I. P. Borovinskaya

A New Class of Combustion Processes

Abstract A short information is given on the results of work carried out at the Branch of the Institute of Chemical Physics, U.S.S.R. Academy of Sciences, since 1967 on studying the combustion processes caused by the interaction of chemical elements in the condensed phase and leading to the formation of refractory compounds. New phenomena and processes are described which are revealed when investigating the combustion of the systems of this class, viz solid-phase combustion.fast combustion in the condensed phase, filtering combustion, combustion in liquid nitrogen, spinning combustion, self-oscillating combustion and repeated combustion. A new direction in employment of combustion processes is discussed, viz. a self-propagating high-temperature synthesis of refractory nitrides, carbides, borides, silicides and other compounds.

Chemical classes of the SHS processes and materials

The paper concerns chemical issues of self-propagating high temperature synthesis. Chemical classes of the SHS processes and products are considered. The results of research into SHS chemistry are discussed according to the classification suggested by the author: solid solutions of nonmetals in metals, binary compounds (carbides, borides, silicides, nitrides, hydrides, oxides, chalcogenides, etc. 1, chemical mechanism of formation in combustion mode, and intermediate products. Single - phase multicomponent SHS systems involving complex carbides, oxides, nitrides, carbonitrides, hydrocarbides, and hydronitrides are also treated. An extended class of the SHS products comprises complex heterogeneous mixtures of refractory inorganic compounds, as well as hard alloys. The reactions used for synthesizing SHS products of different classes are presented. The use of SHS for producing organic and polymer compounds was experimentally shown to be possible. Finally, the discussion of some issues related to the crystalline structure and particle size of SHS products is presented which is important for understanding the process itself, as well as for practical application of SHS compounds of different classes.

Dynamics of phase forming processes in the combustion of metalgas systems

Abstract The dynamics of phase formation in the combustion wave of the titaniumnitrogen, titaniumoxygen, and titanium-air systems have been investigated. The experiments were on an installation composed of a position detector and a standard x-ray apparatus. The combustion wave in the TiN 2 system has been found to propagate as a result of the formation of titanium nitride, which is the final product. The phase formation process in the TiO 2 system is caused by oxidation of titanium to titanium dioxide without formation of any intermediate oxide phases. In the Ti-air system the combustion front propagation is induced by the formation of the intermediate phase, titanium nitride, which converts to the final product, titanium dioxide, via two oxynitride intermediates.

Combustion Synthesis of Aluminum Nitride

Experimental data are presented on the self-propagating high-temperature synthesis of aluminum nitride from Al + AlN starting charges, containing no gas-generating additions. The effect of the charge composition on the combustion temperature, powder morphology, and the chemical composition of the synthesis products is analyzed. The results are used to optimize the process conditions.

Preparation of Ultrafine Boron Nitride Powders by Self-propagating High-Temperature Synthesis

The possibility of preparing ultrafine boron nitride powders by self-propagating high-temperature synthesis (SHS) is examined. The results demonstrate that, by varying SHS conditions (starting-mixture composition, combustion rate, cooling rate, and intermediate grinding time), one can control the morphology and particle size of the resultant hexagonal BN powder. Systematic data are presented on the chemical dispersion of SHS products for separating nanometer-sized particles. The effect of chemical dispersion (composition of the dispersion medium, process temperature, and dispersion time) on the particle size, morphology, and chemical and phase compositions of boron nitride powders is analyzed. Hexagonal boron nitride powders are obtained with a purity of 99.5+%, particle size of 0.1–0.2 μm, and specific surface of up to 65 m2 /g.

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.

Laws and mechanism of diffusional surface burning of metals

As shown in [1], burning metal powders in a nitrogen medium is an effective means of obtaining the highmelting nitrides widely used in modern technology. Accordingly, the study of the combustion mechanism of metal-gas systems is of considerable interest . The laws of diffusional metal combustion were first studied in [2, 3]. On the basis of the experimental data obtained certain ideas about the combustion mechanism were formulated. The limit regime of diffusional surface burning, in which gas is supplied to the combustion front solely through the ends of the specimen, and the lateral surface is impermeable, was subjected to a detailed theoretical study in [ 4-6].