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Article title NANOSTRUCTURED POWDERS OF AL2O3+хFe COMPOSITES FOR SYNTHESIS OF THERMOCRYSTALS
Authors O.V. Karban, E.I. Salamatov
Section SECTION III. NANOTECHNOLOGY
Month, Year 04, 2017 @en
Index UDC 546.621/623:17
DOI
Abstract A new method for the preparation of powders of Al2O3 + xFe metal composites is proposed, which, in subsequent heat treatment, can realize the required material structure that forms a gap in the phonon spectrum. The method involves mechanoactivation of a mixture of aluminum oxide and a carbonyl iron preliminarily transferred to the nanostructured state. The process of changing the structure and phase composition of a mixture of alumina powder and nanocrystalline iron during mechanoactivation stops at the first stage: particle dispersion and formation of a layered laminar structure that is formed by nanosized alfa-Fe grains separated by oxide phase regions of less than 2 nm in thickness. The particle size of the powder is 50-200 nm and is independent of the iron content in the sample. According to the data of X-ray diffraction and Mossbauer spectroscopy, the chemical interaction between alumina and iron during mechanical processing does not occur for 40 minutes. The average size of the iron crystallites is about 10 nm. The grains of the metallic phase are under the Al2O3 layer, which contributes to the preservation of their phase state and inhibits the metal phase from oxidation in air upon heating. On the surface of the particles other than aluminum atoms, there is oxygen and carbon, which is a part of the adsorbed hydrocarbons. For samples of a mechanical mixture of aluminum oxide and carbonyl iron not subjected to preliminary transfer to the nanostructured state, after 40 minutes of exposure, the average particle size of the mechanically activated mixture is 13.6 μm without forming a nanostructured state. The use of the method of magneto-pulse compaction for compacting does not lead to structural-phase changes in compacts in comparison with a mechanical mixture of powders. Sintering at a temperature of 1300 ° C leads to the formation of the  phase of corundum and the spinel phase of FeAl2O4. The crystallite size of Al2O3 is 50-140 nm and does not depend monotonically on the iron content, the particle size of iron particles is 40-100 nm. The content of the spinel phase does not exceed 3 wt.% and is practically independent of the Fe content of the original powder. The formation of spinel particles on the surface due to interaction with adsorbed oxygen, which is a barrier layer, inhibits further oxidation of metallic inclusions. It is shown that the introduction of the metallic phase leads to a decrease in the thermal diffusivity and conductivity of the thermal crystals, which may be due not only dimensional effects, but also the properties of the interfacial regions.

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Keywords Thermocouples; Composites Al2O3 + Fe; Mechanoactivation, nanoceramics.
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