V. R. Khrustov

Nanostructured composite ceramic materials in the ZrO2-Al2O3 system

The composite ceramic materials based alumina and zirconia in the monoclinic form (10 wt %) or the tetragonal form stabilized by yttrium (7–45 wt %) are produced from metastable nanopowders through magnetic pulsed compaction followed by free sintering. The specific features revealed in the shrinkage are investigated at temperatures in the range 1400–1500°C. The development of the microstructure and polymorphic transformations are analyzed using X-ray diffraction and atomic-force microscopy. The fracture toughness and the microhardness of the samples are determined by the indentation technique.

Phase transition in mayenite Ca12Al14O33

A phase transition in Ca12Al14O33 has been discovered and investigated by thermogravimetric analysis, differential scanning calorimetry, dilatometry, and high-temperature X-ray diffraction. The phase transition occurs at 922 ± 45 K (ΔH = −406 ± 13 kJ/mol, ΔS = −440 ± 14 J/(mol K)) and is presumably a first-order one. It does not change the symmetry of the cationic subsystem. The phase transition is difficult to reveal because the material changes its mass, probably by releasing water bound in several different ways.

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.

Metal-Ceramic Cathodes for Electron Accelerators

Institute of Electrophysics, Ural Division, Russian Academy of Sciences, Komsomol’skaya ul. 34, Yekaterinburg, 620219 Russia 1. Nanosecond pulse generators with the pulse recurrence frequency as high as 1 kHz and the resource up to 1010–1011 pulses and electron accelerators with such generators [1, 2], which are of considerable promise for technological applications [3, 4], have been developed on the basis of semiconductor current switches. In connection with this, the problem of creating simple, inexpensive, and high-resource cathodes with high and stable emissivity in the course of the exploitation becomes urgent. Metal–dielectrics–metal cathodes [5], in which electrons are emitted from plasma arising at a point of contact of special electrodes with ceramics (triple points), most fully conform to these requirements. Among disadvantages of these cathodes are a complexity of the structure and limited number of emitting points. This causes the erosion of igniting electrodes and the ceramics under them, a inhomogeneity in the current distribution at the output foil of a vacuum diode, and a reasonably high impedance of the diode for small dimensions of the cathode unit.

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.

Properties of the translucent ceramics Nd: Y2O3 prepared by pulsed compaction and sintering of weakly aggregated nanopowders

A translucent cubic yttria ceramic material doped by neodymium, namely, 1Nd: Y2O3, with particles of micrometer size (5–17 μm) and clearly defined boundaries is synthesized from nanopowders prepared by laser-induced evaporation with the use of magnetic pulsed compaction and vacuum sintering. Owing to the high activity of nanoparticles, the sintering is performed at temperatures below 1750°C without densifying additives. An increase in the sintering temperature to 900°C leads to an increase in the visual transparency of the ceramic materials and a decrease in the radiation attenuation coefficient. The samples of the translucent ceramics are characterized by rather large values of the microhardness (11.8 GPa) and the fracture toughness as compared to those of single crystals of the same composition. The fracture toughness of the ceramic material increases by a factor of approximately 2.5 with a decrease in the average crystallite size from 5.0 to 0.6 μm.

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 .

Nozzles from Alumina Ceramics with Submicron Structure Fabricated by Radial Pulsed Compaction

By means of magnetic pulsed compaction and sintering of weakly aggregated alumina based nanopowders the jet forming nozzle samples for hydroabrasive cutting were fabricated. The ceramics were obtained from pure alumina, as well as from alumina, doped by TiO2, MgO and AlMg. The ceramic properties of the channel surface and the nozzle volume were investigated. It was shown that the samples sintered from AlMg doped Al2O3 powder had the best desired mechanical properties and structural characteristics: relative density ~0.97, channel microhardness – 18-20 GPa, channel surface roughness ~0.7 /m, average crystallite size ~1 /m.