Electromotive force for solid-flame combustion of heterogeneous systems in loose and pressed states

Abstract

By an example of condensed 3Zr + 2WO 3 and Al + Ni systems, it is demonstrated that the electromotive force of solid-flame combustion measured by probing in loose systems is significantly higher than that in the same systems in a pressed state. An explanation for this phenomenon is offered, based on the difference in electrical conductivity in different zones of the combustion wave in loose and pressed condensed systems.Key words: solid-flame combustion, electromotive force of combustion, condensed systems, electrical conductivity in the combustion wave.Electrical phenomena in combustion have attracted researchers' attention for a long time [1][2][3][4]. Many papers deal with studying the electromotive force (e.m.f.) of combustion in condensed systems. These papers describe the general ideas on the nature of origination of the combustion-induced e.m.f. in condensed systems [5][6][7][8][9][10][11][12][13] and show that the e.m.f. of combustion can be used as a tool for studying combustion properties [14][15][16]. It should be admitted, however, that the detailed mechanism of combustion-induced e.m.f. formation in condensed systems has not been clearly understood yet. In particular, it is not clear which mechanism is responsible for the influence of the initial density of the condensed system on the e.m.f. of solid-flame combustion.Loose systems display rather high values of the combustion-induced e.m.f., which reaches ≈900 mV in some individual systems [5][6][7]10]. At the same time, the e.m.f. values registered for pressed systems in [11,12] were lower approximately by an order of magnitude (less than 100 mV). We should recall that the constant component of the e.m.f. obtained in [9] in measuring the e.m.f. in the combustion wave in the pressed Co + S system reached 5 V, and the value of the variable and pulsed e.m.f. exceeded 110 V. The results of [9] seem to be anomalous, and their alternative interpretation will be presented below. It is also of interest to note that the amplitude of the e.m.f. induced by combustion of a mixture of tetrazole and sodium tetrazolate monohydrate in a pressed state is ≈900 mV [17].Of particular interest is to compare the results obtained for the Al + Ni system in [5,6,[10][11][12]. Typical values of the combustion-induced e.m.f. in this system in a loose state are 560-570 mV [5,6,10], and the corresponding values for the same system in a pressed state are less than several millivolts [11,12]. Though these data were obtained by different authors for different mixtures of Al and Ni powders and with the use of different measurement electrodes (which can affect the results obtained [18]), the main reason for the difference in the values of the combustion-induced e.m.f. seems to be the difference in initial densities of the burning systems.To check this assumption, we performed experiments to estimate the combustion-induced e.m.f. on a setup (Fig. 1), which allows experimentation with condensed systems in a loose state with the use of the same measurement electrodes and powder mixtures...

Electromotive force measurements in the combustion wave front during layer-by-layer surface laser sintering of exothermic powder compositions

“…In contrast, for electromotive force (EMF) measurements the electrical probes must be situated inside of the reaction zone (area of laser synthesis). In order to reduce parasitic thermo-EMF 13,14 the probe material was chosen to be similar to the synthesized material (hence nickel metal). The thermocouple junctions were located near each probe.…”

“…Several experimental studies have revealed that hightemperature front propagation during the combustion synthesis of some compounds generates a voltage of up to 1.5 V. [5][6][7][8][9][10][11][12][13][14] At present the mechanism of these phenomena has not yet been established. Previously we have developed and refined the testing scheme for electro-thermal phenomena studies, which can directly influence the combustion wave front propagation under conditions 15 of SHS-SLS.…”