Chemical Dispersion as a Method for Segregation of Ultrafine and Nanosized Powders of SHS Refractory Compounds
Overviewed are some results of our recent works aimed at the synthesis of ultrafine and nanosized refractory powders by self-propagating high-temperature synthesis (SHS) combined with chemical dispersing of combustion products. The possible application of SHS for preparing ultrafine and nanosized powders of refractory compounds is examined. Specific features of SHS afford to affect not only the chemical/phase composition of products but also the morphology and size of synthesized particles. Quenching experiments have shown that the primary product particles formed in the combustion wave may have a size of below 0.1-0.2 μm. By etching a ground SHS cake with appropriate solutions, one can dissolve the defect-rich layers between the crystallites and remove impurities. As a result, the sinter cake breaks into crystalline particles of the same size as the primary crystallites, without compositional changes. This process was termed chemical dispersion. The phase and chemical composition, specific surface and grain size of SHS refractory compounds powders are detected before and after treatment. The influence of the synthesis conditions and the dispersion agent on the morphology and particle size of the resultant powders are ascertained. Successive chemical dispersing of raw products (carbides, nitrides, silicides, borides, oxides of metals and nonmetals) with solutions of different acids, alkalis, and salts leads to an additional grinding of the product, an increase in the specific surface and a decrease in the particle size. So, the possibility to control the SHS processes allows changing the particle size and structure of the synthesized compounds and provides obtaining high-quality products of the preset structure. The following thermochemical treatment of the ground cake by special solutions (chemical dispersion) allows to segregate ultra- and nanosized powders of refractory compounds. Our experience in preparation, separation, and purification of SHS-produced nanopowders can be used as a basis for elaboration of general methodology for pilot-scale production of nanostructured compounds and composites.
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