Assessment of structurally stable cubic Bi12TiO20 as intermediate temperature solid oxide fuels electrolyte

Colloid processing and subsequent pressure filtration were used to prepare 14.3 mol% TiO₂ doped Bi₂O₃ (Bi₁₂TiO₂₀, 14BTO) as solid oxide fuel cell electrolyte. Materials characterization and electrical behaviors of 14BTO samples were investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM) and two-point probe DC conductivity. A pure 14BTO with a cubic sillenite single phase was prepared at the sintering process of 850 °C with a high relative sintered density of 96.82%. In situ and batch-type long-term conductivity measurements at 600 °C were carried out to verify the possible reason of degradation. Additional reduction-oxidation tests under CH₄ atmosphere by thermogravimetric analysis (TGA) revealed possible application temperature of 14BTO electrolytes below 700 °C.     

Long-term degradation of Ta2O5-doped Bi2O3 systems

Bismuth oxide in δ-phase is a well-known high oxygen ion conductor and can be used as an electrolyte for intermediate temperature solid oxide fuel cells (IT-SOFCs). 5-10 mol% Ta₂O₅ are doped into Bi₂O₃ to stabilize δ-phase by solid state reaction process. One Bi₂O₃ sample (7.5TSB) was stabilized by 7.5 mol% Ta₂O₅ and exhibited single phase δ-Bi₂O₃-like (type I) phase. Thermo-mechanical analyzer (TMA), X-ray diffractometry (XRD), AC impedance and high-resolution transmission electron microscopy (HRTEM) were used to characterize the properties. The results showed that holding at 800-850 °C for 1 h was the appropriate sintering conditions to get dense samples. Obvious conductivity degradation phenomenon was obtained by 1000 h long-term treatment at 650 °C due to the formation of α-Bi₂O₃ phase and Bi₃TaO₇, and 〈1 1 1〉 vacancy ordering in Bi₃TaO₇ structure.     

The effect of Bi2O3 compensation during thermal treatment on the crystalline and electrical characteristics of bismuth titanate thin films

Bismuth titanate thin films are deposited on ITO/glass substrates by rf magnetron sputtering at room temperature using a Bi₄Ti₃O₁₂ ceramic target. The deposited Bi₄Ti₃O₁₂ films are annealed in a conventional furnace in ambient air for 10 min at temperatures ranging from 550 to 640 °C. One specimen is annealed in a crucible containing additional Bi₂O₃ compensation powder, while the other specimen is annealed in ambient air. XRD analysis shows that the crystal phases of films annealed with Bi₂O₃ powder are better than those of films annealed without Bi₂O₃ powder. Furthermore, the EDS results reveal that the bismuth weight percentage of the former is higher than that of the latter. SIMS analysis shows that the bismuth decreases near the surface of Bi₄Ti₃O₁₂ film annealed without Bi₂O₃ powder, but reveals a stable distribution throughout the film annealed with Bi₂O₃ powder. These results imply that bismuth is readily evaporated during the thermal treatment process, particularly from the region near the film surface. Finally, the dielectric and polarization properties of the thin films annealed with Bi₂O₃ powder are found to be superior to those of the films annealed in ambient air.     

Effect of Y2O3 and Er2O3 co-dopants on phase stabilization of bismuth oxide

Bi₂O₃ compositions were prepared to investigate the effect of rare earth metal oxides as co-dopants on phase stability of bismuth oxide. Compositions containing 9-14 mol% of Y₂O₃ and Er₂O₃ were synthesized by solid state reaction. The structural characterization was carried out using X-ray powder diffraction. The XRD results show that the samples containing 12 and 14 mol% total dopants had cubic structure, whereas the samples with lower dopant concentrations were tetragonal. Comparing the lattice parameters of the cubic phases of (Bi₂O₃)_0.88(Y₂O₃)_0.06(Er₂O₃)_0.06 and (Bi₂O₃)_0.86(Y₂O₃)_0.07(Er₂O₃)_0.07 revealed that lattice parameter decreases by increasing the dopant concentration. The XRD pattern and the powder density results indicated the formation of solid solution in the studied systems. After annealing samples with cubic phase at 600 °C for various periods of time, phase transformation to tetragonal and rhombohedral occurs.     

Densification, microstructural evolution and microwave dielectric properties of fluxed sintered 12R-Ba(Ti0.5Mn0.5)O3 ceramics

Doped hexagonal BaTiO₃ (h-BaTiO₃) ceramics have recently been identified as potential candidates for use in microwave dielectric resonators. However, similar to other common microwave ceramics, doped h-BaTiO₃ ceramics require a sintering temperature higher than 1400 °C. In this study, the effects of Bi₂O₃ and Li₂CO₃ on the densification, microstructural evolution and microwave properties of hexagonal 12R-Ba(Ti_0.5Mn_0.5)O₃ ceramics were examined. Results indicate that Bi₂O₃ and Li₂CO₃ are able to effectively reduce the sintering temperature of 12R-Ba(Ti₀₅Mn_0.5)O₃ ceramics through liquid phase sintering while retaining the hexagonal structure and the microwave dielectric properties. The best results were obtained for the 12R-Ba(Ti_0.5Mn_0.5)O₃ with the additions of 5 wt% Bi₂O₃ sintered at 1200 °C (?_r: 36.0, Qf_r: 6779 GHz, and τ_f: 25.3 ppm/°C), and 5 wt% Li₂CO₃ sintered at 1200 °C (?_r: 28.1, Qf_r: 5304 GHz, and τ_f: 35.3 ppm/°C).     

Synthesis of a TiO2 ceramic membrane containing SrCo0.8Fe0.2O3 by the sol-gel method with a wet impregnation process for O2 and N2 permeation

A SrCo_0.8Fe_0.2O₃ impregnated TiO₂ membrane (TiO₂-SrCo_0.8Fe_0.2O₃ membrane) was successfully prepared using a sol-gel method in combination with a wet impregnation process. The membrane was subjected to a single gas permeance test using oxygen (O₂) and nitrogen (N₂). The TiO₂ membrane was immersed in the SrCo_0.8Fe_0.2O₃ solution, dried and then calcined to affix SrCo_0.8Fe_0.2O₃ into the membrane. The effect of the acid/alkoxide (H⁺/Ti⁴⁺) molar ratio of the TiO₂ sol on the TiO₂ phase transformation was investigated. The optimal molar ratio was found to be 0.5, which resulted in nanoparticles with a mean size of 5.30 nm after calcination at 400 °C. The effect of calcination temperature on the phase transformation of TiO₂ and SrCo_0.8Fe_0.2O₃ was investigated by varying the calcination temperature from 300 to 500 °C. X-ray diffraction spectroscopy (XRD) and Fourier transform infrared (FTIR) analysis confirmed that a calcination temperature of 400 °C was preferable for preparing a TiO₂-SrCo_0.8Fe_0.2O₃ membrane with fully crystallized anatase and SrCo_0.8Fe_0.2O₃ phases. The results also showed that polyvinyl alcohol (PVA) and hydroxypropyl cellulose (HPC) were completely removed. Field emission scanning electron microscopy (FESEM) analysis results showed that a crack-free and relatively dense TiO₂ membrane (∼0.75 μm thickness) was created with a multiple dip-coating process and calcination at 400 °C. The gas permeation results show that the TiO₂ and TiO₂-SrCo_0.8Fe_0.2O₃ membranes exhibited high permeances. The TiO₂-SrCo_0.8Fe_0.2O₃ membrane developed provided greater O₂/N₂ selectivity compared to the TiO₂ membrane alone.     

The growth of interfacial compounds between titanium dioxide and bismuth oxide

The growth of interfacial compounds between TiO₂ and Bi₂O₃ during transient liquid phase bonding at 900, 1000 and 1100 °C for various times was investigated. The microstructures and compositions of compounds in joints were analyzed by means of SEM and EPMA. It was found that the compound Bi₄Ti₃O₁₂ forms initially and replaces the Bi₂O₃ interlayer. Bi₂Ti₄O₁₁ then arises at the interface between Bi₄Ti₃O₁₂ and TiO₂ and the metastable Bi₂Ti₂O₇ phase appears last at the interface between Bi₄Ti₃O₁₂ and Bi₂Ti₄O₁₁. The modes and activation energies of the growth of Bi₄Ti₃O₁₂ and Bi₂Ti₄O₁₁ were determined respectively. Holes in the middle of the joint heated at 1100 °C for 24 h were also found.     

Synthesis and reaction mechanism of Ti3SiC2 ternary compound by carbothermal reduction of TiO2 and SiO2 powder mixtures

The present study aims to investigate synthesis of Ti₃SiC₂ from TiO₂ and SiO₂ powder mixtures by carbothermal reduction method. Equilibrium TiO₂–SiO₂–C ternary phase diagram was used to predict the conditions for the formation of Ti₃SiC₂ at 1800 K under Ar atmosphere. A reactant mixture with a TiO₂:SiO₂ molar ratio of 1.5 and a C content of 68.75 mol% (26.86 wt%) was initially selected among the thermodynamically favorable reactant compositions for the experimental studies. Two different C sources, graphite flakes and pyrolytic C coating, were used to synthesize Ti₃SiC₂ at 1800 K under Ar atmosphere. When graphite flakes were used, the products contained a trace amount of Ti₃SiC₂ phase along with major TiC and minor SiC phases. Whereas, pyrolytic C coating on the oxide particles resulted in the products with much higher Ti₃SiC₂ contents owing to the close contact between the reactants. Optimal C concentration for the C coated oxide mixtures with a TiO₂:SiO₂ molar ratio of 1.5 was determined to be 30.05 wt% under the experimental conditions studied. Ti₃SiC₂ content of the products obtained from this reactant was observed to increase with reaction time to 31 wt% at 75 min beyond which it gradually decreased. XRD studies indicated that the product with the highest ternary carbide content also contained TiC and a trace amount of SiC. SEM-EDS analyses showed that this sample essentially consisted of spherical fine TiC particles and Ti₃SiC₂ nanolaminates. Equilibrium thermodynamic analysis of the TiO₂–SiO₂–C system suggested that the reaction of solid Ti₂O₃ with SiO and CO gases may play a dominant role in the formation of Ti₃SiC₂.     

Effects of the starting materials and mechanochemical activation on the properties of solid-state reacted Li4Ti5O12 for lithium ion batteries

Li₄Ti₅O₁₂ was synthesized by a solid-state reaction between Li₂CO₃ and TiO₂ for applications in lithium ion batteries. The effects of the TiO₂ phase and mechanochemical activation on the Li₄Ti₅O₁₂ particles as well as the corresponding electrochemical properties were investigated. Rutile TiO₂ was more desirable in acquiring high purity Li₄Ti₅O₁₂ than anatase due to the anatase to rutile phase transformation, which was found to be more rigid in the solid-state reaction than the intact rutile phase. Mechanochemical activation of the starting materials was effective in decreasing the reaction temperature and particle size as well as increasing the Li₄Ti₅O₁₂ content. The specific capacity depended significantly on the Li₄Ti₅O₁₂ content, whereas the rate capability improved with decreasing particle size due to the enhanced contact area and reduced diffusion path. Overall, a 200 nm-sized Li₄Ti₅O₁₂ powder with a specific capacity of 165 mAh/g could be synthesized by optimizing the milling method and starting materials.     

Effect of TiO2 on phase evolution and microstructure of MgAl2O4 spinel in different atmospheres

The effect of TiO₂ on the formation and microstructure of magnesium aluminate spinel (MgAl₂O₄) at 1600 °C in air and reducing conditions were investigated. Under reducing conditions, stoichiometric MgAl₂O₄ spinel shifted toward alumina-rich types owing to volatilization of MgO, resulting in an increase in the porosity of fired samples. Addition of graphite to mixtures of MgO and Al₂O₃ intensified the reducing conditions and accelerated the formation of non-stoichiometric MgAl₂O₄. For TiO₂-containing samples on addition of MgAl₂O₄, magnesium aluminum titanium oxide (Mg_xAl_2(1−x)Ti_(1+x)O₅, x = 0.2 or 0.3) was detected as a minor phase. Under reducing conditions, XRD peak shifts were smaller for TiO₂-containing samples than for samples without TiO₂ owing to the formation of a solid solution of TiO₂ in MgAl₂O₄ and establishment of alumina-rich spinel, which have opposite effects on increasing the lattice parameter. In bauxite-containing samples, MgAl₂O₄ spinel, corundum, magnesium orthotitanate spinel (Mg₂TiO₄) and amorphous phases were identified. Mg₂TiO₄ spinel formed a complete solid solution with MgAl₂O₄ spinel but Mg₂TiO₄ remained as a distinct phase owing to the heterogeneous microstructure of bauxite-containing samples. Also dense microstructure established in air fired TiO₂ containing samples. The results are discussed with emphasis on the application and design of alumina-magnesia-carbon refractory materials, which are used in the steel industry.     

Microstructures and dielectric properties of spark plasma sintered Ba0.4Sr0.6TiO3/CaCu3Ti4O12 composite ceramics

(1 − x)Ba_0.4Sr_0.6TiO₃/xCaCu₃Ti₄O₁₂ composite ceramics were prepared by spark plasma sintering. Sintering behavior, microstructures and dielectric properties of the composite ceramics were investigated by XRD, SEM, EDS and dielectric spectrometer. Dense composite ceramics consisting of Ba_0.4Sr_0.6TiO₃ phase and CaCu₃Ti₄O₁₂ phase were prepared at 800 °C for 0 min. The dielectric loss of the composite ceramic decreased with increasing amount of Ba_0.4Sr_0.6TiO₃, and the high dielectric constant were retained. Moreover, the better temperature stability of dielectric constant was obtained. These improvements of dielectric characteristics have great scientific significance for potential application.     

Synthesis of Bi0.5(Na0.7K0.2Li0.1)0.5TiO3 powders through the molten salt method

The Bi_0.5(Na_0.7K_0.2Li_0.1)_0.5TiO₃ powder synthesis through molten salt method was investigated in the temperature range of 650–700 °C for 2–4 h. The XRD results indicated that the optimal synthesizing temperature for molten salt method was 700 °C, significantly lower than that for conventional processing route of solid state reaction method, where a calcining temperature of 850 °C was needed. The SEM results revealed better crystallization of the powders obtained through molten salt method, compared with those through the conventional processing route of solid state reaction method.     

All-solid-state lithium battery with a three-dimensionally ordered Li1.5Al0.5Ti1.5(PO4)3 electrode

To fabricate all-solid-state Li batteries using three-dimensionally ordered macroporous Li_1.5Al_0.5Ti_1.5(PO₄)₃ (3DOM LATP) electrodes, the compatibilities of two anode materials (Li₄Mn₅O₁₂ and Li₄Ti₅O₁₂) with a LATP solid electrolyte were tested. Pure Li₄Ti₅O₁₂ with high crystallinity was not obtained because of the formation of a TiO₂ impurity phase. Li₄Mn₅O₁₂ with high crystallinity was produced without an impurity phase, suggesting that Li₄Mn₅O₁₂ is a better anode material for the LATP system. A Li₄Mn₅O₁₂/3DOM LATP composite anode was fabricated by the colloidal crystal templating method and a sol-gel process. Reversible Li insertion into the fabricated Li₄Mn₅O₁₂/3DOM LATP anode was observed, and its discharge capacity was measured to be 27 mA h g⁻¹. An all-solid-state battery composed of LiMn₂O₄/3DOM LATP cathode, Li₄Mn₅O₁₂/3DOM LATP anode, and a polymer electrolyte was fabricated and shown to operate successfully. It had a potential plateau that corresponds to the potential difference expected from the intrinsic redox potentials of LiMn₂O₄ and Li₄Mn₅O₁₂. The discharge capacity of the all-solid-state battery was 480 μA h cm⁻².     

Phase diagram of the Al2O3-HfO2-Y2O3 system

The phase diagram of the Al₂O₃-HfO₂-Y₂O₃ system was first constructed in the temperature range 1200-2800 °C. The phase transformations in the system are completed in eutectic reactions. No ternary compounds or regions of appreciable solid solution were found in the components or binaries in this system. Four new ternary and three new quasibinary eutectics were found. The minimum melting temperature is 1755 °C and it corresponds to the ternary eutectic Al₂O₃ + HfO₂ + Y₃Al₅O₁₂. The solidus surface projection, the schematic of the alloy crystallization path and the vertical sections present the complete phase diagram of the Al₂O₃-HfO₂-Y₂O₃ system.     

Low-temperature synthesis of Bi2Fe4O9 nanoparticles via a hydrothermal method

Bi₂Fe₄O₉ (BFO) nanoparticles were successfully synthesized by a hydrothermal method at a temperature as low as 100 °C. The as-prepared powders, characterized by X-ray diffraction (XRD), energy dispersive spectroscopy (EDS), transmission electron microscope (TEM) and physical property measurement system (PPMS), exhibited a pure BFO phase about 100 nm size with uniform sheet-like shape and exhibited an AF order at room temperature. It was found that high alkali concentration and alkali ion Na⁺ played a key role in the formation of BFO nanoparticles at a low temperature of 100 °C.     

Ti addition effect on α-Al2O3 flakes synthesis using a mixture of boehmite and potassium sulfate

Single-crystal α-Al₂O₃ hexagonal flakes with a diameter of about 200 nm and 20 nm in thickness were obtained by mixing different molar ratios of potassium sulfate to boehmite and heating at 1000 °C. Co-doping 1 mol% TiO₂ can increase the shape anisotropy of α-Al₂O₃ hexagonal flakes, increasing the diameter to 400 nm. The effects of potassium sulfate, Fe₂O₃ and TiO₂ on the phase transformation and morphology development of alumina were investigated using X-ray diffraction analysis (XRD), differential thermal analysis (DTA) and transmission electron microscopy (TEM). The results indicate that co-doping potassium sulfate, Fe³⁺ and Ti⁴⁺ can promote γ → α-Al₂O₃ phase transformation and change the morphology from a vermicular structure into hexagonal platelets. The shape anisotropy of α-Al₂O₃ hexagonal flakes can be increased by adding TiO₂ due to the segregation of Ti⁴⁺ ions onto the surfaces of basal planes of α-Al₂O₃ single crystal particle.     

Preparation of columbite MgNb2O6 and ZnNb2O6 ceramics by reaction-sintering

Columbite MgNb₂O₆ (MN) and ZnNb₂O₆ (ZN) ceramics produced by the reaction-sintering process were investigated. Secondary phases Mg_0.652Nb_0.598O_2.25 and Mg_0.66Nb_11.33O₂₉ were found in MgNb₂O₆ pellets. After 1250 °C sintering for 2 h, a density 4.85 g/cm³ (97.1% of the theoretical value) was obtained in MgNb₂O₆ pellets. In ZnNb₂O₆ pellets, no secondary phase formed. The maximum density 5.55 g/cm³ (98.7% of the theoretical value) occurs at 1200 and 1180 °C sintering for 2 and 4 h, respectively.     

Properties of sol-gel SnO2/TiO2 electrodes and their photoelectrocatalytic activities under UV and visible light illumination

A visible light active binary SnO₂-TiO₂ composite was successfully prepared by a sol-gel method and deposited on Ti sheet as a photoanode to degrade orange II dye. Titanium and SnO₂ can promote the development of rutile phase of TiO₂ and inhibit the formation of anatase phase of TiO₂. Formation of SnO₂ crystalline is insignificant even when the calcination temperature increases to 700 °C. Heterogenized interface between SnO₂ and TiO₂ inhibits growth of TiO₂ linkage and leads to the particle-filled surface morphology of SnO₂-containing films. The carbonaceous, Ti-O-C bonds and Ti³⁺ species are likely to account for the photoabsorption and photoelectrocatalytic (PEC) activity under visible light illumination. The electrode with 30% SnO₂ exhibits higher photocurrent when compared with those in the region of 0-50%. The 600 °C-calcined SnO₂-TiO₂ electrode indicates higher activity when compared with those at 400, 500, 700 and 800 °C. PEC degradation of orange II follows the Langmuir-Hinshelwood model and takes place much effectively in a solution of pH 3.0 than those in pH 7.0 and pH 11.0.     

Correlation between sintering conditions and water contact angles for Ti–O thick films screen printed on an alumina substrate

TiO₂, TiO_2−x and Ti₃O₅ thick-film structures on corundum Al₂O₃ substrates were prepared using screen-printing technology. The screen-printed deposits were sintered up to 1500 °C in oxidising and reducing atmospheres to vary the Ti⁴⁺/Ti³⁺ ratio and consequently water contact angle. The structure of the thick films was studied with an X-ray powder diffractometer (XRD). The microstructural characteristics and the chemical composition were checked with a scanning electron microscope, equipped with an energy-dispersive spectrometer (EDS). The Ti–O films, up to 55 μm thick, exhibited excellent adhesion to the substrate and had uniform grain- and pore-size distributions. Ti₃O₅ and Al₂O₃ were found to be compatible phases up to 1500 °C in a reducing atmosphere. However, rutile-type TiO₂ and Al₂O₃ are not compatible compounds at temperatures up to 1400 °C, in either oxidising or reducing atmospheres. TiO₂ and TiO_2−x form two types of reaction products with Al₂O₃. These reaction products were found to have various Ti/Al ratios.     

Preparation, characterization and dielectric properties of CaCu3Ti4O12 ceramics

CaCu₃Ti₄O₁₂ nano-sized powders were successfully prepared by sol-gel technique and calcination at 600-900 °C. The thermal decomposition process, phase structures and morphology of synthesized powders were characterized by IR, DSC-TG, XRD, TEM, respectively. It was found that the main weight-loss and decomposition of precursors occurred below 450 °C and the complex perovskite phase appeared when the calcination temperature was higher than 700 °C. Using above synthesized powders as starting materials, CCTO-based ceramics with excellent dielectric properties (?₂₅ = 5.9 × 10⁴, tan δ = 0.06 at 1.0 kHz) were prepared by sintering at 1125 °C. According to the results, a conduction mechanism was proposed to explain the origin of giant dielectric constant in CCTO system.