Crystallization in a SHS wave

The crystallization of nonequilibrium amorphous SHS products increases the wave velocity. The regimes of such an acceleration are studied.Institute of Structural Makrokinetics, Russian Academy of Sciences, Chernogolovka 14232. Translated from Fizika Goreniya i Vzryva, Vol. 31, No. 3, pp. 19–21, May–June, 1995.     

Emission phenomena in a SHS combustion wave

There are numerous experimental data indicating that the action of electromagnetic and magnetic fields on the combustion wave of self-propagating high-temperature synthesis (SHS) changes the process kinetics and the structure and properties of the reaction products. Emission phenomena in the combustion wave have received less attention. High rates of chemical energy dissipation in SHS (10¹² W/m³) are accompanied by physical phenomena such as the occurrence of a potential difference between the combustion front and reaction products, free electron emission from the combustion wave, and acoustic emission. Detailed studies of these nonequilibrium phenomena provide a deeper understanding of the reaction mechanism in solid flames to use nonthermal methods of combustion control. __________ Translated from Fizika Goreniya i Vzryva, Vol. 45, No. 4, pp. 121–127, July–August, 2009.     

Mechanoactivation of SHS systems and processes

Data on the mechanoactivation (MA) of SHS processes and systems obtained over the past 15 years are overviewed. Analysis of these data is used to formulate some recommendations for appropriate application of MA to SHS.      

Numerical investigation of the thermal processes associated with quenching of a material in a wave of SHS

The rate of cooling is estimated for different regimes of heat removal from the surface of an SHS sample. The extinction of the combustion front is studied numerically for the example of the system titanium carbon. The temperature dependences of the rate of cooling in different zones of the combustion wave are calculated.Minsk. Translated from Fizika Goreniya i Vzryva, Vol. 27, No. 6, pp. 64–72, November–December, 1991.     

Thermal stability of FeAl intermetallics prepared by SHS sintering

The microstructural evolution of dense nanostructured FeAl produced by mechanical and field activation was investigated over a range of annealing temperature from room temperature to 750°C. In-situ and exsitu X-ray diffraction experiments were performed to determine the evolution of the crystallite size and microdistorsion rate during annealing in an inert atmosphere. The results show the existence of two temperature domains: (i) below 500°C, the crystallite size remained relatively unaffected while the microdistorsions were eliminated and (ii) above 500°C, the crystallites exhibited significant growth leading to the destruction of the nanostructure. In addition, the microhardness changes versus the crystallite size was consistent with prediction of the Hall-Petch effect.      

New-generation refractory concretes. Thermal processes,structure and high-temperature properties

Regularities of the processes of drying and heat treatment (firing) of new refractory concretes (ceramic concretes, low-cement refractory concretes, etc.) are considered. The conditions for the appearance of the effect of their explosive cracking (destruction) in drying are analyzed. Changes in the porous structure and some characteristics of the concretes (porosity, strength, shrinkage, expansion) are considered as functions of the temperature of their preliminary treatment. Regularities in the formation of the zonal structure and the stress state in use of refractory concretes are described. Data on the thermal conductivity and thermal expansion of refractory concretes of the Al₂O₃-SiO₂ system are presented for a wide temperature range.Translated from Ogneupory, No. 6, pp. 5–12, June, 1995.     

Passage of SHS wave through a gap: Mathematical modeling

Stability of combustion wave during its passage through a gaseous gap between two reactive layers was studied by mathematical modeling. The character of transient processes in the gap was explored as a function of thermophysical parameters of the layers, kinetic parameters, and gap width at a predominant role of radiative heat transfer. The passage of combustion wave through the gap was found to be accompanied by the attainment of superadiabatic temperatures within the transition zone. A critical temperature for burning failure at the interface was determined.     

Study of structure formation of products in SHS nickel-aluminum-alloying component systems

A previously proposed model for the interaction in theNi-Al system is developed as applied to ternary reaction systems, which are promising for preparing materials for gasothermal application of protective coatings. In this investigation a correlation is made between the features of the thermal profile of a combustion wave with structural analysis data for the interaction products and the quenching zone. The developed qualitative models of the interaction are shown to fit adequatly the structure and phase composition of products obtained over a wide range of concentrations of dopants(Fe, Ti), which form constitution diagrams of various types with the components of the base system, and also the initial conditions of synthesis.Translated from Fizika Goreniya i Vzryva, Vol. 32, No. 2, pp. 55–63, March–April, 1996.     

Phase and structure formation mechanisms of SHS synthesized composite coatings

High performance composite coatings were synthesized by self-propagation high-temperature synthesis followed by gravitational-separation process based on thermite reaction. The phase, structure and composition of generated composite coatings were investigated, and formation mechanism was studied by thermodynamic analysis. Results showed that phase composition of Al-Fe₂O₃ reaction system consisted of Al₂O₃, Fe and FeAl₂O₄. In Al-Fe₂O₃/Al-Cr₂O₃ composite reaction system, Fe-Cr alloy was formed and FeAl₂O₄ phase disappeared, which could improve the corrosion resistance of composite coatings. Furthermore, the addition of SiO₂ in SHS reaction favored the formation of low-melting point phase Al₂O₃·SiO₂, which filled into voids of Al₂O₃ dendrites and reduced the porosity of composite coatings, thus improving their strength and densification level. Moreover, the generated transition structure in different reaction systems could buffer the residual stress to promote the binding between the composite coating and steel pipe.     

钛酸盐陶瓷固化Sr2+核素的SHS热力学

探索采用燃烧合成(SHS)技术制备包容锶核素(Sr2 )的钛酸盐陶瓷固化体,根据自蔓延高温合成热力学分析,计算钛酸盐固化体的绝热燃烧温度Tad大于临界绝热燃烧温度,说明反应能自发进行并合成致密的CaTiO3固化体。     

Modelling of shock wave processes in unidirectional composites

On the basis of an earlier-formulated model, we analyze the shock wave processes in unidirectional composites. The effects of splitting of shock waves, which are due to the difference in velocities of compression pulse propagation over the materials of composite components, are shown. The effects are associated with the earlier-observed peculiarities of diagrams for dynamic compression of composite materials. Novosibirsk. Translated from Fizika Goreniya i Vzryva, Vol. 29, No. 5, pp. 72–76, September–October, 1993.     

SHS amidst other new processes for in-situ synthesis of Al-matrix composites: A review

The in-situ synthesized Al-matrix composites exhibited better mechanical properties than the composites prepared by ex-situ synthesis. Several in-situ processes for production of carbide-reinforced Al-matrix composites were comparatively discussed. The benefits of SHS process as compared to other insitu techniques are highlighted. The use of halide salts replacing elemental Ti in SHS reactions turned helpful for in-situ generation of nano particles in the Al matrix. The size, shape, and distribution of in-situ prepared reinforcing particles in Al-matrix are uniform and homogeneous. The reinforced surface is compatible with the undoped matrix surface, thus providing strong bonding. The formation of particle clusters and agglomerates can be minimized or eliminated by proper choice of in-situ processing conditions. Also addressed are the problems concerning the formation of Al₃Ti and Al₄C₃ intermetallics as byproducts.     

Thermal analysis of solids production processes

The present work investigates the energy consumption of a solids production process. Such a process consists of three different operation units: crystallization/precipitation, solid-liquid separation by centrifuges or filters and drying of wet crystals. Each unit is analyzed with the objective of minimizing the energy consumption. Operating conditions, process design and other characteristic parameters (crystal size, porosity and physical proporties) are discussed with the aim to evaluate their influence on the process.     

Combustion wave structure in heterogeneous solid propellants

On the basis of describing physicochemical processes in a mixed solid propellant combustion wave, mathematical techniques are suggested for heterogeneous media which make it possible to consider the heterogeneous and gas-phase combustion regimes for individual components in generalized chemical reaction kinetics and to consider their effect on combustion wave velocity. The agglomeration process for Al powder on a hot surface is studied experimentally by high-speed photorecording. A mathematical model is constructed for Al agglomeration in a combustion wave based on agglomeration mechanisms observed in experiments. The dependence of combustion rate on deformation is determined on the basis of an improved method for optical recording of combustion front movement for loaded specimens. The mechanism of the effect of stress on mixed solid propellant combustion rate is connected with activation of chemical bonds of the polymer matrix, which increases its destruction rate. Use of kinetic theory for the durability of polymers made it possible to obtain an analytical equation expressing the dependence of relative combustion rate on measured specimen deformation.Novosibirsk. Translated from Fizika Goreniya i Vzryva, Vol. 29, No. 3, pp. 8–16, May–June, 1993.