SHS Systems Research Articles

Mechanoactivation of SHS systems and processes

Data on the mechanoactivation (MA) of SHS processes and systems obtained over the past 15 years are overviewed. Analysis of these data is used to formulate some recommendations for appropriate application of MA to SHS.

Chemical furnace for preheating in dynamic consolidation experiments

A new method is developed for specimen preheating with subsequent loading by shock waves and conservation of the products of shock consolidation; the method is based on the use of the chemical furnace energy produced by a thermal explosion or combustion of SHS systems. The use of the thermal explosion expands the range of pre-shock heating temperatures up to 3000 °C and the range of mean pressures in the cylindrical ampoule up to 25 GPa. Two variants of preheating are considered: an inert specimen is heated by heat release in a reacting SHS system or the specimen under study itself is an SHS system.

Solid‐State Combustion in Mechanically Activated SHS Systems. II. Effect of Mechanical Activation Conditions on Process Parameters and Combustion Product Composition

The effect of mechanical activation conditions in a planetary ball mill on the main parameters of SHS‐processes and combustion product composition was studied for Ni + 13 wt. % Al and Ni + 45 wt. % Ti compositions. From results of experiments at elevated initial temperatures, it is concluded that because of reverse quenching, the defects produced by mechanical activation are not annealed in the heating zone and are retained in the sample until the beginning of chemical interaction in the leading zone of the SHS wave. The process of annealing of the activated samples was studied in situ on a transmission electron microscope and a synchrotron radiation diffractometer. The energy stored in the samples as a result of mechanical activation is estimated by calorimetric studies.

Solid‐State Combustion in Mechanically Activated SHS Systems. I. Effect of Activation Time on Process Parameters and Combustion Product Composition

The factors responsible for the transition from reagent interaction involving a liquid phase in ordinary SHS powder mixtures to solid‐state combustion after preliminary activation of these mixtures in an energy‐intensive planetary mill were studied for Ni + 13 wt. % Al and Ni + 45 wt. % Ti compositions. The dependences of the burning rate and temperature on the duration and conditions of mechanical activation were determined. It is found that the occurrence of solid‐state SHS in a powdermixture is due to the formation of “laminated composites,” in which the reagents are ground to ultrafine size, the area of their contact increases severalfold, and the concentration of nonequilibrium defects is high. In activated samples, heat release proceeds in several stages and at lower temperature than in powder mixtures.

Enthalpy approach to the investigation of the combustion process. II results of thermal analysis of some combustible systems

Thermal parameters of the critical cross section of the steady-state combustion wave in gaseous mixtures, explosives, nitrocellulose powders, and pyrotechnic and SHS systems are determined from analysis of experimental combustion rates.

Some features of the chemical transformation of a highly exothermic ore concentrate-based mixture in a combustion wave

Use of inexpensive ore concentrates as initial components in SHS metallurgy [1–3] brings about some specific features in the process and the properties of the materials obtained. Ore reagents contain a considerable amount of impurities (Al2O3, SiO2, Fe2O3) that decrease the energetics of SHS systems and the temperature and the rate of combustion and create the limits of combustion and phase separation. In addition, the impurities participate in chemical reactions and enter the desired product. In many cases the presence of silicon, iron, and aluminum in a cast product does not prevent use of this product for practical purposes.

Interactions of oxides in heating and combustion

Special features of the component interaction in two groups of SHS systems are investigated: I) oxideoxidizer+oxide-reducer and II) oxide mixtures in which the reaction of formation of a complex oxide is not redox. It is shown that the components of mixtures I first transform into the components of the corresponding mixture II and only after that into the final products. Mass transfer of the reagents in all steps involves eutectic melts as well as gaseous intermediate products whose formation is caused by impurities. Basic macrosteps of the interaction processes of Ba and Sr oxides with WO3 and MoO3 are proposed and confirmed experimentally.

Mathematical simulation of combustion of three-component SHS systems

Based on the concepts of the mechanics of heterogeneous media, a model of the combustion of a three-component mixture is constructed. The model describes the macrostructural laws that govern the process of combustion and that are associated with the processes of heterogeneous chemical transformation at the microlevel. The dynamics of the concentration and temperature fields in the process of ignition was studied as a function of variations in the characteristics of the initial mixture.

Technological fundamentals of SHS compacting

Different versions of the shock-wave treatment of SHS systems are considered. It is shown that a combination of SHS with explosive compaction is the most promising method as regards development of new materials. This method was used to produce high-density materials with diverse properties such as refractory hard alloys, high-temperature superconductors, etc. The method allows the structure formation to be controlled during the material synthesis, and this is demonstrated by producing fine-grained ceramics.

Initiation of SHS processes

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Chemical classes of the SHS processes and materials

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Oxidation of metal particles in the heating zone during the combustion of shs systems

This article examines, in relation to the combustion of simple binary M + X yields MX systems, where M is a metal, X a nonmetal and MX the product, the problem of the variation of the impurity oxygen concentration in metal particles during their dwell time in the heating zone (metal particle oxidation). On the basis of the results obtained, the experimental data on the synthesis of titanium carbide by the SHS (selfpropagating high-temperature synthesis) method is discussed both in a vacuum and in a constant-pressure bomb.

Thermal conductivity of certain aluminum-based SHS systems

This paper presents the results of an experimental investigation of the thermal conductivity of metal powder mixtures based on aluminum, establishes the fundamental principles of its porosity dependence, and offers recommendations for predicting the thermal conductivity of molded metal-powder materials.

Self-cleaning of SHS titanium carbide from impurity oxygen

Analysis of numerous refractory compounds obtained by the SHS method shows that they self-clean from impurities during synthesis of the final products; the content of many gaseous and volatile impurities turns out to be substantially less in the final product than in the original charge. A singly directed experimental investigation of the regularities for self-cleaning from impurity oxygen in SHS systems is reported on in this paper for the example of the simplest system, Ti + C. Titanium brand PTS was used in the tests. The soot traditionally used in the synthesis of TiC by the SHS method was replaced by graphite powder obtained by grinding up rods from spectrally pure graphite with subsequent separation into fractions. The mass content of impurity oxygen in graphite powder is -0.4%, and in titanium powder is (0.4-0.6)%.