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).     

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.     

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.     

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.     

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.     

Synthesis and microwave dielectric properties of Bi2Ge3O9 ceramics for application as advanced ceramic substrate

During the synthesis of Bi₂Ge₃O₉ ceramics using Bi₂O₃ + 3GeO₂ powders, the Bi₄Ge₃O₁₂ phase was formed at low temperature (≤800 °C). Bi₄Ge₃O₁₂ preferentially adopted GeO₂-excess phase, and this phase was consistently present in the sintered Bi₂Ge₃O₉ ceramic as a secondary phase. Therefore, Bi₄Ge₃O₁₂ powder was first calcined and subsequently reacted with GeO₂ powder to obtain the pure Bi₂Ge₃O₉ ceramic through the following reaction: 1/2Bi₄Ge₃O₁₂ + 3/2GeO₂ → Bi₂Ge₃O₉. Formation of the Bi₂Ge₃O₉ phase was initiated at temperature of 850 °C. The pure Bi₂Ge₃O₉ ceramic sintered at 875 °C for 8 h had a dense microstructure with an average grain size of 2.7 μm. Furthermore, the pure Bi₂Ge₃O₉ ceramic exhibited promising microwave dielectric properties for the advanced ceramic substrate: ε_r = 9.7, Q × f = 48,573 GHz and τ_f = −29.5 ppm/°C.     

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.     

Crystal structure and microwave dielectric properties of garnet-type Ca2YZr2-xTixAl3O12 ceramics for dual-band bandpass filters

Microwave dielectric ceramics with a high-quality factor are vital materials for substrates, dielectric resonators, and filters in millimeter-wave communication systems. Here, a novel microwave dielectric ceramic based on a garnet-type Ca₂YZr₂Al₃O₁₂ compound for bandpass filter was prepared using the solid-state reaction method. Sintering the Ca₂YZr₂Al₃O₁₂ ceramics at 1600 ℃ for 5 h resulted in excellent microwave dielectric properties of ε_r = 10.81 ± 0.16, Qf = 87,628 ± 4000 GHz and τ_f = ?34.3 ± 0.5 ppm/℃. Increasing the Ti⁴⁺ content of the Ca₂YZr_2-xTi_xAl₃O₁₂ ceramics significantly improved the sintering process. Superior microwave dielectric properties (ε_r = 11.93 ± 0.15, Qf = 121,930 ± 2600 GHz and τ_f = ?30.8 ± 0.4 ppm/℃) were obtained for Ca₂YZr_2-xTi_xAl₃O₁₂ (x = 0.3) because of its dense microstructure, large grain size and high lattice energy. The Ca₂YZr_2-xTi_xAl₃O₁₂ (x = 0.3) ceramics were used to fabricate a dual-band bandpass filter with a hairpin structure that exhibited a large return loss (|S₁₁| > 12 dB) and a small insertion loss (|S₂₁| < 0.73 dB). The high performance of the Ca₂YZr_2-xTi_xAl₃O₁₂ ceramics and the corresponding bandpass filter makes this material a potential candidate for millimeter-wave devices.     

Effects of structural transition on microwave dielectric properties of Sr3(Ti1-xSnx)2O7 ceramics

Sr₃(Ti_1-xSn_x)₂O₇ (x = 0–1.0) ceramics were prepared via a standard solid-state reaction method. X-ray diffraction patterns and Rietveld refinement results indicated a composition induced onset of octahedral tilting when x > 0.2, and the crystal structure transformed in sequence: tetragonal (I4/mmm) → coexistence of tetragonal and orthorhombic (I4/mmm + Amam) → orthorhombic (Amam). The τ_f value could be successfully tuned towards zero and the effects of octahedral tilting on the evolution of τ_f value were emphasized. Meanwhile, the role of tolerance factor in tailoring the resultant τ_ε of the present ceramics was revealed and compared with the empirical rule for complex perovskites. Qf value decreased monotonously with increasing x, which could be elucidated by the variations of extrinsic parameters and intrinsic dielectric loss extrapolated from the infrared reflectivity spectra. The optimum microwave dielectric properties were achieved at x = 0.8 (ε_r = 18.6, Qf = 45,250 GHz, τ_f =–14 ppm/^oC).     

Synthesis of a low-firing BaSi2O5 microwave dielectric ceramics with low dielectric constant

In this study, BaSi₂O₅ ceramics with an orthorhombic structure were synthesized by using a traditional solid-state method at a low temperature by doping with Li₂O–B₂O₃–CaO–CuO (LBCC) glass. The phase composition, mechanism of low-temperature sintering, microwave dielectric properties, and changes in the mesophase during the heating of low-temperature sintered BaSi₂O₅ ceramics were examined by performing an X-ray diffraction analysis. A compact matrix of BaSi₂O₅ can be wetted by the liquid phase of the formed LBCC glass. Therefore, LBCC glass with different doping percentages can effectively reduce the sintering temperature of BaSi₂O₅. The microwave dielectric properties of BaSi₂O₅ ceramics sintered at 900 °C at 4 wt% of LBCC glass were determined: ε_r = 7.32, Q × f = 19,002 GHz, and τ_f = ?35.8 ppm/°C. The chemical compatibility of the samples with Ag was studied at 4 wt% doping with LBCC glass, and the samples were fired for 4 h at 900 °C.     

Effects of MnO2 and WO3 co-doping and sintering temperature on microstructure,microwave dielectric properties of Ba2Ti9O20 microwave ceramics

Ba₂Ti₉O₂₀ (short for B2T9) ceramics doped with 0.9 mol% MnO₂ and y mol% WO₃ were prepared by solid-state reaction. The influence of sintering temperature, content of WO₃ dopant and the molar ratio x of TiO₂: BaCO₃ on crystal structure, microstructures as well as microwave dielectric properties of B2T9 ceramics was systematically investigated. The major phase of all samples is B2T9, and the minor phase is BaWO₄, respectively. The content of impurity TiO₂ alternates with the variation of compositions and sintering temperature, which also leads to different microwave dielectric properties. With the continuous increase of the sintering temperature, the B2T9 phase grains gradually grow larger and transform from rod grains to plate-like grains. The enlargement and flattening of grains also result in the decrease of compactness and deterioration of microwave dielectric properties. It is found that B2T9 ceramics possess better performance when the sintering temperature is 1340°C, which is related to lower TiO₂ content, BaWO₄, B2T9 grain size, aspect ratio of B2T9 phase and high compactness. When x = 4 and y = 0.2, the relative dielectric constant, quality factor and the temperature coefficient of resonant frequency are 38, 23758 and 7 ppm/°C, respectively.     

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.     

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.     

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.     

Crystallographic characteristics and microwave dielectric properties of Ni-modified MgTa2O6 ceramics

Ni²⁺ modified MgTa₂O₆ ceramics with a trirutile phase and space group P4₂/mnm were obtained. The correlations between crystallographic characteristics and microwave dielectric performance of MgTa₂O₆ ceramics were systematically studied based on the chemistry bond theory (PVL theory) for the first time. The results indicate that the introduction of Ni²⁺ causes a change in polarizability and the Mg–O bond ionicity, which contributes to the variation of dielectric constant. Moreover, the lattice energy, and packing fraction, full width at half maximum of the Raman peak of Ta–O bond, as the quantitative characterization of crystallographic parameters, regulate the dielectric loss of MgTa₂O₆ ceramics in GHz frequency band. In addition, the study of sintering behavior shows that the densification and micromorphology are the crucial factors affecting the microwave dielectric performance. Typically, Ni²⁺ doping on the A-site of MgTa₂O₆ can effectively promote the Q × f values to 173,000 GHz (at 7.43 GHz), which ensures its applicability in 5G communication technology.     

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 3 h 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 ε_r anomaly peak at lower temperature and increases the ε_r (or frequency) stability against temperature. The Zn²⁺-rich BaZnSiO₄ phase has a τ_f value of −181 ppm/°C, whereas the τ_f value of Zn²⁺-free BaSiO₃ phase decreases to −35.4 ppm/°C. The Zn²⁺ deficiency in Ba₂ZnSi₂O₇ composition could inhibit the presence of BaZnSiO₄ phase and improve the τ_f value, whereas excessive Zn²⁺ cations prompt the formation of the BaZnSiO₄ phase to deteriorate significantly the τ_f value.     

Low-temperature firing and microwave dielectric properties of MgNb2-xVx/2O6-1.25x ceramics

MgNb_2-xV_x/2O_6-1.25x (0.1≤x≤0.6) ceramics with orthorhombic columbite structures were prepared at low-temperature by a solid-phase process. The phase component, microscopic morphology, low-temperature sintering mechanism and microwave dielectric performance of MgNb_2-xV_x/2O_6-1.25x ceramics were comprehensively investigated. Low-temperature sintering densification of dielectric ceramics was achieved via the nonstoichiometric substitution of vanadium (V) at the Nb-site. In contrast to pure MgNb₂O₆ ceramics, the sintering temperature of MgNb_2-xV_x/2O_6-1.25x (x = 0.2) ceramics was reduced by nearly 300 °C owing to the liquid-phase assisted sintering mechanism. The liquid phase arises from the autogenous low-melting-point phase. Meanwhile, MgNb_2-xV_x/2O_6-1.25x (x = 0.2) samples with nonstoichiometric substitution could achieve a more than 900% improvement in the Q × f value, compared with stoichiometrically MgNb_2-xV_xO₆ (x = 0.1, 0.2) ceramics. Finally, MgNb_2-xV_x/2O_6-1.25x dielectric ceramics possess outstanding microwave dielectric properties: ε_r = 20.5, Q × f = 91000, and τ_f = -65 ppm/°C when sintered at 1030 °C for x = 0.2, which provides an alternative material for LTCC technology and an effective approach for low-temperature sintering of Nb-based microwave dielectric ceramics.     

Modulation of microwave dielectric properties in melilite-structured SrREGa3O7 (RE = La,Pr) ceramics

Gallium-based SrREGa₃O₇ (RE = La, Pr) melilite ceramics were prepared and selected to modify their microwave dielectric properties. Sintered at 1425 °C for 6 h, SrLaGa₃O₇ (SLGO) and SrPrGa₃O₇ (SPGO) ceramics exhibited high relative densities of 98.33 and 95.23%, low ε_r values of 11.8 and 10.9, Q×f values of 32,500 GHz (at 12.1 GHz) and 26,400 GHz (at 12.7 GHz), negative τ_f values of ?32 and ?54 ppm/°C. As a compensator, CaTiO₃ can tune the τ_f values of SLGO and SPGO to near zero (+2 and ?4 ppm/°C). In SrREGa₃O₇ melilite ceramics, the ε_r and τ_f values are mainly dependent on ionic polarizability, crystal structure and the “rattling” effect. The micromorphology, XPS, Raman spectrum and A-site bond valence (V_RE) of SrREGa₃O₇ (RE = La, Pr) microwave dielectric ceramics have also been comprehensively reported.