Self-spreading high-temperature synthesis (SHS). SHS-based technology

Abstract

The principle of combustion without heating, which is the basis of SHS, has eliminated the main drawback of traditional production processes: high energy capacity and low productivity. Owing to this, it has become possible to create highly effective procedures for refractory nonoxygen and oxide compounds and materials, semiconducting compounds, hydrides, intermetaUides, and other materials and products based on them. In some cases the technology is completely waste-free. A feature of SHS is the high purity of the products, which is due to the complete use of reagents and self-purification to remove impurities during synthesis.The technical potential of SHS is determined by the variety of the materials used and the possibility of specific combinations of reactants and products not available for other methods. The chemical nature of the reactions of combustion used for the synthesis is also variable.Depending on the combustion temperature it is possible to identify three main trends in the use of SHS in technology: the combustion temperature T c is less than the melting temperature Trn for the products of synthesis (T e < Tin) -the technology of powdered materials; T e is close to T m -the technology for articles combining synthesis and deformation of the products (stamping, rolling, extrusion); T e > T m -the technology of refractories and other articles.The common features of all these trends are the following operations: batching and mixing original powdered COmponents (where necessary, pelletizing the prepared batch); carrying out SHS. All operations can be automated and the process made continuous.Powder Technology. To apply SHS we take dry powders with grain sizes of 0.01-1.0 mm and mix them to a uniform distribution. The mixture, freely poured or prepressed into pellets, is placed in an open or sealed reactor. It can be evacuated or filled with the required gas, including liquid nitrogen, for which the following equipment is used: vacuum (10-4-10 -1 Pa), cylinder (0.1-15 MPa), high-pressure (50-500 MPa), or cryogenic. Inert gases and vacuum are used for mixed systems in which the oxidizer and fuel are in the solid powdered state and are mixed together. In hybrid systems the gas is used as a reagent. Combined versions are also possible.In the simplest case the reactor may consist of spalling-resistant magnesia, corundum, or graphite crucibles. The crucible (Fig. 1) is filled to three-fourths of its height with the reaction mixture, which is ignited with a chemical fuse, an incandescent nichrome spiral, or an electric arc. After combustion is complete the crucible is cooled, the synthesized product is milled, and is then classified according to its purpose. The trade name for such materials contains the abbreviation SHS. They are most often used as abrasives, powders for plasma and gas-plasma melting, and also as a component of composite alloys with special properties [i]. Powdered coatings with SHS materials, 1-4 mm thick, made by the authors for cast and Welded augers of various vacuum presses in ceramics pr...

Producing NiCrAl alloy by the SHS method for use in thermal spray powder manufacturing

“…In the SHS process, after initiation, a highly exothermic reaction becomes self-sustaining and propagates throughout the SHS mixture, yielding the desired product. In this study, since the resulting alloys include Ni 3 Al intermetallic formations and the resulting slags consist of alumina-based multicomponent oxides, the whole process can be considered an SHS process [8]. For such a process, the adiabatic combustion temperature (T ad ) value dictates whether the reaction can self-propagate or not.…”