The nonlinear chemical reaction kinetics of NH₄ClO₄ + Mg + K₂Cr₂O₇ system. I. Experimental phenomenon of solid - phase oscillatory combustion

Under normal circumstances the reaction system of NH₄ClO₄ + Mg + K₂Cr₂O₇ does not burn at a constant velocity, it burns oscillatorily. The nonlinear chemical properties of the system are studied in the present paper. New formulas for the solid-phase oscillatory combustion are introduced. In these formulas, the ratio of composition is changed in order to obtain the new oscillatory combustion phenomenon. The experimental results are recorded by X-Y digital recorder and Kodak EM 1020 high speed movement analysis apparatus. The waveforms show that the formula in this study can produce oscillatory combustion. From the experimental study, it can be seen that with the increasing of NH₄ClCO₄ and Mg contents, the frequency increases. Moreover, the change of the K₂Cr₂O₇ content greatly affects the oscillation frequency. The experiment results and reaction mechanisms are analyzed by the methods of the nonlinear chemical reaction kinetics. It can be seen that oscillatory combustion occurs mainly as a result of the competing reactions among the three phases of Mg with oxygen. 1. Solid-phase Mg(s) reacts with oxygen and releases energy. In open systems, if the energy cannot maintain the temperature of the system, the reaction will stop and combustion ceases. When Mg(s) is at the vapor temperature, if the heat released from phases change of Mg is equal to the heat required for heating the mixture, the temperature is steady, and then the reaction occurs at a smouldering rate. If more heat is generated during the heat production than that consumed, the reaction rate will increase. At this moment there are two states: one is that the production rate of Mg(g) is slower than the consumption rate, and deflagration will not appear; the other is that the production rate of Mg(g) is greater than that of consumed and deflagration appears, or in the vapor state and the temperature is high. From the experiment, it can be seen that, the deflagration time of the reaction is longer than the smouldering time with the increasing of the Mg content. The formulas of this work make the reaction in a middle state between deflagration and smouldering, it is shown that the concentration of Mg controls the reaction process. 2. At the smouldering stage, the major reaction is part of Mg(s) reacting with oxidant gas. At the deflagration stage, the major reaction is the gas-gas reaction of Mg(g) and oxidant gas. There are two different reaction mechanisms replacing each other that result in the pulse oscillatory combustion. 3. Because the vaporizing rate and vapor concentration of Mg(s) have remarkable influence on the frequency of smouldering and deflagration, the endothermic materials generated in the reaction can greatly regulate the vapor rate of Mg(s) and effectively control the pulse frequency of pyrotechnics. The frequency-modulating species K₂Cr₂O₇ can control the vapor of Mg by forming the condensed phase layer at smouldering stage, and it is an essential condition for an oscillatory combustion to occur.