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138SHS CERAMICS: HISTORY AND RECENT ADVANCES
A. S. Mukasyan
University of Notre Dame, Notre Dame, Indiana, 46556 USA National University of Science and Technology MISiS, Moscow, 119049 Russia
e-mail: amoukasi@nd.edu
DOI: 10.24411/9999-0014A-2019-10104
The fundamental paradigm for self-propagating high-temperature synthesis (SHS) of ceramics consists of exothermic self-sustained reactions that, without any external heat sources, lead to formation of non-metallic inorganic solid materials comprising metal and non-metal atoms primarily held by ionic and covalent bonds. The term 'ceramics' comes from Greek word 'Keramos', which is related to an old Sanskrit root meaning "to burn". Thus, SHS of ceramics is probably the closest technology to the root meaning of the term, since it uses combustion, i.e. 'burning', for both synthesis and consolidation of the materials.
History of the SHS is overviewed in many publications (c.f. [1-3]). No doubt that, while having predecessors, including works by N.N. Beketov and H. Goldschmidt, only after thorough fundamental research initiated by A.G. Merzhanov and co-workers, this approach became the widely recognized technology for synthesis of almost any type of compounds, and ceramics were among the first materials fabricated by SHS method. Statistical analysis of publications shows that the CS method for fabrication of materials attracts attention of more and more researchers and engineers, currently scientists in 117 countries are involved in the CS field and number of related publications increases exponentially (Fig. 1).
2000
in c o
= 1500
03
o
3
CL
o 1000
500-
1995 1998 2001 2004 2007 2010 2013 2016 2019 Year
Fig. 1. Dynamic of the citations in the field of combustion synthesis of materials.
Conventional reaction sintering (RS) method for ceramic fabrication also involves synthesis and consolidation. However, in the latter case external heat source (e.g. furnace) is used to assist these processes. Moreover, during RS one has to avoid conditions (e.g. local overheating), which may lead to the "uncontrollable" events associated with self-sustained interactions. Thus, SHS can be considered as a limiting route of RS, when preheating conditions lead to local self-initiation of the reaction followed by its propagation along the media.
Advantages and disadvantages of RS and SHS are as follows. On the one hand, during RS, one may essentially independently vary processing parameters, such as temperature, pressure
0
and time, which, by trial-and-error strategy, allows optimization of the conditions for fabrication of materials with desired microstructure and thus properties. However, the SR is long (hours) and energy consuming method, which requires an expensive and complicated equipment. On the other hand, rapid (seconds), energy saving SHS method needs simple equipment. Nevertheless, all parameters of SHS process are strongly correlated and it is impossible to change one of them keeping the others unaffected. Thus, the issue of process controllability becomes critical. After years of research, we now may confidently state, that based on the fundamental knowledge of the mechanism for self-sustained heterogeneous reactions, one may control the SHS process. Indeed, many examples demonstrate the durability of SHS for controlling of the microstructure and properties of the materials (c.f. [4-7]). This is an important nowadays conclusion.
All systems, which lead to the SHS of ceramics, can be subdivided into two main classes:
• reactive solid powder mixtures in inert or reactive gaseous atmosphere - heterogeneous SHS;
• reactive aqueous solutions in inert or reactive gaseous atmosphere - solution combustion synthesis (SCS).
Heterogeneous SHS can be sub-divided into several groups. First group involves solid-solid systems in which compounds are produced from the elements. Examples of ceramics which are produced by SHS include, carbides (metal: TiC, ZrC, HfC, NbC, TaC and nonmetal: SiC, B4C) and borides (TiB, TiB2, ZrB, ZrB2, NbB, NbB2, TaB2 etc.). Contribution of V. Shkiro, who is one of the co-founder of the solid flame discovery, in investigation of the combustion mechanism in Ti-C system is difficult to overestimate. These works led to the development of technology for fabrication of the SHS/TiC-based polishing pastes, which found wide commercial applications. Titanium boride was one of the first compound synthesized by combustion method. Powder of TiB2 also was produced on industrial level by SHS method.
Second group consists of solid-gas systems and permits SHS of oxides, nitrides and hydride-based ceramics. It is worth noting that, because of their extremely high exothermicity, the SHS of oxides by gas-solid reactions is probably not effective approach. Solution combustion synthesis is typically used to fabricate different nano-sized oxide powders. However, we want to outline works by M. Nersesyan on developing of continuous SHS-based technology for production of ferrites, which found industrial application. Special case is SHS of nitride-based ceramics, including metallic nitride (TiN, ZrN, HfN, TaN, and AlN etc.), nonmetallic nitrides (Si3N4, h-BN, c-BN etc.) and oxinitides (AlONs, SiAlONs etc.) [8]. An important concept of infiltration combustion has been established based on pioneering experimental works on metal combustion in nitrogen by I. Borovinskaya, Yu. Volodin, A. Pityulin, V. Loryan and theoretical studies by A. Aldushin. SHS-based technology for nitriding of ferrovanadium alloy developed by M. Ziatdinov found wide industrial application. Contributions of V. Martinenko and V. Prokudina in establishing of SHS technologies for production of silicon and aluminum nitrides should be outlined. Industrial production of silicon nitride had history on the SHS production plant in Spain.
Third group of SHS systems involves so-called thermite type of reaction systems that include stage during which one element (typically Mg, Al or C) reduce metal (e.g. TiO2) or nonmetal (B2O3) oxides to produce elements (e.g. Ti and B), which may be followed by reaction between the elements to produced ceramics (e.g. TiB2 or BN). In our opinion, this approach is preferable for fabrication of fine non-oxide ceramic powders by using combustion-based technology. The necessity of additional chemical leaching of MgO from combustion product does not make this method too complex. However, as it was shown in works by S. Mamyan, this method requires detailed optimization of the composition for the initial reactive powder mixture. The extremely high exothermicity of some thermite systems allows one to produce large corundum crystals when using Al as a reducing element.
Forth group of SHS systems includes so-called displacement reactions, in which one element displaces another from a compound. It is wort noting that thermite reactions represent a specific type of displacement reactions, but historically they made a separate class of combustion system. Metathesis self-sustained reactions that sometimes called double replacement or double decomposition reactions, which involves the exchange of bonds between two chemical compounds, also belong to this group of SHS systems. On the best of our knowledge, metathesis systems were brought to SHS field by I. Parkin from University College London and, as it is will be shown in this presenation, currently such recations are widely used to fabricate different type of bulk SHS ceramics, including borodes, nitrides and carbides.
Solution combustion synthesis (SCS) involves propagation of self-sustained exothermic reactions along an aqueous or sol-gel homogeneous media. SCS allows for the synthesis of nanoscale materials and coatings (cf. [7]). It provides easy formation of high-quality multielement compounds with complex crystal structures, such as perovskites, garnets, spinels, silicates, and phosphates. SCS also permits for efficient doping of materials, even with a trace amount of elements. Additional details on the historical achievements in SHS filed can be found in the Concise Encyclopedia of Self-Propagating High-Temperature Synthesis [9].
Recent works in the SHS field demonstrate new capabilities of SHS approach. We will focus on the following novel directions:
- Fabrication of 1D, 2D, and 3D nanopowder (Fig. 2, see also review [6]);
- Direct production of bulk ceramics by combination of SHS and spark plasma sintering (Fig. 3, see review also [5, 10]);
- SHS of non-equilibrium phases;
- High entropy ceramics by SHS;
- Nano-sized materials synthesized by combination of mechanical activation and SHS processes [6-8];
- Ultra-high temperature ceramics [10];
- Ceramics by SHS and shock wave [11];
- SCS ceramics, including, inorganic phosphorous, functional ceramics, electro- and magnetic- ceramics, coatings and bio ceramics [8].
Fig. 2. Typical microstructure of SHS nanopowder: (a) 3D-SiC; (b) 1D-Si3N4; (c) 2D-BN.
ZrB2 P-Si6-zAlzOzNg-z B4C-TiB2
Fig. 3. Typical microstructure of SHS bulk ceramics.
It is concluded that combustion-based methods allow fabrication of a wide variety of ceramics: powders, bulk materials, coatings and net shape articles. Nanomaterials of any dimensions can be also produced through self-sustained reactions. Several advantages of the approach can be outlined. The unique conditions of the combustion approaches (e.g. high temperatures, rapid self-heating and cooling) facilitate formation of non-equilibrium phases. Other specific feature of SHS is its ability to produce multi element compounds with complex crystal structures. In addition, the SHS permits scale of materials production. Indeed, increasing the amount of reactive mixture leads to more adiabatic conditions and thus more steady-state combustion regime. Finally, the SHS allows continuous schemes for processing of different ceramics.
However, several limitations of SHS should to be resolved, before this method can be widely implemented in large-scale production of advanced ceramics. The main issues are related to the uniformity of the morphology and microstructure for the produced materials and controllability of SHS process. In this regards it is important that many recent works showed that by investigating the mechanisms of the combustion reactions and structural transformations, which occur during SHS, one might establish effective ways to control the structure of the ceramic materials. Indeed, during the last decade, CS direction made several significant steps ahead in finding new routes of material synthesis. From completely not controlled thermal explosion by heating in furnace, to precisely controlled steady state self-propagating mode. From agglomerates with non-uniform microstructures, to super fine (less than 10 nm) nanoparticles. From powders to thin films and 1D and 2D crystals. From porous bulk materials to pore-free nanostructured net-shape articles.
This work was supported by the Department of Energy, National Nuclear Security Administration; under the award number DE-NA0002377 as part of the Predictive Science Academic Alliance Program II. We acknowledge the financial support of the Ministry of Science and Higher Education of the Russian Federation in the framework of Increase Competitiveness Program of NUST «MISiS» (no. K2-2017-083), implemented by a governmental decree dated 16 the of March 2013, N 211.
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