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.     

Low‐Temperature Sintering and Microwave Dielectric Properties of ZnNb2O6–TiO2 Ceramics with BaCu(B2O5) Additions

The influence of BaCu(B₂O₅) (BCB) on densification, phases, microstructure and microwave dielectric properties of ZnNb₂O₆–xTiO₂ (x = 1.70–1.90) composite ceramics have been investigated. Undoped ZnNb₂O₆–1.8TiO₂ ceramics sintered at 1200°C exhibit temperature coefficient of resonant frequency (τ_f) ~9.25 ppm/°C. When BaCu(B₂O₅) was added, the sintering temperature of the ZnNb₂O₆–1.8TiO₂ composite ceramics was effectively reduced to 950°C. The results indicated that the permittivity and Q × f were dependent on the sintering temperature and the amounts of BaCu(B₂O₅). Addition of 3.0 wt% BaCu(B₂O₅) in ZnNb₂O₆–1.8TiO₂ ceramics sintered at 950°C showed excellent dielectric properties of ε_r = 40.9, Q × f = 12,200 GHz (f = 5.015 GHz) and τ_f = +0.3 ppm/°C.     

Novel ultra-low loss and low-fired Li8MgxTi3O9+xF2 microwave dielectric ceramics for resonator antenna applications

Low temperature sintered Li₈Mg_xTi₃O_9+xF₂ microwave dielectric ceramics with x = 2−7 were developed based on a newly designed pseudo ternary phase diagram of the Li₂TiO₃–MgO–LiF system. Dense solid solution ceramics (of relative density >96 %) with cubic rock-salt structure, accompanied by a small amount of secondary phase MgO, were obtained in the temperature range of 800−925 °C. With increasing Mg²⁺ content, the value of ε_r decreased, whereas that of τ_f remained nearly constant, and the Q × f increased to a maximum at x = 5. The Li₈Mg₅Ti₃O₁₄F₂ ceramic sintered at 875 °C exhibited superior microwave dielectric properties with ε_r = 16.8, Q × f = 119,700 GHz, and τ_f = −41.6 ppm/°C. The good compatibility with Ag electrodes highlights the promising prospects of this ceramic in low-temperature co-fired ceramic technology. Furthermore, a dielectric resonator antenna fabricated based on a Li₈Mg₅Ti₃O₁₄F₂ ceramic exhibited an outstanding S₁₁ of −34.7 dB and a broad bandwidth of 360 MHz at the desired resonant frequency of 5.98 GHz.     

Enhanced dielectric properties and chemical bond characteristics of ZnNb2O6 ceramics due to zinc oxide doping

Regarding advanced 5G mobile communication, microwave dielectric ceramics are considered as the most potential materials to develop new-generation base station resonators. Herein, ZnNb₂O₆ ceramics with ε_r of approximately 24 have been prepared using the solid-state reaction method, with tailored extra ZnO of x mol% (x = 1, 2 and 3). We have for the first time applied the P-V-L chemical bond theory to investigate ZnNb₂O₆ ceramics with ZnO doping, by exploring the relationship of dielectric properties and chemical bond characteristics. Particularly, the Raman spectra demonstrates that the full width at half maximum of υ₁ (A_g) vibration mode can exhibit significant correlation with the quality factor (Q × f ). To further support the experimental study, we have also conducted the first-principle calculation of electron density difference via CASTEP package, which further confirms the change of temperature coefficient of resonance frequency (τ_f ). Our newly designed ZnNb₂O₆ ceramics doped with 1 mol% ZnO exhibit excellent dielectric properties, i.e., ε_r = 23.74, Q × f = 102,824 GHz and τ_f = ?55.38 ppm/°C, which demonstrates great potential to construct miniaturized 5G base station with advanced ceramic dielectrics.     

A cofired trilayer architecture to regulate temperature stability while maintaining high-Q: The case study of Ba(Mg1/3Nb2/3)O3 – Mg4Nb2O9 system

Trilayer architectures were designed and investigated to further improve the microwave dielectric properties of the Ba(Mg_1/3Nb_2/3)O₃ (BMN) – Mg₄Nb₂O₉ (MN) system, namely, to achieve temperature stability while maintaining high-Q. The calculated phase fractions in randomly distributed (1-x)BMN-xMN ceramics deviated from the designed composition (where the composition with x = 0.045, 0.056, 0.125 and 0.98 was respectively referred to as S1, S2, S3 and MN'), thus allowing it difficult to obtain near-zero τ_f as expected. In densification studies, doping a little MN was shown to effectively promote the sintering of BMN and provide the possibility for layer-cofired optimization. Fortunately, undesired differences in composition and performance could be suppressed with the weakened ion diffusion occurring at narrow interfaces with a width of ～2.5 μm in the S1/MN'/S1 trilayer architecture. Considering the influence of cofired compatibility and stress of dielectric layers, the compositionally optimized S3/MN'/S3 ceramics sintered at 1340 °C exhibited excellent microwave dielectric properties of ε_r = 21.95, Q × f = 110,482 GHz (f₀ = 6.870 GHz), and τ_f = 0.965 ppm/°C. Moreover, the dielectric response mechanism of layered ceramics was clarified by establishing the relationship between the layered architecture, dielectric properties and electric field distribution using the finite element method and high-frequency structure simulator (HFSS). This suggests that layered architectures like S1-3/MN'/S1-3 could effectively compensate for the dielectric properties and hold a promising application prospect of 5G wireless communication.     

Effect of zirconium deficiency on structure characteristics,morphology and microwave dielectric properties of Li2Mg3Zr1-xO6 ceramics

To suppress the impurity phases and porous microstructure caused by lithium volatilization, the Li₂Mg₃ZrO₆ and Zr-deficiency Li₂Mg₃Zr_1-xO₆ (x = 0.02, 0.04, 0.06, 0.08, 0.10) ceramics were successfully synthesized via the solid-state method assisted with atmosphere-controlled sintering. The influences of non-stoichiometric on structure characteristics, morphology and microwave dielectric properties of Li₂Mg₃Zr_1-xO₆ ceramics were investigated. The XRD and SEM results proved that the Zr-deficiency restrained the formation of impurity phases and remarkably improved the densification of Li₂Mg₃Zr_1-xO₆ samples. The variation trend of dielectric constant (ε_r) was explained by the relative density and theoretical dielectric polarizability. The quality factor (Q × f) was strongly associated with the impurity phases and relative density. Additionally, the temperature coefficient of resonant frequency (τ_f) showed the same trend as the total bond energy E_total, indicating the bond energy might play vital roles in thermal stability of Li₂Mg₃Zr_1-xO₆ samples. Typically, the Li₂Mg₃Zr_1-xO₆ sample obtained at x = 0.06 possessed remarkable dielectric performances: ε_r = 13.13, Q × f = 116,400 GHz (10.14 GHz) and τ_f = ?26.30 ppm/°C.     

Enhanced temperature stability of Mg2TiO4-based ceramics by LCB additive

The Mg₂TiO₄-xwt% LiF–2CaF₂–2B₂O₃ (LCB, 3.0 ≤ x ≤ 10.0) ceramics were fabricated to study the relationship among LCB additive and the sintering behavior, phase composition, micro-structure and dielectric performance of ceramics in this study. The sintered ceramics are mainly Mg₂TiO₄ phase, accompanied by small amount of second phase CaTiO₃ and Mg₃B₂O₆. They are generated from the chemical reaction of CaF₂ and B₂O₃ with the matrix material, respectively. Appropriate LCB additive significantly enhanced sintering ability and dielectric performance of Mg₂TiO₄-based ceramics. Sintered at 1175 °C, Mg₂TiO₄-7.5 wt% LCB ceramics exhibited a dielectric performance: ε_r of 15.3, Q × f of 32 950 GHz and τ_f of +1.96 ppm/°C, which is expected to be an alternative material for communication component.     

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.     

Structure and microwave dielectric properties of Li2Mg3Ti1-x(Al1/2Nb1/2)xO6 ceramics

Aiming to establish relationships between intrinsic structure factors and dielectric characteristics, a series of Li₂Mg₃Ti_1-x(Al_1/2Nb_1/2)_xO₆ (x = 0.0, 0.04, 0.08, 0.12, 0.16, 0.20) ceramics were synthesized to investigate the influences of (Al_1/2Nb_1/2)⁴⁺ substitution on the dielectric properties of Li₂Mg₃TiO₆ ceramics. The XRD and SEM results revealed that the pure rock salt phase (space group: Fm-3m) with a dense microstructure could be obtained with increasing the (Al_1/2Nb_1/2)⁴⁺ concentration, which is accompanied by an increase in the grain size from 11.69 to 22.81 μm. Meanwhile, some intrinsic factors, such as the average ionic polarizability, bond energy, packing fraction and lattice energy were calculated according to the complex chemical bond theory and refinement results. The unusual change in the dielectric constant (ε_r) was explained by the combined effects of the average ionic polarizability and relative density. The variation in the quality factor (Q × f) was ascribed to the packing fraction and lattice energy. The temperature coefficient of the resonant frequency (|τ_f|) reduced gradually with the increase in the octahedral bond energy, which enhanced the system thermal stability. Particularly, the Li₂Mg₃Ti_0.92(Al_1/2Nb_1/2)_0.08O₆ sample exhibited outstanding dielectric characteristics:ε_r = 15.256, Q × f = 174,300 GHz and τ_f = −19.97 ppm/°C.     

Low-loss and ultra-low temperature sintering microwave dielectrics of pure and Ag-modified K2Mg2(MoO4)3 ceramics

Novel K_2–2xAg_2xMg₂(MoO₄)₃ (x = 0–0.09) ceramics were synthesized by conventional solid-state sintering method. Based on the X-ray diffraction (XRD) patterns, all samples were identified to belong to an orthorhombic structure with a space group of P212121(19). The pure phase K₂Mg₂(MoO₄)₃ specimen when sintered at 590 °C revealed the favorable microwave dielectric properties: ε_r of 6.91, Q×f of 21,900 GHz and τ_f of ?164 ppm/°C. The substitution of Ag⁺ for K⁺ in K_2–2xAg_2xMg₂(MoO₄)₃ (x = 0.01–0.09) ceramics led to the more stable structure and dramatically enhanced the Q×f to a value of 54,900 GHz at 500 °C. The microwave dielectric properties were related to the relative density, microstructure, ionic polarization, lattice energy, packing fraction, and bond valence of the ceramics. It was suggested that for ultra-low temperature co-fired ceramic (ULTCC) applications, K_1.86Ag_0.14Mg₂(MoO₄)₃ ceramic could be sintered at 500 °C, which revealed an excellent combination of microwave dielectric properties (ε_r =7.34, Q×f =54,900 GHz and τ_f =–156 ppm/°C) and good chemical compatibility with aluminum electrodes.     

Solid-phase reaction mechanism and microwave dielectric properties of Mg2GeO4-MgAl2O4 composite ceramics

In this paper, a solution for simplifying the manufacture of Mg₂GeO₄-MgAl₂O₄ ceramics is reported. The Mg₂GeO₄-MgAl₂O₄ ceramics were obtained from MgO, Al₂O₃ and GeO₂ powders using the solid phase reaction approach. The phase structure, microscopic morphology and elemental composition of the Mg₂GeO₄-MgAl₂O₄ ceramics were specifically analyzed with XRD, SEM and EDS. The samples contain Mg₂GeO₄ and MgAl₂O₄ phases and no other phases are formed. The ceramics have a homogeneous and dense microstructure. This simplified preparation method saves preparation costs and improves the firing behavior of the ceramics, enhancing the microwave dielectric properties of Mg₂GeO₄-MgAl₂O₄ ceramics. Excellent microwave dielectric properties with ε_r = 8.0, Q × f = 150000 GHz and τ_f = ?34 ppm/°C were attained for Mg₂GeO₄-MgAl₂O₄ ceramics sintered at 1600 °C.     

Three-phase borate solid solution with low sintering temperature,high-quality factor,and low dielectric constant

The sintering and microwave dielectric properties of a ceramic material based on the mixing of Mg₃B₂O₆ and Zn₃B₂O₆ have been widely studied using first-principles calculations and experimental solid-state reactions. Characterization methods include the Network Analyzer, X-ray, Raman diffraction, scanning electron microscopy, energy-dispersive spectroscopy, and differential-thermal and thermo-mechanical analyzer. The increasing amount of Mg²⁺ results in the appearance of Mg₂B₂O₅ and ZnO, and the mutual substitution (Mg²⁺ and Zn²⁺) phenomenon has emerged in Zn₃B₂O₆ and Mg₂B₂O₅. The mechanisms have been explained with the help of DFT calculations. The bond parameters and electron distributions of the ZnO₄ tetrahedron and MgO₆ octahedron have been modified due to substitution. The sintering, substitution, and phase formation properties have been analyzed quantitatively through the energy parameters. The best dielectric properties were obtained for x = 0.20 sintered at 950°C, ε_r = 6.47, Q × f = 89 600 GHz (15.2 GHz), τ_f = −48.6 ppm/°C, relative density = 96.7%. The mixing of Zn₃B₂O₆ and Mg₃B₂O₆ ceramics is a feasible method to obtain a ceramic with low sintering temperature and excellent dielectric properties.     

Ultrahigh Q values and atmosphere-controlled sintering of Li2(1+x)Mg3ZrO6 microwave dielectric ceramics

Ultrahigh-Q Li_2(1+x)Mg₃ZrO₆ microwave dielectric ceramics were successfully prepared by means of atmosphere-controlled sintering through simultaneously adopting double crucibles and sacrificial powder. This technique played an effective role in suppressing the lithium volatilization and further promoting the formation of the liquid phase, as evidenced by the X-ray diffraction, microstructural observation and the density measurement. Both dense and even microstructure, and the suppression of detrimental secondary phases contributed to low-loss microwave dielectric ceramics with Q×f values of 150,000–300,000 GHz. Particularly, desirable microwave dielectric properties of ε_r=12.8, Q×f=307,319 GHz (@9.88 GHz), and τ_f=−35 ppm/°C were achieved in the x=0.06 sample as sintered at 1275 °C for 6 h.     

Effects of adding B2O3 on microwave dielectric properties of 0.9625MgTiO3-0.0375(Ca0.5Sr0.5)TiO3 composite ceramics

A series of 0.9625MgTiO₃-0.0375(Ca_0.5Sr_0.5)TiO₃ composite ceramics added with different amounts of B₂O₃ (1-5 wt%) were prepared via the solid state sintering method using the pre-synthesized raw MgTiO₃ and (Ca_0.5Sr_0.5)TiO₃ powders by molten-salt reaction. The sintering temperature of 0.9625MgTiO₃-0.0375(Ca_0.5Sr_0.5)TiO₃ composite ceramics can be reduced from 1275°C to 1175°C due to the liquid phase sintering effect of B₂O₃. When the adding amount of B₂O₃ was more than 2 wt%, a new phase MgTi₂O₅ could be detected by X-ray diffraction, which would substantially degrade the dielectric properties of the obtained ceramics. Resultantly, the quality factor (Q·f) and dielectric constant (ε_r) of the samples increase first and decrease later with increasing addition amount of B₂O₃. In addition, the temperature coefficient of resonant frequency (τ_f) progressively increases with increasing content of B₂O₃. By sintering at 1175°C for 4 hours, the obtained 0.9625MgTiO₃-0.0375Ca_0.5Sr_0.5TiO₃ composite ceramics with 2 wt% B₂O₃ possess the optimal microwave dielectric properties of ε_r = 18.9, Q·f = 57 000 GHz and τ_f = −1.2 ppm/°C.     

Significant improvement on quality factor in Bi3+/Ta5+ codoped Ce2Zr3-3xBi1.5xTa1.5xMo9O36 microwave dielectric ceramics with ultra-low sintering temperatures

Molybdenum oxide-based ceramics have attracted intense interest due to ultra-low sintering temperatures. However, low quality factors (Q × f) hinder their practical applications. Although Q × f can be improved by ions doping, the sintering temperature is greatly increased. Accordingly, it is still a challenge to obtain high Q × f ceramics sintered at ultra-low temperatures (<660 °C). Herein, (Bi_0.5Ta_0.5)⁴⁺ ions are utilized to tackle this issue in the Ce₂Zr₃(MoO₄)₉ ceramic as a prototype. Density and scanning electron microscope (SEM) results uncover good sintering states, and X-ray diffraction (XRD) results reveal the formation of solid solutions. Interestingly, the Ce-O bonds exhibit a dominant contribution to the bond ionicity (f_i), while Mo-O bonds play an important role in the lattice energy (U), the bond energy (E) and the thermal expansion coefficient (α). The remarkable increase of Q × f can be interpreted by the enhancement of the packing fraction and the mean U of Mo-O bonds. Moreover, the variations of the dielectric constant (ε_r) and the temperature coefficient of the resonance frequency (τ_f) can be explained by the variations of the intrinsic parameters. More interestingly, a negative correlation between Q × f and τ_f is first found. Typically, the CZ_0.98B_0.02 ceramic sintered at 650 °C exhibits optimum microwave dielectric properties: ε_r = 9.92, Q × f = 110,670 GHz, and τ_f = −19.20 ppm °C⁻¹. Notably, Q × f of the Ce₂Zr_2.94Bi_0.03Ta_0.03Mo₉O₃₆ (CZ_0.98B_0.02) ceramic is about 6 times larger than that of the matrix while retaining a low sintering temperature of 650 °C and a low ε_r of 9.92, making it a promising candidate for ultra-low temperature cofired ceramics (ULTCC) applications.     

Ge-doped Li3+xMg2Nb1-xGexO6 ceramics with enhanced low loss and high temperature stability properties

New high-performance materials have attracted much attention due to ever-increasing demands for advanced communication technologies. In present work, Ge-doped Li_3+xMg₂Nb_1-xGe_xO₆ (0 ≤ x ≤ 0.08) ceramics are prepared via solid-state reaction route. Microstructural analysis and crystal structure refinement reveal that moderate substitution can promote grain growth and modify crystal structure, thus enhancing microwave dielectric properties of composites. In that sense, special attention is paid to the behavior of dielectric constant ε_r, quality factor Q×f, and frequency temperature coefficient τ_f of final products. In these systems, ε_r parameter depends on the density, miscellaneous phases, and polarizability; Q×f value is shown to be influenced by Nb-O bond energy, grain size, and bulk density; finally, τ_f characteristic refers to Nb-O bond valence and NbO₆ octahedral distortion. Among above ceramics, Li_3.02Mg₂Nb_0.98Ge_0.02O₆ composite sintered at 1250 °C exhibits outstanding microwave absorption performance with ε_r = 15.32, Q×f = 969 88 GHz, and τ_f = ?8.25 ppm/°C.     

Tuning of microwave dielectric properties of garnets Ca3Mg2CV2O12 (C = Si,Ti) through compression and expansion effects

Two novel low-ε_r Ca₃Mg₂CV₂O₁₂ (C = Si, Ti) ceramics with the garnet structure were synthesized by a traditional solid-state reaction method. Rietveld refinements based on XRD patterns show both the compounds crystallized into a cubic structure with the Ia-3d space group. Outstanding microwave dielectric properties (ε_r = 9.70, Q × f = 35,680 GHz, and τ_f = ?60.1 ppm/°C for Ca₃Mg₂SiV₂O₁₂ ceramic; ε_r = 11.70, Q × f = 48,530 GHz, and τ_f = ?43.7 ppm/°C for Ca₃Mg₂TiV₂O₁₂ ceramic) were obtained at 1240 °C and 1260 °C, respectively. The bond valence calculations reveal that Ca²⁺ at the A-site and Mg²⁺ at the B-site are slightly compressed, in combination with “rattling” Si⁴⁺ and “compressed” V⁵⁺ in the C-site of Ca₃Mg₂SiV₂O₁₂ compared to “rattling” V⁵⁺and “compressed” Ti⁴⁺ in Ca₃Mg₂TiV₂O₁₂, resulting in a negative deviation of ?7.16% for Ca₃Mg₂SiV₂O₁₂ and a positive value of 5.66% for Ca₃Mg₂TiV₂O₁₂ between the porosity corrected ε_r(Corr) (10.11 and 11.95) and theoretical ε_th (10.89 and 11.31) calculated by the C-M equation. The overall “rattling” effect in Ca₃Mg₂TiV₂O₁₂ results in a higher ε_r and a nearer to zero τ_f compared to Ca₃Mg₂SiV₂O₁₂. Besides, the Q × f values of Ca₃Mg₂CV₂O₁₂ (C = Si, Ti) ceramics were correlated with relative density and Raman mode (A_1g).     

Structure,microwave dielectric performance,and infrared reflectivity spectrum of olivine-type Mg2Ge0.98O4 ceramic

An ultra-low dielectric loss ceramics Mg₂Ge_0.98O₄ with olivine structure was fabricated by conventional solid-state route. The phase composition, crystal structure, and microwave dielectric properties were investigated. The phase of Mg₂Ge_0.98O₄ is formed to the orthorhombic forsterite structure with a space group Pmnb (62). The dense microstructure and excellent microwave dielectric properties of Mg₂Ge_0.98O₄ ceramic were obtained at 1360°C for 4 hours, with relative density ~96.4%, ε_r ~ 7.3, Q × f = 112 400 GHz, and τ_f ~ −64.6 ppm/°C. The conductive mechanism of Mg₂Ge_0.98O₄ in the low frequency (<1 MHz) was studied by the dielectric spectroscopy and the result with E_dc = 0.93 eV demonstrates that the defect was contributed to the double ionized oxygen vacancies. The intrinsic dielectric properties of Mg₂Ge_0.98O₄ in the microwave region were obtained by infrared reflectivity spectra with ε_r ~ 7.13, Q × f = 120 400 GHz. And, acceptable τ_f (~+2.6 ppm/°C) of 0.92Mg₂Ge_0.98O₄–0.08CaTiO₃ composite ceramic was obtained by adding the CaTiO3.     

Influence of sintering temperature on dielectric properties and crystal structure of corundum-structured Mg4Ta2O9 ceramics at microwave frequencies

Microwave dielectric properties of corundum-structured Mg₄Ta₂O₉ ceramics were investigated as a function of sintering temperatures by an aqueous sol–gel process. Crystal structure and microstructure were examined by X-ray diffraction (XRD) technique and field emission scanning electron microscopy (FE-SEM). Sintering characteristics and microwave dielectric properties of Mg₄Ta₂O₉ ceramics were studied as a function of sintering temperature from 1250 °C to 1450 °C. With increasing sintering temperature, the density, ε_r and Qf values increased, saturating at 1300 °C with excellent microwave properties of ε_r=11.9, Qf=195,000 GHz and τ_f=?47 ppm/°C. Evaluation of dielectric properties of Mg₄Ta₂O₉ ceramics were also analyzed by means of first principle calculation method and ionic polarizability theory.     

Effects of ionic polarizability and crystal structure on microwave dielectric properties of RE2O3 (RE = La,Eu) ceramics with opposite τf and high Q

Hexagonal La₂O₃ and monoclinic Eu₂O₃ ceramics were prepared, and their microwave dielectric properties were investigated. La₂O₃ sintered at 1400 °C exhibited promising microwave dielectric properties of ε_r = 18.6, Q×f = 71,400 GHz, and a negative τ_f of − 35.1 ppm/°C, while Eu₂O₃ sintered at 1500 °C possessed relative lower ε_r and Q×f values of 17.9 and 35,000 GHz, respectively, with an abnormally positive τ_f of + 19.6 ppm/°C. The difference in their microwave dielectric properties is mainly due to lattice-induced strain, which can be characterized by bond valence. To investigate the degradation of RE₂O₃ (RE = La, Eu) ceramics in air, a series of La_2−xEu_xO₃ (x = 0.5, 1, and 1.5) ceramics were prepared. The results of the present study suggest that the introduction of Eu³⁺ effectively prevents the decomposition of La₂O₃.