Microwave Dielectric Ceramics Research Articles

Low-temperature sintering and microwave dielectric properties of CaMg1\u2212xLi2xSi2O6 (x = 0\u22120.3) ceramics

In this study, low-temperature fired CaMg1−xLi2xSi2O6 microwave dielectric ceramics were prepared via the traditional solid-state reaction method. In this process, 0.4 wt% Li2CO3-B2O3-SiO2-CaCO3-Al2O3 (LBSCA) glass was added as a sintering aid. The results showed that ceramics consisted of CaMgSi2O6 as the main phase. The second phases were CaSiO3 always existing and Li2SiO3 occurring at substitution content x > 0.05. Li+ substitution effectively lowered sintering temperature due to 0.4 wt% LBSCA and contributed to grain densification, and the most homogeneous morphology could be observed at x = 0.05. The effects of relative density, the second phase, and ionic polarizability on dielectric constant (εr) were investigated. The quality factor (Q × f) varied with packing fraction that concerned the second phase. Moreover, the temperature coefficient of the resonant frequency (τf) was influenced by MgO6 octahedral distortion and bond valence. Excellent dielectric properties of the CaMg1−xLi2xSi2O6 ceramic was exhibited at x = 0.05 with εr = 7.44, Q × f = 41,017 GHz (f = 15.1638 GHz), and τf = −59.3 ppm/°C when sintered at 900 °C. It had a good application prospect in the field of low-temperature co-fired ceramic (LTCC) substrate and devices.

Sintering Behavior and Microwave Dielectric Properties of Low-Permittivity SrMgSi2O6 Ceramic

Low-permittivity SrMgSi2O6 microwave dielectric ceramics were synthesized via a conventional solid-state reaction method. Further, their sintering behavior as well as structural and microwave dielectric properties were studied. The x-ray diffraction patterns of the SrMgSi2O6 and Sr2MgSi2O7 ceramics were compared; both the samples were confirmed to be tetragonal. Meanwhile, the microwave dielectric properties of the samples were related to their microscopic morphology. In addition, the SrMgSi2O6 ceramics had a low sintering temperature of 1125°C; they exhibited good microwave dielectric performances with a relative permittivity of er = 6.7, a quality factor Q × f = 25,800 GHz, and temperature coefficient of the resonator frequency τf = −46 ppm/°C. Therefore, the ceramics exhibit potential for application in microwave devices.

Effect of Mg/B ratio and Sr2+ substitution for Mg2+ on the sintering, phase composition and microwave dielectric properties of Mg3B2O6 ceramics

Mg3B2O6 is a typical microwave dielectric ceramic with a high Qf value, but the pure Mg3B2O6 ceramic is difficult to be obtained due to the volatilization of B2O3 resulting from the high temperature during the fabrication process. We reveal the effects of Mg/B ratio and Sr2+ substitution for Mg2+ on the phase composition, sintering behavior, microstructure, and microwave dielectric properties of Mg3B2O6. The pure Mg3B2O6 ceramic can be obtained with Mg/B ratio equal to 1.2. Proper excessive free-MgO is benefit to the improvement of Qf value of Mg3B2O6, while it is detrimental to the densification of Mg3B2O6. No second phase was detected in the Sr2+ substituted Mg3B2O6 and excellent microwave dielectric properties (εr = 6.9, Qf = 110820 GHz, and τf = −32.4 ppm/°C at 12.8 GHz) were obtained in the (Mg0.998Sr0.002)3B2O6 (x = 0.2 mol.%) ceramic sample sintered at 1250 °C for 3h, making it a hopeful candidate for wireless communications. These results bring forward the underlying insights needed to direct the design of microwave dielectric ceramics with high Qf values for personal communication applications.

TiO2-(Zn1/3Nb2/3)O2-ZrO2 microwave dielectric ceramics with near-zero temperature coefficient of resonant frequency

(56.33-x)TiO2-x(Zn1/3Nb2/3)O2-43.67ZrO2 (x = 2.60-37.66) ceramics with 0.5%wt CuO addition were prepared by the mixed metal oxide route. Effects of sintering temperature were investigated on the sintering behavior and microwave dielectric properties of ceramics. For all ceramics with x = 2.60-37.66, the optimal sintering temperature range was determined to be around 1280 °C. XRD analysis showed that the main crystalline phase of ceramics sintered at 1280 °C changed from ZrTiO4 type at x = 2.60-30.20 to ZrTi2O6 type at x = 30.20-37.66. In addition to the main crystalline phase, trace amounts of secondary phases ZrO2 and ZnCuTiO4 were also observed from SEM and EDS analysis for all ceramics. The permittivity decreased with the increasing x in the range of x = 2.60-30.20 and then remained stable at x = 30.20-37.66, corresponding to the density and covalency bond due to the substitution of (Zn1/3Nb2/3)4+ for Ti4+ and phase transition at x = 30.20. Furthermore, it was surprisingly found that the temperature coefficient of resonant frequency was near-zero (τf = -9.2 to +9.9 ppm/°C) in a very wide range of x = 10.80-30.20.

Phase structure, sintering behaviour and microwave dielectric properties of Ln2MoO6 (Ln = La and Y) ceramics

Microwave dielectric ceramics of Ln2MoO6(Ln = La and Y) were fabricated via a conventional solid-state reaction route. XRD, Rietveld refinement, SEM and vector network analysis were applied to analyse the crystal structural, microstructural and dielectric properties of the ceramics. The Y2MoO6 ceramic was crystallized in a monoclinic crystal structure, whereas the La2MoO6 ceramic could form a pure tetragonal structure. All components can be sintered at 1200–1500 °C with a relative permittivity of 14.1–17.1 and a high quality factor. The La2MoO6 ceramic exhibits satisfactory dielectric performance with Q × f = 67,090 GHz, εr = 16.6 and τf = −50.1 ppm/°C, while Q × f = 27,760 GHz, εr = 14.6, and τf = −37.4 ppm/°C for Y2MoO6; hence, these ceramics could be candidates for 5G technology.

Ultra-high-Q and wide temperature stable Ba(Mg1/3Tax)O3 microwave dielectric ceramic for 5G-oriented dielectric duplexer adhibition

With the deployment of 5G (5th generation wireless systems) worldwide, the development of dielectric duplexers more suitable for high-frequency applications is in full swing. Thus, the key material of dielectric duplexers, microwave dielectric ceramics, have attracted widespread attention from scholars. Here, high-performance microwave dielectric ceramics were prepared by adjusting the stoichiometry of Ta5+ of Ba(Mg1/3Tax)O3 within 0.67–0.68. Typically, excellent resonance frequency stability (resonance frequency offset is less than 1.2 MHz) in a wide temperature range (−55–85 °C) is obtained by all samples, which extends to the negative temperature range compared to previous studies. Optimally, the Ba(Mg1/3Ta0.675)O3 ceramic sintered at 1620 °C/12h and annealed at 1550 °C/10h is provided with the microwave dielectric properties of εr∼25.021, Q × f∼244,155 GHz, τf ∼ -0.74 ppm/°C, achieving an 11% increase in Q × f value (compared to pure BMT). Comprehensive and excellent performance makes Ba(Mg1/3Ta0.675)O3 ceramics furnish powerful material support for 5G dielectric duplexer applications.

Novel low-permittivity microwave dielectric ceramics in garnet-type Ca4ZrGe3O12

Novel low-permittivity microwave dielectric ceramics Ca4ZrGe3O12 with garnet structure were fabricated by a traditional solid-state route. A part of the Ca2+ cations are located at the octahedral sites, a quite unusual coordination for this cation in oxygen compounds. The structural characteristics and vibrational modes for the ceramics were studied based on structure refinement and Raman spectra analysis. Owning to the octahedral site is occupied by equivalent Ca2+ and Zr4+ simultaneously, the deviation between εtheo and εr could be related to the rattling effect of smaller Zr4+ in the octahedron. Optimum microwave dielectric properties were obtained for Ca4ZrGe3O12 ceramic sintered at 1340 °C with εr = 10.68, Q × f = 76900 GHz, and τf = −41.3 ppm/°C.

Influence of substituting Na+ for Mg2+ on the crystal structure and microwave dielectric properties of Mg1-xNa2xWO4 ceramics

The Mg1-xNa2xWO4 (0 ≤ x ≤ 0.14) microwave dielectric ceramics were prepared through traditional solid-state reaction and sintered at 875−925 °C for 4 h. Compared with pure MgWO4 ceramic, the samples with substituting Na+ for Mg2+ could be sintered at a lower temperature, the quality factor values and the densification of microstructure were obviously improved. Raman spectroscopy proved that the relative permittivity was related to the multiple phases and molecular polarizability. In addition, the distortion of[MgO6] octahedron also changed, resulting in resonance frequency temperature coefficient varied nonlinearly from x = 0 to 0.14. Considering the applications of low temperature co-fired ceramics technology, the Mg1-xNa2xWO4 (x = 0.06) ceramic sintered at 875 °C had the best performance. Not only did it have excellent microwave dielectric properties of εr = 10.474, Q × f = 45,868 GHz, and τf = −69 ppm /ºC, but also presented great chemical compatibility with the commonly used Ag electrode.

A novel low sintering temperature scheelite-structured CaBiVMoO8 microwave dielectric ceramics

Abstract Novel medium-permittivity scheelite-structured CaBiVMoO8 ceramics were prepared by a solid-state reaction route. The effects of sintering temperature on the phase composition, microstructure and microwave dielectric properties of CaBiVMoO8 ceramics were investigated systematically. The Rietveld refinements of X-ray data revealed all samples were well crystallized in space group I41/a. All the samples exhibited a closely packed grain morphology and discernable grain boundaries. The relationship between microwave dielectric properties and bulk density of samples at different sintering temperatures was further researched. Besides, the lattice energy was one of the important factors affecting the Q×f values of CaBiVMoO8 ceramics. The temperature coefficient of resonant frequency (τf) could be slightly affected by sintering temperature. The CaBiVMoO8 ceramics were expected to be applied to low temperature co-fired ceramics technology. In this paper, the CaBiVMoO8 ceramics exhibited excellent properties sintered at 850 °C for 4h: er ∼ 40.55, Q×f ∼ 16,670 GHz, τf ∼ +55.90 ppm/°C.

A facile chemical route to prepare Nd[(Zn0.7Co0.3)0.5Ti0.5]O3 powders and microwave dielectric materials

High-purity Nd[(Zn0.7Co0.3)0.5Ti0.5]O3 powders with perovskite structure, which were employed to prepare microwave dielectric ceramics, were produced by a facile chemical route from a solution containing neodymium oxide, zinc nitrate hexahydrate, cobalt nitrate hexahydrate, tetrabutyl titanate, citric acid, and ethylene glycol based on a citrate precursor process. Furthermore, the sintering behavior, microstructure, and microwave dielectric properties of Nd[(Zn0.7Co0.3)0.5Ti0.5]O3 ceramics were evaluated. XRD and FT-IR results showed that the polymeric precursor could directly transform into Nd[(Zn0.7Co0.3)0.5Ti0.5]O3 powders without forming any intermediate phase. TEM results revealed that the as-obtained powders after calcination treatment at 800 °C were approximately spherical in shape and slightly agglomerated with an average particle size of 15 nm. Typical microwave dielectric properties of er = 35.26, Q × f = 236,800 GHz (f = 9.52 GHz), and τf = –28.6 ppm/°C were achieved for the Nd[(Zn0.7Co0.3)0.5Ti0.5]O3 ceramics with a relative density of 98.7% sintered at 1250 °C for 2 h.

Crystal structure, Raman spectra and microwave dielectric properties of novel temperature-stable LiYbSiO4 ceramics

Olivine−type structure microwave dielectric ceramics LiYbSiO4 with near-zero τf were fabricated by solid-state reaction process for the first time. The relationships among structural parameters, sintering behavior, vibrational modes and microwave dielectric properties for the ceramics were studied. The variation in εr could be correlated with Raman shift. The variation in Q×f values was inversely correlated to FWMH and average cation covalency. The τf values were explained with the bond valence sum of cations. Single phase LiYbSiO4 ceramics could be obtained at the range of 1100–1140 °C and showed promising microwave dielectric properties with εr = 7.36 – 7.42, Q×f = 19081 – 25276 GHz and τf = +4.52 – +8.03 ppm/°C.

A novel high-Q and low-temperature sintering Li2Mg3Ti1-x(Al1/2Nb1/2)xO6-4wt%LiF microwave dielectric ceramics

A novel system Li2Mg3Ti1-x(Al1/2Nb1/2)xO6-4wt%LiF(0.02 ≤ x ≤ 0.08) ceramics were synthesized by the solid stated reaction method to improve the microwave dielectric properties of the ceramics. The phase composition, sintering characteristics and microwave dielectric properties of Li2Mg3Ti1-x(Al1/2Nb1/2)xO6-4wt%LiF(0.02 ≤ x ≤ 0.08) ceramics were then explored. The results demonstrated that at the optimal sintering temperature(900 °C), with the (Al1/2Nb1/2)4+ ions increased, the εr value decreased and the absolute value of τƒ augmented. Moreover, the Q × f value of ceramics was in accord with the relative density, which enlarged at first until the doping content reached the solid solubility limit (0.04 mol) of the ceramics. In total, the Li2Mg3Ti1-0.96(Al1/2Nb1/2)0.04O6-4wt%LiF ceramics sintered at 900 °C for 6 h showed the best microwave dielectric properties of εr~15.54, Q × f~154,000 GHz and τƒ~-38.3 ppm/°C.

Microwave dielectric properties of glass-free CaMg0.9-xLi0.2ZnxSi2O6 ceramics for LTCC applications

Herein, glass-free microwave dielectric ceramics CaMg0.9-xLi0.2ZnxSi2O6 (x = 0–0.36) with excellent microwave dielectric properties were prepared via traditional solid-state reaction method. Phase composition and crystalline structure were identified using X-ray diffraction (XRD), which confirmed that the ceramics contained more than one phase. Rietveld refinement method was utilized to analyze XRD data and obtain structural information, such as lattice parameters. SEM photographs showed that Zn2+ substitution could effectively promote densification of the ceramics. Microwave dielectric properties were obviously influenced by the combination of multiple phases. Combined with Raman spectra, influences of Raman shift and FWHM of vibration peaks on microwave dielectric properties were determined. The CaMg0.9-xLi0.2ZnxSi2O6 ceramic exhibited high performance at x = 0.28 with: Q × f = 68,190 GHz, εr = 7.51 and τf = −68 ppm/°C, when sintered at 900 °C. The studied ceramics are promising candidates for LTCC application.

The structure evolution and microwave dielectric properties of MgAl2-x(Mg0·5Ti0.5)xO4 solid solutions

In the present work, MgAl2-x(Mg0·5Ti0.5)xO4 (x = 0.02, 0.04, 0.06, 0.08, 0.10) solid solutions were synthesized via the traditional solid-state reaction route. The valence state of Ti ions, crystal structural characteristics, and microwave dielectric properties were discussed. A solid solution with spinel structure was revealed by the Rietveld refinement results. The partial substitution of (Mg0·5Ti0.5)3+ for Al3+ lowered the sintering temperature and improved the Q × f value of MgAl2O4 ceramic. The MgAl2-x(Mg0·5Ti0.5)xO4 solid solutions with x = 0.06 can be well sintered at 1425 °C in an oxygen atmosphere for 8 h and exhibits excellent microwave dielectric properties with εr = 9.1, Q × f = 98,000 GHz, τf = −61.36 ppm/°C. The sintering temperature of MgAl1·94(Mg0·5Ti0.5)0.06O4 microwave dielectric ceramics was approximately 200 °C lower than that of conventional MgAl2O4 ceramics.

A3Y2Ge3O12 (A = Ca, Mg): Two novel microwave dielectric ceramics with contrasting τf and Q × f

Two garnet-structured microwave dielectric ceramics of A3Y2Ge3O12 (A = Ca, Mg; called CYG and MYG, respectively) were synthesized. CYG was crystallized in a normal cubic garnet structure with Ca2+ fully occupying the dodecahedral (A) site, whereas MYG was an inverse garnet with mixed distribution of Mg2+/Y3+ at the A site. The difference in ionic occupation resulted in anomalies in microwave dielectric properties, with dielectric constant (εr) = 14.1, quality factor (Q × f) = 12,600 GHz and positive temperature coefficient of resonant frequency (τf) =120.5 ppm/°C for MYG ceramic and εr = 10.8, Q × f = 97,126 GHz and τf = −40.6 ppm/°C for CYG ceramic. The large deviations in measured εr from theoretical εth possibly resulted from the garnet structural constraints leading to ‘rattling’ Mg2+ and Y3+ in A site of MYG. Infrared reflectivity spectra analysis revealed ion polarization contributed mostly to the permittivity of both ceramics in microwave frequency ranges.

Synthesis of NdAlO3 with good microwave dielectric properties by stearic acid method

In this study, NdAlO3 with perovskite structure was synthesized by the stearic acid method at relatively low temperature. The structural characteristics of the as-synthesized product were identified by TG–DSC, XRD, FT–IR, SEM, and TEM techniques. Using the powders as starting materials, NdAlO3 bulk microwave ceramics were prepared, and the corresponding densification process, microstructural and dielectric properties were studied. The XRD and FI–IR results confirmed that single phase NdAlO3 could be prepared at low temperature by the stearic acid method. A unique two-dimensional platelike morphology with an unevenly dispersed bubble shape structure was observed in the calcined powder. However, the TEM result revealed that the powder calcined at 800 °C had a good dispersity accompanied with narrow particle size distribution within a range of 20–35 nm. The average particle size of 27.3 nm was in accordance with that calculated from the XRD data. Using the powder calcined at 800 °C as raw materials, the as-obtained NdAlO3 ceramics sintered at 1500 °C for 4 h possessed the highest density and favorable combined microwave dielectric properties (i.e., εr = 23.02, Q × f = 65320 GHz, and τf = −32.4 ppm/°C). The present work developed a fast, energy-efficient approach to synthesize NdAlO3 powder used as promising raw materials of microwave dielectric ceramics.

Bond analysis of novel MnZrTa2O8 microwave dielectric ceramics with monoclinic structure

A new type of ceramic, namely MnZrTa2O8, was synthesized after sintering at high temperature in this work. The possible dielectric loss mechanism was discussed by Raman spectroscopy and chemical bond theory. X-ray diffraction indicated that MnZrTa2O8 formed through a reaction between ZrO2 and intermediate MnTa2O6. After sintering at 1350 °C, a monoclinic structure with cell parameters a = 4.8370(3) A, b = 5.7163(1) A, c = 5.1398(5) A, β = 91.7219° was gained for ceramic. Among all bonds, Ta–O with the greatest bond ionicity and lattice energy was the dominant factor that influenced the microwave dielectric properties. The temperature coefficient of the resonant frequency τf changed from − 50.55 to − 41.21 ppm/ °C, which was related to the lattice energy. The effect of porosity on dielectric loss was also checked and found to be significant. MnZrTa2O8 ceramic exhibited relative permittivity er ~ 23.0 and enhanced quality factor Q × f~48103 GHz (at 8.97 GHz), which provided a promising candidate for electric components.

High-performance ZnTiNb2O8 microwave dielectric ceramics produced from ZnNb2O6–TiO2 nano powders

Abstract Ixiolite-structured ZnTiNb2O8 (ZTN) ceramics were prepared by the simple but effective route of reaction sintering ZnNb2O6 and TiO2 nano powders. The effect of sintering temperature and soaking time on the densification behavior, microstructure evolution and microwave dielectric properties of the synthesized ZTN ceramics were systematically studied. Nano ZnNb2O6 and TiO2 raw materials were mixed, pressed and sintered directly into ceramic bodies without any calcination step involved. The sintered samples were characterized by Field-Emission Scanning Electron Microscopy (FESEM), X-ray Diffraction (XRD), and microwave dielectric measurements. Single-phase ZnTiNb2O8 ceramics with relative densities up to 97% theoretical density were achieved by sintering at 975–1075 °C. The ixiolite ZTN ceramics reaction sintered at 1050 °C for 3 h exhibited excellent microwave dielectric properties with a dielectric constant ( e r ) = 35.5, quality factor (Q × f0) = 52,500 GHz and a temperature coefficient of resonant frequency ( τ f ) = −60 ppm/°C. These results demonstrate that high-performance ZTN microwave dielectric ceramics can be successfully produced by the simple and efficient reaction-sintering method.

Structure, spectral analysis and microwave dielectric properties of novel x(NaBi)0.5MoO4-(1-x)Bi2/3MoO4 (x = 0.2 ∼ 0.8) ceramics with low sintering temperatures

Herein, the x(NaBi)0.5MoO4-(1-x)Bi2/3MoO4 (xNBM-(1-x)BMO, x = 0.2 ∼ 0.8) microwave dielectric ceramics with low sintering temperatures were prepared via the traditional solid-state method to adjust the τf value and dielectric constant. The crystal structure was determined using X-Ray diffraction and Raman spectroscopy, the microstructure was investigated using scanning electron micrograph and energy disperse spectroscopy, and the dielectric properties were studied using a network analyser and infrared spectroscopy. For the xNBM-(1-x)BMO composite ceramics, the (NaBi)0.5MoO4 tetragonal phase coexisted with the Bi2/3MoO4 monoclinic phase. With the rise of x value, the permittivity increased from 23.7–29.8, and the τf value shifited from -53.3 ppm/°C to -13.7 ppm/°C. The 0.8NBM-0.2BMO ceramic sintered at 680 °C possessed excellent microwave dielectric properties with a εr = 29.8 (6.7 GHz), a Qf = 11,800 GHz, and a τf = -13.7 ppm/°C. These results made the xNBM-(1-x)BMO composite ceramics great candidates in low temperature co-fired ceramics technology.

Ca3Al2O6: novel low-permittivity microwave dielectric ceramics with abnormally large negative τf

Ca3Al2O6 ceramics are synthesized using a standard solid-state reaction method and their microwave dielectric properties are systematically investigated. Pure phases with space group of Pa_3 are confirmed from X-ray diffraction patterns (XRD) and Rietveld refinements. er and Qf values of the present ceramics are mainly determined by the porosity, and the optimal values (er = 14.5, Qf = 24,500 GHz) are achieved when sintered at 1425 °C. Abnormally large negative τf values in the range of − 322 to − 354 ppm/°C are obtained, which are rare and extremely important in adjusting the positive τf values of other material systems. In addition, a large temperature sensitivity of − 4.19 MHz/°C is also confirmed in the present ceramics, which indicates great potential for temperature sensor applications. Moreover, the dielectric spectra measured under various radio frequencies reveal similar positive τe values of 715.5 ppm/°C, which could account for the large negative τf values in the operating temperatures.