Microwave dielectric properties of CaWO4–Li2TiO3 ceramics added with LBSCA glass for LTCC applications

In this work, 0.86CaWO₄–0.14Li₂TiO₃ ceramics were prepared via a traditional solid-state process. The effects of Li₂O–B₂O₃–SiO₂–CaO–Al₂O₃ (LBSCA) addition on the phase formation, sintering character, microstructure and microwave dielectric properties of the ceramics were investigated. A small amount of LBSCA addition could effectively lower the sintering temperature of the ceramics. X-ray diffraction analysis revealed that CaWO₄ and Li₂TiO₃ phases coexisted without producing any other crystal phases in the sintered ceramics. The dielectric constant and Qf values were related to the amount of LBSCA addition and sintering temperatures. All specimens could obtain near-zero temperature coefficient (τ_f) values through the compensation of the positive τ_f of Li₂TiO₃ and the negative τ_f of CaWO₄. The 0.86CaWO₄–0.14Li₂TiO₃ ceramic with 0.5 wt% LBSCA addition and sintered at 900 °C for 3 h exhibited excellent microwave dielectric properties of ε_r=12.43, Qf=76,000 GHz and τ_f=−2.9 ppm/°C.     

Preparation and microwave dielectric properties of low-loss MgZrNb2O8 ceramics

A novel low-loss microwave dielectric material MgZrNb₂O₈ was reported for the first time. Single-phase MgZrNb₂O₈ was prepared by a conventional mixed-oxide route and sintered in the temperature range of 1280–1360 °C. The microstructure and microwave dielectric properties were investigated systematically. The X-ray diffraction results showed that all samples exhibit a single wolframite structure. When the sintering temperature was lower than 1340 °C, the Q×f value mainly depended on the relative density. However, when the sintering temperature was above 1340 °C, the Q×f value mainly relied on the grain morphology in addition to the density. The MgZrNb₂O₈ ceramic sintered at 1340 °C for 4 h exhibited excellent microwave dielectric of ε_r=26, Q×f=120,816 GHz (where f=6.85 GHz), and τ_f=?50.2 ppm/°C. These results demonstrate that MgZrNb₂O₈ could be a promising candidate material for the application of highly selective microwave ceramic resonators and filters.     

Novel ultra-low loss and low-fired Li8MgxTi3O9+xF2 microwave dielectric ceramics for resonator antenna applications

Low temperature sintered Li₈Mg_xTi₃O_9+xF₂ microwave dielectric ceramics with x = 2−7 were developed based on a newly designed pseudo ternary phase diagram of the Li₂TiO₃–MgO–LiF system. Dense solid solution ceramics (of relative density >96 %) with cubic rock-salt structure, accompanied by a small amount of secondary phase MgO, were obtained in the temperature range of 800−925 °C. With increasing Mg²⁺ content, the value of ε_r decreased, whereas that of τ_f remained nearly constant, and the Q × f increased to a maximum at x = 5. The Li₈Mg₅Ti₃O₁₄F₂ ceramic sintered at 875 °C exhibited superior microwave dielectric properties with ε_r = 16.8, Q × f = 119,700 GHz, and τ_f = −41.6 ppm/°C. The good compatibility with Ag electrodes highlights the promising prospects of this ceramic in low-temperature co-fired ceramic technology. Furthermore, a dielectric resonator antenna fabricated based on a Li₈Mg₅Ti₃O₁₄F₂ ceramic exhibited an outstanding S₁₁ of −34.7 dB and a broad bandwidth of 360 MHz at the desired resonant frequency of 5.98 GHz.     

Microwave dielectric properties of LiNb3O8 ceramics with TiO2 additions

The microwave dielectric properties of LiNb₃O₈ ceramics were investigated as a function of the sintering temperature and the amount of TiO₂ additive. LiNb₃O₈ ceramics, which were calcined at 750 °C and sintered at 1075 °C for 2 h, showed a dielectric constant (ɛ_r) of 34, a quality factor (Q × f₀) of 58,000 GHz and a temperature coefficient of resonance frequency (τ_f) of −96 ppm/°C, respectively. The density of the samples influenced the properties of these properties. As the TiO₂ content increased in the LiNb₃O₈–TiO₂ system, ɛ_r and τ_f of the material were increased due to the mixing effect of TiO₂ phase, which has higher dielectric constant and larger positive τ_f. The 0.65LiNb₃O₈–0.35TiO₂ ceramics showed a dielectric constant ɛ_r of 46.2, a quality factor (Q × f₀) of 5800 GHz and a temperature coefficient of resonance frequency τ_f of near to 0 ppm/°C.     

Microwave dielectric properties of Re3Ga5O12 (Re: Nd,Sm, Eu,Dy and Yb) ceramics and effect of TiO2 on the microwave dielectric properties of Sm3Ga5O12 ceramics

Re₃Ga₅O₁₂ (Re: Nd, Sm, Eu, Dy and Yb) garnet ceramics sintered at 1350–1500 °C had a high quality factor (Q × f) ranging from 40,000 to 192,173 GHz and a low dielectric constant (ɛ_r) of between 11.5 and 12.5. They also exhibited a relatively stable temperature coefficient of resonant frequency (τ_f) in the range of −33.7 to −12.4 ppm/°C. In order to tailor the τ_f value, TiO₂ was added to the Sm₃Ga₅O₁₂ ceramics, which exhibited good microwave dielectric properties. The relative density and grain size increased with addition of TiO₂, resulting in the enhancement of Q × f value. The τ_f increased with the addition of TiO₂. Excellent microwave dielectric properties of ɛ_r = 12.4, Q × f = 240,000 GHz and τ_f = −16.1 ppm/°C were obtained from the Sm₃Ga₅O₁₂ ceramics sintered at 1450 °C for 6 h with 1.0 mol% TiO₂. Therefore, Re₃Ga₅O₁₂ ceramics, especially TiO₂-added Sm₃Ga₅O₁₂ ceramics are good candidates for advanced substrate materials in microwave integrated circuits (MICs) applications.     

Temperature stability and high-Qf of low temperature firing Mg2SiO4–Li2TiO3 microwave dielectric ceramics

In this work, a series of low-temperature-firing (1−x)Mg₂SiO₄–xLi₂TiO₃–8 wt% LiF (x = 35–85 wt%) microwave dielectric ceramics was prepared through conventional solid state reaction. X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) analyses showed that the Li₂TiO₃ phase was transformed into cubic phase LiTiO₂ phase and secondary phase Li₂TiSiO₅. Partial substitution of Mg²⁺ ions for Ti³⁺ ions or Li⁺Ti³⁺ ions increased the cell volume of the LiTiO₂ phase. The dense microstructures were obtained in low Li₂TiO₃ content (x ≤ 65 wt%) samples sintered at 900 °C, whereas the small quantity of pores presented in high Li₂TiO₃ content (x ≥ 75 wt%) samples sintered at 900 °C and low Li₂TiO₃ content (x = 45 wt%) sintered at 850 and 950 °C. Samples at x = 45 wt% under sintering at 900 °C for 4 h showed excellent microwave dielectric properties of ε_r = 10.7, high Q × f = 237,400 GHz and near-zero τ_f = − 3.0 ppm/°C. The ceramic also exhibited excellent chemical compatibility with Ag. Thus, the fabricated material could be a possible candidate for low temperature co-fired ceramic (LTCC) applications.     

Influence of Li2O–B2O3–SiO2 glass on the sintering behavior and microwave dielectric properties of BaO–0.15ZnO–4TiO2 ceramics

This paper reports the investigation of the performance of Li₂O–B₂O₃–SiO₂ (LBS) glass as a sintering aid to lower the sintering temperature of BaO–0.15ZnO–4TiO₂ (BZT) ceramics, as well as the detailed study on the sintering behavior, phase evolution, microstructure and microwave dielectric properties of the resulting BZT ceramics. The addition of LBS glass significantly lowers the sintering temperature of the BZT ceramics from 1150 °C to 875–925 °C. Small amount of LBS glass promotes the densification of BZT ceramic and improves the dielectric properties. However, excessive LBS addition leads to the precipitation of glass phase and growth of abnormal grain, deteriorating the dielectric properties of the BZT ceramic. The BZT ceramic with 5 wt% LBS addition sintered at 900 °C shows excellent microwave dielectric properties: ε_r=27.88, Q×f=14,795 GHz.     

Microwave dielectric properties of novel temperature stable high Q Li2Mg1−xZnxTi3O8 and Li2A1−xCaxTi3O8 (A = Mg,Zn) ceramics

The Li₂Mg_1?xZn_xTi₃O₈ (x = 0–1) and Li₂A_1?xCa_xTi₃O₈ (A = Mg, Zn and x = 0–0.2) ceramics are synthesized by solid-state ceramic route and the microwave dielectric properties are investigated. The Li₂MgTi₃O₈ ceramic shows ?_r = 27.2, Q_u × f = 42,000 GHz, and τ_f = (+)3.2 ppm/°C and Li₂ZnTi₃O₈ has ?_r = 25.6, Q_u × f = 72,000 GHz, and τ_f = (?)11.2 ppm/°C respectively when sintered at 1075 °C/4 h. The Li₂Mg_0.9Zn_0.1Ti₃O₈ dielectric ceramic composition shows the best dielectric properties with ?_r = 27, Q_u × f = 62,000 GHz, and τ_f = (+)1.1 ppm/°C. The effect of Ca substitution on the structure, microstructure and microwave dielectric properties of Li₂A_1?xCa_xTi₃O₈ (A = Mg, Zn and x = 0–0.2) has also been investigated. The materials reported in this paper are excellent in terms of dielectric properties and cost of production compared to commercially available high Q dielectric resonators.     

Improvement of the dielectric properties of rutile-doped Al2O3 ceramics by annealing treatment

The microwave dielectric properties of alumina (Al₂O₃) ceramics were studied. The objectives were to improve the large negative temperature coefficient of the resonant frequency (τ_f) of Al₂O₃ ceramics and to obtain a relatively large quality factor (Qf) through the addition of rutile (TiO₂), which has a large positive τ_f, and an annealing treatment. A near-zero τ_f (+1.5 ppm/°C), excellent Qf (148,000 GHz) and ɛ_r (12.4) were obtained in 0.9 Al₂O₃–0.1 TiO₂ ceramics sintered at 1350 °C for 2 h, followed by annealing at 1100 °C for 12 h in air.     

Microstructure and microwave dielectric properties of (1 − x)Ca0.6La0.267TiO3–xCa(Mg1/3Nb2/3)O3 ceramics

(1 ? x)Ca_0.6La_0.267TiO₃–xCa(Mg_1/3Nb_2/3)O₃ ceramics were prepared by a conventional solid-state ceramic route. The microstructure and microwave dielectric properties were investigated as a function of composition and sintering temperature. As the content of Ca(Mg_1/3Nb_2/3)O₃ increased, the temperature coefficient of resonant frequency (τ_f) value decreased gradually. By appropriately adjusting the x value in the present ceramic system, a near-zero τ_f value could be achieved. The appropriate increase of sintering temperature could significantly improve Q·f value by influencing the grain growth. The optimal microwave dielectric properties with a dielectric constant (?_r) of 52.4, Q·f of 36,428 GHz (at 5.8 GHz), and τ_f of 3.4 ppm/°C were obtained for the specimen 0.5Ca_0.6La_0.267TiO₃–0.5Ca(Mg_1/3Nb_2/3)O₃ sintered at 1490 °C for 4 h.     

Microwave dielectric properties and compatibility with silver electrode of novel low-fired Ba4CuTi11O27 ceramic

A novel low-fired microwave dielectric ceramic with composition of Ba₄CuTi₁₁O₂₇ was prepared by a conventional solid-state reaction method. A single-phase Ba₄CuTi₁₁O₂₇ ceramic could be well densified after sintering above 950 °C for 4 h in air. A refinement using X-ray powder diffraction data was carried out in the Rietveld method using the parameters of Ba₄Ti₁₂O₂₇ as a starting model. Ba₄CuTi₁₁O₂₇ ceramic sintered at 975 °C has a monoclinic structure (C12/m1) with lattice parameters of a=19.8061(4) Å, b=11.4456(2) Å, c=9.9131(2) Å, β=108.8988(15) Å, V=2126.08(8) Å³, Z=4. The Ba₄CuTi₁₁O₂₇ ceramics exhibited a low sintering temperature (~975 °C) and good microwave dielectric properties with Q×f value of 15,040 GHz, ε_r of 36.3 and τ_f value 11.9 ppm/°C. More importantly, the Ba₄CuTi₁₁O₂₇ dielectrics demonstrated good chemical compatibility with Ag when sintered at 950 °C, keeping excellent microwave dielectric properties with Q×f=12,130 GHz, ε_r=36.1, τ_f=12.1 ppm/°C, which indicates that Ba₄CuTi₁₁O₂₇ ceramic is a candidate for LTCC devices.     

Effect of B2O3 and CuO additives on the sintering temperature and microwave dielectric properties of Ba(Zn1/3Nb2/3)O3 ceramics

The B₂O₃ added Ba(Zn_1/3Nb_2/3)O₃ (BBZN) ceramic was sintered at 900 °C. BaB₄O₇, BaB₂O₄, and BaNb₂O₆ second phases were found in the BBZN ceramic. Since BaB₄O₇ and BaB₂O₄ second phases have an eutectic temperature around 900 °C, they might exist as the liquid phase during sintering at 900 °C and assist the densification of the BZN ceramics. Microwave dielectric properties of dielectric constant (ɛ_r) = 32, Q × f = 3500 GHz, and temperature coefficient of resonance frequency (τ_f) = 20 ppm/°C were obtained for the BZN with 5.0 mol% B₂O₃ sintered at 900 °C for 2 h. The BBZN ceramics were not sintered below 900 °C and the microwave dielectric properties of the BBZN ceramics sintered at 900 °C were very low. However, when CuO was added, BBZN ceramic was well sintered even at 875 °C. The liquid phase related to the BaCu(B₂O₅) second phase could be responsible for the decrease of sintering temperature. Good microwave dielectric properties of ɛ_r = 36, Q × f = 19,000 GHz and τ_f = 21 ppm/°C can be obtained for CuO doped BBZN ceramics sintered at 875 °C for 2 h.     

The effect of sintering temperature and time on microwave properties of Sm(Zn1/2Ti1/2)O3 ceramics for resonators

The microwave dielectric properties of Sm(Zn_1/2Ti_1/2)O₃ ceramics have been investigated. Sm(Zn_1/2Ti_1/2)O₃ ceramics were prepared by conventional solid-state route with various sintering temperatures and times. The prepared Sm(Zn_1/2Ti_1/2)O₃ exhibited a mixture of Zn and Ti showing 1:1 order in the B-site. Higher sintered density of 7.01 g/cm³ can be produced at 1310 °C for 2 h. The dielectric constant values (ɛ_r) of 22–31 and the Q × f values of 4700–37,000 (at 8 GHz) can be obtained when the sintering temperatures are in the range of 1250–1370 °C for 2 h. The temperature coefficient of resonant frequency τ_f was a function of sintering temperature. The ɛ_r value of 31, Q  ×  f value of 37,000 (at 8 GHz) and τ_f value of −19 ppm/°C were obtained for Sm(Zn_1/2Ti_1/2)O₃ ceramics sintered at 1310 °C for 2 h. For applications of high selective microwave ceramic resonator, filter and antenna, Sm(Zn_1/2Ti_1/2)O₃ is proposed as a suitable material candidate.     

Effect of BaCu(B2O5) on the sintering and microwave dielectric properties of Ca0.4Li0.3Sm0.05Nd0.25TiO3 ceramics

The sintering behaviors and microwave dielectric properties of the Ca_0.4Li_0.3Sm_0.05Nd_0.25TiO₃ (abbreviated CLSNT) ceramics with different amounts of BaCu(B₂O₅) addition were investigated in this paper. Adding BaCu(B₂O₅) to CLSNT lowered its sintering temperature from 1300 °C to 925 °C. No secondary phase was observed in the CLSNT ceramics and complete solid solution of the complex perovskite phase was confirmed. The CLSNT ceramics with small amounts of BaCu(B₂O₅) addition could be well sintered at 925 °C without much degradation in the microwave dielectric properties. Especially, the 1.75 wt.% BaCu(B₂O₅)-doped CLSNT ceramic sample sintered at 925 °C for 3 h had optimum microwave dielectric properties of ε_r = 93.5 ± 3.2, Q × f = 6486 ± 434 GHz, and τ_f = 5 ± 1.5 ppm/°C (at 3–4 GHz), enabling it a promising candidate material for LTCC applications. Obviously, BaCu(B₂O₅) could be a suitable sintering aid to facilitate the densification and microwave dielectric properties of the CLSNT ceramics.     

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.     

Crystal structure,mixture behavior,and microwave dielectric properties of novel temperature stable (1 − x)MgMoO4 − xTiO2 composite ceramics

Novel temperature stable MgMoO₄–TiO₂ microwave dielectric ceramics were prepared by a solid state reaction process at low temperature (950 °C). As TiO₂ content increases, the relative permittivity increases while the Q × f value decreases, and the variation mechanisms are proposed, respectively. The temperature coefficient of resonant frequency (τ_f) shifts to the positive direction as TiO₂ is added. The mixture mechanisms of τ_f value for two-phase composite materials are supposed. A near-zero τ_f value (3.2 ppm/°C) is obtained when x = 0.3, with ε_r = 9.13 ± 0.03 and Q × f = 11,990 GHz. The 0.7MgMoO₄–0.3TiO₂ composites are considered to be appropriate as a low temperature co-fired ceramic material for microwave wireless communication applications.     

Low-dielectric-constant LiAlO2 ceramics combined with LBSCA glass for LTCC applications

This study used Li₂O–B₂O₃–SiO₂–CaO–Al₂O₃ (LBSCA) glass to reduce the sintering temperature of LiAlO₂ ceramics and to realise the low dielectric constants (ɛ_r<5) of low-temperature co-fired ceramic (LTCC) materials. LBSCA glass remarkably enhanced the densification of LiAlO₂ ceramics. X-ray diffraction patterns indicated that only the γ-LiAlO₂ phase occurred within the doping range of 1 wt% to 3.5 wt%. Scanning electron microscopy images showed dense and uniform grains in samples with 3.0 wt% LBSCA glass. These samples also exhibited low dielectric constants and low dielectric loss when sintered at 900 °C and 950 °C (i.e., ɛ_r=4.48, Qf=35,540 GHz and τ_f=−53 ppm/°C at 900 °C; ɛ_r=4.50, Qf=38,979 GHz and τ_f=−55 ppm/°C at 950 °C, respectively). The material prepared was chemically compatible with silver and showed potential in applications of high-frequency LTCC microwave substrates.     

Structure and dielectric properties of novel low temperature co-fired Bi2O3-RE2O3-MoO3 (RE=Pr,Nd, Sm,and Yb) based microwave ceramics

Novel monoclinic Bi₂O₃-xRE₂O₃-yMoO₃ (RE=Pr, Nd, Sm, and Yb) based low temperature co-fired ceramics (LTCC) systems with high sintering density and low microwave dielectric loss are synthesized by conventional solid state reaction technique. The structure and dielectric properties of Bi₂O₃-xRE₂O₃-yMoO₃ ceramics are investigated. Dense BiNdMoO₆ ceramics sintered at 900 °C for 8 h in air have a low dielectric constant ε_r=~7.5, a high quality factor Q×f=~ 24, 800 GHz at 7.0 GHz, and τ_f=~−16 ppm/̊C. Especially, good chemical compatibility of BiNdMoO₆ with Ag electrodes is represented as well. In contrast, BiSmMoO₆ ceramics sintered at 1000 °C for 8 h show enhanced Q×f=~43, 700 GHz at 7.8 GHz with ε_r=~8.5 and τ_f=~−27 ppm/°C. Bi₂O₃-xRE₂O₃-yMoO₃ (RE=Pr, Nd, Sm, and Yb) based ceramics could be considered as promising microwave ceramics for LTCC applications.     

Synthesis and properties of TiO2 added NiNb2O6 microwave dielectric ceramics using a simple process

TiO₂ added NiNb₂O₆ ceramics produced using a reaction-sintering process were investigated. Pure columbite NiNb₂O₆ could be obtained without TiO₂ addition. With 30 and 40 mol% TiO₂ addition, a phase with the same structure of Ni_0.5Ti_0.5NbO₄ formed. Grain growth is easier in pellets with 30 and 40 mol% TiO₂ addition than in the NiNb₂O₆ pellets. Microwave dielectric properties: ?_r = 20.7, Q × f = 19,800 GHz (at 9 GHz) and τ_f = ?31.9 ppm/°C were obtained for NiNb₂O₆ pellets sintered at 1300 °C/2 h. ?_r around 45, Q × f = 5400–7700 GHz (at 6 GHz) and τ_f ～ 73 ppm/°C were obtained in pellets with 30 mol% TiO₂ addition. ?_r around 50, Q × f = 3800–5700 GHz (at 6 GHz) and τ_f ～ 99 ppm/°C were obtained in pellets with 40 mol% TiO₂ addition.     

Temperature stable and high-Q microwave dielectric ceramics in the Li2Mg3−xCaxTiO6 system (x=0.00–0.18)

Microwave dielectric properties of Li₂Mg_3−xCa_xTiO₆ (x=0–0.18) ceramics were studied using a conventional solid-state route to find temperature stable and high Q microwave ceramics. As the calcination temperature was 500 °C, the Li₂TiO₃ phase with monoclinic rock salt structure in C2/c space group started to form. When the samples were calcined from 600 °C to 900 °C, the XRD patterns exhibited a remarkable chemical reaction between the MgO and Li₂TiO₃ phases, which eventually formed the Li₂Mg₃TiO₆ phase. The results indicated the Li₂Mg₃TiO₆and CaTiO₃ co-existed with each other and formed a stable composite system when the calcium content was added. The SEM photographs indicated that the pores caused by the Li evaporation could be effectively reduced due to the appearance of CaTiO₃. As x was increased from 0 to 0.18, the relative density was significantly improved due to the elimination of pores. As the Ca content increased, the dielectric constant (εr) increased from 14.8 to 20.6; the quality factor (Q×f) decreased from 148,713 GHz to 79,845 GHz, and the temperature coefficient of resonant frequency (τf) significantly increased from −42.4 to +10.8 ppm/°C due to the increased amount of CaTiO₃. Therefore, at x=0.12, the LMCxT ceramics sintered at 1280 °C for 6 h displayed excellent comprehensive properties of εr=17.8,Q×f=102,246 GHz and τf=−0.7 ppm/°C.