Microwave Dielectric Ceramics Research Articles

A wideband high-gain dielectric resonator antenna based on mullite microwave dielectric ceramics

In this paper, the high-quality factor mullite microwave dielectric ceramic is employed to introduce in dielectric resonator antenna (DRA). The microwave dielectric properties of ɛr ∼ 5.8, Q × f ∼ 31 240 at 9.66 GHz, and loss tangent tanδ = 1/Q ∼ 2.7 × 10−4 can be obtained by the solid-state reaction method in mullite microwave dielectric ceramic sintered at 1700 °C. On this basis, the resonant modes of the proposed broadband high-gain rectangular DRA are analyzed by using a dielectric waveguide theoretical model and the antenna performance is evaluated by a commercial CST Microwave Studio 2021® software. It has been found that the proposed rectangular DRA can provide a broad bandwidth of 35.9%, a maximum gain of 10 dBi at 3.61 GHz, and a radiation efficiency of 90% in the range of 3.13–4.42 GHz. For demonstration, adequate consistency between the simulation results and the measurement results was achieved. The proposed DRA has great application potential and value in 5G communication scenarios requiring wideband and high-gain antenna.

Two K20 microwave dielectric ceramics SrLnAlO4 (Ln = Eu, Gd) with near-zero τf and contrasting Q×f

K20 microwave dielectric ceramics SrLnAlO4 (Ln = Eu, Gd, and Ho) were prepared at 1440–1520 °C. SrEuAlO4 and SrGdAlO4 crystallized into tetragonal K2NiF4-type structures. While SrHoAlO4 was a mixture (SrHo2Al2O7, Ho2O3, and Sr3HoAl2O7.5) due to the too-small radius of Ho3+ to bond in [HoO9] dodecahedron. SrEuAlO4 and SrGdAlO4 exhibited near-zero τf values of +1.7 and +4.3 ppm/°C and low εr (19.37 and 19.14) along with contrasting Q×f values (70,030 and 5560 GHz). Compared with the modified molecular polarizability (αCorr) based on the bond valence deviation, the αobs values of SrEuAlO4 and SrGdAlO4 are much higher, indicating that their dielectric polarizabilities were underestimated, and this phenomenon was mainly influenced by rattling cations, which also causes their τf with near-zero values. Due to the magnetic subsystem adversely affecting microwave dielectric losses, the worsened Q×f value of SrGdAlO4 compared to that of SrEuAlO4 is due to the high magnetic moment of Gd3+ (7.94 μB).

Enhanced the temperature stability of low-fired Li3Mg2NbO6 microwave dielectric ceramics for LTCC application

Enhanced the temperature stability of low-fired Li3Mg2NbO6 microwave dielectric ceramics for LTCC application

Structure evolution and τf influence mechanism of Bi1–xHoxVO4 microwave dielectric ceramics for LTCC applications

Bi1–xHoxVO4 (0.1 ≤ x ≤ 0.9) ceramics were prepared via a solid-state reaction method, and all the ceramics could be well densified in the 920–980 °C range. The ceramics with 0.1 ≤ x < 0.4 were composed of both monoclinic scheelite (M) and tetragonal zircon (T) phases, and a single M phase could be obtained in the range of x ≥ 0.4. The measured εr decreased from 58.9 (x = 0.1) to 14.7 (x = 0.9), so do the calculated values (εr(C−M) = 34.3–12.1), and the main reason for εr > εr(C−M) was the rattling of Ho3+ in the dodecahedron. Two points with zero τf appeared in Bi1–xHoxVO4 (0 ≤ x ≤ 1) ceramics, and the best microwave dielectric properties with εr = 16.6, Q × f = 18,400 GHz (f = 10.69 GHz), and τf = +3.29 ppm/°C were obtained in the Bi0.2Ho0.8VO4 ceramic. The change in temperature coefficient of ionic polarizability (ταm) caused by the rattling effect of cations is the physical essence that affects τf. Therefore, the rattling effect can be used as an effective mechanism to regulate τf in low-εr materials. Furthermore, there was no chemical reaction between Bi1–xHoxVO4 and Ag electrode, which indicates potential applications in low-temperature co-fired ceramic (LTCC) technology.

Sintering characteristics, phase structure and microwave dielectric properties of novel BaCeO3 ceramics

It is a known fact that BaCeO3 (BC) is a material widely used in catalysts and electrochemistry, but its microwave dielectric performances have not been explored. Herein, novel BC microwave dielectric ceramics are prepared by the solid-state reaction method. The single-phase BC ceramic with orthorhombic structure (Pnma space group) is confirmed via Rietveld refinements and synthesized. Raman analysis reveals that the Q × f value of ceramics is negatively correlated with the full width at half maximum of the Raman peak. High surface densification and uniform grain size are obtained at 1650 °C and exhibit decent microwave dielectric properties: εr = 14.68, Q × f = 27,037 GHz(at 9.32 GHz), and τf = -63.67 ppm/ °C. The XPS results show that the oxygen vacancy content is the lowest and the Q × f value is the largest at the sintering temperature of 1650 °C. The results indicate that the ceramic is a good candidate for microwave dielectric materials.

Sm2-xCexO3+x/2 Composite microwave dielectric ceramics with tunable τf and enhanced Q×f

Dense Sm2-xCexO3+x/2 (x = 0.10−0.25) ceramics were obtained by the solid-state method. Main-phase Sm2O3 with monoclinic structure and a small amount of fluorite-structured CeO2 were achieved. As the x value increased, the τf significantly reduced from 18.8 ± 2.0 ppm/°C to −20.3 ± 2.0 ppm/°C and the Q×f enhanced from 21,382 ± 300–32,927 ± 300 GHz. The τf variating from positive to negative was related to the increasing temperature coefficient of ion polarizability (ταm) from 20.6 ppm/°C to 27.9 ppm/°C. The rise in Q×f was attributed to the reduced AC conductivity resulting from the decreasing oxygen vacancy concentration. The addition of Ce could effectively inhibit the decomposition of Sm2O3 ceramics in atmospheric condition. Notably, the Sm1.80Ce0.20O3.10 ceramic sintered at 1475 °C demonstrated optimal microwave dielectric properties of εr = 19.2 ± 0.2, Q×f = 28,033 ± 300 GHz, and τf = 1.8 ± 2.0 ppm/°C.

Experimental and theoretical study on the relationship between structure and properties of NiO substituted BaZnP2O7 microwave dielectric ceramics with enhanced quality factor for LTCC application

In this work, we incorporated Ni2+ ions into BaZnP2O7 ceramics with a low dielectric constant to improve their performance. Enhancement of microwave dielectric properties was observed for BaZn0.94Ni0.06P2O7 sintered at 900 °C for 3 h: εr = 7.43(±0.04), Q×f = 102,538(±3012) GHz, and τf = -64(±2.4) ppm/ °C. XRD, SAED, and EDS mapping indicated that a solid solution was formed in all ceramic samples. The Ni2+ doping promoted grain growth and densification of the samples. The density was a major factor affecting dielectric constant εr, and the pore-corrected εrc was explained by the molecular polarizability. The vibrational characteristics were analyzed by Raman and FTIR spectroscopies. The FWHM of Raman peaks had a marked effect on the Q×f and τf values. Furthermore, the higher lattice energy and larger band gap lead to the higher Q×f value of the BaZn0.94Ni0.06P2O7 ceramic with lower density. The excellent performance of the prepared ceramics makes them a good candidate material for low-temperature co-fired ceramic technology.

Microwave dielectric properties of (Na0.5Bi0.5)MoO4–BaMoO4 composite ceramics with ultralow sintering temperature

AbstractThe rapidly ever‐growing wireless communication demands development of low loss, ultralow sintering temperature dielectric ceramics with excellent microwave properties. In this work, fabrication and properties of novel microwave dielectric composite ceramics (1 − x)(Na0.5Bi0.5)MoO4–xBaMoO4 (x = 0.3–0.6), in short as ((1 − x)NBM–xBM), synthesized via solid‐state reaction method with an ultralow sintering temperature is reported. X‐ray diffraction analysis confirms the coexistence of both (Na0.5Bi0.5)MoO4 and BaMoO4 scheelite phases. Scanning electron microscopy analysis demonstrated dense microstructures of ceramics sintered at 650°C and 700°C. The dielectric properties of (1 − x)NBM–xBM (x = 0.3–0.6) ceramic are significantly tailored by incorporation of BaMoO4. With the increase in x from 0.3 to 0.6, the (1 − x)(Na0.5Bi0.5)MoO4–xBaMoO4 displays noticeable change in microwave permittivity (varying from 25.4 to 16.6), quality factor values, and temperature coefficient of frequency (range tunable from +37.6 ppm/°C to −14.2 ppm/°C). The microwave dielectric ceramic, 0.5(Na0.5Bi0.5)MoO4–0.5BaMoO4, has optimum dielectric properties (, = 15 164 GHz, τf = −0.2 ppm/°C) at sintering temperature of 650°C. The dielectric resonator antenna designed using (1 − x)(Na0.5Bi0.5)MoO4–xBaMoO4 ceramics demonstrated excellent radiation performance at resonance frequency of 5.905 GHz, gain of ∼5.8 dBi and 140 MHz bandwidth, which validates its microwave applications utilization within the field of ultralow temperature cofired ceramics technology.

The ultralow loss Li2Mg3Ti1−xAl4x/3O6 microwave dielectric ceramics and its design of dielectric resonator antenna

The Li2Mg3Ti1−xAl4x/3O6 (0 ≤ x ≤ 0.1) ceramics are synthesized using the solid phase method, showing excellent microwave dielectric properties with εr = 14.05, Q × f = 144 500 GHz, and τf = −43 ppm/°C at x = 0.04. Rietveld refinements, based on the XRD data, reveal that the Li2Mg3Ti1−xAl4x/3O6 ceramics crystallize in the rock salt structure with Fm-3m space group. The relationship between the crystal structure and intrinsic microwave dielectric properties is studied in detail. The variation in the dielectric constant is related to ionic polarization and relative density. The quality factor (Q × f) demonstrates a strong correlation with lattice energy, relative density, and concentration of Ti3+. The variation of τf is influenced by the bond thermal expansion coefficient and bond valence. The dielectric resonant antenna, manufactured from Li2Mg3Ti0.96Al0.0533O6-0.09TiO2 ceramic with near-zero τf, shows a bandwidth of 230 MHz at 7.40 GHz and a high radiant efficiency, which indicates that high-quality factor Li2Mg3Ti0.96Al0.0533O6 ceramic has significant potential in relevant fields such as 5G and smart driving.

(Ni1/3Nb2/3)4+-Improved High-K Ba3Ti4Nb4O21 Microwave Dielectric Ceramics for Miniaturized All-Ceramic Radomes with 5G Beam-Splitting Function.

With the rapid development of 5G communication technology, microwave dielectric ceramics with high dielectric constants are very conducive to the miniaturization of passive devices. Here, Ba3Ti4-x(Ni1/3Nb2/3)xNb4O21 (BTN ∼ NN, 0.03 ≤ x ≤ 0.15) ceramics with hexagonal phases are synthesized via the solid-phase route. The composite (Ni1/3Nb2/3)4+ ion substitution strategy can substantially improve the microwave dielectric properties of the Ba3Ti4Nb4O21 (BTN) ceramic. The εr and Q × f values depend on the ionicity (Nb-O bonds) and lattice energies (Nb(1)-O3 and Nb(1)-O2(1) bonds). The microwave dielectric properties of the BTN ∼ NN (x = 0.09) ceramic sintered at 1250 °C are εr = 60.3, Q × f = 22073 GHz, and τf = 78.1 ppm/°C. A miniaturized all-ceramic radome (@400 mm × 400 mm × 8 mm) for 5G beam-splitting function is designed and demonstrated using this ceramic. Compared to other radomes designed for other work utilizing low εr, the size of this radome has been reduced by 3/7. The reflection coefficients of the beam splitting function are all 0.73, and the phase shifts are all 360°. This work contributes to the development of miniaturized passive devices from a materials point of view.

Calcium Manganese Vanadate Microwave Dielectric Ceramics with Ni2+ Partial Substitution for LTCC Applications

Calcium Manganese Vanadate Microwave Dielectric Ceramics with Ni2+ Partial Substitution for LTCC Applications

Structural and microwave dielectric properties of LiZnSbO4 ceramcis for dielectric resonator antenna applications

New microwave dielectric ceramics of LiZnSbO4 were prepared by the reactive sintering method. Within the sintering temperature zone of 1350∘C–1385∘C, a single-phase LiZnSbO4 with an orthorhombic structure was identified by XRD refinement, which contains 10 molecular formula per unit cell (i.e., [Formula: see text]). The relative density and average grain size had a main effect on [Formula: see text] and [Formula: see text] value of the LiZnSbO4 ceramics, whereas the phase assemblage was responsible for its [Formula: see text]. The 1375∘C-sintered LiZnSbO4 specimens owned comparable low [Formula: see text], but ultra-high [Formula: see text][Formula: see text]GHz (nearly three times) compared to LiZnPO4 and LiZnVO4 ultra-high [Formula: see text][Formula: see text]GHz (nearly three times) compared to LiZnPO4 and LiZnVO4 manufactured, which showed a return loss as low as 37.4[Formula: see text]dB, maximum gain of 5.7[Formula: see text]dB and a fractional bandwidth of 3.9% (11.95–12.43[Formula: see text]GHz).

Crystal structure and chemical bond characteristics for Li2Zn2(MoO4)3 microwave dielectric ceramics for ULTCC application

Li2Zn2(MoO4)3 (LZMO) ceramics were prepared via solid-state method. XRD analysis showed LZMO phase had an orthorhombic structure with Pnma space group, representing the only crystalline phase. Ceramic sintered at 600 °C exhibited high relative density (96.2 %) and dense microstructure, demonstrating optimal microwave dielectric properties: εr = 10.38, Q×f = 69,400 GHz, and τf = -72.81 ppm/°C. Further investigations revealed close correlation between εr and polarizability, emphasizing greater influence of relative density on Q×f than packing fraction. Increase in distortion of [Li/ZnO6] octahedra resulted in deterioration of ceramics' τf. Computation based on P-V-L theory revealed significant impact of ionicity, lattice energy, and bond energy on εr, Q×f, and τf, with Mo-O bonding dominating ceramics' microwave dielectric properties. Raman spectroscopy analysis showed majority of Raman bands attributed to vibrational modes of Mo-O bonds. LZMO ceramics have great potential in ULTCC field.

Machine learning enhanced prediction of permittivity of spinel microwave dielectric ceramics compared to traditional C-M calculation

Abstract Microwave dielectric ceramic (MWDC) is crucial in advancing the development of 5G technology and the communication field. The prediction or calculation of its properties is of great significance for accelerating the design and development of MWDCs. Therefore, the prediction of permittivity of spinel MWDCs based on machine learning was investigated in this work. Firstly, we collected 327 single-phase spinel MWDC entries and constructed feature engineering, which includes feature generation and feature selection (five dominant features, including Mpo, Dar, Mmbe, Aose and Dgnve, were selected from 208 generated features). Next, seven commonly used algorithms were utilized during the training process of machine learning models. The extreme gradient boosting (XGBoost) model shows the best performance, achieving R-squared (R 2) of 0.9095, mean absolute error of 1.02 and root mean square error of 1.96 on the train and test dataset. In addition, the machine learning models, especially the XGBoost model, show enhanced prediction (calculation accuracy) of the permittivity of spinel MWDCs compared to the traditional Clausius–Mossotti equation, which can provide a guide for the design and development of spinel MWDCs applied for wireless communication.

Effects of Ga2O3 doping on the microstructure and dielectric properties of SrTiO3–SmAlO3 ceramics for microwave applications

xSrTiO3-(1-x)SmAlO3 (ST-SA) (x = 0.4, 0.5, 0.6) microwave dielectric ceramics were prepared by solid-state reaction method and the microwave dielectric properties (εr, Q × f, τf) were studied. The 0.4ST-0.6SA component with better Q × f and τf was selected and the effects of different ywt % Ga2O3 (y = 0.5, 1.0, 1.5, 2.0, 2.5) doping on the phase structure, microstructure, and microwave dielectric properties of ceramics were investigated. X-ray diffraction analysis showed that Ga3+ mainly replaced Ti4+ site when y ≤ 0.5, and when y > 0.5, Ga3+ start to replace Al3+ site. The best grain uniformity of the ceramics was observed at y = 2.0 by scanning electron microscopy testing. Raman spectroscopy shows that the stability of the oxygen octahedron is improved after Ga2O3 doping with increased Q × f value. The change of Ga2O3 doping amount also affects the dielectric properties of ST-SA ceramics. The optimum Q × f value of the ceramic is improved by about 50% from 10,278 GHz (7.465 GHz, 0.4ST-0.6SA) to 15,712 GHz (6.950 GHz, 0.4ST-0.6SA-2.0 wt% Ga2O3) with improved dielectric properties (εr = 36.14，τf = 4.17 ppm/°C).

Li2(Mg1-xZnx)3SnO5F2 (x=0.00–0.08): A novel low-temperature sintered microwave dielectric ceramics with low loss

Through solid-state reaction method, a novel oxy-fluoride Li2(Mg1-xZnx)3SnO5F2 (LMZSOF, x = 0.00 - 0.08) ceramic with low-temperature sintering were acquired. Based on the detection methods, X-ray diffraction and scanning electron microscopy, the polyphase structure and surface morphology of ceramics were tested, the LMZSOF ceramics are a multiphase structure with dense microstructure. LMZSOF ceramics possess a low temperature for sintering densification. The dielectric constants of ceramics were predominantly influenced by the second phase and density. While the Zn-addition amount increases from 0.00 to 0.06, the quality factor appears a trend of increasing first and then decreasing, reaching its maximum value at x = 0.04. The resonant frequency temperature coefficient fluctuated around ∼ -65 ppm/°C. Li2(Mg0.96Zn0.04)3SnO5F2 ceramics with superb microwave dielectric properties were obtained at 950 °C, where εr is 9.19, Q × f is 82,783 GHz, τf is -65.6 ppm/°C.

Acceleration of optimizing the dielectric properties of (Ca0.7Nd0.2)1−x(Li0.5Nd0.5)xTi1−y(Mg1/3Nb2/3)yO3 with the aid of machine learning

AbstractThe three key properties required for microwave dielectric ceramics are suitable dielectric constant (εr), high quality factor (Q × f), and near‐zero temperature coefficient of resonant frequency (τf). Due to the intricate coupling relationship among these three properties, regulating one often leads to the deterioration of the remaining two. In this study, the synergetic regulation of dielectric properties of Ca0.7Nd0.2TiO3 was investigated to reduce τf to near‐zero while maintaining its high εr by A‐ and B‐site substitutions. To avoid the seesaw effect and improve the efficiency of composition optimization, machine learning method was introduced in this study. First, the models for predicting dielectric properties were fitted based on a small amount of high‐quality data gathered via a uniform experimental design. Then the dielectric properties of 1037 compositions of (Ca0.7Nd0.2)1−x(Li0.5Nd0.5)xTi1−y(Mg1/3Nb2/3)yO3 were predicted, from which 69 microwave dielectric ceramics with near‐zero τf and adjustable εr of 95–125 were quickly picked out, and 5 of them were proved by experiments with very high prediction accuracy. This work was finished within a period of 1 month, which proves the obvious acceleration of composition designing process of microwave dielectric ceramics with the aid of machine learning.

Effects of Zr4+ and Hf4+ substitution on the structure and dielectric response of Ba4Sm9.33Ti18O54 ceramics

Microwave dielectric ceramics with high relative permittivity (εr) are commonly used for the miniaturation of microwave devices. The dielectric loss mechanism of microwave dielectric ceramics, as well as the property modification, has been a concerned topic among scholars. Herein, the lattice structure, defects, and broadband dielectric response of Zr4+ and Hf4+ substituted Ba4Sm9.33Ti18O54 (BST) ceramics are systematically examined. The negative temperature coefficient of resonant frequency (τf) of BST ceramics can be tuned to near zero via ion substitution, owing to the Sm2Ti2O7 second phase with a large positive τf. Optimal microwave dielectric properties of εr = 81.3, Q × f = 10,800 GHz, and τf = −4.6 ppm/°C are achieved. The lattice vibration behavior and defect concentration are analyzed by Raman spectra and thermally stimulated depolarization currents (TSDC), respectively. The increasing volume of the unit cell and TiO6 octahedra, which is because of the large-radius substitution ions, leads to an increase in lattice vibration damping factor and the generation of more oxygen vacancies (VO∙∙). Therefore, the dielectric losses in the microwave and terahertz (THz) bands increase after substitution. Meanwhile, the low-frequency dielectric losses at high temperatures also increase due to the weak restraint of carriers by the lattice. The broadband loss mechanism of Zr4+ and Hf4+ substituted BST ceramics related to lattice vibration and defects might be referenced for developing low-loss dielectric ceramics.

Spinel-trirutile microwave dielectric ceramics with high Q and excellent temperature stability based on MgO-Al2O3-Ta2O5 ternary systems

(1-x)MgAl2O4-xMgTa2O6 composite ceramics based on the MgO-Al2O3-Ta2O5 pseudoternary phase diagram were prepared by the solid-state reaction. MgAl2O4 and MgTa2O6 could coexist due to their different crystal structures. MgAl2O4 inhibited the rapid growth of MgTa2O6 grains, and dense microstructures (ρ > 95%) with a uniform grain size distribution was obtained, with an optimal densification temperature of 1550 - 1575 °C. The temperature coefficient of resonant frequency (τf) and quality factor (Q×f) were optimized by introducing MgTa2O6 phases with low dielectric loss and positive τf. In addition, the partial solid solutions of Al3+ into Ta5+ sites resulted in loosening bonds within [Al/TaO6] octahedra and abnormally large polarizabilities, which was confirmed by the decrease in VTa-O and the redshift of A1 g(Ta-O) mode. Consequently, the relative permittivity (εr) and τf values of MgAl2O4-MgTa2O6 composite ceramics were further enhanced, exceeding the predicted ones. The 0.6MgAl2O4-0.4MgTa2O6 composition sintered at 1550 °C achieved excellent performance, with a near-zero τf value of 3.3 ± 1.7 ppm/°C, a low εr of 16.4 ± 0.3 and an ultra-high Q×f of 179,000 ± 6700 GHz @ 8.94 ± 0.04 GHz. These findings hold promising applications in the field of 5 G/6 G high-frequency communication.

Sintering behavior and mechanism of Bi‐Zn‐Nb‐O microwave dielectric ceramics

AbstractBi‐Zn‐Nb‐O‐based microwave dielectric ceramics are promising candidates for wireless communication. Investigations have concentrated on their fabrication methods and doping modification. In this work, the densification mechanism and grain growth kinetics of the Bi‐Zn‐Nb‐O‐based microwave dielectric ceramics were explored. A significantly wide sintering temperature window (800–1000°C) of the monoclinic Bi2Zn2/3Nb4/3O7 (BZN) was found. High‐density stacking faults were observed in BZN grains at the initial stage of sintering, which could act as paths for mass transport, thus remarkably accelerating the densification process and decreasing the densification temperature. On the other hand, BZN possesses a high grain growth exponent and activation energy, which indicates a slow grain growth rate and a high threshold for abnormal grain growth, corresponding to the wide sintering temperature window. Moreover, the BZN ceramics display good microwave dielectric properties stability over a wide temperature range (25–450°C).