Electrical Characterization of a Joined Electroceramic, La0.85Sr0.15MnO3

Pieces of saw-cut La_0.85Sr_0.15MnO₃ were joined at 1150° and 1250°C under a compressive stress. The strains to form the joints were ∼0.1. Joints formed by plastic deformation were examined using scanning electron microscopy, and they were indistinguishable from the bulk. The room-temperature direct-current resistivity of the joined pieces was identical to that measured in the bulk material. This indicated that a sound, electrically conducting joint could be formed using plastic deformation (grain-boundary sliding) with little surface preparation.     

Electrophoretic Deposition of Dense La0.8Sr0.2Ga0.8Mg0.115Co0.085O3−δ Electrolyte Films from Single-Phase Powders for Intermediate Temperature Solid Oxide Fuel Cells

La_0.8Sr_0.2Ga_0.8Mg_0.115Co_0.085O_3−δ (LSGMC) powders were prepared by polymeric precursor synthesis, using either polyvinyl alcohol (PVA) or citric acid (CA) as complexing agents. The powders were synthesized using different ratios between the complexing agent and the cations dissolved in solution. The obtained polymer gel precursors were dried and calcined at temperatures between 1000° and 1450°C. Single-phase LSGMC powders were obtained at a firing temperature of 1450°C, using PVA and a molar ratio between the hydroxylic groups and the total cations of 3:1. Phase-pure LSGMC powders were used to sinter (1490°C, 2 h) thick pellets. The functional properties of LSGMC pellets were assessed by electrochemical impedance spectroscopy. The electrical conductivity values and the apparent activation energies in different transport regimes were in agreement with literature data. The same LSGMC powders were deposited by electrophoretic deposition (EPD) on a green membrane containing lanthanum-doped ceria (La_0.4Ce_0.6O_{2− x} , LDC), a binder, and carbon powders. The LSGMC/LDC bi-layer obtained by EPD was cofired at 1490°C for 2 h. A dense and crack-free 8-μm-thick LSGMC film supported on a porous skeleton of LDC was obtained. The combined use of proper powder synthesis and film processing routes has thus proven to be a viable way for manufacturing anode-supported LSGMC films.     

Crystal Structure Refinement of La0.683(Ti0.95Al0.05)O3 Perovskite by the Rietveld Method

The structural parameters of orthorhombic and tetragonal phases (space group P / mmm and P 4/ mmm ) of A-site deficient La_0.683(Ti_0.95Al_0.05)O₃ perovskite have been refined by Rietveld analysis through the X-ray powder diffraction patterns measured in the temperature range from 15° to 500°C. With an increase in temperature the unit-cell parameters a , b , and c increased, while the b / a ratio decreased and became unity between 200° and 400°C. No significant changes were observed for atomic coordinates throughout the temperature studied. These results strongly suggest that the phase transition is induced by lattice distortion.     

Structural Analysis of a New Layered Compound: La0.05Sr0.95MnO3

We have found a new phase of La_0.05Sr_0.95MnO₃ with a 30-layer rhombohedral structure by using electron microscopy. The lattice constants were hexagonal axes of a = 0.5444 nm and c = 6.7582 nm. Both weak and strong intensities appeared in selected area diffraction (SAD) patterns. The strong intensities were caused by the periodicity of 15 (Sr,La)O₃ layers that had a new stacking sequence of (cchch)₃. However, the weak intensities indicated that the 15-layer structure has modulation along the c -direction that is twice as long as that of the structure indicated by the strong intensities. We concluded that the modulation of the 30-layer structure was produced by the introduction of two kinds of oxygen octahedra, Mn³⁺O₆ and Mn⁴⁺O₆.     

Electrical Conduction Behavior of La1+xSr1−xGa3O7–δ Melilite-Type Ceramics

Ceramics of the melilite-type compound La_{1+ x} Sr_{1− x} Ga₃O_7−δ were prepared by conventional ceramic processing. Samples prepared represented the entire homogeneity region of the phase (i.e., x =−0.15 to 0.60). Electrochemical characterization under variable temperature and atmospheric conditions in the vicinity of air entailed four-point direct-current conductivity measurements and electromotive force measurements. La_{1+ x} Sr_{1− x} Ga₃O_7−δ samples exhibited a p -type behavior with generally increased conductivity with increased substitution of lanthanum for strontium, which reached a saturation value of ∼10⁻¹ S·cm⁻¹ at 950°C.     

Ferromagnetism and Magnetoresistance in La0.67Sr0.33Fe0.07Mn0.93O3

A bulk ceramic sample La_0.67Sr_0.33Fe_0.07Mn_0.93O₃ (LSFMO) with a rhombohedral structure has been prepared from a coprecipitated carbonate precursor in this study. Ferromagnetism and a negative, isotropic magnetoresistance (MR) as large as 11% have been observed in a ceramic sample of LSFMO. There are two resistivity transition peaks on the resistivity versus temperature curves. The resistivity peak and MR have been related to the ferromagnetic state in LSFMO.     

Electrical Conduction in Pure and Li-Substituted Co3O4

Electrical conductivity and thermoelectric power measurements on pure and Li-substituted Co₃O₄ were performed at 200° to 900°C. Impurity-induced p-type semiconduction was observed at low temperatures and intrinsic behavior at high temperatures. A hopping scheme for electron hole conduction was confirmed, and site exchange of cations in the spinel structure was proposed to explain the electrical conduction at high temperatures.     

Neutron Powder Diffraction Study of a Phase Transition in La0.68(Ti0.95Al0.05)O3

Crystal structures and structural changes of the compound La_0.68(Ti_0.95Al_0.05)O₃ have been studied using neutron powder diffraction data and the Rietveld method in the temperature range from 25° to 592°C. The Rietveld profile-fitting analyses of the neutron data and the synchrotron diffraction profile revealed that the crystal symmetry of the low-temperature phase of La_0.68(Ti_0.95Al_0.05)O₃ is orthorhombic Cmmm (2 a _p× 2 a _p× 2 a _p; p: pseudo-cubic perovskite). The unit-cell and structural parameters were successfully refined with the orthorhombic Cmmm for the intensity data measured at 25°, 182°, and 286°C, and with the tetragonal P 4/ mmm ( a _p× a _p× 2 a _p) for intensity data obtained at 388° and 592°C. The P 4/ mmm -to- Cmmm phase transition was found to be induced by tilting of the (TiAl)O₆ octahedron. The tilt angle decreased with increasing temperature, reaching 0° at the Cmmm – P 4/ mmm transition temperature.     

Synthesis and Electrical Properties of Dense Ce0.9Gd0.1O1.95 Ceramics

Uniform spherical powders of Ce_0.9Gd_0.1O_1.95 with an average diameter of 250 nm were obtained at 700°C from a sol-gel process of mixing nitrates and ethylene glycol. Broadening of the X-ray peaks of the fluorite structure reveals a small crystallite size within the powders. Sintering in air of pressed pellets of the powders at 1585°C gives ceramics of 99% theoretical density with grain sizes 1-10 µm and a cubic unit cell a = 5.422 ± 0.034 Å. The electrical conductivity σ=σ_o+σ_e has two components. In air or argon, the electronic component σ_e is negligible and the oxide-ion conductivity σ_o is not described by a classical Arrhenius equation; a pronounced curvature at T similar/congruent T * has been observed in the Arrhenius plot of the bulk conductivity. The system could be modeled by a condensation of mobile oxygen vacancies into ordered clusters below a temperature T * similar/congruent 583 ± 45°C and a motional enthalpy Δ H _m= 0.63 ± 0.01 eV for the vacancies. A measured trapping energy Δ H _t(1 - T/T *) has Δ H _t= 0.19 ± 0.01 eV = 2.57 kT *. In a reducing atmosphere, σ_e exhibits a small-polaron motional enthalpy Δ H _p= 0.40 ± 0.08 eV for transfer of a 4 f electron from a Ce³⁺ to a Ce⁴⁺ ion and a Δ H _p+Δ H _pt= 0.51 ± 0.04 eV at T < T *.     

Electrical Conductivity of the High-Temperature Proton Conductor BaZr0.9Y0.1O2.95

The impedance of the cubic perovskite BaZr_0.9Y_0.1O_3-δ has been systematically investigated in dry and wet atmospheres at high and low oxygen partial pressures. In the grain interior, conductivity contributions from oxygen ions, electron holes, and protons can be identified. Below 300°C, proton conduction dominates and increases linearly with the frozen-in proton concentration. The proton mobility, with an activation energy of 0.44 ± 0.01 eV is among the highest ever reported for a perovskite-type oxide proton conductor. For dry oxygen atmos-pheres, electron hole conduction dominates with an activation energy of ∼0.9 eV. At temperatures <500°C, the grain-boundary conductivity can be separated and increases upon incorporation of protons. The high electrical conductivity and chemical stability make acceptor-doped barium zirconate a good choice for application as a high-temperature proton conductor.     

Electrical Conduction in β-Bi2O3 Doped with Sb2O3

Electrical conduction in tetragonal β-Bi₂O₃ doped with Sb₂O₃ was investigated by measuring electrical conductivity, ionic transference number, and Seebeck coefficient. The β-Bi₂O₃ doped with 1 to 10 mol% Sb₂O₃ was stable up to 600°C and showed an oxygen ionic and electronic mixed conduction, where the electron conduction was predominant at low oxygen pressures. The oxygen-ion conductivity showed a maximum at 4 mol% Sb₂O₃, whereas the activation energy for the ionic conduction remained unchanged for 4 to 10 mol% Sb₂O₃-doped specimens. These results were interpreted in terms of the oxygen vacancy concentration and the distortion of the tetragonal structure. The electron conductivity and its oxygen pressure dependence decreased with increasing Sb₂O₃ content. The fact that Sb⁵⁺ is partially reduced by excess electrons in heavily doped β specimens at low oxygen pressures is explained.     

Electrophoretic Codeposition of La0.6Sr0.4Co0.8Fe0.2O3−δ and Carbon Nanotubes for Developing Composite Cathodes for Intermediate Temperature Solid Oxide Fuel Cells

Carbon nanotubes (CNTs)/La_0.6Sr_0.4Co_0.8Fe_0.2O_3−δ (LSCF) composite films have been fabricated by electrophoretic codeposition on Ce_0.9Gd_0.1O_1.95 (CGO) substrates. CNTs are used as a sacrificial phase to produce ordered porous LSCF cathodes for intermediate temperature solid oxide fuel cells. The synthesis of LSCF powder by a modified sol–gel route is presented. The possible mechanism of formation of CNT/LSCF composite nanoparticles in suspension is discussed. Moreover the optimal suspension composition and the conditions for achieving successful electrophoretic deposition (EPD) of CNTs/LSCF composite nanoparticles were evaluated. Experimental results showed that the CNTs were homogeneously distributed and mixed with LSCF nanoparticles forming a mesh-like structure, which resulted in a highly porous LSCF film when the CNTs were burned out during heat treatment in air at 800°C for 2 h. Scanning electron microscopy (SEM), transmission electron microscopy (TEM), and X-ray diffraction (XRD) techniques were employed to characterize the microstructure of the precursors and of the composite films.     

Mesoscale Organization of Flower-Like La2O2CO3and La2O3 Microspheres

Well-dispersed spherical lanthanum oxycarbonate (La₂O₂CO₃) and La₂O₃ particles with a flower-like morphology were synthesized via a simultaneous polymerization–precipitate reaction, metamorphic reconstruction, and mineralization under a hydrothermal condition as well as subsequent calcination. The La₂O₂CO₃ and La₂O₃ microspheres obtained consist of about 20 or 40 nm thick nanosheets as petals, respectively. They have an open three-dimensional mesoporous and hollow structure and possess a high surface area, large pore volume, and marked thermal stability. These novel porous structural lanthanum-based materials could be used as catalyst supports for many reactions.     

Preparation of Dense Platinum-Yttria Stabilized Zirconia and Yttria Stabilized Zirconia Films on Porous La0.9Sr0.1MnO3 (LSM) Substrates

A sol–gel process has been developed to prepare fine powder of La_0.9Sr_0.1MnO₃ (LSM) with an average particle size ∼40 nm. The LSM powder is pressed to pellets, on which a uniform green yttria-stabilized zirconia (YSZ) film is deposited using an electrophoretic deposition process. A green composite film of platinum and YSZ (Pt-YSZ) is prepared on the top of the green YSZ film using a colloidal process, followed by filling the pores using a sol–gel process. The three-layered structure, a green Pt-YSZ film, and a green YSZ film on a green LSM substrate, is fired at 1250°C for 3 h, resulting in a dense Pt-YSZ and YSZ film supported by a porous LSM substrate. Electrical measurements show that the sensors with the three-layered structure display a well-defined diffusion-limited current in gases containing partial pressure of oxygen up to ∼9 vol%, implying that the quality of the Pt-YSZ and YSZ film is adequate for the sensor application.     

Optical and Electrical Properties of Amorphous and Nanocrystalline (La0.8Sr0.2)0.9MnO3 Thin Films Prepared from Low-Temperature Processing Technique

The results of a study on the optical and electrical properties of (La_0.8Sr_0.2)_0.9MnO₃ (LSM) thin films obtained by a polymeric precursor spin coating technique were presented. This method allowed preparation of optical quality thin films at annealing temperatures around 800°C. Amorphous and crystalline LSM thin films were studied by optical and electrical conductivity measurements. The energy-dependent absorption coefficients for the crystalline specimen were calculated from optical spectra and extra absorption was observed in the range of 1.8–2.5 eV with the exchange-gap excitation behavior in the 3–5 eV range. In comparison to the amorphous specimens, the electrical conductivity of the nanocrystalline specimen increased two to three orders of magnitude with decreasing activation energy. The charge carrier absorption model suggested an increase of the carrier concentration in the nanocrystalline specimen which may be a reason for the change in the electrical conductivity.     

New Compound in the System La2O3-Al2O3

A new compound, 5La₂O₃-2Al₂O₃, is formed from an amorphous material prepared by the simultaneous hydrolysis of lanthanum and aluminurn alkoxides. It has an orthorhombic unit cell with a=0.9704 nm, b=0.5967 nm, and c=1.5473 nm. The structure contains tetrahedral AlO₄ groups and octahedral AlO₆ groups.     

Structural and Electric Properties of Sol–Gel-Derived Ba0.9Sr0.1TiO3 Multilayers

Crack-free Ba_0.9Sr_0.1TiO₃ (BST) and Mn-doped Ba_0.9Sr_0.1TiO₃ (BSTM) multilayers with thickness over 2 μm have been prepared by chemical solution deposition based on one single precursor. Both multilayers exhibit good performance as Bragg reflectors. Mn doping tends to suppress the leakage current in BST multilayers effectively by smoothing the layers and the reduction of the charge carries. The Mn-doped BST multilayer displays an excellent ferroelectric property, with an average remnant polarization ( P _r⁺– P _r⁻)/2 of 12.69 μC/cm² and an average coercive field ( E ⁺ – E ⁻ )/2 of about 72.95 kV/cm under an applied field of 440 kV/cm.     

Phases in the Binary Systems PbO—La2O3, Gd2O3, and Sm2O3

In the binary system PbO–La_zO₃ only one compound, 4PbO.La₂O₃, exists; it is flanked by two eutectics. The structure of the compound, although of lower symmetry, is intimately related to the C modification of the rare earths. Below 800° to 1000°C, metastable solid solutions are formed from oxide mixtures coprecipitated from mixed solutions of the nitrates, the cubic parameter a = 5.66 A, if extrapolated to pure La₂O₃, corresponding to half the a parameter of the C form of La₂O₃. The solid solutions existing between the compositions La₂O₃–2Pb0 and pure La₂O₃ have a cubic face–centered lattice and obey Vegard's rule. The systems of PbO with Sm₂O₃ and Gd₂O₈ are quite similar to that with La₂O₃. The compound Sm₂O₃.4Pb0 decomposes at 1000°C with evaporation of PbO; Sm₂O₃ remains in the B modification.