Minireviews



MR-1: Experimental Diagnostics of SHS Processes

A.S. Rogachev

Institute of Structural Macrokinetics and Materials Science

142432, Chernogolovka, Russia

E-mail: rogachev@ism.ac.ru
 
 

 Diagnostics of SHS is defined as a whole complex of experimental methods explored in order to measure fundamental characteristics of the process and the products of synthesis. In the present overview, we consider both historical retrospective and newest results of elaborating of unique experimental techniques and exploring of modern analytical methods in the field of SHS. The historical view shows us, that originating of new fundamental ideas in this direction usually implies to the creation of unconventional methods of diagnostics and, from the other hand, developing of the experimental tools often leads to a breakthrough in the fundamental understanding of the process. Present-day concept of SHS-diagnostics aimed to solution of such fundamental problems as:

- expanding of the combustion theory in an attempt to include all phase and microstructure transformations as well as micro-heterogeneous nature of SHS-systems;

- study of the kinetics and mechanisms of high-exothermic fast chemical reactions in heterogeneous media;

- understanding of mechanisms of the product formation (including microstructure and crystal structure) in the high-temperature reaction waves;

- outgrowth of the theory of thermal explosion to make it applicable for the volume reaction mode of SHS-process.

Following these principle directions, evolution of the experimental methods is treated, and some ideas for the future efforts are proposed. The most attention is paid to the methods of in situ diagnostics, such as high-speed micro-video recording, micro-thermometry and pyrometry, time-resolved X-ray diffraction and others. In the conclusion, some examples of applying of the diagnostic results to solve practical problems are discussed.

This work is supported by the Russian Foundation of Basic Research, grant 98-03-32137.
 
 

MR-2: Mathematical Modeling and Computer-Assisted Simulation

of SHS Processes

V.Hlavacek1, H. Viljoen2

1 Dept. of Chem.Engng. SUNY/Buffalo, Buffalo, N.Y. 14031, USA

2 Dept. of Engng. Univ. of Nebraska, Linkoln, NE 68588, USA
 
 

 Many exothermic chemical reactions can propagate through a mixture of initial components as self-sustained reaction waves. For gaseous mixtures this phenomenon has been studied for a long time, thermite reactions of the type solid-solid were discovered at the end of last centure, however, the real boost to the theory and practice of self-propagating reactions has been given by the Russian school of prof. Merzhanov in 60s. This paper attempts to review the most impotant discoveries in the field of modeling and computer simulation of the exothermic “solid-solid” and “solid-gas” non-catalytic chemical reactions, frequently referred to as SHS reactions.

The SHS reactions can propogate as a constant pattern Wave, however, under certain conditions fingering, rotating and chaotic waves have been observed. Typical SHS wave moves with a speed of few mm or sm/sec, however, waves with speeds much higher have been observed. Accordingly, a “solid-solid” or “solid-gas” reaction can occur in one of the following reactions regimes: 1. Kinetic, 2. Slow deflagration, 3. Fast deflagration, 4. Detonation. A typical SHS combustion process, used for synthesis of ceramic materials, can be described as a slow deflagration wave. Fast deflagration waves can move with a speed 10-1,000m/sec, SHS detonation wave can propogate with a speed 1-10 km/sec.

The reaction mixture is usually considered as quasi-continuum and transport equations are formulated in terms of continuum description asa set of parabolic nonlinear differential equations. However, recent experiments of Varma, Rogachev and Mukasyan indicate that a more complicated space pattern occurs. Evidently, a discrete system shoud be descpibed by a discrete model. A discussion of such model Will be presented. A major problem in modelling SHS reactions is representation of the radiation term. Recent theoretical analysis carried out by the authors of this review revealed that for SHS reactions occurring in a detonation regime a hyperbolic desciption of the heat transfer must be used to get qualitative agreement with the experimental observations. Experimental observations indicate that reaction front propagation may be assisted by the formation of cracks in the preheated region. Historically, stress-related effects were neglected; a better model requires a mechano-chemical description of the process. Consideration of stress coupling in internal energy and kinetics, thermal and concentration induced stresses and equation of state for the solid phases is essential to describe phenomena like solid phase detonation, mechanical activation, cryogenic explosions and temperature independend reaction rates. Analytical methods of solution and bifurcation methods of analysis will be discussed. In particular results obtained by the Merzhanov School, Matkowski and Viljoen will be reported. Classical pioneering papers by Merzhanov and Gontovskaja describing theory of thermal ignition and breaking of symmetry of a propagating thermal front will be discussed in detail. Numerical solution of two and three dimensional case indicates that the mathematical model of quasi-continuumwith a strongly exothermic non catalytic reaction is capable of predicting breaking of symmetry and that rotating, fingering or chaotic wave can be calculated.
 
 

MR-3: Chemistry and Technology in SHS R&D

I.P. Borovinskaya

Institute of Structural Macrokinetics and Materials Science,

Russia Academy of Science, Chernogolovka, 142 432 Russia

E-mail: inna@ism.ac.ru

 Main directions of evolution in SHS chemistry and technology are associated with elaboration of methods for producing refractory inorganic compounds, materials and items for various applications, with studying possibility of synthesizing some organic compounds. Those methods are based on well-known exothermic reactions of interaction between metals and nonmetals, metals and metals, nonmetals and nonmetals, as well as reactions at which various compounds (salts, azides, hydrides, oxides, hydrocarbons, etc.) participate as starting components. Reactions of elements are the most widespread in SHS. However, recently chemical reactions involving oxides as reagents (with a metal-reducer) have composed a large separated group of processes that are responsible for formation of SHS products with specific features (fine-dispersed powders, cast materials and items). Reactions involving several elements, inorganic salts and organic compounds are intensively studied. Reactions of metals that form intermetallic compounds have generated great practical interest.

Chemical mechanism of reactions proceeding during synthesis is also studied in SHS research. Great interest is shown in experiments carried out with using novel investigation methods (quenching of intermediate combustion products, X-ray diffraction analysis using synchrotron emission, dynamic X-raying and others). However, the number of such works is not large.

Peculiarities of the course of interaction in each of above-mentioned reaction types (solid-phase reactions, with fusion, with evaporation, etc.) have significantly reflected on the variety of SHS technologies: powdered, producing porous, compact or cast materials, gas transport coatings, etc.

In the report some types of exothermic reactions that are most important for producing products by SHS are exemplified. Ideas of different authors of mechanism of reactions proceeding at combustion are discussed, problems of stage formation of phases at which intermediate products and solid solutions participate are considered.

Great attention is paid to the works searching for optimal SHS conditions of producing high quality products (powders, materials and items). SHS methods used on a pilot or an industrial scale in different countries for producing various inorganic materials and items that are important for practical applications are exemplified.
 
 

MR-4: State-of-the Art in R&D of SHS Materials in the World

Y.Miyamoto

Joining and Welding Research Institute, Osaka University

Ibaraki, Osaka 567-0047, Japan
 
 

 Since the SHS was first developed in Russia in 1967, various processes of SHS have been developed in the world that enables synthesis of ceramic, intermetallics and composites in powder, solid and laminate forms. TiNi shape memory and superelastic alloys, TiAl intermetallic powders, ceramic powders, SiC/C composite components, and alumina lining steel pipes formed by centrifugal SHS have been commercialized in some countries. The simultaneous synthesis and consolidation of materials is a uniqueness of SHS. Usually it is necessary to apply pressure for consolidation during or just after the SHS. A variety of techniques using uniaxial, isostatic, quasi-isostatic, shock, and centrifugal force pressings were used and numerous ceramic/metal composites fabricated in homogeneous and heterogeneous forms. The SHS compaction is one of promising methods to fabricate functionally graded materials because the rapid process can sustain the compositional gradients designed without long range migration of metal constituents at high temperatures. New reactive processes with mild SHS reactions are increasing in recent years that are more controllable in product phases and structures. A history of R&D of SHS materials is briefly reviewed and some recent topics on the new direction of SHS materials are introduced and discussed in respect to the future of SHS materials.