Self-propagating high-temperature reactions

The technique, now named self-sustaining high-temperature synthesis (SHS) - on the grounds that long names drive out short ones - was later taken up in the West, and has gradually become more sophisticated. The synthesis of TiC, by Holt and Munir (1986) marks the beginning of detailed analysis of heat generation and 

A particularly striking recent application was by Deevi and Sikka (1997) they developed an industrial process for casting intermetallics, especially nickel alumi-nides, so designed (by modifying the furnace-loading sequence) that the runaway temperature rise which had made normal casting particularly dangerous was avoided. 

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This same calculation can be made for the self-propagating high-temperature reaction if the functional form of the variable-temperature heat capacity is known. From this calculation we see how adding additional product salt reduces the temperature that the reaction can reach by increasing the heat capacity of the system. 

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. 

The ignition process initiates a self-propagating, high-temperature chemical reaction at the surface of the mixture. The rate at which the reaction then proceeds through the remainder of the composition will depend on the nature of the oxidizer and fuel, as well as on a variety of other factors. "Rate"... 

In addition to conventional sintering, reactive powder processing, also called combustion synthesis or self-propagating high-temperature synthesis (SHS), can be used if the target compounds can be synthezised from the starting powder mixture (Stangle and Miyamoto, 1995). This process comprises a rapid and exothermic chemical reaction to simultaneously synthesize some or all of the constituent phases in the FGM and density the component. 

Self-Propagating High-Temperature Synthesis (SHS) Conventional methods of synthesizing materials via solid reactions involve... 

Solid-phase reactions are usually activated by high-temperature treatment [1-4]. However, the practical efficiency of this process is rather low since the diffusion rate through a product layer is small, no tight contacts between the particles of components are provided, a particle size distribution is non-uniform, etc. Therefore, the search for new methods of performance of solid-phase reactions is carried out. Some new methods are successfully used for these purposes. These methods include, for example, self-propagating high-temperature synthesis [5], shock waves [6,7], mechanical activation of mixtures in grinding apparatus [8-15]. The latter method becomes more widely used at present due to its relative simplicity and availability. 

Many borides can be synthesized by novel synthetic methods, inclnding shock-induced chemical reactions, mechanical alloying by ball milling, and self-propagating high-temperature synthesis. In these methods, intimately mixed elemental powders are brought to react by a rapid pressure increase, mechanical deformation including local... 

Several different processes have been used, the simplest being by the reaction of hydrogen sulphide with molybdenum pentachloride, or the reaction of sulphur vapour with molybdic oxide or molybdenum metal. The last of these processes has been called the SHS process (Self-Propagating High-Temperature Synthesis) and Russian workers have reported that the product is less contaminated with impurities and has almost identical lubricating properties to natural molybdenum disulphide. The crystal structure is considered in more detail later, but it seems probable that the initial product of syntheses has a disordered... 

Four different routes were employed for the synthesis of iron-oxide-based redox water-sphtting materials Solid-State Synthesis (SSS), Self-Propagating High-Temperature Synthesis (SHS), Gel Combustion (GC) and Aerosol Spray Pyrolysis (ASP) [8]. These synthesis methods were chosen with the rationale to exploit particular characteristics of each one for the synthesis of products with tunable oxygen vacancies concentration. The synthesis details have been reported previously [8], therefore only the general reaction concepts are reported below, where A and B denote the bivalent dopant metals Ni, Mn or Zn. 

Combustion synthesis (CS) can occur by two modes self-propagating high-temperature synthesis (SHS) and volume combustion synthesis (VCS). A schematic diagram of these modes is shown in Fig. 1. In both cases, reactants may be pressed into a pellet, typically cylindrical in shape. The samples are then heated by an external source (e.g., tungsten coil, laser) either locally (SHS) or uniformly (VCS) to initiate an exothermic reaction. 

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