A. V. Nikonov

Densification of Nano-Sized Alumina Powders under Radial Magnetic Pulsed Compaction

An influence of aggregation degree of alumina nanopowder and of radial magnetic pulsed compaction conditions on the powder densification and on the density distribution inside compacts has been investigated experimentally. The usage of radial magnetic pulsed compaction allows to compact alumina nanopowders up to high densities at low degree of aggregation. The compacts homogeneity is attained by appropriate compaction pulse duration and by the usage of powder packing prior to compaction until the condition with high local sound speed is achieved.

Co-doping effect on the properties of scandia stabilized ZrO2

The influence of low concentrations (1 mol %) of few co-dopants (Y2O3, La2O3, CeO2, Gd2O3, Er2O3, ZnO) on the structure and characteristic of 10 mol % scandia stabilized zirconia was studied. Sintering kinetics and thermal expansion coefficients of synthesized solid eletrolytes were determined. It was found that co-doping increased the conductivity of electrolytes at temperature below 550°C. However, at high temperatures, the introduction of co-dopants decreased the conductivity; moreover, this reduction was more severe the more the ionic radii of Zr4+ and co-dopant differ.

Electrochemical cell with solid oxide electrolyte and oxygen pump thereof

The paper presents the scientific basis and technical implementation of a method for obtaining oxygen by extraction from air using an electrochemical cell based on a solid oxide cell (SOC) with anion-conducting solid electrolyte. A nanopowder of a weak aggregate of the YSZ solid electrolyte and LSM fine powder was used to manufacture SOC. The electrolyte-electrode SOC structure was formed as a tube by joint pressing of functional layers and the further co-sintering at the temperature of 1200°C. The characteristics of an electrochemical cell of the oxygen pump based on a thin-wall tube of the YSZ supporting electrolyte (150 μm) with symmetrical electrodes based on LSM (∼20 μm) are studied. A prototype of a compact oxygen generator (oxygen pump) is developed and manufactured with an electrochemical part based on three serially connected SOCs. The connection is implemented in the form of metallic couplings of the Crofer 22 APU steel. The method of reaction magnetron sputtering was used to protect current leads from corrosion by applying a coating based on a MnxCo3 − xO4 spinel. The efficiency of a demonstration prototype at 800°C was 9 l/h at the power consumption of 50 W. The current density through SOC was 1.1 A/cm2. The prototype was designed to contain no noble metal components. It is shown that the engineering approach applied allows manufacturing effective nanostructural SOCs and devices on their basis.

Effect of structural parameters of Ni-ScSZ cermet components on the SOFC anodes characteristics

The effect of the degree of dispersion and the ratio of initial components of metalloceramic composites based on Ni and Sc2O3-stabilized ZrO2 (Ni/ScSZ) on the kinetics of sintering, conductivity, and polarization resistance of the corresponding anodes in solid-oxide fuel cells (SOFC) is studied. The composites are prepared from nano- and submicrosized powders of NiO and ScSZ (10.5 mol % Sc2O3) containing particles with the average size of 0.02–0.33 μm. Anode composites of three types differing in the ratio of initial components (NiO-ScSZ) with different degrees of dispersion: micro-micro, nano-micro, and nano-nano are studied. Due to the ratio of particle sizes, the anodic composites of the nano-nano type demonstrate the preferential electronic conduction (the percolation threshold) starting from the Ni content of about 35 vol %, in contrast to the other two types of anodic composites for which this threshold is achieved at 30 vol %. The lowest polarization resistance is typical of anode composites with the Ni content of about 40 vol %. The use of one or both components in the nanosized state makes it possible to decrease the anodic polarization up to two times. It is demonstrated that an active cermet anode for SOFC can be fabricated in the form of a planar three-layer structure Ni/ScSZ-ScSZ-Ni/ScSZ prepared from nanosized powders by the tape casting technique and cosintering.

Electrical conductivity and thermal expansion of La1 – xSrxFe1 – yGayO3 – δ (x = 0.2–0.5; y = 0–0.4)

Lanthanum–strontium gallate–ferrites are thought of as perspective electrode materials. A series of La1 –xSrxFe1 –yGayO3 – δ (x = 0.2–0.5; y = 0–0.4) compositions is synthesized by self-propagating high-temperature synthesis. The solid solutions’ existence domain is identified. The linear expansion thermal coefficient and the electric conductivity of the materials in the form of ceramics are studied for single-phase samples.

Characteristics of the La0.6Sr0.4Fe0.8Co0.2O3 − δ electrode in contact with the lanthanum gallate-based electrolyte

Lowering the working temperature of solid oxide fuel cells (SOFCs) is the main trend in their development, which requires selection of materials for electrolyte and electrodes. A highly conducting lanthanum gallate-based electrolyte is a promising material for creating medium-temperature SOFCs. The electrochemical characteristics of the La0.6Sr0.4Fe0.8Co0.2O3 − δ cathode that contacted with the La0.88Sr0.12Ga0.82Mg0.18O2.85 electrolyte subject to electrode formation temperatures have been investigated. It was found that at optimum bake-on temperatures of 1200–1250°C, the cathode polarization resistance at 800°C was ∼0.08 Ohm cm2, which is comparable to the world’s best achievements.

Electrical conductivity of zirconia-based solid electrolyte with submicron grain size

The electrical conductivity of the Zr(Y)O2 electro� lyte ceramics with the highest density and the grain size from 90 to 800 nm was studied by impedance spectroscopy. The ceramics was obtained by magnetic pulse compacting of weakly aggregated nanopowders and subsequent sintering. The grain boundary con� ductivity was shown to depend on the grain size. The grain boundary resistance was found to increase with an increase in the grain size in the range of 90- 800 nm. In the grain size range of 1-18 µm, according to the data obtained previously, the grain boundary resistance in the ceramics of this composition mono� tonically decreased with an increase in the grain size. This indicates that a maximum exists in the size dependence of the grainboundary resistance. One of the factors that influence the efficiency of devices with solid oxide electrolytes are the losses due to the voltage drop in the electrolyte. In this connec� tion, the technological problem of decreasing the thickness of the electrolyte layer becomes increasingly urgent. However, in the framework of the ceramic method, the thickness of membranes cannot be decreased without the corresponding decrease in the grain size. This dictates the practical significance of the present work. Attempts to obtain nanosized oxide materials with a facecentered cubic (fcc) lattice of the fluorite type are known in the literature. For ceria� based materials, it was shown (1) that these microand nanosized ceramic samples differ in both the conduc� tivity values and the character of the temperature and oxygen pressure dependences of the conductivity. The models were developed (2, 3) to describe the depen� dences observed by regarding the grain boundaries in the ceramic as a second phase. This concept is obvious for describing heterogeneous materials, for example, a mixture of a cationconducting solid electrolyte and an insulator, as it was done in the classic works of J. Maier (see, for example, (3)), but should be eluci� dated in the general case of homogeneous ceramics. On the other hand, the presence of the Ce 3+ /Ce 4+ redox system complicates the interpretation of the phenomena observed. In this connection, it is of inter� est to study the zirconiabased ceramics. Dense nano� sized Zr(Y)O2 ceramic samples with fcc lattice have not been previously obtained. It turned out to be a very difficult technical task. To succeed, we used magnetic pulse compaction of weakly agglomerated nanopow� ders obtained by laser evaporation. This method allows one to reach pressures up to 15-17 t/cm 2 .

Fabrication of Microtubular Solid Oxide Fuel Cells by Film Compaction and Co-Sintering

The microtubular design of solid oxide fuel cells (SOFCs), which are promising electrochemical power sources, has a number of significant advantages over traditional planar and tubular designs: increased resistance to the cell (stack) heating rate and packing density of cells in a stack. The paper presents results on the development of a microtubular SOFC (MT-SOFC) fabrication method based on compaction and co-sintering a set of films. The formation of an anode-supported MT-SOFC having a Ni-cermet collector (support) and functional layers of about 300 and 50 μm thick, respectively; a Zr0.84Y0.16O2–δ solid electrolyte layer (40 μm); and a cathode based on La0.7Sr0.3MnO3–δ has been developed. The outer diameter and length of the MT-SOFC were 3.9 and 12 mm, respectively. The maximum specific power generated by the MT-SOFC at 850°C was 0.21 W/cm2.

Pore-Forming Agents for the Supporting Ni-Based SOFC Anode

The effect of the type (semolina, starch, and ashless filter paper) and amount (5 and 10 wt %) of pore-forming agents on the structure of supporting Ni-cermet anodes and characteristics of solid-oxide fuel cells on their basis has been studied. It has been found that the use of paper fibers as the pore-forming agent doubles the open porosity of the anode as compared with equal amounts of semolina or starch. It has been shown that the long cylindrical pores formed by fibers allow fast transport of gases through the anode bulk, which has a positive effect on the fuel cell characteristics.

Fabrication of multilayer ceramic structure for fuel cell based on La(Sr)Ga(Mg)O 3 –La(Sr)Fe(Ga)O 3 cathode

Fabrication by co-sintering method of a multilayer pore-free electrode–electrolyte structure promising for use in solid-oxide fuel cell and its characteristics have been studied. A material with high ionic conductivity of La0.88Sr0.12Ga0.82Mg0.18O3–δ (LSGM) served as electrolyte. The composite electrode was formed from a 1: 2 mixture of LSGM and LSFG (La0.7Sr0.3Fe0.95Ga0.05O3–δ). The maximum temperature of the materials co-sintering ability is 1250°C. It was shown by the impedance spectroscopy that the polarization resistance of the LSGM–LSFG electrode is 0.14 Ω cm2 at 800°C.