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.     

Low‐Temperature‐Fired ReVO4 (Re = La,Ce) Microwave Dielectric Ceramics

We report a series of ReVO₄ (Re = La, Ce) microwave dielectric ceramics fabricated by a standard solid‐state reaction method. X‐ray diffraction and scanning electron microscopy measurements were performed to explore the phase purity, sintering behavior, and microstructure. The analysis revealed that pure and dense monoclinic LaVO₄ ceramics with a monazite structure and tetragonal CeVO₄ ceramics with a zircon structure could be obtained in their respective sintering temperature range. Furthermore, LaVO₄ and CeVO₄ ceramics sintered at 850°C and 950°C for 4 h possessed out‐bound microwave dielectric properties: ε_r = 14.2, Q × f = 48197 GHz, τ_f = ?37.9 ppm/°C, and ε_r = 12.3, Q × f = 41 460 GHz, τ_f = ?34.4 ppm/°C, respectively. The overall results suggest that the ReVO₄ ceramics could be promising materials for low‐temperature‐cofired ceramic technology.     

Microwave Dielectric Properties of MLi2Ti6O14 [M=Ba and Sr] Ceramics and Their Compatibility with Sliver

Microwave dielectric properties of Li‐containing orthorhombic compounds with the composition of MLi₂Ti₆O₁₄ (M = Ba and Sr) were investigated. The ceramics were synthesized by the conventional solid‐state reaction route. The optimized sintering temperatures for the BaLi₂Ti₆O₁₄ and SrLi₂Ti₆O₁₄ ceramics are 1025°C and 1000°C, respectively. Favorable microwave dielectric properties were obtained with moderate ε_r of 31.7 and 33.6, quality factor Q × f values of 23 300 (at 7.3 GHz) and 8700 GHz (at 6.8 GHz), and low‐temperature coefficient of resonant frequency (τ_f) values of ?15.4 and ?2.7 ppm/°C for BaLi₂Ti₆O₁₄ and SrLi₂Ti₆O₁₄ ceramics, respectively. The addition of BaCu(B₂O₅) can effectively reduce the sintering temperature below 930°C without degrading the microwave dielectric properties. Compatibility with Ag electrode indicates these materials could be applied to low‐temperature cofired ceramic devices.     

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.     

Ba5Li2W3O15: A New Li‐Containing Perovskite‐Type Microwave Ceramic with High Q and Low τf

A new Li‐containing microwave ceramic Ba₅Li₂W₃O₁₅ with hexagonal perovskite structure was prepared through a solid‐state ceramic route. Small amount of scheelite BaWO₄ appeared as a second phase during sintering. The Ba₅Li₂W₃O₁₅ could be well densified at 1120°C and exhibits good microwave dielectric properties with permittivity (ε_r) of 25.4, high Q × f value about 39 000 GHz, and low temperature coefficient of resonate frequency (τ_f) of 10 ppm/°C. The addition of BaCu(B₂O₅) can effectively lower the sintering temperature from 1120°C to 900°C and does not induce degradation of the microwave dielectric properties. These results indicate that the Ba₅Li₂W₃O₁₅ ceramic might be a promising candidate in microwave dielectric resonators.     

A Novel Low‐Firing and Low Loss Microwave Dielectric Ceramic Li2Mg2W2O9 with Corundum Structure

The crystal structure and microwave dielectric properties of a novel low‐firing compound Li₂Mg₂W₂O₉ were investigated in this study. The phase purity and crystal structure were investigated using X‐ray diffraction analyses and Rietveld refinement. The best microwave dielectric properties of the ceramic with a low permittivity (ε_r) ~11.5, a quality factor (Q × f) ~31 900 GHz (at 10.76 GHz) and a temperature coefficient of the resonant frequency (τ_f) ~ ?66.0 ppm/°C were obtained at the optimum sintering temperature (920°C). CaTiO₃ was added into the Li₂Mg₂W₂O₉ ceramic to obtain a near zero τ_f, and 0.93Li₂Mg₂W₂O₉–0.07CaTiO₃ ceramic exhibited improved microwave dielectric properties with a near‐zero τ_f ~ ?1.3 ppm/°C, a ε_r ~21.6, a high Q_u × f value ~20 657 GHz. The low sintering temperature and favorable microwave dielectric properties make it a promising candidate for LTCC applications.     

Synthesis of LiBGeO4 using compositional design and its dielectric behaviors at RF and microwave frequencies

Borates are promising candidates as dielectric substrate materials in low temperature cofired ceramics technology (LTCC) due to their relative low sintering temperatures and relative permittivities compared to their counterparts. However, synthesizing borates having single-phase is still challenging because of the volatility and hydrophilicity of boron resources. In this work, a compositional design was utilized to synthesize single-phase LiBGeO₄ ceramics over a broad temperature range from 600 to 840 °C. Radio-frequency dielectric behaviours featured a strong temperature dependence, especially at high temperatures (>400 °C), which is related to the thermally activated polarizations. LiBGeO₄ ceramic sintered at 820 °C has optimum microwave dielectric properties with the relative permittivity (ε_r) of 6.28, a quality factor (Q × f) of 21,620 GHz, and a temperature coefficient of resonance frequency (τ_f) of -88.7 ppm/^°C. LiBGeO₄ also showed chemical inertness when cofired with silver (Ag), provided an evidence for its utilization in LTCC technology. Overall, this work provides a strategy for facile synthesis of phase pure borates, via the proposed two-step process to obtain stable boron resources.     

Microwave Dielectric Properties of Novel Glass‐Free Low‐Firing Li2CeO3 Ceramics

Novel glass–free low temperature firing microwave dielectric ceramics Li₂CeO₃ with high Q prepared through a conventional solid‐state reaction method had been investigated. All the specimens in this paper have sintering temperature lower than 750°C. XRD studies revealed single cubic phase. The microwave dielectric properties were correlated with the sintering conditions. At 720°C/4 h, Li₂CeO₃ ceramics possessed the excellent microwave dielectric properties of ε_r = 15.8, Q × f = 143 700 (GHz), and τ_f  = ?123 ppm/°C. Li₂CeO₃ ceramics could be excellent candidates for glass‐free low‐temperature co‐fired ceramics substrates.     

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.     

A Novel Magnetodielectric Solid Solution Ceramic 0.4LiFe5O8–0.6Li2MgTi3O8 with Excellent Microwave Dielectric Properties

In this study, a novel spinel solid solution ceramic of 0.4LiFe₅O₈–0.6Li₂MgTi₃O₈ (0.4LFO–0.6LMT) has been developed and investigated. It is found that the 40 mol% LiFe₅O₈ and 60 mol% Li₂MgTi₃O₈ are fully soluble in each other and a disordered spinel phase is formed. The ceramic sample sintered at 1050°C/2 h exhibits both good magnetic and dielectric properties in the frequency range 1–10 MHz, with a permeability between 29.9~14.1 and magnetic loss tangent between 0.12~0.67, permittivity between 16.92~16.94 and dielectric loss tangent between 5.9 × 10^?3–2.3 × 10^?2. The sample also has good microwave dielectric properties with a relative permittivity of 16.1, a high quality factor (Q × f) ~28 500 GHz (at 7.8 GHz). Furthermore, 3 wt% H₃BO₃–CuO (BCu) addition can effectively lower the sintering temperature to 925°C and does not degrade the magnetodielectric properties. The chemical compatibility with silver electrode indicates that this kind of ceramics is a good candidate for the low‐temperature cofired ceramic (LTCC) application.     

Structure,Microwave Dielectric Properties,and Novel Low‐Temperature Sintering of xSrTiO3–(1−x)LaAlO3 Ceramics with LTCC Application

xSrTiO₃–(1?x)LaAlO₃ ceramics with ZnO–B₂O₃ sintering aid were prepared by solid‐state reaction method leading to a significant decrease in sintering temperature from 1550°C to 1050°C. The structure, microwave dielectric properties, and low‐temperature sintering behavior were systematically investigated. The results revealed the relationships among ionic size, ionic polarizability and cell volume. With increasing additive, chemical ordering of B‐site cations was indicated with selected area electron diffraction (SAED) patterns, HRTEM images and Raman spectrum, which contributed to the greatly enhanced microwave dielectric properties. Particularly, the 0.7Sr_0.85 Mg_0.15TiO₃–0.3LaAlO₃ ceramics modified with 10 wt % ZnO‐B₂O₃ can further decrease the sintering temperature down to 950°C without deteriorating its performance. Thermal tests implied ceramics featured good chemical compatibility with Cu/Ag electrode. Thus, they can be cofired with internal Cu/Ag electrodes in special patterns to fulfill different electrical functions for LTCC (low‐temperature cofired ceramic) application.     

Low‐Temperature Sintering and Microwave Dielectric Properties of CaMoO4‐Based Temperature Stable LTCC Material

The CaMoO₄–xY₂O₃–xLi₂O ceramics were prepared by the solid‐state reaction method. The sintering behavior, phase evolution, microstructure, and microwave dielectric properties were investigated. CaMoO₄ solid solution was obtained when x = 0.030, and two‐phase system including tetragonal CaMoO₄ phase and cubic Y₂O₃ phase formed when 0.066 ≤ x ≤ 1.417. A temperature stable CaMoO₄‐based microwave dielectric ceramic with ultralow sintering temperature (775°C) was obtained in the CaMoO₄–xY₂O₃–xLi₂O system when x = 0.306, which showed good microwave dielectric properties with a low permittivity of 9.5, a high Q_f value of 63 240 GHz, and a near‐zero temperature coefficient of resonant frequency of +7.2 ppm/°C.     

Sintering Behavior and Dielectric Properties of Ultra‐Low Temperature Fired Silver Molybdate Ceramics

Ag₂MoO₄ ceramic was prepared by using the solid‐state reaction method, which could be sintered at 450°C for 2 h, having a relative permittivity of 8.08, a Qf value of 17 000 GHz, and a temperature coefficient of resonance frequency about ?133 ppm/°C. Ag₂MoO₄ ceramic was chemically compatible with silver but reacted seriously with aluminum to form (Ag_0.5Al_0.5)MoO₄ during the sintering. The fitting of infrared spectra and the Shannon's additive rule were employed to study intrinsic dielectric behaviors of the ceramics at microwave region. Ionic displacive polarization and the electronic polarization contributed almost equally to the dielectric permittivity of the ceramic at microwave region. The Ag₂MoO₄ ceramics could be a good candidate for ultra‐low temperature co‐fired microwave devices.     

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.     

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.     

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.     

Plastic deformation and effects of water in room‐temperature cold sintering of NaCl microwave dielectric ceramics

NaCl ceramics were prepared by room‐temperature cold sintering using moistened NaCl powder with 4 wt% water and dry pressing using dehydrated powder. When the applied uniaxial pressure is low, the relative density of dry‐pressed NaCl ceramic is significantly lower than that of cold‐sintered ceramic, while the former is 98.5%‐99.3% and much higher than the latter (94.3%‐94.6%) for high applied pressure of 200‐300 MPa. The uniaxial pressure‐induced plastic deformation dominates the densification of dry‐pressed NaCl ceramic, and also plays a role during cold sintering as well as the dissolution‐precipitation process. The lower density of cold‐sintered NaCl ceramic under high applied pressure is attributed to the trapped water in ceramic body during cold sintering. Besides, the presence of water always promotes the microstructural homogeneity, which is responsible for the much higher Qf value of cold‐sintered NaCl ceramic. The optimal microwave dielectric properties with ε_r = 5.55, Qf = 49 600 GHz, and τ_f = ?173 ppm/°C are obtained in cold‐sintered NaCl ceramic under the applied pressure of 300 MPa, indicating that it is a promising candidate as a microwave dielectric material.     

Novel Temperature Stable Li2MnO3 Dielectric Ceramics with High Q for LTCC Applications

Novel microwave dielectric ceramics in the Li₂MnO₃ system with high Q prepared through a conventional solid‐state route had been investigated. All the specimens exhibited single phase ceramics sintered in the temperature range 1140°C–1230°C. The microwave dielectric properties of Li₂MnO₃ ceramics were strongly correlated with sintering temperature and density. The best microwave dielectric properties of ε_r = 13.6, Q × f = 97 000 (GHz), and τ_f = ?5.2 ppm/°C could be obtained as sintered at 1200°C for 4 h. BaCu(B₂O₅) (BCB) could effectively lower the sintering temperature from 1200°C to 930°C and slightly induced degradation of the microwave dielectric properties. The Li₂MnO₃ ceramics doped with 2 wt% BaCu(B₂O₅) had excellent dielectric properties of ε_r = 11.9, Q × f = 80 600 (GHz), and τ_f = 0 ppm/°C. With low sintering temperature and good dielectric properties, the BCB added Li₂MnO₃ ceramics are suitable candidates for LTCC applications in wireless communication system.     

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.     

Microwave dielectric properties of SnO‐SnF2‐P2O5 glass and its composite with alumina for ULTCC applications

Ultralow‐temperature sinterable alumina‐45SnF₂:25SnO:30P₂O₅ glass (Al₂O₃‐SSP glass) composite has been developed for microelectronic applications. The 45SnF₂:25SnO:30P₂O₅ glass prepared by melt quenching from 450°C has a low T_g of about 93°C. The SSP glass has ε_r and tanδ of 20 and 0.007, respectively, at 1 MHz. In the microwave frequency range, it has ε_r=16 and Q_u × f=990 GHz with τ_f=?290 ppm/°C at 6.2 GHz with coefficient of thermal expansion (CTE) value of 17.8 ppm/°C. A 30 wt.% Al₂O₃ ‐ 70 wt.% SSP composite was prepared by sintering at different temperatures from 150°C to 400°C. The crystalline phases and dielectric properties vary with sintering temperature. The alumina‐SSP composite sintered at 200°C has ε_r=5.41 with a tanδ of 0.01 (1 MHz) and at microwave frequencies it has ε_r=5.20 at 11 GHz with Q_u × f=5500 GHz with temperature coefficient of resonant frequency (τ_f)=?18 ppm/°C. The CTE and room‐temperature thermal conductivity of the composite sintered at 200°C are 8.7 ppm/°C and 0.47 W/m/K, respectively. The new composite has a low sintering temperature and is a possible candidate for ultralow‐temperature cofired ceramics applications.