Alexander S. Mukasyan

Combustion Synthesis of Advanced Materials: Principles and Applications

Combustion synthesis is an attractive technique to synthesize a wide variety of advanced materials including powders and near-net shape products of ceramics, intermetallics, composites, and functionally graded materials. This method was discovered in the former Soviet Union by Merzhanov et al. (1971). The development of this technique by Merzhanov and coworkers led to the appearance of a new scientijc direction that incorporates both aspects of combustion and materials science. At about the same time, some work concerning the combustion aspects of this method was also done in the United States (Booth, 1953; Walton and Poulos, 1959; Hardt and Phung, 1973). However, the full potential of combustion synthesis in the production of advanced materials was not utilized. The scientijc and technological activity in thejeld picked up in the United States during the 1980s. The signijcant results of combustion synthesis have been described in a number of review articles (e.g., Munir and Anselmi-Tamburini, 1989; Merzhanov, I990a; Holt and Dunmead, 1991; Rice, 1991; Varma and Lebrat, 1992; Merzhanov, 1993b; Moore and Feng, 1995). At the present time, scientists and engineers in many other countries are also involved in research and further development of combustion synthesis, and interesting theoretical, experimental, and technological results have been reported from various parts of the world (see SHS Bibliography, 1996). This review article summarizes the state of the art in combustion synthesis, from both the scientijc and technological points of view. In this context, we discuss wide-ranging topics including theory, phenomenology, and mechanisms of product structure formation, as well as types and properties of product synthesized, and methods for large-scale materials production by combustion synthesis technique.

Perovskite membranes by aqueous combustion synthesis: synthesis and properties

The objective of this work is to identify optimum synthesis, compacting and sintering conditions in order to achieve a pure phase fully densified La0.8Sr0.2CrO3 (LSC) perovskite membrane. The aqueous combustion synthesis of LSC powders was investigated over a wide range of synthesis conditions by using the metal nitrates (oxidizer)‐glycine (fuel) system. The powders were pressed and sintered to create dense materials, which were characterized. It was shown that depending on fuel/oxidizer ratio, , the reaction can proceed in three different modes: Smoldering Combustion Synthesis (SCS), 0.7, with maximum temperature, Tm 600°C; Volume Combustion Synthesis (VCS), 0.71.2, 1150°CTm1350°C; Self-propagating High-temperature Synthesis (SHS), 1.2 1.6, 800°CTm1100°C. In turn, the characteristics of synthesized powders depend on the combustion mode. The crystalline structure of as-synthesized powders becomes more defined as increases (amorphous for SCS; crystalline for VCS and SHS). The specific surface area decreases slightly when mode changes from SCS ( 25 m 2 g − 1 )t o VCS (20 m 2 g −1 ), however, it increases substantially under SHS conditions (up to 45 m 2 g − 1 ). It was also shown that calcination is beneficial only for SCS powders, while VCS and SHS powders may be sintered directly as synthesized, thus bypassing the time and energy consuming calcination step. The measured oxygen permeation values for the membranes are comparable with the best candidate materials reported in the literature.

Novel Approaches to Solution-Combustion Synthesis of Nanomaterials

Solution-combustion is an attractive approach to synthesis of nanomaterials for a variety of applications, including catalysts, fuel cells, and biotechnology. In this paper, several novel methods based on the combustion of a reactive solution are presented. These methods include self-propagating sol-gel combustion and combustion of impregnated inert and active supports. It was demonstrated that, based on the fundamental understanding of the considered combustion processes, a variety of extremely high surface area materials could be synthesized. The controlling process parameters are defined and discussed. Examples of materials synthesized by the above methods are presented. For the first time, a continuous technology for production of nanopowders by using the solution combustion approach is demonstrated.

Thermal explosion in Ni-Al system: influence of reaction medium microstructure

The thermal explosion (TE) phenomenon was investigated in the Al-Ni system (Al-rich compositions) with various reactant medium microstructures. The parameters of TE (e.g., ignition temperature, reaction times, etc.) obtained for Al particles clad by Ni were compared with those for Al+Ni powder mixtures having different particle sizes. The heating rate was varied in the range 100 to 900 K/min, and its influence on TE characteristics was also studied. The results show that ignition process is related to either phase (melting of reactant or first product formed) or microstructural (breaking of the Ni shell in case of clad particles) transformations and depends on the reactant medium microstructure. The latter influences the process of initial product formation, which in turn affects the interaction rate during the post-ignition stages. The obtained results were discussed and compared with those reported in the literature. In general, it is suggested that TE in heterogeneous gasless systems differs from explosions in conventional homogeneous media. In the latter, the operating conditions define ignition temperature, while in the former, it is related to the system phase diagram and thus is a characteristic property of the system.

Thermal explosion in Al-Ni system: influence of mechanical activation.

The influence of short-term (5-15 min) highly energetic ball milling on the ignition characteristics of a gasless heterogeneous Ni-Al reactive system has been investigated. By using Al-Ni clad particles (30-40 microm diameter Al spheres coated by a 3-3.5 microm layer of Ni, that corresponds to a 1:1 Ni/Al atomic ratio), it was shown that such mechanical treatment leads to a significant decrease in the self-ignition temperature of the system. For example, after 15 min of ball milling, the ignition temperature appears to be approximately 600 K, well below the eutectic (913 K) in the considered binary system, which is the ignition temperature for the initial clad particles. Thus, it was demonstrated that the thermal explosion process for mechanically treated reactive media can be solely defined by solid-state reactions. Additionally, thermal analysis measurements revealed that mechanical activation results in a substantial decrease in the effective activation energy (from 84 to 28 kcal/mol) of interaction between Al and Ni. This effect, that is, mechanical activation of chemical reaction, is connected to a substantial increase of contact area between reactive particles and fresh interphase boundaries formed in an inert atmosphere during ball milling. It is also important that by varying the time of mechanical activation one can precisely control the ignition temperature in high-density energetic heterogeneous systems.

Kinetics of high temperature reaction in Ni-Al system: influence of mechanical activation.

High temperature (>1000 K) reaction kinetics in the stoichiometric (1:1 by molar ratio) Al-Ni system was investigated by using the, so-called, electrothermal analysis (ETA) method. ETA is the only technique that allows studying kinetics of a heterogeneous gasless reaction at temperatures above the melting points of the precursors. Special attention was focused on methodological aspects of the ETA method. Two different reaction systems were studied: (i) initial Al/Ni clad particles; (ii) the same powders but after 15 min of high energy ball milling. Analysis of the obtained results leads to the conclusion that such mechanical treatment decreases the apparent activation energies of the reaction in the Ni-Al system, from 47 +/- 7 kcal/mol for the initial powder to 25 +/- 3 kcal/mol after ball milling. Comparison of these data with those reported previously was also made.

Dynamics of phase forming processes in the combustion of metalgas systems

Abstract The dynamics of phase formation in the combustion wave of the titaniumnitrogen, titaniumoxygen, and titanium-air systems have been investigated. The experiments were on an installation composed of a position detector and a standard x-ray apparatus. The combustion wave in the TiN 2 system has been found to propagate as a result of the formation of titanium nitride, which is the final product. The phase formation process in the TiO 2 system is caused by oxidation of titanium to titanium dioxide without formation of any intermediate oxide phases. In the Ti-air system the combustion front propagation is induced by the formation of the intermediate phase, titanium nitride, which converts to the final product, titanium dioxide, via two oxynitride intermediates.

Combustion Joining of Refractory Materials

This paper overviews heterogeneous exothermic reactive systems as they apply to the joining of materials. Techniques that are investigated fall under two general schemes: so-called Volume Combustion Synthesis (VCS) and Self-Propagating High-Temperature Synthesis (SHS). Within the VCS scheme, applications that are considered include Reactive Joining (RJ), Reactive Resistance Welding (RRW), and Spark Plasma Sintering (SPS). Under the SHS scheme, Combustion Foil Joining (CFJ) and Conventional SHS (CCJ) are discussed. Analysis of the relevant works show significant potential, particularly for the RJ, RRW, and CFJ approaches, in the joining of a variety of materials which are difficult, or impossible, to bond using conventional techniques. More specifically, it is shown that these methods can be successfully applied to the joining of: (i) dissimilar materials such as ceramics and metals and (ii) refractory materials, such as graphite, carbon-carbon composites, W, Ta, Nb, etc.

Combustion synthesis of advanced materials: Fundamentals and applications

The combustion synthesis (CS) of materials is an advanced approach in powder metallurgy. The number of products synthesized by CS has increased rapidly during recent years and currently exceeds 1,000 different compounds. The same features, such as high temperatures and rates, self-sustained manner of microstructure formation in non-equilibrium conditions, that make CS an attractive technology also define difficulties to study the nature and mechanisms of this process, which in turn are essential to control the properties of the synthesized materials. In this survey paper, we present results of our recent work both in fundamental studies of mechanisms for rapid reaction wave propagation in heterogeneous media and in using the CS approach to synthesize different types of advanced materials, including bio-alloys and nano-sized powders.

Dynamics of self-propagating reactions in heterogeneous media: experiments and model

Abstract The extreme reaction conditions during combustion synthesis (CS) process require new methods for controlling the microstructure (and hence, the properties) of the desired materials. It appears that the structural changes which determine the microstructure and phases are influenced by local variations in reactant composition and temperature. A direct observation of the microstructural transformations during CS was made using a novel digital high-speed microscopic video recording technique, with imaging rates up to 12,000 frames/s and magnification up to 800×. It was shown that over a wide range of experimental conditions, macroscopically steady reaction fronts in heterogeneous mixtures exhibit random microscopic fluctuations in shape and instantaneous velocity, which are directly related to the microstructure of the reaction mixture. These fluctuations were correlated with the initial heterogeneity of the reaction medium. In conjunction with these experiments, a micro-heterogeneous cell model was developed to describe the local reaction wave propagation in a heterogeneous porous medium. The calculated combustion front profiles compared well with the experimental results.