Novel low-εr and lightweight LiBO2 microwave dielectric ceramics with good chemical compatibility with silver

High-speed signal propagation systems require dielectric ceramics with low relative permittivity (ε_r) and a high-quality factor (Qxf). In this paper, a novel low-permittivity borate ceramic (LiBO₂) was synthesized using a conventional solid-state reaction method. Based on the X-ray diffraction and Rietveld refinement, the LiBO₂ crystallized into a monoclinic structure with a space group of P21/c. Dense and single-phase ceramic was obtained at 640 °C with comprehensive microwave dielectric properties: a low relative permittivity (ε_r) of 5.3, a moderate quality factor (Q×f) of 18,200 GHz at 16.3 GHz, and a temperature coefficient of resonant frequency (τ_f) of ? 66.2 ppm/°C. Good chemical compatibility with Ag electrode and thermal expansion coefficient of 25.4 ppm/°C was achieved demonstrating the potential applications as dielectric resonances in wireless communications and substrates in low-temperature cofired ceramics.     

A low-sintering temperature microwave dielectric ceramic for 5G LTCC applications with ultralow loss

In next-generation mobile and wireless communication systems, low sintering temperature and excellent dielectric properties are synergistic objectives in the application of dielectric resonators/filters. In this work, Li₂Ti_0·98Mg_0·02O_2·96F_0.04–1 wt% Nb₂O₅ (LTMN) ceramics were fabricated, and their sintering temperature was successfully lowered from 1120 °C to 750 °C by adjusting the mass ratio of B₂O₃–CuO (BC) additive. The optimum dielectric properties (ԑ_r ~ 24.44, Q × f ~ 60,574 GHz and τ_f ~ 22.8 ppm/°C) were obtained in BC-modified LTMN ceramics sintered at 790 °C. Even if their sintering temperature was lowered to 750 °C, the lowest temperature among the Li₂TiO₃-based dielectric ceramics currently used for LTCC technology, excellent dielectric properties (ԑ_r ~ 23.77, Q × f ~ 51,636 GHz) were still maintained. Additionally, no extra impurity phase was detected in BC-modified LTMN ceramics co-fired with Ag at 790 °C, indicating that BC-modified LTMN ceramics have a bright prospect in high-performance LTCC devices for 5G applications.     

Low-temperature-sintered MgO-based microwave dielectric ceramics with ultralow loss and high thermal conductivity

With the development of 5G/6G communication, the requirements of portable devices for miniaturization and multifunction make low-temperature co-fired ceramic (LTCC) more and more important. In the area of high-frequency high-density passive integration, microwave dielectric ceramics with a low dielectric loss and high thermal conductivity are urgently needed to ensure the effective signals transmission and system reliability. However, most microwave dielectric ceramics with a low dielectric loss were not applicable for the LTCC technology due to the high sintering temperature. In this work, a series of MgO-based ceramics [(100 − x) wt.% MgO–x wt.% (0.2SrF₂–0.8LiF) (x = 5,7,10)] were prepared by solid-state reaction method. The addition of sintering aid 0.2SrF₂–0.8LiF (S₂L₈) decreased the sintering temperature below 880°C without degrading the microwave dielectric properties of ceramics. Microwave dielectric properties of ceramics, including quality factor Q × f, relative permittivity ε_r, and temperature coefficient of resonant frequency τ_f, were investigated to find the optimum composition and sintering temperature. In general, MgO–7 wt.% S₂L₈ ceramic sintered at 860°C exhibits outstanding properties of Q × f = 180 233 GHz, ε_r = 9.11, τ_f = −40.33 ppm/°C, and a high thermal conductivity of 24.02 W/(m K). This series of ceramics are suitable to be co-fired with Ag electrodes. With all those great properties, this series of MgO-based ceramics are expected to be the candidates for LTCC applications in 5G/6G technology.     

Localized Temperature Stability in Low‐Temperature Cofired Ceramics

Low‐temperature cofired ceramic (LTCC) is a multilayer 3D packaging, interconnection, and integration technology. For LTCC modules targeting radio and microwave frequency (RF and MW) applications, a low or near 0 ppm/°C temperature coefficient of resonant frequency (τ_f) ensures temperature stability of embedded resonator and filter functions. The base dielectrics of most commercial LTCC systems have a τ_f in the range ?50 to ?80 ppm/°C. This study explored a method to achieve a zero τ_f on stripline (SL) resonators by locally cofiring, in a multilayer LTCC structure, compensating dielectrics (CD) with an opposite τ_f to that of the host dielectric. The formulation, synthesis, dielectric properties, and microstructure of SrTiO₃ (STO)‐based low‐fire τ_f CD are presented. Chemical interactions and physical compatibility between the compensating and the host LTCC dielectrics are investigated for cofireability. The dependence of τ_f compensation on the wt% of STO, the printed thickness, and the location of the CD in multilayer LTCC are discussed. The most effective τ_f compensation is achieved by integrating CD next to the resonator lines, and can be explained by the concentration of electromagnetic energy via total internal reflection of electromagnetic waves inside the CD layer.     

Ultra-low fired fluoride composite microwave dielectric ceramics and their application for BaCuSi2O6-based LTCC

A total of 14 fluoride composite ceramics were prepared through solid-state method and their microwave dielectric properties were investigated. Among the fluoride composite ceramics, 0.36LiF–0.39MgF₂–0.25SrF₂ (LMS) had the lowest sintering temperature (600°C) and presented a dielectric constant (ε_r) of 6.24 ± 0.05, a quality factor (Q × f) of 33 274 ± 900 GHz, and a temperature coefficient resonant frequency (τ_f) of −86.74 ± 8 ppm/°C. As the LMS ceramic had a low melting point (646°C), it can be used as sintering aid for LTCC applications. The sintering temperature of BaCuSi₂O₆ decreased from 1050°C to 875°C with 2 wt% LMS doped and excellent microwave dielectric properties of ε_r = 8.16 ± 0.04, Q × f = 24 351 ± 300 GHz, and τ_f = −9.74 ± 1 ppm/°C were obtained. Moreover, BaCuSi₂O₆-2 wt% LMS can be co-fired with Ag powders, which makes it a potential new candidate for LTCC applications.     

A new type of microwave dielectric ceramic based on K2O–SrO–P2O5 composition with high quality factor and low sintering temperature

A new type of microwave dielectric ceramics with low dielectric loss was fabricated through a traditional solid-phase method. X-ray diffraction and density tests showed that KSrPO₄ ceramics with a single orthorhombic phase could be synthesized and densified at 950 °C, and the crystal structure of KSrPO₄ was further confirmed by Rietveld refinement analysis. The densification temperature of KSrPO₄ was lower than 961 °C, indicating the ceramics could be used in LTCC devices. Additionally, based on the complex chemical bond theory, some internal parameters of KSrPO₄ ceramics were calculated and the effects of these parameters on the properties of KSrPO₄ were systematically analyzed for the first time. Furthermore, the composite dielectric constant and loss of KSrPO₄ ceramics were analyzed by infrared reflectance spectroscopy, and the theoretical loss and the actual loss were compared. Finally, a vector network analyzer was employed to measure the microwave dielectric properties of all samples. The results showed that KSrPO₄ sintered at 950 °C obtain the best microwave dielectric properties, including ε_r = 7.85, Q·f = 34,527 GHz (at 10.43 GHz) and τ_f = ?14.82 ppm/°C.     

Improved microstructure and high quality factor of Li2Ti0.9(Zn1/3Ta2/3)0.1O3 microwave ceramics with LiF additive for LTCC applications

In this study, LiF was utilized to decrease sintering temperature, improve microstructure, enhance Q×f, and regulate τ_f of Li₂Ti_0.9(Zn_1/3Ta_2/3)_0.1O₃ (abbreviated as LTZT) ceramics. A complete solid solution together with a phase transition from monoclinic to cubic rock salt structure occurred. The cell volume of LTZT ceramics decreased as the LiF content increased. Relatively dense and uniform microstructures were observed for the ceramics as the LiF content was not less than 2 wt%. The dielectric constant of LTZT ceramics initially increased and then decreased with the increasing LiF content. The FWHM of the Raman band at about 808 cm^?1 was closely related to the Q×f value. Notably, the samples with 3 wt% LiF exhibited the highest relative density of 97.4 % and satisfactory microwave dielectric properties of ε_r = 23.14 ± 0.16, Q×f = 110,090 ± 1100 GHz, and τ_f = +3.25 ± 1.45 ppm/°C when sintered at 950 °C. Good chemical compatibility with silver indicated the ceramic is a promising candidate in LTCC applications.     

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.     

Phase analysis and microwave dielectric properties of LTCC TiO2 with glass system

Glass–ceramic composites containing TiO₂ (anatase, rutile) and modified borosilicate glasses were prepared and their sintering behaviour, phase evolution, interface reactions, and microwave dielectric properties were investigated as new candidates for low-temperature cofired ceramic (LTCC) materials. It was found that the addition of small amounts of borosilicate glasses lowered the sintering temperature of TiO₂ from 1400 to 900 °C. X-ray diffraction results showed that second phases, including Zn₂SiO₄, were formed when TiO₂+zinc-borosilicate glass was used, while no crystalline phase except rutile could be found using unmodified borosilicate glass. High-density TiO₂+zinc borosilicate glass material showed promising microwave dielectric properties: relative dielectric constant (ε_r)=74, quality factor (Q×f)=8000 GHz, and temperature coefficient of resonant frequency (τ_f)=340 ppm/°C. The effect of borosilicate glasses on the anatase–rutile phase transition was also investigated.     

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‐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.     

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.     

Two novel ultralow temperature firing microwave dielectric ceramics LiMVO6 (M = Mo,W) and their chemical compatibility with metal electrodes

Low temperature cofired ceramics technology (LTCC) has been widely studied and used in wireless communication because of their outstanding capability for device miniaturization and integration. However, many commercial microwave dielectric materials have high sintering temperatures that pose challenge for cofiring with inner electrodes. Herein, two brannerite vanadate LiMVO₆ (M = Mo, W) ceramics with intrinsically low sintering temperatures were prepared. Dense and stable LiMVO₆ (M = Mo, W) ceramics could obtained at 640 °C for LiMoVO₆ and 700 °C for LiWVO₆. Favorable microwave dielectric properties were also obtained with ε_r = 13.3, Q × f = 12,460 GHz, and τ_f = +101.0 ppm/°C for LiMoVO₆ and ε_r = 11.5, Q × f = 13,260 GHz, and τ_f = +163.8 ppm/°C for LiWVO₆. Moreover, the relationship between crystal structure and microwave dielectric properties was studied by means of packing fraction, bond valence, and octahedral distortion. Their chemical compatibility with the metal electrodes were confirmed.     

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.     

A cofired trilayer architecture to regulate temperature stability while maintaining high-Q: The case study of Ba(Mg1/3Nb2/3)O3 – Mg4Nb2O9 system

Trilayer architectures were designed and investigated to further improve the microwave dielectric properties of the Ba(Mg_1/3Nb_2/3)O₃ (BMN) – Mg₄Nb₂O₉ (MN) system, namely, to achieve temperature stability while maintaining high-Q. The calculated phase fractions in randomly distributed (1-x)BMN-xMN ceramics deviated from the designed composition (where the composition with x = 0.045, 0.056, 0.125 and 0.98 was respectively referred to as S1, S2, S3 and MN'), thus allowing it difficult to obtain near-zero τ_f as expected. In densification studies, doping a little MN was shown to effectively promote the sintering of BMN and provide the possibility for layer-cofired optimization. Fortunately, undesired differences in composition and performance could be suppressed with the weakened ion diffusion occurring at narrow interfaces with a width of ～2.5 μm in the S1/MN'/S1 trilayer architecture. Considering the influence of cofired compatibility and stress of dielectric layers, the compositionally optimized S3/MN'/S3 ceramics sintered at 1340 °C exhibited excellent microwave dielectric properties of ε_r = 21.95, Q × f = 110,482 GHz (f₀ = 6.870 GHz), and τ_f = 0.965 ppm/°C. Moreover, the dielectric response mechanism of layered ceramics was clarified by establishing the relationship between the layered architecture, dielectric properties and electric field distribution using the finite element method and high-frequency structure simulator (HFSS). This suggests that layered architectures like S1-3/MN'/S1-3 could effectively compensate for the dielectric properties and hold a promising application prospect of 5G wireless communication.     

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.     

Low-temperature firing and microwave dielectric properties of MgNb2-xVx/2O6-1.25x ceramics

MgNb_2-xV_x/2O_6-1.25x (0.1≤x≤0.6) ceramics with orthorhombic columbite structures were prepared at low-temperature by a solid-phase process. The phase component, microscopic morphology, low-temperature sintering mechanism and microwave dielectric performance of MgNb_2-xV_x/2O_6-1.25x ceramics were comprehensively investigated. Low-temperature sintering densification of dielectric ceramics was achieved via the nonstoichiometric substitution of vanadium (V) at the Nb-site. In contrast to pure MgNb₂O₆ ceramics, the sintering temperature of MgNb_2-xV_x/2O_6-1.25x (x = 0.2) ceramics was reduced by nearly 300 °C owing to the liquid-phase assisted sintering mechanism. The liquid phase arises from the autogenous low-melting-point phase. Meanwhile, MgNb_2-xV_x/2O_6-1.25x (x = 0.2) samples with nonstoichiometric substitution could achieve a more than 900% improvement in the Q × f value, compared with stoichiometrically MgNb_2-xV_xO₆ (x = 0.1, 0.2) ceramics. Finally, MgNb_2-xV_x/2O_6-1.25x dielectric ceramics possess outstanding microwave dielectric properties: ε_r = 20.5, Q × f = 91000, and τ_f = -65 ppm/°C when sintered at 1030 °C for x = 0.2, which provides an alternative material for LTCC technology and an effective approach for low-temperature sintering of Nb-based microwave dielectric ceramics.     

NaTaO3 microwave dielectric ceramic a with high relative permittivity and as an excellent compensator for the temperature coefficient of resonant frequency

NaTaO₃ ferroelectric ceramics sintered at 1550 °C exhibit high relative permittivity and outstanding microwave dielectric properties of ε_r = 113.76, Q × f = 8824 GHz, and τ_? = +645 ppm/°C in the low-frequency band (3.5195 GHz). The high relative permittivity is advantageous for the miniaturization of modern wireless communication devices. A near zero temperature coefficient of the resonance frequency (τ_? = +1.05 ppm/°C) and good microwave dielectric properties (Q × f = 54,680 GHz, ε_r = 19.42) were obtained for 0.92MgTiO₃-0.08NaTaO₃ ceramics. Moreover, the sintering temperature of MgTiO₃ could be decreased from 1350 °C to 1200 °C. Hence, NaTaO₃ is a high-potential microwave dielectric material for adjusting systems with a negative temperature coefficient of resonance frequency.     

5G array antenna substrate for electromagnetic beam splitting via cobalt-substituted zinc molybdate low temperature co-fired ceramics

A novel Zn_1-xCo_xMoO₄ (ZCMO) (x = 0.03) ceramic with low-dielectric constant, low-loss, and low-sintering temperature for 5G electromagnetic beam splitting is developed by solid-state reaction method. This ceramic exhibits excellent microwave dielectric properties with ε_r = 8.0, Q × f = 57682 GHz, τ_f = ? 54.9 ppm/°C at a sintering temperature of 825 °C. An array antenna for electromagnetic beam splitting at 5.4 GHz is designed by using this ceramic for the first time. The highest efficiency and the best electromagnetic beam splitting effect can be jointly controlled by the dielectric constant and dielectric loss of the ceramic. The normalized reflection amplitude of each array unit cell is above 98 %, and the reflection phase covers 360°. The function of electromagnetic beam splitting is verified by the far-field pattern of the electric field. This work helps to promote the development of LTCC and broaden the application scope of microwave dielectric ceramics.     

Significant improvement on quality factor in Bi3+/Ta5+ codoped Ce2Zr3-3xBi1.5xTa1.5xMo9O36 microwave dielectric ceramics with ultra-low sintering temperatures

Molybdenum oxide-based ceramics have attracted intense interest due to ultra-low sintering temperatures. However, low quality factors (Q × f) hinder their practical applications. Although Q × f can be improved by ions doping, the sintering temperature is greatly increased. Accordingly, it is still a challenge to obtain high Q × f ceramics sintered at ultra-low temperatures (<660 °C). Herein, (Bi_0.5Ta_0.5)⁴⁺ ions are utilized to tackle this issue in the Ce₂Zr₃(MoO₄)₉ ceramic as a prototype. Density and scanning electron microscope (SEM) results uncover good sintering states, and X-ray diffraction (XRD) results reveal the formation of solid solutions. Interestingly, the Ce-O bonds exhibit a dominant contribution to the bond ionicity (f_i), while Mo-O bonds play an important role in the lattice energy (U), the bond energy (E) and the thermal expansion coefficient (α). The remarkable increase of Q × f can be interpreted by the enhancement of the packing fraction and the mean U of Mo-O bonds. Moreover, the variations of the dielectric constant (ε_r) and the temperature coefficient of the resonance frequency (τ_f) can be explained by the variations of the intrinsic parameters. More interestingly, a negative correlation between Q × f and τ_f is first found. Typically, the CZ_0.98B_0.02 ceramic sintered at 650 °C exhibits optimum microwave dielectric properties: ε_r = 9.92, Q × f = 110,670 GHz, and τ_f = −19.20 ppm °C⁻¹. Notably, Q × f of the Ce₂Zr_2.94Bi_0.03Ta_0.03Mo₉O₃₆ (CZ_0.98B_0.02) ceramic is about 6 times larger than that of the matrix while retaining a low sintering temperature of 650 °C and a low ε_r of 9.92, making it a promising candidate for ultra-low temperature cofired ceramics (ULTCC) applications.