Microwave Dielectric Ceramics Research Articles

Low-fired Li3Mg2NbO6 microwave dielectric ceramics by adding LMBCS glass for LTCC application

Low-fired Li3Mg2NbO6 microwave dielectric ceramics by adding LMBCS glass for LTCC application

An UHF band planar resonator temperature sensor constructed from high-performance titanium dioxide system microwave dielectric ceramics: Toward integrated ceramic-based sensor devices

With the rapid fusion of temperature sensing technology and microwave technology, microwave temperature sensors have become the protagonist of competing research. We propose a planar resonator temperature sensor that combines substrate material modifications with sensor structure design. To realize this concept, high-performance TiO2-xwt. % ZnO (0 ≤ x ≤ 3) microwave dielectric ceramics are prepared. The various factors influencing dielectric properties, including crystal structure, phase composition, Raman vibration, microstructure, element valence, and oxygen vacancy, are completely investigated. The TiO2-0.7 wt. % ZnO ceramic exhibiting exceptional properties (εr = 106.6, Qf = 46 000 GHz, τf = 426.0 ppm/°C) is selected for substrate fabrication. The frequency and temperature dependence of εr and tan δ are analyzed at 2–4.5 GHz from −50 to 100 °C, revealing a good linearity between εr and temperature. A CSRR temperature sensor employing this substrate material is designed, simulated, fabricated, and validated from −50 to 90 °C. This sensor generates two resonance frequencies (around 0.5 and 1.4 GHz) in the UHF band, demonstrating sensitivities of 2.2 MHz/10 °C and 6.3 MHz/10 °C at the first and second resonance frequencies, along with an outstanding normalized sensitivity of approximately 0.045. Through a comprehensive analysis of the physical mechanisms affecting the sensor's sensitivity and quality factor, the design of the sensor is strengthened from the perspective of optimizing the performance of microwave dielectric ceramics. The regulation mechanism of dielectric characteristics is enriched and clarified, thereby achieving a synergistic improvement in sensor performance. This work expands the application scope of microwave dielectric ceramics and provides an innovative approach to environmental monitoring.

Near-Zero Temperature Coefficient of Resonance Frequency in Rare-Earth Titanates via the Phase Transition Strategy.

This study presents an approach to achieve a near-zero temperature coefficient of resonance frequency (τf) in rare-earth titanate microwave dielectric ceramics (MWDCs) by inducing a phase transition. By Zr4+ substitution at the B site, a series of Sm2Ti1-xZrxO5 (0.02 ≤ x ≤ 0.55) ceramics are synthesized using the solid-state method to intentionally alter the radius ratio of the A/B sites, realizing in a controlled phase transition from orthorhombic (Pnma) to biphasic coexistence and ultimately to cubic (Fd3̅m) structure. The phase composition is rigorously identified through X-ray diffraction (XRD) Rietveld refinement, high-resolution transmission electron microscopy (HRTEM), selected-area electron diffraction (SAED), and Raman spectroscopy. A comprehensive analysis is conducted to elucidate the relationships between factors such as ionic polarizability, packing fraction, bond valence, complex chemical bonding, and far-infrared reflectivity spectra with microwave dielectric properties. The results demonstrate that these ceramics exhibit a broad range of permittivity (14.30-23.18), high-quality factors (14,828-22,300 GHz), opposite temperature coefficient of resonance frequency (-16.0 to + 22.4 ppm/°C), and nice thermal conductivity (1.81-2.76 W·m-1·K-1), particularly at x = 0.30 with a near-zero τf value of +1.6 ppm/°C. The findings not only provide insights into designing MWDCs with a near-zero τf but also offer a promising route for developing advanced microwave materials with improved performance and reliability.

Crystal structures, lattice vibrational characteristics, and dielectric responses of ZnWO4 microwave dielectric ceramics sintered at different temperatures

The study meticulously examined the impact of varying sintering temperatures on the phase transformations, crystallographic configurations, dielectric response mechanisms, and lattice vibrational characteristics of the ZnWO4 ceramics. Far-infrared reflectance spectra were fitted using a four-parameter semiquantum model to obtain the intrinsic dielectric properties. Moreover, the contribution to the dielectric properties for each vibrational mode was analyzed, indicating that the lower-frequency mode had the most contribution. The relationship linking the structural aspects to the properties of ZnWO4 ceramics was elucidated through an analysis of the Raman spectral modes across a range of sintering temperatures, overcoming the limitation of solely textual description without mathematical quantification. When sintered at 1070 °C, the ZnWO4 ceramic had the best dielectric properties: εr = 14.81, Q×f = 34,239 GHz (f = 9.69 GHz), with the highest relative density of 96.95 %.

Influence of structural characteristics of Ca1-xNdxW1-xVxO4 ceramics on microwave dielectric properties

Dense Ca1-xNdxW1-xVxO4 (x = 0−0.4) microwave dielectric ceramics were obtained when sintered at 1150 °C for 4 h. A tetragonal scheelite could be formed in the range of x ≤ 0.3, while a composite region with tetragonal scheelite and tetragonal zircon was found at x ≥ 0.4. The relative permittivity (εr) increased from 10.15 to 13.66, mainly attributed to an increase in the ionic polarizability. The quality factor (Q×f) decreased from 61,060 to 32,204 GHz, correlated to the variations in lattice energy and the Raman FWHM. The temperature coefficient of resonant frequency (τf) gradually increased from −54.03 to −23.50 ppm/℃, mainly affected by the rattling effect of W6+/V5+ and the temperature coefficient of ion polarizability (ταm). Ca0.7Nd0.3W0.7V0.3O4 with excellent comprehensive properties (εr = 13.21, Q×f = 35,570 GHz, and τf = −23.50 ppm/°C) were obtained at 1150 °C.

Ultrafast synthesis and luminescence properties of rare earth orthoniobates RENbO4 (RE = La, Eu, Gd, Yb, Lu)

Rare earth orthoniobates (RENbO4) are one kind of important functional materials due to its applications in solid-state phosphors, thermal barrier coatings, and microwave dielectric ceramics. The synthesis of rare earth niobates often needs high reaction temperatures (1300 °C–1700 °C) and long processing times (from hours to tens of hours) in solid-state reactions, which can increase the study time of the relationship between structure and properties. In this work, we used ultrafast high-temperature sintering method to synthesize RENbO4 (RE = La, Eu, Gd, Yb, Lu), and found specific structure and properties in these materials obtained with specific synthetic techniques. Based on the electronegativity scale, the charge transfer energy of lanthanide ions in the YNbO4 crystal was calculated. The rapid synthesis of RENbO4 in a vacuum atmosphere generated more oxygen vacancies, and the structures of [REO8] and [NbO6] were distorted. The shortening of the fluorescence lifetime of LaNbO4 and EuNbO4 was related to the formation of self-trapped excitons facilitated by lattice distortion. The emission peak of LuNbO4 at about 530 nm is attributed to the oxygen vacancy in the niobate group. The reported synthetic methods can provide a fast materials screening route for high melting point inorganic materials.

Structures and properties of LiBaF3 microwave dielectric ceramics for ULTCC applications

Perovskite structural LiBaF3 ceramics with excellent microwave dielectric properties were synthesized using traditional solid-state method. By regulating the sintering temperatures from 600 °C to 675 °C, LiBaF3 ceramics exhibit tunable dielectric constant (εr) from 12.42 to 13.72, adjustable Q × f value from 55000 GHz to 73000 GHz, and regulable temperature coefficient of resonance frequency (τf) from −36.78 ppm/°C to −34.68 ppm/°C. The properties are strongly associated with the density, and can be further optimized by 0.5 wt%TiO2 addition (εr = 14.43, Q × f = 56000 GHz, τf = -1.26 ppm/°C). The LiBaF3 ceramics also exhibit excellent compatibility with Ag and Al electrodes. Furthermore, relationships between multiscale structures and microwave dielectric properties of the ceramics were revealed. Oxidation behaviors between LiBaF3 and O2 would result in the lattice shrinkage, the introduction of defect level, the decrease of bandgap and density, and then deteriorating the microwave dielectric properties. All the findings make the LiBaF3 ceramic a promising candidate for ULTCC application, and may facilitate the development of other novel microwave dielectric materials.

Temperature-stabilized Zn0.15Nb0.3Ti0.55O2 microwave dielectric ceramics achieved via (Nd1/2Nb1/2)4+ ionic co-substitution

Temperature-stabilized Zn0.15Nb0.3Ti0.55O2 microwave dielectric ceramics achieved via (Nd1/2Nb1/2)4+ ionic co-substitution

Low-firing and temperature stability regulation of tri-rutile MgTa2O6 microwave dielectric ceramics

Low-firing and temperature stability regulation of tri-rutile MgTa2O6 microwave dielectric ceramics

Near-zero temperature coefficient in Mg2+–Ti4+ co-doped Sm2O3–CaSmAlO4 microwave dielectric ceramics

Near-zero temperature coefficient in Mg2+–Ti4+ co-doped Sm2O3–CaSmAlO4 microwave dielectric ceramics

Effects of Zn2+ substitution on the dielectric properties, chemical bonding properties, and crystal structure of Mg3(PO4)2 ceramics

A series of (Mg1-xZnx)3(PO4)2 (x = 0.02–0.10) microwave dielectric ceramics were fabricated by the solid-state reaction method and investigated in terms of crystal structure, chemical bond properties, and dielectric properties were analyzed. The XRD data indicates that (Mg1-xZnx)3(PO4)2 samples belong to the monoclinic crystal with P21/c space group and no detectable secondary phases. The Rietveld refinement was employed to obtain crystal parameters. In addition, the results of chemical bond properties reveal that the lattice energy and ionicity of Mg (2)–O (3) bonds play a primary effect on the dielectric loss and dielectric constant, respectively. The bond energy of Mg(l)-O (2) bonds plays a dominant role in thermal stability. The far-infrared spectroscopy was employed to explore the intrinsic dielectric parameters, and the results showed that peaks below 400 cm-l contributed 78.9 % to ε′ and 99.1 % to ε″. The Raman data demonstrated that the Raman shift and FWHM exhibit an important influence on Q × f. The optimal performance was achieved in (Mg0.94Zn0.06)3(PO4)2 ceramics: εr = 5.00, Q × f = 84,674 GHz, τf = −59.98 ppm/°C.

Preparation and microwave dielectric properties of calcite (CaCO3) ceramics

Calcite, as the thermodynamically stable phase of calcium carbonate, is a promising low-cost candidate as low-εr microwave dielectric ceramics because of its large natural reserve and the strong covalency of C–O bond, while it is difficult to be densified due to the decomposition at about 627 °C. In the present work, calcite ceramics with the relative density of 87.8 % and low Qf value of 5,600 GHz were prepared by the room-temperature cold sintering, and the pressure-less sintering at 400–600 °C was conducted subsequently. The pressure-less sintering only increases the relative density slightly, while the microstructural evolution including the formation of uniform grains and significant grain growth were observed with increasing the sintering temperature, leading to a dramatic increase in Qf value. The best microwave dielectric properties with εr = 7.60, Qf = 20,200 GHz, and τf = −154 ppm/°C at around 12 GHz were obtained in the calcite ceramic sintered at 600 °C, indicating the applicable possibility of carbonate ceramics in microwave communications.

Low temperature sintering and improvement of temperature Stability of Ba3P4O13 microwave dielectric ceramics by BCB additions

In our previous study, Ba3P4O13 ceramic was found to exhibit good microwave dielectric performance with a low permittivity (εr), a high Q × f (Q = 1/dielectric loss, f = resonant frequency) value but a large positive τf value (TCF, temperature coefficient of resonant frequency), which limits its application in low-temperature co-fired ceramic (LTCC) technology. BaCu(B2O5) (BCB) additions were selected in this study to adjust τf value of Ba3P4O13 ceramic samples. It was found that as BCB contents increased, sintering temperatures decreased from 840 °C (without BCB addition) to 820 °C (with 2 wt. % BCB), meanwhile τf values of ceramic samples were reduced from + 48 ppm/ °C to − 2 ppm/ °C. Moreover, BCB added Ba3P4O13 ceramic showed a good chemical compatibility with Cu electrode. These results indicate that BCB added Ba3P4O13 ceramics are promising candidate for LTCC applications.

Effects of sintering temperatures on crystal structures, dielectric properties, and phonon characteristics of Sr2V2O7 microwave ceramics

Sr2V2O7 (SVO) microwave dielectric ceramics were fabricated through a conventional solid-state synthesis method and sintered at 900–1000 °C for 10 h. The crystal structures and morphologies of the SVO samples were examined via X-ray diffraction and scanning electron microscopy. These analyses revealed that the SVO sample sintered at 950 °C exhibited high crystallinity and a triclinic crystal system. Through the lens of lattice dynamics, the phonon characteristics of the SVO samples were investigated via Raman spectroscopy and far-infrared spectroscopy to elucidate the dielectric response mechanics and microstructural origins of these responses. Raman spectra were classified into three sections corresponding to the vibration of Sr2+ ions, the V–O or V–O–V telescopic vibration, and the internal elongation of the tetrahedral structure composed of V–O, respectively. The inherent dielectric properties obtained from the far-infrared spectra were analyzed using a four-parameter semi-quantum model with the assistance of the Focus software. This analysis aligned with the measured property values. The relationships between the crystal structures and the dielectric properties were established using the Raman modes. Particularly, the SVO sample sintered at 950 °C exhibited excellent dielectric properties, characterized by a dielectric constant (εr) of 9.49, a quality factor (Q × f) of 35,189 GHz, and a resonant frequency (f) of 12.48 GHz.

Crystal structure, Raman spectroscopy and microwave dielectric properties of novel (1-x)MgGa2O4-xBaCu(B2O5) based LTCC materials

A novel series of (1-x)MgGa2O4-xBaCu(B2O5) (x = 0.1–0.5) microwave dielectric ceramics were prepared by solid phase method. By adding BaCu(B2O5) additives, the sintering temperature of MgGa2O4 ceramics was reduced from 1410 °C to 925 °C. Through an analysis of the trend in packing fraction and FWHM value of T2g Raman peak, the underlying influence mechanism of various intrinsic factors on the dielectric loss of ceramic were revealed. And the density, as the external factors, exhibits a positive correlation with the dielectric constant and Q × f value. In addition, the high bond strength of Ga-site reduces lattice distortion and consequently enhances the frequency temperature coefficient. The microwave dielectric properties of (1-x)MgGa2O4-xBaCu(B2O5) (x = 0.3) are found to be superior at a sintering temperature of 925 °C (εr = 9.42, Q × f = 35,480 GHz, τf = −22.7 ppm/°C, at 14 GHz). Moreover, the chemical compatibility of ceramics cofiring with Ag electrode makes them a highly promising substrate for microwave communications.

Phase composition, microstructure, and microwave dielectric properties of Mg1-x(Li0.5Y0.5)xMoxW1-xO4 ceramics with low sintering temperatures

Abstract Generally, doping is an effective method to improve the microwave dielectric properties and regulate the sintering behavior of microwave dielectric ceramics. In this study, the temperature-stable Mg1- x (Li0.5Y0.5) x Mo x W1- x O4 ceramics were prepared by a conventional solid-state reaction method, and the effects of doping on the phase, density, microstructure, and microwave dielectric properties of the samples were investigated. The presence of secondary phase Li0.5Y0.5MoO4 successfully improved the sintering behavior of the sample, achieving densification at low sintering temperatures, while effectively adjusting the temperature coefficient of resonant frequency and improving the temperature stability of the sample. Notably, when sintered at 875°C, Mg1- x (Li0.5Y0.5) x Mo x W1- x O4 ceramics exhibited satisfactory microwave dielectric properties: ε r = 14.83, Q×f = 19, 887 GHz, and τ f = 4.11 ppm/ºC, making the ceramics as candidates for LTCC technology.

Structure property correlation of (1-x)MgTiO3 - xSrTiO3 microwave dielectric ceramics for dielectric resonator antenna applications

The current paper reports on the crystal structure, microwave dielectric properties as well as the antenna parameters of (1-x)MgTiO3 - xSrTiO3 (abbreviated as (1-x)MT-xST) for x = 0.025, 0.05, 0.075, 0.1 ceramic based dielectric resonator antennas (DRAs). The (1-x) MT - xST samples have been prepared with the help of a solid-state reaction route. The SrTiO3 was introduced to MgTiO3 as a τf compensator for developing a tunable τf material. Rietveld refinement results of X-ray diffraction data suggest that all the samples exhibit a combination of two crystallographic phases i.e., rhombohedral (R-3) + cubic (Pm-3m). The SEM micrographs show clear and well-defined grains for (1-x)MT - xST (for x = 0.025–0.1) ceramics. The modes of vibration of the crystal structures of the materials have been identified by the Raman spectroscopy method. The bond strength, bond valency, bond energy, and tolerance factor have been determined for (1-x)MT - xST (for x = 0.025–0.1) materials. The microwave dielectric performances of ceramics have been analyzed over a frequency band of 1–10 GHz. It has been found that the dielectric constant is almost constant along the microwave frequency range. The behavioral change of tuned temperature coefficient of resonant frequency (τf) along with quality factor have been reported. Besides, DRAs have been formed using all ceramic compositions, and simulations were performed to determine the various antenna properties, such as efficiency, input impedance, radiation properties, gain, and reflection coefficient. This study supports the use of microwave dielectric ceramics in antenna fields and highlights the enormous potential of (1-x)MT - xST for (x = 0.05) materials in communication systems.

Structure evolution and microwave dielectric characteristics of Hf1−xSnxO2 ceramics

AbstractHigh Q (Hf1−xSnx)O2 ceramics were prepared via a standard solid‐state reaction route, and the microwave dielectric characteristics were investigated systematically together with the structure evolution. With increasing x, the structure changed from HfO2 (monoclinic in space group P21/c, x = 0 and 0.03) to Hf0.77Sn0.23O2 (orthorhombic in space group Pbcn, x = 0.23), while the two‐phase structure: HfO2 major phase and Hf0.77Sn0.23O2 secondary phase, was determined for x = 0.05–0.21, and Hf0.77Sn0.23O2 major phase combined with SnO2 tetragonal secondary phase in space group P42/mnm was confirmed for x = 0.30–0.40. The Qf value was significantly increased from 24,500 to 167,650 GHz by Sn‐substitution, and the best combination of microwave dielectric characteristics was obtained in Hf0.77Sn0.23O2 ceramics: εr = 17.2, Qf = 167 650 GHz at 9.6 GHz and 230 710 GHz at 26.1 GHz, and τf = −52.6 ppm/°C. The dielectric loss was deeply linked with the structure stability and lattice‐distortion. Also, the phase composition, the bond strength, as well as the degree of covalency, had a tight connection with dielectric loss. The present ceramics were expected to be the promising candidates as low‐εr microwave dielectric ceramics.

Effect of LiF addition strategy on the sintering of Li2MgTiO4 microwave dielectric ceramics with cubic rock salt structure

Recently, microwave dielectric ceramics with an oxyfluoride cubic rock salt structure have been widely studied due to their excellent microwave dielectric properties. However, the sintering mechanism of this material, especially the role of F-ions, is not clear. Here, the ceramic samples with two different LiF addition strategies were prepared by a conventional solid-phase reaction method. The sintering mechanisms of both samples were discussed in detail by analyzing the sintering activation energy, F-ions distribution, and concentration of oxygen vacancies by linear shrinkage, SEM-EDS, and XPS. The best microwave dielectric properties were obtained for Li2MgTiO4 +4 wt% LiF (LiF-B) ceramics sintered at 950°C: εr = 18.0, Q×f = 82,100 GHz, and τƒ = −32.7 ppm/°C.

On the τf measurement of microwave dielectric ceramics

As a key parameter of microwave dielectric ceramics, the temperature coefficient of resonant frequency (τf) determines the temperature stability of microwave dielectric ceramics and relevant microwave devices. The τf of a microwave dielectric ceramic is typically calculated from the measured resonant frequencies of the system, consisting of the ceramic sample plus other dielectric elements, at two temperatures ∼60 – 70 °C apart. This measurement process assumes that τf is a constant; however, it is generally temperature-dependent based on both theoretical and experimental results. Furthermore, the as-measured τf is actually the τf of the dielectric resonator system, which is different from that of the sample itself. In this paper, these two overlooked problems during τf measurement are discussed in detail, and suggestions are given to avoid their possible misleading consequences and thus ensure the reliability of τf measurements.