Synthesis combustion

43 Combustion Synthesis. Besides solid-state reactions requiring heating even for several days at high temperatures to be completed, fast reactions are also known which give their products in minutes or seconds. Several nearly equivalent names are used for these reactions such as solid-state combustion, combustion synthesis, self-propagating (self-sustained) high-temperature synthesis. Solid state direct combinations and solid-state metathesis have been described. 

Fast reactions between pure metal powders (A1 + Ni, A1 + Ti and A1 + Ni + Ti) have been studied by Javel etal. (1997) by using time-resolved X-ray diffraction with the help of synchrotron radiation. The sample (20 X 10 X 2 to 3 mm3 was prepared under purified argon by cold pressing the metal powders mixed in the required proportion. It was then placed in a reaction chamber kept under He gas. A mylar window allows the incident and diffracted X-ray beams to pass in and out. Two small heating devices (tungsten coils on alumina supports) were included. The first one was used to keep the sample at a uniform temperature before ignition and the second one to start the self-propagating reaction at one end of the sample. X-rays irradiated the centre of the specimen. 

Specimens of different stoichiometries were prepared, in some cases with the addition of 20 mass% alumina powder introduced as heat sink to reduce the reaction violence. For the 1 1 NiAl alloy a record for a total time of 4.3 seconds was reported. Diffraction lines of Ni (111, 200) and A1 (111) were detected at the beginning then, when the reaction front penetrated the irradiated zone, A1 melted and its line disappeared Ni was wetted (and the intensities of its lines decreased) and the temperature jumped from 660°C to about 1400°C, as shown by the shift of the diffraction lines toward lower angles, with the appearance of the NiAl diffraction lines and subsequent cooling. 

Effects of particle size and pressure on the reactive sintering were studied by Gobran et al. (2004). The synthesis of RuAl was described it was observed that reactive systems containing low-melting constituents such as aluminium are assisted in densification by the formation of a transient liquid phase. Reactive sintering using 

Self-propagating reduction reactions Several oxide reduction solid-state reactions have been known for a long time. Starting materials are considered which react together highly exothermically. Once the reaction is initiated, enough heat is produced for very high temperature to be attained, and complete reaction occurs rapidly. The so-called thermite process corresponds to the reactions  

In order to carry out combustion synthesis, the powdered mixture of reactants (0.1-100 pm particle size) is generally placed in an appropriate gas medium that favours an exothermic reaction on ignition (in the case of oxides, air is generally sufficient). The combustion temperature is anywhere between 1500 and 3500K depending on the reaction. Reaction times are very short since the desired product results soon after the combustion. A gas medium is not always necessary. This is so in the synthesis of borides, silicides and carbides, where the elements are quite stable 

Essentials cf Inorganic Materials Synthesis, First Edition. C.N.R. Rao and Kanishka Biswas. 2015 Jdin Wiley Sons, Inc. Published 2015 by John WQey Sons, Inc. 

FIGURE 5.1 Combustion reaction during the preparation of a cuprate. 

FIGURE 5.2 YjFCjOjj powder resulting from the combustion reaction. 

TABLE 5.1 Typical Materials Prepared by the Combustion Method 

Of the various synthetic methods, combustion synthesis (GS) has special significance because CS processes are characterized by high temperatures, fast heating rates and short reaction times. These features make CS an attractive method for the manufacture of materials. In this method, the exothermicity of the redox (reduction-oxidation or electron transfer) chemical reaction is used to produce useful materials. -  

Different types of combustion techniques are employed for the synthesis of nanomaterials. These methods are classified on the basis of the physical nature of the reaction media as (i) flame synthesis or gas phase combustion, (ii) solution combustion synthesis 

Fig e 8.19 Solution combustion process before, during and after the reaction. Reprinted from Mukasyan et al. with permission from Springer. 

In a trivial application of this method, one could consider the synthesis of carbon dioxide from methane as a combustion synthesis. This trivial example provides some framework for additional insights into the solid-state case. The balanced reaction for methane reacting with oxygen is 

This reaction could be viewed in terms of a double displacement reaction One of the oxygen atoms and one of the hydrogen atoms have displaced each other. One of the most famous examples of this type of exchange reaction, a single exchange, occurs between iron oxide and aluminum  

The topic combustion synthesis was first studied by Alexander Merzhavov (b. 1931) in the 1970s has been reviewed by Gillan and Kaner 1996. A typical reaction occurs between a metal halide and an alkali or alkaline earth main group 

Zirconium chloride is a white powder, and lithium nitride is a black powder. When this reaction proceeds, the lithium chloride forms as a liquid just as the iron forms as a liquid in the thermite reaction. The melting point of lithium chloride is 605°C (the melting point of iron is 1536°C ) The temperature of the zirconium chloride/lithium nitride reaction reaches 1370°C less than 1 second after the reaction is initiated. This liquid plays the same role as the flux agents already discussed It provides a medium for rapid mixing of reagents. 

MgO + ZnO + 2Fe + Fe203 + NaC104 - 2Mg0.5Zno.5Fe204(s) + NaCl 

The observed particle size was 10-30 nm the ODH fuel led to smaller particles than the TFTA process. 

The Science of Nanomaterials is proving to be one of the most attractive and promising fields for technological development in this century. In the scientific literature several terms related to Nanoscience can be found, of which it is worth highlighting nanoparticles, nanocrystals, nanofibers, nanotubes and nanocomposites. In fact, all these are related to nanostractured materials, which have well-defined structural features. The physical and chemical properties of materials at the nanometer scale (usually set in the range of 1-100 nm) are of immense interest and increasing importance for future technological applications. Nanostractured materials often exhibit different properties when compared to other materials. 

The relationship between particle size and properties has been known since the nineteenth century, when Faraday showed that the color of colloidal Au particles can be modified depending on their size. However, despite the long history of their discovery, the interest in nanoparticles has only increased significantly in the last 15 years. The research activities related to this area were driven by the ability to control material properties by controlling the size of the particles. 

The nanotechnology wave will likely change the way materials and devices are produced in the future. The abiUty to synthesize crystallites at the nanometer scale with precisely controlled size and composition and to assemble them into large structures with unusual properties and functions, will revolutionize all segments of material manufacturing for industrial applications. Among the main techniques for generating nanoparticles via the wet chemical route, combustion synthesis is the one that stands out. 

Solution combustion synthesis (SCS) is an effective method for the synthesis of nanoscale materials and has been used in the production of various ceramic powders for a variety of advanced applications. 

Ceramic oxide powders at the nanoscale using SCS can be prepared by the combination of metal nitrates in an aqueous solution with a fuel. Glycine and urea, in particular, are suitable fuels because they are amino acids that can act as a complexing agent of the metal ion in the solution and also serve as fuel for the synthesis of nanocrystalUne metal oxides. This method can directly produce the 

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Apart from simple compounds, composite materials may also be prepared by combustion synthesis. Thus a composite of TiB2 and MgO can be formed... 

Controlled and selective combustion of components via thermal or chemical routes Calcination. Thermal detemplation of organic templates in micro- and mesoporous materials. Chemical detemplation protocols. Solution combustion synthesis... 

A recent comparative investigation of the NO + CO reaction shows a significant rate enhancement in the formation of N2 on Ce0.98Pda02O2 8 prepared via a combustion synthesis method in comparison with conventional Pd-based catalysts supported on alumina... 

Civera, A., Pavese, M., Saracco, G. et al. (2003) Combustion synthesis of perovskite-type catalysts for natural gas combustion, Catal. Today, 83, 199. 

Julien, C., Camacho-Lopez, M. A., Mohan, T., Chitra, S., Kalyani, P., Gopukumar, S., Combustion synthesis and characterization of substituted lithium cobalt oxides in lithium batteries, Solid State Ionics 135, 241-248 (2000). 

CNTs can also be produced by diffusion flame synthesis, electrolysis, use of solar energy, heat treatment of a polymer, and low temperature solid pyrolysis. In flame synthesis, combustion of a portion of the hydrocarbon gas provides the elevated temperature required, with the remaining fuel conveniently serving as the required hydrocarbon reagent. Hence, the flame constitutes an efficient source of both energy and hydrocarbon raw material. Combustion synthesis has been shown to be scalable for a high volume commercial production. 

Arvind Varma, Alexander S. Rogachev, Alexandra S. Mukasyan, and Stephen Hwang, Combustion Synthesis of Advanced Materials Principles and Applications J. A. M. Kuipers and W. P. M. van Swaaij, Computional Fluid Dynamics Applied to Chemical Reaction Engineering... 

Deshpande K, Mukasyan A, and Varma A. Aqueous combustion synthesis of strontium-doped lanthanum chromite ceramics. J. Am. Ceram. Soc. 2003 86 1149-1154. 

Bansal NP and Zhong Z. Combustion synthesis of Sm0 5Sr0 5Co03 x and Lao 6Sr04CoO3 x nanopowders for solid oxide fuel cell cathodes. J. Power Sources 2006 158 148-153. 

According to the aggregation state of the component elements and the method selected for starting and performing their reaction, several preparative procedures can be considered, such as melting (direct reaction in the liquid state), solid-state synthesis (mechanical alloying), combustion synthesis, etc. 

Intermetallic compound formation may be observed as the result from the diffusion across an interface between the two solids. The transient formation of a liquid phase may aid the synthesis and densification processes. A further aid to the reaction speed and completeness may come from the non-negligible volatility of the component(s). An important factor influencing the feasibility of the reactions between mixed powders is represented by the heat of formation of the desired alloy the reaction will be easier if it is more exothermic. Heat must generally be supplied to start the reaction but then an exothermic reaction can become self-sustaining. Such reactions are also known as combustion synthesis, reactive synthesis, self-propagating high-temperature synthesis. 

In the field of high-temperature combustion synthesis, metals have been reacted with nitrogen, both in the gaseous and liquid phases, to form refractory nitrides [2], In most cases, this nitriding process is heterogeneous. 

Modem computational programs [4] and thermodynamic tables [5] now make it possible to explicitly calculate metal-oxygen flame temperatures, thereby opening up a unique aspect of combustion thermodynamics that could be important in the consideration of metal as fuels and as reactants in combustion synthesis. 

Saita et al. [215] used hydriding combustion synthesis for a direct production of TiFe. In the experiments, an exothermic reaction of Ti with hydrogen (Ti -i- i = TiHj + 144 kJ) was utilized for HCS of TiFe because the adiabatic flame temperature of this reaction was estimated to be 2,000°C, which is sufficiently high for melting both iron and titanium. A 1 1 molar mixture of elemental Ti and Fe pow-... 

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