Phase compositions and crystal structure of Sn-deficient Ca2Sn2Al2O9 microwave dielectric ceramics with high Q×f values

The phase compositions and microwave dielectric properties of Sn-deficient Ca₂Sn₂Al₂O₉ ceramics in this study were explored. The CaSnO₃ and SnO₂ second phases existed at Ca₂Sn_2-xAl₂O_9-2x (x = 0) ceramic. Single-phase Ca₂Sn₂Al₂O₉ ceramics were obtained at 0.08 ≤ x ≤ 0.1, and the orthorhombic structure with the Pbcn space group of Ca₂Sn₂Al₂O₉ was verified. For multi-phase Ca₂Sn_2-xAl₂O_9-2x (0 ≤ x ≤ 0.06) ceramics, their microwave dielectric properties were mainly affected by second phase contents, and their Q × f values increased gradually with the rise in x. High Q × f (105,700 GHz at 12.99 GHz) was obtained by the Ca₂Sn_2-xAl₂O_9-2x (x = 0.08) ceramic with high intrinsic Q × f (175,000 GHz). The large deviation between measured Q × f values and fitted intrinsic Q × f values could be ascribed to the Sn⁴⁺ vacancies of the Sn-deficient Ca₂Sn₂Al₂O₉ ceramics. The Ca₂Sn_2-xAl₂O_9-2x (0 ≤ x ≤ 0.1) ceramics presented large negative τ_f values, and this τ_f was mainly affected by τ_ε. Meanwhile, the Ca₂Sn_2-xAl₂O_9-2x (x = 0.08) ceramic achieved optimal microwave dielectric properties (ε_r = 8.3, Q × f = 105,700 GHz at 12.99 GHz and τ_f = ?63.7 ppm/°C), indicating the good feature of this material for millimetre-wave applications.     

The relationship between crystal structure and modified microwave dielectric properties of Ca3SnSi2-xGexO9 ceramics

Ca₃SnSi_2-xGe_xO₉ (0 ≤ x ≤ 0.8) and (1–y) Ca₃SnSi_1.6Ge_0.4O₉ – y CaSnSiO₅ – 2 wt% LiF (y = 0.4 and 0.5) microwave dielectric ceramics were prepared by traditional solid-state reaction through sintering at 1250°C–1425°C for 5 h and at 875°C for 2 h, respectively. Ge⁴⁺ replaced Si⁴⁺, and Ca₃SnSi_2-xGe_xO₉ (0 ≤ x ≤ 0.4) solid solutions were obtained. At 0.1 ≤ x ≤ 0.4, the Ge⁴⁺ substitution for Si⁴⁺ decreased the sintering temperature of Ca₃SnSi_2-xGe_xO₉ from 1425 to 1300°C, the SnO₆ octahedral distortions, and the average CaO₇ decahedral distortions, which affected the τ_f value. The large average decahedral distortions corresponded with nearer-zero τ_f values at Ca₃SnSi_2-xGe_xO₉ (0.1 ≤ x ≤ 0.4) ceramics. The τ_f value and sintering temperature of Ca₃SnSi_2-xGe_xO₉ (x = 0.4) ceramic were adjusted to near-zero by CaSnSiO₅ and decreased to 875°C upon the addition of 2 wt% LiF. The (1 – y) Ca₃SnSi_1.6Ge_0.4O₉ – y CaSnSiO₅ – 2 wt% LiF (y = 0.5) ceramic sintered at 875°C for 2 h exhibited good microwave dielectric properties: ε_r = 10.3, Q × f = 14 300 GHz (at 12.2 GHz), and τ_f = ‒5.8 ppm/°C.     

Crystal structure,densification, and microwave dielectric properties of Li3Mg2(Nb(1−x)Mox)O6+x/2 (0 ≤ x ≤ 0.08) ceramics

A novel system Li₃Mg₂(Nb_(1−x)Mo_x)O_6+x/2 (0 ≤ x ≤ 0.08) microwave dielectric ceramics were fabricated by the solid-state method. The charge compensation of Mo⁶⁺ ions substitution for Nb⁵⁺ ions was performed by introducing oxygen ions. The X-ray diffraction patterns and Rietveld refinements indicated Li₃Mg₂(Nb_(1−x)Mo_x)O_6+x/2 ceramics with single phase and orthorhombic structure. Micro-structure and density confirmed that the grain of Li₃Mg₂(Nb_(1-x)Mo_x)O_6+x/2 ceramics grew well. In addition, the permittivity of Li₃Mg₂(Nb_(1−x)Mo_x)O_6+x/2 ceramics with the same trend as density decreased slightly with increasing Mo⁶⁺ ions content. However, the Q*f and τ_f were obviously improved with an appropriate amount of Mo⁶⁺ ions. When x ≤ 0.04, the Q*f was closely related to the bond valence of samples, while when x ≥ 0.06, the Q*f was closely related to the density of samples. The variations of τ_f and oxygen octahedral distortion were the opposite. In conclusions, the Li₃Mg₂(Nb_0.98Mo_0.02)O_6.01 ceramic sintered at 1200°C for 6 hours exhibited outstanding properties: ε_r ~ 15.18, Q*f ~ 116 266 GHz, τ_f ~ −15.71 ppm/^oC.     

Optimized sintering behavior and microwave dielectric properties of Ca1+2xSnSi2x+yO3+6x+2y by composition modulation

Ca_1+2xSnSi_2x+yO_3+6x+2y (0.1 ≤ x ≤ 0.9; 0.1 ≤ y ≤ 0.9) microwave dielectric ceramics were prepared through traditional solid-state reaction sintered at 1450°C–1500°C for 5 hours. The Ca₃SnSi₂O₉ second phase replaced the SnO₂ second phase of the Ca_1+2xSnSi_2xO_3+6x (x = 0, y = 0) ceramics by controlling the ratio of Ca:Sn:Si. The cracks of CaSnO₃ (x = 0, y = 0) ceramic were inhibited, the microwave dielectric properties were optimized by introducing the Ca₃SnSi₂O₉ second phase, and the CaSnO₃-Ca₃SnSi₂O₉ mixture system existed at (0.1 ≤ x ≤ 0.9, y = 0). The CaSnSiO₅ phase with positive τ_f value was related to the Si-rich in CaSnSi_yO_3+2y (x = 0; 0.1 ≤ y ≤ 0.9), and the coexistence of three and four phases was obtained at CaSnSi_yO_3+2y (0.1 ≤ y ≤ 0.9) ceramics. The CaSnSiO₅ phase appeared at CaSnSi_yO_3+2y (0.3 ≤ y ≤ 0.9) ceramics. The CaSnSi_yO_3+2y (y = 0.8) ceramic with 49.2 wt% CaSnSiO₅ phase exhibited excellent microwave dielectric properties: ε_r = 11.06, Q × f = 57,500 GHz (at 11.5 GHz), and τ_f = +8.1 ppm/°C.     

Crystal structure,phase compositions,and microwave dielectric properties of malayaite-type Ca1−xSrxSnSiO5 ceramics

Low-permittivity Ca_1−xSr_xSnSiO₅ (0 ≤ x ≤ 0.45) microwave dielectric ceramics were prepared via traditional state-reaction at 1400°C-1450°C for 5 hours. Moreover the microwave dielectric properties of SnO₂ ceramic were obtained for the first time. SnO₂ ceramic was difficult to densify, and SnO₂ ceramic (ρ_rel = 65.1%) that was sintered at 1525°C exhibited the optimal microwave dielectric properties of ε_r = 5.27, Q × f = 89 300 GHz (at 14.5 GHz), and τ_f = −26.7 ppm/°C. For Ca_1−xSr_xSnSiO₅ (0 ≤ x ≤ 0.15) ceramics, Sr²⁺ could be dissolved in the Ca²⁺ site of Ca_1−xSr_xSnSiO₅ to form a single phase, and the partial substitution of Ca²⁺ by Sr²⁺ could improve the microwave dielectric properties of CaSnSiO₅ ceramic. Secondary phases (SnO₂ and SrSiO₃) appeared at 0.2 ≤ x ≤ 0.45 and could adjust the abnormally positive τ_f value of CaSnSiO₅ ceramic. The highest Q × f value (60 100 GHz at 10.4 GHz) and optimal microwave dielectric properties (ε_r = 9.42, Q × f = 47 500 GHz at 12.4 GHz, and τ_f = −1.2 ppm/°C) of Ca_1−xSr_xSnSiO₅ ceramics were obtained at x = 0.05 and 0.45, respectively.     

Effect of TiO2 Doping on the Structure and Microwave Dielectric Properties of Cordierite Ceramics

The (1?x)Mg₂Al₄Si₅O₁₈–xTiO₂ |(1?x)MAS‐xT| (0 ≤ x ≤ 0.35) cordierite ceramics are fabricated by solid‐state reaction method for obtaining near‐zero temperature coefficient of resonant frequency (τ_f). The XRD and SEM results show that (1?x)MAS‐xT (0 ≤ x ≤ 0.10) ceramics exhibit single cordierite solid solution, whereas as 0.15 ≤ x ≤ 0.35, present composite phases of Mg₂Al₄Si₅O₁₈ solution and TiO₂. Rietveld refinements of XRD data suggest that the [(Si₄Al₂)O₁₈] hexagonal shape in cordierite structure happens to alternate change from nonsymmetrical hexagonal rings to almost centrosymmetrical equilateral rings as x increases to 0.10 comparing to that of x = 0. As Ti⁴⁺ ions squeeze into the [(Si₄Al₂)O₁₈] rings structure, the orientation and shapes of the rings begin to rotate and expand from initial state of [1–20] (x = 0) to near [210] direction (x = 0.10), and then continue to expand toward close to [110] direction (x = 0.25). Due to centrosymmetry adjustment of [(Si₄Al₂)O₁₈] hexagonal rings and of other microstructure factors improvement, the (1?x)MAS‐xT (x = 0.10) cordierite solution achieves optimum quality factor Q_f: ε_r = 6.3, Q_f = 55 400 GHz (17.6 GHz), τ_f = ?21 ppm/°C. The (1?x)MAS‐xT (x = 0.25) composites obtain a near‐zero temperature coefficient of resonance frequency: ε_r = 6.8, Q_f = 37 800 GHz (18.4 GHz), τ_f = ?0.2 ppm/°C.     

Structure Evolution and Enhanced Microwave Dielectric Characteristics of (Sr1−xCax)La2Al2O7 Ceramics

(Sr_1?xCa_x)La₂Al₂O₇ (0.1 ≤ x ≤ 0.5) ceramics were prepared by a standard solid‐state reaction method. Their densification behavior and microwave dielectric properties were investigated together with the structural evolution. X‐ray diffraction analysis indicated that the major phase of Ruddlesden–Popper structure with n = 2 was obtained for all the compositions investigated here. Partial Ca substitution improved the sintering behavior of SrLa₂Al₂O₇ ceramics. More importantly, microwave dielectric characteristics were enhanced in (Sr_1?xCa_x)La₂Al₂O₇ ceramics with compositions of x = 0.1~0.3. The stacking fault was confirmed by TEM observation in the present ceramics, and the microwave dielectric loss was influenced by it. The best combination of microwave dielectric characteristics was achieved for the composition of x = 0.1: ε_r = 19.9, Qf = 135 400 GHz and τ_f = ?18.5 ppm/°C.     

Lattice evolution and microwave dielectric properties of La-doped yttrium aluminum garnet ceramics

In this study, Y_3−xLa_xAl₅O₁₂ (0 ≤ x ≤ 0.09) ceramics were synthesized, and the phase composition, lattice evolution, and microwave dielectric properties were investigated in detail. Scanning electron microscopy confirms that the addition of moderate amounts of La₂O₃ improves the grain development of YAG ceramics, but excessive doping destabilizes the crystal structure. Transmission electron microscopy characterization shows that the variation of the dielectric properties of the samples with x-value is related to the occurrence of benign dislocation structures caused by modifications in the type and content of the A-site rare-earth ions. The variations in relative density, dielectric constant, and quality factor remain basically coordinated. The optimum microwave dielectric properties of La³⁺ doped YAG samples are exhibited as ε_r = 10.61, Q × f = 187, 542 GHz, τ_f = −31.2 ppm/°C when La₂O₃ is doped at x = 0.015.     

Improved sinterability and microwave dielectric properties of [Zn0.5Ti0.5]3+-doped ZnAl2O4 spinel solid solution

Low-permittivity ZnAl_2-x(Zn_0.5Ti_0.5)_xO₄ ceramics were synthesized via conventional solid-state reaction method. A pure ZnAl₂O₄ solid-state solution with an Fd-3m space group was achieved at x ≤ 0.1. Results showed that partial substitution of [Zn_0.5Ti_0.5]³⁺ for Al³⁺ effectively lowered the sintering temperature of the ZnAl₂O₄ ceramics and remarkably increased the quality factor (Q × f) values. Optimum microwave dielectric properties (ε_r = 9.1, Q × f = 115,800 GHz and τ_f = −78 ppm/°C) were obtained in the sample with x = 0.1 sintered at 1400°C in oxygen atmosphere for 10 h. The temperature used for the sample was approximately 250°C lower than the sintering temperature of conventional ZnAl₂O₄ ceramics.     

Effect of Ce3+ doping on phase and microwave dielectric properties of 0.94MgTiO3−0.06(Ca0.8Sr0.2)TiO3 ceramics

In this study, 0.94Mg_(1-3x/2)Ce_xTiO₃−0.06(Ca_0.8Sr_0.2)TiO₃ (MCe_xT−CST, 0≤x≤0.01) composite ceramics were prepared at a low temperature of 1175°C by using the 50-nm-sized powders. The effects of Ce³⁺ doping on crystalline phase, microstructure, and microwave dielectric properties of MCe_xT−CST were studied. A main ilmenite (Mg,Ce)TiO₃ phase and a minor perovskite (Ca_0.8Sr_0.2)TiO₃ phase coexist well with the appearance of impurity MgTi₂O₅ phase in MCe_xT−CST. The dielectric properties of MCe_xT−CST are affected by the molecular polarizability, the impurity phase, and the Ce³⁺ doping. The replacement of Mg²⁺ by high valence Ce³⁺ could effectively inhibit the formation of oxygen vacancy, resulting in the enhancement of Q×f. When x = 0.005, MCe_xT−CST exhibits microwave dielectric properties with a moderate ε_r of 21.5, a high Q×f of 67 000 GHz, and a near-zero τ_f of −0.74 ppm/°C. The results reveal that the Ce³⁺ substitution is a prospective approach to optimize the microwave dielectric properties of MgTiO₃-based ceramics.     

Polyaminocarboxylate promoted synthesis of Hafnium/ Zirconium substituted anion excess In2O3: Structure,optical and electrical conductivity properties

The current study aimed to generate Hf/Zr substituted In₂O₃ with the ultimate aim of realizing a potential transparent conducting oxide. We applied a co-complexation method to bring the reactively dissimilar In and Hf/Zr together in one oxide network. We prepared an EDTA complex containing an equimolar concentration of In and Hf/Zr and examined their characteristics with FTIR and TG-DSC traces. Rietveld refinement results of calcined complexes and their Raman spectra confirmed the formation of anion excess bixbyite structure for (In_1-xM_x)₂O_3+δ (M = Hf, Zr, and x = 0.50). The lattice expanded after substituting with Hf/Zr, and the optical bandgap increased from 2.87 eV (In₂O₃) to 3.20–3.60 eV. The high percentage reflectance in the visible region and absorbance in the UV region fulfilled some of the prerequisites of transparent conducting oxide. Electrical resistivity reduced up to two orders in magnitude with increasing temperature for Hf and Zr incorporated In₂O₃.     

Improved sintering behavior and microwave dielectric properties of SnO2-based ceramics and their LTCC materials with ultrawide temperature stability

Microwave dielectric ceramics with simple composition, a low permittivity (ε_r), high quality factor (Q × f) and temperature stability, specifically in the ultrawide temperature range, are vital for millimetre-wave communication. Hence, in this study, the improvements in sintering behavior and microwave dielectric properties of the SnO₂ ceramic with a porous microstructure were investigated. The relative density of the Sn_1-xTi_xO₂ ceramic (65.1%) was improved to 98.8%, and the optimal sintering temperature of Sn_1-xTi_xO₂ ceramics reduced from 1525 °C to 1325 °C when Sn⁴⁺ was substituted with Ti⁴⁺. Furthermore, the ε_r of Sn_1-xTi_xO₂ (0 ≤ x ≤ 1.0) ceramics increased gradually with the rise in x, which can be ascribed to the increase in ionic polarisability and rattling effects of (Sn_1-xTi_x)⁴⁺. The intrinsic dielectric loss was mainly controlled by r_c (Sn/Ti–O), and the negative τ_f of the SnO₂ ceramic was optimised to near zero (x = 0.1) by the Ti⁴⁺ substitution for Sn⁴⁺. This study also explored the ideal microwave dielectric properties (ε_r = 13.7, Q × f = 40,700 GHz at 9.9 GHz, and τ_f = ?7.2 ppm/°C) of the Sn_0.9Ti_0.1O₂ ceramic. Its optimal sintering temperature was decreased to 950 °C when the sintering aids (ZnO–B₂O₃ glass and LiF) were introduced. The Sn_0.9Ti_0.1O₂-5 wt% LiF ceramic also exhibited excellent microwave dielectric properties (ε_r = 12.8, Q × f = 23,000 GHz at 10.5 GHz, and τ_f = ?17.1 ppm/°C). At the ultrawide temperature range (?150 °C to +125 °C), the τ_ε of the Sn_0.9Ti_0.1O₂-5 wt% LiF ceramic was +13.3 ppm/°C, indicating excellent temperature stability. The good chemical compatibility of the Sn_0.9Ti_0.1O₂-5 wt% LiF ceramic and the Ag electrode demonstrates their potential application for millimetre-wave communication.     

(Sr1-xCax)2TiO4 microwave dielectric ceramics with R-P structure (x = 0~0.15)

(Sr_1-xCa_x)₂TiO₄ ceramics (0 ≤ x ≤ 0.15) were synthesized by a standard solid state reaction route. Tetragonal Ruddlesden-Popper (R-P) solid solutions (Sr_1-xCa_x)₂TiO₄ with space group I4/mmm were obtained in the composition range of x = 0~0.15, while Sr₃Ti₂O₇ has been observed as secondary phase in x ≤ 0.10. In the present ceramics, both the dielectric constant ɛ_r and temperature coefficient of resonant frequency τ_f decreased at first and turned to increase again with increasing x, while the quality factor Qf decreased slightly at first and turned to decrease sharply after x = 0.1. The improved microwave dielectric properties with the best combination were obtained in (Sr_1-xCa_x)₂TiO₄ ceramics at x = 0.075: ɛ_r = 39.3, Qf = 93 550 GHz, τ_f = 119 ppm/°C The obvious improvement was achieved in temperature coefficient of resonant frequency (from 140 to 119 ppm/°C) with Ca²⁺-substitution for Sr²⁺ in Sr₂TiO₄ ceramics due to the increased B-site bond valance.     

An evaluation of the impact of crystal structure on microwave dielectric properties of novel titanite-type CaMGeO5 ceramics

Novel titanite-type CaMGeO₅ (M = Ge, Hf, Zr) microwave dielectric ceramics were prepared using solid-state reaction. A CaGe₂O₅ single-phase ceramic with a triclinic structure was obtained, and the main phase compositions and crystal structure of CaMGeO₅ (M = Hf, Zr) ceramics were confirmed. The CaGe₂O₅ single-phase ceramic exhibited a high Q×f value (92,400 GHz), and its Q×f values were controlled by the r_c (Ge-O) and the packing fraction. For CaMGeO₅ (M = Hf, Zr) multi-phase ceramics, the Q×f values were deteriorated by the second phase. The τ_f and τ_ε values of the CaMGeO₅ ceramics were directly controlled by the ε_r anomaly peaks, and the phase transitions were controlled by ^?δ. The τ_f values of the CaMGeO₅ (M = Ge, Hf, Zr) ceramics were mainly related to ^?δ, and the small ^?δ corresponded to the nearer-zero τ_f values for the titanite-type CaMGeO₅ ceramic with negative τ_f values. The optimal microwave dielectric properties (ε_r = 10.57, Q×f = 92,400 GHz at 13.22 GHz, and τ_f = 68.3 ppm/°C) were obtained by the CaGe₂O₅ ceramic.     

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.     

Ultra low loss (Mg1 − xCax)2SiO4 dielectric ceramics (x = 0 to 0.15) for millimeter wave applications

(Mg_{1 − x}Ca_x)₂SiO₄ dense ceramics (x ≥ 0.15) were prepared, and their microwave dielectric characteristics were investigated together with the structure evolution. The sintering temperature for Mg₂SiO₄ ceramics was reduced significantly with Ca²⁺substitution. (Mg_{1 − x}Ca_x)₂SiO₄ ceramics exhibited a small increase in dielectric constant (ε_r) correlated with increased crystallite size, and ultra-high quality factor Qf value was achieved throughout the compositional range. Temperature coefficient of resonant frequency (τ_f) was considerably tuned from −70 ppm/°C to −33 ppm/°C, and this improvement was deeply linked with the decreased bond valance. At x = 0.075, (Mg_{1 − x}Ca_x)₂SiO₄ ceramics exhibited the best combination of microwave dielectric characteristics: ε_r = 7.2, Qf = 199,800 GHz at 26 GHz, τ_f = −33 ppm/°C. The present ceramics could be expected as promising candidate of dielectric materials for millimeter wave applications.     

Phase composition,crystal structure,and microwave dielectric properties of LiIn1-xAlxO2 ceramics

A series of LiIn_1-xAl_xO₂ (x =0.05, 0.10, 0.15, 0.20, 0.25) microwave dielectric ceramics with low permittivity were synthesized via a solid-state reaction method. XRD, Raman spectra, and SEM analysis reveal that a single LiInO₂ tetragonal structure phase could be obtained at the x < 0.10, and with the x increased further to 0.15–0.25, the diffraction peaks of the secondary phase LiAlO₂ were detected. In the LiIn_1-xAl_xO₂ ceramics, the τ_f was closely related to the ε_r, and the relative density, microstructure, and microwave dielectric properties were effectively improved by the Al³⁺ substitution for In³⁺. Bond valence theory analysis demonstrates that the Al³⁺ entered the In³⁺ site exhibits a strength rattling effect, which is beneficial to the increase of ε_r. While Al³⁺ substitution for In³⁺ simultaneously lowers the average ionic polarizability, resulting in a decrease in ε_r. A near-zero τ_f (0.74 ppm/°C) combined with ε_r approximately 12.83, Q × f = 58 200 GHz, was obtained in LiIn0_.85Al0_.15O₂ ceramic sintered at 970°C.     

Microwave dielectric properties of silico-carnotite Ca3M2Si3O12 (M= Yb,Y) ceramics synthesized via high energy ball milling

The Ca₃M₂Si₃O₁₂ (M = Yb, Y) ceramics with orthorhombic silico-carnotite structure were fabricated via high-energy ball milling and solid-state reaction route. Dense Ca₃Yb₂Si₃O₁₂ and Ca₃Y₂Si₃O₁₂ ceramics sintered at 1260 °C and 1240 °C revealed promising microwave dielectric properties with ε_r = 9.2 and 8.7, Q×f = 56,400 GHz and 29,094 GHz, τ_f = −77.5 ppm/°C and −76.8 ppm/°C, respectively. The connection between crystal structure and Q×f values of Ca₃M₂Si₃O₁₂ (M = Yb, Y) ceramics was discussed with respect to the packing fraction, and their intrinsic microwave dielectric properties were examined using the infrared reflectivity spectra analysis. The thermal stability of Ca₃Yb₂Si₃O₁₂ was improved successfully by forming 0.91Ca₃Yb₂Si₃O₁₂‐0.09CaTiO₃ composite ceramics with τ_f = +2.9 ppm/°C, ε_r = 12.93 and Q×f = 26,729 GHz.     

Structure and microwave dielectric characteristics of Sr2[Ti1−x(Al0.5Nb0.5)x]O4 (x ≤ 0.50) ceramics

Sr₂[Ti_1−x(Al_0.5Nb_0.5)_x]O₄ (x = 0, 0.10, 0.25, 0.30, 0.5) ceramics were synthesized by a standard solid-state reaction process. Sr₂[Ti_1−x(Al_0.5Nb_0.5)_x]O₄ solid solutions with tetragonal Ruddlesdon-Popper (R-P) structure in space group I4/mmm were obtained within x ≤ 0.50, and only minor amount (1-2 wt%) of Sr₃Ti₂O₇ secondary phase was detected for the compositions x ≥ 0.25. The temperature coefficient of resonant frequency τ_f of Sr₂[Ti_1−x(Al_0.5Nb_0.5)_x]O₄ ceramics was significantly improved from 132 to 14 ppm/°C correlated with the increase in degree of covalency (%) with increasing x. The dielectric constant ɛ_r decreased linearly with increasing x, while high Qf value was maintained though it decreased firstly. The variation tendency of Qf value was dependent on the trend of packing fraction combined with the microstructure. Good combination of microwave dielectric properties was achieved for x = 0.50: ɛ_r = 25.1, Qf = 77 580 GHz, τ_f = 14 ppm/°C. The present ceramics could be expected as new candidates of ultra-high Q microwave dielectric materials without noble element such as Ta.     

Crystal structure and microwave dielectric properties of garnet-type Ca2YZr2-xTixAl3O12 ceramics for dual-band bandpass filters

Microwave dielectric ceramics with a high-quality factor are vital materials for substrates, dielectric resonators, and filters in millimeter-wave communication systems. Here, a novel microwave dielectric ceramic based on a garnet-type Ca₂YZr₂Al₃O₁₂ compound for bandpass filter was prepared using the solid-state reaction method. Sintering the Ca₂YZr₂Al₃O₁₂ ceramics at 1600 ℃ for 5 h resulted in excellent microwave dielectric properties of ε_r = 10.81 ± 0.16, Qf = 87,628 ± 4000 GHz and τ_f = ?34.3 ± 0.5 ppm/℃. Increasing the Ti⁴⁺ content of the Ca₂YZr_2-xTi_xAl₃O₁₂ ceramics significantly improved the sintering process. Superior microwave dielectric properties (ε_r = 11.93 ± 0.15, Qf = 121,930 ± 2600 GHz and τ_f = ?30.8 ± 0.4 ppm/℃) were obtained for Ca₂YZr_2-xTi_xAl₃O₁₂ (x = 0.3) because of its dense microstructure, large grain size and high lattice energy. The Ca₂YZr_2-xTi_xAl₃O₁₂ (x = 0.3) ceramics were used to fabricate a dual-band bandpass filter with a hairpin structure that exhibited a large return loss (|S₁₁| > 12 dB) and a small insertion loss (|S₂₁| < 0.73 dB). The high performance of the Ca₂YZr_2-xTi_xAl₃O₁₂ ceramics and the corresponding bandpass filter makes this material a potential candidate for millimeter-wave devices.