Ba5Li2W3O15: A New Li‐Containing Perovskite‐Type Microwave Ceramic with High Q and Low τf

A new Li‐containing microwave ceramic Ba₅Li₂W₃O₁₅ with hexagonal perovskite structure was prepared through a solid‐state ceramic route. Small amount of scheelite BaWO₄ appeared as a second phase during sintering. The Ba₅Li₂W₃O₁₅ could be well densified at 1120°C and exhibits good microwave dielectric properties with permittivity (ε_r) of 25.4, high Q × f value about 39 000 GHz, and low temperature coefficient of resonate frequency (τ_f) of 10 ppm/°C. The addition of BaCu(B₂O₅) can effectively lower the sintering temperature from 1120°C to 900°C and does not induce degradation of the microwave dielectric properties. These results indicate that the Ba₅Li₂W₃O₁₅ ceramic might be a promising candidate in microwave dielectric resonators.     

Microwave hybrid sintering of 0.95(Mg0.95Zn0.05)TiO3-0.05CaTiO3 ceramics and its microwave dielectric properties

Microwave dielectric ceramic powder of 0.95(Mg_0.95Zn_0.05)TiO₃-0.05CaTiO₃ (MCT) has been prepared by solid-state reaction method through single-step calcination at 1150 °C. The green bodies prepared from the calcined powder have been sintered by conventional, susceptor-aided, and hybrid microwave sintering techniques followed by annealing. XRD of calcined and sintered ceramics show (Mg,Zn)TiO₃ as a major phase with CaTiO₃ as a minor secondary phase. Fractographs of fired ceramics obtained by SEM show similar features in conventional and hybrid microwave types of sintering. Microwave dielectric properties such as relative permittivity(ε_r), temperature coefficient of resonant frequency(τ_f), and unloaded quality factors (Q_u) for conventional sintered at 1325 °C for 4 h are—ε_r~19.8, τ_f< –6 ppm/°C and Q_u.f 69,600 GHz at 6 GHz. Ceramics obtained through susceptor-aided microwave sintering at 1325 °C for 4 h show poor fired density. But ceramics got by microwave-hybrid sintering (resistive + microwave) at the same temperature show ε_r~20.6, Q_u.f~81,600 GHz at 6 GHz and τ_f~?6.9 ppm/°C. The effect of hybrid microwave sintering on the dielectric properties of MCT ceramics is found to be more subtle than microstructural.     

Liquid‐phase sintering,microstructural evolution,and microwave dielectric properties of Li2Mg3SnO6–LiF ceramics

The liquid‐phase sintering behavior and microstructural evolution of x wt% LiF aided Li₂Mg₃SnO₆ ceramics (x = 1‐7) were investigated for the purpose to prepare dense phase‐pure ceramic samples. The grain and pore morphology, density variation, and phase structures were especially correlated with the subsequent microwave dielectric properties. The experimental results demonstrate a typical liquid‐phase sintering in LiF–Li₂Mg₃SnO₆ ceramics, in which LiF proves to be an effective sintering aid for the Li₂Mg₃SnO₆ ceramic and obviously reduces its optimum sintering temperature from ~1200°C to ~850°C. The actual sample density and microstructure (grain and pores) strongly depended on both the amount of LiF additive and the sintering temperature. Higher sintering temperature tended to cause the formation of closed pores in Li₂Mg₃SnO₆‐x wt% LiF ceramics owing to the increase in the migration ability of grain boundary. An obvious transition of fracture modes from transgranular to intergranular ones was observed approximately at x = 4. A single‐phase dense Li₂Mg₃SnO₆ ceramic could be obtained in the temperature range of 875°C‐1100°C, beyond which the secondary phase Li₄MgSn₂O₇ (<850°C) and Mg₂SnO₄ (>1100°C) appeared. Excellent microwave dielectric properties of Q × f = 230 000‐330 000 GHz, ε_r = ~10.5 and τ_f = ~?40 ppm/°C were obtained for Li₂Mg₃SnO₆ ceramics with x = 2‐5 as sintered at ~1150°C. For LTCC applications, a desirable Q × f value of ~133 000 GHz could be achieved in samples with x = 3‐4 as sintered at 875°C.     

Low‐temperature sintering and thermal stability of Li2GeO3‐based microwave dielectric ceramics with low permittivity

A low‐permittivity dielectric ceramic Li₂GeO₃ was prepared by the solid‐state reaction route. Single‐phase Li₂GeO₃ crystallized in an orthorhombic structure. Dense ceramics with high relative density and homogeneous microstructure were obtained as sintered at 1000‐1100°C. The optimum microwave dielectric properties were achieved in the sample sintered at 1080°C with a high relative density ~ 96%, a relative permittivity ε_r ~ 6.36, a quality factor Q × f ~ 29 000 GHz (at 14.5 GHz), and a temperature coefficient of resonance frequency τ_f ~ ?72 ppm/^°C. The sintering temperature of Li₂GeO₃ was successfully lowered via the appropriate addition of B₂O₃. Only 2 wt.% B₂O₃ addition contributed to a 21.2% decrease in sintering temperature to 850°C without deteriorating the dielectric properties. The temperature dependence of the resonance frequency was successfully suppressed by the addition of TiO₂ to form Li₂TiO₃ with a positive τ_f value. These results demonstrate potential applications of Li₂GeO₃ in low‐temperature cofiring ceramics technology.     

Novel Temperature Stable Li2MnO3 Dielectric Ceramics with High Q for LTCC Applications

Novel microwave dielectric ceramics in the Li₂MnO₃ system with high Q prepared through a conventional solid‐state route had been investigated. All the specimens exhibited single phase ceramics sintered in the temperature range 1140°C–1230°C. The microwave dielectric properties of Li₂MnO₃ ceramics were strongly correlated with sintering temperature and density. The best microwave dielectric properties of ε_r = 13.6, Q × f = 97 000 (GHz), and τ_f = ?5.2 ppm/°C could be obtained as sintered at 1200°C for 4 h. BaCu(B₂O₅) (BCB) could effectively lower the sintering temperature from 1200°C to 930°C and slightly induced degradation of the microwave dielectric properties. The Li₂MnO₃ ceramics doped with 2 wt% BaCu(B₂O₅) had excellent dielectric properties of ε_r = 11.9, Q × f = 80 600 (GHz), and τ_f = 0 ppm/°C. With low sintering temperature and good dielectric properties, the BCB added Li₂MnO₃ ceramics are suitable candidates for LTCC applications in wireless communication system.     

Structure‐Dependent Microwave Dielectric Properties and Middle‐Temperature Sintering of Forsterite (Mg1–xNix)2SiO4 Ceramics

The crystal structure, microstructure, and microwave dielectric properties of forsterite‐based (Mg_1–xNi_x)₂SiO₄ (x = 0.02–0.20) ceramics were systematically investigated. All samples present a single forsterite phase of an orthorhombic structure with a space group Pbnm except for a little MgSiO₃ secondary phase as x > 0.08. Lattice parameters in all axes decrease linearly with increasing Ni content due to the smaller ionic radius of Ni²⁺ compared to Mg²⁺. The substitution of an appropriate amount of Ni²⁺ could greatly improve the sintering behavior and produce a uniform and closely packed microstructure of the Mg₂SiO₄ ceramics such that a superior Q × f value (152 300 GHz) can be achieved as x = 0.05. The τ_f value was found to increase with increasing A‐site ionic bond valences. In addition, various additives were used as sintering aids to lower the sintering temperature from 1500°C to the middle sintering temperature range. Excellent microwave dielectric properties of ε_r~6.9, Q × f~99800 GHz and τ_f~?50 ppm/°C can be obtained for 12 wt% Li₂CO₃‐V₂O₅‐doped (Mg_0.95Ni_0.05)₂SiO₄ ceramics sintered at 1150°C for 4 h.     

Silver Co‐Firable Li2ZnTi3O8 Microwave Dielectric Ceramics with LZB Glass Additive and TiO2 Dopant

New dielectric ceramics are prepared by the conventional solid‐state ceramic route. Effects of LZB glass on sintering, phase purity, microstructure, and dielectric properties of Li₂ZnTi₃O₈ ceramics have been investigated. Adding LZB lowers sintering temperature from 1050°C to 875°C, and does not induce much degradation of dielectric properties. The 1.0 wt% LZB glass‐added ceramic has better properties of ε_r = 23.9, Q × f = 31,608 GHz, τ_f = ?14.3 ppm/°C. Additions of TiO₂ markedly improve microwave properties. Typically, the Li₂ZnTi₃O₈ + 1 wt%LZB + 3.5 wt%TiO₂ sintered at 900°C shows ε_r = 26.1, Q × f = 45,168 GHz, τ_f = ?4.1 ppm/°C. Compatibility with Ag electrode indicates that this material may be applied to LTCC devices.     

Li4Mg3Ti2O9: A novel low-loss microwave dielectric ceramic for LTCC applications

Low-loss novel Li₄Mg₃Ti₂O₉ dielectric ceramics with rock-salt structure were prepared by a conventional solid-state route. The crystalline structure, chemical bond properties, infrared spectroscopy and microwave dielectric properties of the abovementioned system were initially investigated. It could be concluded from this work that the extrinsic factors such as sintering temperatures and grain sizes significantly affected the dielectric properties of Li₄Mg₃Ti₂O₉ at lower sintering temperatures, while the intrinsic factors like bond ionicity and lattice energy played a dominant role when the ceramics were densified at 1450 °C. In order to explore the origin of intrinsic characteristics, complex dielectric constants (ε^’ and ε^’’) were calculated by the infrared spectra, which indicated that the absorptions of phonon oscillation predominantly effected the polarization of the ceramics. The Li₄Mg₃Ti₂O₉ ceramics sintered at 1450 °C exhibited excellent properties of ε_r=15.97, Q·f=135,800 GHz and τ_f=−7.06 ppm/°C. In addition, certain amounts of lithium fluoride (LiF) were added to lower the sintering temperatures of matrix. The Li₄Mg₃Ti₂O₉−3 wt% LiF ceramics sintered at 900 °C possessed suitable dielectric properties of ε_r=15.17, Q·f =42,800 GHz and τ_f=−11.30 ppm/°C, which made such materials promising for low temperature co-fired ceramic applications (LTCC).     

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.     

Preparation and Microwave Dielectric Properties of Ultra‐low Temperature Sintering Ceramics in K2O–MoO3 Binary System

A series of microwave dielectric ceramics in the compositions of K₂Mo₂O₇, K₂Mo₃O₁₀, and K₂Mo₄O₁₃ in K₂O–MoO₃ binary system with ultra low sintering temperatures were prepared using the solid‐state reaction method. Their synthesis, phase composition, compatibility with metal electrodes, microstructures, and microwave dielectric properties were investigated. The K₂Mo₂O₇ ceramic sintered at 460°C with a triclinic structure has a relative permittivity of 7.5, a Q × f value of 22 000 GHz, and a τ_f value of ?63 ppm/°C. The X‐ray diffraction patterns indicate that K₂Mo₂O₇ does not react with Ag and Al electrodes at the co‐fired temperatures. The K₂Mo₃O₁₀ ceramic can be sintered well at 520°C with a relative permittivity of 5.6, a Q × f value of 35 830 GHz, and a τ_f value of ?92 ppm/°C. It has compatibility with Ag electrode. The K₂Mo₄O₁₃ ceramic sintered at 540°C possesses good microwave dielectric properties with a relative permittivity of 6.8, a Q × f value of 39 290 GHz and a τ_f value of ?67 ppm/°C and it is compatible with Al electrode. For K₂Mo₂O₇ and K₂Mo₄O₁₃, it is found that the grain sizes and the number of grain boundaries play an important role in the dielectric loss. From this study, it can be seen that the three ceramics in K₂O–MoO₃ system have good microwave dielectric properties, ultra‐low sintering temperatures, non‐toxic, and low‐cost characteristics. So they can be potentially applied to ultra‐LTCC devices.     

Design and fabrication of a C-band patch antenna using novel low permittivity SrGa2Si2O8 microwave dielectric ceramic

A novel microwave dielectric ceramic of SrGa₂Si₂O₈ was synthesized using the traditional solid-state method. Its phase composition, microstructure, and microwave dielectric properties were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), Raman spectroscopy, and network analyzer. XRD results indicated that the space group of the ceramic transformed from I₂/c to P₂₁/a at 700 °C. A combination of good microwave dielectric properties was obtained at 1260 °C with ε_r = 6.3, Q×f= 96,600 GHz and τ_f = −45.2 ppm/°C at 16.5 GHz. The negative τ_f can be tuned to near zero by adding 15 mol% CaTiO₃. The densification temperature can be reduced to 940 °C by adding 4 wt% LiF. Moreover, the SrGa₂Si₂O₈ ceramic had good chemical compatibility with the Ag electrode. A patch antenna was designed using the 0.85SrGa₂Si₂O₈ + 0.15CaTiO₃ + 4 wt% LiF ceramic. The antenna had a high radiation efficiency of 99.2 % and a gain of 2.988 dBi at the center frequency of 4.261 GHz. All results indicated that the SrGa₂Si₂O₈ ceramic has promising potential for applications in 5 G wireless communication technology.     

Novel Low‐Firing Microwave Dielectric Ceramic LiCa3MgV3O12 with Low Dielectric Loss

Novel low temperature firing microwave dielectric ceramic LiCa₃MgV₃O₁₂ (LCMV) with garnet structure was fabricated by the conventional solid‐state reaction method. The phase purity, microstructure, and microwave dielectric properties were investigated. The densification temperature for the LCMV ceramic is 900°C. LCMV ceramic possessed ε_r = 10.5, Q_u × f = 74 700 GHz, and τ_f = ?61 ppm/°C. Furthermore, 0.90LiCa₃MgV₃O₁₂–0.10CaTiO₃ ceramic sintered at 925°C for 4 h exhibited good properties of ε_r = 12.4, Q_u × f = 57 600 GHz, and τ_f = 2.7 ppm/°C. The LCMV ceramic could be compatible with Ag electrode, which makes it a promising ceramic for LTCC technology application.     

Silver Co-Firable ZnTiNb2O8 Microwave Dielectric Ceramics with Li2O–ZnO–B2O3 Glass Additive

The effects of Li₂O–ZnO–B₂O₃ glass additive on the sintering behavior, phase formation, microstructure, and microwave dielectric properties of ZnTiNb₂O₈ ceramics have been investigated. The sintering temperature of ZnTiNb₂O₈ ceramics can be effectively reduced from 1200°C to 875°C by adding a small amount of Li₂O–ZnO–B₂O₃ glass, while no obvious degradation of the microwave dielectric properties was induced. Typically, the 2.0 wt% Li₂O–ZnO–B₂O₃ glass-added ceramic sintered at 875°C has better microwave dielectric properties of ɛ_r=31.8, Q×f=25,013 GHz, and τ_f=−62 ppm/°C. In addition, the ceramics can be co-fired well with an Ag electrode.     

A Novel Magnetodielectric Solid Solution Ceramic 0.4LiFe5O8–0.6Li2MgTi3O8 with Excellent Microwave Dielectric Properties

In this study, a novel spinel solid solution ceramic of 0.4LiFe₅O₈–0.6Li₂MgTi₃O₈ (0.4LFO–0.6LMT) has been developed and investigated. It is found that the 40 mol% LiFe₅O₈ and 60 mol% Li₂MgTi₃O₈ are fully soluble in each other and a disordered spinel phase is formed. The ceramic sample sintered at 1050°C/2 h exhibits both good magnetic and dielectric properties in the frequency range 1–10 MHz, with a permeability between 29.9~14.1 and magnetic loss tangent between 0.12~0.67, permittivity between 16.92~16.94 and dielectric loss tangent between 5.9 × 10^?3–2.3 × 10^?2. The sample also has good microwave dielectric properties with a relative permittivity of 16.1, a high quality factor (Q × f) ~28 500 GHz (at 7.8 GHz). Furthermore, 3 wt% H₃BO₃–CuO (BCu) addition can effectively lower the sintering temperature to 925°C and does not degrade the magnetodielectric properties. The chemical compatibility with silver electrode indicates that this kind of ceramics is a good candidate for the low‐temperature cofired ceramic (LTCC) application.     

Effects of ZnO and B2O3 on the Microstructure and Microwave Dielectric Properties of 5Li2O‐1Nb2O5‐5TiO2 Ceramics

The effects of ZnO and B₂O₃ addition on the sintering behavior, microstructure, and the microwave dielectric properties of 5Li₂O‐1Nb₂O₅‐5TiO₂ (LNT) ceramics have been investigated. With addition of low‐level doping of ZnO and B₂O₃, the sintering temperature of the LNT ceramics can be lowered down to near 920°C due to the liquid phase effect. The Li₂TiO₃ss and the “M‐phase” are the two main phases, whereas other phase could be observed when co‐doping with ZnO and B₂O₃ in the ceramics. And the amount of the other phase increases with the ZnO content increasing. The addition of ZnO does not induce much degradation in the microwave dielectric properties but lowers the τ_f value to near zero. Typically, the good microwave dielectric properties of ε_r = 36.4, Q × f = 8835 GHz, τ_f = 4.4 ppm/°C could be obtained for the 1 wt% B₂O₃ and 4 wt% ZnO co‐doped sample sintered at 920°C, which is promising for application of the multilayer microwave devices using Ag as internal electrode.     

A new Li-based ceramic of Li4MgSn2O7: Synthesis,phase evolution and microwave dielectric properties

A pure-phase Li₄MgSn₂O₇ (L₄MS) was successfully synthesized through optimizing the calcination condition. Microwave dielectric properties of the L₄MS ceramic with the phase evolution were investigated together with its low-temperature sintering. The sample maintains a single L₄MS phase as sintered below 1200?°C, such that τ_f remains a constant value of ～12.4?ppm/°C. Accompanied by the appearance of impurity phases (Li₂SnO₃)_ss and especially (MgO)_ss at higher sintering temperatures, excellent microwave dielectric properties of ε_r?=?13.1–13.5, Q?×?f?=?106,800–126,810?GHz and τ_f ?=?0–?4.2?ppm/°C are obtained in samples sintered at 1215–1260?°C for 4?h. Reduction of sintering temperature using LiF sintering aid also helps achieve pure-phase dense L₄MS ceramic. The L₄MS?+?x wt.% LiF ceramic exhibits ε_r～13.7, Qxf～97,000?GHz (x?≤?3) and τ_f ～8–13?ppm/°C sintered at 850?°C for potential LTCC applications, and ε_r ～13.9, Qxf～146,000?GHz and τ_f ～1.5–6?ppm/°C (x?≥?4) as sintered 1000?°C, exhibiting large potentials for microwave dielectric candidates.     

Microwave Dielectric Properties of Novel Glass‐Free Low‐Firing Li2CeO3 Ceramics

Novel glass–free low temperature firing microwave dielectric ceramics Li₂CeO₃ with high Q prepared through a conventional solid‐state reaction method had been investigated. All the specimens in this paper have sintering temperature lower than 750°C. XRD studies revealed single cubic phase. The microwave dielectric properties were correlated with the sintering conditions. At 720°C/4 h, Li₂CeO₃ ceramics possessed the excellent microwave dielectric properties of ε_r = 15.8, Q × f = 143 700 (GHz), and τ_f  = ?123 ppm/°C. Li₂CeO₃ ceramics could be excellent candidates for glass‐free low‐temperature co‐fired ceramics substrates.     

Effects of Magnesium–Tungsten co-substitution on crystal structure and microwave dielectric properties of CaTi1-x(Mg1/2W1/2)xO3 ceramics

CaTi_1-x (Mg_1/2W_1/2)_xO₃ (x = 0, 0.02, 0.04, 0.06, 0.08) dielectric ceramics were synthesized via the traditional solid-state reaction method. Crystal structure and microwave dielectric properties of CaTi_1-x (Mg_1/2W_1/2)_xO₃ system were systematically investigated based on chemistry bond theory (P–V-L theory) for the first time. The pure perovskite phase was obtained for all doped samples, as confirmed through the XRD and Rietveld refinement results. The lattice characteristics were closely related to the microwave dielectric properties. The bond ionicity, lattice energy, and bond energy affected the dielectric constant, quality factor, and temperature stability of the ceramic material. Through the use of (Mg_1/2W_1/2)⁴⁺ doped on B-site, the CaTi_1-x (Mg_1/2W_1/2)_xO₃ system can maintain a high dielectric constant (ε_r > 100) while effectively reducing the τ_f value from 800 ppm/^°C to less than 300 ppm/^°C and improving the Q × f value to 9650 GHz (at 3.76 GHz).     

Novel Low‐Firing Forsterite‐Based Microwave Dielectric for LTCC Applications

A novel low‐temperature sintering microwave dielectric based on forsterite (Mg₂SiO₄) ceramics was synthesized through the solid‐state reaction method. The effects of LiF additions on the sinterability, phase composition, microstructure, and microwave dielectric properties of Mg₂SiO₄ were investigated. It demonstrated that LiF could significantly broaden the processing window (~300°C) for Mg₂SiO₄, and more importantly the sintering temperature could be lowered below 900°C, maintaining excellent microwave dielectric properties simultaneously. The 2 wt% LiF‐doped samples could be well‐sintered at 800°C and possessed a ε_r ~ 6.81, a high Q×f ~ 167 000 GHz, and a τ_f ~ ?47.9 ppm/°C, having a very good potential for LTCC integration applications.     

Sintering behavior,structural phase transition,and microwave dielectric properties of La1‐xZnxTiNbO6‐x/2 ceramics

La_1‐xZn_xTiNbO_6‐x/2 (LZTN‐x) ceramics were prepared via a conventional solid‐state reaction route. The phase, microstructure, sintering behavior, and microwave dielectric properties have been systematically studied. The substitution of a small amount of Zn²⁺ for La³⁺ was found to effectively promote the sintering process of LTN ceramics. The corresponding sintering mechanism was believed to result from the formation of the lattice distortion and oxygen vacancies by means of comparative studies on La‐deficient LTN ceramics and 0.5 mol% ZnO added LTN ceramics (LTN+0.005ZnO). The resultant microwave dielectric properties of LTN ceramics were closely correlated with the sample density, compositions, and especially with the phase structure at room temperature which depended on the orthorhombic‐monoclinic phase transition temperature and the sintering temperature. A single orthorhombic LZTN‐0.03 ceramic sintered at 1200°C was achieved with good microwave dielectric properties of ε_r~63, Q×f~9600 GHz (@4.77 GHz) and τ_f ~105 ppm/°C. By comparison, a relatively high Q × f~80995 GHz (@7.40 GHz) together with ε_r~23, and τ_f ~?56 ppm/°C was obtained in monoclinic LTN+0.005ZnO ceramics sintered at 1350°C.