Y3NbO7 transparent ceramic series for high refractive index optical lenses

A₂B₂O₇ and A₃BO₇ transparent ceramic families are potential materials for optical lenses because of their high refractive index. Although nonstoichiometry is widely present in these material families, its effect on refractive index and optical properties has not yet been fully studied. In this study, optical properties are reported for the Y₃NbO₇ transparent ceramic series, Y_1−xNb_xO_1.5+x (x = 0.20, 0.22, 0.24, 0.25, 0.26), which were fabricated by a pressureless pre-sintering and a hot isostatic pressing post-sintering treatment. The refractive index increases from 2.04 to 2.10 (at 587.6 nm) as the Nb content x increases, which is mainly attributed to the variation in the oxygen ion/vacancy ratio. The Abbe number is larger than 40, showing a decreasing trend as the Nb content x increases. The specimen with x = 0.24 has the highest inline transmittance, which were 62% and 76% at 587.6 and 2000 nm, respectively, for a 1-mm-thick specimen. Through the approach of nonstoichiometry, Y_1−xNb_xO_1.5+x series exhibit balanced properties of refractive index, Abbe number, and transmittance, which can be considered as a promising candidate for high refractive index optical lenses.     

Preparation,characterization, and giant dielectric permittivity of (Y3+ and Nb5+) co–doped TiO2 ceramics

The microstructure and giant dielectric properties of Y³⁺ and Nb⁵⁺ co–doped TiO₂ ceramics prepared via a chemical combustion method are investigated. A main rutile–TiO₂ phase and dense ceramic microstructure are obtained in (Y_0.5Nb_0.5)_xTi_1-xO₂ (x = 0.025 and 0.05) ceramics. Nb dopant ions are homogeneously dispersed in the microstructure, while a second phase of Y₂O₃ particles is detected. The existence of Y³⁺, Nb⁵⁺, Ti⁴⁺ and Ti³⁺ as well as oxygen vacancies is confirmed by X–ray photoelectron spectroscopy and X–ray absorption near edge structure analysis. The sintered ceramics exhibit very high dielectric permittivity values of 10⁴–10⁵ in the frequency range of 40–10⁶ Hz. A low loss tangent value of ≈0.08 is obtained at 40 Hz. (Y_0.5Nb_0.5)_xTi_1-xO₂ ceramics can exhibit non–Ohmic behavior. Using impedance spectroscopy analysis, the giant dielectric properties of (Y_0.5Nb_0.5)_xTi_1-xO₂ ceramics are confirmed to be primarily caused by interfacial polarization.     

Transport of potassium ions in niobium phosphate glasses

The influence of Nb₂O₅ on the structure and ionic conductivity of potassium phosphate glasses was investigated in glasses with composition xNb₂O₅–(100-x)[0.45K₂O–0.55P₂O₅], x = 10–47 mol%. The Raman spectra of glasses reveal a transition from predominantly orthophosphate to predominantly niobate glass network with increasing Nb₂O₅ content. In the glass structure, niobium forms NbO₆ octahedra which are interlinked with phosphate units for the glass containing 10 mol% Nb₂O₅, but for higher Nb₂O₅ content they become mutually interconnected via Nb-O-Nb bonds. The transport of potassium ions was found to be strongly dependent on the structural characteristics of the glass network. While the mixed niobate-phosphate glass network hinders the diffusion of potassium ions by providing traps that immobilize them and/or by blocking the conduction pathways, predominantly niobate glass network exhibits a rather facilitating effect which is evidenced in the trend of DC conductivity as well as in the features of the frequency-dependent conductivity and typical hopping lengths of potassium ions.     

High-temperature thermoelectric properties of polycrystalline CaMn1-xNbxO3-δ

The structural and thermoelectric (TE) properties of polycrystalline CaMn_1-xNb_xO_3-δ (0.025?≤?x?≤?0.25) were studied with X-ray diffraction (XRD), Fourier transform infrared (FT-IR) spectroscopy, X-ray photoelectron spectroscopy (XPS), field emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM), and electrical transport measurements, with an emphasis placed on the Nb⁵⁺ content. The CaMn_1-xNb_xO_3-δ crystallized in an orthorhombic perovskite structure of the Pnma space group. The density and grain size of the CaMn_1-xNb_xO_3-δ samples gradually decreased when Nb⁵⁺ ions substituted Mn⁴⁺ ions. The CaMn_0.95Nb_0.05O_3-δ sample contained charge-ordered domains, stacking faults, and micro-twins. The substitution of Nb⁵⁺ for Mn⁴⁺ up to x?=?0.15 led to an increase in electrical conductivity, mainly due to an increased electron concentration. The CaMn_1-xNb_xO_3-δ samples with low Nb⁵⁺ contents (0.025?≤?x?≤?0.15) showed metallic behavior, whereas those with high Nb⁵⁺ contents (0.2?≤?x?≤?0.25) showed semiconducting behavior. The Nb⁵⁺ substitution lowered the absolute value of the Seebeck coefficient for the CaMn_1-xNb_xO_3-δ samples due to an increased electron concentration. The largest power factor (1.19?×?10^?4 W?m^?1 K^?2) was obtained for CaMn_0.95Nb_0.05O_3-δ at 800?°C. The partial substitution of Nb⁵⁺ for Mn⁴⁺ in CaMnO_3-δ proved to be highly effective for improving high-temperature TE properties.     

Effect of Ta5+ doping on the thermal physical properties of defective fluorite Y3NbO7 ceramics

The effects of substituting the B cation in A₃BO₇ ceramics on their thermal physical properties were investigated by applying a large mass difference. Y₃(Nb_1-xTa_x)O₇ (x = 0, 0.1, 0.2, 0.3, 0.4, and 0.5) ceramics were synthesized, and their structural characteristics were determined. All the fabricated Y₃(Nb_1-xTa_x)O₇ ceramics showed defective fluorite structures and glass-like low thermal conductivity (1.18−2.04 W/m∙K at 25°C) because of the highly distorted crystal structure and significant mass difference. Substitution with Ta⁵⁺ enhanced the sintering resistance, leading to superior thermal-insulating performance via grain boundary scattering. Furthermore, the ceramics exhibited excellent coefficients of thermal expansion, implying the promising applicability of Y₃(Nb_1-xTa_x)O₇ as new thermal barrier materials. The effect of mass difference on the thermomechanical properties of the ceramics was examined, suggesting a simple strategy for engineering the chemical composition of new thermal barrier materials.     

Improved wear resistance and low thermal radiative conductivity of a middle-entropy tantalate Y0.5Gd0.5Ta0.5Nb0.5O4

Ferroelastic YTaO₄ is a promising material for thermal barrier coatings (TBCs), and its thermal and mechanical properties can be further optimized by various methods. In this study, a ferroelastic middle-entropy ceramic Y_0.5Gd_0.5Ta_0.5Nb_0.5O₄ was synthesized by solid-phase sintering, and its microstructures and thermophysical properties were key points. We determined the contributions of phonons and photons to total thermal conductivity, and a low thermal conductivity was obtained by inhibiting high-temperature thermal radiation. The thermal expansion coefficients were improved by introducing of Nb atom into the lattice of the prepared middle-entropy ceramic, which meets the requirements of TBCs. The improved wear resistance originated from relatively high Young's modulus and the hardness of Y_0.5Gd_0.5Ta_0.5Nb_0.5O₄, and it would contribute to the service performance of the corresponding coatings. This work motivates the engineering applications of ferroelastic RETaO₄ and RENbO₄ (RE is rare-earth element) as high-temperature TBCs.     

Improvement on thermophysical and mechanincal properties of LaMgAl11O19 magnetoplumbite by Nd2O3 and Sc2O3 co-doping

LaMgAl₁₁O₁₉-type magnetoplumbite holds great promise to be used above 1300 °C as thermal barrier coatings (TBCs), but its practical application has been restricted because of inferior thermophysical properties. Herein, we focus on optimizing the thermophysical properties of LaMgAl₁₁O₁₉ by simultaneously substituting La³⁺ and Al³⁺ ions with Nd³⁺ and Sc³⁺ ions, respectively. Results show that the effects of co-substitution on reducing thermal conductivity are pronounced. The thermal conductivities of La_1-xNd_xMgAl_11-xSc_xO₁₉ (x = 0, 0.1, 0.2, 0.3) ceramics decrease progressively with dopant concentration and a lowest thermal conductivity of 2.04 W/(m·K) is achieved with x = 0.3 at 1000 °C, which is a value superior to pure LMA and even lower than YSZ. The mechanisms behind the lowered thermal conductivity are investigated. Increase of the thermal expansion coefficient is also realized (8.53 × 10⁻⁶ K⁻¹ for pure LMA, 9.07 × 10⁻⁶ K⁻¹ for x = 0.3, 1300 °C). Most importantly, Nd³⁺ and Sc³⁺ combination doping indeed facilitates mechanical properties of La_1-xNd_xMgAl_11-xSc_xO₁₉ solid solutions as well. It should be noted that Sc³⁺ doping at Al³⁺ site plays more effective role in improving thermal properties than Nd³⁺ does at La³⁺ site. This work provides a path to simultaneously integrate low thermal conductivity, good phase stability, moderate thermal expansion behavior and excellent mechanical properties on LMA for the next generation TBCs.     

Fabrication of Lu2Ti2O7-Lu3NbO7 solid solution transparent ceramics by spark plasma sintering and their electrical conductivities

The Lu₂Ti₂O₇-Lu₃NbO₇ system, belonging to A₂B₂O₇ with a cubic structure, is attractive for tailored properties by substitution. In this study, Lu_2+0.25xTi_2−0.5xNb_0.25xO₇ (x = 0–4) transparent ceramics were fabrication by reactive spark plasma sintering using commercially available Lu₂O₃, TiO₂ and Nb₂O₅ powders. The phase evolution, microstructure, density, transmittance and electrical conductivity were investigated as a function of composition parameter x. The results showed that Lu_2+0.25xTi_2−0.5xNb_0.25xO₇ transparent ceramic had a pyrochlore structure at x = 0 and 1, while preserved a defect-fluorite structure at x = 2–4. The lattice parameter and theoretical density increased linearly, while the average grain size decreased steadily with increasing composition parameter x. All the specimens exhibited a dense microstructure and the highest in-line transmittance was 64% at 550 nm for x = 4. The bulk conductivity increased with increasing x, reaching a maximum value of 4.2 × 10⁻² S m^-1 for Lu₃NbO₇ at 1073 K.     

Blocking effect in promising proton conductors based on Ba3Ca1.18Nb1.82-xRxO9-δ (R = Y3+, Gd3+, Sm3+, Nd3+) ordered perovskites for PC-SOFCs

High proton conductivity and good chemical stability are keys to development of new electrolytes for PC-SOFCs as the next-future energy generation systems. However, the extensive use of new polycrystalline materials as solid electrolytes is still avoided, since the grain boundary response usually leads to a decrease in total conductivity due to electrical blocking effect. Here, we present our results on the space-charge modeling of impedance spectroscopy data obtained for Ba₃Ca_1.18Nb_1.82-xR_xO_9-δ proton conducting ceramics, where x?=?0, 0.30 and R =?Y³⁺, Gd³⁺, Sm³⁺, Nd³⁺ are doping agents. Non-stoichiometric barium calcium niobate perovskites have received much attention as potential solid electrolytes for proton conducting solid oxide fuel cells. We show that despite their increased grain conductivity, the doped ceramics possess Schottky barriers that are higher than those observed for undoped Ba₃Ca_1.18Nb_1.82O_9-δ. In view of the space-charge model, proton depletion at the space-charge layer is the reason for the reduction of grain boundary conductivity in the doped compositions. Our findings are important for the understanding of proton conduction mechanisms in polycrystalline materials, which may allow future optimization of new doped electrolytes based on barium calcium niobate perovskites.     

Anomalous electrical performance of A-site double-bivalent-doped Bi0.49Na0.5TiO3-δ ceramics from nominal oxygen deficiency to excess

Perovskite structural Bi_0.5Na_0.5TiO₃ (BNT) ferroelectrics can exhibit considerable ionic conductivity, giving a new area for the application as oxygen ion conductors. Through acceptor doping and A-site nonstoichiometry, an ionic conductive mechanism associated with well-resolved arcs in the complex impedance spectra is proposed. Different dielectric responses in ceramics can be deduced to probe the electrical inhomogeneities in separative regions, such as bulk, grain boundary, and electrode, regions. Generally, ionic conductivity only arises at the nominal oxygen deficiency composition of BNT based ceramics. Although large current leakage can exist in the BNT ferroelectrics, they overall display an insulative nature and dominate the electronic conductive contribution, and only a large main arc can be identified in the Nyquist plots. In this work, A-site bivalent doped Bi_0.49-x(SrBa)_xNa_0.5TiO_3-δ and Bi_0.49-x(SrCa)_xNa_0.5TiO_3-δ ceramics ranging from oxygen deficiency to excess are investigated. However, anomalous ionic conductive characteristics are achieved at nominal oxygen excess composition. Herein, the polarization and conductive mechanisms are discussed to elucidate the effect of compositions and their phase and microstructure on the AC impedance, dielectric, and ferroelectric performances. The inhomogeneous distribution of oxygen vacancies, resulting from element segregation or distorted phases in the respective electroactive domains, is a crucial issue in the design of BNT based dielectrics or ionic conductors.     

Effect of niobium valence on the mechanochemical activation of niobium oxides–hematite magnetic ceramic nanoparticles

Three types of nanostructured systems: xNbO·(1?x)α-Fe₂O₃, xNbO₂·(1?x)α-Fe₂O₃, and xNb₂O₅·(1?x)α-Fe₂O₃ were synthesized by ball milling at different molar concentrations (x=0.1, 0.3, 0.5, and 0.7). The effect of Nb valence and milling time on mechanochemical activation of these systems were studied by X-ray diffraction and the Mössbauer spectroscopy measurements. In general, Nb-substituted hematite was obtained at lower molar concentrations for all Nb oxides. For the NbO–Fe₂O₃ system the favorable substitution of Fe²⁺ for Nb²⁺ in the octahedral sites in the NbO lattice was observed after 12 h milling for x=0.7. In the case of the NbO₂–Fe₂O₃ and Nb₂O₅–Fe₂O₃ systems a formation of orthorhombic FeNbO₄ compound was observed, in which Fe³⁺ cations were detected. For the highest concentration of NbO₂ (x=0.7) iron was completely incorporated into the FeNbO₄ phase after 12 h milling. The molar concentrations of x=0.3 and 0.5 were the most favorable for the formation of ternary FeNbO₄ compound in the Nb₂O₅–Fe₂O₃ system. Influence of ball milling on thermal behavior of the powders was investigated by simultaneous DSC–TG measurements up to 800 °C.     

Effect of Y3+-O2- partial substitution with Ca2+-F- on the luminescence enhancement of Y2Mo3O12:Sm3+ red-emitting phosphors

To improve the luminescence property of Sm³⁺ in Y₂Mo₃O₁₂, partial Ca²⁺-F^- co-substituted Y₂Mo₃O₁₂:Sm³⁺ phosphor, namely Y_2-xCa_xMo₃O_12-xF_x:Sm³⁺, was prepared using a solid-state method. The effect of introducing Ca²⁺-F^- ion pairs on structure and luminescence properties of Y₂Mo₃O₁₂:Sm³⁺ was studied in depth. XRD patterns not only manifested that all as-prepared Y_2-xCa_xMo₃O_12-xF_x:Sm³⁺ samples had standard Y₂Mo₃O₁₂ structure, but also indicated the introduction of Ca²⁺-F^- ion pairs did not cause the change of crystal structure. Under the near ultraviolet excitation of 404 nm, the emission peaks of Y₂Mo₃O₁₂:Sm³⁺ were located at 567 nm, 605 nm and 652 nm, respectively, resulting from the 4f→4f electron transitions of Sm³⁺ ions. Furthermore, the luminescence intensity of Sm³⁺ was obviously enhanced through the co-substitution of Y³⁺-O^2- ions with Ca²⁺-F^- ions in Y₂Mo₃O₁₂ structure, and the chromaticity coordinates moved towards red region, which due to the environmental effect of crystal field around Sm³⁺. Besides, the red LED device was manufactured for suitable chromaticity parameters. All results indicated that the as-prepared Y_1.84Ca_0.06Mo₃O_11.94F_0.06:0.10Sm³⁺ red-emitting phosphor could become a promising candidate for application of white light-emitting diodes and plant illumination.     

Electrical conduction behavior in nonstoichiometric BaBixNb5O15±δ tungsten bronze ceramics

The BaBi_xNb₅O_15±δ (BB_xN, 0.98 ≤ x ≤ 1.02) ceramics were synthesized via solid-state reaction to investigate the effect of Bi³⁺ nonstoichiometry on their microstructure and electrical conduction behavior. The study of the relationship between structure and conductive behavior revealed two main conclusions: (1) as the concentration of Bi³⁺ content increased, from deficiency to excess, the oxygen vacancies decreased, and the lattice unit volume V gradually increased; (2) there was a low-frequency Warburg electrode response besides the medium-frequency grain boundary response and high-frequency grain response, and the Bi³⁺ introduction could reduce conductivity. In addition, the dielectric anomalies indicated by the T₁ peak and the T₂ peak at 300 °C and 500 °C are related to the Warburg electrode response and to the Bi vacancy and oxygen vacancy defects, respectively.     

Optical and electrical properties of pressureless sintered transparent (K0.37Na0.63)NbO3-based ceramics

Lead-free (1−x)(K_0.37Na_0.63)NbO₃-xCa(Sc_0.5Nb_0.5)O₃ (x=0.050, 0.070, 0.090, 0.095 and 0.100) transparent ferroelectric ceramics have been fabricated by pressureless sintering procedure. Transmittance of 0.91(K_0.37Na_0.63)NbO₃-0.09Ca(Sc_0.5Nb_0.5)O₃ ceramics sintered in sealed alumina crucible was 15% higher than those sintered unsealed in air. By increasing the content of Ca(Sc_0.5Nb_0.5)O₃, the phase structure of (K_0.37Na_0.63)NbO₃ ceramics transformed from orthorhombic to tetragonal symmetry first and then to pseudo cubic symmetry. The 0.91(K_0.37Na_0.63)NbO₃-0.09Ca(Sc_0.5Nb_0.5)O₃ ceramics exhibited high density (98%), high transmittance (60%) in the near-IR region and relatively good electrical properties (ε_r=1914, tanδ=0.037, T_c=147 °C, P_r=6.88 μC/cm², E_c=8.49 kV/cm). Meanwhile, the introduction of Ca(Sc_0.5Nb_0.5)O₃ induced a composition fluctuation in the (K_0.37Na_0.63)NbO₃ lattice and made the ceramics more relaxor-like, which would lead to a further reduction of light scattering. These results demonstrated that 0.91(K_0.37Na_0.63)NbO₃-0.09Ca(Sc_0.5Nb_0.5)O₃ could be promising lead-free transparent ferroelectric ceramics.     

Ionic/electronic mixed conduction relations in perovskite-type oxides by defect structure

Perovskite-type oxides La_1-aA_aM_1-bB_bO_3-x with A=Sr²⁺, Ln³⁺, Ce⁴⁺ and M=Fe, Co, Ga; B=Co, Fe, Mg were prepared in the concentration range a=0·1 to 1 mol and b=0·1 to 0·5 mol. Additionally, A-substoichiometric compositions were prepared. Preparation conditions for monophase materials and structure types of the perovskite were determined by X-ray investigation. The electrical conductivity as a function of pO₂ in the range 10⁵>pO₂>10⁻¹⁴ Pa and temperature (500 to 1000°C) was measured on ceramic shapes by a dc four-point technique in combination with solid electrolyte coulometry. The ionic part of conductivity in mixed conductors was determined by oxygen permeation measurements. The II–III-perovskites Sr(Co,Fe)O_3-x in their stabilized form are excellent mixed conductors (maximum 500 S cm⁻¹ at 400°C) and have up to 2 orders of magnitude higher oxygen ionic conductivity than the preferred III–III-perovskite La(Sr)Mn(Co)O_3-x. The oxygen ionic conductivity of the electrolyte La(Sr)Ga(Mg)O_3-x, was increased by doping with 0·1 mol Co. By applying higher Co or Fe doping concentrations the lanthanum gallate, becomes a mixed conductor.     

Using yttrium,tungsten, and niobium-oxide additives to improve the electrical resistance of strontium zirconate ceramics

Conclusions It is possible to increase the electrical resistance of materials with vacancy transfer of oxygen anions by filling some of the oxygen vacancies with oxygen admitted by means of additives forming a solid solution.To bring in additional oxygen the cation of the additive should have a higher degree of oxidation than the cation it is replacing in the strontium zirconate.In terms of the effective influence on enhancing the conductivity of strontium zirconate, the additives are arranged in accordance with the amount of admitted oxygen in the series: Y₂O₃ + Nb₂O₅, SrO + WO₃, Y₂O₃+2 WO₃, 2SrO+ Nb₂O₅, Y₂O₃+2ZrO₂, Y₂O₃.Additions of yttrium, tungsten, and niobium oxides will improve the working properties of ceramics based on strontium zirconate, since they increase its electrical resistance by 10 or more times at room temperature, and 4–7 times at 1500°C, which indicates the preservation, up to quite high temperatures, of the marked influence on conductivity of the transfer of oxygen through the oxygen vacancies.Translated from Ogneupory, No. 8, pp. 11–13, August, 1993.     

Magneto-dielectric laminated Ba(Fe0.5Nb0.5)O3-Bi0.2Y2.8Fe5O12 composites with high dielectric constant and high permeability

Magneto-dielectric laminated ceramic composites of xBa(Fe_0.5Nb_0.5)O₃-(1-x)Bi_0.2Y_2.8Fe₅O₁₂(BFN-BYIG) with high volume fractions of the giant dielectric constant material BFN (x=10, 30, 50, 70 wt%) were fabricated by the solid-state sintering method. Microstructure, dielectric and magnetic properties of the composites were investigated. The composites possess stable dielectric properties in the frequency range from 100 Hz to 1 MHz with high dielectric constant and low dielectric loss. The maximum permeability of the magneto-dielectric laminated composites reaches up to about 25. And the magnetic behaviors are strongly dependent on the mass ratio of BYIG. The results indicate that such multilayer structures of BFN/BYIG can enhance the permeability and decrease the dielectric and magnetic loss efficiently.     

Investigation on phase structure,spectral characteristics,microstructure and microwave dielectric properties of Li2Zn[Ti1-x(Co1/3Nb2/3)x]3O8 (0.0 ≤ x ≤ 0.4) ceramics

In this paper, a series of Li₂Zn[Ti_1-x(Co_1/3Nb_2/3)_x]₃O₈ (0.0 ≤ x ≤ 0.4) ceramics were prepared via the conventional solid-state method. The influences of (Co_1/3Nb_2/3)⁴⁺ complex ions on the phase composition, spectral characteristics, microstructure, and microwave dielectric properties of Li₂Zn[Ti_1-x(Co_1/3Nb_2/3)_x]₃O₈ ceramics were studied systematically. XRD analysis accompanied with Rietveld refinements showed that pure Li₂ZnTi₃O₈ solid solution ceramics with the cubic spinel structure were obtained at x = 0.2–0.4. New Raman-active mode of about 858 cm⁻¹ should be attributed to the vibrations of NbO₆ due to the high bond energy of Nb–O bonds, exerting a certain impact on the structure and performance of Li₂Zn[Ti_1-x(Co_1/3Nb_2/3)_x]₃O₈ ceramics. XPS results indicated that Nb⁵⁺ ion donor suppressed the deoxidation process and therefore resulted in the disappearance of Ti³⁺ ion and oxygen vacancy. The downward trend variation in the ε_r value with the increase of (Co_1/3Nb_2/3)⁴⁺ content could be explained by the presence of “compressed” cations and “rattling” cations effect. In addition, the Q × f of the current ceramics was closely dependent on relative density, grain size, FWHM, and oxygen vacancy. Good combined microwave dielectric properties of ε_r = 24.5, Q × f = 91,250 GHz, and τ_f = −16.8 ppm/°C were achieved for the Li₂Zn[Ti_0.8(Co_1/3Nb_2/3)_0.2]₃O₈ ceramic sintered at 1120 °C. High quality factor gives evidence that the Li₂Zn[Ti_0.8(Co_1/3Nb_2/3)_0.2]₃O₈ ceramic is an appealing candidate for highly selective microwave devices.     

Transport properties of proton conductive Y-doped BaHfO3 and Ca or Sr-substituted Y-doped BaZrO3

An electrolyte in fuel cells requires not only high ionic conductivity, but also high transport numbers of ionic conduction. Although Y-doped BaZrO₃ is regarded to be the most promising candidate as the electrolyte in protonic ceramic fuel cells (PCFCs), significant hole conduction generates in wet oxygen at high temperatures. With the aim to increase the transport number of ionic conduction, in this work, Sr and Ca were introduced to partially substitute Ba in BaZr_0.8Y_0.2O_3-δ. The results revealed that a single cubic perovskite phase was obtained for Ba_0.95Ca_0.05Zr_0.8Y_0.2O_3-δ and Ba_1-xSr_xZr_0.8Y_0.2O_3-δ (x = 0.05, 0.10, 0.15, 0.20 or 0.40). However, replacing Ba with Sr resulted in almost no increase in the transport number of ionic conduction in wet oxygen atmosphere, but drastic decrease in proton conductivity at all replacement levels. In addition, Ba_0.95Ca_0.05Zr_0.8Y_0.2O_3-δ shows no meaningful change in the transport number of ionic conduction, compared with BaZr_0.8Y_0.2O_3-δ. Incorporating Ca or Sr into the Ba-site of BaZr_0.8Y_0.2O_3-δ appears to impart no positive influence on electrochemical properties. These interesting results also indicate that the hole conductivity decreases with the decrease in proton conductivity, and will aid to consider the hole conduction mechanism. BaHfO₃ doped with 10 and 20 mol% Y was also prepared. A bimodal microstructure was observed for BaHf_0.9Y_0.1O_3-δ, whereas BaHf_0.8Y_0.2O_3-δ shows uniform grain size after sintering at 1600°C for 24 hours. The transport numbers of ionic conduction and bulk conductivity in such Y-doped BaHfO₃ samples are close to those of BaZrO₃ doped with the same amount of Y.     

Products and microwave dielectric properties of ceramics with nominal composition (Ba0.9Ca0.1)(YxB′1/2)O(3x+4.5)/2 (B′=Nb5+, Ta5+)

The products and microwave dielectric properties of ceramics with nominal composition (Ba_0.9Ca_0.1)(Y_xB′_1/2)O_(3x+4.5)/2 (B′=Nb⁵⁺, Ta⁵⁺) are investigated. When x=0.5, i.e. (Ba_0.9Ca_0.1)(Y_1/2B′_1/2)O₃ (B′=Nb⁵⁺, Ta⁵⁺), the product contains a considerable amount of Y₂O₃ as well as the main perovskite phase. When x=0.3 the product is single phase, equivalent to Ba(Ca_1/9Y_3/9Nb_5/9)O₃ or Ba(Ca_1/9Y_3/9Ta_5/9)O₃. The lattice parameters of these new compounds are smaller than those of Ba(Y_1/2Nb_1/2)O₃ and Ba(Y_1/2Ta_1/2)O₃. The relative permittivities (ε_r) of these new compounds are larger than those of Ba(Y_1/2B′_1/2)O₃ (B′=Nb⁵⁺, Ta⁵⁺). The increase in ε_r of the Nb-system is about 4 times larger than that of the Ta-system. The Q f values of the present ceramics are larger than the Ca-containing perovskite in the (Ba_1−xCa_x)(Mg_1/3Ta_2/3)O₃ system. The sharp increase of ε_r in this study cannot be explained by the Ca²⁺ rattling ion model at the A-site, which applies to the case of the (Ba_1−xCa_x)(Mg_1/3Ta_2/3)O₃ system. A new method to explain the increase in ε_r is discussed.