Microwave dielectric properties and its compatibility with silver electrode of Li2MgTi3O8 ceramics

The effects of BaCu(B₂O₅) (BCB) additions on the sintering temperature and microwave dielectric properties of Li₂MgTi₃O₈ ceramic have been investigated. The pure Li₂MgTi₃O₈ ceramic shows a relative high sintering temperature (∼1000 °C) and good microwave dielectric properties as Q × f of 40,000 GHz, ?_r of 27.2, τ_f of 2.6 ppm/°C. It was found that the addition of a small amount of BCB can effectively lower the sintering temperature of Li₂MgTi₃O₈ ceramics from 1025 to 900 °C and induce no obvious degradation of the microwave dielectric properties. Typically, the 0.5 wt% BCB added Li₂MgTi₃O₈ ceramic sintered at 900 °C for 2 h exhibited good microwave dielectric properties of Q × f = 36,200 GHz (f = 7.31 GHz), ?_r = 26 and τ_f = −2 ppm/°C. Compatibility with Ag electrode indicates this material can be applied to low temperature-cofired ceramics (LTCC) devices.     

Mg-substituted ZnNb2O6-TiO2 composite ceramics for RF/microwaves ceramic capacitors

Microstructure and microwave dielectric properties of Mg-substituted ZnNb₂O₆-TiO₂ microwave ceramics were investigated. Mg acted as a grain refining reagent and columbite phase stabilization reagent. With an increasing Mg content, the amount of ixiolite (Zn, Mg) TiNb₂O₈ decreased, and the amount of (Zn_0.9Mg_0.1)_0.17Nb_0.33Ti_0.5O₂ and columbite increased. ZnO-Nb₂O₅-1.75TiO₂-5 mol.%MgO exhibited excellent dielectric properties (at 950 °C): ?_r = 35.6, Q × f = 16,000 GHz (at 5.6 GHz) and τ_f = −10 ppm/°C. The material was applied successfully to make RF/microwaves ceramic capacitor, whose self-resonance frequency was 19 GHz at low capacitance of 0.13 pF.     

Low temperature sintering and microwave dielectric properties of (Mg0.7Zn0.3)0.95Co0.05TiO3 ceramics with BaCu(B2O5) additions

The effects of BaCu(B₂O₅) additives on the sintering temperature and microwave dielectric properties of (Mg_0.7Zn_0.3)_0.95Co_0.05TiO₃ ceramics were investigated. The (Mg_0.7Zn_0.3)_0.95Co_0.05TiO₃ ceramics were not able to be sintered below 1000 °C. However, when BaCu(B₂O₅) were added, they were sintered below 1000 °C and had the good microwave dielectric properties. It was suggested that a liquid phase with the composition of BaCu(B₂O₅) was formed during the sintering and assisted the densification of the (Mg_0.7Zn_0.3)_0.95Co_0.05TiO₃ ceramics at low temperature. BaCu(B₂O₅) powders were produced and used to reduce the sintering temperature of the (Mg_0.7Zn_0.3)_0.95Co_0.05TiO₃ ceramics. Good microwave dielectric properties of Q × f = 35,000 GHz, ?_r = 18.5.0 and τ_f = −51 ppm/°C were obtained for the (Mg_0.7Zn_0.3)_0.95Co_0.05TiO₃ ceramics containing 7 wt.% mol% BaCu(B₂O₅) sintered at 950 °C for 4 h.     

A new temperature stable microwave dielectric ceramics: ZnTiNb2O8 sintered at low temperatures

The phases, microstructure and microwave dielectric properties of ZnTiNb₂O₈-xTiO₂ composite ceramics with different weight percentages of BaCu(B₂O₅) additive prepared by solid-state reaction method have been investigated using the X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy-dispersive spectroscopy (EDS). The results showed that the microwave dielectric properties were strongly dependent on densification, grain sizes and crystalline phases. The sintering temperature of ZnTiNb₂O₈ ceramics was reduced from 1250 °C to 950 °C by doping BaCu(B₂O₅) additive and the temperature coefficient of resonant frequency (τ_f) was adjusted from negative value of −52 ppm/°C to 0 ppm/°C by incorporating TiO₂. Addition of 2 wt% BaCu(B₂O₅) in ZnTiNb₂O₈-xTiO₂ (x = 0.8) ceramics sintered at 950 °C showed excellent dielectric properties of ?_r = 38.89, Q × f = 14,500 GHz (f = 4.715 GHz) and τ_f = 0 ppm/°C, which represented very promising candidates as LTCC dielectrics for LTCC applications.     

Densification and mechanical properties of fine-grained Al2O3-ZrO2 composites consolidated by spark plasma sintering

Alumina matrix composites containing 5 and 10 wt% of ZrO₂ were sintered under 100 MPa pressure by spark plasma sintering process. Alumina powder with an average particle size of 600 nm and yttria-stabilized zirconia with 16 at% of Y₂O₃ and with a particle size of 40 nm were used as starting materials. The influence of ZrO₂ content and sintering temperature on microstructures and mechanical properties of the composites were investigated. All samples could be fully densified at a temperature lower than 1400 °C. The microstructure analysis indicated that the alumina grains had no significant growth (alumina size controlled in submicron level 0.66-0.79 μm), indicating that the zirconia particles provided a hindering effect on the grain growth of alumina. Vickers hardness and fracture toughness of composites increased with increasing ZrO₂ content, and the samples containing 10 wt% of ZrO₂ had the highest Vickers hardness of 18 GPa (5 kg load) and fracture toughness of 5.1 MPa m^1/2.     

Low-loss microwave dielectrics using rock salt oxide Li2MgTiO4

Rock-salt-structured Li₂MgTiO₄ ceramic was prepared by the conventional mixed oxide route and its microwave dielectric properties were investigated. The microstructures of the ceramics were characterized by SEM. The dielectric properties of the ceramics exhibited a significant dependence on the sintering condition and crystal structure. A new microwave dielectric material, Li₂MgTiO₄ sintered at 1360 °C has a dielectric constant (?_r) of ∼17.25, a Q × f of ∼97,300 GHz (where f = 9.86 GHz, is the resonant frequency) and a τ_f of ∼-27.2 ppm/°C. The microwave dielectric properties of the ceramic are reported for the first time.     

Microwave dielectric properties of (1 − x)ZnMoO4-xTiO2 composite ceramics

(1 − x)ZnMoO₄-xTiO₂ (x = 0.0, 0.05, 0.158, 0.25, and 0.35) composite ceramics were synthesized by the conventional solid state reaction process. The sintering behavior, phase composition, chemical compatibility with silver, and microwave dielectric properties were investigated. All the specimens can be well densified below 950 °C. From the X-ray diffraction analysis, it indicates that the triclinic wolframite ZnMoO₄ phase coexists with the tetragonal rutile TiO₂ phase, and it is easy for silver to react with ZnMoO₄ to form Ag₂Zn₂(MoO₄)₃ phase and hard to react with TiO₂. When the volume fraction of TiO₂ (x value) increasing from 0 to 0.35, the microwave dielectric permittivity of the (1 − x)ZnMoO₄-xTiO₂ composite ceramics increases from 8.0 to 25.2, the Q_f value changes in the range of 32,300-43,300 GHz, and the temperature coefficient τ_f value varies from −128.9 to 157.4 ppm/°C. At x = 0.158, the mixture exhibits good microwave dielectric properties with a ?_r = 13.9, a Q_f = 40,400 GHz, and a τ_f = +2.0 ppm/°C.     

Microwave dielectric properties of temperature stable Li2ZnxCo1−xTi3O8 ceramics

The Li₂Zn_xCo_1−xTi₃O₈ (x = 0.2-0.8) solid solution system has been synthesized by the conventional solid-state ceramic route and the effect of Zn substitution for Co on microwave dielectric properties of Li₂CoTi₃O₈ ceramics has also been investigated. The microwave dielectric properties of these ceramics show a linear variation between the end members for all compositions. The optimized sintering temperatures of Li₂Zn_xCo_1−xTi₃O₈ ceramics increase with increasing content of Zn. The specimen with x = 0.4 sintered at 1050 °C/2 h exhibits an excellent combination of microwave dielectric properties with ?_r = 27.7, Q_u × f = 57,100 GHz and τ_f = −1.0 ppm/°C.     

Synthesis, structural and microwave dielectric properties of Al2W3−xMoxO12 (x = 0-3) ceramics

Low dielectric ceramics in the Al₂W_3−xMo_xO₁₂ (x = 0-3) system have been prepared through solid state ceramic route. The phase purity of the ceramic compositions has been studied using powder X-ray diffraction (XRD) studies. The microstructure of the sintered ceramics was evaluated by Scanning Electron Microscopy (SEM). The crystal structure of the ceramic compositions as a result of Mo substitution has been studied using Laser Raman spectroscopy. The microwave dielectric properties of the ceramics were studied by Hakki and Coleman post resonator and cavity perturbation techniques. Al₂Mo_xW_3−xO₁₂ (x = 0-3) ceramics exhibited low dielectric constant and relatively high unloaded quality factor. The temperature coefficient of resonant frequency of the compositions is found to be in the range −41 to −72 ppm/°C.     

Improved dielectric and non-ohmic properties of Ca2Cu2Ti4O12 ceramics prepared by a polymer pyrolysis method

Non-ohmic and dielectric properties of Ca₂Cu₂Ti₄O₁₂ (CaCu₃Ti₄O₁₂/CaTiO₃ composite) ceramics prepared by a polymer pyrolysis method (PP-ceramic samples) are investigated. The PP-ceramics show a highly dense structure and improved non-ohmic and dielectric properties compared to the ceramics obtained by a solid state reaction method (SSR-ceramic samples). ?′ (tan δ) of the PP-ceramic samples is found to be higher (lower) than that of the SSR-ceramic samples. Interestingly, the PP-ceramic sintered at 1050 °C for 10 h exhibits the high ?′ of 2530 with weak frequency dependence below 1 MHz, the low tan δ less than 0.05 in the frequency range of 160 Hz-177 kHz, and the little temperature coefficient, i.e., |Δ?′| ≤ 15 % in the temperature range from −55 to 85 °C. These results indicate that the CaCu₃Ti₄O₁₂/CaTiO₃ composite system prepared by PP method is a promising high-?′ material for practical capacitor application.     

Microwave properties of Ba(Zn1/3Ta2/3)O3 dielectric resonators

Ba(Zn_1/3Ta_2/3)O₃ (BZT) dielectric resonators were prepared by solid-state reaction. The starting materials were BaCO₃, ZnO, and Ta₂O₅ powders with high purity. The double calcined BZT pellets were sintered in air at temperatures of 1575, 1600, 1625, and 1650 °C for 4 h. The X-ray diffraction data allowed the study of the unit cell distortion degree and the presence of the secondary phases. A long-range order with a 2:1 ratio of Ta and Zn cations on the octahedral positions of the perovskite structure was observed with the increase of the sintering temperature. The dielectric constant of BZT resonators measured around 6 GHz was between 26 and 28. High values of Q × f product (120 THz) were obtained for BZT resonators sintered at 1650 °C/4 h. The temperature coefficient of the resonance frequency exhibits positive values less than 6 ppm/°C. The achieved dielectric parameters recommend BZT dielectric resonators for microwave and millimeter wave applications.     

High-dielectric-constant and low-loss microwave dielectric in the (1 − x)La(Mg0.5Ti0.5)O3-x(Ca0.8Sr0.2)TiO3 solid solution system

The microwave dielectric properties and microstructures of (1 − x)La(Mg_0.5Ti_0.5)O₃-x(Ca_0.8Sr_0.2)TiO₃ ceramics, prepared by a mixed oxide route, have been investigated. The forming of solid solutions was confirmed by the XRD patterns and the measured lattice parameters for all compositions. A near zero τ_f was achieved for samples with x = 0.5, although the dielectric properties varied with sintering temperature. The Q × f value of 0.5La(Mg_0.5Ti_0.5)O₃-0.5(Ca_0.8Sr_0.2)TiO₃ increased up to 1475 °C, after which it decreased. The decrease in dielectric properties was coincident with the onset of rapid grain growth. The optimum combination of microwave dielectric properties was achieved at 1475 °C for samples where x = 0.5 with a dielectric constant ?_r of 47.12, a Q × f value of 35,000 GHz (measured at 6.2 GHz) and a τ_f value of −4.7 ppm/°C.     

Structure and dielectric characterization of xNd(Mg1/2Ti1/2)O3-(1 − x)Ca0.6La0.8/3TiO3 in the microwave frequency range

The crystal structures, phase compositions and the microwave dielectric properties of the xLa(Mg_1/2Ti_1/2)O₃-(1 − x)Ca_0.8Sr_0.2TiO₃ composites prepared by the conventional solid state route have been investigated. The formation of solid solution is confirmed by the XRD patterns. Doping with B₂O₃ (0.5 wt.%) can effectively promote the densification and the dielectric properties of xNd(Mg_1/2Ti_1/2)O₃-(1 − x)Ca_0.6La_0.8/3TiO₃ ceramics. It is found that xNd(Mg_1/2Ti_1/2)O₃-(1 − x)Ca_0.6La_0.8/3TiO₃ ceramics can be sintered at 1375 °C, due to the liquid phase effect of B₂O₃ addition observed by Scanning Electronic Microscopy. At 1375 °C, 0.4Nd(Mg_1/2Ti_1/2)O₃-0.6Ca_0.6La_0.8/3TiO₃ ceramics with 1 wt.% B₂O₃ addition possesses a dielectric constant (?_r) of 49, a Q × f value of 13,000 (at 8 GHz) and a temperature coefficients of resonant frequency (τ_f) of 1 ppm/°C. As the content of Nd(Mg_1/2Ti_1/2)O₃ increases, the highest Q × f value of 20,000 GHz for x = 0.9 is achieved at the sintering temperature 1400 °C.     

Elucidating the dielectric properties of Mg2SnO4 ceramics at microwave frequency

This study investigated the potential applications of microwave dielectric properties of Mg₂SnO₄ ceramics in mobile communication. Mg₂SnO₄ ceramics were prepared using a conventional solid-state method. The X-ray diffraction patterns of the Mg₂SnO₄ ceramics revealed no significant variation of phase with sintering temperature. A maximum density of 4.62 g/cm³, a dielectric constant (?_r) of 8.41, a quality factor (Q × f) of 55,100 GHz, and a temperature coefficient of resonant frequency (τ_f) of −62 ppm/ °C were obtained when Mg₂SnO₄ ceramics were sintered at 1550 °C for 4 h.     

Effect of addition of Ba(W0.5Cu0.5)O3 in Pb(Mg1/3Nb2/3)O3-Pb(Zn1/3Nb2/3)O3-Pb(Zr0.52Ti0.48)O3 ceramics on the sintering temperature, electrical properties and phase transition

In this work, we report on the Pb(Mg_1/3Nb_2/3)O₃-Pb(Zn_1/3Nb_2/3)O₃-Pb(Zr_0.52Ti_0.48)O₃ (PMN-PZN-PZT) ceramics with Ba(W_0.5Cu_0.5)O₃ as the sintering aid that was manufactured in order to develop the low-temperature sintering materials for piezoelectric device applications. The phase transition, microstructure, dielectric, piezoelectric properties, and the temperature stability of the ceramics were investigated. The results showed that the addition of Ba(W_0.5Cu_0.5)O₃ significantly improved the sintering temperature of PMN-PZN-PZT ceramics and could lower the sintering temperature from 1005 to 920 °C. Besides, the obtained Ba(W_0.5Cu_0.5)O₃-doped ceramics sintered at 920 °C have optimized electrical properties, which are listed as follows: (K_p = 0.63, Q_m = 1415 and d₃₃ = 351 pC/N), and high depolarization temperature above 320 °C. These results indicated that this material was a promising candidate for high-power multilayer piezoelectric device applications.     

Influence of MnO2 on the sintering behavior and magnetic properties of NiFe2O4 ferrite ceramics

The samples with small amounts of MnO₂ (0, 0.5, 1.0, 1.5, 2.0, and 2.5 wt%, respectively) were prepared via ball-milling process and two-step sintering process from commercial powders (i.e. Fe₂O₃, NiO and MnO₂). Microstructural features, phase transformation, sintering behavior and magnetic properties of Mn-doped NiFe₂O₄ composite ceramics have been investigated by means of scanning electron microscopy (SEM), differential thermal analyzer, X-ray diffraction (XRD), thermal dilatometer and vibrating sample magnetometer (VSM) respectively. The XRD analysis and the result of differential thermal analysis indicate that the reduction of MnO₂ into Mn₂O₃ and the following reduction of Mn₂O₃ into MnO existed in sintering process. No new phases are detected in the ceramic matrix, the crystalline structure of the ceramic matrix is still NiFe₂O₄ spinel structure. Morphology and the detecting result of thermal dilatometer show that MnO₂ can promote the sintering process, the temperature for 1 wt% MnO₂-doped samples to reach the maximum shrinkage rate is 59 °C lower than that of un-doped samples. For 1 wt% MnO₂-doped samples, the value of the saturation magnetization (M_s) and coercivity (H_c) is 15.673 emu/g and 48.316 Oe respectively.     

Dielectric properties of low-temperature sintered Ba0.6Sr0.4TiO3 thick films prepared by reactive sintering method

Screen printed Ba_0.6Sr_0.4TiO₃ (BST6/4) thick films were fabricated by reactive sintering at a low temperature below 900 °C. The dielectric properties in radio frequency range were measured on samples of sandwich structure MIM capacitors by impedance analyzer, while that in microwave frequency range were measured on samples of thick films without top and bottom electrodes by split-post dielectric resonator method. The thick films exhibited a low permittivity, while at the same time, maintained a high tunability. The permittivity and dielectric loss at 1 MHz were 228.8 and 0.007, respectively. The corresponding values measured at 9.9 GHz were 82.24 and 0.109, respectively. The tunability was as high as 72.4% (150 kV/cm, 10 kHz). This method provides a simple and effective route to obtain thick films with great potential in applications in Low Temperature Co-fired Ceramic (LTCC) and microwave tunable devices.     

Low-firable high-K dielectric in the Zrx(Zn1/3Nb2/3)1−xTiO4 ceramic system

Composite ceramics in the solid solution of Zr_x(Zn_1/3Nb_2/3)_1−xTiO₄ (x = 0.1-0.4) have been prepared by the mixed oxide route. Formation of solid solution was confirmed by the X-ray diffraction patterns. The microwave dielectric properties, such as dielectric constant (?_r), Q × f value and temperature coefficient of resonant frequency (τ_f) have been investigated as a function of composition and sintering temperature. With x increasing from 0.1 to 0.4, the dielectric constant decreases from 70.9 to 43.2, and the τ_f decreases from 105 to 55 ppm/°C. The Q × f value, however, increases with increasing x value to a maximum 26,600 GHz (at 6 GHz) at x = 0.3, and then decreases thereafter. For low-loss microwave applications, a new microwave dielectric material Zr_0.3(Zn_1/3Nb_2/3)_0.7TiO₄, possessing a fine combination of microwave dielectric properties with a high ?_r of 51, a high Q × f of 26,600 GHz (at 6 GHz) and a τ_f of 70 ppm/°C, is suggested.     

Piezoelectric and dielectric properties of Dy2O3-doped (Bi0.5Na0.5)0.94Ba0.06TiO3 lead-free ceramics

(Bi_0.5Na_0.5)_0.94Ba_0.06TiO₃ + x wt% Dy₂O₃ with x = 0-0.3 ceramics were synthesized by conventional solid-state processes. The effects of Dy₂O₃ on the microstructure, the piezoelectric and dielectric properties were investigated. X-ray diffraction pattern confirmed that the coexistence of tetragonal and rhombohedral phases in the (Bi_0.5Na_0.5)_0.94Ba_0.06TiO₃ composition was not changed by adding 0.05-0.3 wt% Dy₂O₃. SEM images indicate that all the ceramics have pore-free microstructures with high density, and that doping of Dy₂O₃ inhibits the grain growth of the ceramics. The addition of Dy₂O₃ shows the double effects on decreasing the piezoelectric and dielectric properties for 0 < x < 0.15 when Dy³⁺ ions substitute B-site Ti⁴⁺ ions, and increasing the properties for 0.15 < x < 0.3 when Dy³⁺ ions enters into A-site of the perovskite structure. The optimum electric properties of piezoelectric constant d₃₃ = 170 pC/N and the dielectric constant ?_r = 1900 (at a frequency of 1 kHz) are obtained at x = 0.3.     

Effect of CaTiO3 addition on microwave dielectric properties of Mg2(Ti0.95Sn0.05)O4 ceramics

The microwave dielectric properties of CaTiO₃-added Mg₂(Ti_0.95Sn_0.05)O₄ ceramics prepared by the mixed oxide route have been investigated. The combination of spinel-structured Mg₂(Ti_0.95Sn_0.05)O₄ and perovskite-structured CaTiO₃ forms a two-phase system (1 − x)Mg₂(Ti_0.95Sn_0.05)O₄-xCaTiO₃, which was confirmed by the XRD patterns and the EDX analysis and it also leads to a zero τ_f. The microwave dielectric properties of the ceramics can be effectively controlled by varying the x value. For practical applications, a new microwave dielectric material 0.91Mg₂(Ti_0.95Sn_0.05)O₄-0.09CaTiO₃ is suggested and it possesses a good combination of dielectric properties with an ?_r of ∼18.01, a Q × f of ∼92,000 GHz, and a τ_f of ∼0 ppm/°C, which makes it is a very promising candidate material for high frequency applications.