Microwave dielectric ceramics in the BaO–TiO2–ZnO system doped with Nb2O5

The microwave dielectric properties of BaO–TiO₂–ZnO (BTZ) system ceramics were studied as a function of the amount of Nb₂O₅ dopant. With the addition of 0–0.025 mol% Nb₂O₅, the substitution of Ti⁴⁺ ions with Nb⁵⁺ ions decreased the sintering temperature and increased the dielectric constant ε_r and quality factor Q of the system due to the similar atomic radius of Nb⁵⁺ and Ti⁴⁺ ions. When the amount of Nb⁵⁺ increased further (>0.025 mol%), Q was decreased by increasing the titanium vacancies. When the system doped with 0.025 mol% Nb₂O₅ was sintered at 1160 °C for 6 h, the ε_r, Qf₀, and the temperature coefficient of resonant frequency (τ_f) were 36.7, 41,000 GHz, and −5.0 ppm/°C, respectively, at 5 GHz.     

Microstructures and microwave dielectric characteristics of ceramics with the composition BaO·Nd2O3·5TiO2

The microstructures and the microwave dielectric characteristics of BaO·Nd₂O₃·5TiO₂ ceramics were determined as a function of the sintering conditions. The ceramics were confirmed to have a ternary orthorhombic major phase with minor amounts of Nd₂Ti₂O₇ and TiO₂, and nearly equiaxed fine structures and homogeneous columnar structures were obtained at sintering temperatures of 1340 C and 1370 C, respectively, while discontinuous and excess grain growth were observed at sintering temperatures 1350–1360 C and 1380 C, due to the formation of a liquid phase. The desired value of Qf was achieved in ceramics with fine-grained and/or homogeneous microstructures, and the discontinuous and excess grain growth was found to be seriously harmful to the dielectric properties.     

Photoluminescence and scintillation properties of Eu2O3–BaO–Nb2O5–TeO2 glass and glass ceramics

Journal of Materials Science: Materials in Electronics - 1Eu2O3–3BaO–20Nb2O5–76TeO2 glass and the corresponding glass-ceramics were synthesized with the aim to investigate the...     

Microwave dielectric properties of CaO–La2O3–Nb2O5–TiO2 ceramics

A series of ceramics with a general formula Ca_1+xLa_4?xNb_xTi_5?xO₁₇ (0 ≤ x ≤ 4) were fabricated using the solid-state ceramic route. The phase, microstructure, and microwave dielectric properties varied distinctly with composition or the value of x. X-ray diffraction results showed that the two end member phases, CaLa₄Ti₅O₁₇ and Ca₅Nb₄TiO₁₇, crystallized into single phases with orthorhombic and monoclinic crystal structure, respectively. For intermediate compounds with x = 1, 2, and 3, mixture phases CaLa₄Ti₅O₁₇ and Ca₅Nb₄TiO₁₇ coexisted and a trace amount of second phase was detected. The ceramics showed high ε _r in the range of 45–52, relatively high quality factors with Q × f in the range of 9,870–15,680 GHz and τ _f value in the range between ?38 and ?126.4 ppm/°C. τ _f of CaLa₄Ti₅O₁₇ can be tuned to a near-zero value by addition of suitable amount of TiO₂.     

Fluorinated polyimide–silica films with low permittivity and low dielectric loss

A sol–gel process was used to prepare polyimide–silica hybrid films from the fluorinated polyimide precursors (6FDA-ODA) and tetraethylorthosilicate (TEOS) in N,N-dimethyl acetamide. The hybrid film was then treated with hydrofluoric acid to remove the dispersed silica particles, leaving inside the film pores with diameters ranged from 80 nm to 1 μm, which depended on the size of the silica particles. The chemical structures and morphology of the hybrid and porous films were characterized by Fourier transform infrared spectroscopy and scanning electron microscopy, respectively. The synthesized porous fluorinated polyimide films show low relative dielectric permittivity of 1.9, rendering them promising for microelectronic packaging materials.     

Phase Relations in the Na2O–Al2O3–Nb2O5and CaO–Al2O3–Nb2O5Systems

Phase relations in the Na₂O–Al₂O₃–Nb₂O₅and CaO–Al₂O₃–Nb₂O₅systems were studied. The Na₂O system was found to contain neither ternary compounds nor niobate–aluminate solid solutions. In the CaO system, a ternary compound of composition 4CaO · Al₂O₃·Nb₂O₅was identified (cubic structure, a= 7.628 Å, Z= 2, _meas= _x= 4.43 g/cm³).     

Microwave dielectric properties of 3Li2O–Nb2O5–3TiO2 ceramics with Li2O–V2O5 additions

A new Li₂O–Nb₂O₅–TiO₂ (LNT) ceramic with the Li₂O:Nb₂O₅:TiO₂ mole ratio of 3:1:3 has been investigated. The compound is composed of two phases, the Li₂TiO₃ and “M-phase” solid solution phase. The microwave dielectric ceramic has low sintering temperature (∼1100 °C) and good microwave dielectric properties of a relatively high permittivity (∼51), high Q × f value up to 8700, and small temperature coefficient (∼37 ppm/°C). The low-amount doping of 0.83Li₂O–0.17V₂O₅ (LV) can effectively lower the sintering temperature from 1100 to 900 °C and induce no obvious degradation of the microwave dielectric properties. Typically, the 1 wt.% LV-doped ceramic sintered at 900 °C has better microwave dielectric properties of ε_r = 51.3, Q × f = 7235 GHz, τ _f  = 22 ppm/°C, which suggests that the ceramics can be applied in microwave LTCC devices.     

High permittivity and low dielectric loss of Na0.5Bi0.5−xLaxCu3Ti4O12 ceramics

The phase structure, microstructure and dielectric properties of Na_0.5Bi_0.5?xLa_xCu₃Ti₄O₁₂ (NBLCTO) ceramics were investigated. La³⁺ substitution had a great influence on the phase structure and dielectric properties. The results showed that the pure phase could be more easily obtained when substituting La³⁺ for Bi³⁺. Under the same processing condition (970 °C for 7.5 h) and measuring condition (10 kHz around room temperature), NBLCTO ceramics with x = 0.10 possessed the highest permittivity (1.02 × 10⁴) and lowest dielectric loss (0.022). The obtained NBLCTO ceramics with x = 0.10 also had good frequency stability and good temperature stability (?1.87% to +3.27%) from ?60 °C to 120 °C at both 1 and 10 kHz. Complex impedance results revealed that the grain resistance R_g was 7.18 Ω cm and the grain boundary resistance R_gb was 1.19 × 10⁶ Ω cm.     

Phase Relations in the Systems Al2TiO5–Fe2O3 , Al2O3–TiO2–Fe2O3 , and Al2TiO5–Cr2O3

Phase relations in the systems Al₂TiO₅–Fe₂O₃, Al₂TiO₅–Cr₂O₃, and Al₂O₃–TiO₂–Fe₂O₃ are investigated, and the composition ranges of pseudobrookite Al_{2 – 2x} M_2x TiO₅ (M = Fe, Cr) solid solutions are determined.     

Effect of Bi4B2O9 addition on the sintering temperature and microwave dielectric properties of BaO–Nd2O3–4TiO2 ceramics

The effects of Bi₄B₂O₉ addition on the sintering temperature, phase transition and microwave dielectric properties of BaO–Nd₂O₃–4TiO₂ (BNT) ceramics have been investigated. With 10 wt% Bi₄B₂O₉ addition, the sintering temperature of the BNT ceramics can be lowered down to about 1,150 °C. The secondary phase was observed at the level of 15 wt% Bi₄B₂O₉ addition. The Bi₄B₂O₉ addition can significantly affects the microwave dielectric properties. The Q × f ₀ value is a function of the sintering temperature and the Bi₄B₂O₉ content. For the samples sintered at 1,150 °C, Q × f ₀ value varies from 6,300 to 3,300 GHz as the Bi₄B₂O₉ addition increases from 5 to 20 wt%. The addition of Bi₄B₂O₉ does not induce much degradation in ε_r but modified the τ_f value to near zero. Typically, When 10 wt% Bi₄B₂O₉ is added, the τ_f of the ceramics could be tuned to a near-zero value (~1.2 ppm/°C), a substantial ε_r (~86) and Q × f ₀ (~4,670 GHz) could also be achieved simultaneously. The Bi₄B₂O₉ is an efficient sintering additive to decrease the sintering temperature and tune the τ_f value of the microwave dielectric materials for the practical microwave applications.     

Effect of sintering process on electrical properties and ageing behavior of ZnO–V2O5–MnO2–Nb2O5 varistor ceramics

The effect of sintering process on microstructure, electrical properties, and ageing behavior of ZnO–V₂O₅–MnO₂–Nb₂O₅ (ZVMN) varistor ceramics was investigated at 875–950 °C. The sintered density decreased from 5.52 to 5.44 g/cm³ and the average grain size increased from 4.4 to 9.6 μm with the increase of sintering temperature. The breakdown field (E_{1 mA}) decreased from 6991 to 943 V/cm with the increase of sintering temperature. The ZVMN varistor ceramics sintered at 900 °C led to surprisingly high nonlinear coefficient (α = 50). The donor concentration (N_d) increased from 3.33 × 10¹⁷ cm⁻³ to 7.64 × 10¹⁷ cm⁻³ with the increase of sintering temperature and the barrier height (Φ_b) exhibited the maximum value (1.07 eV) at 900 °C. Concerning stability, the varistors sintered at 925 °C exhibited the most stable accelerated ageing characteristics, with %ΔE_{1 mA} = 1.5% and %Δα = 13.3% for DC accelerated ageing stress of 0.85 E_{1 mA}/85 °C/24 h.     

Fabrication and characterization of high permittivity ceramics in the Ba(Ti1−x−ySnxZry)O3 system

Ceramics of solid solution compositions in the system BaTi_1–x–ySn_xZr_yO₃ have been fabricated using conventional mixed oxide processing routes. Samples with y = 0 or 0.01 exhibited maximum relative permittivities, _r ^max of ~ 65000 and ~ 62000 respectively. The Curie peaks became broader and _r ^max decreased with increasing zirconium substitutions. The Curie temperature decreased linearly from 35°C for y = 0 to –40°C for y = 0.11: for the latter, _r ^max was equal to ~ 20000. The results are interpreted on the basis of microstructural features and on the degree of compositional inhomogeneity in the system.     

Influence of Nb2O5 on the varistor behavior of TiO2–Cr2O3 system

This work investigated the influence of Nb₂O₅ dopant on the varistor behavior of the ternary system (99.95 ? x)%TiO₂, 0.05 %Cr₂O₃, x%Nb₂O₅, where x = 0.10; 0.15; 0.20; 0.25 % in mol. The processing was carried out using the conventional oxide mixture method. The initial oxides were homogenized in alcoholic media in a ball mill, for 3 h, dried in oven and isostatically pressed at 210 MPa and sintered at 1,400 °C for 2 h in air atmosphere and cooled at 5 °C/min cooling rate, resulting in pellets with high densification. Electrical measurements in continuous current at different temperatures revealed that for the lowest Nb₂O₅ concentration the breakdown electric field, E_B = 4.41 V/cm and non-linear coefficient, α = 4.6 were obtained, and for the highest Nb₂O₅ concentration the breakdown electric field, E_B = 9.71 V/cm and non-linear coefficient, α = 15.3 were obtained. These low values in the breakdown electric field enable these varistor systems to be used in protection systems for low-voltage energy grids. Changes in the potential barrier present in the grain–grain boundaries could also be observed, in which the height increased and the width decreased with the increase in the dopant concentration. However, for x = 0.25 % in mol Nb₂O₅, a significant reduction in the potential barrier height and the voltage per barrier was obtained. Furthermore, the tendency to increase the volume of the unitary cell with the increase of dopant concentration, which was interrupted in the system with 0.25 % Nb₂O₅, was another evidence that suggested the occurrence of Nb₂O₅ segregation in the grain boundaries, or even that the reduction in the average grain size could possibly dilute Nb₂O₅ concentration in the grain boundaries. The average grain size was calculated through SEM micrographs and ranged from 7 to 16 μm, with larger sizes occurring for lower dopant concentrations and presenting higher porosity and lower uniformity in the grains shape.     

Low loss, temperature stable dielectric ceramics in ZnNb2O6–Zn3Nb2O8 system for LTCC applications

(1 − x)ZnNb₂O₆–xZn₃Nb₂O₈ mixed phase ceramics have been prepared by conventional solid state ceramic route by both mixing ZnO with Nb₂O₅ and by ZnNb₂O₆ with Zn₃Nb₂O₈, respectively. The sintered ceramics have high relative permittivity (ε _r  = 23–25), high quality factor (Q _u xf) up to 95,500 GHz and temperature coefficient of resonant frequency (τ _f ) in the range −55 to −73 ppm/°C. The quality factors are higher for the mixtures when prepared from ZnNb₂O₆ and Zn₃Nb₂O₈. The 0.5ZnNb₂O₆–0.5Zn₃Nb₂O₈ has Q _u xf = 95,500 GHz (at 5.16 GHz), ε _r  = 22.7 and τ _f  = −65 ppm/°C when sintered at 1200 °C. The τ _f of the ceramic has been tuned close to zero by the addition of ZnTa₂O_6, which has a positive τ _f . The ceramic composition (1 − y)[0.5ZnNb₂O₆–0.5Zn₃Nb₂O₈]–yZnTa₂O₆ with y = 0.91 shows ε _r  = 34.7 and Q _u xf = 41,950 GHz (at 4.63 GHz) and zero τ _f . In order to lower the sintering temperature of 0.5ZnNb₂O₆–0.5Zn₃Nb₂O₈ ceramic for low temperature co-fired ceramic applications, low melting additives such as CuO, B₂O₃ and ACuB₂O₅ (A—Ba, Sr, Zn, Ca) have been added. 12 wt% ZnCuB₂O₅ added 0.5ZnNb₂O₆–0.5Zn₃Nb₂O₈ ceramic sintered at 875 °C has Q _u xf = 39,750 GHz (at 5.89 GHz), ε _r  = 18.3 and τ _f  = −88 ppm/°C.     

Piezoelectric properties of Nb2O5 doped and MnO2—Nb2O5 co-doped Pb(Zr0.53Ti0.47)O3 ceramics

The effects of Nb₂O₅ addition and MnO₂Nb₂O₅ co-addition on the piezoelectric properties of Pb(Zr_0.53Ti_0.47)O₃ (PZT) ceramics are investigated. When Nb₂O₅ is added to PZT, the planar coupling factor (K _p) significantly increases but the mechanical quality factor (Q _M) as well as the electrical quality factor (Q _E) decreases. When MnO₂ and Nb₂O₅ are co-doped, Q _M increases remarkably while K _p rarely changes. This increase in Q _M is ascribed to the domain pinning by Mn as is the case of previously reported MnO₂-doped PZT, and the negligible change in K _p is ascribed to the barely changed tetragonality. Even though MnO₂-Nb₂O₅ co-doped PZT shows a smaller induced strain than that of Nb₂O₅- doped PZT, excellent temperature stability is obtained. It is shown that MnO₂Nb₂O₅ co-doped PZT may be a very suitable material for high-power piezoelectric actuators because of its high K _p, Q _M and Q _E, its low dielectric constant and its excellent temperature stability.     

Influence of different nucleating agent additives on phase structure and ferroelectric properties of SrO–BaO–Nb2O5–CaO–SiO2–B2O3 glass-ceramics

Ferroelectric glass-ceramics of the SrO–BaO–Nb₂O₅–CaO–SiO₂–B₂O₃ system with different nucleating agents Cr₂O₃, BaF₂, ZrO₂, CaF₂ and CeO₂, have been prepared by conventional melt casting followed by controlled crystallization processing. The effect of different nucleating agent additions on crystallization mechanism, crystallization behaviors, microstructures and dielectric properties of the glass-ceramics are studied using DTA, XRD techniques, SEM and LCR analyzer. The results show that the addition of different nucleating to the glass-ceramics decreases the crystallization temperatures, accelerates the secondary phase elimination and promotes the uniform distribution of grain size. The well development of microstructure promoted by the nucleating agents can result in the improvement of the dielectric constant as well as energy storage density remarkably. The sample with CeO₂ as the optimum nucleating agent achieves the dielectric constant of 301, breakdown strength of 622 kV/cm and the highest energy storage density of 5.15 J/cm³.     

Development and bioactivity evaluation of bioglasses with low Na2O content based on the system Na2O–CaO–MgO–P2O5–SiO2

Osteoconductive bioglasses, free of K(2)O and Al(2)O(3) and with content of Na(2)O lower than 10?mol%, were designed based on the ratio (SiO(2)?+?MgO)/(P(2)O(5)?+?CaO?+?Na(2)O) in the system Na(2)O-CaO-MgO-P(2)O(5)-SiO(2). The developed glasses have shown a strong potential for the formation of hydroxycarbonated apatite (HCA) in vitro. The particles of HCA aggregates tend to be of finer size with increasing the ratio of (SiO(2)?+?MgO)/(CaO?+?P(2)O(5)?+?Na(2)O) in the glass chemical composition indicating significant bioactivity. Critical size bone defects created in the femurs of albino adult female rats, and grafted with the glass particles for 12?weeks post implantation, were completely healed by filling with mineralized bone matrix without infection showing a strong potential for new bone formation in vivo. Osteoblasts and osteocytes were observed close to the surface of the granular implants with active areas of bone deposition, resorption and remodelling. The bioglass with lowest (SiO(2)?+?MgO)/(CaO?+?P(2)O(5)?+?Na(2)O) ratio has shown the highest bioactivity while the bioglass with the highest (SiO(2)?+?MgO)/(CaO?+?P(2)O(5)?+?Na(2)O) has shown the lowest bioactivity. The newly formed bone in vivo has shown a similar structure to that of the original bone as indicated by the histology and microstructural results. In addition, Ca/P molar ratio of the newly formed bone was found to be (~1.67), which is similar to that of the original bone.