Structure stability,bond characteristics and microwave dielectric properties of co-substituted NdNbO4 ceramics

Structure-property relationship of co-substituted (Mg²⁺_1/4Mo⁶⁺_3/4)⁵⁺, (Al³⁺_1/3Mo⁶⁺_2/3)⁵⁺, (Si⁴⁺_1/2Mo⁶⁺_1/2)⁵⁺, (Zr⁴⁺_1/2Mo⁶⁺_1/2)⁵⁺ for Nb⁵⁺ in NdNbO₄ ceramics was investigated systematically. The remarkable differences in dielectric properties of each composition originated from their bond characteristics and structure stability. The elongated/compressed bonds have an effect on the cell volume and polarization. And the average bond covalency of Nb-O bond was responsible for the development of permittivity. Q×f values and the total lattice energy went up to maximum when (Si_0.5Mo_0.5) occupied Nb-site. Variations of lattice energy together with Nb-O bond energy suggest that a more stable structure was obtained through co-substitution. The optimal microwave dielectric properties is: ε_r =?18.97, Q×f?=?49466?GHz, τ_f =?7.34?ppm/°C for NdNb_0.97(Si_0.5Mo_0.5)_0.03O₄, sintered at 1250?°C.     

Raman spectra,bond characteristics,and microwave dielectric properties of Gd2Mo3O12 ceramics

Gd₂Mo₃O₁₂ ceramics were prepared using the traditional solid-phase reaction method. All samples were found to possess an orthorhombic crystal structure with a space group of Pba2, as revealed by refined XRD results. The ceramic sintered at 1000 °C exhibited a high relative density of 96.95 % and superior microwave dielectric properties, including ε_r of 9.42 ± 0.05,  × f of 49258 ± 1200 GHz, and τ_f of −71.8 ± 0.7 ppm/°C. The results suggest a correlation between an increase in ε_r and higher relative density, and a more compact and uniform microstructure can lead to higher  × f value. Chemical bonding theories and Raman spectroscopy analysis reveal that Mo-O bonds, rather than Gd-O bonds, dominate the microwave dielectric properties. Furthermore, the ε_r of Gd₂Mo₃O₁₂ ceramic was closely related to bond ionicity, while  × f and τ_f were mainly determined by lattice energy and bond energy, respectively.     

Structural evolution and microwave dielectric properties in Sr2(Ti1-xSnx)O4 ceramics

The structure and microwave dielectric properties of Sr₂(Ti_1-xSn_x)O₄ ceramics were determined in the entire composition range of x?=?0–1.0. X-ray diffraction patterns and Raman spectra indicated a composition-induced onset of octahedral tilting at x?=?0.75, and the crystal structure transformed from tetragonal (I4/mmm) to orthorhombic (Pccn). An obvious change of grain morphology was observed in the phase transformation region as well. The variations of the microwave dielectric properties with composition were systematically investigated and the effect of octahedral tilting on the evolution of τ_f value was emphasized. Moreover, the relationship between τ_ε and tolerance factor of the present ceramics was revealed and compared with the empirical rule in perovskite structure. The role of tolerance factor in designing the materials with required performance was highlighted.     

A novel ultralow-loss Sr2CeO4 microwave dielectric ceramic and its property modification

A new ultralow-loss Sr₂CeO₄ microwave dielectric ceramic was prepared via a conventional solid-state method. The X-ray diffraction and Rietveld refinement results demonstrate that pure-phase Sr₂CeO₄ ceramics belong to the orthorhombic structure with a Pbam space group. Scanning electron microscopy analysis reveals dense and homogeneous microstructure. Optimum microwave dielectric properties of ε_r = 14.8, Q × f = 172,600 GHz (9.4 GHz) and τ_f = -62 ppm/°C were obtained as it was sintered at 1270 °C for 4 h. In addition, the substitution of a few amount of Ti⁴⁺ for Ce⁴⁺ was found to have significant influences on the grain morphology, sintering behavior, phase structure and microwave dielectric properties. Among them, the Sr₂Ce_0.65Ti_0.35O₄ ceramic sintered at 1350 °C for 4 h demonstrates near-zero τ_f of -1.8 ppm/°C, ε_r of 20.7 and Q×f of 115,550 GHz (8.1 GHz) because of its two-phase structure, showing large application potentials.     

Sintering behaviour,phase composition,microstructure, and dielectric properties of BaSm2O4 microwave ceramics

BaSm₂O₄ was prepared by the conventional solid-phase reaction method. Single-phase dense BaSm₂O₄ ceramic (space group: Pnam) was obtained at 1500 °C. Crystal refinement results show that BaSm₂O₄ ceramics have a CaFe₂O₄ structure. The change of Q × f is explained by calculating the stack fraction and radial shrinkage of BaSm₂O₄ ceramics. When the sintering temperature was 1500 °C, the packing fraction and radial shrinkage of the BaSm₂O₄ ceramic reached the maximum values of 52.194% and 36%. Due to secondary recrystallization, the relative density of the ceramics increases and then decreases, reaching a maximum of 1500 °C (96.65%). In addition, the τ_? value is affected by SmO₆ octahedral distortion. The ceramics have the best combined dielectric properties after sintering at 1500 °C for 4 h: ε_r = 10.99, Q × f = 54598 GHz, τ_? = ?25.4 ppm/°C. BaSm₂O₄ ceramics have good prospects for applications in the field of mobile communication base stations.     

Room temperature densified H3BO3 microwave dielectric ceramics with ultra-low permittivity and high quality factor for dielectric substrate applications

The densification and microwave dielectric properties of H₃BO₃ ceramics prepared by dry pressing at room temperature were studied. The results show that pressure is the key factor of densification of H₃BO₃ ceramics. No second phase appears in all the as-fabricated H₃BO₃ ceramic samples. A dense H₃BO₃ ceramic (relative density~97.6%) was obtained by uniaxial compression of 384 MPa for 300s and the optimal microwave dielectric properties are ε_r = 2.83, Q × f = 59,400 GHz (f = 16 GHz), τ_f = −91 ppm/°C, which make it as a prospective candidate for microwave and millimeter-wave devices such as substrates for 5G communication technology.     

Low temperature fired CaF2-based microwave dielectric ceramics with enhanced microwave properties

Low-temperature-fired microwave ceramics are key to realizing the integration and miniaturization of microwave devices. In this study, a facile wet chemical method was applied to synthesize homogenous nano-sized CaF₂ powders for simultaneously achieving low-temperature sintering and superior microwave dielectric properties. Pure CaF₂ ceramics sintered at 950 °C for 6 h with good microwave dielectric properties (ε_r = 6.22, Q×f = 36,655 GHz, and τ_f = ?102 ppm/°C) was achieved. The microwave dielectric properties of the CaF₂ ceramics were further improved by introducing LiF as a sintering aid. The sintering temperature of CaF₂-based ceramics was effectively lowered from 950 °C to 750 °C with 10 wt% LiF doping, and excellent microwave dielectric properties (ε_r = 6.37, Q×f = 65,455 GHz, and τ_f = ?71 ppm/°C) were obtained.     

Structure,chemical bond and microwave dielectric characteristics of novel Li3Mg4NbO8 ceramics

A novel Li₃Mg₄NbO₈ compound was fabricated through the process of solid-state reaction. The crystal structure, sinterability and microwave dielectric properties of the Li₃Mg₄NbO₈ ceramics were investigated. XRD refinement and Raman spectra results ascertained that the Li₃Mg₄NbO₈ compound crystallized into an orthorhombic Li₃Mg₂NbO₆-like structure with space group Fddd. The ε_r value was strongly impacted by the relative density and average ionic polarization. The Q × f value was mainly affected by the relative density and average grain size. The Li₃Mg₄NbO₈ ceramics sintered at 1150 ℃ showed outstanding microwave dielectric performance: ε_r = 13.8 ± 0.14, Q × f = 103 400 ± 3500 GHz (at 9.6 GHz), τ_f = −36.0 ± 1 ppm/℃. Additionally, the bond characteristics were calculated for a better understanding of the structure-property correlation for Li₃Mg₄NbO₈ ceramics.     

Structure dependence of microwave dielectric properties in Zn2-xSiO4-x-xCuO ceramics

In this work, the Zn_2-xSiO_4-x-xCuO (x = 0, 0.04, 0.08, 0.12, 0.16 and 0.20) ceramics were synthesized through solid state reaction. The dependence of microwave dielectric properties on the structure was investigated through X-ray diffraction (XRD) with Rietveld refinements, Scanning electron microscope (SEM) and Raman spectra. The melting of CuO can reduce the densification temperature of Zn_2-xSiO_4-x ceramics. In comparison with x = 0, the x = 0.08 ceramics were densified at 1150℃ and the excellent microwave dielectric properties with low dielectric constant (ε_r = 6.01), high quality factor (Qf = 105 500 GHz) and τ_f = ?28 ppm/°C, were obtained. The ε_r, Qf and τ_f value are dominated by covalency of Si-O bond and secondary phase, crystallinity and lattice energy, respectively. This provides a theoretical basis to further adjust the microwave dielectric property (especially τ_f value) from the structural point of view.     

Effects of (Cr0.5Ta0.5)4+ on the microstructure,chemical bond characteristics,and dielectric properties of molybdate-based ceramics at microwave and terahertz frequency

In this study, Ce₂ [Zr_1−x (Cr_0.5Ta_0.5)_x]₃(MoO₄)₉ (x = 0.02–0.10) ceramics were synthesized using the solid-state reaction technique, and the crystalline parameters, sintering behaviors, chemical bond characteristics, infrared reflection spectrum, and dielectric response at microwave and terahertz frequency were examined. X-ray diffraction results demonstrated the crystallization of all ceramics in the trigonal structure (R-3c space group), and additional peaks were not detected. The densification point of ceramic was 875 °C. The addition of (Cr_0.5Ta_0.5)⁴⁺ significantly reduced the dielectric loss in the host ceramic. For Ce₂ [Zr_0.96(Cr_0.5Ta_0.5)_0.04]₃(MoO₄)₉, outstanding microwave properties of ε_r = 10.66, Qf = 79,436 GHz, and τ_f = −19.07 ppm/°C were obtained at 875 °C. The chemical bond characteristics were also parameterized to explore the relationship between Zr(CrTa)–O and microwave properties. Infrared spectral results further indicate that phonon vibrations lower than 400 cm⁻¹ contribute to 80% of the polarization. In our comparison between the infrared spectrum and terahertz time-domain spectrum, we found that the permittivity extracted by the latter is closer to the observed value.     

Structure and microwave dielectric properties of NaSr4V5O17 ceramics for LTCC applications

A new microwave dielectric ceramic, NaSr₄V₅O₁₇ with low firing temperature was fabricated via the traditional mixed oxide method. Rietveld refinement of XRD profiles and Raman spectrum analysis ascertained that the NaSr₄V₅O₁₇ compounds crystallized into Sr₂V₂O₇-like triclinic structure with space group P-1 (2) and Z = 1.6. The variation of Q × f value was explained by the combined effects of mean grain size and cell volume rather than packing fraction and bond valence. The change regulation of ε_r was similar to that of density. The |τ_f| value is mainly related to the cations bond valence. The NaSr₄V₅O₁₇ ceramics sintered at 725 °C showed good compatibility with Ag electrode and superior dielectric properties: ε_r = 8.6, Q × f = 45 900 GHz, τ_f = ?57.0 ppm/K, making it a potential application for LTCC.     

Densification,microstructure evolution,and microwave dielectric properties of Mg1-xCaxZrTa2O8 ceramics

In this study, the effects of the Mg²⁺ ions replaced by Ca²⁺ ions on the microwave dielectric properties of newly developed MgZrTa₂O₈ were investigated. Mg_1-xCa_xZrTa₂O₈ (x = 0–1.0) ceramics were prepared via a solid-state reaction method. Calcination of the mixed powders was performed at 1200 °C and sintering of the powder compacts was accomplished at temperatures from 1200 to 1550 °C. The substitution of Ca²⁺ significantly inhibited the densification of Mg_1-xCa_xZrTa₂O₈, led to the expansion of the unit cells, and triggered the formation of a second phase, CaTa₂O₆. The porosity-corrected relative permittivity increased almost linearly with the x value because of the replacement of the less polarizable Mg²⁺ ions by the more polarizable Ca²⁺ ions. The variation in the Q × f values followed a similar trend as that of the sintered density, and the change trend in the τ_f values was in accordance with that of relative permittivity. The best composition appeared to be Mg_0.9Ca_0.1ZrTa₂O₈, which showed excellent microwave dielectric properties of ε_r = 22.5, Q × f = 231,951 GHz, and τ_f = −32.9 ppm/°C. The Q × f value obtained is the highest among the wolframite dielectric ceramics reported in literature.     

Solubility limits and microwave dielectric properties of Ba6−3xSm8+2xTi18O54 solid solution

Upper and lower solubility limits in Ba_6−3xSm_8+2xTi₁₈O₅₄ tungsten bronze ceramics were determined by Rietveld refinement of XRD data combined with backscattered electron images, and the variation tendency of microwave dielectric characteristics was also investigated. The upper solubility limit was confirmed as x = 2/3, while the lower solubility limit was determined as 1/4 instead of the previously reported one x = 3/10. The dielectric constant of Ba_6−3xSm_8+2xTi₁₈O₅₄ ceramics decreases monotonically with increasing x, while the small temperature coefficient of resonant frequency with complex variation tendency is observed for the compositions 1/2 ≤ x ≤ 4/5. The Qf value increases at first, reaches the maximum around x = 2/3, and turns to decrease for x > 7/10.     

Synthesis and microwave dielectric properties of BaO-Sm2O3-5TiO2 ceramics with NdAlO3 additions

BaO-Sm₂O₃-5TiO₂ (BST5) ceramics with NdAlO₃ additions of up to 15 wt% were produced with a solid state reaction method, and their structural and microwave dielectric properties were determined. Experimental results showed that NdAlO₃ neither merged nor altered the orthorhombic tungsten bronze structure of the main phase of the produced ceramics (except for a shrinkage in the crystal lattice), but it was segregated in distinct grains in the microstructure of the produced ceramics. However, the amount of NdAlO₃ strongly influenced the densification and the microstructure (i.e. grain shape and size) of the produced ceramics. Analysis of the experimental results suggests that the microstructural features can be correlated to the dielectric properties of these ceramics. Accordingly, the dielectric constant (ε_r) and the temperature coefficient of resonant frequency (τ_f) of the produced BLT5 ceramics can be tuned with the amount of NdAlO₃ additions and the sintering process parameters. The best dielectric properties were achieved for BaO-Sm₂O₃-5TiO₂ ceramics with 7.5% NdAlO₃ (ε_r=73.22, Q×f =10,300 GHz, and τ_f=−1.05 ppm/°C).     

Synthesis,lattice energy and microwave dielectric properties of BaCu2-xCoxSi2O7 ceramics

BaCu_2-xCo_xSi₂O₇ solid solutions with orthorhombic structure (Pnma) were prepared by solid-state reaction method. The phase synthesis process, structural evolution and microwave dielectric properties of BaCu_2-xCo_xSi₂O₇ ceramics were investigated. Single BaCu₂Si₂O₇ phase was obtained when calcined at 950 °C for 3 h and was decomposed into BaCuSi₂O₆ phase when calcined at 1075 °C for 3 h. The sintering process was effectively promoted when Cu²⁺ was replaced by Co²⁺ and the maximum solubility of BaCu_2-xCo_xSi₂O₇ was located between 0.15 and 0.20. P-V-L complex chemical bond theory and Raman spectra were used to explain the structure-property correlations of BaCu_2-xCo_xSi₂O₇ ceramics. The corrected dielectric constant (ε_r-corr) of BaCu_2-xCo_xSi₂O₇ ceramics decreased monotonously with the susceptibility (Σχ^μ) and ionic polarizability of primitive unit cell. The quality factor (Q × f) increased with bond strength and lattice energy (U_cal), especially the lattice energy of the Si-O bond. The temperature coefficient of resonant frequency (τ_f) was determined by the susceptibility and lattice energy of the Cu/Co-O bond. The following optimum microwave dielectric properties were obtained at x = 0.15 when sintered at 1000 °C for 3 h: ε_r = 8.45, Q×f =58958 GHz and τ_f = -34.4 ppm/°C.     

Effects of sintering parameters and Nd doping on the microwave dielectric properties of Y2O3 ceramics

Y₂O₃ ceramics with good dielectric properties were prepared via co-precipitation reaction and subsequent sintering in a muffle furnace. The effects of Nd doping and sintering temperature on microwave dielectric properties were studied. With the increase in sintering temperature, the density, quality factor (Q×f), and dielectric constant (ε_r) values of pure Y₂O₃ ceramics increased to the maximum and then gradually decreased. The Y₂O₃ ceramics sintered at 1500 °C for 4 h showed optimal dielectric properties: ε_r=10.76, Q×f=82, 188 GHz, and τ_f=−54.4 ppm/°C. With the addition of Nd dopant, the Q×f values, ε_r, and τ_f of the Nd: Y₂O₃ ceramics apparently increased, but excessive amount degraded the quality factor. The Y₂O₃ ceramics with 2 at% Nd₂O₃ sintered at 1460 °C displayed good microwave dielectric properties: ε_r=10.4, Q×f=94, 149 GHz and τ_f=−46.2 ppm/°C.     

Sintering behavior,phase structure and microwave dielectric properties of CeO2 added CaTiO3-SmAlO3 ceramics prepared by reaction sintering method

Novel 0.695CaTiO₃-0.305SmAlO₃+xwt% CeO₂ (x = 0, 0.5, 1.0, 1.5) ceramics were fabricated using a reaction-sintering (RS) approach. The crystal structure, morphology, and microwave dielectric properties of ceramics were systematically studied. The addition of CeO₂ could effectively improve the sintering behavior of 0.695CaTiO₃-0.305SmAlO₃ (CTSA) ceramics. When x = 0.5 wt%, the ceramics exhibited optimal microwave dielectric properties, with ε_r = 43.9, Q×f = 48 779 GHz, and τ_? = ?0.24 ppm/°C, thereby indicating that the samples prepared via the RS route possess superior dielectric properties compared to those prepared by the conventional solid phase reaction. The results demonstrate that CaTiO₃-SmAlO₃ ceramics can be prepared simply and efficiently through a reaction-sintering process.     

Sintering behavior,structural evolution,and dielectric properties of Li2+xMgTiO4Fx microwave dielectric ceramics

Herein, the sintering behavior, structural evolution, microstructure, and dielectric properties of Li_2+xMgTiO₄F_x (0 ≤ x ≤ 5) ceramics were investigated. At x ≤ 0.75, Li_2+xMgTiO₄F_x ceramics formed a continuous solid solution with a cubic rock salt structure. Subsequently, a composite ceramic of Li_2+xMgTiO₄F_x and LiF was formed. It was found that the maximum mass percentage of LiF required to fully form a solid solution was between 11% and 13%. The Li_2.75MgTiO₄F_0.75 exhibited the best dielectric properties: ε_r = 16.52, Q × f = 123,574 GHz, and τ_f = −18.11 ppm/°C. The substitution of F^- for O^2- resulted in a lower sintering temperature of 875 °C, which slightly suppressed the volatilization of Li, and thus optimized the dielectric properties. The decrease in lattice vibration damping behavior and the increase in electron cloud density resulted in lower dielectric losses. The reduction in molecular polarization rate led to a reduction in ε_r, and the increase in bond energy optimized τ_f. Good chemical compatibility with Ag electrode was demonstrated, indicating that Li_2+xMgTiO₄F_x ceramics have unlimited potential for LTCC applications.     

Correlations between structure and microwave dielectric properties of Co doped MgMoO4 ceramics

Mg_1-xCo_xMoO₄ (x = 0.01–0.15) ceramics were prepared by traditional solid-state methods. The phase composition, crystalline structure, micromorphology, and microwave dielectric properties of Mg_1-xCo_xMoO₄ ceramics were comprehensively studied. Mg_1-xCo_xMoO₄ ceramics present monoclinic wolframite structures from x = 0.01 to x = 0.15 with Co occupying the Mg-site. With the addition of Co²⁺, ε_r of Mg_1-xCo_xMoO₄ ceramics increase. Q × f is maximal at 5 mol% Co²⁺ content. The Mg_0.95Co_0.05MoO₄ ceramic exhibits an optimal microwave dielectric property: ε_r = 7, Q × f = 59247 GHz, τ_f = −68 ppm/°C. The Q × f values increase by 20% compared with the pure MgMoO₄ ceramics (~49149 GHz). Doping Co²⁺ effectively promotes the densification of ceramics and increases ε_r and Q × f. However, when the Co content exceeds 5 mol%, the decreased packing fraction and disorder distribution of ions contribute to the increase in dielectric losses. The correlations between Co²⁺ substitution and wolframite structure have been discussed by Raman spectroscopy, FT-IR spectroscopy and Rietveld refinement.