High oxide ion conduction in sintered Bi2O3 containing SrO,CaO or La2O3

Ionic conduction in sintered oxides of the system Bi₂O₃-SrO was investigated by measuring the conductivity and ion transference number under various conditions. The ion transference numbers were measured by an oxygen concentration cell employing the specimen as the electrolyte.It was found that the solid solution containing 2040 mole% SrO which had a rhombohedral structure was an almost pure oxide ion conductor under a relatively high partial pressure of oxygen, and that the conductivity was several times higher than that of stabilized zirconias at the same temperatures up to 800°C. Oxide ion conduction was confirmed also by quantitative determination of generated O₂ from the anode of the oxygen concentration cell during discharge.The sintered specimens of the systems Bi₂O₃-CaO and Bi₂O₃-La₂O₃ were found also to be oxide ion conductors, and the ion transference numbers were greater than 0.9.     

Oxide ion conduction in the sintered oxides of MoO3-doped Bi2O3

In order to examine the conduction behaviour in the sintered oxides of MoO₃-doped Bi₂O₃, the electrical conductivity in air and the EMF of oxygen gas concentration cell were measured with respect to the phase relation determined by X-ray diffraction.The tetragonal single phase oxide containing 22 mol% MoO₃ was found to be a high oxide ion conductor, the conductivity of which was comparable to those of stabilized zirconias. The partial electronic conduction in this phase was negligibly small at relatively high oxygen pressure. The oxide ion conduction was considered to be attributable to an appreciable amount of oxygen vacancies present in the crystal. In the monoclinic compound 3Bi₂O₃·2MoO₃, the oxide ion conduction was also observed. Although the conductivity of this phase was somewhat lower than that of the tetragonal phase, the activation energy for conduction (53·5 kJ mol^–1) was much lower than the values for usual oxide ion conductors.     

High oxide ion conduction in sintered oxides of the system Bi2O3-Y2O3

The ionic conduction in sintered Bi₂O₂-Y₂O₃ was investigated by measuring the conductivity and the emf of an oxygen concentration cell using the specimen tablet as electrolyte. The face centred cubic phase in this system was found to show high oxide ion conduction accompanied by a little electronic conduction when exposed to air. This phase was stable with a composition of 25 ～ 43 mol % Y₂O₃ over a wide range of temperatures, and the oxide ion conductivity increased with decrease in Y₂O₃. The conductivities of (Bi₂O₃)_0.75 (Y₂O₃)_0.25 were 1.6×10^?1 Ω^?1 cm^?1 at 700°C and 1.2×10^?2 Ω^?1 cm^?1 at 500°C values which are many times higher than those of stabilized zirconia (ZrO₂)_0.90(Y₂O₃)_0.10 at corresponding temperatures. Specimens containing less than 25 mol % Y₂O₃ showed a phase transition at 700 ～ 580°C and the conductivities decreased remarkably below these temperatures. High oxide ion conduction in the fcc phase is attributed to the migration of oxide ion vacancies which were present in an appreciable amount.     

High oxide ion conduction in the sintered oxides of the system Bi2O3-Gd2O3

In order to characterize the conduction behaviour in the sintered oxides of the system Bi₂O₂-Gd₂O₃, the electrical conductivity in air and the emf of the oxygen concentration cell were measured. The new rhombohedral phase found in this system exhibited high oxide ion conduction especially at relatively high oxygen pressure. The rhombohedral phase was stable in the composition range between 10 and 30 mol% Gd₂O₃ below 600° C and was transformed into the face-centred cubic phase with rising temperature, the conduction in which was by oxide ion as in the rhombohedral phase. At concentrations greater than 35 mol% Gd₂O₃, the fcc phase was stable over a wide range of temperature (～900° C) and kept its high oxide ion conduction. The conductivities of rhombohedral (Bi₂O₃)_0.90(Gd₂O₃)_0.10 and fcc (Bi₂O₃)_0.65(Gd₂O₃) _0.35 ^* are 4.5 and 2.4×10^?2 cm^?1 at 600°C, respectively. These are about one order of magnitude higher than that of the well-known yttria-stabilized zirconia at corresponding temperatures. High-oxide ion-conduction in the rhombohedral and fcc phase was considered to be due to the appreciable numbers of oxide ion vacancies in these crystals.     

Conduction in Bi2O3-based oxide ion conductors under low oxygen pressure. I. Current blackening of the Bi2O3-Y2O3 electrolyte

The cathodic current blackening of Bi₂O₃-based oxide ion conductors was examined for the Bi₂O₃-Y₂O₃ electrolyte at low oxygen pressure. In air, more than 500 mA cm^–2 d.c. could be passed at 600° C without causing changes in the electrolyte itself. However, in argon gas, a limiting current of 3 mA cm^–2 was observed and the electrolyte was blackened at the cathode side. The limiting current was ascribed to control by the diffusion of oxygen gas at the cathode. The blackened oxide was found to consist of a mixture of Bi metal and Bi₂O₃-Y₂O₃ solid solution and to exhibit the equilibrium oxygen partial pressure almost corresponding to that of the Bi, Bi₂O₃ mixture.     

Synthesis and ion transport properties of RE3GaO6 (RE = rare earth) oxide ion conductors

First-principles calculations were conducted, which proposed that members of the RE₃GaO₆ (RE = rare earth) system were oxide ion conductors. This study experimentally verified oxide ion conduction in Dy₃GaO₆, Er₃GaO₆, and Nd₃GaO₆. The sintered bodies were synthesized by a solid-state reaction method, and their properties were characterized. The samples with dopants were observed to be mixed electron and oxide ion conductors. Dy_2.85Ca_0.15GaO_6-δ exhibited oxide ion conductivities of 2.1 × 10^?4 S/cm at 973 K, with an oxide ion transport number of 21 % under O₂ gas flow. Additionally, the Rietveld refinement suggested that oxide ion migration might occur via the oxide ion vacancy between the O2 sites. Overall, the oxide ion conductivities of RE₃GaO₆ increased in the following order: Nd > Dy > Er, which was in good agreement with that predicted by using the first-principles calculations. The discrepancy between the experimentally measured and predicted conductivities was caused by the solid-solution limit at the RE site for the dopants.     

Oxide ion and electron mixed conduction in the fluorite-type cubic solid solution in the system Bi2O3-Tb2O3.5

Electrical conduction in sintered oxides of the system Bi₂O₃-Tb₂O_3.5 has been investigated. Oxide ion conduction was observed in the rhombohedral (low temperature) phase and the f c c (high temperature) phase present in the composition range less than 20mol% Tb₂O_3.5. The fcc phase could be stabilized at lower temperatures by adding more than 30 mol % Tb₂O_3.5. In addition to oxide ion conduction, appreciable electronic conduction appeared in this composition range. The oxide ion transport number of this phase decreased with increasing content of Tb₂O_3.5 and the specimens having 40–50 mol % Tb₂O_3.5 showed mixed conduction where electrical conduction was comparably contributed by oxide ions and electrons. Electronic conduction in the fee phase was considered to be due to the change in valence of terbium at high temperatures.     

Bi2O3–Nd2O3–WO3 system: Phase formation,polymorphism, and conductivity

Cubic, tetragonal, and monoclinic (Bi₂O₃)_x (Nd₂O₃)_y (WO₃)_z (x + y + z = 1) solid solutions based on the Bi₂O₃ oxygen ion conductor have been prepared by solid-state reactions in the ternary system Bi₂O₃–Nd₂O₃–WO₃. The field of monoclinic compounds with a Bi_3·24La₂W_0·76O_10.14-type structure has been shown to account for most of the ternary system. Compounds with a cubic fluorite structure exist at the boundary of the monoclinic phase field in two small regions at high (83–91 mol% Bi₂O₃, δ-phase) and low (20–55 mol% Bi₂O₃, δ′-phase) Bi concentrations. The cubic samples of the δ-phase retain their structure only during rapid heating and cooling, but annealing in the range of 300–700 °C results in structure degradation to lower symmetry phases. The monoclinic compounds and Bi-poor cubic compounds (δ′-phase) have good thermal stability. The cubic samples of the δ′-phase are hygroscopic. Their bulk conductivity noticeably increases with atmospheric humidity, suggesting that these materials are potential proton conductors.     

High oxygen ion conduction in sintered oxides of the Bi2O3-Er2O3 system

The phase diagram of the Bi₂O₃-Er₂O₃ system was investigated. A monophasic f c c structure was stabilized for samples containing 17.5–45.5 mol% Er₂O₃. Above and below this concentration range polyphasic regions appear. The f c c phase showed high oxygen ion conduction. The ionic transference number is equal to one for specimens containing 30 mol% Er₂O₃ or less, while an electronic component is introduced at low temperatures for specimens containing 40–60 mol% Er₂O₃. Between 673 K and 873 K a maximum in the conductivity was found at 20 mol% Er₂O₃. (Bi₂O₃)_0.8.(Er₂O₃)_0.20 is found to be the best oxygen ion conductor so far known. The conductivity at 773 K and 973 K is 2.3 ^–1m^–1 and 37 ^–1 m^–1 respectively. These values are 2–3 times higher than the best oxygen ion conductor reported for substituted Bi₂O₃ systems and 50–100 times higher than those of stabilized zirconia (ZrO₂)_0.915(Y₂O₃)_0.085 at corresponding temperatures.     

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.     

Preparation of Bi2WO6 photocatalyst by high-energy ball milled Bi2O3-WO3 mixture

Mechanochemical methods offer a simple and low-cost technique to synthesize nanomaterials. In this work, a new route for preparing Bi₂WO₆ photocatalyst combining high-energy ball milling and solid-state reactions is explored. By using the mechanochemically-activated Bi₂O₃-WO₃ mixture as a starting raw material, the preparation temperature of Bi₂WO₆ is successfully reduced to 400 °C. The obtained Bi₂WO₆ nanopowder as-well-as the mechanochemically-activated Bi₂O₃-WO₃ mixture show a considerable photocatalytic activity for the decomposition of Rhodamine B. Bi₂WO₆ nanopowders calcined at 400 °C exhibited a higher photocatalytic activity respect to powders calcined at a higher temperatures. The small amount of Bi₂O₃ and WO₃ contained in the optimized photocatalysts is thought to be helpful in establishing a complex ternary heterostructure system, allowing for an improved visible-light driven photocatalytic activity. Experimental results show that a combination of high-energy ball milling and solid-state reactions could be a promising technique for fabricating highly efficient photocatalysts.     

Effect of double doping on crystal structure and electrical conductivity of CaO and WO3-doped Bi2O3

The atomic arrangement of WO₃-doped Bi₂O₃ was found similar to that of the fluorite structure. However, the electrical conductivity of WO₃-doped Bi₂O₃ is significantly lower than that of commonly used Y₂O₃-doped Bi₂O₃. The structure and electrical conductivity of samples formulated as (Ca_xW_0.15Bi_0.85−x)₂O_3.45−x (x = 0, 0.1, 0.2 and 0.3) were investigated. The as-sintered (W_0.15Bi_0.85)₂O_3.45 and (Ca_0.1W_0.15Bi_0.75)₂O_3.35 exhibit similar single tetragonal structure that is isostructural with 7Bi₂O₃·2WO₃. Therefore, (W_0.15Bi_0.85)₂O_3.45 and (Ca_0.1W_0.15Bi_0.75)₂O_3.35 formed a superstructure consisting of 10 enlarged cubic fluorite subcells. However, the as-sintered samples consist of a tetragonal structure and tetragonal CaWO₄ for x = 0.2 and 0.3 because the oxygen vacancy concentration increases. The conductivities of (Ca_xW_0.15Bi_0.85−x)₂O_3.45−x (x = 0, 0.1, 0.2 and 0.3) did not exhibit linear dependence with x value. The best conductivity is 2.35 × 10⁻² S cm⁻¹ at 700 °C for x = 0.1 that is higher than that of Ca-free (W_0.15Bi_0.85)₂O_3.45. The higher conductivity of (Ca_0.1W_0.15Bi_0.75)₂O_3.35 than (W_0.15Bi_0.85)₂O_3.45 may result from the higher anion vacancy concentration and more symmetrical structure.     

Bi1.4Er0.6O3‐(La0.74Bi0.10Sr0.16) MnO3‐δ Cathodes Fabricated By Ion‐impregnating Method For IT‐SOFCs

Bi_1.4Er_0.6O₃‐(La_0.74Bi_0.10Sr_0.16)MnO_3‐δ (ESB‐LBSM) composite cathodes were fabricated by impregnating the ionic conducting ESB matrix with the LBSM electronic conducting materials. The ion‐impregnated ESB‐LBSM cathodes were beneficial for the O₂ reduction reactions, and the performance of these cathodes was investigated at temperatures below 700 °C by AC impedance spectroscopy and the results indicated that the ion‐impregnated ESB‐LBSM system had an excellent performance. At 700 °C, the lowest cathode polarisation resistance (R_p) was only 0.07 Ω cm² for the ion‐impregnated ESB‐LBSM system. For the performance testing of single cells, the maximum power density was 1.0 W cm^–2 at 700 °C for a cell with the ESB‐LBSM cathode. The results demonstrated that the unique combination of the ESB ionic conducting matrix with electronic conducting LBSM materials was a valid method to improve the cathode performance, and the ion‐impregnated ESB‐LBSM was a promising composite cathode material for the intermediate‐temperature solid oxide fuel cells.     

Equilibrium phases in the Bi2O3–TiO2–WO3 system

Phase relations in the Bi₂O₃–TiO₂–WO₃ ternary system were evaluated for different compositions calcined in air at 850 °C by means of XRD techniques. A solid solution area was observed for Bi₄Ti₃O₁₂ compositions with a small amount of WO₃. The experimental results suggest the existence of a new phase with a nominal composition close to Bi₃Ti_2.5W_0.5O₁₁. This phase allows the definition of four triangles of compatibility at 850 °C in the ternary system. The new phase was characterized by XRD, SEM and EDS.     

Bi2O3 induced tremella-like Bi2WO6 with visible light catalytic performance

The development of single phase photocatalyst is expected to realize clean energy and pollution treatment. Herein, we reported a novel Tremella-like Bi₂WO₆ catalyst which was obtained by facile hydrothermal technique. The formation of Tremella-like Bi₂WO₆ strongly depended on introduction of Bi₂O₃. Based on the Kirkendall effect, Bi₂O₃ induced Bi(NO₃)₃·5H₂O to form biscuit-like Bi₆O₆(OH)₃(NO₃)₃·3H₂O particles which provided templates and reacted simultaneously with WO₄^2? to synthesize Tremella-like Bi₂WO₆. The Tremella-like Bi₂WO₆ exhibited remarkable visible-light catalytic performance. The degradation rate of RhB dye reached 100% with 10 min, the reduction rate of CO₂ was 5.5 times higher than pure Bi₂WO₆. Moreover, the Tremella-like Bi₂WO₆ catalyst displayed excellent stability during the recycle experiments. The high catalytic activity makes single phase Bi₂WO₆ catalyst great potential in environmental protection field.     

Effect of Bi2O3 on the electrochemical performance of LaBaCo2O5+δ cathode for intermediate-temperature solid oxide fuel cells

The LaBaCo₂O_5+δ−x wt.% Bi₂O₃ (LBCO-xBi₂O₃, x=10, 20, 30, and 40) were prepared as composite cathodes for intermediate-temperature solid oxide fuel cells (IT-SOFCs) via the conventional mechanical mixing method. The effect of Bi₂O₃ on polarization resistance, overpotential, and long-term stability of the LBCO cathode was investigated. An effective sintering aid for LBCO cathode, Bi₂O₃ not only lowers its sintering temperature by ~200 °C, but also improves the electrochemical performance within the intermediate temperature range of 600–800 °C. Electrochemical impedance spectroscopy measurements showed that the addition of 20 wt% Bi₂O₃ to LBCO exhibited the lowest area-specific resistance of 0.020 Ω cm² at 800 °C in air, which was about a seventh of that of the LBCO cathode at the same condition. At a current density of 0.2 A cm⁻², the cathodic overpotential of LBCO-20Bi₂O₃ was about 12.6 mV at 700 °C, while the corresponding value for LBCO was 51.0 mV. Compared to B₂O₃–Bi₂O₃–PbO frit, the addition of Bi₂O₃ significantly improved the long-term stability of cathode. Therefore, LBCO-20Bi₂O₃ can be a promising cathode for IT-SOFCs.     

Structure and electronic properties of δ-Bi2O3 tuned by vacancy and doping: A first-principles study

Pure Bi₂O₃ with high ionic conductivities is considered as a candidate material for an electrolyte in solid oxide fuel cells and oxygen separation membranes. However, its lower structural and thermal stability prevent it application in ion conductivity and photocatalysis at suitable temperatures. Metal oxides are usually used to stabilize its structure to lower temperatures and the underlying mechanism is still unclear. To shed light on the issue, vacancy ordered structures of pure and doped δ-Bi₂O₃ have been studied by first-principles calculations. It have been shown that the structure with combined <110> and <111> vacancy arrangements is energetically favorable compared to either <100>, <110> or <111> vacancy ordered structures. Electronic structure analyses have further verified that δ-Bi₂O₃ has a semiconductor character with an energy gap of 2.0 eV, consistent with the experiment results. The site occupation of doping ions is further analyzed by formation energy, geometry and electronic structures. It is evident that the substitution sites of doping ions depend on the type of the doping ions. The ions with large ion sizes tend to occupy the Bi(2) sites while the ions with small ion sizes tend to occupy the Bi(1) sites. At the same time, the probability of the Y ions occupying the oxygen vacancy sites and the optical properties of the Y-doped Bi₂O₃ are explored. Our investigations reveal that the electronic structure of oxides could be tuned by vacancy and interstitial defects for better conductivity, photocatalytic properties.     

Intermediate phases formation during the synthesis of Bi4Ti3O12 by solid state reaction

In this work, the formation of Bi₄Ti₃O₁₂ by solid state reaction from Bi₂O₃ and TiO₂ starting powders has been studied. The Bi₄Ti₃O₁₂ formation occurs through an intermediate Bi₁₂TiO₂₀ sillenite phase formed at temperatures sligthly over 300 °C. This sillenite phase is stable up to ∼750 °C, but in the presence of TiO₂ reacts to form Bi₄Ti₃O₁₂ at temperatures >500 °C. Raman spectroscopy has been used to evidence the amorphization of TiO₂, demonstrating that the Bi₄Ti₃O₁₂ formation occurs through the reaction of sillenite Bi₁₂TiO₂₀ and TiO₂.     

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.     

Influence of WO3‐Doping on the Microstructure and Electrical Properties of ZnO–Bi2O3 Varistor Ceramics Sintered at 950°C

The phase evolution, microstructure, and electrical properties of WO₃‐doped ZnO–Bi₂O₃‐based varistors were investigated for different amounts x (0 ≤ x ≤ 1.60 mol%) of the dopant. When x was less than 0.40, the dissolved W⁶⁺ in the β‐Bi₂O₃ acted as a donor in the grain boundaries and reduced the electrical properties of the ZnO varistors. However, when x was 0.40 mol%, which meant an amount of WO₃ equal to that of Bi₂O₃, the electrical properties dramatically increased, which means the W⁶⁺ donor effect is removed at the grain boundaries because a new Bi₂WO₆ phase was formed in the grain‐boundary regions. The Bi₂WO₆ phase has high oxygen conductivity at high temperatures; it transfers more oxygen to the grain boundaries in order to further enhance the electrical properties. For x values higher than 0.40 (i.e., an addition of WO₃ that is greater than the content of Bi₂O₃), the electrical properties were steadily reduced in comparison to the composition with x = 0.40. This could be explained by the reduced amount of Co, Mn, and Al at the grain boundaries and in the ZnO grains as a result of their incorporation into the ZnWO₄ phase. The electrical properties of the ZnO grains and the grain boundaries were in agreement with the results of the impedance spectroscopy analysis.