Nonstoichiometric microwave dielectric ceramics [(Na0.5-xBi0.5+x/3)0.5Ca0.5]MoO4 with low sintering temperatures

In this work, [(Na_0.5-xBi_0.5+x/3)_0.5Ca_0.5]MoO₄ (x = ±0.03, ±0.06, ±0.09, ±0.12) microwave dielectric ceramics prepared by the solid-state reaction method are investigated. All the samples can be sintered well below 800℃. The sintered ceramics show a scheelite structure without any secondary phase, indicating that a solid solution is formed in nonstoichiometric [(Na_0.5-xBi_0.5+x/3)_0.5Ca_0.5]MoO₄. While x value increases from -0.12 to 0.12, the relative permittivity rises from 16.7 to 21.0, TCF value is improved from -21 ppm/℃ to +1 ppm/℃, and Q × f value varies in the range of 17,000 GHz and 34,000 GHz. The Raman analysis reveals that one of the external modes is attributed to be the main factor affecting the performances. When x = 0.09 and 0.12, high performance microwave dielectric ceramics can be well densified at low sintering temperatures (750−775℃) with relative permittivities of 20.9–21.0, improved Q × f values of 31,400−33,000 GHz, and near-zero temperature coefficients of resonate frequency (|TCF| ≈ ±2 ppm/℃).     

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.     

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.     

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.     

Raman Spectra,Infrared Spectra,and Microwave Dielectric Properties of Low‐Temperature Firing [(Li0.5Ln0.5)1−xCax]MoO4 (Ln = Sm and Nd) Solid Solution Ceramics with Scheelite Structure

In this work, a series of scheelite solid solution [(Li_0.5Ln_0.5)_1?xCa_x]MoO₄ (Ln = Sm and Nd; x = 0.20, 0.40, 0.60, 0.70, 0.80, 0.85, 0.90) ceramics were prepared by conventional solid‐state reaction method. The sintering temperature was lowered to 925°C by (Li_0.5Ln_0.5)²⁺ substituting for Ca²⁺ in the solid solution without any secondary phase. Compared with that of scheelite CaMoO₄ (?57 ppm/°C), the temperature coefficient of resonant frequency (TCF or τ_f) of the scheelite solid solution was modified to near zero (about +4.3 ppm/°C) at x = 0.8 with a dielectric constant 11.0 and the quality factor (Q × f value) of 18 695 GHz. The Raman spectra, showed the degree of disordering increased with x value, which resulted in decrease in the permittivities and increase in the Q × f values. The infrared spectra were analyzed using the classical harmonic oscillator model and were extrapolated to the microwave range. The chemical compatibility with silver electrode indicated that the reported series of ceramics were good candidates for the low‐temperature cofired 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.     

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.     

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.     

Enhancement of structural and microwave properties of Zn2+ ion-substituted Li2MgSiO4 ceramics for LTCC applications

In this study, Zn²⁺-substituted Li₂MgSiO₄ ceramics (Li₂(Mg_1-xZn_x)SiO₄, x = 0.00, 0.05, 0.10, 0.15, and 0.20) were synthesized using a traditional solid-state method. A fixed amount of LiF sintering aid (1.5 wt%) was added to the ceramics for decreasing the sintering temperature and adjusting their microwave dielectric properties. X-ray diffraction (XRD) results revealed no secondary phases, and scanning electron microscopy (SEM) data suggest that the Zn²⁺ ion substitution increased the size and uniformity of the grains, thereby affecting the densification of the prepared ceramics. The maximum bulk density (2.94 g/cm³) was found in a Zn²⁺ ion-substituted ceramic with x = 0.10 at a relative density of 94.2% (compared with the XRD theoretical density). Excellent microwave dielectric properties (ε_r = 6.28, Q × f = 50400 GHz, and τ_f = ?145 ppm/°C) can also be obtained at this zirconium content. We believe that the developed ceramics are promising for use as antenna substrates or transmit/receive modules in low-temperature co-firing ceramic applications.     

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.     

Crystal structure,infrared-reflectivity spectra,and microwave dielectric properties of novel Ce2(MoO4)2(Mo2O7) ceramics

Novel Ce₂(MoO₄)₂(Mo₂O₇) (CMO) ceramics were prepared by a conventional solid-state method, and the microwave dielectric properties were investigated. X-ray diffraction results illustrated that pure Ce₂(MoO₄)₂(Mo₂O₇) structure formed upon sintering at 600 °C-725 °C. [CeO₇], [CeO₈], [MoO₄], and [MoO₆] polyhedra were connected to form a three-dimensional structure of CMO ceramics. Analysis based on chemical bond theory indicated that the Mo–O bond critically affected the ceramics’ performance. Furthermore, infrared-reflectivity spectra analysis revealed that the primary polarisation contribution was from ionic polarisation. Notably, the optimum microwave dielectric properties of ε_r = 10.69, Q·f = 49,440 GHz (@ 9.29 GHz), and τ_f = −30.4 ppm/°C were obtained in CMO ceramics sintered at 700 °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.     

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.     

The spectra analysis and microwave dielectric properties of [Ca0.55(Sm1-xBix)0.3]MoO4 ceramics

A series of low-temperature firing ceramics with scheelite structure, [Ca_0.55(Sm_1-xBi_x)_0.3]MoO₄ (x = 0.2–0.95), were prepared via solid-state reaction. The sintering temperature ranges from 660 to 760°C. A standard tetragonal scheelite phase was formed without secondary phase. When the x value was 0.95, the temperature coefficient of resonant frequency (τ_f) moved to a near zero value (−2.1 ppm/°C) with a dielectric constant 13.7 and the quality factor (Qf) of 33 200 GHz. The Raman spectra shows that the more vibration modes appeared with x value, which is due to the increasing of Bi concentration and results in increase in permittivities and decrease in Qf values. The classical harmonic oscillator model is used in the infrared spectra and extrapolate to the microwave range. The [Ca_0.55(Sm_1-xBi_x)_0.3]MoO₄ ceramics show high-performance microwave dielectric properties at low-sintering temperature.     

Structure and microwave dielectric properties of Li(Al1-xLix)SiO4-x ceramics

Li(Al_1-xLi_x)SiO_4-x (x = 0.005, 0.01, 0.015, and 0.02) ceramics were synthesized via a traditional solid phase reaction method with different sintering temperatures. To determine the positions occupied by Li⁺ in the lattice, the defect formation energies and total energies of various sites of LiAlSiO₄ (LAS) occupied by Li⁺ were examined, and the energy of LAS systems were calculated using density functional theory of first-principle with the CASTEP module. The results demonstrated that the Al-sites occupied by Li⁺ had the lowest formation energies and total energy, so Li ⁺ should substitute Al³⁺. The impacts of replacing Al³⁺ with Li⁺ on the bulk density, sintering properties, phase composition, microstructure, and microwave dielectric properties of Li(Al_1-xLi_x)SiO_4-x (0 = x ≤ 0.02) ceramics were thoroughly studied. With Li⁺-doping, the sintering temperature decreased from 1300 °C (x = 0) to 1175 °C (x = 0.02), while the Q × f and τ_f values of LAS ceramics significantly increased. The Li(Al_0.99Li_0.01)SiO_3.99 ceramic was fully sintered at 1250 °C for 10 h to obtain excellent microwave dielectric properties: ε_r = 3.49, Q × f = 51,358 GHz, and τ_f = ?51.48 × 10^?6 °C^?1.     

A new BaB2O4 microwave dielectric ceramic for LTCC application

To satisfy the requirements of miniaturization and integration of microwave devices, microwave dielectric ceramics with low sintering temperatures and good microwave dielectric properties are particularly important for LTCC materials. In this study, low-cost BaB₂O₄ ceramics were prepared with different Ba/B ratios using a solid-phase method. Combined with the Raman spectra, the effects of the Raman shift and FWHM of the vibration peaks on the microwave dielectric properties were determined. As a novel microwave dielectric ceramic, BaB₂O₄ consists of a highly dense structure with optimal microwave dielectric properties (ε_r = 4.06, Q×f = 23845 GHz, and τ_f = −7.2 ppm/℃) at a low sintering temperature (840 ℃). In addition, BaB₂O₄ ceramic is chemically compatible with Ag, making it a promising candidate substrate for microwave communications.     

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.     

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.     

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.