Microwave Dielectric Ceramics Research Articles

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

A novel Zn1-xCoxMoO4 (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 εr = 8.0, Q × f = 57682 GHz, τf = − 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.

Improved Co-substituted zinc vanadate ceramics based on LTCC for enhanced polarization converters

A novel low-loss and low-sintering temperature Zn3-xCox(VO4)2 (ZCVO) (x = 0.25) ceramic for enhanced polarization converters is developed by solid-state reaction methodology. This ceramic has excellent microwave dielectric properties (εr = 13.6, Q×f = 32173 GHz (@10.1 GHz), τf = −74.5 ppm/℃) and good chemical compatibility with Ag. According to microstructural analysis, Raman spectroscopy, and first-principles calculations, it is revealed that an appropriate amount of Co2+ substitution can enhance the density, regulate the dielectric constant (εr), and improve the Q×f and τf values. Based on this ceramic, an enhanced polarization converter is designed. The maximum efficiency is as high as 90.38%. And the performance of this polarization converter can be regulated by the εr and dielectric loss of ZCVO ceramics. The mechanism and function of the polarization converter are revealed according to the electric field strength, current direction, and far-field pattern. This work expands the application of microwave dielectric ceramics.

Novel high εr MNdTiNbO7 (M = Ca, Sr) microwave dielectric ceramics: preparation, phase composition, microstructure, and dielectric performance

Novel high εr MNdTiNbO7 (M = Ca, Sr) microwave dielectric ceramics: preparation, phase composition, microstructure, and dielectric performance

Effects of LiF additive on crystal structures, lattice vibrational characteristics and dielectric properties of CaWO4 microwave dielectric ceramics for LTCC applications

In this paper, CaWO4-x wt.% LiF (x = 0.5–3.0) microwave dielectric ceramics are prepared by conventional solid-state reaction at 850 °C. All the X-ray diffraction peaks are attributed to CaWO4, combined with the results of X-ray photoelectron spectroscopy and scanning electron microscope. They indicate that LiF forms liquid phase at high temperature, exists in amorphous phase, and the CaWO4-x wt.% LiF samples belong to the composite ceramics. With the increase of the LiF content, the liquid phase fills the pores of the ceramics and increases the compactness. Therefore, the quality factor increases from 59,055 GHz (x = 0.5) to 77,855 GHz (x = 3.0), and the dielectric constant decreases from 9.26 (x = 0.5) to 8.41 (x = 3.0). The lattice vibration characteristics of the CaWO4-x wt.% LiF samples are studied by Raman and infrared spectroscopy. The external modes associated with the vibration of the Ca–O bonds have the greatest influence on the dielectric response, as shown by the fitting results based on the four-parameter semi-quantum (FPSQ) model. In addition, the simulated intrinsic properties and measured values are in general agreement. When the x value increases, the dielectric constant decreases with the decrease of the bond length. The quality factor has a negative correlation with the FWHM value of the Bg mode and a positive correlation with the packing fraction. Finally, the CaWO4-3 wt.% LiF sample has good compatibility with the silver powders at 850 °C, and high application value in the LTCC field.

Cold sintering optimized SrF2 microwave dielectric ceramics for the development of dielectric resonator antenna at 5G millimeter-wave band

SrF2 is a promising low-εr fluoride with outstanding microwave dielectric properties, while densification of SrF2 ceramics is challenging via traditional thermal sintering (TTS). In this work, dense SrF2 ceramics with 93.4%–97.2% relative density have been obtained via cold sintering (300 MPa–900 MPa, 150 °C, 1 h) and subsequent post-annealing at 950 °C/3 h. The pretreated cold sintering process is beneficial for microstructure optimization, where the maximum Qf value (62,037 GHz) is obtained at 750 MPa, nearly three times higher than the TTS sample (21,080 GHz). An ultra-low dielectric constant (εr) of 5.94 is simultaneously obtained, together with a temperature coefficient of resonant frequency τf = −78.26 ppm/°C. Good chemical compatibility between SrF2 ceramics and silver is verified, indicating great promise for their use in LTCC technology. Moreover, the low-εr and high Qf values of the cold sintering optimized SrF2 ceramics suggest great potential in the 5G millimeter-wave antenna systems. A SrF2-based dielectric resonator antenna is designed and fabricated, which resonates at the desired 24.5 GHz and exhibits an outstanding S11 of −43.95 dB and a broad bandwidth of 4.51 GHz.

Dense LiF microwave dielectric ceramics with near-zero linear shrinkage during sintering

In order to improve the densification of LiF ceramics, LiF green compacts were prepared by room-temperature cold sintering through uniaxially pressing the mixture of LiF powder and water. With increasing the compacting pressure from 100 to 800 MPa, the relative density of mechanically robust LiF compacts increased dramatically from 69.4% to 92.4%, which was attributed to the plastic deformation and dissolution-precipitation mechanism with the aid of high pressure and water. The relative density and microstructural homogeneity could be further improved by sintering the compacts at 825 °C, and the highest relative density of 95.4% was obtained in LiF ceramic with the compacting pressure of 800 MPa, which also exhibited the optimum microwave dielectric properties of εr = 8.53, Qf = 118,400 GHz and τf = −154 ppm/°C. Moreover, the linear shrinkage of LiF ceramics during sintering could be significantly reduced by increasing the compacting pressure and relative density of the green compacts, and it was as low as 1.1% for the compacting pressure of 800 MPa.

The role of CuO addition in the phase evolution and properties of BaTi5O11 microwave dielectric ceramics prepared by the solid-state reaction method

The role of CuO addition in the phase evolution and properties of BaTi5O11 microwave dielectric ceramics prepared by the solid-state reaction method

Low temperature sintering of Al2O3 microwave dielectric ceramics co-doped with 1.5wt%CuO-3wt%Nb2O5-0.5wt%ZrO2

Low temperature sintering of Al2O3 microwave dielectric ceramics co-doped with 1.5wt%CuO-3wt%Nb2O5-0.5wt%ZrO2

High-Qf value and temperature stable Zn2+-Mn4+ cooperated modified cordierite-based microwave and millimeter-wave dielectric ceramics

Cordierite-based dielectric ceramics with a lower dielectric constant would have significant application potential as dielectric resonator and filter materials for future ultra-low-latency 5G/6G millimeter-wave and terahertz communication. In this article, the phase structure, microstructure and microwave dielectric properties of Mg2Al4–2x(Mn0.5Zn0.5)2xSi5O18 (0 ≤ x ≤ 0.3) ceramics are studied by crystal structure refinement, scanning electron microscope (SEM), the theory of complex chemical bonds and infrared reflectance spectrum. Meanwhile, complex double-ions coordinated substitution and two-phase complex methods were used to improve its Q×f value and adjust its temperature coefficient. The Q×f values of Mg2Al4–2x(Mn0.5Zn0.5)2xSi5O18 single-phase ceramics are increased from 45,000 GHz@14.7 GHz (x = 0) to 150,500 GHz@14.5 GHz (x = 0.15) by replacing Al3+ with Zn2+-Mn4+. The positive frequency temperature coefficient additive TiO2 is used to prepare the temperature stable Mg2Al3.7(Mn0.5Zn0.5)0.3Si5O18-ywt%TiO2 composite ceramic. The composite ceramic of Mg2Al3.7(Mn0.5Zn0.5)0.3Si5O18-ywt%TiO2 (8.7 wt% ≤ y ≤ 10.6 wt%) presents the near-zero frequency temperature coefficient at 1225 °C sintering temperature: εr = 5.68, Q×f = 58,040 GHz, τf = −3.1 ppm/°C (y = 8.7 wt%) and εr = 5.82, Q×f = 47,020 GHz, τf = +2.4 ppm/°C (y = 10.6 wt%). These findings demonstrate promising application prospects for 5 G and future microwave and millimeter-wave wireless communication technologies.

Ultra-Low Loss Mg2TiO4 Based Dielectric Ceramics for Microwave Applications: An Overview

For several decades, temperature stable, medium permittivity, and low loss dielectric ceramics have been used as resonators, oscillators, filters, and GPS patch antennas for microwave communication systems. Various microwave dielectric ceramics have been proposed and widely used in telecommunication industries. Among the most interesting materials of that kind, magnesium orthotitanate (Mg<sub>2</sub>TiO<sub>4</sub>) is recognized as one of the most promising low loss microwave materials, which played an important role in the field of microwave and millimeter wireless communication industry. It was found that by modifying the Mg<sub>2</sub>TiO<sub>4</sub> compound with other materials, their microwave dielectric properties have changed tremendously. The main purpose of this review is to gather information about Mg<sub>2</sub>TiO<sub>4</sub> - based low loss dielectrics used for microwave applications. The study also helps the researchers and technologists to get compact information for Mg<sub>2</sub>TiO<sub>4</sub> -based compounds all over the world.

Temperature dependence of τf and its origin in MgTiO3–CaTiO3 microwave dielectric composites

As a key parameter of microwave dielectric ceramics, τf is required to be near-zero and usually treated as a temperature-independent constant. However, the strong temperature dependence of τf was observed in (1-x)MgTiO3–xCaTiO3 composites and the constituting phases. Especially for the composite with x = 0.06 and a near-zero average τf (τf,W) of − 2.3 ppm/°C, the normalized resonant frequency (f0̃) first increased and then decreased with temperature, and τf decreased significantly from 8.6 ppm/°C at − 40°C to − 13.4 ppm/°C at 80 °C. The temperature dependence of τf could be evaluated by the average dτfdT, which decreased monotonically from − 0.0206 ppm/°C2 to − 3.442 ppm/°C2 with increasing x from 0 to 1. The undesired temperature dependence of τf should be common in microwave dielectric composites with near-zero τf,W, which was determined by the nonlinear coefficient b2 and −τf2 of the constituting phases, and expected to be suppressed by adopting suitable constituting phases with relatively low |b2| and |τf,W|.

Wideband low-profile H-shaped dielectric patch antennas based microwave dielectric ceramics

Dielectric patch antenna (DPA) has been extensively studied in recent years, especially for the application of 5G mobile communication system. In this paper, we proposed a notch structure of H-shaped DPA (HDPA) with wideband low-profile properties but without additional components. Temperature stable 0.65 CaTiO3–0.35 LaAlO3 microwave dielectric ceramic of high permittivity was utilized for fabricating the H-shaped dielectric patch, and the method of leveraging the multi-modal features of DPA was introduced. Further analysis of the antenna structure was accomplished by the simplified theoretical method. As a result, the H-shaped notch is properly ensembled in the dielectric patch to make the higher-order TM121 mode close to the dominant TM101 mode, greatly expanding the broadband of the HDPA. The accuracy of the proposed HDPA element model is evaluated by the commercial full-wave simulator CST Microwave Studio. For demonstration, a prototype of the proposed HDPA element operating in the microwave band (5-GHz) has been realized and measured.

L2Ti0.85(Mg1/3Nb2/3)0.15O3/MgTiO3/L2Ti0.85 (Mg1/3Nb2/3)0.15O3 tri-layer co-fired microwave dielectric ceramics: A strategy to suppress non-linear variation of resonant frequency with temperature and achieve a high Q value

In many microwave dielectric ceramics systems, like MgTiO3–CaTiO3, MgTiO3–SrTiO3, and tri-layer MgTiO3/TiO2/MgTiO3, the resonant frequency shows a non-linear variation over a wide temperature range (−40 – 105 °C), indicating that the negative τf (−40 – 25 °C) value deviates from zero when the positive τf (25–105 °C) value is tuned to zero. To suppress the non-linear variation of resonant frequency in the range of −40 – 105 °C, a well-established and efficacious method has been proposed by introducing Li2Ti0.85(Mg1/3Nb2/3)0.15O3 ceramic to the MgTiO3 system. Furthermore, a tri-layer architecture ceramic was designed to improve the dielectric properties. Compared with randomly distributed samples, the Q × f value can be increased by up to 51%. Moreover, the electric field distribution was simulated by the finite element method with the aid of the eigenmode solver of high frequency structure simulator (HFSS) to clarify the dielectric mechanism and predict the microwave dielectric characteristics of the tri-layer structure ceramics. Finally, tri-layer Li2Ti0.85(Mg1/3Nb2/3)0.15O3/MgTiO3/Li2Ti0.85(Mg1/3Nb2/3)0.15O3 ceramic with wide temperature stability and high Q × f value was obtained [εr = 18.4, Q × f = 85 000 GHz, τf+ = −3.8 ppm/°C (25–105 °C), τf− = 4.0 ppm/°C (−40 – 25 °C)], which refines the theory of non-linear variation of resonant frequency and holds promise for application in the 5G communication system.

Structure refinement, defects evolution and calcium doping of Sr2CeO4 microwave dielectric ceramics with high quality factor

Sr2-2xCa2xCeO4 (x = 0, 0.025, 0.05, 0.1, 0.2, 0.4, 0.6, 0.8) ceramics were synthesized through cold isostatic pressing and solid-state reaction. The microstructure, defects, microwave dielectric properties, and the effect of Ca2+ doping of Sr2CeO4 ceramics were systematically investigated. As the sintering temperature increased, the densities of Sr2CeO4 ceramics rose, the content of oxygen vacancies increased, and Ce4+ reduction would be enhanced. In addition, the Sr2CeO4 structure had poor compatibility with Ca2+. The major phase could be kept unchanged only when x ≤ 0.1. The reason was that the doping of Ca2+ intensified the distortion of the CeO6 octahedron and induced the structural transformation of the common edges (Sr2CeO4) to the common angles (SrCeO3). With the increase of dopant, the densities of Sr2-2xCa2xCeO4 ceramics increased significantly, while the content of oxygen vacancies also increased. The microwave dielectric properties were mainly influenced by the density, structural symmetry, defects, and the second phase SrCeO3. The dielectric permittivity (εr) of 13.4–15, the quality factor (Qf) of 118,580–52,170 GHz, and the temperature coefficient of resonant frequency (τf) of −58.3 ∼ −47.5 ppm/°C were obtained for Sr2-2xCa2xCeO4 ceramics When x ≤ 0.1. This work has provided a foundation for further research on cerate microwave dielectric ceramics.

Crystal structure and microwave dielectric properties of garnet-type Ca2YZr2-xTixAl3O12 ceramics for dual-band bandpass filters

Microwave dielectric ceramics with a high-quality factor are vital materials for substrates, dielectric resonators, and filters in millimeter-wave communication systems. Here, a novel microwave dielectric ceramic based on a garnet-type Ca2YZr2Al3O12 compound for bandpass filter was prepared using the solid-state reaction method. Sintering the Ca2YZr2Al3O12 ceramics at 1600 ℃ for 5 h resulted in excellent microwave dielectric properties of εr = 10.81 ± 0.16, Qf = 87,628 ± 4000 GHz and τf = −34.3 ± 0.5 ppm/℃. Increasing the Ti4+ content of the Ca2YZr2-xTixAl3O12 ceramics significantly improved the sintering process. Superior microwave dielectric properties (εr = 11.93 ± 0.15, Qf = 121,930 ± 2600 GHz and τf = −30.8 ± 0.4 ppm/℃) were obtained for Ca2YZr2-xTixAl3O12 (x = 0.3) because of its dense microstructure, large grain size and high lattice energy. The Ca2YZr2-xTixAl3O12 (x = 0.3) ceramics were used to fabricate a dual-band bandpass filter with a hairpin structure that exhibited a large return loss (|S11| > 12 dB) and a small insertion loss (|S21| < 0.73 dB). The high performance of the Ca2YZr2-xTixAl3O12 ceramics and the corresponding bandpass filter makes this material a potential candidate for millimeter-wave devices.

Lattice dynamics and terahertz response of microwave dielectrics: A case study of Al-doped Ca0.6Sm0.27TiO3 ceramics

Low-loss microwave dielectric ceramics are of great significance for fifth-generation telecommunication (5G) devices used for higher frequencies, while reducing the dielectric loss in the microwave band is still a technical challenge. Lattice dynamics and terahertz response are closely related to the polarization of dielectrics, and can be used as effective methods to explore the loss mechanism of microwave dielectrics. Here, Al3+ was doped into the conventional high-permittivity (εr) Ca1-xSm2x/3TiO3 (x = 0.4) ceramic system, enhancing the Q×f values significantly. Raman spectra, thermally stimulated depolarization currents (TSDC) and terahertz time-domain spectra (THz-TDS) together showed that the stiffness of cation vibration became stronger when z value was larger, indicating a smaller damping of the lattice vibration, and leading to lower extrinsic losses and higher Q×f values. The relationship between lattice dynamics, defect behavior and microwave dielectric loss will provide a significant reference for the development of low-loss microwave dielectric ceramics.

Low temperature fired CaF2-based microwave dielectric ceramics with enhanced microwave properties

Low-temperature-fired microwave ceramics are key to realizing the integration and miniaturization of microwave devices. In this study, a facile wet chemical method was applied to synthesize homogenous nano-sized CaF2 powders for simultaneously achieving low-temperature sintering and superior microwave dielectric properties. Pure CaF2 ceramics sintered at 950 °C for 6 h with good microwave dielectric properties (εr = 6.22, Q×f = 36,655 GHz, and τf = −102 ppm/°C) was achieved. The microwave dielectric properties of the CaF2 ceramics were further improved by introducing LiF as a sintering aid. The sintering temperature of CaF2-based ceramics was effectively lowered from 950 °C to 750 °C with 10 wt% LiF doping, and excellent microwave dielectric properties (εr = 6.37, Q×f = 65,455 GHz, and τf = −71 ppm/°C) were obtained.

Crystal structure and microwave dielectric properties of (Mg0.2Ni0.2Zn0.2Co0.2Mn0.2)2SiO4 - A novel high-entropy ceramic

Novel (Mg0.2Ni0.2Zn0.2Co0.2Mn0.2)2SiO4 (A5SO) high-entropy microwave dielectric ceramics with olivine structure were prepared in the sintering temperature range of 1100 °C–1300 °C via the solid-phase reaction route. The crystal structure was confirmed by XRD, Raman, and Rietveld refinement. Optimal microwave dielectric properties (εr = 8.02, tanδ = 0.00051 at 14.5 GHz, and τf = −38.2 ppm/°C) were obtained at the sintering temperature of 1250 °C, where a relative density of 95.1% was detected. The complex chemical bonding theory manifests that the εr value of A5SO is mainly affected by the ionicity of A-O (A = Mg, Ni, Zn, Co, Mn) bond, while the dielectric loss is affected by both A-O and Si–O lattice energy. The τf value is mainly influenced by the [A(2)O6] oxygen octahedral distortion (1.8 × 10−3). The experimental results of this study provide both theoretical and practical guidance for high-entropy microwave dielectric ceramic applications.

Microwave dielectric properties, low-temperature sintering mechanism, and defects behaviour of temperature stable (1-x)Li2Zn3Ti4O12-xSr3(VO4)2 ceramics

(1-x)Li2Zn3Ti4O12-xSr3(VO4)2 (0.1 ≤ x ≤ 0.4) microwave dielectric ceramics were fabricated by solid-state sintering technology. The impact of SV addition on the microstructure, dielectric properties, sintering process, and defects behaviour was studied. The formation of SrTiO3 and the glass phase were observed via XRD and TEM, and the latter resulted in a decrease in the sintering temperature. The variations in microwave dielectric properties were consistent with the empirical mixture rules calculated by XRD refinement, and a near-zero τf value was obtained. The Li, Zn and V elements of the glass phase and the liquid phase sintering model were deduced via DSC, TEM and Raman spectroscopy. Then, the defect behaviour, such as oxygen vacancies, Ti3+, and V4+, was investigated by XPS and complex impedance spectroscopy. It was found that the generation and migration of defects occurred much more easily in 0.7LZT-0.3 SV than in LZT, resulting in a higher dielectric loss. Finally, the 0.7Li2Zn3Ti4O12-0.3Sr3(VO4)2 ceramic sintered at 900 °C exhibited excellent microwave dielectric properties of εr = 17.8, Q × f = 41,891 GHz, and τf = −4.4 ppm/°C and good compatibility with silver electrode, showing a good potential application for LTCC.

Characterization of structure and chemical bond in high-Q microwave dielectric ceramics LiM2GaTi2O8 (M = Mg, Zn)

LiM2GaTi2O8 (M = Mg, Zn) ceramics with the Fd-3m space group were synthesized using the solid-state method. In comparison with Mg2+ that fully occupied the tetrahedral (A) site in LiMg2GaTi2O8, LiZn2GaTi2O8 was jointly occupied by Zn2+ and Li+ at the A site. Excellent microwave dielectric properties of Q×f = 133,400 ± 500 GHz, 101,800 ± 500 GHz, εr = 17.1 ± 0.2, 15.8 ± 0.2, and τf = −60.1 ± 3.0 ppm/°C, − 42.2 ± 3.0 ppm/°C for LiZn2GaTi2O8 and LiMg2GaTi2O8 were obtained, respectively. The large deviations (30.3% for LiMg2GaTi2O8 and 19.6% for LiZn2GaTi2O8) between the corrected εcorr and theoretical εth were observed, which might be attributed to the underestimated Shannon’s ionic polarizability of Ti4+ in Ti-containing spinels. Their intrinsic microwave dielectric properties were discussed based on bond valence, lattice energy (U), and B-site bond energy (E). Besides, their large negative τf values were compensated to near-zero by CaTiO3.