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.     

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.     

Novel low-εr and lightweight LiBO2 microwave dielectric ceramics with good chemical compatibility with silver

High-speed signal propagation systems require dielectric ceramics with low relative permittivity (ε_r) and a high-quality factor (Qxf). In this paper, a novel low-permittivity borate ceramic (LiBO₂) was synthesized using a conventional solid-state reaction method. Based on the X-ray diffraction and Rietveld refinement, the LiBO₂ crystallized into a monoclinic structure with a space group of P21/c. Dense and single-phase ceramic was obtained at 640 °C with comprehensive microwave dielectric properties: a low relative permittivity (ε_r) of 5.3, a moderate quality factor (Q×f) of 18,200 GHz at 16.3 GHz, and a temperature coefficient of resonant frequency (τ_f) of ? 66.2 ppm/°C. Good chemical compatibility with Ag electrode and thermal expansion coefficient of 25.4 ppm/°C was achieved demonstrating the potential applications as dielectric resonances in wireless communications and substrates in low-temperature cofired ceramics.     

High-performance 0.9Al2O3-0.1TiO2 microwave dielectric ceramics prepared by digital light processing

In this work, the 0.9Al₂O₃-0.1TiO₂ ceramic sample with good microwave dielectric properties and complex structures can be well fabricated by digital light processing (DLP). A relationship between dispersant content and rheological behavior of 0.9Al₂O₃-0.1TiO₂ slurry was explored. When dispersant content was 3.0 wt%, 0.9Al₂O₃-0.1TiO₂ slurry with high solid loading (50 vol%) and low viscosity (2.9 Pa s) could be obtained. 0.9Al₂O₃-0.1TiO₂ ceramic parts with high accuracy were fabricated successfully by adding 3.0 wt% photoinitiator under 600 mJ/cm² exposure energy. With the increase of sintering temperature from 1400 °C to 1600 °C, relative density, dielectric constant (ε_r), and quality factor (Q × f) of 0.9Al₂O₃-0.1TiO₂ ceramic sample increased first and then decreased, and all reached the maximum value at 1550 °C due to the uniformity and densification of microstructures. The temperature coefficient of resonant frequency (τ_f) value showed an almost monotonous increase, changing from negative to positive, and near-zero τ_f value at 1550 °C. In addition, 0.9Al₂O₃-0.1TiO₂ ceramic samples sintered at 1550 °C fabricated by DLP method presented much better microwave dielectric properties: ε_r = 11.30 ± 0.02, Q × f = 35,345 ± 143 GHz (@～12 GHz), τ_f = 2.16 ± 0.21 ppm/°C than that of by dry pressing method: ε_r = 11.16 ± 0.11, Q × f = 30,195 ± 257 GHz (@～12 GHz), τ_f = 4.45 ± 0.13 ppm/°C, especially the Q × f value achieved a 17% increase. Accordingly, DLP technique, which has advantages of producing relatively high properties and complex geometry of microwave dielectric ceramics as well as without extra high-cost mold, greatly satisfies application requirements.     

Ultra-low temperature sintering and temperature stable microwave dielectrics of phase pure AgMgVO4 ceramics

The present study systematically invesitgates the formation and microwave dielectric properties of novel AgMgVO₄ ceramics fabricated via the solid-state reaction method. The crystal structure of the ceramics is confirmed to be orthorhombic with a space Group of Pnma (62). A high relative density of 96.2% and an excellent combination of microwave dielectric properties with ε_r of ~14.89, Q×f of ~19,400 GHz, and τ_f of ~ ? 2.71 ppm/°C can be achieved for the ceramic sintered at 630 °C. The dielectric constant is mainly influenced by the relative density (porosity) and dielectric polarizability. The Q×f is controlled by the microstructure, packing fraction, and lattice energy, which are also highly related to the unit-cell volume. A smaller unit-cell volume leads to a high Q×f. Variation of the τ_f is strongly correlated to the bond valence of the specimen. Furthermore, the ceramic exhibits good chemical compatibility with aluminum electrodes and is demonstrated as a potent substrate for a band-pass filter with a center frequency of 3.5 GHz. These findings show a great promise for ultra-low temperature co-fired ceramic applications at high frequencies.     

Two novel low permittivity microwave dielectric ceramics Li2TiMO5 (M = Ge,Si) with abnormally positive τf

Two tetragonal natisite structured Li₂TiMO₅ (M = Ge, Si) ceramics fabricated using the conventional solid-state reaction method were investigated in terms of the thermal stability, sintering behavior and dielectric properties at radio (RF) and microwave frequency region. At the optimum sintering temperature of 1140 °C, Li₂TiGeO₅ (LTG) has ε_r ˜ 9.43, Q × f ˜ 65,300 GHz (at 14.7 GHz), and τ_f ˜ +24.1 ppm/°C, while Li₂TiSiO₅ (LTS) sintered at 1180 °C exhibits ε_r ˜ 9.89, Q × f ˜ 38,100 GHz (at 14.2 GHz), and τ_f ˜ +50.1 ppm/°C. The positive τ_f values of the present LTG and LTS are abnormal and extremely important for low-ε_r microwave dielectric ceramics, which could behave as a promising τ_f compensator. Moreover, the dielectric spectra of both ceramics revealed a phase transition at low-temperature, exhibiting a dielectric peak, which could account for the negative τ_ε and positive τ_f in operating temperature ranges.     

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.     

New low-ε r,temperature stable Mg3B2O6-Ba3(VO4)2 microwave composite ceramic for 5G application

Novel low-ε_r, thermal and phase stable of (1-x)Mg₃B₂O₆-xBa₃(VO₄)₂ (x mol% =51, 53, 55, 57, 59) microwave composite ceramics were firstly fabricated and reported using the conventional solid-state reaction method. X-ray diffraction, scanning electron microscopy, energy dispersive X-ray, and Raman spectroscopy confirmed the coexistence of both phases without other phases. Near-zero temperature coefficient of resonant frequency (τ_f ~ +1.2 ppm/°C) is obtained for the 0.43Mg₃B₂O₆-0.57Ba₃(VO₄)₂ composite ceramic, with permittivity (ε_r) of 8.8 and high-quality factor (Q×f) of 45 420 GHz, which is a promising candidate for 5G applications.     

Phase analysis and microwave dielectric properties of LTCC TiO2 with glass system

Glass–ceramic composites containing TiO₂ (anatase, rutile) and modified borosilicate glasses were prepared and their sintering behaviour, phase evolution, interface reactions, and microwave dielectric properties were investigated as new candidates for low-temperature cofired ceramic (LTCC) materials. It was found that the addition of small amounts of borosilicate glasses lowered the sintering temperature of TiO₂ from 1400 to 900 °C. X-ray diffraction results showed that second phases, including Zn₂SiO₄, were formed when TiO₂+zinc-borosilicate glass was used, while no crystalline phase except rutile could be found using unmodified borosilicate glass. High-density TiO₂+zinc borosilicate glass material showed promising microwave dielectric properties: relative dielectric constant (ε_r)=74, quality factor (Q×f)=8000 GHz, and temperature coefficient of resonant frequency (τ_f)=340 ppm/°C. The effect of borosilicate glasses on the anatase–rutile phase transition was also investigated.     

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.     

Chemical bond and microwave dielectric properties of two novel low-εr AGa4O7 (A = Ca,Sr) ceramics

Two novel low-ε_r microwave dielectric ceramics AGa₄O₇ (A = Ca, Sr; called CGO and SGO, respectively) were analyzed by XRD, SEM, and Raman spectroscopy, showed the monoclinic structure with a C2/c space group. Encouraging microwave dielectric properties (CGO: ε_r = 9.1, Q × f = 40 600 GHz (f = 12.4 GHz), and τ_f = ?86.3 ppm °C^?1; SGO: ε_r = 9.2, Q × f = 62 400 GHz (f = 14.6 GHz), and τ_f = ?63.4 ppm °C^?1) are obtained. The PVL chemical bond theory indicated that GaO bonds play a role in affecting the ε_r and Q × f values. The τ_f of AGa₄O₇ (A = Ca, Sr) had adjusted close to zero (0.93CGO-0.07CaTiO₃:ε_r = 11.3, Q × f = 31,043 GHz, and τ_f = 3.97 ppm °C^?1; 0.82SGO-0.18LiCa₂Mg₂V₃O_12：ε_r = 11.8, Q × f = 55,564 GHz, and τ_f = 5.62 ppm °C^?1).     

A novel high-Q oxyfluoride Li4Mg2NbO6F microwave dielectric ceramic with low sintering temperature

Novel low-fired Li₄Mg₂NbO₆F ceramics were synthesised using a conventional solid-state reaction method. X-ray diffraction and Rietveld refinement confirmed that the Li₄Mg₂NbO₆F compound had a face-centred-cubic rock salt structure [Fm-3 m(225)] above 625 °C. Li₄Mg₂NbO₆F ceramics sintered between 875 °C and 950 °C displayed the optimised density (> 97.5 %). The theoretical ε_theo was calculated based on the refined crystal parameters, closing to the measured ε_r. The ceramic sintered at 900 °C exhibited excellent microwave dielectric properties with ε_r of 15.53 ± 0.03, Q × f value of 93,300 ± 1100 GHz (at 7.7 GHz) and τ_f value of ?39.8 ± 0.8 ppm/°C. The compatibility with Ag powders makes the oxyfluoride a potential candidate for LTCC applications.     

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.     

Structural,infrared reflectivity spectra and microwave dielectric properties of the Li7Ti3O9F ceramic

A cubic rock salt structured ceramic, Li₇Ti₃O₉F, was fabricated via the conventional solid-state reaction route. The synthesis conditions, sintering characteristics, and microwave dielectric properties of Li₇Ti₃O₉F ceramics were investigated by X-ray diffraction (XRD), thermal dilatometer, Scanning Electron Microscopy (SEM) accompanied with EDS mapping, and microwave resonant measurements. Rietveld refinement, selected area electron diffraction (SAED) pattern and high-resolution transmission electron microscopy (HRTEM) confirmed that Li₇Ti₃O₉F adopts a cubic rock-salt structure. The ceramic sintered at 950?°C presented the optimal microwave properties of ε_r?=?22.5, Q×f?=?88,200?GHz, and τ_f?=??24.2?ppm/^oC. Moreover, good chemical compatibility with Ag was verified through cofiring at 950?°C for 2?h. These results confirm a large potential for Li₇Ti₃O₉F ceramic to be utilized as substrates in the low temperature cofired ceramic (LTCC) technology. This work provides the possibility to exploit low-temperature-firing ceramics through solid solution between oxides and fluorides.     

Structure,microwave dielectric properties,and infrared reflectivity spectrum of olivine type Ca2GeO4 ceramic

Ca₂GeO₄ dielectric ceramic was prepared using the conventional solid-state reaction method. Sintering behavior, crystal structure, microstructure, and microwave dielectric properties were analyzed by XRD, SEM, Raman, and Infrared reflectivity spectrum. Ca₂GeO₄ was found to crystallize in the olivine structure with a space group of Pnma. A dense and high-performance microwave dielectric property with permittivity ? 6.76 ± 0.02, Q×f value ? 82,400 ± 1800 GHz, and temperature coefficient ? -67 ± 3.4 ppm/°C were obtained in the sample sintered at 1420 °C. Infrared spectral analysis supported that the dielectric contribution for Ca₂GeO₄ at microwave region is dominated by absorption of phonons and there is no contribution from dipolar or other polarization mechanisms. The large negative τ_f values could be compensated by forming composite ceramics with CaTiO₃. A low-ε_r of 9.02 ± 0.03, a high Q×f of 49,880 ± 1400 GHz, and a near-zero τ_f value of +4 ± 0.6 ppm/°C were obtained for 0.92Ca₂GeO₄-0.08CaTiO₃ ceramic at 1420 °C for 4 h. This ceramic could be a good candidate for microwave substrate materials.     

NaCa4V5O17: A low-firing microwave dielectric ceramic with low permittivity and chemical compatibility with silver for LTCC applications

Phase formation, crystal structure and dielectric properties of NaCa₄V₅O₁₇ ceramics fabricated via a solid state reaction route at relatively low temperatures (780–860 °C) were investigated. NaCa₄V₅O₁₇ crystallizes in a triclinic structure. Dielectric properties were measured based on the Hakki-Coleman post resonator method at microwave frequency. Specially, a specimen sintered at 840 °C demonstrated balanced dielectric properties with a permittivity ε_r = 9.72, a quality factor Q×f = 51,000 GHz, and a temperature coefficient of resonance frequency τ_f = −84 ppm/°C. NaCa₄V₅O₁₇ ceramics showed excellent chemical compatibility with Ag metal electrodes. Besides, the thermal stability of resonance frequency was effectively adjusted through formation of composite ceramics between NaCa₄V₅O₁₇ and TiO₂ and a near-zero τ_f ˜ 1.3 ppm/°C accompanied with ε_r = 14.9 and Q×f = 19,600 GHz was achieved when 50% mol TiO₂ was added. All the merits render NaCa₄V₅O₁₇ a potential candidate for multilayer electronic devices.     

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.     

A low-εr and high-Q microwave dielectric ceramic Li2SrSiO4 with abnormally low sintering temperature

A novel low-temperature co-fired ceramic Li₂SrSiO₄ with low-ε_r and high-Q was synthesized through solid-state reaction. XRD results showed that the Li₂SrSiO₄ ceramic formed a single hexagonal structure with a space group of P3₁21. The microwave dielectric properties of Li₂SrSiO₄ ceramic sintered at 880 °C were ε_r = 7.4 ± 0.1, Q × f = 100,700 ± 2000 GHz, and τ_f = −85.4 ± 2.4 ppm/℃. The PVL chemical bond theory indicated that the low sintering temperature of Li₂SrSiO₄ ceramic might be ascribed to the weak covalency (18 %) of SiO bond. A near-zero τ_f (‒1.9 ± 1.8 ppm/℃) combined with ε_r = 8.2 ± 0.1, and Q × f = 63,200 ± 1900 GHz was obtained by adding SrTiO₃ to form 0.94Li₂SrSiO₄ - 0.06SrTiO₃ composite ceramic. Given their chemical compatibility with Ag powders, the Li₂SrSiO₄ ceramics can be a candidate dielectric material for LTCC applications.     

High quality factor cold sintered LiF ceramics for microstrip patch antenna applications

Cold sintering is adopted to pre-densify LiF ceramics, where the relative density increases significantly from 72.1 % at 125 MPa to 88.9 % at 500 MPa. The following post-annealings at 800 °C lead to further optimizations of densification, and near-full densifications with relative densities of 95.6 % and 97.6 % are achieved at 375 and 500 MPa, respectively. Qf value increased with increasing uniaxial pressure until it reaches the maximum value of 134,050 GHz at 375 MPa, which is 1.82 times higher than that via conventional sintering (73,800 GHz). ε_r and τ_f are mainly determined by the relative density, and the optimum microwave dielectric properties are obtained as follows: ε_r = 8.45, Qf = 134,050 GHz, τ_f =–135 ppm/°C. A microstrip patch antenna is designed and fabricated using the LiF ceramic as the substrate, which gives an S11 of –20.3 dB, a simulated high efficiency of 90.5 %, and a gain of 4.25 dB at the resonant frequency of 6.81 GHz.     

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.