Ultra-low temperature sintering and temperature stable microwave dielectrics of phase pure AgMgVO4 ceramics

The present study systematically invesitgates the formation and microwave dielectric properties of novel AgMgVO₄ ceramics fabricated via the solid-state reaction method. The crystal structure of the ceramics is confirmed to be orthorhombic with a space Group of Pnma (62). A high relative density of 96.2% and an excellent combination of microwave dielectric properties with ε_r of ~14.89, Q×f of ~19,400 GHz, and τ_f of ~ ? 2.71 ppm/°C can be achieved for the ceramic sintered at 630 °C. The dielectric constant is mainly influenced by the relative density (porosity) and dielectric polarizability. The Q×f is controlled by the microstructure, packing fraction, and lattice energy, which are also highly related to the unit-cell volume. A smaller unit-cell volume leads to a high Q×f. Variation of the τ_f is strongly correlated to the bond valence of the specimen. Furthermore, the ceramic exhibits good chemical compatibility with aluminum electrodes and is demonstrated as a potent substrate for a band-pass filter with a center frequency of 3.5 GHz. These findings show a great promise for ultra-low temperature co-fired ceramic applications at high frequencies.     

Effect of Mn:Mg ratio on sintering behavior and microwave dielectric properties of (Mn,Mg)V2O6 ceramics at ultra-low sintering temperature

Ultra-low-firing-temperature ceramics (Mn_1−xMg_x)V₂O₆ (x = 0–1) were prepared using the conventional solid-state reaction method. The effects of the Mn:Mg ratio on the crystal structure and microwave dielectric properties of the prepared ceramics were systematically investigated. The results indicated that an appropriate Mn:Mg ratio effectively improves the dielectric properties of the compounds. Specimens with x = 0.01 and x = 0.93 sintered at 630 °C exhibited the following microwave dielectric properties: ε_r = 12.4 and 9.8, high Q×f = 57,000 and 21,000 GHz, and τ_f = –15 and −24 ppm/°C, respectively. This suggests that the (Mn_0.99Mg_0.01)V₂O₆ ceramic is a potential material for ULTCC applications.     

Low-loss and temperature stable (1-x)Ba3P2O8-xMg2B2O5 composite ceramics with low sintering temperature

In this study, the Ba₃P₂O₈ and Mg₂B₂O₅ were fabricated by the solid-state reaction method separately, and the (1-x)Ba₃P₂O₈-xMg₂B₂O₅ (x = 0.2–0.4) low-temperature co-fired ceramic (LTCC) materials were obtained in the sintering temperature range of 880–960 °C. The phase compositions, microstructures, elemental compositions, and microwave dielectric properties of the (1-x)Ba₃P₂O₈-xMg₂B₂O₅ composite ceramics were investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM), energy-dispersive spectroscopy (EDS), and TE_01δ mode dielectric resonator method, respectively. The results revealed that the Mg₂B₂O₅ phase and Ba₃P₂O₈ phase could coexist well in the (1-x)Ba₃P₂O₈-xMg₂B₂O₅ composite ceramics without formation of any new phases. The abnormal grain growth of Ba₃P₂O₈ grains was inhibited by the addition of Mg₂B₂O₅. In addition, through composition of Ba₃P₂O₈ and Mg₂B₂O₅, the temperature coefficient of resonant frequency (τ_f) and quality factor (Q×f) were effectively optimized, and the sintering temperature was reduced to 880–960 °C. The optimal performance of 0.8Ba₃P₂O₈-0.2Mg₂B₂O₅ composite ceramic was achieved at a sintering temperature of 920 °C, τ_{f =} ?1.9 ppm/°C, Q×f = 61,250 GHz, and a low permittivity ε_r = 10.7. The chemical compatibility test demonstrated that the composite ceramic could coexist well with silver, which indicated that the 0.8Ba₃P₂O₈-0.2Mg₂B₂O₅ composite ceramic is a candidate LTCC material with wide application prospects.     

Low-loss microwave dielectric of novel Li1-2xMxVO3 (M = Mg,Zn) (x = 0–0.09) ceramics for ULTCC applications

Novel Li_1?2xM_xVO₃ (M = Mg, Zn) (x = 0–0.09) microwave dielectrics suitable for ULTCC (ultra-low temperature co-fired ceramics) applications were synthesized. The X-ray diffraction patterns revealed that all samples were monoclinic structured with a space group of C2/c. Microstructures, lattice parameters, and Raman spectra of the ceramics were also studied. Q×f (Q: quality factor) was mainly controlled by intrinsic and extrinsic loss, while the variation of τ_f (temperature coefficient of resonant frequency) was related to ceramic bond valence. Poor microwave dielectric properties of pure LiVO₃ can be tremendously enhanced by substituting a minute amount of Mg or Zn in place of Li. Excellent properties could be obtained for specimens sintered at 520 °C with an ε_r of 9.78, a Q×f of 45,600 GHz and an τ_f of –45 ppm/°C for Li_0.98Mg_0.01VO₃, in addition to an ε_r (dielectric constant) of 9.25, a Q×f of 33,100 GHz and an τ_f of –53.6 ppm/°C for Li_0.98Zn_0.01VO₃. Furthermore, the Li_0.98Mg_0.01VO₃ specimen was found to be chemically comparable with the Al electrode. With 2 mol% of TiO₂ added, the specimen at 520 °C achieved excellent characteristics which include an ε_r of 9.2, a Q×f of 30,000 GHz, and an τ_f of –2.8 ppm/°C, making it a very promising ULTCC dielectric for high-frequency 5 G applications.     

Enhanced Na+-substituted Li2Mg2Mo3O12 ceramic substrate based on ultra-low temperature co-fired ceramic technology for microwave and terahertz polarization-selective functions

A novel Li₂Mg_2-xNa_2xMo₃O₁₂ (x = 0.09) ceramic with ultra-low sintering temperature is prepared by the solid-state reaction method. This ceramic (625 °C) exhibits excellent microwave dielectric properties (ε_r = 7.9, Q×f = 43844 GHz, τ_f = ?48.3 ppm/°C), terahertz transmission properties (ε_r¹ = 7.4, tan σ¹ = 0.0158, T_coefficient = 0.598), and chemical compatibility with Ag. For the first time, two polarization selective devices are designed in the microwave and terahertz regions by using this ceramic substrate, respectively. The transmission amplitudes of the right- and left-handed circularly polarized waves of the microwave device at 9.7 GHz are 0.895 and 0.019, respectively. The transmission coefficients of the y- and x-polarized waves of the terahertz device at 0.45 THz are 0.598 and 0.075, respectively. Both functions are verified by the overall far-field radiation pattern. This work promotes the application of dielectric ceramics and ULTCC technology in the microwave and terahertz regions.     

Polarization-dependent multichannel transmission of 5G signals based on Zn0.91(Li0.5Bi0.5)0.09MoO4 low-temperature co-fired ceramics

Microwave communication for 5 G signals is the preferred solution in modern communication networks where fiber optic cables are difficult to deploy or base stations operate incorrectly. Here, a novel Zn_1-x(Li_0.5Bi_0.5)_xMoO₄ (ZLBMO∼xLB, x = 0.09) ceramic with excellent microwave dielectric properties (ε_r = 10.5, Q×f = 43,001 GHz, τ_f = −21.5 ppm/°C) is developed using solid-state reaction method. The sintering temperature is successfully reduced from 850 °C to 725 °C. For the first time, an all-ceramic device for multichannel transmission of 5 G signals is designed using this ceramic material. This device exhibits transverse dual-channel, longitudinal dual-channel, and four-channel transmission under x-, y- and 45°-polarized waves incidence, respectively. The number of channels can be changed by switching the polarization state of the incident wave. This functionality is verified by simulation results of the electric field and phase. This work provides new ideas for the combination of dielectric ceramics and communication devices.     

Significantly enhanced sinterability and temperature stability of Li3Mg4NbO8-based microwave dielectric ceramics with LiF and Ba3(VO4)2 addition

Li₃Mg₄NbO₈-basic composite ceramics were elaborated via the solid-state reaction process, in which LiF and Ba₃(VO₄)₂ were utilized as a sintering aid and reinforcement phase, respectively. The sinterability, phase assemblage, microstructures, and microwave dielectric performances of Li₃Mg₄NbO₈–LiF–Ba₃(VO₄)₂ composite ceramics were thoroughly researched. The co-addition of LiF–Ba₃(VO₄)₂ can simultaneously lower the sintering temperature and improve the thermal stability of Li₃Mg₄NbO₈-basic ceramics. Solid state activated sintering is responsible for the low-temperature densification of the present ceramics. The coexistence of rock-salt structural Li₃Mg₄NbO₈/Li₄Mg₄NbO₈F and hexagonal structural Ba₃(VO₄)₂ phases was demonstrated by the combinational XRD and SEM-EDS analysis results. The 0.65(Li₃Mg₄NbO₈–LiF)-0.35Ba₃(VO₄)₂ ceramics fired at 825 °C/5 h exhibited promising microwave dielectric performances: τ_f = 0.5 ppm/°C along with ε_r = 13.8 and Qxf = 68500 GHz. The good compatibility of the developed ceramics with Ag demonstrates it potential for use in LTCC technology.     

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.     

Effects of MnO2 and WO3 co-doping and sintering temperature on microstructure,microwave dielectric properties of Ba2Ti9O20 microwave ceramics

Ba₂Ti₉O₂₀ (short for B2T9) ceramics doped with 0.9 mol% MnO₂ and y mol% WO₃ were prepared by solid-state reaction. The influence of sintering temperature, content of WO₃ dopant and the molar ratio x of TiO₂: BaCO₃ on crystal structure, microstructures as well as microwave dielectric properties of B2T9 ceramics was systematically investigated. The major phase of all samples is B2T9, and the minor phase is BaWO₄, respectively. The content of impurity TiO₂ alternates with the variation of compositions and sintering temperature, which also leads to different microwave dielectric properties. With the continuous increase of the sintering temperature, the B2T9 phase grains gradually grow larger and transform from rod grains to plate-like grains. The enlargement and flattening of grains also result in the decrease of compactness and deterioration of microwave dielectric properties. It is found that B2T9 ceramics possess better performance when the sintering temperature is 1340°C, which is related to lower TiO₂ content, BaWO₄, B2T9 grain size, aspect ratio of B2T9 phase and high compactness. When x = 4 and y = 0.2, the relative dielectric constant, quality factor and the temperature coefficient of resonant frequency are 38, 23758 and 7 ppm/°C, respectively.     

A novel high-Q oxyfluoride Li4Mg2NbO6F microwave dielectric ceramic with low sintering temperature

Novel low-fired Li₄Mg₂NbO₆F ceramics were synthesised using a conventional solid-state reaction method. X-ray diffraction and Rietveld refinement confirmed that the Li₄Mg₂NbO₆F compound had a face-centred-cubic rock salt structure [Fm-3 m(225)] above 625 °C. Li₄Mg₂NbO₆F ceramics sintered between 875 °C and 950 °C displayed the optimised density (> 97.5 %). The theoretical ε_theo was calculated based on the refined crystal parameters, closing to the measured ε_r. The ceramic sintered at 900 °C exhibited excellent microwave dielectric properties with ε_r of 15.53 ± 0.03, Q × f value of 93,300 ± 1100 GHz (at 7.7 GHz) and τ_f value of ?39.8 ± 0.8 ppm/°C. The compatibility with Ag powders makes the oxyfluoride a potential candidate for LTCC applications.     

Effects of Sn substitution on the crystal structures,microstructures, and microwave dielectric properties of Ce2Zr3(MoO4)9 ceramics

A series of Ce₂(Zr_1?xSn_x)₃(MoO₄)₉ (0.02 ≤ x ≤ 0.1) (CZ_1?xS_xM) ceramics were synthesized to investigate the effect of Sn⁴⁺ doping on the crystal structure, chemical bond parameters, and dielectric properties of Ce₂Zr₃(MoO₄)₉ ceramics. X-ray diffraction patterns illustrated the formation of the single-phase trigonal system solid solution in all samples. Rietveld refinement result showed that the lattice volume decreased linearly, which can be explained by the fact that the effective radius of Sn ion is smaller than that of Zr ion. As the Sn content increased, scanning electron microscope images showed that the CZ_1?xS_xM ceramics transformed from bar-like grains to disk-like grains and the grain size declined gradually. The structure–property correlation was estimated by using P–V-L theory; the descending ε_r was mainly consistent with the reduced polarizability and total bond ionicity. The Q × f was associated with the lattice energy of the Ce–O1 bond. The change of τ_f value was mainly attributed to the bond energy (E_Mo1–O1 and E_Mo1–O4) and the coefficients of thermal expansion (α_Mo1–O1 and α_Mo1–O4). Infrared analysis indicated that the dielectric properties of the CZ_1?xS_xM ceramics were primarily ascribed to the absorption of phonon oscillation. Notably, when x = 0.08, outstanding microwave dielectric properties could be achieved, namely, ε_r = 10.22, Q × f = 72,390 GHz, τ_f = ?7.54 ppm/°C.     

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

Temperature stable xLi₂MoO₄-(1-x)(LiBi)_0.5MoO₄ (x = 0, 40, 50, 60, 70, 100 vol%) microwave dielectric ceramics obtained by cold sintering process (CSP) under a mild sintering condition (120 ℃/30 min/200 MPa) are introduced in this work. The XRD, SEM, and Raman analysis indicate the coexistence of Li₂MoO₄ and (LiBi)_0.5MoO₄ phases. Li₂MoO₄-(LiBi)_0.5MoO₄ ceramics are compatible with Ag and Al electrode materials under cold sintering condition. The specimens exhibit high relative densities and good microwave dielectric properties (relative permittivities: 31.5–5.5; Q×f values: 1900 - 18,500 GHz; TCF values: from +144 ppm/℃ to ?106 ppm/℃), in particular, TCF = +0.7 ppm/℃ can be obtained in the case of x = 50 vol%. The extrapolated microwave dielectric properties obtained by the fitted infrared reflectivity spectrum are close to the measured data, revealing that the dielectric responses of cold sintered Li₂MoO₄-(LiBi)_0.5MoO₄ ceramics in the microwave range are dominated by the polar optical phonons.     

The microwave dielectric properties and crystal structure of low temperature sintering LiNiPO4 ceramics

The LiNiPO₄ ceramic for the LTCC technology was prepared via the traditional solid-state reaction route and its dielectric properties were investigated for the first time. The best dielectric properties of LiNiPO₄ ceramics with a ε_r of 7.18, Q×f value of 27,754?GHz and τ_f of ?67.7?ppm/°C were obtained in samples sintered at 825?°C for 2?h. Rietveld refinement was firstly employed to study the crystal structure and dielectric properties of LiNiPO₄ ceramics. Unfortunately, the relatively large negative τ_f was unfavorable to practical applications. Therefore, we introduced TiO₂, which possessed a considerable positive τ_f, to obtain a desired τ_f value. The prepared LiNiPO₄ ceramics with 15?wt% TiO₂ sintered at 900?°C for 2?h exhibited excellent dielectric properties of ε_r～11.49, Q×f～10,792?GHz, τ_f～?2.8?ppm/°C. The Ag co-fired experiments confirmed the excellent chemical compatibility with LiNiPO₄-TiO₂ ceramics which might be potential dielectric LTCCs for high frequency applications.     

Effect of (Zn1/3Nb2/3)4+ co-substitution on the microwave dielectric properties of Ce2Zr3(MoO4)9 ceramics

Ce₂[Zr_1-x(Zn_1/3Nb_2/3)_x]₃(MoO₄)₉ (CZ_1-x(ZN)_xM) (x = 0.02–0.08) compounds were successfully prepared to scientifically examine the effect of (Zn_1/3Nb_2/3)⁴⁺ doping on phase composition, microstructures, and properties. The XRD results showed that all compounds formed a pure phase with the space group of R-3c. SEM results indicated that all compounds were compact at 675 °C, and the lattice parameters and average grain size decreased with doping. Performance analysis illustrated that ε_r was closely related to the polarizability, and Q?f was affected by the lattice energy of the Mo–O bond. The τ_f was maintained at an excellent level. Far-infrared analysis indicated that the major dielectric contribution to CZ_1-x(ZN)_xM ceramics was related to the absorption of phonon oscillation. The optimum properties (ε_r = 10.72, Q?f = 59,381 GHz, τ_f = ?11.48 ppm/°C) were obtained when x = 0.04.     

Structure,chemical bond and microwave dielectric characteristics of novel Li3Mg4NbO8 ceramics

A novel Li₃Mg₄NbO₈ compound was fabricated through the process of solid-state reaction. The crystal structure, sinterability and microwave dielectric properties of the Li₃Mg₄NbO₈ ceramics were investigated. XRD refinement and Raman spectra results ascertained that the Li₃Mg₄NbO₈ compound crystallized into an orthorhombic Li₃Mg₂NbO₆-like structure with space group Fddd. The ε_r value was strongly impacted by the relative density and average ionic polarization. The Q × f value was mainly affected by the relative density and average grain size. The Li₃Mg₄NbO₈ ceramics sintered at 1150 ℃ showed outstanding microwave dielectric performance: ε_r = 13.8 ± 0.14, Q × f = 103 400 ± 3500 GHz (at 9.6 GHz), τ_f = −36.0 ± 1 ppm/℃. Additionally, the bond characteristics were calculated for a better understanding of the structure-property correlation for Li₃Mg₄NbO₈ ceramics.     

Room temperature densified H3BO3 microwave dielectric ceramics with ultra-low permittivity and high quality factor for dielectric substrate applications

The densification and microwave dielectric properties of H₃BO₃ ceramics prepared by dry pressing at room temperature were studied. The results show that pressure is the key factor of densification of H₃BO₃ ceramics. No second phase appears in all the as-fabricated H₃BO₃ ceramic samples. A dense H₃BO₃ ceramic (relative density~97.6%) was obtained by uniaxial compression of 384 MPa for 300s and the optimal microwave dielectric properties are ε_r = 2.83, Q × f = 59,400 GHz (f = 16 GHz), τ_f = −91 ppm/°C, which make it as a prospective candidate for microwave and millimeter-wave devices such as substrates for 5G communication technology.     

Microwave hybrid sintering of 0.95(Mg0.95Zn0.05)TiO3-0.05CaTiO3 ceramics and its microwave dielectric properties

Microwave dielectric ceramic powder of 0.95(Mg_0.95Zn_0.05)TiO₃-0.05CaTiO₃ (MCT) has been prepared by solid-state reaction method through single-step calcination at 1150 °C. The green bodies prepared from the calcined powder have been sintered by conventional, susceptor-aided, and hybrid microwave sintering techniques followed by annealing. XRD of calcined and sintered ceramics show (Mg,Zn)TiO₃ as a major phase with CaTiO₃ as a minor secondary phase. Fractographs of fired ceramics obtained by SEM show similar features in conventional and hybrid microwave types of sintering. Microwave dielectric properties such as relative permittivity(ε_r), temperature coefficient of resonant frequency(τ_f), and unloaded quality factors (Q_u) for conventional sintered at 1325 °C for 4 h are—ε_r~19.8, τ_f< –6 ppm/°C and Q_u.f 69,600 GHz at 6 GHz. Ceramics obtained through susceptor-aided microwave sintering at 1325 °C for 4 h show poor fired density. But ceramics got by microwave-hybrid sintering (resistive + microwave) at the same temperature show ε_r~20.6, Q_u.f~81,600 GHz at 6 GHz and τ_f~?6.9 ppm/°C. The effect of hybrid microwave sintering on the dielectric properties of MCT ceramics is found to be more subtle than microstructural.     

Effect of MgO/La2O3 additives on sintering behavior and microwave dielectric properties of (Sr,Ca)TiO3-(Sm,Nd)AlO3 ceramics

The effect of MgO/La₂O₃ additives on phase composition, microstructures, sintering behavior, and microwave dielectric properties of 0.7(Sr_0.01Ca_0.99)TiO₃−0.3(Sm_0.75Nd_0.25)AlO₃ (7SCT-3SNA) ceramics prepared via conventional solid-state route were systematically investigated. MgO/La₂O₃ as additives showed no obvious influence on the phase composition of the 7SCT-3SNA ceramics and all the samples exhibited pure perovskite structures. The presence of MgO/La₂O₃ additives effectively reduced the sintering temperature of 7SCT-3SNA ceramics due to the formation of a liquid phase at a relatively low temperature during sintering progress. The 0.5 wt% MgO doped 7SCT-3SNA sample with 0.5 wt% of La₂O₃, sintered at 1320 °C for 4 h, was measured to show superior microwave dielectric properties, with an ε_r of 45.57, a Q×f value of 46205 GHz (at 5.5 GHz), and τ_f value of −0.32 ppm/°C, which showed dense and uniform microstructure as well as well-developed grain growth.