A mathematical model of mechanochemical synthesis will improve the efficiency of SHS processes.

The scientists from the Research Department for Structural Macrokinetics (Tomsk Scientific Center of the Siberian Branch of the Russian Academy of Sciences), Head of the Laboratory of Mathematical Modeling of Physical and Chemical Processes in Heterogeneous Systems Oleg Lapshin and Senior Researcher Oksana Ivanova have published an article "Macrokinetics of mechanochemical synthesis in heterogeneous systems: Mathematical model and evaluation of thermokinetic constants" in the high-ranking journal Materials Today Communications. https://www.sciencedirect.com/science/article/pii/S2352492821006632). The obtained results represent the modeled process of mechanochemical synthesis with regard to main parameters and increase the efficiency of self-propagating high-temperature synthesis (SHS).

Synthesis of multicomponent gas-free systems is among the promising ways to obtain composite materials. As a rule, the transformation of starting components into products proceeds in several reaction stages, and the rate of these reactions depends on many physical and chemical factors, which significantly complicates the analysis of experimental data and optimization of synthesis of composites with the required content of phase components. Mechanochemical treatment or mechanical activation of reagents and their mixtures in a high-energy mill (mechanical activator) provide additional opportunities for controlling the synthesis in multicomponent systems. The method of mechanochemical synthesis belongs to the resource-saving and environmentally friendly technologies that allow the final product to be obtained with the given set of required properties and parameters under safe conditions. Similar technologies can be used for the synthesis of nanoparticles and materials made of them.

The application of mechanical pre-treatment expands the nomenclature of low-caloric systems, such as nickel-aluminum, aluminum-magnesium, titanium-aluminum, titanium-silicon, tantalum-silicon, niobium-silicon, iron-silicon, tantalum-carbon, titanium-carbon, niobium-titanium, in which high-temperature synthesis can be conducted in combustion and thermal explosion modes.

Intensive mechanical treatment of powder mixtures grinds components, increases the interfacial reaction surface by "smearing" the components on each other in the friction contact areas and decreases the effective activation barrier of the reaction due to an increase in the excess energy accumulated in the formed structural defects. In addition, it was found that phases of different structures and morphology can be formed in mechanically synthesized mixtures of the same composition, depending on the parameters of mechanical treatment.

   For example, increasing the pre-activation time at the second stage of mechanical synthesis reduces the self-ignition temperature of the powder mixture (for thermal explosion) and raises the temperature and rate of combustion for layer-by-layer synthesis.

   The article discusses the possibilities of mathematical modeling that can optimize the processes of self-propagating high-temperature synthesis in mechanically pre-activated compositions.   

The scientists concluded that the previously used approaches have a significant disadvantage. The point is that the modeling is carried out without simultaneous consideration of the main mechanochemical synthesis parameters, such as temperature, completeness of chemical transformations, the size of the interfacial surface, and the amount of excess energy stored in activated substances. The proposed macroscopic approach corrects this gap.

   The researchers added new equations determining the dynamics of the reaction surface during grinding and agglomerating reagents to the system of equations traditionally used in studies of SHS processes, as well as the dynamics of defects (excess energy) formed during mechanical activation. The results obtained are important for specialists engaged in mathematical modeling, as well as for materials scientists who create efficient (economically and ecologically) technologies for the production of materials and products with improved properties.