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.     

Sintering characteristics and microwave dielectric properties of ultralow-loss SrY2O4 ceramics

A SrY₂O₄ microwave dielectric ceramic suitable for 5G systems is synthesised via a solid-state reaction in a sintering temperature range of 1425–1525 °C. X-ray diffraction patterns and Rietveld refinement analysis show that the ceramic has an antispinel orthorhombic crystal structure belonging to the Pnma space group. Scanning electron microscopy images show that the ceramic particles are closely connected, the grain boundaries are clear, and the particles are uniform at the optimal sintering temperature of 1475 °C. The optimal microwave dielectric performances are ε_r = 14.78, Q × f = 84090 GHz, τ_? = ?14.98 ppm/°C. The relatively low dielectric constant, high Q × f value, low τ_? value, and easily available raw materials indicate that it is a good choice for 5G equipment.     

Sintering behavior,structure, and microwave properties of novel Li2xCu1-xMoO4 ceramics

In this study, we prepared a novel series of Li_2xCu_1-xMoO₄ (x = 0.02, 0.04, 0.06, 0.08, and 0.10) microwave ceramics. The dynamic sintering behavior, crystal phases, micro-morphologies, and dielectric properties of the samples were studied. The substitution of Li⁺ contributed to refining the crystal grain size, promoting the densification of microstructure, and enhancing the quality factor. Due to different valence substitutions, Cu⁺ ions were created, which were verified by X-ray photoelectron spectroscopy (XPS) and Raman experiments. In addition, the Raman shift, full width at half maximum (FWHM) value of the A_1g peak, and crystal microstrains were analyzed to gain a mechanistic understanding of the influence of structure on the dielectric properties. When x = 0.08, the Li_2xCu_1-xMoO₄ ceramic sintered at 675 °C exhibited optimal comprehensive properties with ε_r = 8.17, Qf = 68 476 GHz, and τ_f = ?25 ppm/°C, and good chemical stability between the ceramic and Al electrode was also achieved. These promising properties make Li_2xCu_1-xMoO₄ (x = 0.08) more suitable for ultra-low temperature co-fired ceramic (ULTCC) applications.     

Low temperature sintered Li6MgTiNb1-xVxO8F microwave dielectric ceramics with high-quality factor

Li₆MgTiNb_1?xV_xO₈F (0 ≤ x ≤ 0.08) ceramics were prepared using a solid-state reaction. The correlations between their sintering characteristics and the microwave dielectric performance as functions of V⁵⁺ substitution and sintering temperature were investigated systematically. Rietveld refinements of the X-ray diffraction data showed that all the samples had a cubic rock-salt structure. The Li₆MgTiNbO₈F ceramic sintered at 1175 °C exhibited an attractive Q × f value of 105,700 ± 1600 GHz. The substitution of V⁵⁺ for Nb⁵⁺ decreased the sintering temperature while improving the relative density and relative permittivity. The V-beared Li₆MgTiNb_0.98V_0.02O₈F ceramic sintered at 850 °C showed outstanding dielectric properties of ε_r = 18.14 ± 0.05, Q × f = 58,300 ± 1300 GHz, and τ_f = ?42.66 ± 0.33 ppm/°C. Good chemical compatibility with Ag electrodes highlighted the potential of the ceramic in low-temperature co-fired ceramic applications.     

Low-temperature sintering and microwave dielectric properties of Ca5Co4(VO4)6 ceramics

A novel low temperature firing high Q microwave dielectric ceramic Ca₅Co₄(VO₄)₆ was prepared by the conventional solid-state reaction method. The phase purity, microstructure, and microwave dielectric properties were investigated. The Ca₅Co₄(VO₄)₆ ceramic sintered at 875 °C exhibited excellent microwave dielectric properties: Qxf = 95,200 GHz (at 10.6 GHz), τ_f = −63 ppm/°C, ɛ_r = 10.1, and its ɛ_r corrected for porosity was calculated as 11.1.     

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.     

Sintering behavior and dielectric properties of KCa2Nb3O10 ceramics

Dense KCa₂Nb₃O₁₀ (KCN) oxides were synthesized and their dielectric properties were investigated. A homogeneous KCN phase was formed in the specimen sintered above 1300 °C, but a CaNb₂O₆ secondary phase was developed in the specimen sintered at 1400 °C, as result of the evaporation of K₂O. The KCN oxides sintered at 1375 °C showed a high relative density that was 97.1% of the theoretical density. Furthermore, liquid-phase-assisted abnormal grain growth occurred during sintering. The dielectric constant of this KCN oxide was 46, with a low dielectric loss of 0.9% at 100 kHz; these values are smaller than those that were previously reported. Complex impedance analysis indicated that the resistivity of the KCN oxide was very low, probably as a result of the presence of K⁺ ions between the layers, and this could be the origin of the low-frequency dispersion of the KCN oxides.     

Sintering characteristic,microstructure and microwave dielectric properties of the borax-added Sr3(VO4)2 ceramics

Na₂B₄O₇·10H₂O (borax) doped Sr₃(VO₄)₂ ceramics for ULTCC applications were synthesized by the solid-state reaction. The influence of borax addition on the sintering characteristic, microstructure, and microwave dielectric properties of Sr₃(VO₄)₂ ceramics was investigated in detail. The result indicated that borax was an effective sintering aid for the Sr₃(VO₄)₂ system and a suitable amount of borax dramatically reduced the sintering temperature of Sr₃(VO₄)₂ ceramics from 1000 to 675 °C. Meanwhile, borax addition prevented the Q × f value from getting degenerated and improved the τ_f value of Sr₃(VO₄)₂ ceramics in the case of ultra-lower sintering temperatures. Novel Sr₃(VO₄)₂ + 1 wt% borax ceramic sintered at 675 °C with optimum properties of Q × f = 19,200 GHz, ε_r = 15.9, and τ_f = 10.9 ppm/°C was achieved in this study.     

The spectra analysis and microwave dielectric properties of [Ca0.55(Sm1-xBix)0.3]MoO4 ceramics

A series of low-temperature firing ceramics with scheelite structure, [Ca_0.55(Sm_1-xBi_x)_0.3]MoO₄ (x = 0.2–0.95), were prepared via solid-state reaction. The sintering temperature ranges from 660 to 760°C. A standard tetragonal scheelite phase was formed without secondary phase. When the x value was 0.95, the temperature coefficient of resonant frequency (τ_f) moved to a near zero value (−2.1 ppm/°C) with a dielectric constant 13.7 and the quality factor (Qf) of 33 200 GHz. The Raman spectra shows that the more vibration modes appeared with x value, which is due to the increasing of Bi concentration and results in increase in permittivities and decrease in Qf values. The classical harmonic oscillator model is used in the infrared spectra and extrapolate to the microwave range. The [Ca_0.55(Sm_1-xBi_x)_0.3]MoO₄ ceramics show high-performance microwave dielectric properties at low-sintering temperature.     

Preparation and microwave dielectric properties of cristobalite ceramics

Dense SiO₂ ceramics with cristobalite phase were prepared by the solid state sintering route, and the microwave dielectric properties were evaluated. The dielectric constant (?_r) and temperature coefficient of resonant frequency (τ_f) of the pure cristobalite ceramics showed little dependence on the sintering temperature. While, the Qf value increased significantly with increasing the sintering temperature, and it was due to the increasing grain size. The optimized microwave dielectric properties with very low ?_r of 3.81, high Qf value of 80,400 GHz and low τ_f of ?16.1 ppm/°C were obtained for the cristobalite ceramics sintered at 1650 °C for 3 h. It was indicated that cristobalite ceramic was a promising candidate as a low-dielectric-constant microwave material for applications in microwave substrates.     

Temperature-dependent dielectric and Raman spectra and microwave dielectric properties of gehlenite-type Ca2Al2SiO7 ceramics

Gehlenite-type Ca₂Al₂SiO₇ ceramics were prepared by the conventional solid-state reaction. Two anomalies were found in the plot of dielectric constant vs temperature, which were associated with space charge polarization. Pure phase crystal structure and no phase transition were observed in the temperature-dependent X-ray diffraction (XRD) patterns and Raman spectra from room temperature (RT) to 900°C. There was relevant relation between Q × f and τ_ƒ with the stretching vibrations of Ca-O bond and O-Ca-O bending in CaO₈ polyhedron. Excellent microwave dielectric properties (ε_r = 8.86, Q × f = 22 457 GHz, and τ_f = −51.06 ppm/°C) were obtained for Ca₂Al₂SiO₇ sintered at 1440°C in air, which had the potential application to use in microwave and millimeter-wave devices such as capacitors and substrates.     

Structure and microwave dielectric properties of Li(Al1-xLix)SiO4-x ceramics

Li(Al_1-xLi_x)SiO_4-x (x = 0.005, 0.01, 0.015, and 0.02) ceramics were synthesized via a traditional solid phase reaction method with different sintering temperatures. To determine the positions occupied by Li⁺ in the lattice, the defect formation energies and total energies of various sites of LiAlSiO₄ (LAS) occupied by Li⁺ were examined, and the energy of LAS systems were calculated using density functional theory of first-principle with the CASTEP module. The results demonstrated that the Al-sites occupied by Li⁺ had the lowest formation energies and total energy, so Li ⁺ should substitute Al³⁺. The impacts of replacing Al³⁺ with Li⁺ on the bulk density, sintering properties, phase composition, microstructure, and microwave dielectric properties of Li(Al_1-xLi_x)SiO_4-x (0 = x ≤ 0.02) ceramics were thoroughly studied. With Li⁺-doping, the sintering temperature decreased from 1300 °C (x = 0) to 1175 °C (x = 0.02), while the Q × f and τ_f values of LAS ceramics significantly increased. The Li(Al_0.99Li_0.01)SiO_3.99 ceramic was fully sintered at 1250 °C for 10 h to obtain excellent microwave dielectric properties: ε_r = 3.49, Q × f = 51,358 GHz, and τ_f = ?51.48 × 10^?6 °C^?1.     

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.     

Low-temperature sintering and microwave dielectric properties of CaWO4 ceramics for LTCC applications

Microwave dielectric properties of CaWO₄ ceramics were investigated as a function of H₃BO₃ and/or Bi₂O₃ content and sintering temperature. For a single addition of H₃BO₃ (1 ≤ x (wt.%) ≤ 5), the density of specimen increased up to 3 wt.% H₃BO₃, and then decreased. The dielectric constant (K) and the quality factor (Q × f) of the specimens sintered at 850 °C showed lower value than those of specimens sintered above 900 °C due to the poor sinterability. With the increase of H₃BO₃ content of 0.5 wt.% Bi₂O₃–yH₃BO₃ (5 ≤ y (wt.%) ≤ 20), the sintering temperature of CaWO₄ ceramics could be effectively reduced from 1100 to 850 °C without degradation of dielectric properties. For the specimens sintered at 850 °C for 30 min, K was not changed remarkably with Bi₂O₃–H₃BO₃ content; however, Q × f value increased up to 9 wt.% H₃BO₃ of 0.5 wt.% Bi₂O₃–yH₃BO₃, and then decreased. The temperature coefficient of resonant frequency (TCF) shifted to the positive value with increasing Bi₂O₃–H₃BO₃ content. Typically, K of 8.7, Q × f of 70,220 GHz and TCF of −15 ppm/°C were obtained for the specimens with 0.5 wt.% Bi₂O₃–9 wt.% H₃BO₃ sintered at 850 °C for 30 min.     

Microwave dielectric properties of Ca1-xBaxMgSi2O6 ceramics

Ca_1-xBa_xMgSi₂O₆(x = 0–0.4) ceramics were prepared through a traditional solid-state reaction sintering route with various sintering temperatures. The effects of substituting Ba²⁺ for Ca²⁺, the relative density, phase composition, crystal morphology, and microwave dielectric properties of Ca_1-xBa_xMgSi₂O₆ (x = 0–0.4) ceramics were thoroughly studied. X-ray diffraction patterns indicate a single phase was formed in the samples when x ≤ 0.2, and the second phase BaMg₂Si₂O₇ appeared at x = 0.4. As the amount of Ba²⁺ substitution increases, the Q×f value first increases and then decreases due to the combined effects of FWHM of peak v₁₁ and atomic packing density, and the ${\epsilon }_r$ value was increased continuously which was closely corrected with the relative density and molecular polarization. The ${\tau }_f$ value improved slightly with the substituting Ba²⁺ for Ca²⁺. Typically, the Ca_0.88Ba_0.12MgSi₂O₆ ceramic can be well sintered at 1275 °C for 4 h with a maximum relative density of 99.3%, and possesses optimal microwave dielectric properties: ${\epsilon }_r=7.49$, $Q\times f=64310$ GHz, ${\tau }_f=-44.02$ ppm/°C.     

Sintering behavior,structural phase transition,and microwave dielectric properties of La1‐xZnxTiNbO6‐x/2 ceramics

La_1‐xZn_xTiNbO_6‐x/2 (LZTN‐x) ceramics were prepared via a conventional solid‐state reaction route. The phase, microstructure, sintering behavior, and microwave dielectric properties have been systematically studied. The substitution of a small amount of Zn²⁺ for La³⁺ was found to effectively promote the sintering process of LTN ceramics. The corresponding sintering mechanism was believed to result from the formation of the lattice distortion and oxygen vacancies by means of comparative studies on La‐deficient LTN ceramics and 0.5 mol% ZnO added LTN ceramics (LTN+0.005ZnO). The resultant microwave dielectric properties of LTN ceramics were closely correlated with the sample density, compositions, and especially with the phase structure at room temperature which depended on the orthorhombic‐monoclinic phase transition temperature and the sintering temperature. A single orthorhombic LZTN‐0.03 ceramic sintered at 1200°C was achieved with good microwave dielectric properties of ε_r~63, Q×f~9600 GHz (@4.77 GHz) and τ_f ~105 ppm/°C. By comparison, a relatively high Q × f~80995 GHz (@7.40 GHz) together with ε_r~23, and τ_f ~?56 ppm/°C was obtained in monoclinic LTN+0.005ZnO ceramics sintered at 1350°C.     

Tuning the microwave dielectric properties of Zn2SnO4 ceramics by adding Ca0.8Sr0.2TiO3

The influence of sintering temperature on the microwave dielectric properties and microstructure of the (1−y)Zn₂SnO₄–yCa_0.8Sr_0.2TiO₃ ceramic system were investigated with a view to their application in microwave devices. A (1−y)Zn₂SnO₄–yCa_0.8Sr_0.2TiO₃ ceramic system was prepared by the conventional solid-state method. The X-ray diffraction patterns of the 0.85Zn₂SnO₄–0.15Ca_0.8Sr_0.2TiO₃ ceramic system did not significantly vary with sintering temperature. A dielectric constant of 9.6, a quality factor (Q×f) of 15,900 GHz, and a temperature coefficient of resonant frequency of −4 ppm/°C were obtained when the 0.85Zn₂SnO₄–0.15Ca_0.8Sr_0.2TiO₃ ceramic system was sintered at 1175 °C for 4 h.     

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.     

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.     

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.