There are numerous experimental data indicating that the action of electromagnetic and magnetic fields on the combustion wave of self-propagating high-temperature synthesis (SHS) changes the process kinetics and the structure and properties of the reaction products. Emission phenomena in the combustion wave have received less attention. High rates of chemical energy dissipation in SHS (10 12 W/m 3 ) are accompanied by physical phenomena such as the occurrence of a potential difference between the combustion front and reaction products, free electron emission from the combustion wave, and acoustic emission. Detailed studies of these nonequilibrium phenomena provide a deeper understanding of the reaction mechanism in solid flames to use nonthermal methods of combustion control. EMISSION OF FREE ELECTRONS AND IONSSpontaneous generation of an electromotive force (EMF) between a SHS front and condensed reaction products was first found in combustion studies [1, 2] using the Ni-Al and Mo-B systems.Later [3], for combustion of Ti-C, Ti-N, and Mo-Si systems, an ionization wave accompanying the combustion wave was recorded. In the opinion of the authors, flow of ions and electrons resulted from the surface ionization of the gas phase. Detailed invstigations [4,5] of combustion of condensed systems (Ni-Al, Co-Al, ZrAl, Mo-B, etc.) in an electric field have shown that the transfer of ions and electrons emitted from the combustion wave occurs in both the condensed phase and pore space. In the latter case, the emission of charged particles by the surface of pores plays a major role. The high value of the emission current observed in the work (up to 200 A/cm 2 ) indicates the presence of high density of charge carriers (up to 10 17 cm −3 ) in the gas medium.According to an estimate obtained using the Saha and Richardson-Dushman equations, the concentration of charge carriers in the plasma is 1-3 orders of magnitude larger than the equilibrium value determined from the laws of thermodynamics and thermal emission.The wave-like dependence of the emission current peak on the coordinate of the combustion front of SHS ( Fig. 1) subjected to a constant voltage is unusual for the well-known emission processes. In addition to low frequency oscillations, high frequency current oscillations with a frequency of up to 100 MHz are recorded by a radio frequency spectrometer (Fig. 2).The energy characteristics of charged particle flow from combustion waves are usually estimated using the electric probe method of plasma measurements -construction of a volt-ampere characteristic [5,6]. The electron energy distribution in the emission flow [see Fig. 2, where f (E e ) is the electron distribution function in the energy E e and f max (E e ) is the maximum value of the distribution function] was calculated by processing experimental dependences of the maximum current through probes on the voltage I max (U 0 ) via double differentiation of the interpolating function. It is seen that the electron energy distribution has a multimodal nature and does n...

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

No abstract

Emission effects of heterogeneous combustion in the region of ionization radiation are studied. By an example of a Ti-B powder system, it is demonstrated that the processes of self-propagating high-temperature synthesis in the thermal explosion regime is accompanied by "soft" X-ray radiation with the quantum energy estimated as ≈5 keV.

Исследован процесс распространения фронта волны горения в двухслойной порошковой системе (Ni + Al)/(PbO2 + B + Al2O3+ стекло). Обнаружен эффект расширения волны экзотермического процесса при добавлении смеси PbO2 + B в нижний слой. Добавление смеси PbO2 + B в нижний слой позволяет снизить толщину слоя NiAl и обеспечить формирование однородного диэлектрического покрытия. Установлены оптимальные соотношения толщин слоев и состава порошковой смеси слоев.