Contrasting microwave dielectric properties of zircon-structured AEuV2O8 (A = Bi,La) ceramics

Two zircon-structured ceramics AEuV₂O₈ (A = Bi, La) were prepared with optimal sintering temperatures of 900 °C and 1375 °C, respectively. They exhibited sharp contrast performances with medium ε_r ～ 28.7 ± 0.1, Q × f ～ 14,000 ± 300 GHz, and large positive τ_f ～ 75.7 ± 2.0 ppm/°C for BiEuV₂O₈, whereas low ε_r ～ 10.4 ± 0.1 Q × f ～ 25,100 ± 300 GHz, and negative τ_f ～ ? 40.7 ± 2.0 ppm/°C for LaEuV₂O₈. The rattling effect at the A-site was more dominant in determining microwave dielectric properties than that of compressed V⁵⁺ at the B-site. It resulted in the higher ε_r, lower Q × f and τ_ε, and larger τ_f of BiEuV₂O₈ than those of LaEuV₂O₈. Besides, their Q × f was related with the relative density, bond valence and FWHM of the B_1 g Raman mode.     

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.     

Densification,microwave dielectric properties and rattling effect of LiYbO2 ceramics with low εr and anomalous positive τf

LiYbO₂ ceramic with tetragonal structure formed only by [LiO₆] and [YbO₆] octahedra, exhibited a ε_r of 13.3 ± 0.2, Q × f of 24,700 ± 500 GHz and an abnormal positive τ_f of + 38.6 ± 3.0 ppm/°C as sintered at 1100 °C with low relative density of 88 %. Dense ceramic with relative density of 95.1 % was obtained with the addition of 1 wt% LiF when sintered at 1080 °C, exhibiting enhanced microwave dielectric properties of ε_r = 14.3 ± 0.2, Q × f = 41,200 ± 500 GHz, τ_f = +56.5 ± 3.0 ppm/°C and α_L = 8.5 × 10^?6/K. The large positive discrepancy as much as + 45.1 % between measured ε_r with porosity correction and calculated ε_r(C-M) using Clausius?Mossotti relations might be due to the rattling effect of Li⁺, which also led to the positive τ_f.     

Structure,far-infrared reflectance spectra,and microwave dielectric properties of Ba2MGa11O20 (M = Bi,La) ceramics

Ba₂MGa₁₁O₂₀ (M = Bi, La; called BBG and BLG, respectively) ceramics with monoclinic space group I2/m were prepared through a solid-state reaction method. BBG ceramic sintered at 1150 °C for 6 h has the best microwave dielectric properties with low ε_r = 10.68, Q × f = 41,756 GHz, and negative τ_f = ?61.3 ppm/°C. BLG ceramic sintered at 1440 °C for 6 h exhibits ε_r = 13.94, Q × f = 45,592 GHz, and near-zero τ_f = ?16.3 ppm/°C. The large deviation between ε_r and ε_th was ascribed to the “rattling” effect of the cations and the existence of lone pair ions of Bi³⁺. The difference in Q × f of the two ceramics was discussed in terms of packing fraction, and the τ_f of BLG was closer to zero than that of BBG due to the smaller τ_ε value. Their intrinsic dielectric properties were analyzed through far-infrared reflectivity spectroscopy.     

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.     

Correlation between crystal structure and microwave dielectric properties of two garnet-type ceramics in rare-earth-free gallates

Two garnet-type rare-earth-free ceramics, Ca₃M₂SiGa₂O₁₂ (M = Sn and Zr), were prepared through a solid-state reaction method. The relationship between crystal structure and microwave dielectric properties was investigated. The larger deviation of ε_r from ε_theo in Ca₃Zr₂SiGa₂O₁₂ could be ascribed to the rattling Zr⁴⁺. The increase in packing fraction and the decrease in FWHM enhance the Q × f value by substituting Zr⁴⁺ with Sn⁴⁺. The smaller oxygen bond valence in Ca₃Zr₂SiGa₂O₁₂ indicates a smaller τ_f value. Good microwave dielectric properties are obtained with ε_r = 9.14 ± 0.02, Q × f = 106,800 ± 1700 GHz and τ_f = -45.8 ± 1.8 ppm/°C for Ca₃Sn₂SiGa₂O₁₂ and ε_r = 11.98 ± 0.03, Q × f = 84,200 ± 1500 GHz, and τ_f = -32.8 ± 1.4 ppm/°C for Ca₃Zr₂SiGa₂O₁₂. Furthermore, near-zero τ_f values of +5.7 ± 1.9 ppm/°C and +4.5 ± 1.6 ppm/°C appear in 0.95Ca₃Sn₂SiGa₂O₁₂-0.05CaTiO₃ and 0.96Ca₃Zr₂SiGa₂O₁₂-0.04CaTiO₃, respectively.     

Effects of ion polarizability and oxygen vacancy on microwave dielectric properties of fluorite-structured Ce1−xCaxO2−x

X-ray diffractometer with Rietveld refinement and Raman spectroscopy were used for phase analysis. Ca doping results in decreased ε_r (24.09–21.52), Q × f (68 914–40 110 GHz), and τ_f (−50.0 ppm/°C to −60.2 ppm/°C) all decrease, which obviously does not conform to the mutual constraint relationship among the three parameters of microwave dielectric properties. The ε_r of Ce_1−xCa_xO_2−x ceramics is affected by the ion polarizability and the Ce rattling and valence states, among which the rattling of Ce cations plays a dominant role. The presence of oxygen vacancies in Ce_1−xCa_xO_2−x ceramics explains the decrease in Q × f. The τ_ε, ε_r, and α_L are responsible for the decrease in τ_f, especially for the τ_αm-3α_L values.     

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.     

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.     

Effects of LiF addition on the densification and microwave dielectric properties of LiInO2 ceramics

Low-temperature co-fired ceramics (LTCC) LiInO₂ + xwt% LiF (x = 0, 1, 2, 3, 4, 5) was synthesized by a traditional solid-state reaction method. XRD, TEM, and SEM show that all specimens form a pure-phase tetragonal structure with a space group of I4₁/amd. The addition of LiF can effectively optimize the microstructure and improve the relative density of LiInO₂ ceramics. As x value increased, the ε_r increased from 9.6 to 13.6, Q×f increased from 39,600 GHz to 52,500 GHz, and all the specimens exhibit a positive τ_f value at the range of 9.6–19.1 ppm/°C. The low-ε_r and positive τ_f in LiInO₂ ceramics was investigated by bond valence and P–V-L chemical bond theory, indicating that it was closely related to the rattling effect of In and Li. The concentration of oxygen vacancies was studied by dielectric spectroscopy, indicating that doping LiF can compensate for the volatilization of Li during high temperature sintering. Notably, excellent microwave dielectric properties (ε_r ~13.6, Q×f = 52,500 GHz, and τ_f ~ 18.1 ppm/°C) were achieved in the LiInO₂ + 3 wt% LiF sintered at 890 °C.     

Effect of Li nonstoichiometry and TiO2 addition on the microwave dielectric properties of Li3PO4 ceramics

Li₃PO₄ ceramic is a promising microwave ceramic material with low dielectric constant. The effect of Li nonstoichiometry on phase compositions, microstructures, and microwave dielectric characteristics of Li₃PO₄ ceramics, on the other hand, has been examined infrequently. Therefore, in the first part of this study, the stoichiometry and Li nonstoichiometry compositions based on Li_3+xPO₄(x = 0, 0.03, 0.06, 0.09, 0.12 and 0.15) were prepared by conventional solid-phase method. The results show that a few nonstoichiometric lithium ions enter the lattice of Li_3+xPO₄. Compared with the chemical content of Li₃PO₄, the sintering characteristics, relative dielectric constants and quality factors of Li_3+xPO₄ ceramics can be improved by slightly excessive Li ions, while the properties of Li₃PO₄ ceramics can be deteriorated by excessive Li ions. Li_3.12PO₄ ceramics sintered at 975 °C for 2 h have good dielectric properties (ε_r = 5.89, Q×f = 44,000 GHz, τ_f = ?206 ppm/^°C). In order to improve its large negative temperature coefficient of resonant frequency, in the following study, rutile nano TiO₂ particles were added as τ_f compensator. Adding TiO₂ powders not only effectively improve the temperature stabilities of the multiphase ceramics, but also make the grain growth more uniform. With the increase of TiO₂ content from 0.40 to 0.60, τ_f increases from ?73.5 ppm/^°C to +42.3 ppm/^°C. The best dielectric property of 0.45Li_3.12PO₄-0.55TiO₂ composite ceramic is ε_r = 13.29, Q×f = 40,700 GHz, τ_f = +8.8 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 and chemical bond characteristics of two low-εr microwave dielectric ceramics LiBO2 (B = Ga,In) with opposite τf

Two novel microwave dielectric materials LiBO₂ (B = Ga, In) were synthesized by a solid-state reaction method. The orthorhombic structured LiGaO₂ sintered at 1075 °C for 4 h exhibits a relative density of 96.1 ± 0.2 % and dielectric properties, namely, ε_r ～ 5.82 ± 0.01 (16.7 ± 0.1 GHz), Q × f = 24,500 ± 2000 GHz, and τ_f ～ -74.3 ± 2 ppm/°C. Meanwhile, the tetragonal structured LiInO₂ sintered at 920 °C for 4 h exhibits a relative density of 85.3 ± 0.2 % and dielectric properties, namely, ε_r ～ 9.6 ± 0.01 (15.2 ± 0.1 GHz), Q × f = 39,600 ± 2000 GHz, and τ_f ～ 9.8 ± 2 ppm/°C. The dielectric properties were investigated by chemical bond theory, and results indicate that the τ_f value is closely related to the bond ionicity. The good chemical compatibility of LiInO₂ ceramics with silver electrodes makes it a potential material for LTCC technology.     

Crystal structure,Raman spectra and microwave dielectric properties of novel temperature-stable LiYbSiO4 ceramics

Olivine−type structure microwave dielectric ceramics LiYbSiO₄ with near-zero τ_f were fabricated by solid-state reaction process for the first time. The relationships among structural parameters, sintering behavior, vibrational modes and microwave dielectric properties for the ceramics were studied. The variation in ε_r could be correlated with Raman shift. The variation in Q×f values was inversely correlated to FWMH and average cation covalency. The τ_f values were explained with the bond valence sum of cations. Single phase LiYbSiO₄ ceramics could be obtained at the range of 1100–1140 °C and showed promising microwave dielectric properties with ε_r = 7.36 – 7.42, Q×f = 19081 – 25276 GHz and τ_f = +4.52 – +8.03 ppm/°C.     

Microwave dielectric properties of silico-carnotite Ca3M2Si3O12 (M= Yb,Y) ceramics synthesized via high energy ball milling

The Ca₃M₂Si₃O₁₂ (M = Yb, Y) ceramics with orthorhombic silico-carnotite structure were fabricated via high-energy ball milling and solid-state reaction route. Dense Ca₃Yb₂Si₃O₁₂ and Ca₃Y₂Si₃O₁₂ ceramics sintered at 1260 °C and 1240 °C revealed promising microwave dielectric properties with ε_r = 9.2 and 8.7, Q×f = 56,400 GHz and 29,094 GHz, τ_f = −77.5 ppm/°C and −76.8 ppm/°C, respectively. The connection between crystal structure and Q×f values of Ca₃M₂Si₃O₁₂ (M = Yb, Y) ceramics was discussed with respect to the packing fraction, and their intrinsic microwave dielectric properties were examined using the infrared reflectivity spectra analysis. The thermal stability of Ca₃Yb₂Si₃O₁₂ was improved successfully by forming 0.91Ca₃Yb₂Si₃O₁₂‐0.09CaTiO₃ composite ceramics with τ_f = +2.9 ppm/°C, ε_r = 12.93 and Q×f = 26,729 GHz.     

Crystal structures,bond characteristics,and dielectric properties of novel middle-εr Ln3NbO7 (Ln = Nd,Sm) microwave dielectric ceramics with opposite temperature coefficients

This study synthesized two novel middle-ε_r Ln₃NbO₇ (Ln = Nd, Sm; named NNO and SNO) microwave dielectric ceramics through the classic solid-state process. The results of XRD and Rietveld refinement show that NNO and SNO ceramics formed pure phases with the space group Cmcm (63) and C222₁ (20), respectively. The properties of Ln-O and Nb–O bonds of NNO and SNO ceramics were calculated based on the P–V–L theory. The Nb–O bonds positively affect the crystal structure stability of the two ceramics. The optimum microwave dielectric properties were obtained (NNO: ε_r = 31.61, Q·f = 6,615 GHz (at 6.10 GHz), and τ_f = ?455.70 ppm/°C; SNO: ε_r = 34.55, Q·f = 11,625 GHz (at 5.77 GHz) and τ_f = 72.59 ppm/°C) when the samples sintered at 1550 °C. Notably, SNO ceramic shows a low dielectric loss and medium dielectric constant, and the opposite τ_f of NNO and SNO ceramics provide the possibility to fabricate microwave dielectric devices with good temperature stability.     

Crystal structural and microwave dielectric properties of Ba4B'Nb3O12 (B′=Yb,Tm, Er,Y, Ho,Dy, Gd) ceramics

The influence of substitution of rare-earth ion (RE = Yb, Tm, Er, Y, Ho, Dy, Gd) for B′-site on phase composition, crystal structure, micromorphology, and microwave dielectric properties of Ba₄RENb₃O₁₂ ceramics are investigated. The results of XRD and Rietveld refinement indicate that the Ba₄RENb₃O₁₂ ceramic is composed of Ba(RE_1/12Nb_9/12)O₃ and Ba (RE_1/2Nb_1/2)O₃ phases. Porosity analysis shows that the relative density of Ba₄RENb₃O₁₂ ceramics can reach more than 95.25%. The variations of permittivity and temperature coefficient are associated with ionic polarization and cell polarization, respectively. The A_1g Raman‐active modes at 754 cm^?1 are oxygen-octahedron stretch vibration, reflecting the variation of the structure. Optimal microwave dielectric properties (ε_r = 38.75, Q × f = 41251 GHz, τ_f = 71.57 ppm/°C) of Ba₄RENb₃O₁₂ ceramics for RE = Yb are obtained at 1425 °C for 6 h.     

A3Y2Ge3O12 (A = Ca,Mg): Two novel microwave dielectric ceramics with contrasting τf and Q × f

Two garnet-structured microwave dielectric ceramics of A₃Y₂Ge₃O₁₂ (A = Ca, Mg; called CYG and MYG, respectively) were synthesized. CYG was crystallized in a normal cubic garnet structure with Ca²⁺ fully occupying the dodecahedral (A) site, whereas MYG was an inverse garnet with mixed distribution of Mg²⁺/Y³⁺ at the A site. The difference in ionic occupation resulted in anomalies in microwave dielectric properties, with dielectric constant (ε_r) = 14.1, quality factor (Q × f) = 12,600 GHz and positive temperature coefficient of resonant frequency (τ_f) =120.5 ppm/°C for MYG ceramic and ε_r = 10.8, Q × f = 97,126 GHz and τ_f = −40.6 ppm/°C for CYG ceramic. The large deviations in measured ε_r from theoretical ε_th possibly resulted from the garnet structural constraints leading to ‘rattling’ Mg²⁺ and Y³⁺ in A site of MYG. Infrared reflectivity spectra analysis revealed ion polarization contributed mostly to the permittivity of both ceramics in microwave frequency ranges.     

Relationship between Rattling Mg2+ ions and anomalous microwave dielectric behavior in Ca3-xMg1+xLiV3O12 ceramics with garnet structure

Ca_3-xMg_1+xLiV₃O₁₂ (0 ≤ x ≤ 1) ceramics with cubic-garnet-structure were synthesized by the solid-phase reaction. The replacement of Ca²⁺ with the smaller Mg²⁺ exerted an increasingly strong “rattling” effect at the A-site of Ca_3-xMg_1+xLiV₃O₁₂. With increased Mg²⁺, ε_r increased from 10.5 ± 0.1 to 15.4 ± 0.1 and ε_corr increased from 11.0 ± 0.1 to 16.0 ± 0.1, whereas the theoretical ε_th decreased from 12.0 ± 0.1 to 10.5 ± 0.1. ε_corr was higher than ε_th due to the “rattling” effect. The enhanced “rattling” effect caused τ_f to increase rapidly from –64.1 ± 1.0 ppm/°C to +267.2 ± 1.0 ppm/°C. With increased x, Q × f decreased from 74,700 ± 500 GHz to 15,370 ± 500 GHz, due to the decreased packing fraction, increased FWHM of the A_1g modes, and enhanced “rattling” effect. Additionally, the chemical compatibility between Ca_2.75Mg_1.25LiV₃O₁₂ and Ag electrodes was confirmed, indicating this material’s potential for LTCC.     

Effect of CuO and TiO2 on the sintering temperature and dielectric properties of BaWO4 for LTCC applications

A series of (1-x) BaWO₄-xCuO low-temperature sintered composite ceramics were prepared via co-firing the mixture of BaWO₄ and CuO in the first part of this study. The sintering temperature of BaWO₄ was decreased from 1150 °C to 950 °C when a little amount of CuO was used as sintering aids. The 0.95BaWO₄-0.05CuO sample heated at 950 °C for 4 h had good dielectric performance (Q × f ~ 31,847 GHz, ε_r ~ 7.99, τ_f ~ −81.7 ppm/°C). However, the negative τ_f was too large to apply to practice. To solve this problem, the rutile phase TiO₂ nano-particle with a large positive temperature coefficient of +450 ppm/°C was added as τ_f compensator in the second part of the study. TiO₂ powders not merely improved the temperature stability, but promoted the grain growth. However, the ε_r value increased from 10.5 to 12.6 and Q×f value decreased slightly as TiO₂ content increased from 0.30 to 0.45. The 0.65 (0.95BaWO₄-0.05CuO) −0.35TiO₂ composite ceramic displayed an optimum performance (Q × f ~ 22,012 GHz, ε_r ~ 11.21, τ_f ~ −2.9 ppm/°C). Such a sample was chemically compatible with silver, implying that it can be implemented on LTCC applications.