Microwave Radiation from Combustion of an Iron-Aluminum Thermite Mixture

Abstract

Nonequilibrium microwave radiation from the combustion wave of the Fe 2 O 3 -Al powder system in the frequency range of 3.4-37.5 GHz was recorded. It is found that the radiation power is proportional to the free-surface area of the reaction products and is 4-7 orders of magnitude higher than the thermal-radiation level in the same wavelength interval. A possible mechanism of the electromagnetic emission is the Bremsstrahlung of the mobile electrons released by the condensed-phase surface.It is known that in the combustion waves of NiAl, Co-S, FeO-TiO 2 -Al, and Ti-Si systems and similar heterogeneous systems that yield condensed products, there is gas ionization due to the emission of electrons and ions from the condensed-phase surface [1][2][3][4][5]. Because of the special nature of ionization of this type of systems, which includes direct chemical excitation of charged particles [3,4], it is of interest to analyze the microwave radiation due to dissipative processes in plasmas. Previously, this radiation has been recorded during combustion of gases and organic solid propellants [6] and explosions of metal-containing mixtures [7].In the present study, the object of research was the Fe 2 O 3 -Al powder system, whose choice was motivated by high values of the heat effect, the maximum combustion temperature (up to 3200 K [8]), and the predominant course of the reaction inside the condensed phases; for stoichiometric component ratios, the reaction follows the schemeUnder normal pressure, the partial participation of gases in the process involves the boiling of aluminum (T boil = 2793 K [9]), the decomposition of Fe 2 O 3 (T dec = 1835 K [9]), the gasification of volatile impurities, etc.For the experiments, we prepared powder mixtures of ASD-4 aluminum and Fe 2 O 3 iron oxide (chemically pure), which were then shaped into disk-like specimens about 10 mm thick. Combustion was initiated by a wire spiral heated by an electric current in air.The gas release due to the reaction leads to dispersion of the specimen with formation of a cloud of suspended material. According to videorecords, during combustion of the mixture, a high-temperature core of the cloud forms, which expands to a diameter D m = (50-150) · 10 −3 m, where the chemical transformations of the system are apparently completed. Subsequently, the core relaxes to form a scattering zone of the cooling particles of the final product. The latter, as shown by powder X-ray diffraction analysis of stoichiometric compositions, is a composition of Fe, FeO, and FeAl 2 O 4 . The presence of iron oxides is explained by the fact that in addition to the oxidation-reduction scheme (1), the reaction involves atmospheric oxygen. The full dispersion of the specimen into particles of size (0.1-0.5) · 10 −3 m occurs for (75-70%) Fe 2 O 3 + (25-30)% Al compositions of bulk density, where the expansion and relaxation stages of 0010-5082/05/4104-0481

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