V. V. Panichkina

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.

Effect of Microwave Heating on the Mass Transfer, Phase Formation, and Microstructural Transformations in the Y2O3–Al2O3 Diffusion Couple

The phase composition, phase growth kinetics, and structures of diffusion zones formed under microwave heating (24 GHz) (MWH) and conventional heating (CH) in two-layer Al2O3–Y2O3 samples are studied by optical and scanning electron microscopy and electron microprobe analysis. Diffusion annealing was conducted at 1700°C for 5 h in vacuum, the heating rate being 10°C/min in all experiments. The diffusion couples included alumina layers, such as coarse-grained polycore or sintered Al2O3–5 vol.% ZrO2 layers, and yttria layers, such as sintered coarse-grained samples or fine Y2O3 powder layers on the Al2O3 surface. It is shown that the phases formed during reactive diffusion do not uniquely correspond to the phase diagram, but depend on the initial structure of contacting layers and the type of heating. This is attributed to the contribution of kinetic factors to the competitive phase growth, particularly to the structural sensitiveness of diffusion coefficients whereby the diffusive phases grow and the stresses appearing when new phases form. It is found that MWH influences the competitive phase growth in the Al2O3–Y2O3 system, which involves both the change in the phase composition of the diffusion zone compared to that formed under CH and the acceleration of phase growth. The maximum effect of the phase growth acceleration under MWH is observed for the YAG phase, which is 30 times as fast as that under CH. It is suggested that the structure of grain boundaries changes and, accordingly, their permeability increases under MWH. The accelerated GB diffusion under MWH promotes the YAG phase growth in both oxides as a result of opposite diffusion flows of Al and Y ions along GBs. Under TH the YAG phase is formed only in Y2O3 oxide because of the unipolar diffusion of Al3+ ions to Y2O3. The validity of the proposed mechanism is confirmed by numerical evaluations.

Properties of molybdenum powders obtained by reduction in moving beds

We have studied the process of decomposition of ammonium paramolybdate and reduction of the molybdenum oxides in moving beds with rotation of the working chamber. We have shown that agglomerated disperse molybdenum powders are formed under these conditions. Movement of powder beds during reduction ensures continuous contact between the oxides being reduced and the hydrogen, rapid removal of water vapor from the reaction zone, and establishment of nearly kinetic reduction conditions. As a result the metallic molybdenum powders are strong, highly porous agglomerates, similar to the MoO3 particles in size and shape. The powders have high bulk (as-poured) density and good flowability, and their dispersity depends only on the reduction temperature.

Mechanical properties and structure of tungsten-rhenium powder metallurgy deformed alloys

The properties of tungsten-rhenium alloys, obtained by compact sintering of dispersed powders which formed a tungsten-rhenium solid solution at relatively low temperatures, were investigated. Three systems were compared: a tungsten alloy with 2% rhenium, an alloy with dispersed additions of yttrium and hafnium oxides, and commercially pure tungsten. The properties included ductile-to-brittle transition temperature, bend strength, yield strength, and crack resistance. Structures were determined by scanning electron microscopy. Tungsten alloyed with small additions of rhenium possessed higher plasticity and in other mechanical properties was similar to cast and deformed alloys of the same composition.

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.

Interdiffusion and Structural Changes in the Cr2O2–Al2O3(ZrO2) Diffusion Couple under Microwave Heating

The interdiffusion and microstructural evolution of the Cr2O3–Al2O3 (5 vol.% ZrO2) diffusion couple are studied in the temperature range 1600–1800°C under microwave heating (24 Hz) and, for comparison, under traditional heating using electron microprobe analysis and microscopic analysis. It is found that the concentration of chromium is distributed differently in Al2O3 in diffusion zones under microwave and traditional heating. This is due to greater contribution of grain-boundary diffusion to the effective diffusion flux under microwave heating. Bulk diffusion and average grain-boundary diffusion coefficients are calculated. The grain size in the diffusion zone toward Al2O3 is smaller after microwave heating. Traditional heating induces grain growth by recrystallization, whereas two processes, recrystallization and polygonization, are superimposed during microwave heating. The polygonization is due to the generation of dislocations under thermal stresses originating from nonuniform temperature distribution in the diffusion zone with variable concentrations of the components. The calculated bulk and grain-boundary diffusion coefficients can be used to predict the kinetics of various diffusion mass-transfer processes in Al2O3 and Cr2O3 oxides and their mixtures.

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.

Nature of changes in the microstructure and properties of fine-grained iron-copper composites under heat treatment

We examine the nature of the changes that occur in the microstructure and properties of fine-grained iron-copper composites with 30 mass % (27.3 vol. %) Cu during solid-phase heat treatment and when passing through the melting point of copper. Quantitative studies of the microstructure were made during sintering of mixtures of the highly dispersed powders of the initial metals and during heating of sintered high-density fine-grained specimens. The process of microstructure transformations during liquid-phase sintering and heating of high-density fine-grained composites above the melting point of copper was found to have three stages: recovery of the crystal structure and formation of large-angle boundaries in the Fe component, an increase in Fe grain size, and formation of solid solutions by mutual diffusion of components; penetration of the liquid phase along Fe grain boundaries with a decrease in grain size because of disintegration; and a secondary growth of Fe grains and formation of a Cu matrix structure or, more likely, a matrix structure of solid solution of Fe in Cu begins to form.

Compaction and grain growth in alumina ceramics under microwave sintering

The laws governing the transformations of the porous and grain structures that occur in Al2O3-based ceramics during microwave sintering are investigated. These processes are very similar to analogous processes that occur during coherent compaction of powders. Under microwave sintering the ceramic preforms retain a wide range of porosity and microstructure for a long time. As a result strong capillary forces exist throughout the process, causing active compaction. On the basis of the experimental data we suggest that the heating rate and the bulk mode of the heating are not the only factors that accelerate the consolidation of the ceramics under microwave sintering.

Segregation of solutes to the grain boundaries of tungsten-copper alloyed pseudoalloys

The authors examine the effect of silicon additions on the redistribution of solutes in the struture of a tungsten network in tungsten-copper pseudoalloys and determine their ductility characteristics in tensile loading. The fracture surfaces of the specimens were examine using a JAMP10S Auger microprobe, and the tensile test was carried out using the standard procedure. Data show that the introduction of silicon greatly reduces the content of carbon and oxygen both in the volume and at the grain boundaires of tungsten.