Enhanced Microwave Dielectric Properties Research Articles

Phase composition, crystal structures and enhanced microwave dielectric properties of Mg2+/Zr4+ co-doped Al2Mo3O12 ceramics

In this work, the substitution of Al3+ by Zr/Mg in Al2Mo3O12 was designed to enhance the quality factor while simultaneously adjusting the thermal stability of the resonant frequency. The Al2–2x(Zr0.5Mg0.5)2xMo3O12 (AMZM) (0.1 ≤ x ≤ 0.5) ceramics were synthesized using a simple solid-state method, resulting in all components exhibiting single-phase crystallinity and a dense structure. A phase transition temperature below room temperature was observed when x ≥ 0.4. Furthermore, the quality factor of the synthesized samples, compared to pure-phase Al2Mo3O12 (εr = 6.79, Q×f = 28471 GHz, τf = −48.77 ppm/℃), increased by nearly threefold. A detailed investigation of the crystal structure of the samples was conducted through Rietveld refinement. The composition with x = 0.3 (monoclinic phase) exhibited optimal dielectric performance after sintering at 800 ℃ for 6 h: εr = 7.84, Q×f = 86511 GHz, τf = −42.08 ppm/℃. By forming a composite ceramic with (Li1/2Sm1/2)MoO4, the negative τf value was further adjusted to near zero.

Enhanced microwave dielectric properties of Al2O3 ceramics via a cold sintering assisted two-step sintering route

Enhanced microwave dielectric properties of Al2O3 ceramics via a cold sintering assisted two-step sintering route

Enhanced microwave dielectric properties in LiAlO2 ceramics by Li+ substitution for Al3,+

Low permittivity LiAl1-xLixO2-x (x = 0–0.02) ceramics were prepared by conventional solid-state reaction method. XRD analysis indicated that the system remained single phase solid solution in the whole range of x examined. Li + heterovalent substitution for Al3+ generated oxygen vacancy Ovac, and enhanced the relative density from 94.33% to 97.13%. The dielectric constant εr showed near linear variations versus sintering temperature and x value. Their relations mainly depended on the combined effects of relative density and dielectric polarizability. The quality factor Q×f was more than doubled from 15,647 GHz to 31,627 GHz, due to the packing fraction enlargement and covalent bond strengthening. The variation of temperature coefficient of resonance frequency τf was explained from the view of oxygen bond valence. When x = 0.01, the ceramic exhibited the best microwave dielectric properties with εr = 5.37, Q×f = 31,627 GHz and τf = −104.2 ppm/°C when sintered at 1300 °C for 5 h.

Crystal structure, bond characteristics and enhanced microwave dielectric properties of Sr0.6Ca0.4LaAl1-x(Zn0.5Ti0.5)xO4 ceramics

SrLaAlO4-based ceramics have attracted intense interest due to their high quality factor (Qf) at high frequency. However, the poor resonant frequency temperature stability (τf) limits their practical applications. Therefore, obtaining near-zero τf while guaranteeing high Qf values is challenging. (Zn0.5Ti0.5)3+ is utilized in Sr0.6Ca0.4LaAlO4 ceramics to improve the τf value effectively. The relationship between the structural features and microwave dielectric properties of Sr0.6Ca0.4LaAl1-x(Zn0.5Ti0.5)xO4 (x = 0–0.5) ceramics has been systematically investigated by XRD, Rietveld refinement, SEM, Raman spectroscopy, TEM and P–V-L theory. XRD and Raman spectroscopy reveal the effect of (Zn0.5Ti0.5)3+ substitution on the evolution of the crystal structure. According to the chemical bond theory, (Zn0.5Ti0.5)3+ substitution leads to the increase of the dielectric constant, which is related to the increase of the average bond ionicity, with the La–O bond contributing predominantly to the bond ionicity. The variation of the Qf values can be attributed to the densification of the ceramics, the lattice energy of the Al/Zn/Ti–O bond, and the FWHM. The τf value is strongly affected by the coefficient of thermal expansion of the lattice and the Al/Zn/Ti–O bond valence. It is noteworthy that excellent microwave dielectric properties are obtained for Sr0.6Ca0.4LaAl1-x(Zn0.5Ti0.5)xO4 ceramics at x = 0.4: εr = 21.62, Qf = 87,700 GHz (at 7.23 GHz), and τf = −1.81 ppm/°C. The above work provides deeper insights into establishing the relationship between the K2NiF4-type crystal structure and dielectric properties from the perspective of chemical bond.

Enhanced microwave dielectric properties in one-step cold sintered LiF ceramics via pre-calcination treatment

Cold sintering has proven effective in densifying microwave dielectric ceramics, but the resulting properties are significantly influenced by the crystallinity of the raw powder. In this study, lithium fluoride (LiF) raw powder underwent pre-calcination at temperatures ranging from 650 °C to 800 °C for 3 h to enhance grain crystallinity. The calcined powder was then mixed with 10 wt.% deionized water and cold-sintered at 150 °C under 900 MPa for 1 h to achieve dense LiF ceramics. XRD results confirmed the preservation of LiF phase composition throughout both processes. Increasing calcination temperature exhibited a positive correlation with enhanced relative density in cold-sintered ceramics, reaching up to 98.6 % at 800 °C. SEM images revealed controlled grain growth with higher calcination temperatures. εr increased with temperature, corresponding to the variation in relative density. Significantly, Qf value experienced substantial enhancement with increasing calcination temperature, ultimately achieving optimal microwave dielectric properties (εr = 8.80, Qf = 33,300 GHz, and τf = −138 ppm/°C) through cold sintering of LiF powder pre-calcinated at 800 °C.

Effects of Ag+ substitution on the enhanced microwave dielectric properties of Na4Co(MoO4)3 ceramics

Novel Na4-xAgxCo(MoO4)3 (x = 0–0.09) compounds were prepared using a solid state ceramic route. The investigation focused on the impact of intrinsic and extrinsic factors on the microwave dielectric properties of the samples. All samples were identified as belonging to the monoclinic structure with the C2/c space group. An optimal combination of microwave dielectric properties (εr = 8.55, Q × f = 30,300 GHz and τf = –30 ppm/°C) can be obtained for pure Na4Co(MoO4)3 specimen fired at 660 °C. Partially replacing Na+ with Ag+ in Na4-xAgxCo(MoO4)3 ceramics resulted in a more stable structure and significantly increased the Q × f value to 64,000 GHz at x = 0.03. Additionally, the study suggested that Na3.95Ag0.05Co(MoO4)3, sintered at 600 °C, could be a very promising candidate for ULTCC applications at microwave and millimeter wave frequencies due to its high thermal stability. This compound exhibited tremendous microwave dielectric properties (εr = 9.2, Q × f = 53,000 GHz and τf = –9.8 ppm/°C), which fit well within the criteria for today's 5G/6G applications, and demonstrated good compatibility with Al pastes. The designed and fabricated hairpin filter on the Na3.95Ag0.05Co(MoO4)3 substrate also demonstrated an excellent frequency response for 5G/6G applications.

Cracking behavior of Sm2O3 ceramics and enhanced microwave dielectric properties with MgO addition: Insights from chemical bond characteristics and microstructure

Monoclinic Sm2O3 ceramics were prepared using the conventional solid-state sintering method. The influences of sintering temperature on structure, morphology, and performance of Sm2O3 ceramics were studied systematically. The excellent microwave dielectric properties (εr = 21.95, Q × f = 47,000 GHz, and τf = +30.15 ppm/°C) were achieved in Sm2O3 ceramic sintered at 1550 °C for 4 h. Interestingly, varying degrees of cracking were observed after the sintered samples were placed for a short time, with the extent depending on the sintering temperatures. The reasons for cracking were analyzed through chemical bond characteristics and microstructure. The analysis of bond valence and microstructure revealed that Sm2O3 ceramics sintered at higher temperatures exhibited larger bond strain index, global instability index, and interplanar spacings. These results suggest that the cracking of Sm2O3 ceramics originates from inner stress resulting from structural variations caused by variations in sintering temperatures. The effect of MgO addition on microwave dielectric properties and stability of Sm2O3 ceramics was also investigated. Phase identification and elemental distribution indicated that MgO existed as a secondary phase. The Sm2O3 ceramic with 20 wt% MgO addition exhibited optimal microwave dielectric properties: εr = 17.72, Q × f = 61,100 GHz, and τf = +2.4 ppm/°C. Importantly, cracking was not observed when MgO addition amounts were 5 wt% or higher. The attractive microwave dielectric properties and favorable stability confirm the possibility of the 5G applications for Sm2O3/MgO composite ceramics.

Structure, bond chemistry and enhanced microwave dielectric properties of La2−xYxMo2O9 ceramics with low sintering temperature

Low-temperature sintered La2−xYxMo2O9 ceramics (x = 0–0.12) were synthesized by solid-state method. XRD analysis confirmed pure La2Mo2O9 with a cubic structure (space group P 21 3). Sintering at 925 °C resulted in La2Mo2O9 ceramic with favorable microwave dielectric properties: εr = 17.8, Q × f = 24300 GHz, τf = −35.1 ppm/°C. Notably, adding Y3+ greatly improved Q × f and τf. Sintering La1.88Y0.12Mo2O9 at 900 °C significantly enhanced microwave dielectric properties: εr = 17.4, Q × f = 34100 GHz, τf = −5.5 ppm/°C. εr was primarily influenced by ionic polarizability, while Q × f correlated positively with packing fraction. Analysis based on P-V-L theory showed that increased ionicity, lattice energy, and bond energy led to higher εr, Q × f, and τf values. Raman spectroscopy revealed that most observed peaks were associated with Mo-O bond vibrations. These findings highlight La2−xYxMo2O9 ceramics as promising candidates for LTCC materials.

Enhanced Microwave Dielectric Properties of the Ba2TiSi2O8 Ceramic by the Addition of TiO2

Enhanced Microwave Dielectric Properties of the Ba2TiSi2O8 Ceramic by the Addition of TiO2

Effects of F− substitution on the enhanced microwave dielectric properties of MgTa2O6 ceramics

Ion substitution engineering is an effective procedure in refining the crystal structure and performance for the microwave dielectric ceramics. In this study, MgTa2O6-xF2x (x = 0.1, 0.2, 0.3, 0.4, 0.5, 0.6) ceramics were synthesized using the conventional solid-phase reaction method. The effects of F− substitution for O2− in MgTa2O6 matrix were systematically investigated. The correlations between the crystal structure and the microwave dielectric properties were analyzed. The solution of F− into the oxygen sites was confirmed by the XRD and Raman spectra. The introduction of F− improved the morphology as well as the stability of the crystal structure significantly, which successfully depressed the electromagnetic wave loss through the bulk and enhanced the quality factor (Q × f) by nearly 50%, reaching up to 179,000 GHz (7.15 GHz) in the sample of x = 0.4 at the sintering temperature of 1425 °C.

Microstructure evolution, crystal structure, Raman analysis, bond characteristics and enhanced microwave dielectric properties of Zn1-xCuxZrNb2O8 solid solution ceramics

The Structural factors of ceramic materials significantly affect microwave dielectric properties. Consequently, the structure–performance mechanism is essential for providing insight into material discovery and performance refinement. In the present work, Zn1-xCuxZrNb2O8 ceramics were synthesized through oxygen-assisted sintering procedures. The controversy of lattice vibrations was clarified and complete Raman vibration with complete mode assignments were provided for the first time through density functional theory and symmetry analysis. Intrinsic properties, including lattice energy, bond ionicity, thermal expansion coefficient, Raman vibrational states and bond energy, were calculated and the structure–performance relationships were established based on crystal structure refinement, density functional theory, chemical bond theory and Raman analyses. X-ray diffraction patterns and crystal structure refinement indicate a pure phase with the wolframite structure. The smaller ionic radii of Cu2+ ions led to a decrease in cell volume, which could account for the blueshift of the Ag mode at around 340 cm−1. Through Cu2+ substitution, the grain growth was further promoted and morphology evolution from polyhedral grain to rod shape was observed, which may be attributed to grain amalgamation. From chemical bond theory, Cu2+ substitution leads to the decrease in the dielectric constant, which is related to a decrease in the Nb–O bond ionicity (fiNb-O). The improvement in Q×f value is attributed to the high lattice energy of Nb–O bond (UNb–O), low FWHM at 845 cm−1 and large grain size. τf values are strongly impacted by the bond energies of the Zn/Cu–O bonds in the Zn1-xCuxZrNb2O8 ceramic crystals. Remarkably, microwave dielectric performance of the composition x = 0.06, sintered at 1175 °C, where εr = 27.9, Q×f = 73,200 GHz (@7.14 GHz) and τf = −40 ppm/°C, provide potential for millimeter-wave applications.

Correlation between crystal structure, bond characteristics, Raman vibrations, and improved microwave dielectric properties of high-performance Zn0.5Zr0.5NbO4 ceramics: First principle calculation and experiment

The structure–performance mechanism provides new insights into performance modification and materials discovery. Herein, the electronic structure, Raman vibration, chemical bond factors and enhanced microwave dielectric properties of Zn0.5Zr0.5NbO4 ceramics through oxygen-assisted reaction sintering were investigated by Raman spectroscopy, first-principle calculations, and complex P–V-L theory. Pure-phase Zn0.5Zr0.5NbO4 ceramics were synthesized under oxygen-assisted reaction sintering, confirmed by XRD refinement and Raman analysis. Systematic vibration analysis was first introduced to provide complete mode assignments and Raman shift. Optimized microstructure with full density was obtained through morphology and EDS analysis. First-principle calculations indicated that the d orbit exerts the main contribution to Fermi energy with energy gap of 3.51 eV and Nb–O bonds may possess strong vibration, exhibiting a remarkable effect on dielectric loss. P–V-L results showed that Nb–O bonds have a significant influence on the dielectric constant and Q×f value while the Zn–O bonds dominate the τf value. In addition, high-performance Zn0.5Zr0.5NbO4 ceramics were fabricated through oxygen-assisted reaction sintering at 1250 °C, with εr = 28.4, Q×f = 79,800 GHz, and τf = −47.7 ppm/°C, exhibiting tremendous superiorities for commercial production.

A Study of Densification and Enhanced Microwave Dielectric Properties of Al2O3–Polystyrene Ceramic Composites

A Study of Densification and Enhanced Microwave Dielectric Properties of Al2O3–Polystyrene Ceramic Composites

Densification, microwave dielectric properties and rattling effect of LiYbO2 ceramics with low εr and anomalous positive τf

LiYbO2 ceramic with tetragonal structure formed only by [LiO6] and [YbO6] octahedra, exhibited a εr of 13.3 ± 0.2, Q × f of 24,700 ± 500 GHz and an abnormal positive τf of + 38.6 ± 3.0 ppm/°C as sintered at 1100 °C with low relative density of 88 %. Dense ceramic with relative density of 95.1 % was obtained with the addition of 1 wt% LiF when sintered at 1080 °C, exhibiting enhanced microwave dielectric properties of εr = 14.3 ± 0.2, Q × f = 41,200 ± 500 GHz, τf = +56.5 ± 3.0 ppm/°C and αL = 8.5 × 10−6/K. The large positive discrepancy as much as + 45.1 % between measured εr with porosity correction and calculated εr(C-M) using Clausius−Mossotti relations might be due to the rattling effect of Li+, which also led to the positive τf.

Low temperature sintered Ca1-x(Li0.5Bi0.5)xWO4 ceramics with enhanced microwave dielectric properties for LTCC applications

This research explored microwave dielectric properties of Ca1-x(Li0.5Bi0.5)xWO4 (x = 0–0.15) ceramics obtained by traditional solid-phase reaction method. Effects of replacing Ca2+ with (Li0.5Bi0.5)2+ on crystal structure, phase composition, grain size, and dielectric characteristics were investigated via X-ray diffraction (XRD), Rietveld refinement analysis, scanning electron microscopy, and Raman spectroscopy. According to results, the substitution of Ca2+ with (Li0.5Bi0.5)2+ caused samples to form only single phase and also reduced optimal sintering temperature of CaWO4 from 1150 °C to 900 °C. Q × f value under low temperature sintering was optimized from 27,624 to 46,049 GHz. In addition, τf value changed from −53 to −36 ppm/°C, indicating that temperature stability improved to some extent. Ca0.97(Li0.5Bi0.5)0.03WO4 sample sintered at 900 °C exhibited satisfactory dielectric characteristics with εr = 10.2, Q × f = 46,049 GHz, and τf = −36 ppm/°C. Therefore, Ca1-x(Li0.5Bi0.5)xWO4 composite ceramics are potential materials for low-temperature co-fired ceramics technology.

Raman vibration, bond chemistry and enhanced microwave dielectric properties of Li3Mg2NbO6 ceramics under an oxygen atmosphere

An effective synthesis provides a significant influence on microwave dielectric ceramics for commercial production. In this work, single-phase Li3Mg2NbO6 ceramics were synthesized under an oxygen atmosphere to depress oxygen vacancies. Raman vibration, bond characteristics and crystal structure were analyzed to calculate the inherent features of the Li3Mg2NbO6 ceramics. The Raman spectra and group theory indicated that the mode at 772 cm−1 reflected the symmetric vibrations of Nb–O bonds in NbO6 octahedra and dominated the Raman vibrations. XRD patterns and Rietveld refinement indicated an orthorhombic rock-salt phase under an oxygen assisted sintering process. Well-distributed and homogeneous microstructures were obtained through SEM and grain distribution. From the chemical theory calculation, Nb–O bonds dominated the dielectric properties in consideration of their largest bond ionicity, lattice energy, and bond energy. The Li3Mg2NbO6 ceramics sintered at 1100 °C with an oxygen atmosphere exhibited remarkable performance: εr = 15.3, Q × f = 109,600 GHz and τf = −17.3 ppm/°C, providing enormous potential for communication applications.

Structure characteristics and enhanced microwave dielectric properties of Ba(Mg1/3Nb2/3)O3–Mg2SiO4 composite ceramics with a tunable τf

The performance of microwave communication devices widely used in 5G network is strongly determined by the properties of dielectric ceramics used in the fabrication, which are required to have a suitable dielectric constant (εr), high Q × f value and near-zero temperature coefficient of resonant frequency (τf). In this work, (1-x)Ba(Mg1/3Nb2/3)O3–xMg2SiO4 (x = 0.1, 0.2, 0.3, 0.4, 0.5) composite ceramics meeting the application requirements have been developed and investigated. Ba(Mg1/3Nb2/3)O3 and Mg2SiO4 could coexist without any unexpected phases due to the different crystal structures and coordination relationship. The sintering behavior and the structural changes of Ba(Mg1/3Nb2/3)O3 were specifically relevant to the microwave dielectric properties. Introducing appropriate Mg2SiO4 content promoted the sintering of Ba(Mg1/3Nb2/3)O3 and the maximum Q × f value of 110,000 GHz was obtained at the sample with x = 0.3 sintered at the optimized temperature of 1420 °C. Due to the low dielectric loss and the opposite sign τf value of Mg2SiO4, the composite ceramic successfully adjusted the τf value to near zero while maintaining a high Q × f value. However, the experimentally measured values of εr and τf were significantly different from the theoretically calculated values due to the “rattling” effect of B-site cations and the enhanced symmetry of oxygen octahedra in Ba(Mg1/3Nb2/3)O3. These structural changes were confirmed by the results of bond valence analysis and Raman vibration spectra. The 0.6Ba(Mg1/3Nb2/3)O3–0.4Mg2SiO4 ceramic sintered at 1420 °C exhibits competitive microwave dielectric properties: εr = 18.9, Q × f = 91,000 GHz (at 8.016 GHz) and τf = 2.6 ppm/°C, which indicates it a promising application prospect of microwave communication.

Enhanced microwave dielectric properties of Bi6B10O24 ceramics as ultra-low temperature co-fired ceramics materials

Enhanced microwave dielectric properties of Bi6B10O24 ceramics as ultra-low temperature co-fired ceramics materials

Enhanced Microwave Dielectric Properties and Sintering Behaviors of Mg2SiO4-Li2TiO3-LiF Ceramics by Adding CaTiO3 for LTCC and GPS Antenna Applications

Mg2SiO4 holds promise for its application in the microwave communication field due to its low dielectric constant and high Q×f value. However, its high negative τf and high sintering temperature limit its application in low-temperature co-fired ceramic (LTCC) devices. In this work, Li2TiO3 and CaTiO3 were introduced to improve the τf, and LiF was chosen to decrease the sintering temperature. According to XRD patterns and SEM micrographs, the ceramic systems displayed a complex-phase structure, and the microstructure was densified when CaTiO3 was added. All of the relative densities, dielectric constants, and Q×f values first increased and then decreased as the sintering temperature increased. The MLLC11.5 ceramics sintered at 800 °C could be obtained with the highest Q×f value of 54,581 GHz (at 8.06 GHz), εr of 14.13, and τf of + 5.81 ppm/°C. Furthermore, it was proven that the MLLC11.5 powders could be co-fired without any reaction with Ag powders at 800 °C, indicating its potential for LTCC application. The MLLC11.5 composition was used to prepare a GPS antenna and showed good prospects for its application in electronic communications.

Crystal structure and enhanced microwave dielectric properties of the Ce2[Zr1\u2212x(Al1/2Ta1/2)x]3(MoO4)9 ceramics at microwave frequency

Dense microwave dielectric ceramics of Ce2[Zr1−x(Al1/2Ta1/2)x]3(MoO4)9 (CZMAT) (x = 0.02–0.10) were prepared by the conventional solid-state route. The effects of (Al1/2Ta1/2)4+ on their microstructures, sintering behaviors, and microwave dielectric properties were systematically investigated. On the basis of the X-ray diffraction (XRD) results, all the samples were matched well with Pr2Zr3(MoO4)9 structures, which belonged to the space group Rbar 3c. The lattice parameters were obtained using the Rietveld refinement method. The correlations between the chemical bond parameters and microwave dielectric properties were calculated and analyzed by using the Phillips—Van Vechten—Levine (P—V—L) theory. Excellent dielectric properties of Ce2[Zr0.94(Al1/2Ta1/2)0.06]3(MoO4)9 with a relative permittivity (εr) of 10.46, quality factor (Q × f) of 83,796 GHz, and temperature coefficient of resonant frequency (τf) of −11.50 ppm/°C were achieved at 850 °C.