Crystal structure,far-infrared spectra,and microwave dielectric properties of bazirite-type BaZr(Si1-xGex)3O9 ceramics

Novel BaZr(Si_1-xGe_x)₃O₉ (0 ≤ x ≤ 1.0) microwave dielectric ceramics were prepared by solid-state reaction sintering at 1200–1450 °C for 5 h Ge⁴⁺ ions occupied the Si⁴⁺ positions, and BaZr(Si_1-xGe_x)₃O₉ solid solutions were obtained. The BaZr(Si_1-xGe_x)₃O₉ (0 ≤ x ≤ 1.0) ceramics exhibited hexagonal structures with P-6c2 space groups and octahedral layers and [Si/Ge₃O₉]^6- rings. Owing to these structural characteristics, the ceramics exhibited low permittivity. With an increase in x, the relative permittivity (ε_r) values of the BaZr(Si_1-xGe_x)₃O₉ (0 ≤ x ≤ 1.0) ceramics increased from 7.68 (x = 0) to 9.45 (x = 1.0), while their quality factor (Q × f) values first increased and then decreased. The Q × f value (10,300 GHz at 13.43 GHz) of the BaZrSi₃O₉ (x = 0) ceramic improved with the substitution of Si⁴⁺ by Ge⁴⁺. A high Q × f value (36,100 GHz at 13.81 GHz) was obtained for the BaZr(Si_1-xGe_x)₃O₉ (x = 0.2) ceramic, and the Q × f values of the BaZr(Si_1-xGe_x)₃O₉ ceramics could be controlled by varying the Si/Ge-site bond valence. The temperature coefficient of resonance frequency (τ_f) values of the BaZr(Si_1-xGe_x)₃O₉ ceramics were mainly affected by the O2-site bond valence, and the optimum τ_f value (?22.8 ppm/°C) was achieved for the BaZrSi₃O₉ ceramic. The BaZr(Si_1-xGe_x)₃O₉ (x = 0.2) ceramic showed the optimum microwave dielectric properties (ε_r = 8.36, Q × f = 36,100 GHz at 13.81 GHz, and τ_f = ?34.5 ppm/°C).     

Study on phase structures and compositions,microstructures, and dielectric characteristics of (1-x)NdGaO3-xBi0.5Na0.5TiO3 microwave ceramic systems

(1–x)NdGaO₃-xBi_0.5Na_0.5TiO₃ [(1–x)NdGaO₃-xBNT, 0.05 ≤ x ≤ 0.7)] ceramic systems were fabricated using a conventional solid-state reaction, and their phase structures, microstructures, and microwave dielectric characteristics were systematically investigated. The XRD patterns results showed an orthorhombic perovskite structure as the main phase with a Pbnm space group at x = 0.05. In a range of x = 0.1–0.3, the main Nd₃Ga₅O₁₂ cubic structure phases (Ia-3d space group) had been formed due to the increase in the cation vacancies resulting from the Na and Bi ion volatilization at the higher densification sintering temperatures (1400–1450 °C). As (Na_0.5Bi_0.5)²⁺ ions increased and the sintering temperatures decreased, the orthorhombic perovskite-type Pnma space group solid solutions (the main phases) were detected at x = 0.4 and 0.5, accompanied with a certain amount of second phase Bi₂O₃ in the samples at x = 0.6 and 0.7. The results of Raman spectroscopy analysis, EDS data analysis, and surface morphologies observations were basically in keeping well with the XRD analysis results, wherein the Raman-active modes also implied that the crystal structure of the main phase actually had a tendency to change the perovskite structure when x = 0.3. The ε_r value increased gradually as the x value increased from 0.05 to 0.5 because of the increase in the ionic polarizability, and increased slowly at x = 0.6, then decreased slightly as the x value further increased to 0.7 due to the formation of the second phase. The Q × f values of the (1-x)NdGaO₃-xBNT (x = 0.05–0.7) ceramic systems were strongly influenced by the densification, lattice defects, and phase composition and content. The τ_f values could be well predicted by replacing Na, Bi and Ti ions with suitable substitutions for these ceramic systems. As a result, the new temperature-stable ceramics with optimal dielectric properties of ε_r ~43.1, Q × f ~6700 GHz (at 5.85 GHz), and τ_f ~ −24.5 ppm/°C and ε_r ~ 44.5, Q × f ~ 5600 GHz (6.12 GHz), and τ_f ~ +2.4 ppm/°C were obtained in the (1-x)NdGaO₃-xBNT composite series at x = 0.5 and x = 0.6 sintered at 1320 °C and 1250 °C for 4 h, respectively.     

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 structure evolution and lattice energy of spinel-structured Li(Mg0.5Ti0.5)xGa5−xO8 microwave dielectric ceramics

The microwave dielectric properties of spinel-structured Li(Mg_0.5Ti_0.5)_xGa_5−xO₈ (0 ≤ x ≤ 1) ceramics were researched together with their microstructures. The X-ray diffraction and Raman spectroscopic revealed that an ordered spinel structure in 1: 3 B-site ordering with space group P4₃32 was formed in the composition range of 0 = x ≤ 0.25, and a disordered spinel with space group Fd-3m was formed in 0.5 = x ≤ 1. All the ceramics were compact with uniform grain, clear grain boundaries and high relative density (ρ_relative ≥ 95 %). With the substitution of [Mg_0.5Ti_0.5]³⁺ for Ga³⁺ increased, the dielectric constant (ε_r) increased from 10.48 to 11.28, which was related to the increased molar ionic polarizability (α_theo/V_m) and B-site bond ionicity. The temperature coefficient of the resonant frequency (τ_f) slightly increased from −66.27 ppm/°C to −61.45 ppm/°C, due to the decrease of B-site bond valence. The Q × f value firstly decreased from 125,400 GHz to 50,381 GHz and then increased to 85,360 GHz, which was affected by the intrinsic loss analyzed by lattice energy. The optimal microwave dielectric properties were obtained for LiMg_0.5Ti_0.5Ga₄O₈ ceramic (x = 1) sintered at 1260 °C with ε_r = 11.28, Q × f = 85,360 GHz and τ_f = −61.45 ppm/°C.     

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.     

The effect of co-substitution on the microwave dielectric properties of Ba6-3xNd8+2xTi18O54(x=2/3) ceramics

In this work, the microwave dielectric properties of Ba₄(Nd_1-yBi_y)_28/3Ti_18-x(Al_1/2Ta_1/2)_xO₅₄(0≤x≤2, 0.05≤y≤0.2) ceramics co-substituted by A/B-site were studied. Firstly, (Al_1/2Ta_1/2)⁴⁺ was used for substitution at B-site. At 0≤x≤1.5, the above mentioned ceramic was found to exist in single-phase tungsten bronze structure, but at x = 2.0, the secondary phase appeared. Although the dielectric constant decreased by doping the (Al_1/2Ta_1/2)⁴⁺, but the quality factor was observed to improve by 40% and the temperature coefficient of resonant frequency decreased by 75%. Based on the above results, Bi³⁺ was introduced to Ba₄Nd_28/3Ti₁₇(Al_1/2Ta_1/2)O₅₄. The introduction of Bi³⁺ reduced the sintering temperature, greatly improved the dielectric constant, and ultimately decreased the temperature coefficient of resonant frequency, but it led to deterioration of quality factor. At last, with appropriate site-substitution content control (x = 1.0,y = 0.15), excellent comprehensive properties (ε_r = 89.0, Q × f = 5844 GHz @ 5.89 GHz，TCF = +8.7 ppm/°C) were obtained for the samples sintered at 1325 °C for 4 h.     

Microwave dielectric properties of Re3Ga5O12 (Re: Nd,Sm, Eu,Dy and Yb) ceramics and effect of TiO2 on the microwave dielectric properties of Sm3Ga5O12 ceramics

Re₃Ga₅O₁₂ (Re: Nd, Sm, Eu, Dy and Yb) garnet ceramics sintered at 1350–1500 °C had a high quality factor (Q × f) ranging from 40,000 to 192,173 GHz and a low dielectric constant (ɛ_r) of between 11.5 and 12.5. They also exhibited a relatively stable temperature coefficient of resonant frequency (τ_f) in the range of −33.7 to −12.4 ppm/°C. In order to tailor the τ_f value, TiO₂ was added to the Sm₃Ga₅O₁₂ ceramics, which exhibited good microwave dielectric properties. The relative density and grain size increased with addition of TiO₂, resulting in the enhancement of Q × f value. The τ_f increased with the addition of TiO₂. Excellent microwave dielectric properties of ɛ_r = 12.4, Q × f = 240,000 GHz and τ_f = −16.1 ppm/°C were obtained from the Sm₃Ga₅O₁₂ ceramics sintered at 1450 °C for 6 h with 1.0 mol% TiO₂. Therefore, Re₃Ga₅O₁₂ ceramics, especially TiO₂-added Sm₃Ga₅O₁₂ ceramics are good candidates for advanced substrate materials in microwave integrated circuits (MICs) applications.     

Phase composition,crystal structure,and microwave dielectric properties of Nb-doped and Y-deficient yttrium aluminum garnet ceramics

Nb-doped and Y-deficient yttrium aluminum garnet ceramics were designed and synthesized using the solid-state reaction method according to the chemical equation Y_3?xAl₅Nb_xO_12+x (0 ≤ x ≤ 0.16). The phase composition, sintering behavior, microstructure, and microwave dielectric properties were investigated as functions of the composition and sintering temperature. A single-phase solid solution of yttrium aluminum garnet structure formation was observed in the range of 0 ≤ x ≤ 0.1. Further increments in x prompted the precipitation of the YNbO₄ secondary phase at the grain boundary of Y₃Al₅O₁₂. The complexity of the phase composition degrades the micromorphology and dielectric properties of the ceramics to varying degrees. Transmission electron microscopy results show that the lattice exhibits additional symmetry, which is closely related to the ultrahigh Q×f values of the ceramics. Effectively improving the sintering behaviour and suppressing the secondary phase by simultaneously doping with Nb⁵⁺ and reducing the yttrium stoichiometry. Finally, excellent microwave dielectric properties of ε_r ~ 10.99, Q×f ~ 280,387 GHz (13.5 GHz), and τ_f ~ ? 34.7 ppm/°C can be obtained in x = 0.1 (Y_2.9Al₅Nb_0.1O_12.1) sintered at 1700 °C for 6 h.     

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.     

Fabrication of high-efficiency dielectric patch antennas from temperature-stable Sr3−xCaxV2O8 microwave dielectric ceramic

A novel high-efficiency dielectric patch antenna was fabricated using Sr_3-xCa_xV₂O₈ ceramics. A typically temperature-stable Sr_3-xCa_xV₂O₈ was achieved by tailoring the Ca²⁺ substitution to 30 mol% (x = 0.3), where well-balanced microwave dielectric properties were obtained (a near-zero value of +5.2 ppm/°C, a low ε_r ～ 13.4, and a moderate Q×f ～ 18,500 GHz). To manifest the application potentiality in wireless communication, a patch antenna was fabricated from the x = 0.3 ceramic based on the simulated result using the CST Microwave Studio software, and it showed a high simulated radiation efficiency (99.7%) and a gain (5.35 dBi) at 3.421 GHz. All results indicate that the Sr_3-xCa_xV₂O₈ ceramics have promising application potential for 5 G technology due to their prominent microwave dielectric properties, lightweight, and low cost.     

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.     

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.     

A/B-site cosubstituted Ba4Pr28/3Ti18O54 microwave dielectric ceramics with temperature stable and high Q in a wide range

High permittivity Ba₄(Pr_1-xSm_x)_28/3Ti_18-yAl_4y/3O₅₄(0.4≤x ≤ 0.7, 0≤y ≤ 1.5) ceramics were synthesized using a standard solid-state method. The effects of Sm³⁺ substitution into the A-site and Sm³⁺/Al³⁺ cosubstitution into the A/B-sites on the microstructure, crystal structure, Raman spectra, infrared reflectivity (IR) spectra and dielectric characteristics were investigated in a Ba₄Pr_28/3Ti₁₈O₅₄ solid solution. In the ceramic samples of Ba₄(Pr_1-xSm_x)_28/3Ti₁₈O₅₄(0.4≤x ≤ 0.7), Sm³⁺ partial substitution for Pr³⁺ could improve the quality factor (Qf) value and reduce the TCF value. Nevertheless, the quality factor (Qf~10,000GHz) needed further improvement and the TCF values (+12.3~+35.4 ppm/°C) were still too large. Therefore, Al³⁺ was introduced for further optimization of the TCF values and Qf values of the Ba₄(Pr_1-xSm_x)_28/3Ti₁₈O₅₄ ceramics. Sm/Al cosubstitution led to a good combination of high ε_r (ε_r ≥ 70), high Qf (Qf ≥ 12,000 GHz), and near-zero TCF (−10 < TCF < +10 ppm/°C) in a wide range (0.4≤x ≤ 0.7). Infrared reflectivity (IR) spectra indicated that A-TiO₆ vibration modes gave the primary contribution rather than Ti–O bending and stretching modes. The decrease in the degree of B-site cations order could be confirmed by Raman spectra. XPS results demonstrated that the improvement of quality factor (Qf) value was strongly related to the suppression of Ti³⁺. Excellent dielectric properties were achieved in Ba₄(Pr_1-xSm_x)_28/3Ti_18-yAl_4y/3O₅₄ microwave ceramics with x = 0.5 and y = 1.25: ε_r = 72.5, Qf = 13,900GHz, TCF = +1.3 ppm/°C.     

Low loss and good chemical compatibility with silver electrodes of Cu2+-substituted Ba3Ti4Nb4O21 ceramics realizing low temperature co-fired ceramics technology: Crystal structure,vibration modes and chemical bonds studies

New low loss and low-sintering temperature co-fired Ba_3-xCu_xTi₄Nb₄O₂₁ (BCTN, 0 ≤ x ≤ 0.12) ceramics with 0.60 wt% Li₂O-B₂O₃-SiO₂-CaO-Al₂O₃ (LBSCA) glass were prepared by solid-state reaction methodology. This work showed that CuO and LBSCA were effective sintering aid, which improved the densification and decreased sintering temperature. Thus, the excellent microwave dielectric properties of BCTN ceramics (x = 0.08) were obtained after sintering at 925 ℃ with ε_r ~ 44.18, Q×f ~ 17,860 GHz (@ 5.6 GHz) and τ_f ~ 94.76 ppm/℃. Q×f value was increased nearly 3-fold compared to pure BTN ceramics (~ 6090 GHz). Based on the P-V-L bond theory, the Ti-O and Nb-O bonds together contributed greatly to ε_r. The Nb-O bonds was the main factor affecting the internal loss on Q×f. The τ_f closely related to the oxygen octahedron [Ti1/Nb1O₆]. The BCTN ceramics would not react with Ag electrodes, and had great potential to be used in LTCC microwave devices.     

Cation distribution of high-performance Mn-substituted ZnGa2O4 microwave dielectric ceramics

In current study, only 5?mol% Mn²⁺ was applied to fabricate high performance microwave dielectric ZnGa₂O₄ ceramics, via a traditional solid state method. The crystal structure, cation distribution and microwave dielectric properties of as-fabricated Mn-substituted ZnGa₂O₄ ceramics were systematically investigated. Mn²⁺-substitution led to a continuous lattice expansion. Raman, EPR and crystal structure refinement analysis suggest that Mn²⁺ preferentially occupies the tetrahedral site and the compounds stay normal-spinel structure. The experimental and theoretical dielectric constant of Zn_1-xMn_xGa₂O₄ ceramics fit well. In all, this magnetic ion, Mn²⁺, could effectively adjust the τ_f value to near zero and double the quality factor from 85,824?GHz to 181,000?GHz of Zn_1-xMn_xGa₂O₄ ceramics at the meantime. Zn_1-xMn_xGa₂O₄ (x?=?0.05) ceramics sintered at 1400?°C for 2?h exhibited excellent microwave dielectric properties, with ε_r =?9.7(@9.85?GHz), Q?×?f?=?181,000?GHz, tanδ?=?5.44?×?10^?5,and τ_f =???12?ppm/°C.     

Effect of Ga3+ substitution on the microwave dielectric properties of 0.67CaTiO3–0.33LaAlO3 ceramics

0.67CaTiO₃–0.33La(Al_1−xGa_x)O₃ (0≤x≤0.4) (CTLAG) ceramics with pure perovskite structure were prepared by a conventional two-step solid-state reaction process. The effect of Ga³⁺ substitution for Al³⁺ on the microwave dielectric properties of the ceramics was subsequently investigated. As Ga content increased, the ionic polarizability increased and led to an increase of the dielectric constant (ε_r). Meanwhile, both the tolerance factor (t) of CTLAG ceramics and A-site bond valence were considered to have effect on the temperature coefficient of the resonant frequency (τ_f) with the increase of Ga content. Results also showed that the quality factor (Q×f ) varied with increasing Ga³⁺ content because of not only intrinsic factor but also extrinsic factors such as the bimodal grain size distribution, the variation of relative density, and the packing fraction. Excellent microwave dielectric properties with ε_r≈45.81, Q×f≈34,152 GHz, and τ_f≈3.09 ppm/°C were achieved for 0.67CaTiO₃–0.33La(Al_0.9Ga_0.1)O₃ ceramics sintered at 1420 °C for 4 h.     

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.     

Degree of inversion of A/B lattice sites and microwave/millimeter wave/terahertz dielectric properties of MgAl2-x(Zn0.5Mn0.5)xO4 ceramics

In this work, spinel-structured MgAl_2-x(Zn_0.5Mn_0.5)_xO₄ (0 ≤ x ≤ 0.08) single-phase ceramics were prepared through a solid-state reaction route. The substitution of (Zn_0.5Mn_0.5)³⁺ for Al³⁺ at the octahedral site affected the degree of inversion of A/B lattice sites, bond length/strength/valence, and covalency of metal-oxygen bond in the tetrahedron and hence microwave dielectric properties of MgAl₂O₄. The variation in ε_r and tanδ of ceramics is investigated in the millimeter wave-terahertz frequency band by combining infrared reflection spectrum and terahertz time-domain spectroscopy. A high Q×f value of 111,010 GHz @ 12.01 GHz, low ε_r = 8.3, and slightly lower τ_f = −60 ppm/°C is obtained for MgAl_1.98Zn_0.01Mn_0.01O₄ ceramics, which is tuned by adding a small amount of SrTiO₃. The composite ceramics exhibited a near-zero τ_f (2.8 ppm/°C), high Q×f (55,400 GHz @ 11.15 GHz), and low ε_r (= 8.5), showing a great potential application prospect for 5G/6G wireless communication.     

Tunability of τf in garnet-structured Y3Ga5O12 microwave dielectric ceramics

The τ_f of Y₃Ga₅O₁₂ ceramics was adjusted by Ca²⁺/Ti⁴⁺ co-doping for Y³⁺ at A-site and Ga³⁺ at C-site to form Ca_xY_3-xTi_xGa_5-xO₁₂ (0 ≤ x ≤ 2.0) ceramics by the solid-state reaction. When 0 ≤ x ≤ 1.7, all the samples remained as single garnet-structured with Ia-3d space group. The ε_r increased from 10.4 ± 0.1 to 15.9 ± 0.1, and τ_f optimized from −54.7 ± 1.0 to −8.3 ± 1.0 ppm/℃, attributing to the increase in polarizability per volume and “rattling” effect. Meanwhile, the Q × f slightly decreased from 98,200 ± 500 GHz to 65,300 ± 500 GHz due to the decrease in packing fraction. As x increased to 2.0, the properties dramatically deteriorated owing to the second phase CaTiO₃. Optimal properties were obtained at x = 1.7, with ε_r = 15.9 ± 0.1, Q × f = 65,300 ± 500 GHz and τ_f = −8.3 ± 1.0 ppm/℃.     

Modified Ba6−3xNd8+2xTi18O54 microwave dielectric ceramics

In order to improve the microwave dielectric properties of Ba_6−3xNd_8+2xTi₁₈O₅₄ solid solution ceramics, the effects of Bi₂O₃ and Bi₄Ti₃O₁₂ additives were determined. The results of SEM and EDS analyses suggested that the present ceramics with Bi₄Ti₃O₁₂ additives consisted of Ba_6−3xNd_8+2xTi₁₈O₅₄ solid solution matrix phase, and secondary phase of BaTi₄O₉, but this was not the situation of Bi₂O₃ added ceramics. XRD analysis also revealed that the unit cell volume of the matrix phase increased with increasing the amount of Bi₄Ti₃O₁₂ additive. With addition of Bi₄Ti₃O₁₂ into the present ceramics, the dielectric constant increased and the temperature coefficient of resonator frequency decreased, while the Qf value slightly decreased. The excellent microwave dielectric characteristics (ε=94·9, Qf=5620 GHz, τ_f=21·4 ppm/°C could be achieved in the present ceramics through the microstructure control.