T. I. Ignat’eva

SHS of ultrafine and nanosized refractory powders: An autoreview

Overviewed are some results of our recent works aimed at the synthesis of ultrafine and nanosized refractory powders by SHS combined with chemical dispersing of combustion products. The microstructure of synthesized powders was found to depend on the type and amount of added modifying agents. Successive chemical dispersing of raw products with solutions of different acids, alkalis, and salts leads to an increase in the specific surface and a decrease in the particle size caused by dissolution and wash-out of the impurities and intermediate products present in raw products. 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.

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.

Self-propagating high-temperature synthesis (SHS) of a pyrochlore-based ceramic for immobilization of long-lived high-level waste

A Y2Ti2O7-based ceramic of pyrochlore-type structure as a host material for immobilization of actinide-containing HLW was prepared by SHS compaction. Up to 25 at. % Zr was introduced into the pyrochlore structure instead of Ti to enhance the chemical and radiation resistance. The ceramic matrices exhibit low porosity, good water resistance, and high mechanical strength. The study was performed with simulated HLW.

Self-propagating high-temperature synthesis for immobilization of high-level waste in mineral-like ceramics: 1. Synthesis and study of titanate ceramics based on perovskite and zirconolite

Preparation of ceramics based on synthetic perovskite and zirconolite (analogs of titanate minerals) using self-propagating high-temperature synthesis (SHS) was studied. Ceramics was prepared as matrix material for immobilization of high-level waste (HLW). Using model HLW, the optimal synthetic conditions were determined which allow preparation of compact low-porosity material (in the form of cylindrical blocks) exhibiting high strength and low rate of leaching of Cs, Sr, Y, Ce, and La into double-distilled water. The phase composition and micro structure of the resulting materials were studied. As found, immobilization of Cs is accompanied by significant loss of this element.

Self-propagating high-temperature synthesis for immobilization of high-level waste in mineral-like ceramics: 2. Immobilization of cesium in ceramics based on perovskite and zirconolite

Immobilization of Cs in ceramics based on perovskite and zirconolite by self-propagating high-temperature synthesis (SHS) was studied using model systems. The ceramics was obtained as cylindrical blocks. Cesium can be completely immobilized in ceramics at 1250°C in the form of synthetic mineral pollucite and, in this case, it is uniformly distributed through the bulk of the ceramic sample. The optimal synthetic conditions were determined, and the phase composition of the ceramics was evaluated. The leaching rates of Cs from ceramics in double-distilled water at 20°C were determined.

Mineral-like ceramics for immobilization of nuclear wastes by forced SHS compaction

Dense cylindrical samples based on Y2Ti2O7 ceramics with a pyrochlore structure were produced by forced SHS compaction and characterized as a matrix for immobilization of actinide-containing nuclear wastes. Synthesized materials were found to exhibit high hydrolytic resistance and mechanical strength.

SHS of mineral-like ceramics for immobilization of radioactive nuclear wastes

Synthetic analogs of titanate ceramics (perovskite and zirconolite) designed for use as a matrix for immobilization of high-level nuclear fuel reprocessing wastes (HLW). Such ceramics have been prepared by SHS method from a mixture of titanate ceramics and non-radioactive model oxides. Synthesis conditions have been optimized. The synthesized low-porosity cylindrical compacts exhibited a high strength and low rate for leaching Cs, Sr, Y, Ce, and La in bidistilled water. The phase composition and microstructure of synthesized products have been characterized. The immobilization of Cs was found to be accompanied by a marked loss of this element.

EFFECT OF A MASS FORCE ON THE COMBUSTION OF THE NI-AL SYSTEM

The effect ofg-load (in the range of 1 to 1000g) on the combustion of the Ni−Al system was studied experimentally. It is shown that a mass force has a significant effect on the burning rate of the mixture, compaction, and the chemical and phase compositions of the combustion products. A mechanism for the effect ofg-load on the burning rate and compaction of the system is proposed.

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.