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.     

Correlation between structural characteristics and microwave dielectric properties of walstromite BaCa2M3O9 (M = Si,Ge) ceramics

Novel BaCa₂M₃O₉ (M = Si, Ge) microwave dielectric ceramics were prepared via solid-state reaction with sintering at 1125°C–1275°C for 5 h. Single-phase BaCa₂M₃O₉ (M = Si, Ge) ceramics were obtained according to stoichiometry. The single-phase BaCa₂Ge₃O₉ ceramic was confirmed through Rietveld refinement and high-resolution transmission electron microscopy/selected area electron diffraction and synthesized for the first time. The BaCa₂M₃O₉ (M = Si, Ge) exhibited a triclinic structure with a P$\overline{1}$ space group and good microwave dielectric properties. The ε_r, Q × f, and τ_f values of BaCa₂M₃O₉ (M = Si, Ge) ceramics are mostly dominated by the relative density, ionic polarizability, relative covalence, and bond energy of M–O bond, respectively. A high Q × f value (61 800 GHz at 16.3 GHz) was obtained in BaCa₂Ge₃O₉ ceramic due to its high r_c (Ge–O) and low intrinsic dielectric loss. The BaCa₂Si₃O₉ ceramic exhibited small |τ_f| value (‒36.4 ppm/°C) due to its large E_Si-O. Excellent microwave dielectric properties (ε_r = 8.31, Q × f = 61 800 GHz, and τ_f = ‒58.7 ppm/°C) were obtained for the BaCa₂Ge₃O₉ ceramic.     

Lattice structure and microwave dielectric properties of La[Al1−x(Mg0.5Ti0.5)x]O3 (x = 0-0.2)-based ceramics

La[Al_1−x(Mg_0.5Ti_0.5)_x]O₃ (LAMT, x = 0-0.2) ceramics were synthesized by the conventional solid-state reaction method and formed a solid solution. The pure solid solutions were recorded by X-ray diffraction (XRD) in every range. Relative permittivity (ε_r) and structural stability were greatly affected because the Al³⁺ site was replaced by [Mg_0.5Ti_0.5]³⁺. The total ionic polarizability gradually increased with x, and ε_r gradually increased. The trend of τ_f is due to the change in structural stability. The variation in Q × f value increased firstly and then decreased due to the change in the symmetric stretching mode of Al/MgTi–O. The optimum microwave dielectric properties of LAMT were obtained at x of 0.1 after sintering at 1650°C for 5 hours, and ε_r = 24.9, Q × f = 79 956 GHz, and τ_f = −33 ppm/°C. The CaTiO₃ have a large positive τ_f (+800 ppm/°C), thus, the τ_f achieved near zero when CaTiO₃ and LAMT (x = 0.1) ceramics were mixed with a certain molar mass, and the optimum microwave dielectric properties of 0.65CaTiO₃–0.35LaAl_0.9(Mg_0.5Ti_0.5)_0.1O₃ were as follows: ε_r = 44.6, Q × f = 32 057 GHz, and τ_f = +2 ppm/°C.     

A low-permittivity microwave dielectric ceramic BaZnP2O7 and its performance modification

Complex pyrophosphates compounds have attracted much attention as promising candidates for substrate applications. In the work, a low-permittivity BaZnP₂O₇ ceramic was synthesized through solid-state reaction. The pure phase BaZnP₂O₇ was crystallized in the triclinic P−1 space group. Excellent microwave dielectric properties of the BaZnP₂O₇ ceramic with ε_r = 8.23, Qf = 56170 GHz, and τ_f = −28.7 ppm/°C were obtained at 870°C for 4 h. The substitution of Mg²⁺ for Zn²⁺ was found to have positive effects on grain morphology and dielectric properties. Optimized performance of ε_r = 8.21, Qf = 84760 GHz, and τ_f = −21.9 ppm/°C was yielded at 900°C for the BaZn₀.₉₈Mg_0.02P₂O₇ ceramic. Intrinsic dielectric properties of BaZn_1-xMg_xP₂O₇ ceramics were studied via Clausius–Mossotti equation and complex chemical bond theory.     

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.     

Ultra low loss (Mg1 − xCax)2SiO4 dielectric ceramics (x = 0 to 0.15) for millimeter wave applications

(Mg_{1 − x}Ca_x)₂SiO₄ dense ceramics (x ≥ 0.15) were prepared, and their microwave dielectric characteristics were investigated together with the structure evolution. The sintering temperature for Mg₂SiO₄ ceramics was reduced significantly with Ca²⁺substitution. (Mg_{1 − x}Ca_x)₂SiO₄ ceramics exhibited a small increase in dielectric constant (ε_r) correlated with increased crystallite size, and ultra-high quality factor Qf value was achieved throughout the compositional range. Temperature coefficient of resonant frequency (τ_f) was considerably tuned from −70 ppm/°C to −33 ppm/°C, and this improvement was deeply linked with the decreased bond valance. At x = 0.075, (Mg_{1 − x}Ca_x)₂SiO₄ ceramics exhibited the best combination of microwave dielectric characteristics: ε_r = 7.2, Qf = 199,800 GHz at 26 GHz, τ_f = −33 ppm/°C. The present ceramics could be expected as promising candidate of dielectric materials for millimeter wave applications.     

MSO4 (M = Ca,Sr, Ba) microwave dielectric ceramics with low dielectric constant

MSO₄ (M = Ca, Sr, Ba) ceramics with relative densities exceeding 96% were prepared by solid-state sintering at low sintering temperatures of 625–875°C, and their structures and microwave dielectric properties at 10–19 GHz were characterized. MSO₄ ceramics crystallized in orthorhombic symmetry with the space groups of Amma for CaSO₄ and Pnma for SrSO₄ and BaSO₄. The optimal microwave dielectric properties were obtained with ε_r = 5.85, Qf = 57 000 GHz, τ_f,W = −98.8 ppm/°C for CaSO₄, ε_r = 10.95, Qf = 15 500 GHz, τ_f,W = 101.6 ppm/°C for SrSO₄, and ε_r = 9.42, Qf = 38 200 GHz, τ_f,W = −4.7 ppm/°C for BaSO₄. The increased ε_r and τ_f,W while decreased Qf value in the order of CaSO₄, BaSO₄, and SrSO₄ were attributed to the enhanced rattling effect of M²⁺. Besides, the temperature dependence of τ_f was weak for CaSO₄ and BaSO₄, whereas much stronger for SrSO₄. As most low-ε_r microwave dielectric ceramics are of large negative τ_f, the near-zero τ_f of BaSO₄ and positive τ_f of SrSO₄ are rare, indicating they are potential candidates for millimeter-wave communication as a temperature-stable dielectric ceramic and a compensator for tuning negative τ_f to near-zero, respectively.     

Improvement of microwave dielectric properties of the LiNb0.6Ti0.5O3 M-phase by doping with (Co1/3Nb2/3)4+

Structural evolution and microwave dielectric properties of LiNb_0.6(Ti_1-x[Co_1/3Nb_2/3]_x)_0.5O₃ (.05≤x≤.2) ceramics have been studied in this paper. Although the doped compositions maintain the M-phase solid solutions, compositional fluctuation due to nonuniform dispersion of minor dopants could be observed as x < .05, and trace amount of Li₂TiO₃-based solid solution (Li₂TiO₃ss) secondary phase presents in the x > .05 compositions. The microwave dielectric properties could be remarkably improved by the doping of (Co_1/2Nb_1/2)⁴⁺ in comparison to the undoped counterpart. Optimized microwave dielectric properties with Q × f = ∼6500 GHz, ε_r = ∼74 and τ_f = +8.2 ppm/°C could be obtained at x = .10 after sintering at 1050°C/2 h. The sintering temperature could be further reduced to 900°C/2 h by adding .2 wt% B₂O₃ without affecting significantly its microwave dielectric properties: ε_r = 73, Q × f = 6000 GHz, τ_f = +8.5 ppm/°C. The LiNb_0.6(Ti_1-x[Co_1/3Nb_2/3]_x)_0.5O₃ ceramics obtained in this case exhibit large dielectric permittivity coupled with much improved Q × f values, near zero τ_f, and low sintering temperature simultaneously, which makes it a promising high-k microwave dielectric material for low temperature cofired ceramic applications.     

A Novel Low‐Firing and Low Loss Microwave Dielectric Ceramic Li2Mg2W2O9 with Corundum Structure

The crystal structure and microwave dielectric properties of a novel low‐firing compound Li₂Mg₂W₂O₉ were investigated in this study. The phase purity and crystal structure were investigated using X‐ray diffraction analyses and Rietveld refinement. The best microwave dielectric properties of the ceramic with a low permittivity (ε_r) ~11.5, a quality factor (Q × f) ~31 900 GHz (at 10.76 GHz) and a temperature coefficient of the resonant frequency (τ_f) ~ ?66.0 ppm/°C were obtained at the optimum sintering temperature (920°C). CaTiO₃ was added into the Li₂Mg₂W₂O₉ ceramic to obtain a near zero τ_f, and 0.93Li₂Mg₂W₂O₉–0.07CaTiO₃ ceramic exhibited improved microwave dielectric properties with a near‐zero τ_f ~ ?1.3 ppm/°C, a ε_r ~21.6, a high Q_u × f value ~20 657 GHz. The low sintering temperature and favorable microwave dielectric properties make it a promising candidate for LTCC applications.     

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.     

Microwave dielectric properties of Ca1.15RE0.85Al0.85Ti0.15O4 (RE=Nd,La, Y) ceramics prepared by the reaction sintering method

Ca_1.15RE_0.85Al_0.85Ti_0.15O₄ (RE = Nd, La, Y) ceramics were prepared by a reaction sintering method. The sintering behavior, phase composition, microstructure and microwave dielectric performances of ceramics were investigated. X-ray diffraction patterns illustrated that both the Ca_1.15Nd_0.85Al_0.85Ti_0.15O₄(CNAT) and Ca_1.15Y_0.85Al_0.85Ti_0.15O₄(CYAT) ceramics are single-phase structures, and the Ca_1.15La_0.85Al_0.85Ti_0.15O₄(CLAT) ceramic contain LaAlO₃ and CaO phases. The apparent morphology and elemental distribution of the ceramic samples were analyzed by using scanning electron microscope and energy dispersive spectrometer. When the sintering temperature is 1500 °C, the CNAT and CYAT ceramics have the best microwave dielectric properties with ε_r = 19.2, Q × f = 74924 GHz, τ_f = ?1.21 ppm/°C and ε_r = 17.5, Q × f = 27440 GHz, τ_f = ?5.79 ppm/°C, respectively. And the best microwave dielectric properties of ε_r = 17.5, Q × f = 22568 GHz, τ_f = ?14.69 ppm/°C were obtained for the CLAT ceramic sintered at 1525 °C. The reaction sintering method provides a low-cost, economical and straightforward method for the preparation of the Ca_1.15RE_0.85Al_0.85Ti_0.15O₄ (RE = Nd, La, Y) ceramics, which has promising potential for application.     

Activated treatment of magnesite for preparing size-controlled Mg2B2O5 whiskers and their microwave dielectric properties

In this study, magnesite was activated through hydration and balling treatment, and the received MgO with high activity and H₃BO₃ were used as raw materials for synthesizing Mg₂B₂O₅ whiskers with the aid of KCl salt. Attention was paid to the effects of B/Mg ratio and reaction temperature on the phase transformation and morphology evolution of the produced whiskers, and their growth mechanism was proposed as well. The Mg₂B₂O₅ ceramics were prepared based on the whiskers with different diameters via traditional pressing-sintering. Their physical properties, such as bulk density, apparent porosity and Vickers hardness (Hv), were studied. Particular emphasis was placed on the relationship between microstructure and microwave dielectric properties (ε, Q × f, and τ_f) of the Mg₂B₂O₅ ceramics. According to the results, the higher B/Mg ratio and the lower reaction temperature were more suitable for the growth of Mg₂B₂O₅ whiskers with high aspect ratio conforming to the liquid-solid (LS) mechanism. In particular, the Mg₂B₂O₅ ceramics based on the whiskers with average diameter (AD) = 198 nm and sintered at 1250 °C exhibited good microwave dielectric properties (ε = 6.18, Q × f = 18,597 GHz, and τ_f = −82 ppm/°C). This new design principle provides a guidance for fabricating high value-added magnesia-based ceramics from magnesite.     

Low-loss microwave dielectric of novel Li1-2xMxVO3 (M = Mg,Zn) (x = 0–0.09) ceramics for ULTCC applications

Novel Li_1?2xM_xVO₃ (M = Mg, Zn) (x = 0–0.09) microwave dielectrics suitable for ULTCC (ultra-low temperature co-fired ceramics) applications were synthesized. The X-ray diffraction patterns revealed that all samples were monoclinic structured with a space group of C2/c. Microstructures, lattice parameters, and Raman spectra of the ceramics were also studied. Q×f (Q: quality factor) was mainly controlled by intrinsic and extrinsic loss, while the variation of τ_f (temperature coefficient of resonant frequency) was related to ceramic bond valence. Poor microwave dielectric properties of pure LiVO₃ can be tremendously enhanced by substituting a minute amount of Mg or Zn in place of Li. Excellent properties could be obtained for specimens sintered at 520 °C with an ε_r of 9.78, a Q×f of 45,600 GHz and an τ_f of –45 ppm/°C for Li_0.98Mg_0.01VO₃, in addition to an ε_r (dielectric constant) of 9.25, a Q×f of 33,100 GHz and an τ_f of –53.6 ppm/°C for Li_0.98Zn_0.01VO₃. Furthermore, the Li_0.98Mg_0.01VO₃ specimen was found to be chemically comparable with the Al electrode. With 2 mol% of TiO₂ added, the specimen at 520 °C achieved excellent characteristics which include an ε_r of 9.2, a Q×f of 30,000 GHz, and an τ_f of –2.8 ppm/°C, making it a very promising ULTCC dielectric for high-frequency 5 G applications.     

A novel low-permittivity LiAl0.98(Zn0.5Si0.5)0.02O2-based microwave dielectric ceramics for LTCC application

Phase composition, morphology, and microwave dielectric properties of (1−x) LiAl_0.98(Zn_0.5Si_0.5)_0.02O₂ + x CaTiO₃ (0.05 ≤ x ≤ 0.20) materials synthesized via the solid state reaction method were investigated. All these densified materials were obtained at a sintering temperature of 1150°C. All compositions showed a major LiAlO₂ phase that was accompanied by a minor CaTiO₃ phase. The ε_r value increased gradually from 10.88 to 11.60, whereas the Q × f value remarkably decreased from 33 251 GHz to 13 511 GHz. The τ_f value changes from −85 ppm/°C to 212 ppm/°C, thereby indicating that CaTiO₃ could effectively adjust this value. HBO₃-doping was used to further decrease the sintering temperature to 900°C. The optimum value was obtained at 7 wt.% HBO₃ doped with microwave dielectric properties of ε_r = 9.39, Q × f = 10 224 GHz, and τ_f = −7.8 ppm/°C. This material also exhibited chemical compatibility with silver, making it a candidate for low temperature co-fired ceramics applications.     

Mechanism study of the Mn-substituted magnesium borate: Decreased sintering temperature and improved dielectric property

The influence of manganese substitution on the sintering and dielectric properties of Mg₃B₂O₆ (MBO) was studied, and the detailed mechanism of variation was discussed using density functional theory calculations. The characterization method involved a network analyser, differential–thermal analyses, thermomechanical analyses, X-ray diffraction, and scanning electron microscopy. Manganese substitution formed a two-phase system (MBO and Mg₂B₂O₅), improved the dielectric property, densified the microstructure, lowered the activation energy, decreased the crystallite size, modified the band structure property, and strengthened the covalency of MgO₆ octahedron. The 7% mole substitution of manganese to magnesium improved the dielectric properties of MBO to 7.06 for ε_r and 61 100 GHz at 15 GHz for Q × f, −54.8 ppm/°C for τ_f. The intrinsic densification temperature decreased from 1350 to 1175°C with 97.7% relative density.     

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.     

The relationship between crystal structure and modified microwave dielectric properties of Ca3SnSi2-xGexO9 ceramics

Ca₃SnSi_2-xGe_xO₉ (0 ≤ x ≤ 0.8) and (1–y) Ca₃SnSi_1.6Ge_0.4O₉ – y CaSnSiO₅ – 2 wt% LiF (y = 0.4 and 0.5) microwave dielectric ceramics were prepared by traditional solid-state reaction through sintering at 1250°C–1425°C for 5 h and at 875°C for 2 h, respectively. Ge⁴⁺ replaced Si⁴⁺, and Ca₃SnSi_2-xGe_xO₉ (0 ≤ x ≤ 0.4) solid solutions were obtained. At 0.1 ≤ x ≤ 0.4, the Ge⁴⁺ substitution for Si⁴⁺ decreased the sintering temperature of Ca₃SnSi_2-xGe_xO₉ from 1425 to 1300°C, the SnO₆ octahedral distortions, and the average CaO₇ decahedral distortions, which affected the τ_f value. The large average decahedral distortions corresponded with nearer-zero τ_f values at Ca₃SnSi_2-xGe_xO₉ (0.1 ≤ x ≤ 0.4) ceramics. The τ_f value and sintering temperature of Ca₃SnSi_2-xGe_xO₉ (x = 0.4) ceramic were adjusted to near-zero by CaSnSiO₅ and decreased to 875°C upon the addition of 2 wt% LiF. The (1 – y) Ca₃SnSi_1.6Ge_0.4O₉ – y CaSnSiO₅ – 2 wt% LiF (y = 0.5) ceramic sintered at 875°C for 2 h exhibited good microwave dielectric properties: ε_r = 10.3, Q × f = 14 300 GHz (at 12.2 GHz), and τ_f = ‒5.8 ppm/°C.     

Crystal structure,far-infrared spectra,and microwave dielectric properties of bazirite-type BaZr(Si1-xGex)3O9 ceramics

Novel BaZr(Si_1-xGe_x)₃O₉ (0 ≤ x ≤ 1.0) microwave dielectric ceramics were prepared by solid-state reaction sintering at 1200–1450 °C for 5 h Ge⁴⁺ ions occupied the Si⁴⁺ positions, and BaZr(Si_1-xGe_x)₃O₉ solid solutions were obtained. The BaZr(Si_1-xGe_x)₃O₉ (0 ≤ x ≤ 1.0) ceramics exhibited hexagonal structures with P-6c2 space groups and octahedral layers and [Si/Ge₃O₉]^6- rings. Owing to these structural characteristics, the ceramics exhibited low permittivity. With an increase in x, the relative permittivity (ε_r) values of the BaZr(Si_1-xGe_x)₃O₉ (0 ≤ x ≤ 1.0) ceramics increased from 7.68 (x = 0) to 9.45 (x = 1.0), while their quality factor (Q × f) values first increased and then decreased. The Q × f value (10,300 GHz at 13.43 GHz) of the BaZrSi₃O₉ (x = 0) ceramic improved with the substitution of Si⁴⁺ by Ge⁴⁺. A high Q × f value (36,100 GHz at 13.81 GHz) was obtained for the BaZr(Si_1-xGe_x)₃O₉ (x = 0.2) ceramic, and the Q × f values of the BaZr(Si_1-xGe_x)₃O₉ ceramics could be controlled by varying the Si/Ge-site bond valence. The temperature coefficient of resonance frequency (τ_f) values of the BaZr(Si_1-xGe_x)₃O₉ ceramics were mainly affected by the O2-site bond valence, and the optimum τ_f value (?22.8 ppm/°C) was achieved for the BaZrSi₃O₉ ceramic. The BaZr(Si_1-xGe_x)₃O₉ (x = 0.2) ceramic showed the optimum microwave dielectric properties (ε_r = 8.36, Q × f = 36,100 GHz at 13.81 GHz, and τ_f = ?34.5 ppm/°C).     

Phase evolution,crystal structure,and microwave dielectric properties of gillespite-type ceramics

A gillespite-structured MCuSi₄O₁₀ (M = Ba_1-xSr_x, Sr_1-xCa_x) ceramics with tetrahedral structure (P4/ncc) were prepared by solid-state reaction method. X-ray diffraction and thermogravimetry with differential scanning calorimetry (TG-DSC) were employed to study the phase synthesis process of BaCuSi₄O₁₀. Pure BaCuSi₄O₁₀ phase was obtained at 1075°C and decomposed into BaSiO₃, BaCuSi₂O₆, and SiO₂ when calcined at 1200°C. The relationships between the crystal structure and microwave dielectric properties of MCuSi₄O₁₀ ceramics were revealed based on the Rietveld refinement and P-V-L complex chemical bond theory. The dielectric constant (ε_r) decreased linearly with decreasing total bond susceptibility and ionic polarizability. Quality factor (Q × f) was closely dependent on bond strength and lattice energy. The temperature coefficient of resonant frequency (τ_f) was controlled by the stability of [CuO₄]⁶⁻ plane in MCuSi₄O₁₀. Optimum microwave dielectric properties were obtained for SrCuSi₄O₁₀ when sintered at 1100°C for 3 hours with a ε_r of 5.59, a Q × f value of 82 252 GHz, and a τ_f of −41.34 ppm/°C. Thus, SrCuSi₄O₁₀ is a good candidate for millimeter-wave devices.     

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.