The thermal decomposition of sodium nitrate and the effects of several oxides, such as silica, titania, zirconia, alumina, and magnesia, on the decomposition were studied up to 900 °C by means of the thermal analysis, gas analysis, and chemical analysis of the reaction products. The reaction of sodium nitrate and silica was especially investigated in some detail over a wide composition range. The thermal decomposition of sodium nitrate started at about 450 °C. The gases formed were O2, NO, and N2, the formation of N2 being detected above 680 °C The thermal decomposition of sodium nitrate was supposed to consist of the following three successive or concurrent reaction processes, according to the degree of the reaction: (I) the decomposition of sodium nitrate to nitrite and oxygen, (II) the first-order liquid-phase reaction, with some kind of quantitative relationship between nitrate and nitrite, and (III) the formation reaction of sodium oxide, expressed by the Avrami-Erofe’ef equation. The (II) and (III) reaction processes can be reasonably interpreted if the existence of the peroxide ion is assumed. Acidic oxides, such as silica and titania, were supposed to lower the activation energy of the (III) reaction process by forming stable salts with sodium nitrate and/or its intermediate reaction products. The effects of these oxides on the thermal decomposition of sodium nitrate could be interpreted by the relative scale of the acidity of oxides.

The reaction of sodium nitrite with silica has been investigated over the relatively wide composition range (Na/Si=0 to 10.0) below 800 °C. In an argon atmosphere, sodium nitrite reacts with oxygen to form sodium nitrate above 400 °C and decomposes to sodium oxide via other oxide species above 500 °C, irrespective of the coexistence of silica. The reaction of sodium nitrite with silica in an argon atmosphere consists of two stages. In the early stage (<600 °C), the main gaseous product is NO, while in the later stage nearly equimolar amounts of NO and O2 are formed. The early stage is further divided into two reaction processes: Process I, in which peroxide is considered to be formed in the melt by generating NO, and Process II, in which nitrate and silicates are formed by generating NO and a trace amount of O2. Peroxide is supposed to be stabilized at the solid-liquid interface, even if no silicates have been formed at relatively low temperatures. In the later stage, the reaction process or mechanism can be regarded as similar to that of sodium nitrate with silica, as the concentration of nitrite in the melt is very low.

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The reaction and the thermal changes of the ternary system NaNO3-B203-SiO2 have been investigated by means of high-temperature thermogravimetry, derivative thermogravimetry, microscopy, differential thermal analysis, gas chromatography, X-ray diffractometry and I R spectrometry. The combination of these techniques provides information on the reaction process. The major reactions which take place in this system are as follows: several sodium borates are preferentially formed in a liquid-phase reaction after NaNO 3 and B20 3 melt, with the generation of 0 2, NO and N 2. SiO 2 does not take part in the reaction below 550 ~ Sodium borates with B/Na = 9 to 1 are formed successively with the generation of 0 2 and NO as the temperature increases. The reaction products around 700 ~ are mixtures of borates and silicates. Borosilicates are formed above 750 ~ J. ThermalAnal. 27, I983