Microwave Dielectric Properties of a Low‐Firing Ba2BiV3O11 Ceramic

A low‐firing microwave dielectric ceramic Ba₂BiV₃O₁₁ was prepared via solid‐state reaction method. Ba₂BiV₃O₁₁ ceramic could be well sintered at 840°C–880°C, with a ε_r ~14.2, a high Q × f value ~68 700 GHz (at 8.7 GHz), and a negative temperature coefficient of ?81 ppm/°C. τ_f of Ba₂BiV₃O₁₁ was tuned to be near zero by formation of a composite with TiO₂. 0.7Ba₂BiV₃O₁₁–0.3TiO₂ ceramic sintered at 910°C showed improved properties with ε_r = 15.7, Q × f = 53 200 GHz, and τ_f  = ?2 ppm/°C.     

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.     

Effect of Glass Addition on the Microwave Dielectric Properties of CaMgSi2O6 Ceramics

CaMgSi₂O₆ (CMS) ceramics prepared by the solid-state ceramic route have a sintering temperature of 1300°C/2 h. The sintering temperature of CMS was reduced below the melting point of Ag using low-melting LBS and LMZBS glasses. In the case of CMS+15 wt% LMZBS sintered at 900°C/2 h, the dielectric properties obtained were ɛ_r=8.2, Q_u×f=32,000 GHz (10.15 GHz), and τ_f=–48 ppm/°C. The CMS+15 wt% LBS composite, sintered at 925°C/2 h, showed ɛ_r=8, Q_u×f=15,000 GHz (10.17 GHz), and τ_f=–49 ppm/°C. The chemical compatibility of Ag with the ceramic–glass composites was also investigated for low-temperature co-fired ceramic applications.     

Facile Synthesis of “Quench‐Free Glass” and Ceramic‐Glass Composite for LTCC Applications

The 10 mol% ZnO–2 mol% B₂O₃–8 mol% P₂O₅–80 mol% TeO₂ (ZBPT) glass was prepared by quenching as well as slowly cooling the melt. The ZBPT glass prepared by both methods show similar microwave dielectric properties. ZBPT glass has an ε_r of 22.5 (at 7 GHz), Q_u × f of 1500 GHz, and τ_f of ?100 ppm/°C. The ceramic‐glass composites of Sr₂ZnTeO₆ (SZT) and ZBPT is prepared through two convenient methods: (a) conventional way of co‐firing the ceramic with ZBPT glass powder and (b) a nonconventional facile route by co‐firing the ceramic with precursor oxide mixture of ZBPT glass at 950°C. In the former route, SZT + 5 wt% ZBPT composite sintered at 950°C showed moderately good microwave dielectric properties (ε_r = 13.4, Q_u × f = 4500 GHz and τ_f = ?52 ppm/°C). Although the SZT + 5 wt% ZBPT composite prepared through the nonconventional method also showed similar microwave dielectric properties (ε_r = 13.8, Q_u × f = 5300 GHz and τ_f = ?50 ppm/°C), the synthesis procedure is much simplified in the latter case. The composites are found to be chemically compatible with Ag. The composite containing 5 wt% ZBPT prepared through conventional and nonconventional ways shows linear coefficients of thermal expansion of 7.0 ppm/°C and 7.1 ppm/°C, respectively. Both the composites have a room‐temperature thermal conductivity of 2.1 Wm^?1 K^?1.     

Low‐Temperature Sintering and Microwave Dielectric Properties of (Mg0.95Zn0.05)2(Ti0.8Sn0.2)O4–(Ca0.8Sr0.2)TiO3 Composite Ceramics

0.9(Mg_0.95Zn_0.05)₂(Ti_0.8Sn_0.2)O₄–0.1(Ca_0.8Sr_0.2)TiO₃ (MZTS–CST) ceramics were prepared by a conventional solid‐state route. The MZTS–CST ceramics sintered at 1325°C exhibited ε_r = 18.2, Q × f = 49 120 GHz (at 8.1 GHz), and τ_f = 15 ppm/°C. The effects of LiF–Fe₂O₃–V₂O₅ (LFV) addition on the sinterability, phase composition, microstructure, and microwave dielectric properties of MZTS–CST were investigated. Eutectic liquid phases 0.12CaF₂/0.28MgF₂/0.6LiF and MgV₂O₆ were developed, which lowered the sintering temperature of MZTS–CST ceramics from 1325°C to 950°C. X‐ray powder diffraction (XRPD) and energy dispersive spectroscopy (EDS) analysis revealed that MZTS and CST coexisted in the sintered ceramics. Secondary phase Ca₅Mg₄(VO₄)₆ as well as residual liquid phase affected the microwave dielectric properties of MZTS–CST composite ceramics. Typically, the MZTS–CST–5.3LFV composite ceramics sintered at 950°C showed excellent microwave dielectric properties: ε_r = 16.3, Q × f = 30 790 GHz (at 8.3 GHz), and τ_f = ?10 ppm/°C.     

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.     

Low‐Temperature‐Fired ReVO4 (Re = La,Ce) Microwave Dielectric Ceramics

We report a series of ReVO₄ (Re = La, Ce) microwave dielectric ceramics fabricated by a standard solid‐state reaction method. X‐ray diffraction and scanning electron microscopy measurements were performed to explore the phase purity, sintering behavior, and microstructure. The analysis revealed that pure and dense monoclinic LaVO₄ ceramics with a monazite structure and tetragonal CeVO₄ ceramics with a zircon structure could be obtained in their respective sintering temperature range. Furthermore, LaVO₄ and CeVO₄ ceramics sintered at 850°C and 950°C for 4 h possessed out‐bound microwave dielectric properties: ε_r = 14.2, Q × f = 48197 GHz, τ_f = ?37.9 ppm/°C, and ε_r = 12.3, Q × f = 41 460 GHz, τ_f = ?34.4 ppm/°C, respectively. The overall results suggest that the ReVO₄ ceramics could be promising materials for low‐temperature‐cofired ceramic 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.     

Microwave Dielectric Properties of Ultralow‐Temperature Cofirable Ba3V4O13 Ceramics

Ultralow‐temperature sinterable Ba₃V₄O₁₃ ceramics have been prepared through solid‐state ceramic route. Structural properties of the ceramic material are studied using powder X‐ray diffraction. Ba₃V₄O₁₃ ceramic has monoclinic structure and the existence of [V₄O₁₃]^6? polyhedra is confirmed through Laser Raman studies. The sample sintered at 600°C for 1 h shows dense microstructure with microwave dielectric properties of ε_r = 9.6, Q × f = 56 100 GHz, and τ_f = ?42 ppm/°C. The ceramics under study show good chemical compatibility with aluminum during cofiring.     

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.     

Structure and Microwave Dielectric Properties of Low Firing Bi2Te2W3O16 Ceramics

Dense Bi₂Te₂W₃O₁₆ ceramics were prepared by the conventional solid‐state reaction route. X‐ray diffraction data show the room‐temperature (RT) crystal symmetry of Bi₂Te₂W₃O₁₆ to be well described by the centrosymmetric monoclinic C2/c space group [a = 21.280(5) Å, b = 5.5663(16) Å, c = 12.831(3) Å and β = 124.014(19)° and Z = 4]. Raman spectroscopy analyses are in broad agreement with space group assignment, but also revealed the presence of Bi₂W₂O₉ as a secondary phase. This phase is present as plate‐like grains embedded on a fine‐grained equiaxed matrix, as revealed by scanning electron microscopy. From the fitting of infrared reflectivity data the relative permittivity, ε_r, was estimated as 34.2, and the intrinsic quality factor, Q_u × f as 57 500 GHz. At RT and microwave frequencies, Bi₂Te₂W₃O₁₆ ceramics sintered at 720°C for 6 h exhibit ε_r ~ 34.5, Q_u × f = 3173 GHz (at 7.5 GHz), and temperature coefficient of resonant frequency, τ_f = ?92 ppm/°C. This shows a good agreement between the estimated and measured ε_r values, but also shows that, in principle, the dielectric losses of the ceramics are of extrinsic origin.     

Microstructures and Microwave Dielectric Properties of Bi2O3‐Deficient Bi12SiO20 Ceramics

The Microstructure and microwave dielectric properties of Bi₂O₃‐deficient Bi₁₂SiO₂₀ ceramics were investigated. A small amount of unreacted Bi₂O₃ phase melted during sintering at 825°C and assisted with densification and grain growth in all samples. The melted Bi₂O₃ reacted with remnant SiO₂ during cooling to form a Bi₄Si₃O₁₂ secondary phase. The nominal composition of Bi_11.8SiO_19.7 ceramics sintered at 825°C for 4 h had a high relative density of 97% of the theoretical density, and good microwave dielectric properties: ε_r = 39, Q × f = 74 000 GHz, and τ_f = ?14.1 ppm/°C. Moreover, this ceramic did not react with Ag at 825°C.     

Structure and microwave dielectric properties of ALn4(MoO4)7 (A = Ba,Sr, Ca,Ln = La,Pr, Nd,and Sm) ceramics

ALn₄(MoO₄)₇ (A = Ba, Sr, Ca, Ln = La, Pr, Nd, Sm) ceramics are prepared by solid state ceramic route and the structural properties have been studied using powder X-ray diffraction and laser Raman spectroscopy. All the ceramics under study are phase pure except BaLn₄(MoO₄)₇ (Ln = Pr, Nd, Sm). Scanning electron micrographs of the sintered ceramics show closely packed microstructure with phase homogeneity. BaLa₄(MoO₄)₇ ceramic has a maximum density of 4.5 g/cm³ at 710°C together with ԑ_r = 11.8, Q_u x f = 39 300 GHz, and τ_f = −68 ppm/^oC. SrLa₄(MoO₄)₇ ceramic exhibited a maximum density of 4.4 g/cm³, ԑ_r = 11.7, Q_u x f = 44 200 GHz, and τ_f = −83 ppm/^°C at 740°C whereas CaLa₄(MoO₄)₇ ceramic possess a maximum density of 4.2 g/cm³, ԑ_r = 11.4, Q_u x f = 30 200 GHz and τ_f = −90 ppm/^oC at 750°C at microwave frequencies. The chemical compatibility of the BaLa₄(MoO₄)₇, SrLa₄(MoO₄)₇ and CaLa₄(MoO₄)₇ ceramics with silver electrode have been studied using powder X-ray diffraction of the co-fired samples and is further examined with energy dispersive X-ray spectroscopy.     

Sintering Mechanism and Microwave Dielectric Properties of Bi12TiO20 Ceramics

A homogeneous Bi₁₂TiO₂₀ phase was developed in a specimen that was calcined at 700°C without the formation of a secondary phase. A small amount of the Bi₁₂TiO₂₀ phase melted during sintering and assisted the densification of the specimen. The Bi₂O₃ and Bi₈TiO₁₄ secondary phases were found in all specimens. All the specimens that were sintered at temperatures ≥775°C exhibited high relative densities above 98% of the theoretical density. The Q × f value of the Bi₁₂TiO₂₀ ceramics was influenced by the grain size. The Bi₁₂TiO₂₀ ceramics sintered at 800°C for 5 h showed promising microwave dielectric properties of ε_r = 41, Q × f = 10 400 GHz, and τ_f = ?10.8 ppm/°C.     

Structure,Microwave Dielectric Properties,and Low‐Temperature Sintering of Acceptor/Donor Codoped Li2Ti1−x(Al0.5Nb0.5)xO3 Ceramics

The structure, microwave dielectric properties, and low‐temperature sintering behavior of acceptor/donor codoped Li₂TiO₃ ceramics [Li₂Ti_1?x(Al_0.5Nb_0.5)_xO₃, x = 0–0.3] were investigated systematically. The x‐ray diffraction confirmed that a single‐phase solid solution remained within 0 < x ≤ 0.2 and secondary phases started to appear as x > 0.2, accompanied by an order–disorder phase transition in the whole range. Scanning electron microscopy observation indicated that the complex substitution of Al³⁺ and Nb⁵⁺ produced a significant effect on the microstructural morphology. Both microcrack healing and grain growth contributed to the obviously enhanced Q×f values. By comparison, the decrease of ε_r and τ_f values was ascribed to the ionic polarizability and the cell volume, respectively. Excellent microwave dielectric properties of ε_r ~ 21.2, Q×f ~ 181 800 GHz and τ_f  ~ 12.8 ppm/°C were achieved in the x = 0.15 sample when sintered at 1150°C. After 1.5 mol% BaCu(B₂O₅) additive was introduced, it could be well sintered at 950°C and exhibited good microwave dielectric properties of ε_r ~ 20.4, Q×f ~ 53 290 GHz and τ_f ~ 3.6 ppm/°C as well. The cofiring test of the low‐sintering sample with Ag powder proved its good chemical stability during high temperature, which enables it to be a promising middle‐permittivity candidate material for the applications of low‐temperature cofired ceramics.     

Microstructural and Microwave Dielectric Properties of Bi12GeO20 and Bi2O3‐Deficient Bi12GeO20 Ceramics

Bi₁₂GeO₂₀ ceramics sintered at 800°C had dense microstructures, with an average grain size of 1.5 μm, a relative permittivity (ε_r) of 36.97, temperature coefficient of resonance frequency (τ_f) of ?32.803 ppm/°C, and quality factor (Q × f) of 3137 GHz. The Bi_12‐xGeO_20‐1.5x ceramics were well sintered at both 800°C and 825°C, with average grain sizes exceeding 100 μm for x ≤ 1.0. However, the grain size decreased for x > 1.0 because of the Bi₄Ge₃O₁₂ secondary phase that formed at the grain boundaries. Bi_12‐xGeO_20‐1.5x (x ≤ 1.0) ceramics showed increased Q × f values of >10 000 GHz, although the ε_r and τ_f values were similar to those of Bi₁₂GeO₂₀ ceramics. The increased Q × f value resulted from the increased grain size. In particular, the Bi_11.6GeO_19.4 ceramic sintered at 825°C for 3 h showed good microwave dielectric properties of ε_r = 37.81, τ_f = ?33.839 ppm/°C, and Q × f = 14 455 GHz.     

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.     

Molten Salt Synthesis,Polymorphism, and Microwave Dielectric Properties of Ba8NiTa6O24 Perovskite

Molten salt synthesis (MSS) of the eight‐layer hexagonal perovskite Ba₈NiTa₆O₂₄ was performed using mixed KCl–NaCl salts in comparison with solid‐state synthesis (SSS). In the SSS, the hexagonal Ba₈NiTa₆O₂₄ formed at 1300°C via a reaction between cubic Ba₃NiTa₂O₉ and hexagonal Ba₅Ta₄O₁₅. While the MSS did not lower the synthesis temperature of the hexagonal Ba₈NiTa₆O₂₄ but stabilized an unusual A‐ and B‐site‐deficient cubic perovskite polymorph of Ba₈NiTa₆O₂₄ below 1350°C as an intermediate phase prior to transforming into the hexagonal phase. This cubic polymorph contains ~3% A‐site and ~9.5% B‐site vacancies plus ~3% Ba cations in the B sites and demonstrated remarkable stability below 1350°C when without presence of the molten salt. The cubic polymorph displayed larger ε ~ 36 and τ_f ~ 110 ppm/°C than the hexagonal polymorph from the MSS (ε ~ 29 and τ_f ~ 67 ppm/°C). The hexagonal SSS‐processed ceramics showed advantageous dielectric properties (Q_f ~ 52 000 GHz, τ_f ~ 30 ppm/°C) over both cubic and hexagonal MSS‐processed ones (Q_f ~ 18 600–20 000 GHz), while displaying anisotropic grain growth. The anisotropic grain growth was suppressed significantly by the MSS processing.     

Low‐firing and temperature stable microwave dielectric ceramics: Ba2LnV3O11 (Ln = Nd,Sm)

Low‐firing and temperature stable microwave dielectric ceramics of Ba₂LnV₃O₁₁ (Ln = Nd, Sm) were prepared by solid‐state reaction. X‐ray diffraction (XRD) and scanning electron microscopy (SEM) were used to investigate the phase purity, crystal structure, sintering behavior, and microstructure. The XRD patterns indicated that Ba₂LnV₃O₁₁ (Ln = Nd, Sm) ceramics belong to monoclinic crystal system with P2₁/c space group in the whole sintering temperature range (800°C ‐900°C). Both ceramics could be well densified at 880°C for 4 hours with relative densities higher than 96%. The Ba₂LnV₃O₁₁ (Ln = Nd, Sm) samples sintered at 880°C for 4 hours exhibited excellent microwave dielectric properties: ε_r = 12.05, Q × f = 23 010 GHz, τ_f = ?7.7 ppm/°C, and ε_r = 12.19, Q × f = 27 120 GHz, τ_f = ?16.2 ppm/°C, respectively. Besides, Ba₂LnV₃O₁₁ (Ln = Nd, Sm) ceramics could be well co‐fired with the silver electrode at 880°C.     

A new low‐firing and high‐Q microwave dielectric ceramic Li9Zr3NbO13

The microwave dielectric ceramic Li₉Zr₃NbO₁₃ was found and investigated. Prepared via the solid‐state reaction method, the Li₉Zr₃NbO₁₃ formed as a Li₂ZrO₃‐type solid solution at 880‐900°C, with monoclinic structure in C2/c space group and Z = 4. Typically, the Li₉Zr₃NbO₁₃ sintered at 900°C exhibited the excellent microwave dielectric properties of ε_r = 21.3, Q×f = 43 600 GHz (at 7.4 GHz), τ_f = 7.3 ppm/°C.