Reaction-sintering method for ultra-low loss (Mg0.95Co0.05)TiO3 ceramics

Microwave dielectric properties and microstructures of (Mg_0.95Co_0.05)TiO₃ ceramics prepared by a new sintering method (reaction-sintering method) were investigated. A pure phase of (Mg_0.95Co_0.05)TiO₃ was obtained by the new method and excellent dielectric properties were observed due to uniformities of the microstructure and the phase. In contrast, the secondary phase (Mg_0.95Co_0.05)Ti₂O₅ was observed in samples prepared by conventional sintering method. In order to study the influence of secondary phase on the microwave dielectric properties quantitatively, the weight fraction of (Mg_0.95Co_0.05)Ti₂O₅ was calculated on the basis of Rietveld refinement. The pore-free?_r values of specimens prepared by two different methods indicated that porosity plays an important role in the ?_r values of (Mg_0.95Co_0.05)TiO₃ ceramics. Specimens sintered by reaction-sintering method at 1350 °C for 4 h possess excellent dielectric properties with an ?_r of 16.3, a Q × f value of 244,500 GHz, and a τ_f value of −53.5 ppm/°C.     

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.     

Phase evolution and microwave dielectric properties of TiO2-modified (Mg0.95Co0.05)2TiO4 ceramics

Phase evolution and microwave dielectric properties of (1 − x)(Mg_0.95Co_0.05)₂TiO₄-xTiO₂ (x = 0-1) ceramics prepared by the conventional mixed oxide route have been investigated. Increasing the TiO₂ content would lead to a main phase transformation from (Mg_0.95Co_0.05)₂TiO₄ to (Mg_0.95Co_0.05)TiO₃, (Mg_0.95Co_0.05)Ti₂O₅ and then TiO₂. Not only did the TiO₂ addition compensate the τ_f, it also lowered the sintering temperature of specimen. A huge drop of Q × f occurs at a 40-60 mol% TiO₂ addition was attributed to the formation of (Mg_0.95Co_0.05)Ti₂O₅ phase. Specimen with x = 0.78 can possess an excellent combination of microwave dielectric properties: ?_r ∼ 24.77, Q × f ∼ 38,500 GHz and τ_f ∼ −1.3 ppm/°C.     

Enhancement microwave dielectric properties of La(Mg0.5Sn0.5)O3 ceramics by substituting Mg with Ni

The microwave dielectric properties of La(Mg_0.5−xNi_xSn_0.5)O₃ ceramics were examined with a view to their exploitation for mobile communication. The La(Mg_0.5−xNi_xSn_0.5)O₃ ceramics were prepared by the conventional solid-state method at various sintering temperatures. The X-ray diffraction patterns of the La(Mg_0.4Ni_0.1Sn_0.5)O₃ ceramics revealed no significant variation of phase with sintering temperatures. Apparent density of 6.71 g/cm³, dielectric constant (?_r) of 20.19, quality factor (Q × f) of 74,600 GHz, and temperature coefficient of resonant frequency (τ_f) of −85 ppm/°C were obtained for La(Mg_0.4Ni_0.1Sn_0.5)O₃ ceramics that were sintered at 1550 °C for 4 h.     

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.     

Crystal structure and dielectric properties of La(Mg1−xZnx)1/2Ti1/2O3 ceramics at microwave frequencies

Ceramics in the system La(Mg_1−xZn_x)_1/2Ti_1/2O₃ with B₂O₃ additions (1 wt.%) have been investigated by the conventional solid-state route. The XRD patterns of the sintered samples (0.3 ≤ x ≤ 1.0) revealed single phase formation with a structure. The unit cell volume slightly increased with increasing Zn content (x). La(Mg_1−xZn_x)_1/2Ti_1/2O₃ were found to form perovskite solid solutions in the whole compositional range. The maximum values of the dielectric constant and the quality factor multiples resonant frequency (Q × f) can be obtained when the La(Mg_0.7Zn_0.3)_1/2Ti_1/2O₃ with 0.5 wt.% B₂O₃ additive were sintered at 1475 °C for 4 h. The temperature coefficient of resonant frequency τ_f (−63 ppm/°C) was measured for x = 0.7.     

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.     

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.     

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.     

Sinterability and dielectric properties of Ba0.55Sr0.4Ca0.05TiO3-CaTiSiO5-Mg2TiO4 composite ceramics

The composite ceramics of Ba_0.55Sr_0.4Ca_0.05TiO₃-CaTiSiO₅-Mg₂TiO₄ (BSCT-CTS-MT) were prepared by the conventional solid-state route. The sintering performance, phase structures, morphologies, and dielectric properties of the composite ceramics were investigated. The BSCT-CTS-MT ceramics were sintered at 1100 °C and possessed dense microstructure. The dielectric constant was tailored from 1196 to 141 as the amount of Mg₂TiO₄ increased from 0 to 50 wt%. The dielectric constant and dielectric loss of 40 wt% Ba_0.55Sr_0.4Ca_0.05TiO₃-10 wt% CaTiSiO₅-50 wt% Mg₂TiO₄ was 141 and 0.0020, respectively, and the tunability was 8.64% under a DC electric field of 8.0 kV/cm. The Curie peaks were broadened and depressed after the addition of CaTiSiO₅. The optimistic dielectric properties made it a promising candidate for the application of tunable capacitors and phase shifters.     

Dielectric properties and crystal structure of La(Mg1/2Ti1/2)O3 ceramics with Mg substituted by Co

Solid solutions of (1 − x)La(Co_1/2Ti_1/2)O₃-xLa(Mg_1/2Ti_1/2)O₃ were used to prepare La(Mg_1−xCo_x)_1/2Ti_1/2O₃ using solid-state synthesis. X-ray diffraction patterns of the sintered samples revealed single phase formation. A maximum density of 6.01 g/cm³ was obtained for La(Mg_1−xCo_x)_1/2Ti_1/2O₃ (x = 1) ceramics sintered at 1375 °C for 4 h. The maximum values of the dielectric constant (?_r = 29.13) and the quality factor (Q × f = 80,000 GHz) were obtained for La(Mg_1−xCo_x)_1/2Ti_1/2O₃ with 1 wt% ZnO additive sintered at 1375 °C for 4 h. The temperature coefficient of resonant frequency τ_f was −59 ppm/°C for x = 0.3.     

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.     

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.     

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.     

Dielectric and piezoelectric properties of Sb doped (NaBi)0.38(LiCe)0.05[]0.14Bi2Nb2O9 ceramics

Sb⁵⁺-doped (NaBi)_0.38(LiCe)_0.05[]_0.14Bi₂Nb₂O₉ (represented as NBNLCS-x, where [] represents A-site vacancies) ceramics were prepared by the conventional solid-state route. The ceramics well sintered to approach ∼98.5% theoretical density and the tetragonality of crystal structure increased with Sb⁵⁺ additions. However, the Curie temperature (T_C) and the piezoelectric coefficient (d₃₃) of Sb⁵⁺-modified ceramics gradually decreased. The 3 mol% Sb⁵⁺-doped samples exhibited optimum properties with a d₃₃ value of ∼22 pC/N planar electromechanical coupling factor (k_p) of ∼11.2% and relatively high T_C of ∼765 °C. These results indicate that NBNLCS-x material is a promising candidate for high-temperature piezoelectric applications.     

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.     

Self-propagating high-temperature synthesis of La(Sr)Ga(Mg,Fe)O3−δ with planetary ball-mill treatment for solid oxide fuel cell electrolytes

This study investigated the combined effects of self-propagating high-temperature synthesis (SHS), planetary ball-mill (PBM) treatment, and sintering temperature on La_0.7Sr_0.3Ga_0.7Mg_0.1Fe_0.2O_3−δ (LSGMF73712) as an electrolyte material for solid oxide fuel cells (SOFC). The SHS products (SHS-LSGMF73712) were compared with that prepared via solid-state reaction (SSR) in terms of sinterability and power generation performance. The SHS products were treated with PBM for 10, 30, 50, and 70 h. The SHS products contained the by-product LaSrGaO₄; however, in the SHS products treated with PBM for longer than 50 h, the by-product disappeared after sintering at 1350 °C for 3 h in air. Among the samples, SHS products treated with PBM for 70 h displayed superior sintering (1350 °C), whereas the SSR product (SSR-LSGMF73712) was successfully sintered at 1450 °C for 3 h in air. Under the cell configuration of Ni-Fe/SHS-LSGMF73712-PBM70 h (0.3 mm thick)/Sm_0.5Sr_0.5CoO₃, the maximum power density was 0.673 W/cm² at 800 °C using humidified hydrogen gas (3 mol% H₂O) as a fuel and air as an oxidizing agent at a flow rate of 100 mL/min, which was almost equivalent to that using SSR-LSGMF73712 (0.629 W/cm² at 800 °C) under the same conditions.     

Sintering behaviors, magnetic and electric properties of Bi-Zn co-doped Co2Y ferrites

The Bi and Zn substitution effects on the sintering behaviors, magnetic and electric properties of hexagonal ferrites with a composition of 2(Ba_1−xBi_xO)·2(Zn_yCo_0.8−yCu_0.2O)·6(Fe_2−x/3Zn_x/3O₃) were investigated. The results showed that the addition of Bi and Zn can significantly promote Co₂Y densification. The Y phase may be triggered to decompose into M and spinel phases at high sintering temperatures (above 1050 °C) for samples with excess Bi (x = 0.2) substitution, which resulted in densification and magnetic properties degradation. Co₂Y ferrites with x = 0.1 and y = 0.4 sintered at 1050 °C show a relative density of 94%, a high initial permeability of 4.5, a quality factor (Q) of 50.     

Study of (1 − x)(Mg0.6Zn0.4)0.95Co0.05TiO3–xCa0.61Nd0.26TiO3 microwave dielectrics

The microwave dielectric properties and the microstructures of the (1 − x)(Mg_0.6Zn_0.4)_0.95Co_0.05TiO₃–xCa_0.61Nd_0.26TiO₃ ceramic system were investigated. In order to achieve a temperature-stable material, we studied a method of combining a positive temperature coefficient material with a negative one. Ca_0.61Nd_0.26TiO₃ has a large positive temperature coefficient of resonant frequency. (Mg_0.6Zn_0.4)_0.95Co_0.05TiO₃ possesses a negative temperature coefficient of resonant frequency. By appropriately adjusting the x value in the (1 − x)(Mg_0.6Zn_0.4)_0.95Co_0.05TiO₃–xCa_0.61Nd_0.26TiO₃ ceramic system, a near-zero τ_f value can be obtained. A new microwave dielectric material of 0.8(Mg_0.6Zn_0.4)_0.95Co_0.05TiO₃–0.2Ca_0.61Nd_0.26TiO₃ possesses the excellent dielectric properties of a dielectric constant of 28.6, a Q × f value of 80,600 GHz and a temperature coefficient of resonant frequency of 4.1 ppm/°C and has a lower sintering temperature of 1250 °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.