**2.1. Self-propagating high-temperature synthesis (SHS)**

The new scientific direction SHS was developed at the interface of three scientific fields: combustion, high-temperature inorganic chemistry and materials science. SHS is an autowave process analogous to propagation of the combustion wave with the chemical reaction being localized in the combustion zone propagating spontaneously along the chemically active medium [15, 16]. The essence of the process is occurrence of exothermic reactions at temperatures developing as a result of self-heating of the substance; the synthesis temperature is up to 4000°C, the temperature growth rate – 103-106 K/s, the combustion velocity – 0.1-10 cm/s.

Thorough fundamental investigations of the SHS process have proved that chemical transformation in combustion waves and product structure formation occur simultaneously with high velocity and at significant temperature gradients. These peculiarities of the process provide practically complete chemical transformation of the mixture and a specific structure of the combustion products. Application of SHS allows avoiding the main disadvantages of conventional technological processes – high power consumption, complicated equipment, low product output.

The extreme terms which are characteristic of SHS of chemical compounds affect chemical and phase composition of the products as well as their morphology and particle size [17, 18]. The experiments in product quenching by special cooling methods immediately after the combustion front propagation have proved that "primary" product particles of 0.1-0.2 μm in size can be formed in the combustion front [19, 20].

The product structure formation during the chemical reaction was called primary structure formation while the structure formed in this case was called the primary structure of the product. The characteristic time of the chemical reaction is 10-3-10-1 s; the time of the primary structure formation being the same. After the chemical reaction the particle size increases as a result of the secondary structure formation process followed by collecting recrystallization [21]. The duration of the process depends on the sample cooling mode and is usually about some or tens seconds.

Transformation of initial reagents to final SHS products is a complicated multiparametric process. There are various ways to govern it. The main types of the occurring processes are solid-flame combustion in the solid-solid system (one of the varieties is combustion with the intermediate melted layer), gas-phase SHS (chain flames, combustion of condensed systems with gaseous intermediate zone), combustion of solid-gas systems (filtration combustion, combustion of gaseous suspensions) [22].

Let us consider the possibilities of these processes.

In order to obtain ultrafine and nanosized products in the processes of solid-flame combustion, one must use the reagents of the same dispersion. In solid-phase systems with the intermediate melted layer the possibility of nano-crystal formation depends on crystallization and recrystallization processes, combustion heat modes and product cooling after the reaction.

Self-Propagating High-Temperature Synthesis of Ultrafine Tungsten Carbide Powders 5

volume of the reactive mass. So it is necessary to carry out some additional operations to

Having analyzed the literature data, we can conclude that in the case of the development of the SHS technology of tungsten carbide the main attention should be paid to detection of the terms of nano-particle formation during the synthesis process. However, investigation of the separation methods of chemically pure ultrafine and nanosized compounds from the

SHS products are cakes or ingots which should be processed for obtaining powders. It can

Mechanical milling (conventional milling by balls, friction milling, planetary milling) is the easiest method for obtaining ultrafine and nano-sized powders. It is possible to obtain fine powders (up to 10-20 nm), but the problems of the long duration of the process, powder contamination with the ball and vessel materials, high power consumption require some

One of the promising methods of obtaining nano-sized powders is the method microparticle dissolution. Recently, the efficiency of the dissolution processes for converting microparticle size to the nano-level has been confirmed. The method is based on the property of particles to decrease their volume uniformly due to their dissolution in acid and alkali media. But simultaneously the structure and the properties of the central part of the substance or phase

The main aim of powder application is to obtain a dense product with homogeneous microstructure after compaction. The common reason restricting the refractory material strength is existence of agglomerates in the powder [27]. So in order to make the powder strong, it is necessary to disintegrate or remove large solid agglomerates from the initial powder. In the case of ultrafine powders the agglomerates are disintegrated by dispergating

The influence of various solutions on the powder structure, dispersion degree and specific surface area has been already studied for SHS powders of boron nitride and aluminum

After synthesis, the materials were mechanically disintegrated and subjected to thermochemical treatment in neutral, acid, and alkali media at temperatures ranging from 20 to 100°C [28]. Such treatment is termed "chemical dispersion" of SHS products, as suggested by Merzhanov [29]. Chemical dispersion in a neutral medium resulted in increased total, outer, and inner specific surfaces. Mean grain size decreased. This implies that chemical dispersion provided for disintegration of the materials, as well as leading to formation of new channels and pores and the appearance of new defects, finally resulting in

sort out the main compound [24, 25].

**2.2. Chemical dispersion** 

additional solution.

remain the same [26].

nitride.

and milling in suitable solutions.

improved specific surface.

synthesis products and their analysis are very important too.

be achieved by either mechanical milling or chemical treatment.

In the case of gas-phase SHS (gas combustion followed by a condensed product formation) the product elemental particles consolidate with each other and form nuclei on the surface of which the following reactions occur. If fast artificial cooling is used, it is possible to arrest the particle size growth at a required stage and obtain nanopowders by depositing the particles from the gas mixture.

At gas-phase combustion the initial reagents, intermediate and final compounds remain in the condensed state (either liquid or solid) during the entire reaction [16, 23].

The SHS method has provided the possibility of obtaining a great number of compounds in the dispersed state (powder). Among the materials for which the technological backgrounds are well developed the main ones are powders of refractory compounds. They are widely used in industry due to their outstanding properties such as hardness, thermal stability, abrasive wear and resistance.

There are several directions of the SHS technologies. The widest and well-developed type of SHS reactions is the synthesis reactions of refractory compounds from elements. It is oxygen-free combustion. Both powders and gaseous elements take part in the chemical reactions. Besides, some regulating additions R are introduced into the initial mixture. They can be synthesis products (as diluents), various inorganic and organic compounds.

Another direction is combination of SHS with thermal reduction (SHS with a reducing stage) when the compounds of elements (oxides, halogenides, etrc.) and metal-reducers – Mg, Ca, Al, Zn, etc. are used for the synthesis. The advantages of this method are a low price and availability of raw materials. Besides, metallothermal powders are characterized by such valuable properties as high dispersion and homogeneous granulometric composition.

The interaction of the reagents in the combustion wave occurs within two stages. The first one (reduction of the main metal oxide) is a metalthermal reaction. The second stage (SHS itself) is the interaction of the reduced metal with a non-metal followed by a refractory compound formation. There are a lot of secondary reactions which should be suppressed when optimum technological terms of the process are worked out. In the complicated systems of oxide – metal-reducer – carbon (hydrocarbon), carbon-containing components take part in carbide formation and reduction of metal oxides as well. It defines the requirements to the choice of the initial components ratio.

As a result of the SHS with a reducing stage a "semiproduct" is obtained which contains the main compound and the secondary products which can often be metal-reducer oxides. In metallothermic powders the secondary product is distributed uniformly in the whole volume of the reactive mass. So it is necessary to carry out some additional operations to sort out the main compound [24, 25].

Having analyzed the literature data, we can conclude that in the case of the development of the SHS technology of tungsten carbide the main attention should be paid to detection of the terms of nano-particle formation during the synthesis process. However, investigation of the separation methods of chemically pure ultrafine and nanosized compounds from the synthesis products and their analysis are very important too.
