V. N. Semenova

Self-propagating high-temperature synthesis of ultrafine and nanometer-sized TiC particles

The feasibility of preparing ultrafine and nanometer-sized titanium carbide particles by self-propagating high-temperature synthesis (SHS) has been studied. Data are presented on the structure formation of TiC powders during SHS with a reduction step. Basic to this process is an exothermic reaction between titanium dioxide, magnesium metal, and carbon. The effects of the composition of the starting mixture, relationship between its components, and the morphology and particle size of the starting TiO2 powder on the particle size of the forming material have been investigated. The TiC powder was recovered from the sinter cake by chemical dispersion, a chemothermal treatment of the synthesis product in different solutions. The results demonstrate that treatment of the sinter cake with appropriate solutions removes impurities and causes imperfect intergranular layers to dissolve. As a result, the cake breaks down into homogeneous single-crystal particles. Subsequent treatment in different solutions further reduces the particle size of the powder. The effect of the composition of the dispersing solution on the particle size of the TiC powder has been studied. Our results made it possible to identify conditions for the preparation of titanium carbide powders containing up to 70% of particles less than 0.3 μm in size by SHS followed by chemical dispersion.

Aluminum oxynitride by SHS in chemical furnace

AlON powders of varied composition were produced from Al–Al2O3 mixtures by SHS without and with B/BN chemical furnace under high pressure of nitrogen gas as a third reactant and characterized by XRD, SEM/EDS, and chemical analysis. The products obtained without chemical furnace were found to contain Al5O6N, Al7O3N5, AlN, as well as unreacted Al and Al2O3. The powders produced by SHS with chemical furnace contained largely the Al5O6N phase with an admixture of Al7O3N5 and AlN. After etching with KOH solution, the Al5O6N powders became single-phased. The etching also leads to marked changes in the microstructure of synthesized powders. Due to their unique microstructure, the synthesized Al5O6N powders can be expected to exhibit unusual electrophysical and optical properties.

Preparation of ultrafine and nanosized MoSi2 particles by self-propagating high-temperature synthesis with a reduction step

We have developed technological principles of the preparation of ultrafine and nanosized MoSi2 particles by self-propagating high-temperature synthesis (SHS) with a reduction step. The effect of synthesis conditions (starting-mixture composition, relative amounts of reactants, and the presence and amount of an inert diluent) on the composition, structure, and particle size of the powders has been studied. The results demonstrate that inert additives reduce the adiabatic temperature. The crystallite size of MoSi2 decreases with increasing additive concentration. The MoSi2 powders obtained by SHS with a reduction step have the form of agglomerates consisting of spherical particles ranging widely in size: from large (several microns) to ultrafine and nanosized. The composition of the powders was checked by chemical analysis, microstructural examination, and X-ray diffraction.

SHS of ultrafine and nanosized MoSi2 powders

Ultrafine MoSi2 powders were prepared by SHS reaction from the elements (under an Ar pressure of 5 atm) followed by optimized diminution process and chemical dispersion (hot alkali etching). Raw SHS products (cakes) were found to contain tetragonal MoSi2 and an admixture of Mo4.8Si3C0.6. Chemical dispersion of ground product yielded MoSi2 powders in the form of porous agglomerates. The precipitates containing 50% MoSi2 and 50% K2MoO4 were separated from etching solutions. The size of MoSi2 particles was below 100 nm.

Extraction of TiAl powder from SHS-produced TiAl–MgO semiproduct by treatment in different solutions

Explored was the extraction of target TiAl powder from SHS-produced TiAl–MgO semiproduct by treatment in acids (H2SO4, HCl,) and NH4Cl solution. Acid leaching was found to be accompanied by marked elution of target TiAl: the yield of TiAl powder was 21 and 33% in case of leaching with H2SO4 and HCl, respectively. But upon leaching with enriched NH4Cl solution at 70–80°C for 30 min, the yield of purified TiAl powder attained a value of 60%.

Ultrafine and nanosized MoSi2 powders by SHS process with a reduction stage

Suggested are basic principles for fabrication of fine MoSi2 powders by SHS process with a reduction stage. Investigated was the influence of green composition, stoichiometric ratio, inert NaCl additive, and synthesis conditions on the structure/properties of thus fabricated MoSi2 powders. With increasing amount of added NaCl, the mean size of MoSi2 crystallites was found to decrease. The fabricated MoSi2 powders represented the agglomerates formed by the crowds of smaller (200 nm) and larger (up to 1–3 μm) particles. The powders were characterized by SEM, XRD, and chemical analysis.

Extraction of Ti Powder from Ti–MgO–Mg(–CaO) Cakes Produced by Magnesiothermic Reduction

The extraction of Ti powder from SHS-produced Ti–MgO–Mg(–CaO) cakes by treatment in leaching solutions (HNO3, HCl, and NH4Cl) was explored and optimized in relation to such factors as concentration of leaching agent, leaching temperature, chemical resistance of target Ti powder, and extent of byproducts extraction. The type of leaching solution was found to affect the size, structure, and morphology of resultant Ti powder. Best results were obtained at 70°C with aqueous solutions of: (1) the nitric acid taken in a 6-fold excess to the Mg content of combustion product and (2) the ammonium chloride taken in a 20-fold excess to nominal Mg content.

AlON powders by aluminothermic SHS under pressure: Synthesis and characterization

Heat-resistant and chemically inert xAlN–yAl2O3 solid solutions, also known as AlONs, find their application in production of optical ceramics [1, 2]. γ-AlON powders as raw materials for fabrication of transparent ceramics are currently obtained by hot pressing and sintering [3], carbothermal reduction [4], solid-phase reaction [5], and spark plasma sintering [6]; the processes of carbothermal reduction and solid-state reaction being most widespread [7]. AlON powders were also prepared by metallothermic SHS from Al–Al2O3 mixtures [8–10] and also from a number of other thermit-type mixtures [11–14] and showed good mechanical and refractory behavior [15].

Combustion and autowave chemical transformations of a highly exothermic CaCrO 4 /Al/B mixture

The combustion of a highly exothermic mixture of calcium chromate with aluminum and boron has been studied. It has been shown that these mixtures are able to burn in a wide range of reactant ratios. The autowave chemical transformation is accompanied by decomposition of calcium chromate, the chemical reaction of the decomposition products with aluminum and boron, the formation of a two-phase melt of the combustion products with its subsequent gravity separation, and crystallization of the layers. The results of the study may be useful for obtaining chromium borides.