Thermolysis in the BaO2-NaClO4 system

Abstract

The mutual influence of barium peroxide and sodium perchlorate on thermolysis of the (2 ⎯ x)BaO2-xNaClO4 system (x = 0-2.0) has been studied. It has been demonstrated that the use of the physicochemical mechanism of this influence makes it possible to carry out heterogeneous combustion in complex oxide systems in the controlled regime by changing the mutual concentration of solid inner reaction oxidizers. An explanation of the thermal and kinetic effects in the thermolysis of the peroxide-per� chlorate system has been suggested. Alkali metal chlorates and perchlorates decompose on heating to evolve oxygen. This makes them suitable for using as oxygen sources in oxidation reactions, including the selfpropagating hightemperature syn� thesis (SHS) of complex oxides, which requires the presence of a fuel and oxidizer in the reaction system (1, 2). In the SHS of complex oxide materials, solid inner reaction oxidizers, such as sodium perchlorate NaClO 4 , can be used as an alternative or complement to gaseous oxygen. Sodium perchlorate contains more than 50 wt % of oxygen, which is completely evolved at relatively low temperatures (3). However, in the com� bustion reaction, the time interval of the vigorous oxy� gen evolution from the solid oxidizer may not coincide with the intense exothermic oxidation reaction (com� bustion) of the metal component. If this is the case, the chemical reaction rate significantly decreases, which leads to incomplete reaction of the components and even to combustion failure. The kinetic parame� ters of sodium perchlorate thermolysis can be changed by both external physical actions (for example, electric field (4)) and various chemical additives. As is known, small additions (up to 4 wt %) of some metal oxides catalyze the thermolysis of potassium perchlorate and sodium chlorate (5). Inasmuch as metal oxides are components of reaction mixtures for the SHS of complex oxide materials, if the character of the influence of some oxide on perchlorate is known, it is possible to find the bath composition at which the process will be controllable and will proceed at defi� nite rates and temperatures. In SHS processes, alkali metal and alkaline earth peroxides are often used as oxide components. For example, barium peroxide BaO 2 is used in the synthesis of hightemperature superconducting ceramics (HTSC—YBa2Cu3O 7- x), barium hexaferrite (BaFe12O19), barium stannate (BaSnO 3 ), etc. In these systems, the fuel is a powder of a corresponding metal: iron, copper, and tin. Thus, to determine optimal component ratios when sodium perchlorate is used as a solid inner reaction oxidizer, it is necessary to know the mechanism of the influence of superstoichiometric NaClO4 , as a chemical addi� tive, on the synthesis in these systems. To determine this mechanism, we considered the mutual influence of NaClO4 and BaO2 on thermolysis of these com� pounds in the reaction mixture.