Combustion Synthesis: Novel Routes to Novel Materials

In this chapter, an introduction to combustion synthesis is provided in the first part. Starting with the definition of a combustion process, the characteristics of combustion synthesis, i.e. a redox reaction for the production of inorganic, solid materials, are explained. Combustion synthesis was first used in prehistoric times to produce carbon black for cave paintings, and has further been developed to an advantageous technique for a wide variety of materials. Combustion synthesis processes are classified by the state of matter of the educts, being gaseous, liquid or solid.

Combustion synthesis in the gas phase, also known as „ flame synthesis“, can be used to produce non-agglomerated nanoparticles and carbon nanotubes (CNT).

In the combustion synthesis in the solid phase, also known as „ gasless combustion“ or “solid flame”, one can distinguish between self-propagating combustion synthesis (self-propagating high-temperature synthesis, SHS) and volume combustion synthesis (VCS). In the prominent SHS mode, combustion travels through the reaction mixture. VCS is also termed “simultaneous combustion” mode or “thermal explosion”.

Combustion synthesis in the liquid phase can be carried out as solution combustion synthesis (SCS) from metal salts in aqueous solution in combination with organic fuels. Molecular mixing ensures excellent product homogeneity, also for multi-component mixtures.

In general, synthesis can start from the elements, or from compounds. The most common type of combustion synthesis is the thermite-type reaction (Goldschmidt process), vis. the reduction of an ore using a metal such as Mg or Al.

More than 700 compounds have been produced using combustion synthesis. Combustion synthesis is carried out in simple reactors rather than furnaces. This chapter also lists typical products and their applications. General advantages of combustion synthesis over traditional techniques are presented.

In the second part of this chapter, all chapters of this E-book are briefly outlined.

The self-propagating high-temperature synthesis (SHS) process involving thermite reactions was introduced as a promising alternative to prepare various Al2O3-reinforced materials, including borides, carbides, and aluminides. The influence of different thermite mixtures on the SHS process is explored in terms of the combustion sustainability, propagation rate of the reaction front, combustion temperature, and phase composition of the synthesized products. On formation of the TiB2-Al2O3 composite, the thermite mixture of Al-TiO2-B2O3 was found to improve the phase conversion when compared with that of Al and TiO2. The addition of the Al-TiO2 thermite mixture to the Ti-Si-C reaction system was demonstrated to produce in situ Ti3SiC2-Al2O3 composites. The thermite reaction of Al and TiO2 can also be applied to prepare the TiAl-Al2O3 composite through combustion synthesis. Two niobium aluminides Nb3Al and Nb2Al were obtained in almost pure form from a thermite-based SHS process with the powder compacts of Al:Nb2O5 = 12:3 and 13:3, respectively. As the thermite reagent of Al-Nb2O5 or Al-Nb2O5-B2O3 was added to the Nb-B combustion system, products were the composites of niobium borides and Al2O3.

The emulsion combustion method (ECM) is a novel powder production process developed to synthesize nanostructured metal-oxide powders. In this process, metal ions present in the aqueous droplets are rapidly oxidized by the combustion of the surrounding flammable liquid. The small reaction field and short reaction period lead to the formation of nano-size ceramic particles. A variety of extremely high surface area materials could be synthesized by the ECM process. For example, hollow spheres of Al2O3 TiO2, ZrO2, and Y2O3 can be prepared by this process using aqueous solutions of aluminum nitrate, TiCl4, zirconium oxynitrate, and yttrium nitrate, respectively. In contrast, solid particles of ZrO2-CeO2, ZnO, Fe2O3, CeO2, MgO and BaTiO3 from aqueous solution of their corresponding nitrates and ZnO/SiO2 from zinc acetate, and silica sol or hexamethyldisiloxane can also be prepared. The powder formation mechanism is discussed in detail. The main aim of this chapter is to introduce emulsion combustion synthesis to the reader.

The nitriding combustion synthesis of nitrogen ceramics has long been an important research area and has found many industrial applications in regarding to the synthesis of a wide range of nitride powders in the past two decades. The energy saving and especially time-saving feature makes the infiltration combustion synthesis technique very attractive in the cost effective fabrication of nitride ceramics. That is why there are continuous efforts devoted to the understanding of the reaction mechanism as well as to the optimizing of the processing parameters on the synthesis of nitride ceramics. In this chapter, the progress on the combustion synthesis of nitride ceramic powders was summarized. It was found that the remarkable achievements obtained in this area could be classified as two aspects. The first aspect is the development of up-scalable cost-effective fabrication process, which follows a direction of decreasing the synthesis pressure, that is, the minimum nitrogen pressure required for implementing the combustion synthesis of nitride ceramics has been decreased from high pressure to normal pressure, and up to now, the nitride ceramic powders could even be synthesized in air. The other aspect is the development of high performance nano-sized nitride powders by combustion synthesis method which showed integrated functional and structural properties. For instance, the Re-doped SiAlON powders could be used as new phosphors for LED lighting and the nano and/or sub-micrometer sized α/β-SiAlON could be densified directly without using any sintering additives. Finally, the regularities for the large-scale synthesis of nitride powders were discussed, and the properties of the as-synthesized nitride powders were systematically characterized.

Recently gel combustion synthesis was widely used to synthesize various forms of simple or complex metal oxides, having a wide range of applications. The chapter, after introductory remarks, begins with the ways of preparation of gelled oxide precursors, their physicochemical characteristics and their ability to burn in terms of chemical composition. Then follows a survey of the synthesis of various ceramics materials (electronic superconductors, ionic conductors, dielectrics, ferroelectrics), catalysts (non-supported and supported) electrode materials for chemical power sources (anode and cathode materials for ion-lithium cells), in a powdery form. Finally, the methods to transform the powdery gel combustion products to other valuable materials, for example to bulk oxide materials, or to films and powders of metals and alloys, will be described.

Combustion synthesis has attracted considerable attention recently for its advantages of low processing cost, high energy efficiency, and high production rate. This chapter presents the preparation of functional oxide materials for energy storage or conversion by cellulose-assisted combustion synthesis. In traditional solid-solution-phase combustion synthesis, e.g., the glycine-nitrate process (GNP), a great quantity of gas is evolved during the synthesis, which can create large amounts of ash by blowing away the products. Natural cotton fibers with a hierarchical pore structure were used as a micro-reactor for the GNP in this study. This novel process is environmentally friendly. Furthermore, the resulting particle size was smaller, which was attributed to the blocking effect of cellulose on inter-particle contact during the synthesis. The method was applied for the synthesis of samaria-doped ceria (SDC) as an electrolyte for solid-oxide fuel cells (SOFCs). SDC powder with a particle size as small as 10 nm was obtained, which was easily sintered to form a dense electrolyte at 1350 °C, several hundred degrees lower than that prepared from the traditional solid-state reaction. La0.6Sr0.4Co0.2Fe0.8O3 perovskite oxide was also prepared and showed higher purity and better cathode performance in SOFCs than that prepared by a sol-gel process. By adopting the same method, phase-pure spinel Li4Ti5O12 could be synthesized at 700 °C. The resulting powder had an excellent rate performance in secondary lithium-ion batteries, with a capacity of 140 mAh g-1 even at a 10 C discharge rate. More importantly, solid TiO2 oxides can also be utilized as the raw materials for this synthesis, making the process highly cost-attractive.

Since combustion chemical reaction generates not only thermal energy but also chemically activated particles such as ions and radicals, thermal plasma vapor deposition and combustion flame diamond deposition process are introduced as related process to combustion synthesis in this chapter. This chapter consists of two sections. In the former section, characteristics of thermal plasma are explained. In the latter section, researches on thermal plasma vapor deposition and/or combustion flame diamond deposition process are explained. As the thermal plasma process, thermal plasma CVD (including DC arcjet CVD and solution precursor thermal spray (SPPS)), thermal plasma PVD (including arc ion plating) are introduced. For example, synthesis mechanisms of the films and/or particles and effects of experimental conditions on the growth of the films (or particles) are discussed in cases of the diamond particle synthesis by combustion flame method, SiC and diamond films deposition by thermal plasma CVD and anatase rich TiO2 film deposition by SPPS.

The combustion temperature of a highly exothermic reaction can be above the melting point of the end products, which results in the formation of melt-casting products. Combustion synthesis melt casting technique possesses remarkable advantages for the low-cost production of structural and function materials with unique properties and characteristics. In this chapter, some combustion synthesis melt-casting reaction systems developed in recent years, such as refractory compounds, intermetallics, as well as advanced ceramics, are introduced, and the solidification mechanisms are discussed.

Synthesis of carbon encapsulated metal nanoparticles by rapid, thermally enhanced synthesis reactions (combustion) is reviewed. Both technical apparatus, the methods, and analysis of the fundamental issues are discussed. In technical issues, the sample preparation procedures with the particular emphasis on their structure and chemical composition is given. Various methods of the reaction ignition and control are assessed. The reaction post processing procedures with particular emphasis on the removal of the by-products is discussed. Different metals that can be encapsulated by the carbon graphite-like layers are mentioned and the reaction parameters are listed. Discussion of the fundamentals of combustion methods includes the principles of the processes, such as their thermodynamic basis, involving the temperature and pressure dependence resulting in different efficiency of the synthesis. The reaction products are characterized including grain size and structure, the crystallographic properties such different phases, change of chemical composition due to segregation, etc. The characterization techniques include the chemical analysis, Transmission Electron Microscopy (TEM), SQUID (Superconducting Quantum Interference Device magnetometer) magnetic measurements and Mossbauer Spectroscopy (in the case of Fe-containing samples). The combustion synthesis is critically compared with other methods. Possible applications in various branches of technology, medicine, environment protection and other areas are listed.

Nanoparticles of iron oxide and ferrites are being currently explored for their diverse range of applications such as magnetic storage media, environment protection, sensors, catalysis, clinical diagnosis and treatment etc. The attention which is being focused on their synthesis and characterization is well-deserved as they have the capability to exhibit certain superior properties as compared to bulk. In the pursuit to prepare nanoparticles of iron oxide and ferrites, a variety of synthesis routes like precursor, precipitation, sol-gel, hydrothermal, combustion, solvent evaporation etc. have been reported. However, most of these methods are associated with some limitations.

In order to overcome the limitations of the existing methods, we have developed a technically simple but cost effective chemical synthetic route for the preparation of single phase α-Fe2O3 and Ni(1-x)ZnxFe2O4 nanopowders. In this method, precursor powders were synthesized by reacting aqueous solutions of metal nitrates with EDTA (ethylene dimmine tetraacetic acid). Pure α-Fe2O3 and Ni(1-x)ZnxFe2O4 nanopowders were obtained by calcining the precursor powders at different temperatures ranging from 250 to 450°C in air. Precursors and calcined powders were characterized by using TG (thermogravimetric) - DSC (Differential Scanning Calorimetry) analysis, XRD (X-Ray Diffraction), TEM (Transmission Electron Microscopy) and SEM (Scanning Electron Microscopy). DC electrical resistivity of the samples was measured from room-temperature to 225°C. Room temperature magnetization measurement was performed by using VSM (Vibrating Sample Magnetometer).

Recently, several groups of alloys have been produced by combustion synthesis. Combustion synthesis of alloys would be an interesting research subject. In this chapter, we will introduce the novel routes to novel materials on combustion synthesis of alloys, containing reactive route design idea, synthesis process, microstructures and properties of the as-prepared alloys.

In the process of traditional solution combustion synthesis, the agglomeration and sintering of the resultant oxide nanoparticles widely exists. To inhibit or reduce the phenomena, a novel and facile salt-assisted solution combustion synthesis process was developed, which is to dissolve some soluble inert inorganic salt into the redox mixture solution. Based on the approach, many interesting materials such as mesoporous ceria-zirconia solid solutions, highly dispersed nanoparticles of perovskite NdCoO3 and spinel CoFe2O4, and LaMnO3 and Re2(Zr1-xSnx)2O7 nanocubes, etc. have been fabricated. The chapter reviews the latest developments in microstructure-controlled synthesis of the mixed metal oxides via a salt-assisted solution combustion synthesis process. In the review, the effects of fuel-to-oxidant ratios, type and amount of added salt on the characteristics of the resultants were investigated by powder X-ray diffractometry (XRD), transmission electron microscopy (TEM) and Brunauer-Emmett-Teller (BET) surface area analysis. The results show that the salt introdution inhibits the formation of hard agglomerates during the combustion synthesis, results in drastic increase in specific surface areas of the resultants and also has apparent effect on the particle morphology. Finally, a mechanism scheme was proposed to illustrate the possible formation processes of highly dispersed nanoparticles and discuss the role of the salt additives. In view of the large number of salt additives available, the SSCS approach would be expected to offer abundant opportunities in materials research and provide an effective strategy to tailor the microstructures and properties of the materials for the conventional combustion synthesis process.

Microwave assisted combustion [MAC] synthesis is a rapid technique being exploited for obtaining fine-scale structural, electronic and bio ceramic powders. The main advantage of microwave energy is its accelerated molecular level reversible heating from core to surface by dielectric polarization that offers size and shape control over the end products. MAC synthesis has been well reported for many ceramic oxides such as Al2O3, ZrO2, mullite, MgAl2O4 and ceramic pigments. We have been using this technique for synthesizing single-phase and sinter-active nanocrystalline yttria (Y2O3), gadolinium doped ceria (Gd-CeO2) and zinc oxide (ZnO) with and without dopants for the potential applications as transparent ceramics, luminous pipes for high-intensity discharge lamps, laser host material, solid oxide fuel cell [SOFC] electrolytes, gas sensors, transparent electronic devices, surge-protection electric power devices (varistors), high frequency electronic and telecommunication devices and ferrite cores. The technique involves combustion of the respective oxidic precursor mixtures under microwave energy [2.45 GHz, 900 W] using organic fuels such as urea, citric acid and glycine. In this book chapter we are presenting an overview of our research on MAC synthesis to synthesize fine-scale electro-ceramic powders in the past few years. The various results such as phase purity by powder X-ray, structural evolution by Fourier transform infra red spectroscopy [FTIR], thermal decomposition by Differential thermal analysis [DTA], thermogravimetry analysis [TGA], differential scanning calorimetry analysis [DSC], and particle size, dilatometry, and morphology by scanning electron microscope [SEM], transmission electron microscope [TEM] are highlighted in addition to the sintering properties. The mechanical properties of sintered ceramics processed through MAC technique are also discussed. Interestingly we observed the mechanical fracture toughness and hardness variations depending upon the fuel systems. The phase analysis supports that the as derived ceramic powders are fully crystalline nano-scale oxides that have high specific surface area. It is also observed that fuel like glycine shows strong influence on the size and shape of the particles. In this type of fuel spherical morphology is usually obtained. The MAC synthesis shows great advantages in chemical homogeneity, high sinterability, fine-sintered grain size, and enhanced mechanical and electrical properties. In this chapter we reviewed our attempts made on MAC synthesis for obtaining electro-ceramic powders for the benefit of the academic researchers and industrial R'.

The advent of so-called “spark plasma sintering” (SPS) has led to a large number of new materials systems, particularly in the realm of nanometric size scales. SPS is a non-equilibrium process usually consisting of directly applied pulsed dc currents and uniaxial pressing with heating rates typically from 100°C/min up to 1500°C/min. The resulting compacts have been found to achieve much smaller grains sizes, often less than 100nm, and higher densities than previously possible with conventional hot-pressing. The ability to produce compacts with diameters up to 75mm with such small grain sizes has produced a revolution in the testing of nanoscale behaviors, both mechanical and functional, that have been historically problematic to produce in even laboratory scale specimens. In this chapter we will cover the history, basic apparatus design, experimental constrains, and the benefits/mechanisms of using SPS techniques to produce novel material properties. These topics will cover metallic and ceramic materials systems, primarily having nanometric size scales. Particular emphasis will be focused on the experimental design in order to reduce the commonly encountered assumptions and caveats in producing SPS parts. Finally, the mechanisms by which SPS achieves its remarkable results will be discussed. These mechanisms, still largely disputed and ill-understood, are nevertheless critical in understanding and predicting the performance of a given material system in both consolidation and final use.

The rapid advances in technology and improved living standard of the society necessitate abundant use of fossil fuels which poses two major challenges to any nation. One is fast depletion of fossil fuel resources; the other is environmental pollution. The Porous Medium Combustion (PMC) has proved to be one of the feasible options to tackle the aforesaid problems to a remarkable extent. PMC has interesting advantages compared with free flame combustion due to the higher burning rates, the increased power dynamic range, the extension of the lean flammability limits, and the low emissions of pollutants. This chapter gives an outline of PMC, right from the history till the current stage, based on a thorough review of the documented investigations in this area.

In recent years solution combustion synthesis is the most widely used process by chemists, physicists and materials scientists world-wide for preparing a variety of oxide materials in both nano and micron size regime. The solution combustion process which has its origin in Indian Institute of Science, India has now spread its combustion wave worldwide. The beauty of this process is the ease and simplicity with which nanosize oxides can be prepared. It involves rapidly heating a solution containing stoichiometric amounts of respective metal nitrates (oxidizer) and a fuel like urea, glycine or hydrazide. All kinds of nanosize oxides have been prepared in nano-size ranging from catalysts, dielectric materials, piezoelectric materials, phosphors, Solid oxide fuel cell (SOFC) materials, pigments, etc. The solution combustion synthesized nanosize oxide catalysts have shown greater promise towards environmental remediation. Among the publications related to synthesis of nanosize oxides, solution combustion synthesis occupies the lion share. In this chapter, the history of solution combustion process, a brief summary on materials prepared along with the recent advances, trends and future prospects is presented.

In this ebook, many challenging and exciting applications of combustion research were highlighted. Combustion Synthesis is a relatively new technology, and it can be expected that its significance will increase in future.