Low-permittivity Microwave Research Articles

Preparation and microwave dielectric properties of temperature-stable low-dielectric (1−x)Li2MoO4−xBi0.4Ce0.6VO4 ceramics for satellite communication applications

In this study, a series of ([Formula: see text]x)Li2MoO4[Formula: see text]xBi0.4Ce0.6VO4 [x = 0.25, 0.35, 0.45, 0.55, ([Formula: see text]x)LM–xBCV] microwave dielectric ceramics, capable of being sintered at low temperatures, were synthesized using the solid-phase method. The temperature coefficient of resonance frequency in LM microwave dielectric ceramics was effectively modulated through the incorporation of BCV with tetragonal zirconia phases, resulting in the formation of composite ceramics. This approach led to the development of a low-permittivity ([Formula: see text]r ∼ 12.6) microwave dielectric ceramic material exhibiting a near-zero temperature coefficient (TCF ∼ +0.5 ppm/∘C). The composite, specifically 0.55LM–0.45BCV, achieved ultra-low temperature sintering below [Formula: see text]C and demonstrated excellent performance as low-temperature co-fired ceramic materials, with good co-firing compatibility with metallic silver. Furthermore, the volatilization of lithium and bismuth elements was effectively mitigated through the burying sintering process. Which resulted in a substantial enhancement of the [Formula: see text] (Q and f denote the quality factor and the resonant frequency, respectively) value for the 0.55LM–0.45BCV ceramics, achieving an increase of nearly threefold (Q × f [Formula: see text] [Formula: see text]GHz). To investigate the application potential of 0.55LM–0.45BCV ceramic, particularly in the context of low-orbit communication satellites, a circularly polarized patch antenna was designed utilizing 0.55LM–0.45BCV ceramic as the dielectric substrate material. The antenna demonstrated a high simulated radiation efficiency of 93.8% and a gain of 4.0[Formula: see text]dBi at a center frequency of 1.559[Formula: see text]GHz, indicating its promising applicability in the domain of orbital satellite communication.

LiGaSiO4: An ultra-low permittivity dielectric material enabling application in patch antenna

A low permittivity microwave dielectric ceramic, Li-containing silicate (LiGaSiO4), was synthesized using the standard solid-state reaction method. The nominal LiGaSiO4 ceramic exhibited a relative permittivity of approximately 4.4. The effects of nonstoichiometric Li on the microstructure, sintering behavior, and microwave dielectric properties of LiGaSiO4 were comprehensively investigated. Excess Li content (6 mol%) significantly improved the relative density (96.1 %) and microwave dielectric properties (εr∼5.34, Q × f ∼55,600 GHz, and τf ∼ -48.6 ppm/°C), while reducing the optimum sintering temperature from 1070 °C to ∼1010 °C. Additionally, the incorporation of 2 wt% LiF further reduced densification temperature (∼940 °C). The regulation of the τf was achieved through the addition of CaTiO3 ceramic, and a patch antenna was designed and fabricated using a modified Li1.06GaSiO4-LiF-CaTiO3 composite ceramic. The resulting patch antenna resonates at 5.87 GHz with an impressive bandwidth of 83.1 MHz, while both simulated and measured results demonstrate excellent agreement.

A novel low permittivity microwave dielectric ceramic Sr2Ga2SiO7 for application in patch antenna

AbstractThe dielectric properties of a Ga‐based melilite type ceramic Sr2Ga2SiO7 via theoretical prediction based on far‐infrared spectroscopy and experimental measurement by the Hakki–Coleman method were studied in this work. Dense and single‐phase ceramics were fabricated via solid‐state reaction at 1330°C and exhibited comprehensive microwave dielectric properties (εr ∼ 7.6, Q × f ∼ 23 600 GHz, and τf ∼ −35.2 ppm/°C) at 14.3 GHz. Chemical modifications were proposed to adjust the thermal stability and reduce the densification temperature. By adding 10 mol% CaTiO3, the negative τf can be compensated to a near‐zero value of −3.8 ppm/°C. The densification temperature was reduced to 940°C by adding 3 wt.% LiF. A patch antenna was designed using Sr2Ga2SiO7 ceramic with a high radiation efficiency of 99.1% and a gain of 2.788 dBi at the center frequency of 4.371 GHz. All results indicate that the Sr2Ga2SiO7 ceramic has promising application potential for 5G wireless communication technology.

Machine learning approaches for permittivity prediction and rational design of microwave dielectric ceramics

Low permittivity microwave dielectric ceramics (MWDCs) are attracting great interest because of their promising applications in the new era of 5G and IoT. Although theoretical rules and computational methods are of practical use for permittivity prediction, unsatisfactory predictability and universality impede rational design of new high-performance materials. In this work, based on a dataset of 254 single-phase microwave dielectric ceramics (MWDCs), machine learning (ML) methods established a high accuracy model for permittivity prediction and gave insights of quantitative chemistry/structure-property relationships. We employed five commonly-used algorithms, and introduced 32 intrinsic chemical, structural and thermodynamic features which have correlations with permittivity for modeling. Machine learning results help identify the permittivity decisive factors, including polarizability per unit volume, average bond length, and average cell volume per atom. The feature-property relationships were discussed. The optimal model constructed by support vector regression with radial basis function kernel was validated its superior predictability and generalization by verification dataset. Low permittivity material systems were screened from a dataset of ∼3300 materials without reported microwave permittivity by high-throughput prediction using optimal model. Several predicted low permittivity ceramics were synthesized, and the experimental results agree well with ML prediction, which confirmed the reliability of the prediction model.

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.

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.

Ca3MgSi2O8: Novel low-permittivity microwave dielectric ceramics for 5G application

In this research, the novel low permittivity Ca3MgSi2O8 microwave dielectric ceramics have been fabricated via conventional solid state process and their synthesis, sintering and microwave dielectric properties were investigated. The thermal behavior of the ball milled stoichiometric mixtures was investigated using simultaneous thermal analysis. Scanning electron microscopy image and particle size distribution of synthesized powder showed that the particle size fell into a range of ~1–11 µm. X-ray diffraction analysis of synthesized and sintered ceramics showed that single phase Ca3MgSi2O8 crystallized in a monoclinic structure. Relative density near 99% of theoretical density, εr = 13.8, quality factor (Q × f) ~ 27000 GHz (at 10.8 GHz) and τf = −62 were obtained for ceramics sintered at 1375 °C. Hence, it is to be hoped that Ca3MgSi2O8 ceramics can be selected as a possible candidate for 5G application and their performance can be improved by optimization of synthesis and sintering process.

Phase compositions and microwave dielectric properties of Sn-deficient Ca2SnO4 ceramics

The phase compositions and low-permittivity microwave dielectric properties of Ca2Sn(1−x)O(4−2x) (0 ≤ x ≤ 1.0) ceramics were investigated through a solid-state reaction at 1350 °C-1450 °C for 5 h. A small Sn-deficiency is beneficial to inhibiting a CaSnO3 second phase and forming a Ca2SnO4 single phase with excellent microwave dielectric properties (εr = 9.9, Q × f = 120,100 GHz, and τf = −45.6 ppm/°C) in the Ca2Sn(1−x)O(4−2x) (x = 0.02) ceramics. With an increase in x from 0.04 to 1.0, the amount of CaO phase increased, and the Ca2SnO4 phase decreased gradually. Finally, a CaO pure phase (εr = 10.6, Q × f = 229,600 GHz, and τf = −37.5 ppm/°C) was obtained at x = 1.0.

Phase composition and microwave dielectric properties of low permittivity AGeO3 (A = Mg, Zn) ceramics

It has become a focus to develop low-permittivity microwave dielectric materials to meet the needs of high-speed transmission in telecommunication devices. Here, we report two Ge-based microwave dielectric ceramics prepared using the conventional solid-state method, MgGeO3 and ZnGeO3, with low permittivity of 6.8 and 6.6, the high-quality factor of 65,200 GHz and 25,200 GHz, and negative temperature coefficient of −45 ppm/°C and −35 ppm/°C. Orthorhombic ilmenite structure of MgGeO3 can be obtained in the range of 1160–1240 °C. However, the single-phase ZnGeO3 is not available in air, and the ceramics sintered at 1040–1120 °C contain two phases of Zn2GeO4 and GeO2.

Two novel low permittivity microwave dielectric ceramics Li2TiMO5 (M = Ge, Si) with abnormally positive τf

Two tetragonal natisite structured Li2TiMO5 (M = Ge, Si) ceramics fabricated using the conventional solid-state reaction method were investigated in terms of the thermal stability, sintering behavior and dielectric properties at radio (RF) and microwave frequency region. At the optimum sintering temperature of 1140 °C, Li2TiGeO5 (LTG) has εr ˜ 9.43, Q × f ˜ 65,300 GHz (at 14.7 GHz), and τf ˜ +24.1 ppm/°C, while Li2TiSiO5 (LTS) sintered at 1180 °C exhibits εr ˜ 9.89, Q × f ˜ 38,100 GHz (at 14.2 GHz), and τf ˜ +50.1 ppm/°C. The positive τf values of the present LTG and LTS are abnormal and extremely important for low-εr microwave dielectric ceramics, which could behave as a promising τf compensator. Moreover, the dielectric spectra of both ceramics revealed a phase transition at low-temperature, exhibiting a dielectric peak, which could account for the negative τε and positive τf in operating temperature ranges.

Optimised phase compositions and improved microwave dielectric properties based on calcium tin silicates

The Ca(1+2y)Sn(1-x)Si(1+y)O(5-2x+4y) low-permittivity microwave dielectric ceramics were prepared through solid-state reaction at 1350–1450 °C for 5 h. The relations between microwave dielectric properties and phase compositions for non-stoichiometric Ca(1+2y)Sn(1-x)Si(1+y)O(5-2x+4y) ceramics have been investigated. A single CaSnSiO5 phase with abnormally positive temperature coefficient of resonant frequency (τf = + 62.5 ppm/°C) was synthesised at 1450 °C. This composition was an effective τf compensator of CaSiO3 and Ca3SnSi2O9 phases with typically negative τf value. The CaSiO3 second phase was related to the Sn deficiency in the CaSn(1-x)SiO(5-2x) (0 <x < 1.0) composition, whereas the Ca3SnSi2O9 second phase was obtained by controlling the Ca:Sn:Si ratios on the basis of the Ca(1+2y)SnSi(1+y)O(5+4y) (0 <y < 1.0) composition. A promising low-permittivity millimetre-wave ceramic with most excellent microwave dielectric properties (εr = 10.2, Q×f = 81,000 GHz and τf = −4.8 ppm/°C) was produced from the Ca(1+2y)SnSi(1+y)O(5+4y) (y = 0.4) ceramic.

Influence of Mg2SiO4 addition on crystal structure and microwave properties of Mg2Al4Si5O18 ceramic system

In this work, (1 − x)Mg2Al4Si5O18–xMg2SiO4 (x = 0–80 wt%) composite dielectric ceramics were prepared via traditional solid-state reaction. The results indicate the sinterability of Mg2Al4Si5O18 ceramics is greatly enhanced with the increasing content of Mg2SiO4 by reducing densified temperature from 1460 to 1340 °C. Rietveld refinement analysis shows a great chemical compatibility between two phases. With the increase of Mg2SiO4 content, the densifications of ceramics can be improved. However, excessive amount of Mg2SiO4 induces abnormal grain growth, which deteriorates the microwave dielectric properties. At x = 50 wt%, low-er dielectric ceramics with high Q × f value was obtained when sintered at 1340 °C: er = 5.73, Q × f = 76,374 GHz and τf = − 24 ppm/ °C. Relative cheap raw materials and adjustable permittivity values, which makes it promising for low-permittivity microwave applications.

Low-temperature sintering and microwave dielectric properties of CaSnxSiO(3+2x)-based positive τf compensator

The sinterability, phase compositions, and microwave dielectric properties of LiF-doped nonstoichiometric CaSnxSiO(3+2x) ceramics prepared by the solid-state reaction were investigated. LiF addition effectively reduced the sintering temperature of CaSnxSiO(3+2x) ceramics and inhibited the volatilization of Sn. A pure monoclinic CaSnSiO5 phase was achieved in the 1.0 wt% LiF-doped CaSn0.94SiO4.88 ceramics sintered at 1175 °C, which exhibited good microwave dielectric properties of εr = 11.6, Q × f = 34000 GHz, and τf = +73.2 ppm/°C. The positive τf value was an atypical and important phenomenon for low-permittivity microwave dielectric ceramics, which could be a promising τf compensator.

LiYGeO4: Novel low-permittivity microwave dielectric ceramics with intrinsic low sintering temperature

Using the solid-state reaction route, a new low-firing microwave dielectric ceramic LiYGeO4 was prepared, and the phase evolution, thermal stability, and dielectric properties were characterized. A single orthorhombic phase LiYGeO4 formed in the sintering temperature range of 920–960 °C decomposed into Y2GeO5, GeO2, and Li2O when the sintering temperature exceeded 960 °C. LiYGeO4 densified at 940 °C/6 h possessed a relative permittivity 9.41, a quality factor 18,860 GHz (at 12.8 GHz), and a temperature coefficient of resonant frequency −27.7 ppm/°C. The negative τf value was compensated by compositing with CaTiO3, and 0.97LiYGeO4-0.03CaTiO3 ceramic exhibited a near-zero τf of −1.37 ppm/°C along with a permittivity of 9.83 and a quality factor of 12,940 GHz (at 13.2 GHz). All merits make LiYGeO4 a promising candidate for high-frequency communication application, and low-temperature co-fired ceramics.

Crystal structure, phase composition and microwave dielectric properties of Ca3MSi2O9 ceramics

Ca3MSi2O9 (M = Zr, Hf, Sn1−xTix; x = 0–0.40) low-permittivity microwave dielectric ceramics with cuspidine structure were prepared using conventional solid-state method at 1325 °C–1400 °C for 10 h in air. The lattice parameters of Ca3MSi2O9 (CMS) linearly decreased when M changed from Zr to Sn0.85Ti0.15, and a single phase was formed. For M = Sn1−xTix (x = 0.20–0.40), a second phase of CaTiO3 (CT) appeared. The relative permittivity (εr), quality factor (Q × f), and temperature coefficient of resonant frequency (τf) were closely related to ionic polarizability, relative covalence of M site, unit-cell volume, and CaTiO3 second phase. As a result, the highest Q × f value of 72840 GHz was obtained for M = Sn0.95Ti0.05, and near zero τf value was achieved for M = Sn0.7Ti0.30. The microwave dielectric properties of which are as follows: εr = 11.07, Q × f = 42400 GHz, and τf = −5.1 ppm/°C.

Li2AGeO4 (A = Zn, Mg): Two novel low-permittivity microwave dielectric ceramics with olivine structure

Two low-permittivity dielectric materials Li2AGeO4 (A = Zn, Mg) were prepared via the solid-state reaction method. X-ray diffraction analysis and Rietveld refinement indicated that both ceramics crystallize in an orthorhombic olivine structure with a space group Pmn21. Dense ceramics with high relative density and homogeneous microstructure were obtained. Li2ZnGeO4 densified at 1200 °C possessed a relative permittivity εr = 6.5, a quality factor Q × f = 35,400 GHz, and a temperature coefficient of resonant frequency. Li2MgGeO4 exhibited εr = 6.1, Q × f = 28,500 GHz, and τf = –74.7 ppm/°C when sintered at 1220 °C. Additionally, the large negative τf values of Li2AGeO4 (A = Zn, Mg) ceramics were successfully adjusted compensated by forming composite ceramics with CaTiO3 and near-zero τf values of +2.9 ppm/°C and +5.8 ppm/°C were achieved in 0.92Li2ZnGeO4-0.08CaTiO3 and 0.90Li2MgGeO4-0.10CaTiO3, respectively.

Temperature-stable BaAl2Si2O8–Ba5Si8O21-based low-permittivity microwave dielectric ceramics for LTCC applications

Abstract Low-temperature sintering (1− x )BaAl 2 Si 2 O 8 – x Ba 5 Si 8 O 21 microwave dielectric ceramics were prepared by solid-state reaction. The effects of LiF (1 wt%) on the sintering behaviour, phase composition and microwave dielectric properties of (1− x )BaAl 2 Si 2 O 8 – x Ba 5 Si 8 O 21 ( x = 0, 0.3 and 1.0) were investigated. The addition of 1 wt% LiF lowers sintering temperature to 900 °C ( x = 0) and 750 °C ( x = 0.3 and 1.0) without changing phase composition and apparently degrading microwave dielectric properties. 0.7BaAl 2 Si 2 O 8 –0.3Ba 5 Si 8 O 21 +1 wt% LiF was well-sintered at 750 °C and showed good microwave dielectric properties with e r = 7.63, Q × f = 12,410 GHz and τ f = −1.2 ppm/°C. This composite exhibited good chemical compatibility with Ag electrodes, making it a promising candidate for practical applications of low-temperature co-fired ceramics.

Weak ferroelectricity and low-permittivity microwave dielectric properties of Ba2Zn(1+x)Si2O(7+x) ceramics

Ba2Zn(1+x)Si2O(7+x) ceramics were prepared using the conventional solid-state method at 1200°C for 3h in air. Apart from the previously reported Ba2Zn(1+x)Si2O(7+x) (x=0) with a monoclinic structure (C 2/c), the end-member compositions at x=−1 and 1 exhibit single-phase β-BaSiO3 with an orthorhombic structure (P212121) and BaZnSiO4 with a hexagonal structure (P63), and possess a coexistence of weak ferroelectricity and low-permittivity microwave dielectric properties. A reduction in Zn2+ content mainly decreases the intensity of the εr anomaly peak at lower temperature and increases the εr (or frequency) stability against temperature. The Zn2+-rich BaZnSiO4 phase has a τf value of −181ppm/°C, whereas the τf value of Zn2+-free BaSiO3 phase decreases to −35.4ppm/°C. The Zn2+ deficiency in Ba2ZnSi2O7 composition could inhibit the presence of BaZnSiO4 phase and improve the τf value, whereas excessive Zn2+ cations prompt the formation of the BaZnSiO4 phase to deteriorate significantly the τf value.

Novel high Curie temperature Ba2ZnSi2O7 ferroelectrics with low-permittivity microwave dielectric properties

Ba2ZnSi2O7 ceramics with a monoclinic structure (C 2/c) were prepared using the conventional solid-state method at 1200°C for 3h in air. The as-prepared ceramics show signs of weak ferroelectricity from the P-E loop and ferroelectric domains with 90° and 103° directions. Although an εr anomaly appears at approximately 510°C, the Curie temperature (Tc) of the Ba2ZnSi2O7 ceramics is 750°C or higher. The ceramics also display low-permittivity microwave dielectric properties (εr=8.09, Q×f=26,634GHz, and τf=−51.46ppm/°C). Therefore, the coexistence of high-Tc ferroelectricity and good microwave dielectric properties is beneficial to develop Ba2ZnSi2O7 ceramics into tunable-microwave and millimeter-wave devices such as capacitors, phase shifters, and oscillators.

Printable Planar Dielectric Antennas

Thick film screen-printing technology is employed to introduce planar dielectric antennas. The micron-size-thick film is made from high-permittivity dielectric paste, which is printed on a low-loss low-permittivity microwave substrate to operate as a magnetic wall boundary. A simplified theoretical model is developed to analyze the antenna structure. The accuracy of this model is evaluated by the results of a commercial three-dimensional (3-D) electromagnetic field solver. The model works well with less than 5% deviation from time-consuming simulations, providing the permittivity contrast between the film and substrate is kept high (e.g., > 30). The impact of dielectric loss of the film in antenna performance is significantly less than that of the substrate, allowing medium-loss films (e.g., ε"= 3) results in highly efficient antennas with over 95% radiation efficiency. A prototype was made using screen printing and experimentally characterized at Ku-band. The experimental results confirm the theoretical achievements. The measured -10 dB impedance bandwidth of the printable planar dielectric antenna having dimensions of 0.2 λ0× 0.2 λ0 is over 10% with a measured gain ranging from 4.5 to 5.5 dBi. Broadside radiation patterns with cross polarization levels of lower than -20 dB are achieved. This process is compatible with low-temperature cofired ceramic (LTCC) technology where similar idea can be applied to other passive microwave components.