Mg3B2O6 microwave dielectric ceramics fabricated by combining cold sintering with post-annealing process

It is difficult to get pure-phase Mg₃B₂O₆ (abbreviated as MBO) ceramics by the traditional high-temperature solid-state reaction method. In this paper, pure-phase MBO ceramics were successfully densified and obtained by combining the cold sintering and post-annealing process. The relative density of MBO ceramics was ∼80% cold sintered at 150°C/90 min/800 MPa, which was further improved to ∼91% by post-annealing at 900°C, 400°C lower than that of the traditional high-temperature sintering process (∼1300°C). X-ray powder diffraction (XRD), scanning electron microscopy (SEM), and Raman results demonstrated that the secondary phase of MgO was effectively eliminated, and dense microstructure was observed by the cold-sintering process plus post-annealing treatment. Finally, the microwave dielectric properties of MBO were evaluated with ε_r: 5.15–6.37, Q×f: 5942–16 686 GHz, τ_f: −48.45–69.72 ppm/°C.     

Investigation of phase composition and microwave dielectric properties of MgO‐Ta2O5 ceramics with ultrahigh Qf value

Four MgO‐Ta₂O₅ ceramics with the MgO/Ta₂O₅ mole ratio x = 1, 2, 3, and 4 were prepared by traditional solid‐state reaction method, and the influence of x on the phase composition, microstructure, and dielectric properties (the dielectric constant ε_r, the temperature coefficient of resonant frequency τ_f and the quality factor Qf) of the materials was investigated using XRD, SEM, etc. The results indicated that the ceramics were composed of two crystalline phases MgTa₂O₆ and Mg₄Ta₂O₉ in the composition range studied, and that the dielectric properties ln ε, 1/Qf, and τ_f changed proportionally to the fraction of main crystal phases, which meet perfectly with the mixing model proposed in this study. It is obvious that the proportion of the two crystal phases could be precisely controlled by x, and thereby, the dielectric properties can be conveniently and precisely tailored. Our research provided a new microwave dielectric ceramic with the composition of 2MgO‐Ta₂O₅, which has an ultrahigh Qf value (211 000 GHz), low dielectric constant ε_r (19.9), and near zero temperature coefficient of resonant frequency τ_f (8 ppm/°C).     

Correlation between the structure and modified microwave dielectric characteristics of Zr4+ substituted Ni0.4Zn0.6Ti(1-x)ZrxNb2O8 ceramics

Microwave ceramics are an important classes of materials that are used in microwave communication systems, especially in the area of 5G wireless communication and the internet of things. In this work, to improve the Q×f values and enhance the temperature stability of Ni_0.4Zn_0.6TiNb₂O₈ ceramics, the influence of the substitution of Zr⁴⁺ ions at the Ti site in Ni_0.4Zn_0.6Ti_(1-x)Zr_xNb₂O₈ ceramics was investigated. The Q×f value increases from 32114 GHz to 45733 GHz and the τ_f value changes from 38.1 ppm/°C to 3 ppm/°C with a slight Zr⁴⁺ ion substitution (x = 0.1). Meanwhile, the sample with the Zr⁴⁺ ion substitution (x = 0.3) that was sintered at 1120 °C shows a very high Q×f value of 92078 GHz. Furthermore, the XRD results reveal that the phase and structure of the Ni_0.4Zn_0.6Ti_(1-x)Zr_xNb₂O₈ ceramics change with the different Zr⁴⁺ ion contents. The substitution of the Zr⁴⁺ ion can promote the sintering process for the Ni_0.4Zn_0.6Ti_(1-x)Zr_xNb₂O₈ ceramics and restrain the Ni_0.5Ti_0.5NbO₄ phase formation. The results obtained from Ni_0.4Zn_0.6Ti_(1-x)Zr_xNb₂O₈ ceramics can offer useful information for the study and application of high-frequency microwaves.     

Ultra-high quality factor and low dielectric constant of (Zn0.5Ti0.5)3+ co-substituted MgAl2O4 ceramic

In this study, MgAl₂O₄-based ceramics with high quality factor (Qf) and low dielectric constant (ε_r ≤ 10) were obtained by fabricating MgAl_2-x(Zn_0.5Ti_0.5)_xO₄ (x = 0–0.5) ceramics via conventional solid-state reaction method. Excellent microwave dielectric properties were achieved for samples at x = 0.5 and sintered at 1550 °C, i.e., ε_r = 9.86, Qf = 263 900 GHz (five times better than that for x = 0 sample) and τ_f = ?92 ppm/°C. The X-ray diffraction (XRD) patterns displayed characteristic peaks of MgAl₂O₄ with spinel structure. MgTi₂O₅ and MgTiO₃ were considered as secondary phases. Scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS) and relative density analysis indicated that ultra-high Qf values were dominated by dense microstructure, secondary phase and cation vacancies; whereas ε_r values were mainly affected by secondary phase and ionic polarizability. MgAl_2-x(Zn_0.5Ti_0.5)_xO₄ ceramics with excellent microwave dielectric properties have potential application in millimeter-wave communication, dielectric filters, dielectric antennas and resonators.     

Improvement of quality factor of SrTiO3 dielectric ceramics with high dielectric constant using Sm2O3

The ceramics with the formula of Sr_1−xSm_2x/3TiO₃ (x = 0-0.5) were prepared by conventional ceramic process. A single phase of SrTiO₃-type with cubic perovskite structure was found companied with a decrease in crystal volume when x < 0.5, which was confirmed by X-ray diffraction results and Rietveld refine. At the level of x = 0.5, the sample consisted of two phases of SrTiO₃-type and Sm₂Ti₂O₇. Raman spectra were used to confirm the change of vibration to explain the variation in microwave dielectric properties. The dielectric constant decreased, the quality factor increased, and the temperature coefficient shifted negatively, when the x value increased from 0 to 0.4. When the x value located between 0.3 and 0.4, the ceramics exhibited high dielectric constant of 140-152, high quality factor (>6000).     

Optimized crystal structure and microwave dielectric properties of BaZnSi3O8-based ceramics

In this study, we synthesized BaZnSi₃O₈-based compounds with monoclinic structures (P21/a) using a solid-state method. The crystal structure, phase composition, and microwave dielectric properties of BaZnSi₃O₈-based ceramics were systematically investigated systematically. X-ray diffraction (XRD) patterns and scanning electron microscopy (SEM) images proved that the maximum solubility of BaZn_1-xMg_xSi₃O₈ ranged between 0.3 and 0.4. Rietveld refinement and Phillips–Van Vechten–Levine complex chemical bond theory were used to illustrate the relationship between the microwave dielectric performance and lattice parameters. To further improve the properties, we substituted Ba²⁺ with Sr²⁺ in BaZn_0.8Mg_0.2Si₃O₈. Ba_1-ySr_yZn_0.8Mg_0.2Si₃O₈ remained in a single-phase as y increased from 0 to 1.0. We achieved thermal stability of the resonance frequency of the BaZnSi₃O₈-based ceramics by adjusting TiO₂ to form composite ceramics. After sintering at 1020°C for 5 h, excellent microwave dielectric properties with ε_r = 7.44, Q×f = 57,400 GHz, and τ_f = − 0.2 ppm/°C were realized in the SrZn_0.8Mg_0.2Si₃O₈+8 wt %TiO₂ system.     

Sm掺杂对Ba_4La_(19.33)Ti_(18)O_(54)陶瓷结构和介电性能的影响分析

主要研究了不同Sm掺杂浓度对Ba4La19.33Ti18O54陶瓷的微波介电性能和微观结构的影响。首先利用常规固相反应技术制备了Sm含量y分别为0.0,0.1,0.3,0.5和0.7的五种Ba4(La1-ySmy)9.33Ti18O54陶瓷样品;室温下在0.3～3.0GHz频率范围内,利用网格分析仪测量了这些样品的介电常数和介电损耗因子;结果表明随着Sm掺杂含量的增大,样品介电损耗明显减小,而介电常数只有微小减少。当Sm掺杂含量y=0.5时,样品的介电性能最好。此外,还利用X射线衍射仪和扫描电子显微镜研究了样品的微观结构及随微波介电性能的变化。     

0.73ZrTi2O6–0.27MgNb2O6 microwave dielectric ceramics modified by Al2O3 addition

0.73ZrTi₂O₆–0.27MgNb₂O₆ ceramics with various Al₂O₃ contents (0‐2.0 wt%) were prepared by conventional ceramic route. The effects of Al₂O₃ on the phase composition, microstructure, conductivity, and microwave dielectric properties were systematically investigated. The coexistence of a disordered α–PbO₂‐type phase and a rutile second phase was found in all compact ceramics with low Al₂O₃ contents (x = 0, 0.5, and 1.0 wt%), while a corundum phase was detected when Al₂O₃ additive increased to 1.5 and 2.0 wt% based on X‐ray diffraction results. With the addition of Al₂O₃, the decreased grain size of the matrix phase was observed using field‐emission scanning electron microscope, accompanied with increased resistivity and band‐gap energy. Additionally, Al₂O₃ additives efficiently improved the quality factor of the ceramics. After sintering at 1360°C for 3 hours, the ceramic with 1.0 wt% Al₂O₃ exhibited excellent microwave dielectric properties: a dielectric constant of 43.8, a quality factor of 33 900 GHz (at 6.6 GHz), and a near‐zero temperature coefficient of resonant frequency (3.1 ppm/^°C).     

Preparation and microwave dielectric properties of low-loss MgZrNb2O8 ceramics

A novel low-loss microwave dielectric material MgZrNb₂O₈ was reported for the first time. Single-phase MgZrNb₂O₈ was prepared by a conventional mixed-oxide route and sintered in the temperature range of 1280–1360 °C. The microstructure and microwave dielectric properties were investigated systematically. The X-ray diffraction results showed that all samples exhibit a single wolframite structure. When the sintering temperature was lower than 1340 °C, the Q×f value mainly depended on the relative density. However, when the sintering temperature was above 1340 °C, the Q×f value mainly relied on the grain morphology in addition to the density. The MgZrNb₂O₈ ceramic sintered at 1340 °C for 4 h exhibited excellent microwave dielectric of ε_r=26, Q×f=120,816 GHz (where f=6.85 GHz), and τ_f=?50.2 ppm/°C. These results demonstrate that MgZrNb₂O₈ could be a promising candidate material for the application of highly selective microwave ceramic resonators and filters.     

Cold sintering and microwave dielectric properties of dense HBO2-II ceramics

HBO₂-II ceramics were prepared by cold sintering with 10wt% dehydrated ethanol as the transient liquid phase. When the processing temperature is 30°C, the relative density of the mechanically robust HBO₂-II ceramics increases from 77.5% to 84.5% with increasing the uniaxial pressure from 200 to 500 MPa. It changes less than 0.2% for higher pressure up to 700 MPa. Under a constant uniaxial pressure of 500 MPa, the relative density further increases to 94.7% for the processing temperature of 120°C. HBO₂-I is observed as the secondary phase when the processing temperature is 150°C. In comparison, the compacts prepared in the absence of ethanol are fragile, and the relative densities are 78.5%-84.5% for the processing temperatures of 30-120°C and uniaxial pressure of 500 MPa. It is indicated that ethanol promotes the densification significantly through the dissolution-precipitation mechanism. The permittivity increases with increasing the processing temperature, while the Qf value decreases. The optimal properties with the relative density of 94.7%, ε_r = 4.21, Qf = 47 500 GHz, and τ_f = −70.0 ppm/°C were obtained in the single-phase HBO₂-II ceramics cold sintered at 120°C under 500 MPa for 10 minutes. The relative density and Qf value are significantly higher than those of the HBO₂-II ceramic prepared by sintering the H₃BO₃ compact at 180°C for 2 hours (70.3% and 32 700 GHz, respectively). The results indicate that the nonaqueous solvent can also be used as the transient liquid phase for cold sintering, so that more materials that are unstable or insoluble in water can be densified by this method.     

Optimized phase compositions and microwave dielectric properties of low loss Ca2Sn2Al2O9-based ceramics by M4+ substitution

The traditional solid-state reaction method was used to prepare Ca₂Sn_2−xM_xAl₂O₉ (M = Ti, Zr, and Hf) ceramics. Then, the impact of an M⁴⁺ substitution of Sn⁴⁺ on the phase transition, crystal structural parameter, and microwave dielectric properties of Ca₂Sn_2−xM_xAl₂O₉ (0 ≤ x ≤ 0.4) ceramics were investigated. Ti⁴⁺ could not replace the Sn⁴⁺ of Ca₂Sn₂Al₂O₉ due to its small ionic radius, and the Al-based second phases of Ca₂Sn_2−xTi_xAl₂O₉ ceramics were confirmed by the X-ray diffractometer and EDS map scanning results. With the Zr⁴⁺ and Hf⁴⁺ substitutions of Sn⁴⁺, the SnO₂ and CaSnO₃ second phases of Ca₂Sn₂Al₂O₉ ceramic were inhibited, and the Ca₂Sn_2−xM_xAl₂O₉ (M = Zr and Hf) (0.05 ≤ x ≤ 0.2) single-phase ceramics with orthorhombic structure (Pbcn space group) were obtained. New MO₂ (M = Zr and Hf) and CaAl₂O₄ second phases appeared in the Ca₂Sn_2−xM_xAl₂O₉ (M = Zr and Hf) (0.3 ≤ x ≤ 0.4) ceramics, and their contents increased gradually with the increase in x. The Ca₂Sn_2−xM_xAl₂O₉ (M = Zr and Hf) (0.05 ≤ x ≤ 0.2) ceramics exhibited high Q × f because of their pure phase compositions, and the Q × f of Ca₂Sn₂Al₂O₉ ceramic was improved to 77 800 GHz (12.6 GHz) in the Ca₂Sn_1.9Zr_0.1Al₂O₉ ceramic. The Q × f values of Ca₂Sn_2−xM_xAl₂O₉ single-phase ceramics were mainly controlled by r_c (Sn/M–O) and r_c (Al–O). The τ_f values of single-phase Ca₂Sn_2−xM_xAl₂O₉ ceramics were related to octahedral distortions. The Zr⁴⁺ and Hf⁴⁺ substitution of Sn⁴⁺ optimized the phase compositions and microwave dielectric properties of the Ca₂Sn_2−xM_xAl₂O₉ ceramics, and the Ca₂Sn_1.9Zr_0.1Al₂O₉ ceramic sintered at optimal temperature exhibited excellent microwave dielectric properties (ε_r = 8.67, Q × f = 77 800 GHz at 12.6 GHz and τ_f = −69.8 ppm/°C).     

Correlation between structural characteristics and microwave dielectric properties of walstromite BaCa2M3O9 (M = Si,Ge) ceramics

Novel BaCa₂M₃O₉ (M = Si, Ge) microwave dielectric ceramics were prepared via solid-state reaction with sintering at 1125°C–1275°C for 5 h. Single-phase BaCa₂M₃O₉ (M = Si, Ge) ceramics were obtained according to stoichiometry. The single-phase BaCa₂Ge₃O₉ ceramic was confirmed through Rietveld refinement and high-resolution transmission electron microscopy/selected area electron diffraction and synthesized for the first time. The BaCa₂M₃O₉ (M = Si, Ge) exhibited a triclinic structure with a P$\overline{1}$ space group and good microwave dielectric properties. The ε_r, Q × f, and τ_f values of BaCa₂M₃O₉ (M = Si, Ge) ceramics are mostly dominated by the relative density, ionic polarizability, relative covalence, and bond energy of M–O bond, respectively. A high Q × f value (61 800 GHz at 16.3 GHz) was obtained in BaCa₂Ge₃O₉ ceramic due to its high r_c (Ge–O) and low intrinsic dielectric loss. The BaCa₂Si₃O₉ ceramic exhibited small |τ_f| value (‒36.4 ppm/°C) due to its large E_Si-O. Excellent microwave dielectric properties (ε_r = 8.31, Q × f = 61 800 GHz, and τ_f = ‒58.7 ppm/°C) were obtained for the BaCa₂Ge₃O₉ ceramic.     

Structure and microwave dielectric characteristics of Hf1-xTixO2 ceramics

Hf_1-xTi_xO₂ dense ceramics were prepared by a standard solid-state reaction process, and the microwave dielectric properties in a wide frequency range were determined together with structure evolution. With increasing x, the structure gradually changed from HfO₂-based solid solution (monoclinic in space group P2₁/c, x ≤ 0.05) to HfTiO₄ (orthorhombic in space group Pbcn, x = 0.5), and the two phase regions were determined for 0.1 ≤x < 0.5 and x = 0.55. The microwave dielectric properties of HfO₂ ceramics at 10 GHz were determined as ε_r = 14, Qf = 24 500 GHz, τ_f = –50 ppm/℃. With Ti-substitution, the microwave dielectric characteristics, especially the Qf value, could be significantly improved, and the best combination of microwave dielectric characteristics was achieved at x = 0.05: ε_r = 17, Qf = 84 020 GHz (at 7.8 GHz) and 151 260 GHz (at 27.9 GHz), and τ_f = –47 ppm/℃. The smallest temperature coefficient of resonance frequency (τ_f = –4.8 ppm/℃) was obtained for x = 0.55 together with a Qf value of 52 370 GHz at 5.0 GHz and 60 390 GHz at 17.9 GHz, where the dielectric constant was 40. Moreover, Hf_1-xTi_xO₂ microwave dielectric ceramics might be very competitive in the future of mobile communication technology due to their excellent compatibility with Si.     

SrZnV2O7: A low-firing microwave dielectric ceramic with high-quality factor

A low-firing SrZnV₂O₇ ceramic was synthesized via a traditional solid-state sintering route. The phase formation, grain morphology, and dielectric performance were studied in detail. Rietveld refinements based on XRD data revealed that SrZnV₂O₇ crystallized in the monoclinic space group P2₁/n, which was further confirmed by TEM analysis. The dense SrZnV₂O₇ ceramic with an ε_r of 11.28, a Qf of 56 660 GHz, and a τ_f of −27.2 ppm/°C was yielded at a low sintering temperature of 740°C for 4 h. Its chemical compatibility with Ag was verified via XRD and SEM-EDS analysis, which rendered SrZnV₂O₇ a prospective candidate for LTCC technology.     

Tunable microwave dielectric properties in SrO-V2O5 system through compositional modulation

Adjustment on resonance frequency stability against the sintering temperature of Sr₃V₂O₈ was realized by adjusting the Sr:V mole ratio. Effects of Sr:V ratio on sintering behavior and dielectric properties of Sr₃V₂O₈ were studied. The sintering temperature was sucessfully reduced to 950°C from 1150°C. With increasing vanadium content, both relative permittivity and quality factor decreased, while the temperature coefficient of resonance frequency shifted from positive to negative values. Especially, a near-zero τ_f of −1.1 ppm/°C along with a low permittivity (ε_r) of 9.8 and a quality factor Q × f of 24 120 GHz was successfully achieved in Sr_3-yV₂O_8-y ceramic (y = 0.6, sintered at 950°C). The wide compositional and processing adjustment window, favorable dielectric performances, and good chemical compatibility with silver render Sr_3-yV₂O_8-y ceramics potential candidates in multilayer electronic devices.     

High-Qf value and temperature stable Zn2+-Mn4+ cooperated modified cordierite-based microwave and millimeter-wave dielectric ceramics

Cordierite-based dielectric ceramics with a lower dielectric constant would have significant application potential as dielectric resonator and filter materials for future ultra-low-latency 5G/6G millimeter-wave and terahertz communication. In this article, the phase structure, microstructure and microwave dielectric properties of Mg₂Al_4–2x(Mn_0.5Zn_0.5)_2xSi₅O₁₈ (0 ≤ x ≤ 0.3) ceramics are studied by crystal structure refinement, scanning electron microscope (SEM), the theory of complex chemical bonds and infrared reflectance spectrum. Meanwhile, complex double-ions coordinated substitution and two-phase complex methods were used to improve its Q×f value and adjust its temperature coefficient. The Q×f values of Mg₂Al_4–2x(Mn_0.5Zn_0.5)_2xSi₅O₁₈ single-phase ceramics are increased from 45,000 GHz@14.7 GHz (x = 0) to 150,500 GHz@14.5 GHz (x = 0.15) by replacing Al³⁺ with Zn²⁺-Mn⁴⁺. The positive frequency temperature coefficient additive TiO₂ is used to prepare the temperature stable Mg₂Al_3.7(Mn_0.5Zn_0.5)_0.3Si₅O₁₈-ywt%TiO₂ composite ceramic. The composite ceramic of Mg₂Al_3.7(Mn_0.5Zn_0.5)_0.3Si₅O₁₈-ywt%TiO₂ (8.7 wt% ≤ y ≤ 10.6 wt%) presents the near-zero frequency temperature coefficient at 1225 °C sintering temperature: ε_r = 5.68, Q×f = 58,040 GHz, τ_f = ?3.1 ppm/°C (y = 8.7 wt%) and ε_r = 5.82, Q×f = 47,020 GHz, τ_f = +2.4 ppm/°C (y = 10.6 wt%). These findings demonstrate promising application prospects for 5 G and future microwave and millimeter-wave wireless communication technologies.     

A low-permittivity microwave dielectric ceramic BaZnP2O7 and its performance modification

Complex pyrophosphates compounds have attracted much attention as promising candidates for substrate applications. In the work, a low-permittivity BaZnP₂O₇ ceramic was synthesized through solid-state reaction. The pure phase BaZnP₂O₇ was crystallized in the triclinic P−1 space group. Excellent microwave dielectric properties of the BaZnP₂O₇ ceramic with ε_r = 8.23, Qf = 56170 GHz, and τ_f = −28.7 ppm/°C were obtained at 870°C for 4 h. The substitution of Mg²⁺ for Zn²⁺ was found to have positive effects on grain morphology and dielectric properties. Optimized performance of ε_r = 8.21, Qf = 84760 GHz, and τ_f = −21.9 ppm/°C was yielded at 900°C for the BaZn₀.₉₈Mg_0.02P₂O₇ ceramic. Intrinsic dielectric properties of BaZn_1-xMg_xP₂O₇ ceramics were studied via Clausius–Mossotti equation and complex chemical bond theory.     

(Cu1/3Nb2/3)4+ ionic substituting effects,low-temperature sintering behaviors,and improved temperature stability of Ba3Nb4Ti4O21 microwave dielectric ceramics

In this study, (Cu_1/3Nb_2/3)⁴⁺ complex cation and BaO–ZnO–B₂O₃ glass frit were adopted to solve the high sintering temperature and poor temperature stability of Ba₃Nb₄Ti₄O₂₁ ceramics. It is shown that pure Ba₃Nb₄Ti₄O₂₁ phase was formed when Ti site was partially replaced by (Cu_1/3Nb_2/3)⁴⁺ cation. The increasing number of dopants decreases the dielectric polarizability, correspondingly, the dielectric constant and temperature coefficient of the resonance frequency values are reduced consistently. The variation of the Q × f value is determined by internal ionic packing fraction and external sintering densification. The (Cu_1/3Nb_2/3)⁴⁺ cation effectively decreases the suitable sintering temperature from 1200 to 1050 °C while greatly improving the temperature stability. BaO–ZnO–B₂O₃ glass was used to further improve the low-temperature sintering characteristics of Ba₃Nb₄Ti₄O₂₁ ceramics. It is proven that the addition of glass frits effectively decreases the temperature to 925 °C with combinational excellent microwave dielectric properties: ε_r ～55.6, Q × f ～5700 GHz, τ_f ～3 ppm/^°C, making the Ba₃Nb₄Ti₄O₂₁ ceramics promising in the applications of low-temperature cofired ceramic technology.     

Phase evolution and microwave dielectric properties of BaTi4O9 ceramics prepared by reaction sintering method

BaTi₄O₉ microwave dielectric ceramics were prepared by reaction sintering method using BaCO₃ and TiO₂ as raw materials. The phase evolution and the chemical reactions were proposed based on the X-ray diffraction results with sintering temperature. The microstructure characteristics were observed using scanning electron microscopy and energy dispersive spectrometer. The compact ceramics with a single phase of BaTi₄O₉ could be prepared successfully by reaction sintering method, exhibiting optimum microwave dielectric properties: a dielectric constant of 36.9, a high quality factor of 52 735 (at 7.5GHz), and a near zero temperature coefficient of resonant frequency of 5.8 ppm/°C, after sintering at 1200°C for 6 hours.