Olga I. Get'man

Effect of the Phase Composition of Barium-Calcium Alumoscandates on the Emission Properties of Impregnated Cathodes

The effect of scandium oxide additions on the emission properties of impregnated tungsten cathodes was investigated. The synthesis of alumoscandates in the BaO ― CaO ― Sc2O3 ― Al2O3 system were studied by x-ray diffraction, thermography, and petrographic analysis. The hygroscopic properties of the emission-active material was determined. Based on the results of emission tests the recommended optimal compositions of emission-active material are 2.4 BaO·0.6 CaO·0.1 Sc2O3·0.9 Al2O3 and 2.6 BaO·1.9 CaO·0.1 Sc2O3·0.9 Al2O3. Cathodes based on these compositions had lifetimes greater than 10000 h operating in the temperature range 900-1000°C at current densities of 15-20 A/cm2 in a vacuum of the order of 10−6 Torr.

The Porous Structure of a Permeable Nickel Material with a Bimodal Pore-Size Distribution

An SIAMS-600 computer system has been used to analyze the biporous structure of a nickel material formed with the use of a pore-forming agent. The pore size distribution is narrower in a material in which the size of the pore-forming agent particles is smaller, which is due to the better mixing of the metal and pore-forming agent powders. These materials have an anisotropic porous structure, with the pores having a shape elongated perpendicular to the pressing direction.

High-Velocity Flame Spraying of Powder Aluminum Protective Coatings

Requirements to industrial processes for the thermal spraying of corrosion-resistant metal coatings on large-scale steel structures are formulated. The promise of high-velocity fuel·air flame spraying for this application is described. Regimes for the deposition of protective aluminum coatings by this process and their adherence characteristics were investigated. Metallographic studies including computer treatment of images of polished cross sections were carried out with the aid of the materials science software SIAMS.

Use of powders of the tungsten—Rhenium alloys for the preparation of impregnated cathode skeletons. II. Emission capabilities of barium containing impregnated cathodes with tungsten—Rhenium alloy skeletons

The dependence of the emission properties of barium-containing impregnated cathodes on the rhenium content, phase composition, and pore structure of the metallic skeleton was studied. Skeletons were prepared from mixtures of elemental W and Re powders, as well as from W—Re alloy powders. The lowest work function (π(1300 K)=1.97–1.99 eV) was observed in cathodes containing 50–80 mass % Re. The use of W—Re alloy powders with additions of highly dispersed aluminum oxide for the preparation of skeletons increased the lifetime of impregnated cathodes by 2–2.5 times. Such cathodes possessed a stable and uniform grain structure which was resistant to recrystallization and grain growth, thus they had a stable skeleton structure.

Sintering of Si3N4 Ceramics with Additives Containing Yttrium and Ytterbium Oxides with Microwave and Traditional Heating

Compaction processes, phase transformation, and porous structure evolution are compared for materials based on silicon nitride containing yttrium and ytterbium oxides under microwave and traditional heating conditions. Experiments by microwave heating are performed in a special gyrotron device operating at a frequency of 30 GHz in a nitrogen atmosphere at normal pressure with a constant heating rate of 30, 60, and 90 deg/min. Traditional sintering is carried out in a tubular continuous furnace with a tungsten muffle in a nitrogen atmosphere at normal pressure with a heating rate of 60 deg/min. It is established that under microwave heating conditions compaction of the test materials and the start of the α →β phase transformation occur at temperatures 100-150 deg lower than those typical for traditional heating. In material with a reduced amount of oxide additives compaction is accompanied by an increase in the proportion of large pores which is the reason for a slow-down in compaction both with traditional heating and with microwave one. It is suggested that the reason for acceleration of mass transfer with microwave heating is the specific nature of the interaction of the microwave field with the liquid phase formed, promoting development of non-diffusion transfer of solid-phase elements dissolved within it and the mechanical effect on its particles.

Mechanism for Improving the Mechanical Properties of Sintered Iron–Copper Composites Alloyed with Molybdenum

The structure of Fe–Cu composites after solid-phase and liquid-phase sintering was studied. It is shown that 2–10 wt.% molybdenum additions have an activating effect on the diffusion processes in densification, grain growth, and recrystallization, as well as on the amount and composition of copper and iron solid solutions. Molybdenum additions to 70 wt.% Fe–30 wt.% Cu composites simultaneously influence their strength and ductility properties. With increasing molybdenum content, the solubility of iron in copper decreases, promoting higher ductility of the composites, and the solid solutions of copper and molybdenum in iron preserve their strength characteristics. Solidphase sintering results in fine-grained FeCuMo samples with high relative density (up to 98.8%) and high ductility.

Effect of the Dimensional Factor on Sintering Composites of the System W – Sc2O3

The effect of adding fine scandium oxide (1, 5, and 10 vol.% in relation to the metal) on shrinkage kinetics during sintering of tungsten powder with an average particle size of 4 μm and 140 nm is studied. Specimens of coarse and fine powders are sintered at 1800-2000 and 1000-1200°C, respectively. The results obtained are explained on the basis of the Skorokhod structural model for composite mixtures that takes account of the dimensional factor of components and the level of the matrix microstructure. It is established that the lower the level of the latter and consequently the greater the level of the carcass formation, the greater is the effect of a slowdown in volumetric shrinkage with the same volume content of oxide addition.

Porous Permeable Polymers

Features of extrusion for mixtures of polyvinyl chloride and a pore-forming agent with the aim of preparing very porous materials are studied. It is shown that extrusion rate depends on the particle size of the pore-forming agent with the same volume content. This is explained by the difference in the structure of mixtures with respect to pore-forming agent. Marked anisotropy is established for the porous structure of materials in the extrusion direction and perpendicular to it. The hydraulic and mechanical properties of porous materials are determined and possible fields for their application (aeration in fish breeding, capillary transport for under-root irrigation, etc.) are suggested.

Size Dependence of Microstructure Characteristics for Chromium Disilicide Thin Films

We have used computer analysis to study linear and bulk characteristics of the microstructure of CrSi2 thin films consisting of crystalline and amorphous phases; we used transmission electron microscopy to determine the microstructure of the films. As the film thickness increases, the linear dimensions of the crystallites and their volume fraction increase, while the average interparticle distance and the specific number of particles decrease. In this case, the microstructure evolves from a matrix type to a matrix-statistical type. It is due to phenomena including recrystallization during film deposition, coalescence of discrete islands of crystallites, and «sintering» of contacting particles.

Sintering a disperse mixture of W-Sc2O3 powders

The effect of additions of scandium oxide (1, 5, and 10 vol. %) on compaction during sintering of a disperse mixture of W-Sc2O3 powder at 2000°C has been studied. It has been found that the scandium oxide particles activate compaction during heating and conversely retard shrinkage of tungsten in the stage of isothermal soaking. The kinetics of compaction during sintering is determined by the geometry of the heterophase system. Depending on how much the tungsten and scandium oxide particles increase in size, the nature of thestructure changes markedly from matrix-statistical to statistical.