S.L. Kharatyan

Synthesis of alumina-silicon carbide composites by chemically activated self-propagating reactions

Abstract The synthesis of alumina/silicon carbide composites by self-propagating reactions activated by Teflon is investigated. It is shown that small amounts of this material (i.e. 0.20–0.25 wt.%) guarantee the occurrence of self-propagation reactions leading to the formation of a composite. In particular, under argon atmosphere, it consists of well-formed crystals of alumina, fine grained silicon carbide and whiskers-shaped crystallites of SiC. On the other hand, aluminium nitride, silicon carbonitride/silicon cyanide, aluminum oxynitrisilicides and mullite are also found under nitrogen atmosphere. The content of SiC whiskers in the final product increases when powdered polymers (polystyrene, polyethylene, polypropylene) are added to the initial mixture together with Teflon.

Influence of high-energy ball milling on reaction kinetics in the Ni-Al system: An electrothermorgaphic study

A new electrothermographic method, viz. high-speed temperature scanning, was applied to kinetic studies of reactions taking place in the Ni-Al system, including those after mechanical activation in a planetary ball mill. Treatment of the temperature profiles taken at different heating rates in terms of the Kissinger-Akahira-Sunose (KAS) approximation gave activation energy E for non-activated mixtures: E = 155 kJ/mol (for temperature range 650–850°C). But for mechanically activated mixtures, the characteristic points (reaction onset temperature, temperature of maximum reaction rate) were found to decrease with increasing heating rate, which makes the KAS method inapplicable to these compositions. It has been concluded that mechanical treatment leads to significant changes in the reaction kinetics, possibly due to splitting the reaction route into two stages the first of which has very low activation energy.

Synthesis of fine boron nitride powders by combining direct boron nitridation with carbothermic boron oxide reduction

Fine boron nitride powders were synthesized by combining direct nitridation of boron with carbothermic reduction of boron oxide. It has been established that the heat released in direct nitridation of boron can be utilized to carry out carbothermic reduction of boron oxide in a mode of self-sustained combustion. Intermediate gaseous products were found to play a key role the SHS reactions under study.

Mechanically and chemically activated SHS in the Mo-Si-C system: Synthesis of MoSi2-SiC composites

Combustion synthesis of various MoSi2-SiC composites was carried out in conditions of combined chemical and mechanical activation upon variation in green composition, ambient gas pressure, duration of mechanical activation, amount of activating additive, etc. In the absence of additive, the SiC content of composites was found to be below 10%. But in the presence of Teflon as an additive, the SiC content of resultant composites was found to attain a value up to 90%. In case of mechanoactivation, the amount of activating additive can be reduced by a factor of four, while the resultant product becomes more finely grained and homogeneous.

Non-isothermal phenomena in Mo/Si diffusion couple: Reaction kinetics and structure formation

Macrokinetic features and mechanism of Mo-Si reaction in conditions of rapid heating on a single particle level (Mo/Si diffusion couple) were studied by high-speed scanning electrothermography. The experiments were conducted in the temperature-time conditions (T = 1000–1600°C, t = 0.01–3.0 s) comparable to those of combustion synthesis of molybdenum disilicide. The rates of chemical heat release (depending on the thickness of silicon layer, δSi) were measured and the processes of phase/structure formation were explored. Over a wide range of δSi, two-stage chemical heat release was associated with occurrence of solid-solid Mo(s) + Si(s) (for T < TmpSi) and solid-liquid Mo(s) + Si(l) reactions (for T > TmpSi), where TmpSi stands for the melting point of Si. Proposed was a new mechanism for siliconization of Mo in non-isothermal conditions.

Reaction Diffusion in Mo-Si System above the Melting Point of Silicon

The results of direct kinetic measurements and SEM observations on formation of silicide phases in Mo-Si system at temperatures 1400-1700о С are presented in the work. It was shown, that the formation of MoSi2 proceeds by two different mechanisms and accordingly, two types of microstructures are formed: (i) a compact layer (usually with expressed columnar structure) by the reaction diffusion mechanism; and (ii) separated fine grains by crystallization in the volume of saturated Me-Si melt. A model of product formation is offered which allows to calculate the relative contributions of the two mechanisms of disilicide phase formation at various stages of interaction and to estimate the role each of them in the total process.

The effect of degassing modes on the self-propagating reactions for the SiO2-Al-C system in the presence of reaction promoters

The combustion synthesis process activated by organic polymers (Teflon and polystyrene) in the SiO2/Al/C system is investigated under the counter-current and co-current degassing conditions. It is found that the combustion parameters, i.e. wave velocity and temperature, and final product microstructure are influenced by the degassing mode considered as well as the type and amount of promoters added to the starting mixture. In particular, the combustion wave results to accelerate during its propagation when adopting the forward infiltration configuration and in absence of polystyrene. In addition, under the same degassing condition, the produced gases are found to provide a specific orientation on the growth direction of thread-like SiC crystals during final product formation.

New isopropanol dehydration catalyst based on tungsten carbide prepared by modified self-propagating high-temperature synthesis

The W2C/C catalytic system with a high specific surface area (55 m2/g) was synthesized for the first time using modified self-propagating high-temperature synthesis (SHS). The bulk and surface properties of the synthesized system were characterized using physicochemical methods and a model reaction of isopropanol conversion. It was found that the conversion of the alcohol with 100% selectivity occurs in the direction of dehydration with the formation of propylene and water. It was shown that active centers are the W(VI) surface ions of the carbide, whose activity is higher than the activity of tungsten as a constituent of the phase of WO3.

Combustion synthesis of lithium cobaltate

A feasibility of combustion synthesis of lithium cobaltate powders using oxide sources was investigated. Cobalt oxide Co3O4 and lithium carbonate Li2CO3 were used as starting reactants. Two different oxidizers were explored: (i) ammonia nitrate NH4NO3 and (ii) lithium nitrate LiNO3. Some organic reducers (ORs)—such as CH-polymers, melamine, urotropine, etc.—were used as a fuel. Investigated was the effect of green composition/density and inert gas pressure on combustion parameters (burning velocity and combustion temperature) and product micro structure/composition. Best results were obtained for LiNO3-Co3O4-OR blends. Relative green density ρ was found to have a crucial effect: good results might be obtained only for ρ > 65%. XRD data testified that the combustion products obtained in optimized conditions contained pure lithium cobaltate LiCoO2. According to SEM data, the obtained powders had a mean particle size of 20 μm.

Electrothermographic Study of Reactive Multilayer Nanofoils

For the first time, the kinetics of heterogeneous reactions in nanofoils was studied by electrothermographic method. During combustion, the electrical resistance of reactive multilayer Ti/Al nanofoils strongly changes, which was used to determine an evolution of the extent conversion during reaction.