Characterization and dielectric behavior of willemite and TiO2-doped willemite ceramics at millimeter-wave frequency

Willemite ceramics (Zn₂SiO₄) have been successfully prepared in the temperature range from 1280 to 1340 °C. It is found that willemite ceramics possess excellent millimeter-wave dielectric properties: a dielectric constant ɛ_r value of 6.6, a quality factor Q × f value of 219,000 GHz and a temperature coefficient of resonant frequency τ_f value of −61 ppm/°C. By adding TiO₂ with large positive τ_f value (450 ppm/°C), near zero τ_f value can be achieved in a wide sintering temperature range. With 11 wt% of TiO₂, an ɛ_r value of 9.3, a Q × f value of 113,000 GHz, and a τ_f value of 1.0 ppm/°C are obtained at 1250 °C. The relationships between microstructure and properties are also studied. Our results show that willemite with appropriate TiO₂ is an ideal temperature stable, low ɛ_r and high Q × f dielectric for millimeter-wave application.     

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.     

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.     

Effect of B2O3 on the sintering and microwave dielectric properties of M-phase LiNb0.6Ti0.5O3 ceramics

The effect of B₂O₃ addition on the sintering, microstructure and the microwave dielectric properties of LiNb_0.6Ti_0.5O₃ ceramics have been investigated. It is found that low-level doping of B₂O₃ (≤2 wt.%) can significantly improve the densification and dielectric properties of LiNb_0.6Ti_0.5O₃ ceramics. Due to the liquid phase effect of B₂O₃ addition, LiNb_0.6Ti_0.5O₃ ceramics could be sintered to a theoretical density higher than 95% even at 880 °C. No secondary phase was observed for the B₂O₃-doped ceramics. There is no obvious degradation in dielectric properties for the ceramics with B₂O₃ additions. In the case of 1 wt.% B₂O₃ addition, the ceramics sintered at 880 °C show good microwave dielectric properties of ɛ_r = 70, Q × f = 5400 GHz, τ_f = −6.39 ppm/°C. It represents that the ceramics could be promising for multilayer low-temperature co-fired ceramics (LTCC) applications.     

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.     

Microwave dielectric properties of Ca(Li1/4Nb3/4)O3–CaTiO3 ceramic systems

The microwave dielectric properties of Ca(Li_1/4Nb_3/4)O₃–CaTiO₃ ceramics have been investigated with regard to calcination temperature and the amount of CaTiO₃ additive. Ca(Li_1/4Nb_3/4)O₃ ceramics with an orthorhombic crystal structure can be synthesized by the conventional mixed oxide method by calcining at 750 °C and sintering at 1275 °C. The dielectric constant (ɛ_r), quality factor (Q × f₀) and temperature coefficient of resonant frequency (τ_f) for Ca(Li_1/4Nb_3/4)O₃ ceramics are 26, 13,000 GHz and −49 ± 2 ppm/°C, respectively. With increase in the CaTiO₃ content, ɛ_r and τ_f are increased and the quality factor decreased due to the solid-solution formation between Ca(Li_1/4Nb_3/4)O₃ and CaTiO₃. The 0.7Ca(Li_1/4Nb_3/4)O₃–0.3CaTiO₃ ceramic exhibits ɛ_r of 44, quality factor (Q × f₀) of 12,000 GHz and τ_f of −9 ± 1 ppm/°C.     

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.     

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.     

A novel low-firing microwave dielectric ceramic Li2ZnGe3O8 with cubic spinel structure

A Li₂ZnGe₃O₈ ceramic was investigated as a promising microwave dielectric material for low-temperature co-fired ceramics applications. Li₂ZnGe₃O₈ ceramic was prepared via the conventional solid-state method. X-ray diffraction data shows that Li₂ZnGe₃O₈ ceramic crystallized into a cubic spinel structure with a space group of P4₁32. Dense ceramic with a relative densities of 96.3% were obtained when sintered at 945 °C for 4 h and exhibited the optimum microwave properties with a relative permittivity (ε_r) of 10.3, a quality factor (Q × f) of 47,400 GHz (at 13.3 GHz), and a temperature coefficient of resonance frequency (τ_f) of −63.9 ppm/°C. The large negative τ_f of Li₂ZnGe₃O₈ ceramic could be compensated by rutile TiO₂, and 0.9Li₂ZnGe₃O₈–0.1TiO₂0·1TiO₂ ceramic sintered at 950 °C for 4 h exhibited improved microwave dielectric properties with a near-zero τ_f of −1.6 ppm/°C along with ε_r of 11.3 and a Q × f of 35,800 GHz (11.6 GHz). Moreover, Li₂ZnGe₃O₈ was found to be chemically compatible with silver electrode when sintered at 945 °C.     

Microwave dielectric properties and low-temperature sintering of Ba3Ti4Nb4O21 ceramics with B2O3 and CuO additions

The low sintering temperature and the good dielectric properties such as high dielectric constant (ɛ_r), high quality factor (Q × f) and small temperature coefficient of resonant frequency (τ_f) are required for the application of chip passive components in the wireless communication technologies. In the present study, the sintering behaviors and dielectric properties of Ba₃Ti₄Nb₄O₂₁ ceramics were investigated as a function of B₂O₃–CuO content. Ba₃Ti₄Nb₄O₂₁ ceramics with B₂O₃ or CuO addition could be sintered above 1100 °C. However, the additions of both B₂O₃ and CuO successfully reduced the sintering temperature of Ba₃Ti₄Nb₄O₂₁ ceramics from 1350 to 900 °C without detriment to the microwave dielectric properties. From the X-ray diffraction (XRD) studies, the sintering behaviors and the microwave dielectric properties of low-fired Ba₃Ti₄Nb₄O₂₁ ceramics were examined and discussed in the formation of the secondary phases. The Ba₃Ti₄Nb₄O₂₁ sample with 1 wt% B₂O₃ and 3 wt% CuO addition, sintered at 900 °C for 2 h, had the good dielectric properties: ɛ_r = 65, Q × f = 16,000 GHz and τ_f = 101 ppm/°C.     

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.     

Microwave dielectric properties of YAG ceramics prepared by sintering pyrolysised nano-sized powders

YAG ceramics with good dielectric properties were prepared via a modified pyrolysis method, with yttrium nitrate as the yttrium source and combined aluminium sulphate and aluminium nitrate as aluminium sources, and subsequent sintering in a muffle furnace. The effects of the different aluminium sources on the powder characteristic and the impact of sintering temperature, sintering aids (TEOS) and additive (TiO₂) on the dielectric properties of the ceramics were studied. The results show that well-dispersed pure YAG nano-powders can be obtained after calcination at 1000 °C with an aluminium sulphate and aluminium nitrate molar ratio of 1.5:2. The relative density, permittivity (ε_r) and quality factor (Q×f) of the YAG ceramics increase with sintering temperature and TEOS addition. TiO₂ can greatly promote τ_f to near-zero but decreases Q×f. The relative density, ε_r, Q×f and τ_f of the YAG–1 wt% TEOS–1 wt% TiO₂ ceramic obtained at 1520 °C are 97.6%, 9.9, 71, 738 GHz and −30 ppm/°C, respectively.     

Microwave dielectric properties of porous Mg2SiO4 filling with TiO2 prepared by a liquid phase deposition process

Forsterite (Mg₂SiO₄) possesses a high quality factor (Q·f) of 270,000 GHz and a low dielectric constant ɛ_r of 6.8. However, it shows a relatively large negative temperature coefficient of resonant frequency τ_f of −73 ppm/°C. For microwave telecommunication, a τ_f of nearly 0 ppm/°C is desirable to keep the frequency stability. In order to improve τ_f, we have tried to produce pure Mg₂SiO₄–TiO₂ composite ceramics with no secondary phases using a liquid phase deposition (LPD) method. Porous Mg₂SiO₄ ceramics was prepared by sintering Mg₂SiO₄ with polymethyl methacrylate (PMMA) particles, and then TiO₂ was filled in the pores of Mg₂SiO₄ by the LPD method. The porosity and microstructure of porous Mg₂SiO₄ was controlled by amount and particle sizes of PMMA and formation process. τ_f of Mg₂SiO₄ filled with TiO₂ by LPD method was improved to −46 ppm/°C.     

Low temperature sintering of BaCu(B2O5)-added BaO–Re2O3–TiO2 (Re = Sm,Nd) ceramics

The sintering temperature of BaSm₂Ti₄O₁₂ (BST) and BaNd₂Ti₅O₁₄ (BNT) ceramics was approximately 1350 °C and decreased to 875 °C with the addition of BaCu(B₂O₅) (BCB) ceramic powder. The presence of the liquid phase was responsible for the decrease of the sintering temperature. The liquid phase is considered to have a composition similar to the BaO-deficient BCB. The bulk density and dielectric constant (ɛ_r) of the specimens increased and reached saturated value with increasing BCB content. The Q-value initially increased with the addition of BCB, but decreased considerably when a large amount of BCB was added, because of the presence of the liquid phase. Good microwave dielectric properties of Q × f = 4500 GHz, ɛ_r = 60 and τ_f = −30 ppm/°C were obtained for the 16.0 mol% BCB-added BST ceramics sintered at 875 °C for 2 h. Moreover, the BST and BNT ceramics containing BCB show good compatibility with silver metal.     

Mixture behavior and microwave dielectric properties of (1 − x)Ca2P2O7–xTiO2

(1 − x)β-Ca₂P₂O₇–xTiO₂ were prepared by solid-state reaction. The mixture behavior and microwave dielectric properties were investigated using X-ray powder diffraction and a network analyzer, respectively. X-ray powder diffraction patterns showed that β-Ca₂P₂O₇ and TiO₂ existed in a mixture form, which was also confirmed by SEM analysis. It was shown that TiO₂, which has positive temperature coefficient of the resonant frequency (τ_f), compensated the negative τ_f of β-Ca₂P₂O₇ (−53 ppm/°C) through mixture formation. The variation of dielectric properties with a function of TiO₂ contents could be explained using mixture rule. In the 0.3 < x < 0.4 regions, τ_f value could be successfully reduced almost zero. In particular, at x = 0.3, good microwave dielectric properties was obtained: Q × f = 44,000, ɛ_r = 10.9, and τ_f = −11 ppm/°C.     

Microwave dielectric properties of CeO2–0.5AO–0.5TiO2 (A = Ca,Mg, Zn,Mn, Co,Ni, W) ceramics

Microwave dielectric ceramic materials based on cerium [CeO₂–0.5AO–0.5TiO₂ (A = Mg, Zn, Ca, Mn, Co, Ni, W)] have been prepared by a conventional solid state ceramic route. The crystal structure was studied by X-ray diffraction, microstructure by scanning electron microscopy (SEM) techniques and the phase composition was studied using energy dispersive X-ray analysis (EDXA). The sintered ceramics had a relative dielectric constant (ɛ_r) in the range 17–65 and quality factor Q_uxf up to 50,000 GHz and a temperature variation of resonant frequency (τ_f) ranging from a negative value (−62 ppm/°C) to a high positive value (+399 ppm/°C). The majority of the synthesized ceramics were of a two phase composite consisting of a fluorite CeO₂ and perovskite ATiO₃ phase. The microwave dielectric properties were further tailored by adding various amounts of dopants of different valencies to the calcined powder. This made it possible to either tune τ_f to zero or improved the quality factor further.     

Effect of dopants on synthesis of BaTi4O9 and Ba2Ti9O20 ceramics prepared by reaction-sintering process

The effect of dopants on BaTi₄O₉ (BT4) and Ba₂Ti₉O₂₀ (B2T9) ceramics by the reaction-sintering process was investigated. CuO addition is more effective in lowering the sintering temperature of BT4 and B2T9 ceramics. MnO₂ and CuO addition are effective to obtain temperature stable BT4 ceramics. With MnO₂ addition, Q × f of BT4 ceramics could be raised. ZrO₂ addition is effective to obtain B2T9 ceramics with higher dielectric constant. With CuO addition, τ_f of B2T9 ceramics shifted toward negative values and 0 ppm/°C could be obtained. Optimum properties in BT4 doped with MnO₂ of ɛ_r = 37.1, Q × f = 51,200 GHz (at 7 GHz) and τ_f = 0 ppm/°C and in B2T9 doped with ZrO₂ of ɛ_r = 37.9, Q × f = 39,700 GHz (at 7 GHz) and τ_f = 5.9 ppm/°C were obtained.     

Mixture behavior and microwave dielectric properties of (1 − x)CaWO4–xTiO2

The microwave dielectric characteristics and mixture behavior of (1 − x)CaWO₄–xTiO₂ ceramics prepared with conventional solid-state route were studied using a network analyzer and X-ray power diffraction, respectively. The CaWO₄ compound had good properties (low permittivity and high quality factor) for microwave applications, but it had a high negative temperature coefficient of resonant frequency (τ_f = −53). Hence, in order to tune the dielectric properties, (1 − x)CaWO₄–xTiO₂ were prepared for different values of x. X-ray powder diffraction and SEM analysis revealed that CaWO₄ and TiO₂ coexisted as a mixture. The mixture formation and dielectric properties could be explained by mixture rule. In particular, at x = 2.6, good microwave dielectric properties were obtained: Qf = 27,000, ɛ_r = 17.48, and τ_f = ∼0 ppm/°C.     

High-Q microwave dielectrics in low-temperature sintered (Zn1−xNix)3Nb2O8 ceramics

The effects of Ni substitution for Zn on microwave dielectric properties of (Zn_1−xNi_x)₃Nb₂O₈ (x = 0.02–0.08) ceramics were investigated in this study. The XRD patterns of the sintered samples reveal single-phase formation with a monoclinic structure. The tremendous improvement of Q × f value can be achieved by a small level of Ni substitution (x = 0.05). The τ_f value was found to decrease with a decreasing A-site bond valence. In addition, B₂O₃ and CuO were used as a sintering aid to lower the sintering temperature from 1180 to 900 °C. Excellent microwave dielectric properties (ɛ_r ∼ 20.7, Q × f ∼ 98,000 GHz and τ_f ∼ −85.2 ppm/°C) and a chemical compatibility with Ag electrodes can be obtained for 4 wt% B₂O₃–CuO doped (Zn_0.95Ni_0.05)₃Nb₂O₈ ceramics sintered at 930 °C for 2 h. This constitutes a very promising material for LTCC applications.     

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.