E. G. Kalinina

Formation of thin YSZ electrolyte films by electrophoretic deposition on porous cathodes

The structure, dispersity, stability, and electrophoretic deposition (EPD) of suspensions of spherical ZrO2 stabilized Y2O3 (YSZ) nanoparticles with a mean size of 10.9 nm onto the porous surface of La0.6Sr0.4MnO3 (LSM) with a pore size of 3–20 μm were studied by electron microscopy, photon correlation spectroscopy, and electroacoustical analysis. The optimum conditions of deposition were attained by using a mixed isopropanol-acetylacetone dispersion medium, which provided the aggregative stability of the suspension with 95% individual particles. The maximum pore size on the covered surface should be up to 0.5 μm if nanoparticles with a mean diameter of 10–20 nm are used. When the pores are larger, the EPD of YSZ will be effective if an additional intermediate LSM layer is formed by EPD to provide the required pore size.

Influence of the degree of deaggregation of YSZ nanopowders in suspension on the process of electrophoretic deposition

Deaggregated nonaqueous suspensions of ZrO2 nanopowders stabilized by Y2O3 (YSZ) with an average diameter of particles of 11 nm obtained by laser evaporation and condensation are studied using dynamic light scattering (DLS). Two types of suspensions with an average size of aggregates of 108 and 66 nm are obtained by ultrasonic machining (USM) and centrifugation. It is shown that the combination of USM and centrifugation allows us to obtain narrow size distributions for the particles in YSZ suspensions. Coatings are obtained using electrophoretic deposition (EPD) from two types of YSZ suspensions with different degrees of deaggregation, and it is shown that no difference in the structure and morphology of suspensions is observed. It is established that, by EPT of the particles, a mechanism of electrochemical coagulation is realized at the electrode from the suspensions with different degrees of deaggregation.

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

We have studied nonaqueous suspensions of Ce0.8Sm0.2O1.9 (CSO), Ce0.8Gd0.2O1.9 (CGO), and Се0.8(Sm0.75Sr0.2Ba0.05)0.2O2–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.

Electrophoretic deposition of Y2O3-stabilized ZrO2 nanoparticles on the surface of dense La0.7Sr0.3MnO3−δ cathodes produced by pyrolysis and solid-state reaction

La0.7Sr0.3MnO3−δ (LSM) cathode materials have been prepared through pyrolysis of liquid precursors and by solid-state reaction. We examined the effect of the cathode material synthesis procedure on the growth of thin films of a solid electrolyte based on Y2O3-stabilized ZrO2 (YSZ) nanopowder with an average geometric size of 10.9 nm on a dense surface of model cathodes by electrophoretic deposition. Using electron microscopy, BET surface area measurements, X-ray diffraction, thermal analysis, and dilatometry, we investigated the structure, specific surface area, phase composition, thermal properties, and sintering-induced volume changes of cathode materials differing in prior history. The results demonstrate that the starting cathode materials differed markedly in properties, but the cathodes produced by sintering them were identical in phase composition: they consisted entirely of a rhombohedral phase. The room-temperature electrical conductivity of the cathodes produced by solid-state reaction was 0.231 ± 0.01 S/cm, exceeding that of the cathodes produced through pyrolysis by an order of magnitude.

Preparation of YSZ/Al2O3 composite coatings via electrophoretic deposition of nanopowders

Using electrophoretic deposition (EPD), we have produced YSZ individual ceramic coatings and YSZ/Al2O3 composite coatings for a wide range of applications in modern materials research. YSZ and Al2O3 nanopowders were prepared by high-energy physical dispersion techniques, namely, by a laser evaporation–condensation process and electroexplosion of wire, respectively. Stable nonaqueous suspensions for the EPD process have been prepared using YSZ and Al2O3 nanopowders with an average particle size of 11 and 22 nm, respectively. The YSZ/Al2O3 composite coating produced by sintering at 1200°C has been shown to have higher density in comparison with the YSZ individual coating produced at the same temperature. X-ray diffraction characterization showed that the YSZ/Al2O3 composite coating consisted of two crystalline phases: α-Al2O3 (corundum) (42 wt %) and cubic ZrO2〈Y2O3〉 (58 wt %). Quantitative analysis of electron micrographs of the surface of the films showed that the YSZ individual coating produced by sintering at 1200°C had a loose structure and contained pores (9%), as distinct from the composite coating, which had a dense, porefree grain structure.

Separation of aluminum oxide powders with different degrees of aggregation by sedimentation in an aqueous medium

The dynamics of change in the average size of particles and their concentration in the sedimentation process of stable aqueous suspensions of two polydisperse nanopowders of aluminum oxide is studied. The specific surface area of nanopowder Al2O3-IAM obtained by the plasma-chemical method was 9 m2/g, which corresponds to a weighted average size of 169 nm; it was nonaggregated in a stabilized aqueous suspension. The specific surface area of nanopowder Al2O3-117ns obtained by the electric explosion method was 20 m2/g; the weighted average size was 80 nm, and it was aggregated in an aqueous suspension to a considerable degree. The method of dynamic light scattering was used to study the size distribution upon the sedimentation of suspensions with a concentration of 5 g/l for 500 h at various depths from the surface; it is shown that the dynamics of decrease in the average size of particles at any depth obeys the classical Stokes law on the assumption of the independence of the sedimentation of individual particles and aggregates. This counts in favor of the fact that no additional aggregation of particles occurs in the sedimentation process of diluted stable nanopowder suspensions. On the other hand, it was experimentally shown that the suspension concentration decreases during sedimentation much faster than it should according to the Stokes law, which indicates that the classical model has restrictions; however, their cause is not quite clear.

A study of the electrophoretic deposition of thin-film coatings based on barium cerate nanopowder produced by laser evaporation

The method of laser ablation of a target, followed by condensation, was used to obtain a weakly aggregated BCSO nanopowder from barium cerate. The dispersity, fraction composition of the nanopowder, electrokinetic potential of its nonaqueous dispersions, and electrokinetic parameters of the electrophoretic deposition process were determined. An ultrasonic treatment produced a stable suspension of the BCSO nanopowder in a mixed isopropanol–acetyl acetone medium (70/30 vol %). The suspension is characterized by a high and positive ζ-potential of +30 mV. The electrophoretic deposition onto a dense model cathode was used to obtain thin-film BCSO coatings that are of interest for the technology of solid-oxide fuel cells. The phase composition of the coating was examined. It was found that the successive annealings of the nanopowder at temperatures of 800–1400°C make it possible to reduce the content of unidentified crystalline phases in BCSO to trace levels (< 5 vol %).

Preparation of alumina nanoparticle suspensions with narrow particle size distribution

Dynamic light scattering (DLS) was applied to the study of the process of the preparing deaggregated water suspensions of alumina nanopowders with specific surface areas of 20–140 m2/g. Nanopowders were prepared by the electric explosion of wire and laser evaporation and, according to electron microscopy (TEM), consisted of nonagglomerated spherical nanoparticles with lognormal size distribution. According to DLS, nonsedimenting water suspensions of alumina nanoparticles, stabilized by sodium citrate at a 5 mM concentration, contain substantial fraction of aggregates. The dynamics of the change in the mean average size of aggregates under exhaustive ultrasound treatment of suspensions with 10 g/l concentration for 1.5–4 h by two types of ultrasonic processors was studied. It was shown that the mean average size of aggregates exponentially diminishes by 1.5–2 times and the fraction of individual particles in suspension enlarges from 45 to 85%. Sequentially centrifuging the suspension at 18000 g separates the remaining aggregates and results in suspensions of individual alumina nanoparticles. Particle size distributions in these suspensions obtained by TEM and DLS are the same within experimental error.

Aggregatively stable suspensions of micrometer powders of doped barium cerate for electrophoretic deposition of thin-film coatings of solid-oxide fuel cells

Potentialities of the method of electrophoretic deposition of thin-film coatings based on micrometer powders of multidoped barium cerate BaCe0.8Sm0.19Cu0.01O3–δ (BCSCuO) and BaCe0.89Gd0.1Cu0.01O3–δ (BCGCuO) were considered. Micrometer powders of BCSCuO and BCGCuO were produced by the methods of solid-phase and citrate-nitrate syntheses, respectively. The dispersity, fraction composition, and electrokinetic potential of nonaqueous suspensions of these powders and the electrokinetic parameters of the electrophoretic deposition process were examined. An ultrasonic treatment and ultracentrifugation produced aggregatively stable suspensions of BCGCuO and BCSCuO micrometer particles in a mixed (70/30 vol %) isopropanol–acetyl acetone medium. These suspensions are characterized by high positive values of the zeta potential (+24 and +28 mV, respectively). Thin film coatings of the electrolyte materials BCSCuO and BCGCuO, which are of interest for the technology of medium-temperature solid-oxide fuel cells, were produced by the electrophoretic deposition onto a dense model cathode.

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 La2NiO4 + 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 Се0.8(Sm0.75Sr0.2Ba0.05)0.2O2–δ), which are of interest for solid oxide fuel cell technology.