Tunable and microwave dielectric properties of Ba0.5Sr0.5TiO3-BaWO4 composite ceramics doped with Co2O3

Co₂O₃ doped BaWO₄-Ba_0.5Sr_0.5TiO₃ composite ceramics, prepared by solid-state route, were characterized systematically, in terms of their phase compositions, microstructure and microwave dielectric properties. Doping of Co₂O₃ promoted grain growth, reduced Curie temperature and broadened phase-transition temperature range of BaWO₄-Ba_0.5Sr_0.5TiO₃, which were attributed mainly to the substitution of Co³⁺ for Ti⁴⁺ at B site in the perovskite lattice. Dielectric diffusion behaviors of the composite ceramics were discussed. The composite ceramics all had dielectric tunability of higher than 10% at 30 kV/cm and 10 kHz, with promising microwave dielectric properties. Specifically, the sample doped with 0.2 wt.% Co₂O₃ exhibited a tunability of 20%, permittivity of 225 and Q of 292 (at 1.986 GHz), making it a suitable candidate for applications in electrically tunable microwave devices.     

Influence of Bi2O3 and CuO addition on low-temperature sintering and dielectric properties of Ba0.6Sr0.4TiO3 ceramics

In this study, we tried to lower the sintering temperature of Ba_0.6Sr_0.4TiO₃ (BST) ceramics by several kinds of adding methods of Bi₂O₃, CuO and CuBi₂O₄ additives. The effects of different adding methods on the microstructures and the dielectric properties of BST ceramics have been studied. In the all additive systems, the single addition of CuBi₂O₄ was the most effective way for lowering the sintering temperature of BST. When CuBi₂O₄ of 0.6 mol% was mixed with starting BST powders and sintered at 1100 °C, the derived ceramics demonstrated dense microstructure with a low dielectric constant (? = 4240), low dielectric loss (tan δ = 0.0058), high tunability (Tun = 38.3%) and high Q value (Q = 251). It was noteworthy that the sintering temperature was significantly lowered by 350 °C compared with no-additive system, and the derived ceramics maintained the excellent microwave dielectric properties corresponding to pure BST.     

Effect of MgWO4 content on properties of Ba0.5Sr0.5TiO3 composite ceramics for tunable microwave applications

xMgWO₄-(1 − x) Ba_0.5Sr_0.5TiO₃ (x = 0.0, 5.0, 15.0, 25.0 and 35.0 wt%) composite ceramics were prepared via solid state reaction processing. Their structural and dielectric properties were systematically characterized. A significant increase in grain size was observed with increasing MgWO₄ content, which was accompanied by obvious variations in dielectric properties of the composite ceramics. It is found that the permittivity peaks of the samples gradually shifted to low temperatures with increasing MgWO₄ content. At the same time, tunabilities of the composite ceramics decreased, but their Q values increased. The sample with 35 wt% MgWO₄ possesses a high tunability of 16.8% (∼10 kHz), a low permittivity of 65 and an appropriate Q value of 309 (∼4.303 GHz), which meet the requirements of high power and impedance matching, thus making it a promising candidate for applications as electrically tunable microwave devices.     

Microstructures and dielectric properties of Ba0.6Sr0.4TiO3-MgO ceramics prepared by non-aqueous gelcasting and dry pressing

Non-aqueous gelcasting and dry pressing were used to prepare 45 wt% Ba_0.6Sr_0.4TiO₃-55 wt% MgO (BSTM) ceramics. The effects of different forming methods on the microstructures and dielectric properties of the BSTM ceramics were investigated. The densities of the BSTM ceramics prepared by non-aqueous gelcasting are lower but more uniform than that of the BSTM ceramics prepared by dry pressing. The XRD analysis illustrates that phase compositions are completely the same no matter what forming method is adopted. The SEM results show that the BSTM green samples and sintered ceramics prepared by non-aqueous gelcasting are more uniform than that prepared by dry pressing. Furthermore, it is found that the BSTM ceramics prepared by non-aqueous gelcasting have higher and more uniform dielectric constant, tunability and loss tangent (measured at 10 kHz and 20 °C). Meanwhile, the BSTM ceramics prepared by non-aqueous gelcasting have higher dielectric constant and lower Q × f value (namely more loss) when they are measured at microwave frequencies.     

High dielectric tunability of Ba0.6Sr0.4TiO3 thin film deposited by radio-frequency magnetron sputtering

In microwave tunable devices, one of the major challenges encountered is the simultaneous minimization of the material's dielectric loss and maximization of dielectric tunability. In this work, Ba_0.6Sr_0.4TiO₃ thin film with the thickness of 300 nm was deposited on Pt/SiO₂/Si substrates using radio-frequency magnetron sputtering technique, and its dielectric properties were investigated. Due to the high temperature annealing process at substrate temperature of 600 °C, well-crystallized Ba_0.6Sr_0.4TiO₃ film was deposited. The dielectric constant and dielectric loss of the film at 100 kHz are 300 and 0.033, respectively. Due to the good crystallinity of the Ba_0.6Sr_0.4TiO₃ films deposited by radio-frequency magnetron sputtering, high dielectric tunability up to 38.3% is achieved at a low voltage of 4.5 V.     

Microwave dielectric properties of LaMgAl11O19

The suitable choice of a substrate material is one of the aims to be fulfilled in high speed microwave technology. LaMgAl₁₁O₁₉ oxide ceramic material, which belongs to the magnetoplumbite family, has been reported earlier as a potential candidate for such applications. This material has been prepared by conventional solid-state ceramic route. The structure has been studied by X-ray diffraction and characterized at microwave frequencies. The effect of dopant and glass addition on the microwave dielectric properties of this material has also been investigated. LaMgAl₁₁O₁₉ has relatively low dielectric constant (ε_r=14), low dielectric loss or high quality factor (Q_u×f>28,000 GHz at 7 GHz) and small temperature variation of resonant frequency (τ_f=−12 ppm/°C) at room temperature (300 K). These properties make LaMgAl₁₁O₁₉ as a good substrate material and as a dielectric resonator to be used in microwave devices operating at relatively high frequencies.     

Optimized microwave dielectric properties of Co- and Ca-substituted Mg0.6Zn0.4TiO3

The system of (1 − y)(Mg_0.6Zn_0.4)_1−xCo_xTiO₃-yCaTiO₃ was investigated to optimize its microwave dielectric properties by adopting appropriate contents of Co and Ca and by controlling sintering conditions. The effect of Co substitution was to enhance densification and Qf value, while the addition of CaTiO₃ resulted in increases of dielectric constant and TCF. As an optimal compositions, 0.93(Mg_0.6Zn_0.4)_0.95Co_0.05TiO₃-0.07CaTiO₃ successfully demonstrated a dielectric constant of 23.04, a Qf of 79,460 GHz and a TCF value of +1.4 ppm/°C after firing at a relatively lower sintering temperature of 1200 °C. The increase of sintering temperature beyond 1200 °C tended to degrade overall microwave dielectric properties presumably due to Zn volatilization as evidenced by the presence of a Zn-deficient phase (MgTi₂O₅) at 1400 °C. An attempt to establish the correlation between microstructure characteristics and dielectric properties was made in this dielectric system where the extensive range of firing temperature up to 1400 °C was evaluated.     

Effect of B2O3-Bi2O3-SiO2-ZnO glass on the sintering and microwave dielectric properties of 0.83ZnAl2O4-0.17TiO2

The 0.83ZnAl₂O₄-0.17TiO₂ (ZAT) ceramics were synthesized by solid state ceramic route. The effect of 27B₂O₃-35Bi₂O₃-6SiO₂-32ZnO (BBSZ) glass on the microwave dielectric properties of ZAT was investigated. The crystal structure and the microstructure of the ceramic-glass composites were studied by X-ray diffraction and scanning electron microscopic techniques. The low frequency dielectric loss was measured at 1 MHz. The dielectric properties of the sintered samples were measured in the microwave frequency range by the resonance method. Addition of 0.2 wt% of BBSZ improved the dielectric properties with quality factor (Q_u × f) > 120,000 GHz, temperature coefficient of resonant frequency (τ_f) = −7.3 ppm/°C and dielectric constant (?_r) = 11.7. Addition of 10 wt% of BBSZ lowered the sintering temperature to about 950 °C with Q_u × f > 10,000 GHz, ?_r = 10 and τ_f = −23 ppm/°C. The reactivity of 10 wt% BBSZ added ZAT with silver was also studied. The results show that ZAT doped with suitable amount of BBSZ glass is a possible material for low-temperature co-fired ceramic (LTCC) application.     

Dielectric properties of (1−x)(Mg0.95Co0.05)TiO3-xCaTiO3 ceramic system at microwave frequency

The microwave dielectric properties and the microstructures of the (1−x)MgTiO₃-xCaTiO₃ ceramic system were investigated. With partial replacement of Mg by Co, dielectric properties of the (1−x)(Mg_0.95Co_0.05)TiO₃-xCaTiO₃ ceramics can be promoted. The microwave dielectric properties are strongly correlated with the sintering temperature. At 1275°C, the 0.95(Mg_0.95Co_0.05)TiO₃-0.05CaTiO₃ ceramics possesses excellent microwave dielectric properties: a dielectric constant ε_r of 20.3, a Q×f value of 107 000 ( at 7 GHz) and a τ_f value of −22.8 ppm/°C. By appropriately adjusting the x value in the (1−x)(Mg_0.95Co_0.05)TiO₃-xCaTiO₃ ceramic system, zero τ_f value can be achieved. With x=0.07, a dielectric constant ε_γ of 21.6, a Q×f value of 92 000 (at 7 GHz) and a τ_f value of −1.8 ppm/°C was obtained for 0.93(Mg_0.95Co_0.05)TiO₃-0.07CaTiO₃ ceramics sintered at 1275°C for 4 h.     

Improved high-Q microwave dielectric resonator using CuO-doped MgNb2O6 ceramics

The microwave dielectric properties and the microstructures of MgNb₂O₆ ceramics with CuO additions (1-4 wt.%) prepared with conventional solid-state route have been investigated. The sintered samples exhibit excellent microwave dielectric properties, which depend upon the liquid phase and the sintering temperature. It is found that MgNb₂O₆ ceramics can be sintered at 1140 °C due to the liquid phase effect of CuO addition. At 1170 °C, MgNb₂O₆ ceramics with 2 wt.% CuO addition possesses a dielectric constant (ε_r) of 19.9, a Q×f value of 110,000 (at 10 GHz) and a temperature coefficient of resonant frequency (τ_f) of −44 ppm/°C. The CuO-doped MgNb₂O₆ ceramics can find applications in microwave devices requiring low sintering temperature.     

Modified tunable dielectric properties by addition of MgO on BaZr0.25Ti0.75O3 ceramics

Phase composition, microstructure and tunable dielectric properties of (1 − x)BaZr_0.25Ti_0.75O₃-xMgO (BZTM) composite ceramics fabricated by solid-state reaction were investigated. It was found Mg not only existed in the matrix as MgO, there was also trace amount of Mg²⁺ ions dissolved in the BZT grains, which led to Curie temperature of the BZTM composites ceramics shifting to below −100 °C. Dielectric permittivity of the BZTM composite ceramics was reduced from thousands to hundreds by manipulating the content of MgO. Johnson's phenomenological equation based on Devonshire's theory was used to describe the nonlinear dielectric permittivity of the ceramics with increasing applied DC field. With increasing content of MgO, anharmonic constant α_(T) increased monotonously. Dielectric permittivity was 672, while dielectric tunability was as high as 30.0% at 30 kV/cm and dielectric loss was around 0.0016 for the 0.6BaZr_0.25Ti_0.75O₃-0.4MgO sample at 10 kHz and room temperature.     

Low-temperature sintering and compatibility with silver electrode of Ba4MgTi11O27 microwave dielectric ceramic

Ba₄MgTi₁₁O₂₇ microwave dielectric ceramic was investigated using X-ray diffraction, scanning electron microscopy and dielectric measurement. The pure Ba₄MgTi₁₁O₂₇ ceramic shows a high sintering temperature (∼1275 °C) and good microwave dielectric properties as Q × f of 19,630 GHz, ?_r of 36.1, τ_f of 14.6 ppm/°C. It was found that the addition of BaCu(B₂O₅) (BCB) can effectively lower the sintering temperature from 1275 to 925 °C, and does not induce much degradation of the microwave dielectric properties. The BCB-doped Ba₄MgTi₁₁O₂₇ ceramics can be compatible with Ag electrode, which makes it a promising ceramic for LTCC technology application.     

Microwave dielectric properties and microstructures of the 11Li2O-3Nb2O5-12TiO2 ceramics with B2O3 addition

The effects of B₂O₃ addition, as a sintering agent, on the sintering behavior, microstructure and microwave dielectric properties of the 11Li₂O-3Nb₂O₅-12TiO₂ (LNT) ceramics have been investigated. With the low-level doping of B₂O₃ (≤2 wt.%), the sintering temperature of the LNT ceramic could be effectively reduced to 900 °C. The B₂O₃-doped LNT ceramics are also composed of Li₂TiO₃ss and “M-phase” phases. No other phase could be observed in the 0.5-2 wt.% B₂O₃-doped ceramics sintered at 840-920 °C. The addition of B₂O₃ induced no obvious degradation in the microwave dielectric properties but increased the τ_f values. Typically, the 0.5 wt.% B₂O₃-doped ceramics sintered at 900 °C have better microwave dielectric properties of ?_r = 49.2, Q × f = 8839 GHz, τ_f = 57.6 ppm/°C, which suggest that the ceramics could be applied in multilayer microwave devices requiring low sintering temperatures.     

Dielectric properties of a new ceramic system (1 − x)Mg4Nb2O9-xCaTiO3 at microwave frequency

The microstructures and the microwave dielectric properties of the (1 − x)Mg₄Nb₂O₉-xCaTiO₃ ceramic system were investigated. In order to achieve a temperature-stable material, CaTiO₃ (τ_f ∼ 800 ppm/°C) was chosen as a τ_f compensator and added to Mg₄Nb₂O₉ (τ_f ∼ −70 ppm/°C) to form a two phase system. It was confirmed by the XRD and EDX analysis. By appropriately adjusting the x-value in the (1 − x)Mg₄Nb₂O₉-xCaTiO₃ ceramic system, near-zero τ_f value can be achieved. A new microwave dielectric material, 0.5Mg₄Nb₂O₉-0.5CaTiO₃ applicable in microwave devices is suggested and possesses the dielectric properties of a dielectric constant ?_r ∼ 24.8, a Q × f value ∼82,000 GHz (measured at 9.1 GHz) and a τ_f value ∼−0.3 ppm/°C.     

Structural, spectroscopic and dielectric investigations on Ba8Zn(Nb6−xSbx)O24 microwave ceramics

Ba₈Zn(Nb_6−xSb_x)O₂₄ (x = 0, 0.3, 0.6, 0.9, 1.2, 1.5, 1.8 and 2.4) ceramics were prepared through the conventional solid-state route. The materials were calcined at 1250 °C and sintered in the range 1400-1425 °C. The structure of the system was analyzed by X-ray diffraction, Fourier transform infrared and Raman spectroscopic methods. The theoretical and experimental densities were calculated. The microstructure of the sintered pellets was analyzed using scanning electron microscopy. The low frequency dielectric properties were studied in the frequency range 50 Hz-2 MHz. The dielectric constant (?_r), temperature coefficient of resonant frequency (τ_f) and the unloaded quality factor (Q_u) are measured in the microwave frequency region using cavity resonator method. The τ_f values of the samples reduced considerably with the increase in Sb concentration. The materials have intense emission lines in the visible region. The compositions have good microwave dielectric properties and photoluminescence and hence are suitable for dielectric resonator and ceramic laser applications.     

Improved microwave dielectric properties of B2O3-doped Nd(Co1/2Ti1/2)O3 ceramics with near zero temperature coefficient of resonant frequency

The microwave dielectric properties and the microstructures of Nd(Co_1/2Ti_1/2)O₃ ceramics prepared by conventional solid-state route have been studied. The prepared Nd(Co_1/2Ti_1/2)O₃ exhibited a mixture of Co and Ti showing 1:1 order in the B-site. It is found that low-level doping of B₂O₃ (up to 0.75 wt.%) can significantly improve the density and dielectric properties of Nd(Co_1/2Ti_1/2)O₃ ceramics. Nd(Co_1/2Ti_1/2)O₃ ceramics with additives could be sintered to a theoretical density higher than 98.5% at 1320 °C. Second phases were not observed at the level of 0.25-0.75 wt.% B₂O₃ addition. The temperature coefficient of resonant frequency (τ_f) was not significantly affected, while the dielectric constants (?_r) and the unloaded quality factors Q were effectively promoted by B₂O₃ addition. At 1320 °C/4 h, Nd(Co_1/2Ti_1/2)O₃ ceramics with 0.75 wt.% B₂O₃ addition possesses a dielectric constant (?_r) of 27.2, a Q × f value of 153,000 GHz (at 9 GHz) and a temperature coefficient of resonant frequency (τ_f) of 0 ppm/°C. The B₂O₃-doped Nd(Co_1/2Ti_1/2)O₃ ceramics can find applications in microwave devices requiring low sintering temperature.     

Dielectric properties of B2O3-doped (1 − x)LaAlO3-xSrTiO3 ceramic system at microwave frequency

The effects of B₂O₃ addition on the microwave dielectric properties and the microstructures of (1−x)LaAlO₃-xSrTiO₃ ceramics prepared by conventional solid-state routes have been investigated. Doping with 0.25 wt.% B₂O₃ can effectively promote the densification and the microwave dielectric properties of (1−x)LaAlO₃-xSrTiO₃ ceramics. It is found that LaAlO₃-SrTiO₃ ceramics can be sintered at 1400°C due to the liquid phase effect of a B₂O₃ addition observed by scanning electronic microscopy (SEM). The dielectric constant as well as the Q×f value decreases with increasing B₂O₃ content. At 1460°C, 0.46LaAlO₃-0.54SrTiO₃ ceramics with 0.25 wt.% B₂O₃ addition possesses a dielectric constant (ε_r) of 35, a Q×f value of 38,000 (at 7 GHz) and a temperature coefficients of resonant frequency (τ_f) of −1 ppm/°C.     

Synthesis of (1 − x)Ca2/5Sm2/5TiO3-xLi1/2Nd1/2TiO3 ceramic powder via ethylenediaminetetraacetic acid precursor

(1 − x)Ca_2/5Sm_2/5TiO₃-xLi_1/2Nd_1/2TiO₃ (CSLNT) ceramic powder was prepared by a liquid mixing method using ethylenediaminetetraacetic acid (EDTA) as the chelating agent. TG, DTA, XRD and TEM characterized the precursors and derived oxide powders. When x = 0.3, perovskite CSLNT was synthesized at 1000 °C for 3 h in air. The CSLNT (x = 0.3) ceramics sintered at 1200 °C for 3 h show excellent microwave dielectric properties of ?_r = 99, Q_f = 6200 GHz and τ_f = 9 × 10⁻⁶ °C⁻¹.     

Preparation, structure and dielectric properties of Ba4LaMNb3O15 (M = Ti, Sn ) ceramics

Two new cation-deficient hexagonal perovskites Ba₄LaMNb₃O₁₅ (M = Ti, Sn) ceramics were prepared by high temperature solid-state reaction route. The phase and structure of the ceramics were characterized by X-ray diffraction, scanning electron microscopy (SEM). The microwave dielectric properties of the ceramics were studied using a network analyzer. The Ba₄LaTiNb₃O₁₅ has high dielectric constant of 52, high quality factors (Q) 3500 (at 4.472 GHz), and temperature variation of resonant frequency (τ_f) +93 ppm °C⁻¹ at room temperature; Ba₄LaSnNb₃O₁₅ has dielectric constant of 39 with high Q value of 2510 (at 5.924 GHz), and τ_f −29 ppm °C⁻¹.     

Synthesis, low-temperature sintering and the dielectric properties of the ZnO-(1 − x)TiO2-xSnO2 (x = 0.04-0.2)

ZnO-(1 − x)TiO₂-xSnO₂ (x = 0.04-0.2) ceramics were prepared by conventional mixed-oxide method combined with a chemical processing. Fine particle powders were prepared by chemical processing to activate the formation of compound and to improve the sinterability. One wt.% of V₂O₅ and B₂O₃ with the mole ratios of 3:1 were used to lower the sintering temperature of ceramics. The effect of Sn content on phase structure and dielectric properties were investigated. The results show that the substituting Sn for Ti accelerates the hexagonal phase transition to cubic phase, and an inverse spinel structure Zn₂(Ti_1−xSn_x)O₄ solid solution forms. The best dielectric properties obtained at x = 0.12. The ZnO-0.88TiO₂-0.12SnO₂ ceramics sintered at 900 °C exhibit a good dielectric property: ?_r = 29 and tan δ = 9.86 × 10⁻⁵. Due to their good dielectric properties, low firing characteristics, ZnO-(1 − x)TiO₂-xSnO₂ (x = 0.04-0.2) can serve as the promising microwave dielectric capacitor.