Effect of Gd and Ti co-doping on electrical performance of Bi2O3 oxygen ion conductor

In this study, a new oxygen ion conductor electrolyte material with high conductivity was reported. Bi2O3 was co-doped with Gd2O3 and TiO2 by solid-phase synthesis method to obtain Bi1-3xGd2xTixO1.5+δ and Bi0.76Gd0.18-xTixO1.5+δ (TGSB) ceramics. The phase composition, surface morphology and electrical properties of TGSB samples were characterized by X-ray diffraction (XRD), scanning electron microscope (SEM) and AC impedance respectively. XRD results show that only 6T12GSB has a single cubic fluorite structure, and impurity phases appear in other samples. The SEM results show that only when the doping concentrations of Gd and Ti are 12 mol.% and 6 mol.% respectively, the particle size is relatively smaller and there are no obvious pores. From the analysis of electrical properties, Gd dopant is helpful to the stability of the Bi2O3 phase than Ti dopant, but not beneficial to higher conductivity. When the doping concentration of Gd is higher than 16 mol.%, it becomes more stable. The conductivity of the 6T12GSB sample is relatively high. The conductivity of the TGSB samples is higher than that of the TLSB samples under the lower temperature condition (< 450 °C), and the electrical performance of Bi2O3-based materials are enhanced especially at low temperature.

Effects of (Li-Na)2CO3 on The Electrical Properties of Calcia-Stabilized Zirconia/Carbonate Composite Electrolytes

Calcia-Stabilized Zirconia (CSZ) is potential as low-cost electrolytes material for Solid oxide fuel cell (SOFC). It is fluorite structure widely known as oxygen ion conductors. Addition of carbonate salt into fluorite-based electrolyte is common to overcome low conductivity of CSZ as well as to decrease sintering temperature. Increase of ionic conductivity in the SOFC electrolyte is also influenced by presence of protons (H+), so it is called mixed-ion electrolytes. In this study, the effect of sintering temperature and carbonate content prepared from mixture of Li2CO3 and Na2CO3 on the relative density, ionic conductivity and microstructure of electrolyte and fuel cell performance was investigated. The sintered CSZ/carbonate samples were examined physically and electrochemically by using SEM, TEM, XRD, and EIS. The unique detail of nanostructure for CSZ/carbonate was investigated by TEM. The XRD is to observed peak associating with CSZ, Li2CO3, and Na2CO3, as well as its crystallinity. Moreover, the electrolyte resistance was measured by EIS so that the proton conductivity and oxygen conductivity of CSZ/carbonate can be calculated. The improvement of low-cost electrolyte material such as CSZ can be realized by providing protons pathway.

Controllable sites and high-capacity immobilization of uranium in Nd2Zr2O7 pyrochlore

As potential nuclear waste host matrices, two series of uranium-doped Nd2Zr2O7 nanoparticles were successfully synthesized using an optimized molten salt method in an air atmosphere. Our combined X-ray diffraction, Raman and X-ray absorption fine-structure (XAFS) spectroscopy studies reveal that uranium ions can precisely substitute the Nd site to form an Nd2–x U x Zr2O7+δ (0 ≤ x ≤ 0.2) system and the Zr site to form an Nd2Zr2–y U y O7+δ (0 ≤ y ≤ 0.4) system without any impurity phase. With increasing U concentration, there is a phase transition from pyrochlore (Fd 3 m) to defect fluorite (Fm 3 m) structures in both series of U-doped Nd2Zr2O7. The XAFS analysis indicates that uranium exists in the form of high-valent U6+ in all samples. To balance the extra charge for substituting Nd3+ or Zr4+ by U6+, additional oxygen is introduced accompanied by a large structural distortion; however, the Nd2Zr1.6U0.4O7+δ sample with high U loading (20 mol%) still maintains a regular fluorite structure, indicating the good solubility of the Nd2Zr2O7 host for uranium. This study is, to the best of our knowledge, the first systematic study on U-incorporated Nd2Zr2O7 synthesized via the molten salt method and provides convincing evidence for the feasibility of accurately immobilizing U at specific sites.

Interface Resistance of an SOFC Cathode with a Pr1-xTbxO2-d Active Layer

We investigated the use of Pr1-xTbxO2-d (x=0.0-1.0) material for active layer in SOFC cathode. Pr1-xTbxO2-d (x=0.0-1.0) in single-phase fluorite structure were successfully synthesized. They are solid solution of Pr6O11 and Tb4O7. When the x is between 0.3 and 0.6, the phase transition between room temperature and 800oC were eliminated Coin cells with GDC electrolyte and Pr1-xTbxO2-d (x=0.0-1.0) active layer and LaNi0.6Fe0.4O3 (LNF) current collecting layer were made to clarify the effect of this active layer. The interface resistance of these cathodes was measured with an AC impedance method at 800oC. The cathodes withPr1-xTbxO2-d (for all composition) active layer performed better than that of reference cathode, which has no active layer (consisting only LNF layer).

Green Synthesis of Nanostructure CeO2 Using Tea Extract: Characterization and Adsorption of Dye from Aqueous Phase

Nanostructure CeO2 powders were synthesized using tea waste extract as gel precursor. The as-prepared samples were characterized by thermogravimetric analyzer (TGA), X-ray diffraction (XRD), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), and Raman spectroscopy. Based on the TGA/DTG analysis, the intermediates of cerium chloride hydrates (CeCl3.4H2O and CeCl3.H2O) and cerium anhydrous (CeCl3) were produced, and the formation temperature of CeO2 was estimated to be 773 K. The cubic fluorite structure of CeO2 was detected to be the predominant species and was completely formed at the calcination temperature of 773K–1073 K with a crystal size between 8.8 and 11.4 nm based on the XRD measurement. Moreover, the main chemical state of ceria on the surface of the synthesized samples was confirmed to be tetravalent ceria by XPS. All samples show a strong Raman signal at a well-defined chemical shift of 463 cm−1 and a significant symmetry feature was observed, suggesting that the tetravalent ceria is the dominant species throughout the bulk sample. All the synthesized CeO2 calcined at different temperatures showed higher adsorption efficiency for Congo red (CR) compared with commercial CeO2. The adsorption efficiency maintained a steady state of more than 95% when the concentration of CR and adsorption temperature were varied in this study. The kinetic analysis showed that the second-order model was the appropriate model to interpret the adsorption behavior of synthesized CeO2. The calculated adsorption capacity derived from the second-order model is in good agreement with the experimental data. The isotherm analysis revealed that the Freundlich and D-R models fit well for the synthesized CeO2 and represent physisorption with a multilayer mechanism. The thermodynamic parameters, including the changes in Gibb’s free energy, enthalpy, and entropy, suggested that the adsorption of CR on the synthesized CeO2 sample was a spontaneous and endothermic process.

Nanopore Formation in CeO2 Single Crystal by Ion Irradiation: A Molecular Dynamics Study

The nanopore formation process that occurs by supplying a thermal spike to single crystal CeO2 has been simulated using a molecular dynamics method. As the initial condition, high thermal energy was supplied to the atoms in a nano-cylinder placed at the center of a fluorite structure. A nanopore was generated abruptly at around 0.3 ps after the irradiation, grew to its maximum size at 0.5 ps, shrank during the time to 1.0 ps, and finally equilibrated. The nanopore size increased with increasing effective stopping power gSe (i.e., the thermal energy deposited per unit length in the specimen), but it became saturated when gSe was 0.8 keV/nm or more. This finding will provide useful information for precise control of the size of nanopores. Our simulation confirmed nanopore formation found in the actual experiment, irradiation of CeO2 with swift heavy ions, but could not reproduce crystalline hillock formation just above the nanopores.

Microstructural evolution and its influence on oxygen diffusion in yttrium-doped ceria thin films

Ceria doped with rare earth ions (Ce1-xMxO2-y (y=x/2)), which contain many oxygen vacancies, have become excellent electrolyte materials for solid oxide fuel cells and important buffer layers for coated conductors. In this paper, Y3+ ions were doped into the lattice of CeO2 to form Ce1-xYxO2-y (CYO) thin films regarded as buffer layers to reduce oxygen diffusion on silicon substrates. It was revealed that the CYO films gradually transformed from a complete fluorite structure into a defective fluorite structure with more and more oxygen vacancies as the proportion of Y3+ ions in CYO films increased from zero, and then the defective fluorite structure transformed into rare earth C-type structure when the proportion of Y3+ ions was beyond 0.5. Moreover, at the beginning, the degree of oxygen diffusion showed an uptrend, but after the proportion of Y3+ ions reached a certain value, the degree of oxygen diffusion turned into a downtrend.

Perspectives on Pyrochlores, Defect Fluorites, and Related Compounds: Building Blocks for Chemical Diversity and Functionality

In this article we provide some perspectives on a range of pyrochlore and defect fluorite type compounds with nominal A2B2O7, A2BO5, ABC2O7, and other stoichiometries. Typically, the phase transformations and stability fields in these systems are mapped as a function of the ionic radii of the A and B-site cations, e.g., the A/B cation radius ratio (rA/rB). This provides a useful guide to compatible structures and compositions for the development of advanced materials. Pyrochlore commonly transforms to a defect fluorite structure at high temperature in many systems; however, it is not uncommon to observe defect fluorite as the initial metastable phase at low temperature. The patterns of order-disorder observed in these materials are primarily due to the energetics of layer stacking, the defect formation and migration energies of cations and anions, or modulations of the parent cubic structure in 3 + n dimensional space. The first lead to predominantly non-cubic derivatives of the parent defect fluorite structure (e.g., zirconolite polytypes), the second control the order-disorder processes, and the latter lead to a variety of subtle additional scattering features within the cubic parent structure. Although the energetics of cation disorder and anion-vacancy disorder have become more accessible via atomistic approaches (e.g., MD and DFT), we continue to find interesting physical-chemical problems in these materials. For example, although there are significant differences in composition (Tb/Zr ratio and O content) between Tb2Zr2O7 and Tb2ZrO5, both of which are defect fluorites, we note that the modulations found in these two compounds by electron scattering are virtually identical with regard to the direction and magnitude of displacement from the normal Bragg diffracted beams. This suggests that neither the A/B cation ratio nor the oxygen stoichiometry have a significant effect on the modulations. The general observations on the systems of compounds noted in this paper rest primarily in the context of industrial materials for nuclear waste disposal, potential applications in inert matrix fuel designs, and other important technological applications such as ionic conductivity, electrical conductivity, and magnetism. Scientific advances in these areas have been underpinned by recent advances in ion irradiation, synchrotron X-ray, neutron scattering, and modelling and simulation capabilities. Furthermore, there has been some renewed interest in natural samples, e.g., Th-U zirconolite and pyrochlore as analogues for potential host phases in nuclear waste forms. In particular, the natural pyrochlores have provided additional details with regard to radiation damage ingrowth, percolation transitions, and the relationships between accumulated dose and physical properties including hardness, elastic modulus. Specific details of the thermal annealing of these samples have also been elucidated in considerable detail.

Structural Aspects of the Superionic Transition in AX2 Compounds With the Fluorite Structure

Some AX2 binary compounds with the fluorite structure (space group Fm3̄m) are well-known examples of materials exhibiting transitions to ionic superconducting phases at high temperatures below their melting points. Such superionic states have been described as either highly defective crystals or part-crystal, part-liquid states where the A ions retain their crystalline order whilst the X ions undergo partial melting. However, no detailed description of the structure of these phases exists. We present here the results of our investigation of the structural changes that occur during these transitions and the structural characteristics of the resulting superionic materials. This work is based on atomic-scale molecular dynamics modelling methods as well as computational diffraction techniques. We employed a set of empirical potentials representing several compounds with the fluorite structure to investigate any potential-dependent effect. We show the importance of small-scale structure changes, with some local environments showing a hexagonal symmetry similar to what is seen in the scrutinyite structure that has been documented for example in UO2.