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.     

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.     

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.     

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.     

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.     

Ge-doped Li3+xMg2Nb1-xGexO6 ceramics with enhanced low loss and high temperature stability properties

New high-performance materials have attracted much attention due to ever-increasing demands for advanced communication technologies. In present work, Ge-doped Li_3+xMg₂Nb_1-xGe_xO₆ (0 ≤ x ≤ 0.08) ceramics are prepared via solid-state reaction route. Microstructural analysis and crystal structure refinement reveal that moderate substitution can promote grain growth and modify crystal structure, thus enhancing microwave dielectric properties of composites. In that sense, special attention is paid to the behavior of dielectric constant ε_r, quality factor Q×f, and frequency temperature coefficient τ_f of final products. In these systems, ε_r parameter depends on the density, miscellaneous phases, and polarizability; Q×f value is shown to be influenced by Nb-O bond energy, grain size, and bulk density; finally, τ_f characteristic refers to Nb-O bond valence and NbO₆ octahedral distortion. Among above ceramics, Li_3.02Mg₂Nb_0.98Ge_0.02O₆ composite sintered at 1250 °C exhibits outstanding microwave absorption performance with ε_r = 15.32, Q×f = 969 88 GHz, and τ_f = ?8.25 ppm/°C.     

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.     

Impedance spectroscopy,B-site cation ordering and structure–property relations of (1 − x) La[Al0.9(Mg0.5Ti0.5)0.1]O3–x CaTiO3 ceramics for 5G dielectric waveguide filters

Dense (1 ? x) La[Al_0.9(Mg_0.5Ti_0.5)_0.1]O₃–x CaTiO₃ ceramics were synthesized via solid-state reaction. The crystal structure and microwave dielectric properties of the ceramics were systematically investigated. Rietveld refinement revealed that when x ≤ 0.2, the ceramics had a rhombohedral structure with an R-3c space group. When x ≥ 0.5, the ceramics had an orthorhombic structure with a Pbnm space group. Selected area electron diffraction and Raman spectroscopy analyses proved that the microwave dielectric ceramics had a B-site order, which accounted for the great improvement in microwave dielectric properties. The content of oxygen vacancies was identified through X-ray photoelectron spectroscopy, and the change rule of Q × f was closely related to oxygen vacancy content. The perturbation of A-site cations had an important influence on dielectric constant. Specifically, with the increase in Ti⁴⁺ content, the perturbation effect of the A-site cations was enhanced and dielectric constant increased. When x = 0.65, the temperature coefficient of resonant frequency of the (1 ? x) La[Al_0.9(Mg_0.5Ti_0.5)_0.1]O₃–x CaTiO₃ microwave dielectric ceramics was near zero. The optimal microwave dielectric properties of 0.35LaAl_0.9(Mg_0.5Ti_0.5)_0.1O₃–0.65CaTiO₃ were ε_r = 44.6, Q × f = 32,057 GHz, and τ_f = +2 ppm/°C.     

Optical and Microwave Dielectric Properties of Zn‐Doped MgAl2O4 Transparent Ceramics Fabricated by Spark Plasma Sintering

(Mg_1?xZn_x)Al₂O₄ transparent ceramics were fabricated by spark plasma sintering technique at 1325°C for 10 min. A small mount of Zn²⁺ addition to MgAl₂O₄ ceramics was very effective to the performance improvement, while further increase in Zn‐doped content would give rise to the optical transmittance deterioration. The optical and microwave dielectric properties of MgAl₂O₄ transparent ceramics were improved by Zn substitution for Mg. The in‐line transmittance of the (Mg_1?xZn_x)Al₂O₄ (x = 0.02) ceramics can be as high as 70% at λ = 550 nm and 86.5% at λ = 2000 nm, respectively. The dielectric constant ε_r of (Mg_1?xZn_x)Al₂O₄ just varied from 8.32 to 8.54, however, the Q × f value increased significantly up to a maximal value of 66,000 GHz at x = 0.02. Moreover, the τ_f of (Mg_1?xZn_x)Al₂O₄ transparent ceramics changed from ?74 to ?65.5 ppm/°C. With the increasing of Zn‐doped content, the average grain size and the porosity increased, which was the primary reason for the change in optical and microwave dielectric properties.     

Crystal structure,microwave dielectric properties and low temperature sintering of (Al0.5Nb0.5)4+ co-substitution for Ti4+ of LiNb0.6Ti0.5O3 ceramics

The phase composition, microstructure, microwave dielectric properties of (Al_0.5Nb_0.5)⁴⁺ co-substitution for Ti site in LiNb_0.6Ti_0.5O₃ ceramics and the low temperature sintering behaviors of Li₂O-B₂O₃-SiO₂ (LBS) glass were systematically discussed. XRD patterns and EDS analysis result confirmed that single phase of Li_1.075Nb_0.625Ti_0.45O₃ solid solution was formed in all component. The increase of dielectric constant (ε_r) is ascribed to the improvement of bulk density. The restricted growth of grain has a negative influence on quality factor (Q×f) value. The τ_f value could be continuously shifted to near zero as the doping content increases. Great microwave dielectric properties were obtained in LiNb_0.6Ti_(0.5-x)(Al_0.5Nb_0.5)_xO₃ ceramics (x?=?0.10) when sintered at 1100?℃ for 2?h: ε_r =?70.34, Q×f =?5144?GHz, τ_f =?4.8?ppm/℃. The sintering aid, LBS glass, can effectively reduce the temperature and remain satisfied microwave performance. Excellent microwave dielectric properties for x?=?0.10 were obtained with 1.0?wt% glass: ε_r =?70.16, Q×f =?4153?GHz (at 4?GHz), τ_f =?-0.65?ppm/℃ when sintered at 925?℃ for 2?h.     

High quality factor microwave dielectric ceramics in the (Mg1/3Nb2/3)O2–ZrO2–TiO2 ternary system

Novel high quality factor microwave dielectric ceramics (1?x)ZrTiO₄?x(Mg_1/3Nb_2/3)TiO₄ (0.325≤x≤0.4) and (ZrTi)_1?y(Mg_1/3Nb_2/3)_yO₄ (0.2≤y≤0.5) with the addition of 0.5 wt% MnCO₃ in the (Mg_1/3Nb_2/3)O₂–ZrO₂–TiO₂ ternary system were prepared, using solid‐state reaction method. The relationship between the structure and microwave dielectric properties of the ceramics was studied. The XRD patterns of the sintered samples reveal the main phase belonged to α‐PbO₂‐type structure. Raman spectroscopy and infrared reflectivity (IR) spectra were employed to evaluate phonon modes of ceramics. The 0.65ZrTiO₄?0.35(Mg_1/3Nb_2/3)TiO₄?0.5 wt% MnCO₃ ceramic can be well densified at 1240°C for 2 hours and exhibits good microwave dielectric properties with a relative permittivity (ε_r) of 42.5, a quality factor (Q×f) value of 43 520 GHz (at 5.9 Ghz) and temperature coefficient of resonant frequency (τ_f) value of ?5ppm/°C. Furthermore, the (ZrTi)_0.7(Mg_1/3Nb_2/3)_0.3O₄?0.5 wt% MnCO₃ ceramic sintered at 1260°C for 2 hours possesses a ε_r of 31.8, a Q×f value of 35 640 GHz (at 6.3 GHz) and a near zero τ_f value of ?5.9 ppm/°C. The results demonstrated that the (Mg_1/3Nb_2/3)O₂–ZrO₂–TiO₂ ternary system with excellent properties was a promising material for microwave electronic device applications.     

Sintering behavior and microwave dielectric properties of a new low-permittivity ceramic system Ca(Mg1−xAlx)(Si1−x/2Alx/2)2O6

The diopside ceramics with a formula of Ca(Mg_1−xAl_x)(Si_1−x/2Al_x/2)₂O₆ (x=0.01–0.3) were synthesized via a traditional solid-state reaction method, and their solid solubility, sintering behavior and microwave dielectric properties were investigated. The results revealed that the solubility limit of Al₂O₃ in Ca(Mg_1−xAl_x)(Si_1−x/2Al_x/2)₂O₆, which is defined as x, was between 0.15 and 0.2, and a second phase of CaAl₂SiO₆ presented when the x value reached 0.2. Appropriate Al³⁺ substitution for Mg²⁺ and Si⁴⁺ could promote the sintering process and lower the densification temperature, and a broadened densification temperature range of 1250–1300 °C was obtained for the compositions of x=0.08–0.15. With the increase of the x value, the dielectric constant (ε_r) increased roughly linearly, and the temperature coefficient of frequency (τ_f) showed a rising trend. The Q×f values increased from 57,322 GHz to 59,772 GHz as the x value increased from 0.01 to 0.08, and then they were saturated in the range of x=0.08–0.2. Further increase of the x value (x≥0.25) deteriorated the microwave dielectric properties. Good microwave dielectric properties of ε_r=7.89, Q×f=59,772 GHz and τ_f=−42.12 ppm/°C were obtained for the ceramics with the composition of x=0.08 sintered at 1275 °C.     

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.     

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.     

Structure and microwave dielectric properties of 1:1 ordered Nd[(Mg1-xZnx)1/2Ti1/2]O3 complex perovskite ceramics

Nd[(Mg_1-xZn_x)_1/2Ti_1/2]O₃ perovskite ceramics (x = 0, 0.2, 0.4, 0.6, 0.8) are prepared by the solid-state reaction method. The effects of Zn²⁺ substitution on the structure, microstructure, especially the B-site 1:1 cation ordering and microwave dielectric properties have been investigated. Sintered Nd[(Mg_1-xZn_x)_1/2Ti_1/2]O₃ ceramics all adopt dense microstructure, along with increased dimensional uniformity as Zn²⁺ substitution. All the ceramics are confirmed to have B-site 1:1 ordered monoclinic perovskite structure with P2₁/n space group. Atomic mass difference of B-site elements might be an important factor affecting the B-site 1:1 cation ordering. HRSTEM observation suggest that the doped Zn²⁺ cations have roughly entered the Mg²⁺ sites to promote 1:1 cation ordering. The degree of the 1:1 cation ordering can be negatively reflected by the full width at half maximum (FWHM) of F_2g(B) mode at 372 cm^?1 in Raman spectra. With Zn²⁺ doping, the degree of the 1:1 cation ordering first increases then decreases, and reaches its maximum at x = 0.6. Meanwhile the best combination of microwave dielectric properties is obtained, as ε_r = 31.4, Q × f = 74,000 GHz, τ_f = ?44 ppm/°C. It is found that the long-range ordering not only decreases the dielectric loss but also affects the dielectric constant, providing a theoretical foundation to understand further the correlation between ionic configuration and microwave dielectric properties.     

Crystal structure and performance modification of a novel triclinic CaMgP2O7 microwave dielectric ceramic with low sintering temperature

In this study, crystal structure and microwave dielectric properties of phosphate CaMgP₂O₇ were comprehensively investigated. As a novel microwave dielectric ceramic, CaMgP₂O₇ consists of highly dense structure with optimal microwave dielectric properties (ε_r = 7.8 ± 0.124, Q×f = 13,165 ± 836 GHz, and τ_f = −85.04 ± 1.205 ppm/℃) at a low sintering temperature (950 ℃). The Rietveld refinement of XRD patterns revealed that CaMgP₂O₇ belongs to a triclinic system with P-1 symmetry type. Moreover, the substitution of Zn²⁺ for Mg²⁺ in CaMgP₂O₇ can further reduce the sintering temperature, effectively promote the densification process, and improve the Q×f value. The effects of porosity (or density) and chemical bond characteristics on the performance of CaMg_1-xZn_xP₂O₇ ceramics were carefully analyzed as well. Outstanding performance (ε_r = 8.05 ± 0.12, Q×f = 20,670 ± 923 GHz, and τ_f = −87.59 ± 3.24 ppm/℃) can be achieved for the CaMg_0.84Zn_0.16P₂O₇ ceramic sintered at 875 ℃ for 3 h.     

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.     

Effect of Mn2+ doping on the lattice and the microwave dielectric properties of MgTa2O6 ceramics

A series of Mn²⁺-doped Mg_1-xMn_xTa₂O₆ (x = 0.02, 0.04, 0.06, 0.08, 0.10, 0.12) ceramics were synthesized by solid-state reaction method. The influence of introducing Mn–O bonds as a partial replacement for Mg–O bonds on the lattice and microwave dielectric properties was systematically investigated. XRD and Rietveld refinement confirm that Mn²⁺ occupies the 2a Wyckoff position and forms a pure trirutile phase. Moreover, based on the chemical bond theory, the dielectric constant is mainly affected by the ionicity of the Ta–O bond. The lattice and dielectric properties remain relatively stable with Mn²⁺ doping below 0.1, but excessive Mn²⁺ doping leads to pronounced distortion of the lattice, which is not beneficial for lattice stability and microwave dielectric properties. Introducing an appropriate amount of Mn–O bonds with high bond dissociation energy facilitates MgO6 octahedron stability, which improves the thermal stability of the lattice. Accordingly, the microwave dielectric properties for 0.06 Mn²⁺-doped MgTa₂O₆ ceramics were determined: ε_r = 28, Q × f = 105,000 GHz (at 7.5 GHz), τ_f = 19.5 ppm/^°C.     

Effect of Ni substitution on the microwave dielectric properties of cordierite

Cordierite (Mg₂Al₄Si₅O₁₈) is one of the silicates with low dielectric constant (ɛ_r), which are expected for good candidate of millimeter wave dielectrics. Microwave dielectric properties of Ni substituted cordierite solid solutions; (Mg_1−xNi_x)₂Al₄Si₅O₁₈ have been investigated. (Mg_1−xNi_x)₂Al₄Si₅O₁₈ with no secondary phase was obtained in the compositions x range from 0 to 0.1. It was found that a very small amount of Ni substitution was effectively increased of the quality factor (Qf) value and the highest Qf value of 99,110 GHz was obtained in the composition x = 0.1. No remarkable composition dependence of the temperature coefficient of resonance frequency (τ_f) was observed in the range from x = 0.05 to 0.2, On the other hand, τ_f abruptly shifted toward negative value with increasing x from 0.3 to 0.5. The correlation between the crystal structure of cordierite and the microwave dielectric properties, particularly Qf is discussed.     

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.