Lattice structure and microwave dielectric properties of La[Al1−x(Mg0.5Ti0.5)x]O3 (x = 0-0.2)-based ceramics

La[Al_1−x(Mg_0.5Ti_0.5)_x]O₃ (LAMT, x = 0-0.2) ceramics were synthesized by the conventional solid-state reaction method and formed a solid solution. The pure solid solutions were recorded by X-ray diffraction (XRD) in every range. Relative permittivity (ε_r) and structural stability were greatly affected because the Al³⁺ site was replaced by [Mg_0.5Ti_0.5]³⁺. The total ionic polarizability gradually increased with x, and ε_r gradually increased. The trend of τ_f is due to the change in structural stability. The variation in Q × f value increased firstly and then decreased due to the change in the symmetric stretching mode of Al/MgTi–O. The optimum microwave dielectric properties of LAMT were obtained at x of 0.1 after sintering at 1650°C for 5 hours, and ε_r = 24.9, Q × f = 79 956 GHz, and τ_f = −33 ppm/°C. The CaTiO₃ have a large positive τ_f (+800 ppm/°C), thus, the τ_f achieved near zero when CaTiO₃ and LAMT (x = 0.1) ceramics were mixed with a certain molar mass, and the optimum microwave dielectric properties of 0.65CaTiO₃–0.35LaAl_0.9(Mg_0.5Ti_0.5)_0.1O₃ were as follows: ε_r = 44.6, Q × f = 32 057 GHz, and τ_f = +2 ppm/°C.     

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.     

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.     

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.     

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 formation and microwave dielectric properties of BiMVO5 (M = Ca,Mg) ceramics potential for low temperature co-fired ceramics application

Two low-firing BiMVO₅ (M = Ca, Mg) ceramics were prepared in the sintering temperature range of 760-850°C. Their differences in phase formation, sintering behavior, and dielectric performances were investigated. BiCaVO₅ formed a single phase with an orthorhombic structure, while BiMgVO₅ crystallized in a monoclinic structure that needs longer dwelling time to obtain single phase. The optimized microwave dielectric properties were obtained with ε_r = 15.70, Q × f = 55 000 GHz (at 10.6 GHz), and τ_f = −71 ppm/°C for BiCaVO₅, ε_r = 18.55, Q × f = 86 860 GHz (at 9.63 GHz), and τ_f = −65 ppm/°C for BiMgVO₅. In addition, the large negative τ_f values of BiMVO₅ (M = Ca, Mg) ceramics were successfully adjusted by forming composite ceramics with CaTiO₃ and near-zero τ_f values of +2 ppm/°C and −3 ppm/°C were obtained in 0.92BiCaVO₅-0.08CaTiO3 and 0.94BiMgVO₅-0.06CaTiO₃, respectively. Both ceramics exhibited good chemical compatibility with Ag electrode. The results demonstrate BiMVO₅ (M = Ca, Mg) ceramics to be attractive candidates in LTCC technology.     

Structure evolution of wolframite-type ZnZrC2O8 (C = Nb,Ta) ceramics in relation to microwave dielectric properties

The relationship among the sintering behavior, crystal structure, chemical bonding properties, and dielectric properties of wolframite-type ZnZr(Nb_1−xTa_x)₂O₈ (0.0 ≤ x ≤ 1.0) ceramics was investigated with the progressive replacement of Nb⁵⁺ by Ta⁵⁺. The optimum sintering temperature increases from 1225 to 1375°C with increasing Ta⁵⁺ content. The ε_r value falls from 27.34 to 22.34 due to a gradual decrease in bond ionicity and a shift in the Raman vibration modes toward higher wave numbers. The Q × f increases from 63 604 GHz (@6.71 GHz) to 115 631 GHz (@7.89 GHz), which is since the increase in the total lattice of chemical bonds. Moreover, the reduction in grain boundary area and the gradual lowering of the full width at half maximum of the Raman vibration modes contribute to the reduction in dielectric losses. First-principles calculations illustrate that the growth in bandgap and electron cloud density in the internal space of the [Zn/ZrO₆] octahedron leads to a reduction in dielectric loss. Furthermore, the reduced degree of oxygen octahedral distortion causes a change in τ_f from −46.56 to −37.40 ppm/°C.     

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.     

Temperature stable dielectric properties of Mg2TiO4–MgTiO3–CaTiO3 Ceramics Over a Wide Temperature range

The zero resonant frequency temperature coefficient (τ_f) of microwave dielectric ceramics (MWDCs) at high and low temperature have attracted great attention in the development of microwave communication equipment. In this work, the Mg₂TiO₄–MgTiO₃–CaTiO₃ (MMC) ceramics with meeting the application requirements of 5G communication were prepared by traditional solid-phase sintering after investigating the relationship among phase compositions of xMg₂TiO₄-(0.931-x)MgTiO₃-0.069CaTiO₃ and 0.34Mg₂TiO₄-0.591MgTiO₃-yCaTiO₃, sintering process, and dielectric properties in detail. The results show that the dielectric properties of MMC ceramics are strongly affected by the phase relative contents of MgTiO₃, Mg₂TiO₄ and CaTiO₃. For instance, MMC ceramics with approximate τ_f = 0 is contributed by mutual compensation of Mg₂TiO₄ and MgTiO₃, in which the Mg₂TiO₄ phase plays an important role in decreasing the τ_f value; and the increase of CaTiO₃ will greatly increase the ε_r value for MMC ceramics, while has a negative effect in the Q × f value. After three-phase regulation, the 0.32Mg₂TiO₄-0.611MgTiO₃-0.069CaTiO₃ microwave dielectric ceramic has a better dielectric temperature stability, associated with dielectric properties of ε_r = 19.7, Q × f = 55,400 GHz (at 8.43 GHz), τ_f- = 4.5 ppm/°C (?40 °C–25 °C), and τ_f+ = ?5.1 ppm/°C (25 °C–90 °C).     

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.     

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.     

Temperature stable K0.5(Nd1−xBix)0.5MoO4 microwave dielectrics ceramics with ultra‐low sintering temperature

K_0.5(Nd_1?xBi_x)_0.5MoO₄ (0.2 ≤ x ≤ 0.7) ceramics were prepared via the solid‐state reaction method. All ceramics densified below 720°C with a uniform microstructure. As x increased from 0.2 to 0.7, relative permittivity (?_r) increased from 13.6 to 26.2 commensurate with an increase in temperature coefficient of resonant frequency (TCF) from – 31 ppm/°C to + 60 ppm/°C and a decrease in Qf value (Q = quality factor; f = resonant frequency) from 23 400 to 8620 GHz. Optimum TCF was obtained for x = 0.3 (?15 ppm/°C) and 0.4 (+4 ppm/°C) sintered at 660 and 620°C with ?_r ~15.4, Q_f ~19 650 GHz, and ?_r ~17.3, Q_f ~13 050 GHz, respectively. Ceramics in this novel solid solution are a candidate for ultra low temperature co‐fired ceramic (ULTCC) technology.     

Preparation and microwave dielectric properties of BaLi1+xF3+x (x = 0–0.01) ceramics

BaLi_1+xF_3+x (x = 0–0.01) were successfully mechanosynthesized by a simple ball-milling process. The effects of excessive LiF and sintering method and/or annealing atmosphere on its sintering behavior, microstructure, and microwave dielectric properties have been investigated in this paper. The mechanosynthesized powder can be densified with relative densities of ∼95 % after sintering at 750–800 °C/2 h in N₂. The obtained ceramics exhibit excellent optimized microwave dielectric properties with ε_r of ∼11.46 ± 0.06, Q×f values of 83175 ± 1839 GHz and τ_f of ∼ − 70 ± 3 ppm/°C at the x = 0.006 composition. Its Q×f value could be improved to 94603 ± 2037 GHz) by post-annealing in N₂ after post annealing at 700 °C/2 h. The Q×f value could be further improved to (120,098 ± 2344 GHz) by hot-pressed sintering (HPS). Sintering in the ambient atmosphere or O₂ leads to lower Q×f values than those of the counterparts sintered in N₂ due to the introduction of F-vacancies by oxidation_, while little variation in ε_r andτ_f.     

Improvement of microwave dielectric properties of the LiNb0.6Ti0.5O3 M-phase by doping with (Co1/3Nb2/3)4+

Structural evolution and microwave dielectric properties of LiNb_0.6(Ti_1-x[Co_1/3Nb_2/3]_x)_0.5O₃ (.05≤x≤.2) ceramics have been studied in this paper. Although the doped compositions maintain the M-phase solid solutions, compositional fluctuation due to nonuniform dispersion of minor dopants could be observed as x < .05, and trace amount of Li₂TiO₃-based solid solution (Li₂TiO₃ss) secondary phase presents in the x > .05 compositions. The microwave dielectric properties could be remarkably improved by the doping of (Co_1/2Nb_1/2)⁴⁺ in comparison to the undoped counterpart. Optimized microwave dielectric properties with Q × f = ∼6500 GHz, ε_r = ∼74 and τ_f = +8.2 ppm/°C could be obtained at x = .10 after sintering at 1050°C/2 h. The sintering temperature could be further reduced to 900°C/2 h by adding .2 wt% B₂O₃ without affecting significantly its microwave dielectric properties: ε_r = 73, Q × f = 6000 GHz, τ_f = +8.5 ppm/°C. The LiNb_0.6(Ti_1-x[Co_1/3Nb_2/3]_x)_0.5O₃ ceramics obtained in this case exhibit large dielectric permittivity coupled with much improved Q × f values, near zero τ_f, and low sintering temperature simultaneously, which makes it a promising high-k microwave dielectric material for low temperature cofired ceramic applications.     

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

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

Ultra-low fired fluoride composite microwave dielectric ceramics and their application for BaCuSi2O6-based LTCC

A total of 14 fluoride composite ceramics were prepared through solid-state method and their microwave dielectric properties were investigated. Among the fluoride composite ceramics, 0.36LiF–0.39MgF₂–0.25SrF₂ (LMS) had the lowest sintering temperature (600°C) and presented a dielectric constant (ε_r) of 6.24 ± 0.05, a quality factor (Q × f) of 33 274 ± 900 GHz, and a temperature coefficient resonant frequency (τ_f) of −86.74 ± 8 ppm/°C. As the LMS ceramic had a low melting point (646°C), it can be used as sintering aid for LTCC applications. The sintering temperature of BaCuSi₂O₆ decreased from 1050°C to 875°C with 2 wt% LMS doped and excellent microwave dielectric properties of ε_r = 8.16 ± 0.04, Q × f = 24 351 ± 300 GHz, and τ_f = −9.74 ± 1 ppm/°C were obtained. Moreover, BaCuSi₂O₆-2 wt% LMS can be co-fired with Ag powders, which makes it a potential new candidate for LTCC applications.     

A novel low permittivity microwave dielectric ceramic Sr2Ga2SiO7 for application in patch antenna

The dielectric properties of a Ga-based melilite type ceramic Sr₂Ga₂SiO₇ via theoretical prediction based on far-infrared spectroscopy and experimental measurement by the Hakki–Coleman method were studied in this work. Dense and single-phase ceramics were fabricated via solid-state reaction at 1330°C and exhibited comprehensive microwave dielectric properties (ε_r ∼ 7.6, Q × f ∼ 23 600 GHz, and τ_f ∼ −35.2 ppm/°C) at 14.3 GHz. Chemical modifications were proposed to adjust the thermal stability and reduce the densification temperature. By adding 10 mol% CaTiO₃, the negative τ_f can be compensated to a near-zero value of −3.8 ppm/°C. The densification temperature was reduced to 940°C by adding 3 wt.% LiF. A patch antenna was designed using Sr₂Ga₂SiO₇ ceramic with a high radiation efficiency of 99.1% and a gain of 2.788 dBi at the center frequency of 4.371 GHz. All results indicate that the Sr₂Ga₂SiO₇ ceramic has promising application potential for 5G wireless communication technology.     

Phase evolution and microwave dielectric properties of novel LiAl5−xZnxO8−0.5x-based (0 ≤ x ≤ 0.5) ceramics

Novel LiAl_5−xZn_xO_8−0.5x microwave dielectric ceramics were synthesized through a solid-state reaction route. Phase evolution of LiAl_5−xZn_xO_8−0.5x was determined by XRD analysis. The XRD results indicated that the phase compositions had a P4₃32 space group when 0 ≤ x ≤ 0.2 and a spinel structure when 0.3 ≤ x ≤ 0.5. The dielectric constant (ε_r) of this series’ solid solutions decreased with the increase in Zn doping content, which was in good agreement with the Clausius-Mossotti relation. Oxygen vacancy and the decreased degree of order degraded the quality factor (Q × f) of the two structures. The deterioration in quality factor was further verified by impedance spectroscopy. The temperature coefficient of the resonant frequency (τ_f) decreased with the increase in x and was correlated with the unit cell volume. Finally, CaTiO₃ was used as a compensation material to obtain a near-zero τ_f of the LiAl₅O₈ ceramic.     

Effects of ion polarizability and oxygen vacancy on microwave dielectric properties of fluorite-structured Ce1−xCaxO2−x

X-ray diffractometer with Rietveld refinement and Raman spectroscopy were used for phase analysis. Ca doping results in decreased ε_r (24.09–21.52), Q × f (68 914–40 110 GHz), and τ_f (−50.0 ppm/°C to −60.2 ppm/°C) all decrease, which obviously does not conform to the mutual constraint relationship among the three parameters of microwave dielectric properties. The ε_r of Ce_1−xCa_xO_2−x ceramics is affected by the ion polarizability and the Ce rattling and valence states, among which the rattling of Ce cations plays a dominant role. The presence of oxygen vacancies in Ce_1−xCa_xO_2−x ceramics explains the decrease in Q × f. The τ_ε, ε_r, and α_L are responsible for the decrease in τ_f, especially for the τ_αm-3α_L values.     

MSO4 (M = Ca,Sr, Ba) microwave dielectric ceramics with low dielectric constant

MSO₄ (M = Ca, Sr, Ba) ceramics with relative densities exceeding 96% were prepared by solid-state sintering at low sintering temperatures of 625–875°C, and their structures and microwave dielectric properties at 10–19 GHz were characterized. MSO₄ ceramics crystallized in orthorhombic symmetry with the space groups of Amma for CaSO₄ and Pnma for SrSO₄ and BaSO₄. The optimal microwave dielectric properties were obtained with ε_r = 5.85, Qf = 57 000 GHz, τ_f,W = −98.8 ppm/°C for CaSO₄, ε_r = 10.95, Qf = 15 500 GHz, τ_f,W = 101.6 ppm/°C for SrSO₄, and ε_r = 9.42, Qf = 38 200 GHz, τ_f,W = −4.7 ppm/°C for BaSO₄. The increased ε_r and τ_f,W while decreased Qf value in the order of CaSO₄, BaSO₄, and SrSO₄ were attributed to the enhanced rattling effect of M²⁺. Besides, the temperature dependence of τ_f was weak for CaSO₄ and BaSO₄, whereas much stronger for SrSO₄. As most low-ε_r microwave dielectric ceramics are of large negative τ_f, the near-zero τ_f of BaSO₄ and positive τ_f of SrSO₄ are rare, indicating they are potential candidates for millimeter-wave communication as a temperature-stable dielectric ceramic and a compensator for tuning negative τ_f to near-zero, respectively.