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.     

Li2+xZrO3Fx (0 ≤ x ≤1.25): A new high-Q × f and temperature-stable microwave dielectric ceramic system for LTCC applications

Here we report a new high-Q × f and temperature-stable oxyfluoride microwave dielectric ceramic system of Li_2+xZrO₃F_x (0 ≤ x ≤ 1.25) sintered at low temperatures (i.e., ≤950°C). The sintering characteristics, phase assemblage, microstructures, and microwave dielectric properties (MDPs) of the Li_2+xZrO₃F_x specimens were studied. The chemical bond theory calculations were employed to analyze the compositional dependence of the MDPs in the Li_2+xZrO₃F_x ceramic system. The optimal MDPs of ε_r = 15.8, Q × f = 65 100 GHz, and τ_f = 1.2 ppm/°C were achieved in the sample with x = 1. Besides, the ceramic samples exhibited high chemical compatibility with Ag electrodes. Therefore, the novel ceramic system of Li_2+xZrO₃F_x is confirmed to be a promising candidate as a low-temperature co-fired ceramic material.     

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.     

Microwave dielectric properties of temperature-stable zircon-type (Bi,Ce)VO4 solid solution ceramics

In the (Bi_1 − xCe_x)VO₄ (0 ≤ x ≤ 1) system, we found that the (Bi_1 − xCe_x)VO₄ (0 ≤ x ≤ 0.1) belongs to the monoclinic scheelite phase and the (Bi_1 − xCe_x)VO₄ (0.7 ≤ x ≤ 1) belongs to the tetragonal zircon phase, while the (Bi_1 − xCe_x)VO₄ (0.1 < x < 0.7) belongs to the mixed phases of both monoclinic scheelite and tetragonal zircon structure. Interestingly, two components with near-zero temperature coefficient of resonant frequency (TCF) appeared in this system. In our previous work, a near-zero TCF of ~+15 ppm/°C was obtained in a (Bi_0.75Ce_0.25)VO₄ ceramic with a permittivity (ε_r) of ~47.9 and a Qf (Q = quality factor = 1/dielectric loss; f = resonant frequency) value of ~18 000 GHz (at 7.6 GHz). Furthermore, in the present work, another temperature-stable microwave dielectric ceramic was obtained in (Bi_0.05Ce_0.95)VO₄ composition sintered at 950°C and exhibits good microwave dielectric properties with a ε_r of ~11.9, a Qf of ~22 360 GHz (at 10.6 GHz), and a near-zero TCF of ~+6.6 ppm/°C. The results indicate that this system might be an interesting candidate for microwave device 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.     

Low‐Temperature Sintering and Microwave Dielectric Properties of CaMoO4‐Based Temperature Stable LTCC Material

The CaMoO₄–xY₂O₃–xLi₂O ceramics were prepared by the solid‐state reaction method. The sintering behavior, phase evolution, microstructure, and microwave dielectric properties were investigated. CaMoO₄ solid solution was obtained when x = 0.030, and two‐phase system including tetragonal CaMoO₄ phase and cubic Y₂O₃ phase formed when 0.066 ≤ x ≤ 1.417. A temperature stable CaMoO₄‐based microwave dielectric ceramic with ultralow sintering temperature (775°C) was obtained in the CaMoO₄–xY₂O₃–xLi₂O system when x = 0.306, which showed good microwave dielectric properties with a low permittivity of 9.5, a high Q_f value of 63 240 GHz, and a near‐zero temperature coefficient of resonant frequency of +7.2 ppm/°C.     

Enhanced Curie temperature and superior temperature stability by site selected doping in BCZT based lead-free ceramics

In this work, Bi³⁺ doped Ba_0.98-3x/2Bi_xCa_0.02Zr_0.02Ti_0.976Cu_0.008O₃ [0 ≤ x ≤ 0.03] lead free ceramics, to be employed for structural, dielectric and ferroelectric studies, have been synthesized via conventional solid state reaction method. Rietveld refinement of the X-ray diffraction (XRD) data evidences the existence of a pure perovskite phase with tetragonal symmetry for all ceramics. The Scanning Electron Microscopy (SEM) reveals that the grain size, which is 16.14 μm for x = 0 reduced to 2.11 μm for x = 0.03. Dielectric studies demonstrate excellent dielectric behavior with high Curie temperature (T_C ～159 °C), high dielectric constant (ε_r ～834, ε_max ～ 3146), and a low dielectric loss (tanδ ～ 0.019), for an optimum value of x = 0.02. The analysis of temperature coefficient of the dielectric permittivity indicates the applicability of these materials in multilayer ceramic capacitors. Impedance studies, conducted to understand the underlying physical mechanisms, are found to be in good agreement with the results of structural and dielectric studies. Furthermore, the ferroelectric measurement confirms the ferroelectric nature for all samples with an energy storage efficiency (η) of ～42% for x = 0.02 composition.     

Thermal stability improvement of dielectric properties and non-ohmic characteristic of CaCu3+xTi4O12 ceramics via a Cu-nonstoichiometric approach

In this work, the effects of Cu composition on the thermal stability of the dielectric and nonlinear properties of CaCu_3+xTi₄O₁₂ (?0.2 ≤ x ≤ 0.2) ceramics obtained via a polymer-pyrolysis chemical process were studied. The mean grain sizes of Cu-stoichiometric (x = 0), Cu-deficient (x < 0) and Cu-excess (x > 0) CaCu_3+xTi₄O₁₂ ceramics were found to be ~3.2, ~3.4 and ~3.7 μm, respectively. Interestingly, very good dielectric properties (0.020 ≤ tanδ ≤ 0.038 and 4000 ≤ ε′ ≤ 7065) were attained in CaCu_3+xTi₄O₁₂ (?0.2 ≤ x ≤ 0.1, excluding x = 0.2) ceramics. Moreover, the variation of dielectric constant (ε′) within a limit of ±15% (Δε′_{± 15%}) over a wide temperature range (T_R) of ?70 – 220 °C with low tanδ < 0.05 (tanδ_<0.05) over a T_R of ?70 to 80 °C were achieved in a CaCu_2.8Ti₄O₁₂ ceramic. These results suggest that this ceramic could be applicable for X9R capacitors and energy storage devices that require high thermal stability. Additionally, the nonlinear properties of Cu-nonstoichiometric ceramics could be improved when compared with those of the Cu-stoichiometric material. The incremental changes of dielectric and nonlinear properties of CaCu_3+xTi₄O₁₂ (?0.2 ≤ x ≤ 0.2) ceramics revealed the significant role of Cu composition on grain boundary resistance (R_gb), which was confirmed by impedance spectroscopy analysis. In addition, XANES results revealed the proper ratios of Cu⁺:Cu²⁺ and Ti³⁺:Ti⁴⁺ found in these ceramics, indicating the semiconducting behavior of these grains.     

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.     

5G array antenna substrate for electromagnetic beam splitting via cobalt-substituted zinc molybdate low temperature co-fired ceramics

A novel Zn_1-xCo_xMoO₄ (ZCMO) (x = 0.03) ceramic with low-dielectric constant, low-loss, and low-sintering temperature for 5G electromagnetic beam splitting is developed by solid-state reaction method. This ceramic exhibits excellent microwave dielectric properties with ε_r = 8.0, Q × f = 57682 GHz, τ_f = ? 54.9 ppm/°C at a sintering temperature of 825 °C. An array antenna for electromagnetic beam splitting at 5.4 GHz is designed by using this ceramic for the first time. The highest efficiency and the best electromagnetic beam splitting effect can be jointly controlled by the dielectric constant and dielectric loss of the ceramic. The normalized reflection amplitude of each array unit cell is above 98 %, and the reflection phase covers 360°. The function of electromagnetic beam splitting is verified by the far-field pattern of the electric field. This work helps to promote the development of LTCC and broaden the application scope of microwave dielectric ceramics.     

Temperature-stable Na0.5Bi0.5TiO3-based ceramics with favorable low-temperature dielectric and energy storage property

In this work, (1 − x)(0.94Na_0.5Bi_0.5TiO₃–0.06BaTiO₃)–xKTaO₃ (x = 0–0.30) ceramics are developed for dielectric capacitor applications. The introduction of KTaO₃ from x = 0 to 0.30 increases the tolerance factor t from 0.984 to 1.005 and causes the decrease of ferroelectric rhombohedral phase in the ceramics. Besides, a gradual structural change toward a higher symmetry can be detected, accompanied by the obvious domain refinement. In the aspect of electrical property, the strengthened dielectric relaxation leads to the greatly enhanced thermal stability of dielectric response. The decline in T_s from 98 to −96°C causes a significant widening of the low-temperature region with temperature-stable dielectric constant ε_r and low dielectric loss tan δ. The x = 0.30 ceramic shows a high ε_r (25°C) of 1094 with the temperature coefficient of capacitance ≤±15% over −70 to 200°C, which exceeds the X9R standard. Meanwhile, tan δ is less than 0.02 in a wide temperature range of −35 to 300°C. In addition, the ultrafine grain size of 290 nm, large bandgap of 3.22 eV, and high resistance of the x = 0.30 ceramic contribute to its electrical breakdown strength. A linear-like P–E loop with the large discharged energy density W_D ∼ 3.50 J/cm³ and high energy efficiency η ∼ 90.1% is obtained under 28 kV/mm at room temperature. The thermal stability of the energy storage performance is also satisfactory with the variation of W_D less than 15% over −40 to 200°C, and the η is higher than 85%.     

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.     

Enhanced Multiferroic and Magnetocapacitive Properties of (1 − x)Ba0.7Ca0.3TiO3–xBiFeO3 Ceramics

The structures, Curie temperature, dielectric relaxor behaviors, ferroelectricity, ferromagnetism, and magnetocapacitance of the (1?x)Ba_0.70Ca_0.30TiO₃–xBiFeO₃ [(1?x)BCT–xBF, x = 0–0.90] solid solutions have been systematically investigated. The ceramics have coexisted tetragonal (T) and orthorhombic (O) phases when x ≤ 0.06, coexisted pseudocubic (PC) and O phases when x = 0.065, coexisted cubic and O phases when 0.07 ≤ x ≤ 0.12, PC phase when 0.21 ≤ x ≤ 0.42, coexisted T and rhombohedral (R) phases when 0.52 ≤ x ≤ 0.70, and R phase when x ≥ 0.75. Significantly, composition‐dependent microstructures and Curie temperature are observed, the average grain size increases from 1.9 μm for x = 0, reaches 12.0 μm for x = 0.67, and then decreases to 1.3 μm for x = 0.90. At room temperature, the ceramics with x = 0.42–0.70 show piezoelectric properties and multiferroic behaviors, characterized by the polarization‐electric field, polarization current intensity–electric field, and magnetization–magnetic field curves, the composition with x = 0.67 has maximum polarization, remnant polarization, maximum magnetization, and remnant magnetization of 15.0 μC/cm², 9.1 μC/cm², 0.33 emu/g, and 0.14 emu/g, respectively. In addition, the magnetocapacitance is evidenced by the increased relative dielectric constant with increasing the applied magnetic field (H). With ΔH = 8 kOe, the composition with x = 0.67 shows the largest values of (ε_r(H) ? ε_r(0))/ε_r(0) = 2.96% at room temperature. The structure–property relationship is discussed intensively.     

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

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

Nickel-modified zinc molybdate low temperature co-fired ceramics for two-dimensional beam splitting of array antenna in X-band

In this work, a new Zn_1-xNi_xMoO₄ (ZNMO) (x = 0.03) ceramic with low-dielectric constant, low-loss, and low-sintering temperature for X-band two-dimensional (2D) beam splitting is developed by solid-state reaction method. This ceramic has excellent microwave dielectric properties of ε_r = 8.5, Q × f = 28192 GHz, τ_f = ?60.2 ppm/°C. The effects of Ni²⁺ substitution on the microwave dielectric properties of the ZnMoO₄ ceramic are studied in detail through crystal structure analysis, Raman spectroscopy, and first-principles calculations. For the first time, an array antenna for X-band 2D electromagnetic beam splitting is designed by using this ceramic as a substrate. The effects of the dielectric constant and dielectric loss on the radiation efficiency of the array antenna are revealed. The normalized reflection amplitude and reflection phase of the unit cell exceed 0.97 and cover 360°, respectively. The function of 2D electromagnetic beam splitting is verified by the overall far-field pattern of the array antenna. This work has the opportunity to promote the development of LTCC and microwave dielectric ceramics.     

A novel low-permittivity LiAl0.98(Zn0.5Si0.5)0.02O2-based microwave dielectric ceramics for LTCC application

Phase composition, morphology, and microwave dielectric properties of (1−x) LiAl_0.98(Zn_0.5Si_0.5)_0.02O₂ + x CaTiO₃ (0.05 ≤ x ≤ 0.20) materials synthesized via the solid state reaction method were investigated. All these densified materials were obtained at a sintering temperature of 1150°C. All compositions showed a major LiAlO₂ phase that was accompanied by a minor CaTiO₃ phase. The ε_r value increased gradually from 10.88 to 11.60, whereas the Q × f value remarkably decreased from 33 251 GHz to 13 511 GHz. The τ_f value changes from −85 ppm/°C to 212 ppm/°C, thereby indicating that CaTiO₃ could effectively adjust this value. HBO₃-doping was used to further decrease the sintering temperature to 900°C. The optimum value was obtained at 7 wt.% HBO₃ doped with microwave dielectric properties of ε_r = 9.39, Q × f = 10 224 GHz, and τ_f = −7.8 ppm/°C. This material also exhibited chemical compatibility with silver, making it a candidate for low temperature co-fired ceramics applications.     

Modification of NdNbO4 microwave dielectric ceramic by Bi substitutions

In the present work, Bi³⁺ was used to substitute for Nd³⁺ in the NdNbO₄ ceramic and pure fergusonite solid solution was formed within 20 mol. % substitutions. Microwave dielectric permittivity of the (Nd_1-xBi_x)NbO₄ (x ≤ 0.2) ceramics increased linearly with x value due to the larger ionic polarizability of Bi³⁺ than Nd³⁺. Excellent microwave dielectric properties with a permittivity (ε_r) ~22.5, a Qf (Q = quality factor, f = resonant frequency) ~50 000 GHz, and a TCF ~−9 ppm/°C were obtained in the (Nd_0.9Bi_0.1)NbO₄ ceramic. This method might work in other fergusonite-type rare-earth ortho-niobates.     

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.     

Effect of excess lithium on the structure and microwave dielectric properties of LiMg0.5Ti0.5O2 ceramics

Herein, Li_1+xMg_0.5Ti_0.5O₂ (0.000 ≤ x ≤ 0.075) ceramics with excess lithium were prepared by solid-state reaction method to compensate for lithium volatilization, thus optimizing dielectric properties. The second phase Mg₂TiO₄ decreases with excess lithium content and eventually disappears at x = 0.05. Of these, Li_1.0375Mg_0.5Ti_0.5O₂ illustrates the best dielectric properties with ε_r = 16.58, Q × f = 120712 GHz (@9.18 GHz), and τ_f = −16.31 ppm/°C. From the point of view of non-intrinsic factors, excess lithium at optimum ratios slightly facilitates the sintering of the ceramics, improves densification, and increases the grain size, resulting in improved dielectric properties. For intrinsic factors, a slight increase in bond ionicity results in a slight improvement in ε_r. First-principles calculations demonstrate that suitable excess lithium increases electron cloud density in the internal space of Li/Mg/TiO₆ octahedron and increases band gap, thus optimizing the dielectric properties. The same results were obtained from the perspective of lattice vibrations. The modified Li_1.0375Mg_0.5Ti_0.5O₂ ceramics offer great potential for future microwave communication technology.