Sr(Ga0.5Nb0.5)1−xTixO3 Low‐Loss Microwave Dielectric Ceramics with Medium Dielectric Constant

The microwave dielectric properties of Sr(Ga_0.5Nb_0.5)_1?xTi_xO₃ (x = 0, 0.1, 0.2 and 0.3) ceramics have been investigated together with their microstructures. Single‐phase solid solutions are achieved in this series of ceramics. The ordering features are comprehensively analyzed by transmission electron microscopy and Raman spectroscopy. Local 1:1 ordering in B‐site leads to a double‐cubic structure with space group , while Ti substitution disrupts this 1:1 ordering between Ga and Nb, and the metastable ordering between Ti and (Ga + Nb) is speculated to form due to their large size difference. The dielectric constant and temperature coefficient of resonant frequency increase nonlinearly as x increases, while the Qf value decreases gradually. The variation trend of Qf value is mainly attributed to the intrinsic loss because of the increasing vibrational anharmonicity by Ti substitution. The ordering transition from short coherence, long‐range ordering to short‐range ordering with increasing Ti content has an agreeable and weak effect on the Qf value. The best combination of microwave dielectric properties is achieved for the composition of x = 0.3: ε_r = 46.6, Qf = 42 200 GHz and τ_f = 5.0 ppm/°C.     

Synthesis and microwave dielectric properties of new high quality Mg2NdNbO6 ceramics

New high‐quality microwave dielectric ceramics Mg₂NdNbO₆ were prepared by conventional solid‐state sintering method. The phases, micro‐structures and microwave dielectric properties of Mg₂NdNbO₆ ceramics were investigated at sintering temperature in the range of 1275°C‐1400°C. The X‐ray diffraction patterns showed that the peaks of the compounds were attributed to two phases, including the main crystalline phase of NdNbO₄ that was indexed as the monoclinic phase and MgO as the second phase. Well‐developed microstructures of Mg₂NdNbO₆ ceramics can be achieved, and the grain size reached the maximum value (1.63 μm) at 1375°C. As the sintering temperature increased, the dielectric constant, temperature coefficient of resonant frequency and apparent density remained almost unchanged, however, the significant change in the quality factor was observed. At 1375°C, Mg₂NdNbO₆ ceramics possessed excellent microwave dielectric properties: ε_r = 16.22, Q × f = 116 000 GHz and τ_f = ?30.96 ppm/°C.     

Srn+1TinO3n+1 (n=1, 2) microwave dielectric ceramics with medium dielectric constant and ultra‐low dielectric loss

Sr_n+1Ti_nO_3n+1 (n=1, 2) ceramics with tetragonal Ruddlesden–Popper structure were prepared via a standard solid‐state reaction process, and their microstructures and microwave dielectric properties were investigated systematically. The phase composition, grain morphology, and densification behavior were explored using X‐ray diffraction (XRD) and scanning electron microscopy (SEM). Outstanding microwave dielectric properties were achieved in the present ceramics: ε_r=42, Q × f=145 200 GHz, τ_f=130 ppm/°C for Sr₂TiO₄, and ε_r=63, Q × f=84 000 GHz, τ_f=293 ppm/°C for Sr₃Ti₂O₇, respectively. The present ceramics might be expected as excellent candidates for next‐generation medium‐permittivity microwave dielectric ceramics after the further optimization of τ_f value.     

Structure dependence of microwave dielectric properties in Zn2-xSiO4-x-xCuO ceramics

In this work, the Zn_2-xSiO_4-x-xCuO (x = 0, 0.04, 0.08, 0.12, 0.16 and 0.20) ceramics were synthesized through solid state reaction. The dependence of microwave dielectric properties on the structure was investigated through X-ray diffraction (XRD) with Rietveld refinements, Scanning electron microscope (SEM) and Raman spectra. The melting of CuO can reduce the densification temperature of Zn_2-xSiO_4-x ceramics. In comparison with x = 0, the x = 0.08 ceramics were densified at 1150℃ and the excellent microwave dielectric properties with low dielectric constant (ε_r = 6.01), high quality factor (Qf = 105 500 GHz) and τ_f = ?28 ppm/°C, were obtained. The ε_r, Qf and τ_f value are dominated by covalency of Si-O bond and secondary phase, crystallinity and lattice energy, respectively. This provides a theoretical basis to further adjust the microwave dielectric property (especially τ_f value) from the structural point of view.     

Effects of TiO2 additive on ultra-low-loss MgO–LiF microwave dielectric ceramics

MgO ceramics have good microwave dielectric properties, but the high sintering temperatures limit its application. The effects of TiO₂ additive on the phase composition and microwave dielectric properties of MgO ceramics with 4mol%LiF were investigated by solid state reaction method. TiO₂ and MgO form Mg₂TiO₄ in a magnesium-rich environment with 4mol%LiF at about 900 °C, which as a solid solution or second phase had a huge impact on MgO ceramic with 4mol % LiF. When the content of TiO₂ less than 2mol %, Mg₂TiO₄ as a solid solution in MgO ceramics, which made the grain of MgO larger. When the content of TiO₂ more than 2mol %, Mg₂TiO₄ as a second phase in MgO ceramics, which made the microwave dielectric properties of MgO ceramics bad. Typically, the MgO-4mol%LiF-0.5mol%TiO₂ ceramic sintered at 1075 °C for 6 h acquired the best dielectric properties: ε_r = 9.7, Qf = 617,000 GHz and τ_f = −59.49 ppm/°C.     

Fundamental mechanisms responsible for the temperature coefficient of resonant frequency in microwave dielectric ceramics

The temperature coefficient of resonant frequency (τ_f) of a microwave resonator is determined by three materials parameters according to the following equation: τ_f=?(½ τ_ε + ½ τ_μ + α_L), where α_L, τ_ε, and τ_μ are defined as the linear temperature coefficients of the lattice constant, dielectric constant, and magnetic permeability, respectively. We have experimentally determined each of these parameters for Ba(Zn_1/3Ta_2/3)O₃, 0.8 at.% Ni‐doped Ba(Zn_1/3Ta_2/3)O₃, and Ba(Ni_1/3Ta_2/3)O₃ ceramics. These results, in combination with density functional theory calculations, have allowed us to develop a much improved understanding of the fundamental physical mechanisms responsible for the temperature coefficient of resonant frequency, τ_f.     

Ultra-high quality factor and low dielectric constant of (Zn0.5Ti0.5)3+ co-substituted MgAl2O4 ceramic

In this study, MgAl₂O₄-based ceramics with high quality factor (Qf) and low dielectric constant (ε_r ≤ 10) were obtained by fabricating MgAl_2-x(Zn_0.5Ti_0.5)_xO₄ (x = 0–0.5) ceramics via conventional solid-state reaction method. Excellent microwave dielectric properties were achieved for samples at x = 0.5 and sintered at 1550 °C, i.e., ε_r = 9.86, Qf = 263 900 GHz (five times better than that for x = 0 sample) and τ_f = ?92 ppm/°C. The X-ray diffraction (XRD) patterns displayed characteristic peaks of MgAl₂O₄ with spinel structure. MgTi₂O₅ and MgTiO₃ were considered as secondary phases. Scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS) and relative density analysis indicated that ultra-high Qf values were dominated by dense microstructure, secondary phase and cation vacancies; whereas ε_r values were mainly affected by secondary phase and ionic polarizability. MgAl_2-x(Zn_0.5Ti_0.5)_xO₄ ceramics with excellent microwave dielectric properties have potential application in millimeter-wave communication, dielectric filters, dielectric antennas and resonators.     

Effect of Ordering on the Microwave Dielectric Properties of Spinel‐Structured (Zn1−x(Li2/3Ti1/3)x)2TiO4 Ceramics

Spinel‐structured (Zn_1?x(Li_2/3Ti_1/3)_x)₂TiO₄ (x = 0–1) microwave dielectric ceramics were manufactured via a conventional mixed‐oxide method. The X‐ray diffraction and Raman spectra revealed that a disordered face‐centered cubic phase was found in the composition range of x < 0.5, and an ordered primitive cubic spinel solid solution was achieved as x was beyond 0.5. Such a disorder–order transition near x = 0.5 was accompanied by the variation of composition‐induced cation occupancy. The Q × f value first kept increasing up to ~160 000 (GHz) in disordered ceramics, and then sharply decreased as an ordered structure appeared at x ≥ 0.5. An obvious decrease in τ_f value was also accompanied by the appearance of an ordered structure. The minimum τ_f value (~ ?20 ppm/°C) was obtained in the x = 0.75 sample with the highest structural order degree. These results demonstrated that microwave dielectric properties of current spinel ceramics could be successfully modified by adjusting their structural order degree, which could be appropriately adopted for the design of spinel‐structured materials with favorable properties.     

Effects of Mg Substitution on Order/disorder Transition,Microstructure, and Microwave Dielectric Characteristics of Ba((Co0.6Zn0.4)1/3Nb2/3)O3 Complex Perovskite Ceramics

Effects of Mg substitution on order/disorder transition, microstructure, and microwave dielectric characteristics of Ba((Co_0.6Zn_0.4)_1/3Nb_2/3)O₃ complex perovskite ceramics have been investigated. The ordered complex perovskite solid solutions are obtained in Ba((Co_0.6?x/2Zn_0.4?x/2Mg_x)_1/3Nb_2/3)O₃ ceramics (x = 0, 0.1, 0.2, and 0.3), and the ordering degree in the as‐sintered dense ceramics increases with increasing Mg‐substitution amount. The significantly improved Qf value is obtained in the present ceramics with increasing x, whereas the dielectric constant decreases slightly together with some increase of temperature coefficient of resonant frequency. The best combination of microwave dielectric characteristics is obtained in the composition of x = 0.3: ε_r = 33.7, Qf = 93 800 GHz, and τ_f = 9.6 ppm/°C. In the Mg‐substituted compositions, clear domain boundaries are obtained and the domain size increases as x increases, the highest Qf value is obtained when the domain size is about 40–60 nm in the ceramics with x = 0.3. The increased ordering degree and the fine ordering domain structure are considered to primarily contribute to the significant increase of Qf value in the Mg‐substituted Ba((Co_0.6Zn_0.4)_1/3Nb_2/3)O₃ complex perovskite ceramics.     

Temperature-Stable and Low-Loss Dielectrics in the Ca(B'1/2Ta1/2)O3 [B'=Lanthanides, Y, and In] System

The Ca(B′_1/2Ta_1/2)O₃ [B′=La, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Y, Er, Yb, and In] double perovskite-type ceramics have been prepared by the conventional solid-state ceramic route. The phase purity and surface morphology of the sintered ceramics have been studied by X-ray diffraction (XRD) and scanning electron microscopy methods. XRD study revealed intersubstitution between Ca and B′ ions. Ca(B′_1/2Ta_1/2)O₃ ceramics have relative permittivity (ɛ_r) in the range 23–30, normalized quality factor (Q_u×f) 20 600–59 200 GHz, and temperature coefficient of resonant frequency (τ_f) −6 to −35 ppm/°C. The dielectric properties of Ca(B′_1/2Ta_1/2)O₃ ceramics have been tailored by the addition of positive τ_f materials such as CaTiO₃, TiO₂, Ba(Y_1/2Ta_1/2)O₃, and Ba(Yb_1/2Ta_1/2)O₃, which form a solid solution or a mixture phase with the parent compound. The 0.7Ca(Y_1/2Ta_1/2)O₃–0.3Ba(Y_1/2Ta_1/2)O₃ ceramic has ɛ_r=27.5, Q_u×f=41 900 GHz, and τ_f=−1.2 ppm/°C, and the 0.6Ca(Yb_1/2Ta_1/2)O₃–0.4Ba(Yb_1/2Ta_1/2)O₃ ceramic has ɛ_r=27.7, Q_u×f=48 000 GHz, and τ_f=1.8 ppm/°C.     

Sintering behavior,structural phase transition,and microwave dielectric properties of La1‐xZnxTiNbO6‐x/2 ceramics

La_1‐xZn_xTiNbO_6‐x/2 (LZTN‐x) ceramics were prepared via a conventional solid‐state reaction route. The phase, microstructure, sintering behavior, and microwave dielectric properties have been systematically studied. The substitution of a small amount of Zn²⁺ for La³⁺ was found to effectively promote the sintering process of LTN ceramics. The corresponding sintering mechanism was believed to result from the formation of the lattice distortion and oxygen vacancies by means of comparative studies on La‐deficient LTN ceramics and 0.5 mol% ZnO added LTN ceramics (LTN+0.005ZnO). The resultant microwave dielectric properties of LTN ceramics were closely correlated with the sample density, compositions, and especially with the phase structure at room temperature which depended on the orthorhombic‐monoclinic phase transition temperature and the sintering temperature. A single orthorhombic LZTN‐0.03 ceramic sintered at 1200°C was achieved with good microwave dielectric properties of ε_r~63, Q×f~9600 GHz (@4.77 GHz) and τ_f ~105 ppm/°C. By comparison, a relatively high Q × f~80995 GHz (@7.40 GHz) together with ε_r~23, and τ_f ~?56 ppm/°C was obtained in monoclinic LTN+0.005ZnO ceramics sintered at 1350°C.     

Structure,Microwave Dielectric Properties,and Low‐Temperature Sintering of Acceptor/Donor Codoped Li2Ti1−x(Al0.5Nb0.5)xO3 Ceramics

The structure, microwave dielectric properties, and low‐temperature sintering behavior of acceptor/donor codoped Li₂TiO₃ ceramics [Li₂Ti_1?x(Al_0.5Nb_0.5)_xO₃, x = 0–0.3] were investigated systematically. The x‐ray diffraction confirmed that a single‐phase solid solution remained within 0 < x ≤ 0.2 and secondary phases started to appear as x > 0.2, accompanied by an order–disorder phase transition in the whole range. Scanning electron microscopy observation indicated that the complex substitution of Al³⁺ and Nb⁵⁺ produced a significant effect on the microstructural morphology. Both microcrack healing and grain growth contributed to the obviously enhanced Q×f values. By comparison, the decrease of ε_r and τ_f values was ascribed to the ionic polarizability and the cell volume, respectively. Excellent microwave dielectric properties of ε_r ~ 21.2, Q×f ~ 181 800 GHz and τ_f  ~ 12.8 ppm/°C were achieved in the x = 0.15 sample when sintered at 1150°C. After 1.5 mol% BaCu(B₂O₅) additive was introduced, it could be well sintered at 950°C and exhibited good microwave dielectric properties of ε_r ~ 20.4, Q×f ~ 53 290 GHz and τ_f ~ 3.6 ppm/°C as well. The cofiring test of the low‐sintering sample with Ag powder proved its good chemical stability during high temperature, which enables it to be a promising middle‐permittivity candidate material for the applications of low‐temperature cofired ceramics.     

Microwave Dielectric Properties of Aluminum‐Substituted Ba6−3xNd8+2xTi18O54 Ceramics

According to solid‐state reaction routine, microwave dielectric ceramics of aluminum‐supplanted Ba_6?3xNd_8+2xTi₁₈O₅₄ (0.5 ≤ x ≤ 0.75) ceramics were synthesized and the effects of composition on microwave dielectric properties were determined. As x value increasing from 0.5 to 0.75, with high‐quality factor values (Q × f > 8000 GHz), both dielectric constant (ε_r) and temperature coefficient of resonant frequency (τ_f) dropped. The X‐ray diffraction patterns showed a single phase for all compositions. Typically, the research gained temperature coefficients at resonant frequency around + 10 ppm/°C, while kept high relative permittivity and quality factor value.     

Structure and Microwave Dielectric Characteristics of Ca[(Ga1/2Nb1/2)1−xTix]O3 Ceramics

The microwave dielectric characteristics of Ca[(Ga_1/2Nb_1/2)_1?xTi_x]O₃ ceramics were investigated together with the structure evolution. The excellent microwave dielectric characteristics were achieved by forming solid solution between Ca(Ga_1/2Nb_1/2)O₃ and CaTiO₃ in the present ceramics. The solid solutions in space group Pbnm with antiphase and inphase tilting were determined for all compositions where minor secondary phase was detected for x = 0–0.47, whereas no B‐site ordering was detected. Owing to the structural modification, the dielectric constant (ε_r) increased with increasing x, and the temperature coefficient of resonant frequency (τ_f) could be tuned from negative to positive, while the decrease of Qf value was acceptable. The best combination of microwave dielectric properties was obtained at x = 0.47: ε_r = 51.6, Qf = 34 100 GHz and τ_f = ?0.3 ppm/°C.     

Low‐Temperature Sinterable (1−x)Ba3(VO4)2–xLiMg0.9Zn0.1PO4 Microwave Dielectric Ceramics

The novel low‐temperature sinterable (1 ? x)Ba₃(VO₄)₂–xLiMg_0.9Zn_0.1PO₄ microwave dielectric ceramics were prepared by cofiring the mixtures of pure‐phase Ba₃(VO₄)₂ and LiMg_0.9Zn_0.1PO₄. The phase structure and grain morphology of the ceramics were evaluated using X‐ray diffraction, Raman spectra, and scanning electron microscopy. The results indicated that Ba₃(VO₄)₂ and LiMg_0.9Zn_0.1PO₄ phases can well coexist in the sintered body. Nevertheless, a small amount of LiZnPO₄ and some vanadate phases with low melting points were observed, which not only can influence the microwave dielectric properties of the ceramic but also can obviously improve the densification behavior at a relatively low sintering temperature. The near‐zero temperature coefficients of the resonant frequency (τ_f) could be achieved by adjusting the relative content of the two phases owing to their opposite τ_f values and simultaneously a desirable quality factor Q × f value can be maintained. No chemical reaction between the matrix ceramic phase and Ag took place after sintering at 800°C for 4 h. The ceramics with 45 vol% LiMg_0.9Zn_0.1PO₄ can be well sintered at only 800°C and exhibit excellent microwave dielectric properties of ε_r ~ 10, Q × f ~ 64 500 GHz, and τ_f ~ ?2.1 ppm/°C, thus showing a great potential as a low‐permittivity low‐temperature cofired microwave dielectric material.     

Microwave dielectric properties of SnO‐SnF2‐P2O5 glass and its composite with alumina for ULTCC applications

Ultralow‐temperature sinterable alumina‐45SnF₂:25SnO:30P₂O₅ glass (Al₂O₃‐SSP glass) composite has been developed for microelectronic applications. The 45SnF₂:25SnO:30P₂O₅ glass prepared by melt quenching from 450°C has a low T_g of about 93°C. The SSP glass has ε_r and tanδ of 20 and 0.007, respectively, at 1 MHz. In the microwave frequency range, it has ε_r=16 and Q_u × f=990 GHz with τ_f=?290 ppm/°C at 6.2 GHz with coefficient of thermal expansion (CTE) value of 17.8 ppm/°C. A 30 wt.% Al₂O₃ ‐ 70 wt.% SSP composite was prepared by sintering at different temperatures from 150°C to 400°C. The crystalline phases and dielectric properties vary with sintering temperature. The alumina‐SSP composite sintered at 200°C has ε_r=5.41 with a tanδ of 0.01 (1 MHz) and at microwave frequencies it has ε_r=5.20 at 11 GHz with Q_u × f=5500 GHz with temperature coefficient of resonant frequency (τ_f)=?18 ppm/°C. The CTE and room‐temperature thermal conductivity of the composite sintered at 200°C are 8.7 ppm/°C and 0.47 W/m/K, respectively. The new composite has a low sintering temperature and is a possible candidate for ultralow‐temperature cofired ceramics applications.     

Pseudo Phase Diagram and Microwave Dielectric Properties of Li2O–MgO–TiO2 Ternary System

Li₂O–MgO–TiO₂ ternary system is an important microwave dielectric ceramic material with excellent properties and prospect in both scientific research and application. A phase diagram of the Li₂O–MgO–TiO₂ ternary system was established in this article, based on earlier research results and our present work. Microwave dielectric properties with compositions in different regions of the phase diagram have been analyzed. We found that the 0.33 Li₂MgTi₃O₈–0.67 Li₂TiO₃ ceramics sintered at 1200°C exhibited excellent dielectric properties: Q × f value = 80 476 GHz (at 7.681 GHz), ε_r = 24.7, τ_f = +3.2 ppm/°C. We also designed two ceramic systems in the Li‐rich region of the Li₂O–MgO–TiO₂ ternary system, which received little attention in the past decades, because many excellent single‐phase ceramics, such as Li₂MgTiO₄, Li₂MgTi₃O₈ and MgTiO₃, have been found in the Ti‐rich region. The ceramic systems have low sintering temperatures but also relatively poor dielectric properties.     

A novel temperature-stable Ba2-xCaxMgTi5O13 microwave dielectric ceramic

The Ba_2-xCa_xMgTi₅O₁₃ (0 ≤ x ≤ 0.3) microwave dielectric ceramics were for the first time prepared via a conventional solid-state reaction method. A small amount of Ca²⁺ can dissolve into the lattice by forming solid solutions with a monoclinic structure (C2/m) and further influence the sintering behavior, grain growth and microwave dielectric properties of Ba_2-xCa_xMgTi₅O₁₃ ceramics. Both increase of ε_r and decrease of Qxf with x should be associated with increased lattice distortion and uneven grain growth although the sample density and the ratio of the ionic polarizability to the molar volume show little variation. Moreover, the A-site bond valence and τ_f indicate a close relation in current study, such that the Ca²⁺substitution can induce an increase of τ_f values. The optimum microwave dielectric properties of ε_r ∼ 29.3, Qxf ∼ 30,870 GHz (6.5 GHz), and a near-zero τ_f ∼ +2.1 ppm/°C can be contained in the x = 0.15 ceramic sintered at 1160 °C.     

SrLa(R0.5Ti0.5)O4 (R=Mg,Zn) microwave dielectric ceramics with complex K2NiF4‐type layered perovskite structure

Dense SrLa(R_0.5Ti_0.5)O₄ (R=Mg, Zn) ceramics were prepared by a standard solid‐state reaction method. The single phase with complex K₂NiF₄‐type layered perovskite structure and I4/mmm space group was revealed by XRD, and the refined structure was analyzed by Rietveld analysis. Significantly improved dielectric constant was obtained in SrLa(R_0.5Ti_0.5)O₄ ceramics compared to the analogues SrLaAlO₄ and SrLaGaO₄, which is attributed to the increasing normalized bond lengths of Sr/La‐O(1) and Sr/La‐O(2a) bonds and the higher polarizability of (R_0.5Ti_0.5)³⁺ than Al³⁺ and Ga³⁺. In addition, τ_f converts to a positive value with the increase in dielectric constant. The following microwave dielectric properties were obtained in the dense ceramics: ε_r=25.5, Qf=72 000 GHz, τ_f=29 ppm/°C for SrLa(Mg_0.5Ti_0.5)O₄, and ε_r=29.4, Qf=34 000 GHz, τ_f=38 ppm/°C for SrLa(Zn_0.5Ti_0.5)O_4. Furthermore, the stability of K₂NiF₄‐type structure in MLnBO₄ [M=Ca, Sr, Ba; Ln=Y, Sm, Nd, La; B=Al, Ga, (Mg_0.5Ti_0.5), (Zn_0.5Ti_0.5)] compounds was discussed in relation to the tolerance factor of perovskite layer and the radius ratio of M²⁺ and Ln³⁺, based on which near‐zero τ_f values are expected to be obtained in SrLa(R_0.5Ti_0.5)O₄‐SrLaAlO₄ and SrLa(R_0.5Ti_0.5)O₄–SrLaGaO₄ unlimited solid solutions.     

Microstructural and Microwave Dielectric Properties of Bi12GeO20 and Bi2O3‐Deficient Bi12GeO20 Ceramics

Bi₁₂GeO₂₀ ceramics sintered at 800°C had dense microstructures, with an average grain size of 1.5 μm, a relative permittivity (ε_r) of 36.97, temperature coefficient of resonance frequency (τ_f) of ?32.803 ppm/°C, and quality factor (Q × f) of 3137 GHz. The Bi_12‐xGeO_20‐1.5x ceramics were well sintered at both 800°C and 825°C, with average grain sizes exceeding 100 μm for x ≤ 1.0. However, the grain size decreased for x > 1.0 because of the Bi₄Ge₃O₁₂ secondary phase that formed at the grain boundaries. Bi_12‐xGeO_20‐1.5x (x ≤ 1.0) ceramics showed increased Q × f values of >10 000 GHz, although the ε_r and τ_f values were similar to those of Bi₁₂GeO₂₀ ceramics. The increased Q × f value resulted from the increased grain size. In particular, the Bi_11.6GeO_19.4 ceramic sintered at 825°C for 3 h showed good microwave dielectric properties of ε_r = 37.81, τ_f = ?33.839 ppm/°C, and Q × f = 14 455 GHz.