microwave permittivity, Q × f, and TCF of the (NaY)_1/2MoO₄ were 10.9, 29 000 GHz and −40.7 ppm/°C, respectively.     

Low‐Temperature Sintering Li2MoO4/Ni0.5Zn0.5Fe2O4 Magneto‐Dielectric Composites for High‐Frequency Application

The coexistence of Li₂MoO₄ (LMO) and Ni_0.5Zn_0.5Fe₂O₄ (NZO) has been proven and their low‐temperature‐sintered magneto‐dielectric composites (1?x)LMO–xNZO (volume fraction factor x = 0.1, 0.3, 0.5, 0.7) were prepared by the conventional solid‐state reaction method and were sintered below 700°C. It is found that the optimal sample (x = 0.5) has good and relatively stable magneto‐dielectric performance in the frequency range from 10 MHz to 1 GHz with permittivity between 7.14 and 6.84, dielectric loss tangent between 0.09 and 0.02, and permeability between 5.23 and 3.30, magnetic loss tangent between 0.06 and 0.65, respectively. Furthermore, the verified chemical compatibility with silver indicates that the LMO–NZO ceramics are potential for low‐temperature cofired ceramic application and their multifunctional magneto‐dielectric properties also make them for potential applications in electronic devices.     

Good dielectric performance and broadband dielectric polarization in Ag,Nb co-doped TiO2

Due to the demand of miniaturization and integration for ceramic capacitors in electronic components market, TiO₂-based ceramics with colossal permittivity has become a research hotspot in recent years. In this work, we report that Ag⁺/Nb⁵⁺ co-doped (Ag_1/4Nb_3/4)_xTi_1−xO₂ (ANTOx) ceramics with colossal permittivity over a wide frequency and temperature range were successfully prepared by a traditional solid–state method. Notably, compositions of ANTO0.005 and ANTO0.01 respectively exhibit both low dielectric loss (0.040 and 0.050 at 1 kHz), high dielectric permittivity (9.2 × 10³ and 1.6 × 10⁴ at 1 kHz), and good thermal stability, which satisfy the requirements for the temperature range of application of X9R and X8R ceramic capacitors, respectively. The origin of the dielectric behavior was attributed to five dielectric relaxation phenomena, i.e., localized carriers' hopping, electron–pinned defect–dipoles, interfacial polarization, and oxygen vacancies ionization and diffusion, as suggested by dielectric temperature spectra and valence state analysis via XPS; wherein, electron-pinned defect–dipoles and internal barrier layer capacitance are believed to be the main causes for the giant dielectric permittivity in ANTOx ceramics.     

Microwave Dielectric Properties of Scheelite Structured PbMoO4 Ceramic with Ultralow Sintering Temperature

In this work, a low‐firing microwave dielectric ceramic PbMoO₄ with tetragonal structure was prepared via a solid‐state reaction method. The sintering temperature ranges from 570°C to 670°C. Ceramic samples with relative densities above 97% were obtained when sintering temperature was around 600°C. The best microwave dielectric properties were obtained in the ceramic sintered at 650°C for 2 h with a permittivity ~26.7, a Q × f value about 42 830 GHz (at 6.2 GHz) and a temperature coefficient value of 6.2 ppm/°C. From the X‐ray diffraction, backscattered electron imaging results of the cofired sample with 30 wt% silver and aluminum additive, the PbMoO₄ ceramic was found not to react with Ag and Al at 630°C. The microwave dielectric properties and low sintering temperature of PbMoO₄ ceramic make it a candidate for low‐temperature cofired ceramic applications.     

Phase transitions and microwave dielectric behaviors of the (Bi1−xLi0.5xY0.5x)(V1−xMox)O4 ceramics

Microwave dielectric ceramics with intrinsic low sintering temperatures are potential candidates for low temperature co-fired ceramics technology. In the present work, the (Li_0.5Y_0.5)MoO₄ ceramic with tetragonal scheelite structures was selected to improve microwave dielectric properties of BiVO₄ ceramics. As proved by X-ray diffraction (XRD) results, scheelite structured solid-solution ceramics were formed with x value ≤0.1 in the (Bi_1−xLi_0.5xY_0.5x)(V_1−xMo_x)O₄. In situ XRD results further confirmed that the addition of (Li_0.5Y_0.5)MoO₄ also lowered transition temperature from distorted monoclinic to tetragonal scheelite structure. When x value increased further, zircon phase was detected by XRD. Room and high-temperature Raman spectra also supported the XRD results. Differences of thermal expansion coefficients of both monoclinic and tetragonal scheelite phases lead to an abnormality at phase transition temperature. Good microwave dielectric properties with permittivity above 70 and Qf (Q = quality factor = 1/dielectric loss and f = frequency) value above 8000 GHz were obtained in the (Bi_1−xLi_0.5xY_0.5x)(V_1−xMo_x)O₄ solid-solution ceramics with x value ≤0.1 sintered below 800°C. However, permittivity peak values at phase transition temperatures lead to large positive or negative temperature coefficient of resonant frequency, and this needs to be modified via composite technologies in the future.     

Ultra‐low temperature sintering microwave dielectric ceramics based on Ag2O–MoO3 binary system

The Ag₂Mo₂O₇ and Ag₆Mo₁₀O₃₃ ceramics for ultra‐low temperature co‐fired ceramic application were prepared by the solid‐state reaction route. The optimized densification temperatures of Ag₂Mo₂O₇ and Ag₆Mo₁₀O₃₃ are 460°C and 500°C, respectively. The phase structures and microstructures of these ceramics were systematically studied. The Ag₂Mo₂O₇ ceramic sintered at 460°C/4 h exhibits excellent microwave dielectric properties with ε_r=13.3, Q×f=25 300 GHz and τ_f=?142 ppm/°C at 9.25 GHz. The Ag₆Mo₁₀O₃₃ ceramic sintered at 500°C/4 h shows the microwave dielectric properties with ε_r=14.0, Q×f=8500 GHz and τ_f=?50 ppm/°C at 9.00 GHz. Moreover, when Ag₂Mo₂O₇ samples are sintered at ultra‐low sintering temperatures of 420°C‐490°C, the Q×f values of them are all above 20 000 GHz. Besides, the Ag₂Mo₂O₇ ceramic does not react with silver powder or aluminum powder. The variation of relative permittivity, resonant frequency, and Q×f values as a function of operating temperature has been also studied. All the results indicate that the Ag₂Mo₂O₇ ceramic is a good candidate for ultra‐low temperature co‐fired microwave devices.     

Improvements and Modifications to Room‐Temperature Fabrication Method for Dielectric Li2MoO4 Ceramics

Dielectric properties of lithium molybdate disks fabricated by moistening water‐soluble Li₂MoO₄ powder, compressing it, and postprocessing the samples at 120°C, were improved by the optimization of powder particle size, sample pressing pressure, and postprocessing time. It appeared that the postprocessing temperature of the Li₂MoO₄ ceramics could be chosen so as to be applicable to the associated integrated materials as long as the postprocessing time was adequately adjusted to ensure the removal of the residual water. In addition, the dielectric properties of Li₂MoO₄ ceramic were modified with an inclusion of suitable additives. For example, at 1 GHz the relative permittivity of Li₂MoO₄ disks fabricated at room temperature and postprocessed at 120°C was increased from 6.4 to 8.8 with an addition of 10 vol% of rutile TiO₂ and to 9.7 with an addition of 10 vol% of BaTiO₃. At the same time the loss tangent value increased from 0.0006 to 0.0014 and to 0.011, respectively.     

Low‐Temperature Sinterable (1−x)Ba3(VO4)2–xLiMg0.9Zn0.1PO4 Microwave Dielectric Ceramics

The novel low‐temperature sinterable (1 ? x)Ba₃(VO₄)₂–xLiMg_0.9Zn_0.1PO₄ microwave dielectric ceramics were prepared by cofiring the mixtures of pure‐phase Ba₃(VO₄)₂ and LiMg_0.9Zn_0.1PO₄. The phase structure and grain morphology of the ceramics were evaluated using X‐ray diffraction, Raman spectra, and scanning electron microscopy. The results indicated that Ba₃(VO₄)₂ and LiMg_0.9Zn_0.1PO₄ phases can well coexist in the sintered body. Nevertheless, a small amount of LiZnPO₄ and some vanadate phases with low melting points were observed, which not only can influence the microwave dielectric properties of the ceramic but also can obviously improve the densification behavior at a relatively low sintering temperature. The near‐zero temperature coefficients of the resonant frequency (τ_f) could be achieved by adjusting the relative content of the two phases owing to their opposite τ_f values and simultaneously a desirable quality factor Q × f value can be maintained. No chemical reaction between the matrix ceramic phase and Ag took place after sintering at 800°C for 4 h. The ceramics with 45 vol% LiMg_0.9Zn_0.1PO₄ can be well sintered at only 800°C and exhibit excellent microwave dielectric properties of ε_r ~ 10, Q × f ~ 64 500 GHz, and τ_f ~ ?2.1 ppm/°C, thus showing a great potential as a low‐permittivity low‐temperature cofired microwave dielectric material.     

Cold sintering co-firing of (Ca,Bi)(Mo,V)O4–PTFE composites in a single step

Because of large differences in the processing temperature windows between ceramics and polymers, the single-step co-sintering of microwave dielectric ceramic–polymer substrates remains challenging. In this work, a dense (Ca_0.65Bi_0.35)(Mo_0.65V_0.35)O₄ (CBMVO) ceramic was first prepared through cold sintering at 150°C, under a uniaxial pressure of 300 MPa for 60 min with Li₂MoO₄ (LMO) as a transient low-temperature solvent. Cold-sintered CBMVO–5 wt% LMO ceramic shows excellent microwave dielectric properties: ε_r ∼ 11.4, Q × f ∼ 7070 GHz, τ_f ∼ −7.4 ppm/°C. Moreover, the optimized cold sintering process enabled the preparation of a layered co-sintered (2–2 type) CBMVO–polytetrafluoroethylene composite, which maintained excellent microwave dielectric properties and showed a good heterogeneous interface bonding. The proposed cold sintering co-firing of ceramic–polymer composites in a single step shows great potential for application in the seamless integration between ceramics and polymer substrates.     

Structure,spectral analysis and microwave dielectric properties of novel x(NaBi)0.5MoO4-(1-x)Bi2/3MoO4 (x = 0.2 ∼ 0.8) ceramics with low sintering temperatures

Herein, the x(NaBi)_0.5MoO₄-(1-x)Bi_2/3MoO₄ (xNBM-(1-x)BMO, x = 0.2 ∼ 0.8) microwave dielectric ceramics with low sintering temperatures were prepared via the traditional solid-state method to adjust the τ_f value and dielectric constant. The crystal structure was determined using X-Ray diffraction and Raman spectroscopy, the microstructure was investigated using scanning electron micrograph and energy disperse spectroscopy, and the dielectric properties were studied using a network analyser and infrared spectroscopy. For the xNBM-(1-x)BMO composite ceramics, the (NaBi)_0.5MoO₄ tetragonal phase coexisted with the Bi_2/3MoO₄ monoclinic phase. With the rise of x value, the permittivity increased from 23.7–29.8, and the τ_f value shifited from -53.3 ppm/°C to -13.7 ppm/°C. The 0.8NBM-0.2BMO ceramic sintered at 680 °C possessed excellent microwave dielectric properties with a ε_r = 29.8 (6.7 GHz), a Qf = 11,800 GHz, and a τ_f = -13.7 ppm/°C. These results made the xNBM-(1-x)BMO composite ceramics great candidates in low temperature co-fired ceramics technology.