Na0.5Bi0.5TiO3 phase relations: Thermodynamics and phase equilibria in the systems Bi2O3 – TiO2, Na2O – TiO2, and Na2O – Bi2O3 – TiO2

The lead-free piezoelectric material sodium bismuth titanate (NBT, Na_0.5Bi_0.5TiO₃) has attracted considerable attention owing to its promising dielectric, piezoelectric, and electrical properties. However, the literature on the binary subsystems is contradictory and there are only limited data for the ternary system. The present work surveys all of the reports of the binary subsystems Bi₂O₃ – TiO₂ and Na₂O – TiO₂ and synthesizes these data into inclusive revised versions. The compatibilities for the ternary system Na₂O – Bi₂O₃ – TiO₂ were determined experimentally, thus enabling the construction of a complete isothermal section at 800 °C. The compatibilities associated with the problematic binary subsystem Na₂O – Bi₂O₃, which experiences extreme volatilisation, were determined through the generation of the absent standard-state thermodynamic functions for the relevant binary and ternary phases, thus providing a full suite of thermodynamic data for this system. The thermodynamic stability diagrams for Na₂O, Bi₂O₃, and TiO₂ thus were calculated. The isothermal section also addresses the contradictions in the literature concerning the formation of solid solutions of Bi₁₂TiO_20-x / Bi_12-xTi_1+xO_20+0.5x, pyrochlore (Bi₂Ti₂O₇ / Na_wBi_2-xTi_2-yO_7-z), BTO (Bi₄Ti₃O₁₂ / Na_xBi₄Ti₃O_12+0.5x), and NBT (Na_0.5Bi_0.5TiO₃ / Bi_1±xNa_xTiO_3.5±x). Further, it was observed that the congruent melting point of NBT, which was determined to be 1225 °C, was preceded by the onset of gradual structural destabilization at 940 °C. Also, the NBT rhombohedral → tetragonal phase transformation was observed at an onset temperature of ∼250 °C. The present work thus provides platform data for the fabrication and reactivities of materials in the ternary system Na₂O – Bi₂O₃· TiO₂ and its binary subsystems.     

A comparison study on the substitution of Y3+−Al3+ by M2+−Si4+(M = Ba,Sr, Ca,Mg) in Y3Al5O12: Ce3+ phosphor

Y_1.94MAl₄SiO₁₂:0.06Ce³⁺ (M = Ba, Sr, Ca, Mg) phosphors were successfully prepared through a classic solid-state reaction method. The crystal structures, photoluminescence spectra, quantum yields, and thermal stabilities of the phosphors were investigated in detail. The results indicate that all Y_1.94MAl₄SiO₁₂:0.06Ce³⁺ phosphors maintain the crystal structure of garnets. The emission peaks of Y_1.94MAl₄SiO₁₂:0.06Ce³⁺ (M = Ba, Sr, Ca, Mg) phosphors are located at 537, 538, 554, and 565 nm, respectively. A red-shift trend of emission peak is observed with decreasing M radius, which can be ascribed to the increase in the crystal-field splitting in the Ce³⁺ 5d level owing to the co-doping of M²⁺−Si⁴⁺. Under 460 nm excitation, the luminescence quantum yields and thermal stabilities of the Y_1.94MAl₄SiO₁₂:0.06Ce³⁺ phosphors decreased with the decrease of M radius. The IQE of the Y_1.94BaAl₄SiO₁₂:0.06Ce³⁺ phosphor is 92.89%, and the resistance to thermal quenching is improved to be 93.32% at 150°C. In addition, the color shifts of Y_1.94MAl₄SiO₁₂: 0.06Ce³⁺ phosphors with increasing temperature are all tiny, which also demonstrates good resistance to thermal quenching of luminescence. The linear shrinkage of Y_1.94MAl₄SiO₁₂:0.06Ce³⁺ phosphors is significantly improved compared with that of YAG: Ce³⁺, which is expected to generate Y_1.94MAl₄SiO₁₂:0.06Ce³⁺ transparent/translucent ceramics and fabricate high-powder w-LEDs for high-quality solid-state lighting in the future.     

A3Y2Ge3O12 (A = Ca,Mg): Two novel microwave dielectric ceramics with contrasting τf and Q × f

Two garnet-structured microwave dielectric ceramics of A₃Y₂Ge₃O₁₂ (A = Ca, Mg; called CYG and MYG, respectively) were synthesized. CYG was crystallized in a normal cubic garnet structure with Ca²⁺ fully occupying the dodecahedral (A) site, whereas MYG was an inverse garnet with mixed distribution of Mg²⁺/Y³⁺ at the A site. The difference in ionic occupation resulted in anomalies in microwave dielectric properties, with dielectric constant (ε_r) = 14.1, quality factor (Q × f) = 12,600 GHz and positive temperature coefficient of resonant frequency (τ_f) =120.5 ppm/°C for MYG ceramic and ε_r = 10.8, Q × f = 97,126 GHz and τ_f = −40.6 ppm/°C for CYG ceramic. The large deviations in measured ε_r from theoretical ε_th possibly resulted from the garnet structural constraints leading to ‘rattling’ Mg²⁺ and Y³⁺ in A site of MYG. Infrared reflectivity spectra analysis revealed ion polarization contributed mostly to the permittivity of both ceramics in microwave frequency ranges.     

Influence of Bi2O3 flux in the structural and photoluminescence properties of SrAl2O4:Eu2+ phosphors

SrAl₂O₄:Eu²⁺ phosphors with various content of Bi₂O₃ flux were synthesized and analyzed. It was observed that the crystallinity and the particle size of the phosphors were increased with the addition of Bi₂O₃ flux. These phenomena are considered to be caused via the melting of the Bi₂O₃ flux particles during the synthesis of the phosphors. The melted Bi₂O₃ flux increased the mobility and homogeneity of solid reactants, thereby enhancing the photoluminescence intensity of the phosphors. SrAl₂O₄:Eu²⁺ phosphors with Bi₂O₃ as the flux exhibited a broad green emission with a peak at 520 nm. The highest photoluminescence emission intensity was observed when 5 mol% Bi₂O₃ flux was added into the phosphors. The emission is due to 4f⁶5d→4f⁷ (⁸S_7/2) transitions of the Eu²⁺ ions. Moreover, Bi₂O₃ flux extended the application of the ultraviolet excited phosphors toward the blue-light excited phosphors. Nevertheless, the influence of Bi₂O₃ on the afterglow and the emission color of SrAl₂O₄:Eu²⁺ phosphors were not significant. This research indicated that Bi₂O₃ flux is effective flux for synthesizing SrAl₂O₄:Eu²⁺ phosphors.     

Effect of codoping Ce3+ on the luminescence properties of Sr9Mg1.5(PO4)7:Eu2+ orange–yellow phosphor

Sr₉Mg_1.5(PO₄)₇:Eu²⁺ has recently been reported as a promising blue light-excited orange–yellow phosphor that can be used in white LED device. Here, Ce³⁺-codoping is found to be an effective strategy to improve the luminescence performance of Sr₉Mg_1.5(PO₄)₇:Eu²⁺ phosphor. The coexistence of Eu²⁺ and Eu³⁺ ions has been verified via photoluminescence spectral analysis. The reduction of Eu³⁺ to Eu²⁺ in Sr₉Mg_1.5(PO₄)₇ lattice cannot be completed in a reducing atmosphere, but can be promoted through codoping with Ce³⁺ ions to a great extent, which finally increase the effective concentration of Eu²⁺ in the crystal lattice. The Eu³⁺−Eu²⁺ reduction mechanism is analyzed using a charge compensation model. This work not only achieves enhanced luminescence of the Sr₉Mg_1.5(PO₄)₇:Eu²⁺ phosphor by codoping with Ce³⁺ ions, but also provides new insights into the design of Ce³⁺/Eu²⁺ codoped luminescent materials.     

Screening sintering aids for 0.88(Bi0.4Na0.2K0.2Ba0.2)TiO3–0.12Sr(Mg1/3Nb2/3)O3 high-entropy dielectric ceramics

The liquid phase screening method was used to search for sintering aids for 0.88(Bi_0.4K_0.2Na_0.2Ba_0.2)TiO₃–0.12Sr(Mg_1/3Nb_2/3)O₃ ceramics. The efficiency of the method was tested by studying the effect of three selected oxides on the densification and properties of the ceramics. The results showed that all three selected sintering aids can improve the sintering ability and energy storage performance. The sample with 1.5 mol% SiO₂ exhibited a recoverable energy storage density and storage efficiency of 2.61 J/cm³ and 77.4% at an electric field of 270 kV/cm, indicating its promising applications in the low-to-moderate fields.     

Optimizing the synthesis of Ag/γ-Al2O3 for selective reduction of NOx with C3H6: Experiments and modelling

Ag/γ-Al₂O₃ is an effective catalyst for the selective reduction of NOx (SCR) using propylene as a reducing agent. The catalyst performance is greatly influenced by the synthesis procedure. Various methods for synthesis of Ag/γ-Al₂O₃ are analyzed, and their performance is examined via packed bed reactor experiments in this work. An optimal one-pot synthesis method, single-step sol–gel (SSG) synthesis, is explored systematically. The SSG-synthesized catalyst shows better performance than those prepared via wet impregnation. The influence of synthesis conditions, specifically pH, on the textural and morphological properties of the SSG-synthesized Ag/γ-Al₂O₃, and therefore the activity for hydrocarbon-based SCR in a packed-bed reactor, are analyzed using experiments and simulations. The optimized catalyst demonstrates excellent performance (90% NOx conversion) for NOx reduction under nominal operating conditions with a wide activity temperature window (300–600°C). The catalyst shows good time-on-stream performance and is effective at higher inlet oxygen concentrations and space velocities. A global kinetic model, which uses synthesis-pH-dependent parameters, is proposed, and its ability to predict the activities of these catalysts is validated.     

(Bi13Co11)Co2O40–Co3O4 nanocomposites: Synthesis,characterization and application as substrate for microstrip patch antenna

The sillenite-cobaltite nanocomposites with chemical formula (Bi₁₃Co₁₁)Co₂O₄₀–Co₃O₄ were synthetized and characterized for application as substrate in a microstrip patch antenna. To prepare the sample, the combustion method using urea as fuel was applied. The structural and vibrational characterizations were done through X-ray diffraction and infrared spectroscopy. The morphology of the sample was studied through FESEM images. The magnetization as a function of magnetic field recorded at 300 K confirmed the paramagnetic character of the sample. Electric characterization was performed from 1.0 GHz to 8.5 GHz aiming a wide frequency spectrum data, presenting steady real relative permittivity around 7, with low losses as results. A microstrip patch antenna was designed by cavity method with the nanocomposite as substrate for application in X-band communication services. Reflection coefficient measurements displayed a good agreement with simulations, achieving only 0.39% error in resonance frequency, validating the performed morphological and electrical characterizations, as antenna design. Simulated radiation pattern is also presented, with coherent results for a directive device.     

Phase composition,microstructure and luminescent property evolutions in “light-scattering enhanced” Al2O3-Y3Al5O12: Ce3+ ceramic phosphors

Classic transparent ceramics for laser gain medium and window materials may benefit from their distinctive features of structural homogeneity and high transparency at large scales. However, this has restricted the ability of the ceramics for local light management. Herein, a strategy for ceramic-phosphor design was conducted in the present work via introducing light-scattering centers into single phased YAG: Ce³⁺ transparent ceramics, and an enhancement of light extraction was experimentally realized through the control of Al₂O₃ grain size. UV–vis-NIR diffuse reflectance spectra further confirmed the important roles of the excrescent Al₂O₃ grains. More importantly, PL characterization shows orange-white light emission with high brightness at high temperature. The results highlight that the unique configuration enables simultaneous control of light propagation, luminous efficiency and thermal stability of luminescence, and the design strategy may create great opportunities for laser lighting and displays with high laser power density.     

Defect cluster engineering in ZnGa2−x(Mg/Ge)xO4:Cr3+ nanoparticles for remarkably improved NIR persistent luminescence

Recently, study on Cr³⁺-doped zinc gallate and zinc gallogermanate persistent phosphors has become a hot topic in persistent luminescence and bio imaging areas, because of their near infrared (NIR) emission and long afterglow. However, regulation of efficient traps and improvement of persistent luminescence through bottom-up design are the key challenge. Here, we recommend a new paradigm of chemical unit co-substitution with [Mg²⁺-Ge⁴⁺] substituting for [Ga³⁺-Ga³⁺] in ZnGa₂O₄, which contributes to the opposite charged and distorted octahedral defects of Mg_Ga′ and Ge_Ga ^· in pair around the Cr_N2 ions. The formed defect clusters of Mg_Ga′-Cr_N2-Ge_Ga ^· , which are closely related to the trap depth, can be accurately regulated through varying the doping content of Mg²⁺/Ge⁴⁺ in the resulting spinel solid solutions of ZnGa_2−x(Mg/Ge)_xO₄:Cr³⁺ (x = 0–1.25). Moreover, the defect clusters cannot only store and recharge visible and UV radiations that contributes to the long lasting NIR persistent luminescence but also can enhance the NIR emission intensity at ~695 nm. The persistent luminescence induced by UV light excitation exhibits an improvement at a deeper trap depth, but it follows an opposite law through visible light excitation. The prepared nanoparticles have the advantages of intense NIR emission, long lasting afterglow, and excellent rechargeability for visible/UV radiations, so they are the potential nanoprobes for long-term bio imaging in living animals.     

Effect of M3+ (M = Bi,Al) co-doping on the luminescence enhancement of Ca2ZnSi2O7:Sm3+ orange-red−emitting phosphors

In this paper, the luminescence properties of Ca₂ZnSi₂O₇:Sm³⁺ phosphors were improved by co-doping with M³⁺ (M = Bi, Al) via the sol-combustion method. The structure and luminescence properties of the Ca₂ZnSi₂O₇:Sm³⁺, M³⁺ samples were investigated in detail, especially the luminescence enhancement effect of Bi³⁺/Al³⁺ co-doping. XRD results indicated that moderate Bi³⁺/Al³⁺ co-doping in the host structure did not change the tetragonal structure of Ca₂ZnSi₂O₇. The series of Ca₂ZnSi₂O₇:Sm³⁺, M³⁺ phosphors could be excited by 402 nm near-ultraviolet light and several significant emission peaks were obtained at 567, 604 and 650 nm, which originated from the electron transitions of Sm³⁺ from ⁴G_5/2 to ⁶H_5/2, ⁶H_7/2 and ⁶H_9/2 levels, respectively. The luminescence intensity of Ca₂ZnSi₂O₇:Sm³⁺ was markedly enhanced through Bi³⁺/Al³⁺ co-doping, which could be explained by Al³⁺ decreasing the crystal field symmetry and greatly increasing the red luminescence intensity, and Bi³⁺ functioning as a sensitizer to increasing the luminescence intensity through energy transfer from Bi³⁺ to Sm³⁺ ions. In conclusion, the excellent Ca₂ZnSi₂O₇:Sm³⁺, M³⁺ phosphors have potential application as red phosphors in white light emitting diodes.     

Formation of strontium-yttrium germanium anionic lacunar apatite (Sr2+δY6.67+(2δ/3)[GeO4]6O2δ) as the intermediate phase of oxygen-rich yttrium-germanium apatite (Y9.333+ε[GeO4]6O2+3/2ε)

The formation of metastable strontium-yttrium(III) (hexakis) germanium(IV) lacunar apatite (P6₃/m, a=11.426 Å, c=8.224 Å and Z=2) was observed during the thermal treatment of SrCO₃, Y₂O₃, and GeO₂ powder precursors using the ceramic method in an oxygen atmosphere. The utilization of an oxygen atmosphere restricts the extensive reduction of GeO₂ and enables us to explain a new potential mechanism for the stabilization of lead-free lacunar apatites. This mechanism involves the use of electrons localized in a neighborhood of oxygen vacancies. Charged vacancies serve as active sites for the ionosorption of oxygen molecules and their subsequent transformation into lattice oxygen anions, which fill the vacancies at the X-site. The estimation of system thermodynamics shows that apatite undergoes a transformation to a more stable state at δ≥0.1, i.e., at the value that corresponds to the composition of prepared apatite. As the temperature and time of the thermal treatment increases, the process of incongruent melting leads to the formation of oxygen-rich yttrium-germanium apatite (YGAp, Y_9.333+ε[GeO₄]₆O_2+3/2ε) and strontium-yttrium-germanium glass. The YGAp phase is also formed during the annealing of lacunar apatite in an oxygen atmosphere.     

Improving the energy storage performance of 0.88(Bi0.4Ba0.2Na0.2K0.2)TiO3–0.12Sr(Mg1/3Nb2/3)O3 high-entropy relaxor ceramics by AlN doping

In this study, we report the effects of AlN additives on the microstructure and energy performance of 0.88(Bi_0.4Ba_0.2Na_0.2K_0.2)TiO₃-0.12Sr(Mg_1/3Nb_2/3)O₃ high-entropy relaxor ceramics. We show that AlN partially reacts with/dissolves into the matrix and only forms a secondary phase when its concentration is above a threshold. AlN doping can only affect the shape of the P-E loop and improve the breakdown strength when it formed the secondary phase. The breakdown strength first increased and then decreased with increasing AlN concentration. When the AlN concentration is 6 mol%, the ceramic has the optimal energy storage performance with a breakdown electric field of 340 kV/cm, a recoverable energy density of 3.85 J/cm³, and an efficiency of 85.8%. The study suggests that the addition of AlN is an effective way to improve the energy storage performance of high-entropy ceramics.     

Cost-effective Li4Ti5O12/C–S prepared by industrial H2TiO3 under a carbon reducing atmosphere as a superior anode for Li-ion batteries

Spinel-type Li₄Ti₅O₁₂ (LTO) is known as a “zero-strain” material due to its negligible structural change during the charge/discharge process. However, high production cost and poor electrical conductivity limit the wide application of spinel-type LTO. Herein, a novel preparation process was developed to prepare a Li₄Ti₅O₁₂/C–S composite using industrial H₂TiO₃ as the titanium source. In the calcination process under a carbon reducing atmosphere, SO₄^2? adsorbed on industrial H₂TiO₃ not only promotes the carbonization of glucose on the surface of LTO but also introduces S heteroatoms into the carbon coating layer in the form of C–S bonds through a series of reactions with glucose, which significantly improves the overall conductivity and the Li⁺ diffusion coefficient of the material. Therefore, the synthesized Li₄Ti₅O₁₂/C–S composite exhibits excellent rate performance and cycling stability, with a specific capacity of 135.88 mAh g^?1 even at 10C and a capacity decay rate of only 0.011% per cycle after 1000 cycles at 5C. This study delivers a novel method for the industrial production of LTO with both cost and performance advantages.     

Relying on energy transfer to enhance the 2.9 μm emission by co-doped Tm3+ ions in Dy:Y2O3 transparent ceramics

2 at.% Tm, xat.% Dy:Y₂O₃ (x = 0, 0.1, 0.5 and 1) transparent ceramics were fabricated via vacuum sintering. The microstructural properties of the prepared ceramics were determined using XRD and SEM. The absorption cross-section of 2 at.% Tm, 1 at.% Dy:Y₂O₃ ceramic was 0.53 × 10^?20 cm² with the FWHM of 43.59 nm. The increased cross-section originates from a large overlapping range appearing in the absorption spectrum of the Dy³⁺:⁶H_15/2 → ⁶F_5/2 and Tm³⁺:³H₆→⁶H₄ transitions. The J-O intensity parameters Ω₂, Ω₄ and Ω₆ and the fluorescence characteristics of the pivotal luminescent level of the Dy³⁺ ions were investigated. Under 793 nm excitation, the emission cross section of the Tm,Dy:Y₂O₃ ceramic at 3094 nm was 3.63 × 10^?21 cm² with the FWHM of 355 nm. The fluorescence lifetimes of Dy³⁺:⁶H_13/2 level of 2 at.% Tm, xat.% Dy:Y₂O₃ (x = 0.1, 0.5 and 1) ceramics were fitted to be 357 μs, 282 μs and 149 μs, respectively. In order to explore the quenching mechanism of Tm³⁺:³F₄ level, the fluorescence lifetimes of Tm³⁺:³F₄ of the 2 at.% Tm, xat.% Dy:Y₂O₃ ceramics (x = 0, 0.1, 0.5 and 1) were measured to be 4.878 ms, 462 μs, 104 μs and 61 μs, respectively. The possible energy transfer mechanisms between Tm³⁺ and Dy³⁺ ions are discussed. The results show that adding Tm³⁺ ions to Dy:Y₂O₃ ceramics can effectively enhance the 2.9 μm MIR through energy transfer.     

The catalytic activity of Ru and Ir supported on Eu2O3 for the reaction,CO2 + CH4 ⇄ 2 H2 + 2 CO: a viable solar-thermal energy system

Methane and carbon dioxide which often co-exist in nature, constitute valuable potential resources both for the storage of solar (thermal) energy and the convenient production of synthesis gas. Ru and Ir supported on Eu₂O₃ are effective catalysts for the reforming of methane with carbon dioxide to produce synthesis gas with no significant coke formation even after prolonged activity. Ru catalysts show onset of activity around 673 K reaching a maximum at about 973 K. Loadings as low as 1% by weight of Ru are highly effective. Ir catalysts show onset around 823 K reaching a maximum conversion in excess of 94% with respect to CO₂ at 1023 K for a 1% loading of the metal. X-ray absorption studies on the spent Ru/Eu₂O₃ catalysts indicate the presence of the reduced metal, primarily in the form of large spherical particles (> 30 Å) of hexagonal structure. 5% Ir supported on Eu₂O₂ was found to transform an equimolar mixture of H₂ and CO to methane and carbon dioxide with a 42% conversion with respect to H₂ at around 848 K and atmospheric pressure. A 5% loading of Ru on Eu₂O₃ produced a 14% conversion with respect to H₂ at 973 K under similar conditions.Commonwealth Academic Fellow on leave from University of Peradeniya, Peradeniya, Sri Lanka.     

Insights in the sol–gel processing of Pb(Mg1/3Nb2/3)O3. The synthesis and crown structure of a new lead magnesium cluster: Pb6Mg12(μ-OAc)6(μ2,η2-OAc)18(μ3,η2-OC2H4OPri)12

The synthesis and molecular structure of a Pb–Mg bimetallic acetatoalkoxide (Pb₆Mg₁₂(μ-OAc)₆(μ₂,η²-OAc)₁₈(μ₃,η²-OC₂H₄OPrⁱ)₁₂, space group R-3, a=b=30.032(2), c=18.855(2) Å, α=β=90°, γ=120°) are discussed in this article. This compound was isolated as an intermediate during the elaboration of Pb(Mg_1/3Nb_2/3)O₃ (PMN) using sol–gel process. It results from the reaction of a bimetallic Mg/Nb species with lead acetate in 2-isopropoxyethanol.     

Catalytic cracking of rubber seed oil using basic mesoporous molecular sieves K<Subscript>2</Subscript>O/MeO-SBA-15 (Me = Ca,Mg or Ba) as heterogeneous catalysts for the production of liquid hydrocarbon fuels

Liquid hydrocarbon fuels obtained from the catalytic cracking animal fats and plant oils have become one kinds of the attractive fuels because of their possible environment benefits and the current concern over the depletion of fossil fuel sources. In this work, using the combined methods of one-pot synthesis and wetness-impregnation, some basic mesoporous molecular sieves K₂O/MeO-SBA-15 (Me?=?Ca, Mg or Ba) were prepared, characterized and used in the catalytic cracking of rubber seed oil (RSO). The results indicated that the catalysts K₂O/MeO-SBA-15 had better catalytic performances than MeO-SBA-15, assigning to their stronger basicity. The catalyst K₂O/MgO-SBA-15 obtained with 15 wt% KNO₃ impregnation concentration showed the excellent catalytic performance with about 93.2% conversion and 78.3% yield of liquid hydrocarbon biofuel. The obtained liquid biofuel had similar chemical composition to diesel-based fuels and showed good cold flow property, high calorific and low acid value. Importantly, the catalyst K₂O/MgO-SBA-15 was of excellent reusability, and it was reused with negligible loss in its catalytic performance for five times, attributing to the MgO layer between silicon skeleton and potassium species which prevents the reaction between silicon in the framework and potassium species.

Graphical Abstract

Open image in new window