Stable suspensions of doped ceria nanopowders for electrophoretic deposition of coatings for solid oxide fuel cells

We have studied nonaqueous suspensions of Ce_0.8Sm_0.2O_1.9 (CSO), Ce_0.8Gd_0.2O_1.9 (CGO), and Се_0.8(Sm_0.75Sr_0.2Ba_0.05)_0.2O_2–δ (CSSBO) nanopowders produced by laser evaporation of a target. The nanoparticles were nearly spherical in shape and their average diameter was 9, 7, and 15 nm, respectively. Using ultrasonic processing, we obtained stable nanopowder suspensions in an isopropanol + acetylacetone mixed medium, investigated their particle size composition, evaluated their zeta potential as a function of pH, and obtained potentiometric titration curves. The starting nanopowder suspensions have been shown to be weakly acidic and have a rather high initial zeta potential. During titration of the nanopowder suspensions with 0.17 N KOH in isopropanol, no isoelectric point was observed. The maximum positive values of the zeta potential, favorable for electrophoretic deposition (EPD), were reached in weakly acidic media in the pH range 4–6. Using EPD, we obtained a coating from a stable self-stabilized CSSBO suspension (ζ = +31 mV, pH 4.0), which was then sintered in air at a temperature of 1400°C. Our results demonstrate that the starting nonaqueous suspensions of the CSO, CGO, and CSSBO nanopowders suit well for producing gas-tight, homogeneous solid oxide fuel cell coatings by EPD.     

Synthesis of Sr- and Fe-doped LaGaO3 perovskites by the modified citrate method

Fine particle strontium and iron substituted lanthanum gallates La_1–x Sr _x Ga_1–y Fe _y O_3–, where x = 0.2, 0.4, and 0.6; y = 0.2, 0.4, 0.6, and 0.8, have been synthesized by a modified citrate method. The formation of these powders was confirmed by the X-ray powder diffraction (XRD) and the fine particle of La_0.6Sr_0.4Ga_0.2Fe_0.8O_3– was investigated by scanning electron microscopy (SEM), and particle size analysis. The single phase of La_0.8Sr_0.2Ga_0.4Fe_0.6O_3–, La_0.6Sr_0.4Ga_0.2Fe_0.8O_3–, and La_0.4Sr_0.6Ga_0.2Fe_0.8O_3– powders could be obtained both with and without calcination. The amount of the secondary phase increased when the amount of Sr in La_1–x Sr _x Fe_0.6Ga_0.4O_3– was more than 0.2 (x > 0.2) and the amount of Fe in La_0.6Sr_0.4Ga_1–y Fe _y O_3– and La_0.4Sr_0.6Ga_0.2Fe_0.8O_3– was less than 0.8 (y < 0.8). The results indicated that in the pH range of 1.36–9.27, the single phase of La_0.6Sr_0.4Ga_0.2Fe_0.8O_3– was formed without calcination and the pH had negligible effects on the structure and lattice parameter. The fine particle of these calcined powders (<4 m) was obtained with the average particle size 1.70 m at pH = 1.36 and with the average particle size between 0.56–0.60 m at pH range between 3.39–9.27, and with a lattice parameter about 3.9 Å.     

Synthesis of layered Li1.2+x[Ni0.25Mn0.75]0.8−xO2 materials (0 ≤ x ≤ 4/55) via a new simple microwave heating method and their electrochemical properties

Li_1.2+x[Ni_0.25Mn_0.75]_0.8−xO₂ (0 ≤ x ≤ 4/55) was prepared by a new simple microwave heating method and the effect of extra Li⁺ content on electrochemistry of Li_1.2Ni_0.2Mn_0.6O₂ (x = 0) was firstly revealed. X-ray diffraction identified that they had layered α-NaFeO₂ structure (space group R-3m). Linear variation of lattice constant as a function of x value supported the formation of solid solution, that is, extra Li⁺ is possibly incorporated in structure of layered Li_1.2Ni_0.2Mn_0.6O₂ (x = 0), accompanying oxidization of Ni²⁺ to Ni³⁺ to form Li_1.2+x[Ni_0.25Mn_0.75]_0.8−xO₂ (0 ≤ x ≤ 4/55). This was confirmed by X-ray photoelectron spectroscopy that Ni³⁺ appeared and increased in content with increasing x value. Charge–discharge tests showed that Li_1.2+x[Ni_0.25Mn_0.75]_0.8−xO₂ (0 ≤ x ≤ 4/55) truly displayed different electrochemical properties (different initial charge–discharge plots, capacities and cycleability). Li_1.2Ni_0.2Mn_0.6O₂ (x = 0) in this work delivered the highest discharge capacity of 219 mAh g⁻¹ between 4.8 and 2.0 V. Increasing Li content (x value in Li_1.2+x[Ni_0.25Mn_0.75]_0.8−xO₂) reduced charge–discharge capacities, but significantly enhancing cycleability.     

Preparation of thin bilayer coatings based on lanthanum,nickel, and cerium mixed oxides by electrophoretic deposition

This paper examines the feasibility of using electrophoretic deposition for producing thin films of cathode materials based on lanthanum nickel oxides (with the lanthanum nickelate La₂NiO_{4 + δ} (LNO) as the predominant phase). We have determined the particle size and zeta potential of nonaqueous suspensions of LNO micro- and nanopowders prepared by solid-state reactions, pyrolysis of liquid precursors, and laser evaporation of a target. Using ultrasonic processing, we have obtained stable LNO nanopowder suspensions in acetylacetone and an isopropanol + acetylacetone mixture, which have zeta potentials of +25 and + 38 mV, respectively. Electrophoretic deposition on a dense model cathode has produced thin bilayer coatings from electrode and electrolyte materials (LNO and Се_0.8(Sm_0.75Sr_0.2Ba_0.05)_0.2O_2–δ), which are of interest for solid oxide fuel cell technology.     

Fabrication of dense LaCrO3-based interconnect thin membrane on anode substrates by co-firing

In order to develop cost-effective method to prepare ceramic interconnect membrane for solid oxide fell cell stacks, the triple-phase composites of NiO/Sm_0.2Ce_0.8O_2−δ/La_0.7Ca_0.3Cr_0.97O_3−δ with the weight ratios of 6:4:x (x = 0, 2, 3, 4, 5) were, respectively prepared, and then examined as novel anode supports. The sintering character, microstructure, electrical conductivity, fracture strength, and thermal expansion coefficient of the anodes were, respectively investigated in detail as a function of La_0.7Ca_0.3Cr_0.97O_3−δ content. Results indicate the above-mentioned performances of NiO/Sm_0.2Ce_0.8O_2−δ/La_0.7Ca_0.3Cr_0.97O_3−δ anode at x ≤ 3 have no significant reduction, and are still acceptable for the practical requirements for solid oxide fell cells. Furthermore, by using a simple and cost-effective drop-coating/co-firing process, dense La_0.7Ca_0.3Cr_0.97O_3−δ interconnect membrane was successfully prepared on the anode support of NiO/Sm_0.2Ce_0.8O_2−δ/La_0.7Ca_0.3Cr_0.97O_3−δ at x = 3. This work presents a simple technical route for developing dense interconnect membranes for Sm_0.2Ce_0.8O_2−δ-based fuel cell stacks.     

Dielectric and magnetoelectric properties of (Ni,Cu)Fe2O4 + [(Ba,Pb)(Ti,Zr)]O3 composites

The observation of magnetoelectric coupling is reported in (x) Ni_0.8Cu_0.2Fe₂O₄ + (1 − x) Ba_0.5Pb_0.5Ti_0.5Zr_0.5O₃ composites synthesized by normal solid state reaction method. The phase formation was confirmed by X-ray diffraction technique and no intermediate or impurity phase is present in all the composites. The dielectric constant (ε′) and dielectric loss (tan δ) were measured as a function of frequency and temperature. The dielectric constant was found to be enhanced with increase in ferrite content in the composite. It was also revealed that magnetoelectric output decreases with increase in ferrite content in the composite due to leakage current. The maximum voltage coefficient of 0.411 mV/cm/Oe was observed in 15% Ni_0.8Cu_0.2Fe₂O₄ + 85% Ba_0.5Pb_0.5Ti_0.5Zr_0.5O₃ composite.     

Dielectric behaviour and a.c. conductivity in Cu x Fe3−x O4 ferrite

The dielectric properties (dielectric constant and loss) for the system Cu _x Fe_3−x O₄ with x = 1.0, 0.8, 0.6, 0.4 and 0.2, were studied in the temperature range 300 ∼ 800 K and also in the frequency range 1 kHz ∼ 1 MHz. A.c. conductivity was derived from dielectric constant and loss tangent data. The conduction in this system is interpreted as due to small polaron hopping. The dielectric relaxation was observed for the compositions with tetragonal structure whereas normal behaviour was observed for cubic structure.     

Magnetoelectric properties of ME particulate composites

Magnetoelectric (ME) composites are biphasic materials consisting of piezoelectric and piezomagnetic materials as the participating constituents. These ME composites when placed under external magnetic field show electrical polarization (ME output). This ME coupling is mediated by mechanical stress. In the present study, we have synthesized particulate composites of Ni_0.8Cu_0.2Fe₂O₄ and Ba_0.8Sr_0.2TiO₃ using conventional ceramic method. The XRD identifications showed that the sintered ceramics retained the presence of distinct ferroelectric and ferrite phases. The dielectric constant was determined as a function of temperature (room temperature to 650 °C) at different test frequencies. The DC resistivity was studied as a function of temperature. The measured ME response, dielectric constant, and resistivity demonstrated strong dependence on the volume fraction of Ni_0.8Cu_0.2Fe₂O₄ in the composite. The ME voltage coefficient strongly depends on the resistivity of the composites.     

Performance of Ce/Fe oxide anodes for SOFC operating on methane fuel

The performance of Ce_1−xFe_xO_2−δ (x = 0.1, 0.2) was studied as anode for solid oxide fuel cell running on methane in this paper. The oxides were prepared by the citrate method. The polarization resistance of electrode was measured in the two-electrode symmetric cell configuration and an electrolyte-supported cell was prepared to assess Ce_0.8Fe_0.2O_2−δ anode performance of running on methane fuel. The results suggested that Ce_0.8Fe_0.2O_2−δ has a significant effect on the electrochemical oxidation of methane. A maximum power density of single cell was 52 mW/cm² at 800 °C and little carbon deposition was shown on the anode operating in humidified methane for 20 h. Therefore, Ce_0.8Fe_0.2O_2−δ is a promising candidate for anode of solid oxide fuel cell running on methane fuel, which breaks a new path to solve the problem of carbon deposition in methane.     

Effect of deposition temperature on dielectric properties of PECVD Ta2O5 thin film

Tantalum oxide film formation by plasma-enhanced chemical vapour deposition (PECVD) using TaCl₅ as a source material was examined. The effects of deposition temperature on the formation, structure and electric properties of the Ta₂O₅ film were investigated for Al/Ta₂O₅/ p-Si (MTS) capacitors. The deposition rate and refractive index increased with increasing deposition temperature. It was found that the structure of Ta₂O₅ deposited by PECVD was amorphous as-deposited. However, crystalline -Ta₂O₅ of hexagonal structure was formed by a 700 °C, 1 h heat treatment in argon. Capacitance and relative dielectric constant of the PECVD Ta₂O₅ were found to be 2.54 fF m^–2 and 23.5, respectively. The PECVD films obtained in this study have higher dielectric constants and remarkably better general film characteristics than those obtained by other deposition methods.     

Microstructure, hysteresis and microwave absorption analysis of Ba(1 − x)SrxFe12O19 ferrite

M-type hexagonal ferrite series, Ba_(1−x)Sr_xFe₁₂O₁₉ (x = 0.0, 0.2, 0.4, 0.6, 0.8, 1.0), has been synthesized by conventional ceramic method. Hysteresis parameters have been investigated at an applied field of 10 kOe and absorption has been studied at X-band as a function of thickness, substitution and frequency. Microstructure and X-ray diffraction confirmed hexagonal structure of ferrite. The substitution causes profound increase in absorption, coercivity and magnetization. The magnetic parameters have been characterized by taking into account microstructure and preferential site occupancy. Curie temperature decreases with substitution due to the formation of spin canting structure.     

Effect of deposition temperature on dielectric properties of PECVD Ta2O5 thin film

Tantalum oxide film formation by plasma-enhanced chemical vapour deposition (PECVD) using TaCl₅ as a source material was examined. The effects of deposition temperature on the formation, structure and electric properties of the Ta₂O₅ film were investigated for Al/Ta₂O₅/ p-Si (MTS) capacitors. The deposition rate and refractive index increased with increasing deposition temperature. It was found that the structure of Ta₂O₅ deposited by PECVD was amorphous as-deposited. However, crystalline -Ta₂O₅ of hexagonal structure was formed by a 700 °C, 1 h heat treatment in argon. Capacitance and relative dielectric constant of the PECVD Ta₂O₅ were found to be 2.54 fF m^–2 and 23.5, respectively. The PECVD films obtained in this study have higher dielectric constants and remarkably better general film characteristics than those obtained by other deposition methods.     

Electrochemical characteristics of Ba0.5Sr0.5Co0.8Fe0.2O3−δ–Sm0.2Ce0.8O1.9 composite materials for low-temperature solid oxide fuel cell cathodes

In this paper, Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3−δ–xSm_0.2Ce_0.8O_1.9 (BSCF–xSDC, x = 0–60 wt.%) composite cathodes were prepared by soft chemical methods, and then examined for potential applications in lower temperature solid oxide fuel cells. Both DC polarization and AC impedance spectroscopy measurements indicated that the addition of SDC electrolyte into BSCF remarkably improved the electrochemical properties. The optimum composition was found to be BSCF–30SDC, which exhibited 5.5 times higher polarization current density and 15.1% polarization resistance, compared with the pure-phase BSCF cathode at 550 °C.     

Piezoelectric, ferroelectric and dielectric properties of La2O3-doped (Bi0.5Na0.5)0.94Ba0.06TiO3 lead-free ceramics

La₂O₃ (0–0.8 wt.%)-doped (Bi_0.5Na_0.5)_0.94Ba_0.06TiO₃ (abbreviated as BNBT6) lead-free piezoelectric ceramics were synthesized by conventional solid-state reaction. The influences of La₂O₃ on the microstructure, the dielectric, ferroelectric and piezoelectric properties of the composites were investigated. X-ray diffraction (XRD) patterns indicate that 0.2-0.8 wt.% of La₂O₃ has diffused into the lattice of BNBT6 ceramics. Consequently, a pure perovskite phase is formed. SEM images show that the microstructure of the ceramics is changed with the addition of a small amount of La₂O₃. The temperature dependence of the relative dielectric constant shows that Curie point decreases with the increase of La₂O₃. At room temperature, the ceramics doped with 0.6 wt.% La₂O₃ show superior performance with high piezoelectric constant (d₃₃ = 167 pC/N), high planar electromechanical coupling factor (k_p = 0.30), high mechanical quality factor (Q_m = 118), high relative dielectric constant (ε_r = 1470) and lower dissipation factor (tanδ = 0.056) at a frequency of 10 kHz.     

Synthesis, microstructure and dielectric properties of zirconium doped barium strontium titanate obtained by solvothermal method

(Ba_0.8Sr_0.2Zr_yTi_1?yO₃) (y = 0.05, 0.1, 0.15 and 0.2) nanosized powders is synthesized by solvothermal method. The formation of (Ba_0.8Sr_0.2Zr_yTi_1?yO₃) was confirmed by XRD, FT-IR and XPS analysis. The purity of the (Ba_0.8Sr_0.2Zr_yTi_1?yO₃) was examined using FT-IR spectroscopy. The X-ray analysis proves that all as-synthesized Ba_0.8Sr_0.2Zr_yTi_1?yO₃ powders have a cubic perovskite structure. The surface characterization indicates the powder contains Zr and Sr elements. SEM investigation shows that the average particle size is reduced with increasing the Zr content and the average particle size is in the range 37–50 nm, the well dispersed nano powders have narrow particle size distribution. The study of dielectric properties shows that the Curie peaks of dielectric constant become broader and shift towards lower temperature on the basis of Ba_0.8Sr_0.2TiO₃ and the dielectric loss decreases with increasing Zr content.     

Electrostatic Spray Deposition of Perovskite-Type Oxides Thin Films with Porous Microstructure

The deposition of perovskite-type oxides thin films [La_0.8Sr_0.2MnO₃ and La_1–xSr_xCo_1–yFe_yO₃ (0 x 0.4 and 0 y 1)] was investigated using the electrostatic spray deposition (ESD) technique. Lanthanum nitrate, strontium chloride and manganese nitrate, or cobalt nitrate and iron nitrate were dissolved into a mixture of 33 vol% ethanol and 67 vol% butyl carbitol, which was used as precursor solution. The effect of process parameters, such as deposition temperature, deposition time and concentration of precursor solution, on the surface morphology and microstructure of thin films were examined with scanning electron microscope (SEM). The deposited La_0.8Sr_0.2MnO₃ and La_1–xSr_xCo_1–yFe_yO₃ thin films were amorphous at the used deposition temperature (573K). Subsequently, the samples were heated at 1173 K for 2 h and were studied using X-ray diffraction (XRD). As the result, the crystal structure of the samples transformed to the desired perovskite phase. The chemical analysis of the thin films was investigated using energy dispersion X-ray (EDX) analysis. The observed chemical compositions of the samples were in a fair agreement with the ones of the starting solutions.     

Crack propagation in layered Al<Subscript>2</Subscript>O<Subscript>3</Subscript>/ZrO<Subscript>2</Subscript> composites prepared by electrophoretic deposition

Crack propagation through layered Al₂O₃/ZrO₂ composites was studied. The specimens were prepared via electrophoretic deposition of alumina and zirconia powders from suspensions with monochloroacetic acid and isopropanol. The kinetics of electrophoretic deposition could be described fully if the electrophoretic mobility and conductivity of suspensions were known. The conductivity of suspensions increased in the course of deposition. Adjusting to properly controlled kinetics of deposition and sintering, deposits were prepared with strongly bonded layers of different pre-defined thicknesses and, consequently, with different magnitudes of residual stress. Cracks, produced by an indentation technique, propagated askew towards layer interfaces deflected towards the interface in the Al₂O₃ layers and away from the interface in the ZrO₂ layers. Changes in the direction of crack propagation were described for the whole range of angles of incidence (0°–90°). The biggest change in the crack propagation was observed for the angle of incidence 45° and was ca. 15°, irrespective of the magnitude of residual stress in the layers.     

Laser Irradiation of Polymer-Doped Cryogenic Matrices

Interactions between intense (10¹⁹ photons/cm² per shot) UV laser light and doped ice matrices were studied by the Matrix-Assisted Pulsed Laser Evaporation (MAPLE) deposition technique. Water, isopropanol, acetone and toluene ice matrices have been used as hosts for the biotechnologically important polymer – polyethylene glycol (PEG). The polymer–matrix system was irradiated under vacuum conditions with high-intensity laser beam at 355 nm in the fluence range 2–10 J/cm². We have explored the ejection of material from the ices in terms of light absorption by the matrix and photochemical interactions between matrix and polymer molecules during the irradiation process. The transfer of polymer from the matrix to a substrate has been studied on the basis of deposition rates measured with a quartz crystal microbalance (QCM) and Fourier-Transform Infrared (FTIR) spectra recorded from the deposits.     

Investigation into the dielectric behavior of ferroelectric superlattices formed by pulsed laser deposition

In an attempt to reproduce the functional properties associated with relaxor electroceramics, pulsed laser deposition has been used to fabricate thin-film capacitor structures in which the dielectric layer is composed of a superlattice of Ba_0.8Sr_0.2TiO₃ and Ba_0.2Sr_0.8TiO₃. The properties of the capacitors were investigated as a function of superlattice periodicity. The dielectric constant was enhanced at stacking periodicities of a few unit cells, consistent with relaxor behavior. However, enhancement of the dielectric constant was found to be associated with high dielectric loss. Analysis of the imaginary permittivity as a function of frequency shows that fine-scale superlattices conform to Maxwell–Wagner behavior. This suggests that the observed enhancement of the real part of the dielectric constant is an artefact produced by carrier migration. A comparison of this data with that already published on dielectric superlattices suggests that previous claims of an enhancement in dielectric constant may also be due to the Maxwell–Wagner effect. The onset of Maxwell–Wagner behavior was attributed to increasing density of defect zones associated with discontinuities in the superlattice structures. In an attempt to exaggerate the influence of such zones, deliberate delays between deposition of successive dielectric layers were introduced. This resulted in reproduction of several features normally associated with relaxors: enhancement of dielectric constants by over an order of magnitude; strong frequency dispersion around and below T_m; migration of T_m with frequency. However, these features were again associated with relatively high loss.     

Effect of copper content on glass formation and superconductivity in the Bi-Pb-Sr-Ca-Cu-O system

High-T _c superconducting ceramics of formula Bi_0.8Pb_0.2SrCaCu _x O _y (x=1.5, 1.8 and 2.0) were prepared by using the melt-quenching method, and the effect of copper content on glass formation and superconductivity was examined. It was found that the composition withx=1.5 had a tendency to form a glass and Bi₂(Sr, Ca)₂CuO_y crystals tended to precipitate easily during the rapid quenching of melts in the compositions withx=1.8 and 2.0. It was found from the temperature dependence of a.c. complex susceptibility that the intergrain coupling of superconducting crystals in the samples obtained was weak, but the weak coupling was improved by increasing the annealing time. The superconducting glass-ceramics Bi_0.8Pb_0.2SrCaCu_1.5O_y exhibited superconductivity with aT _c (zero) of 106 K and aJ _c of 250 Acm^–2.