Influences of Ignition Temperature, Urea-to-Metal Nitrates Molar Ratio, and Further Calcination on Synthesis of C₄AF via the SPCR Method

Abstract

The self-propagating combustion reaction (SPCR) method was employed to synthesize C₄AF using metal nitrates as cation precursors and urea as fuel. Thermal decomposition behavior of dried gels, phase identification and crystallinity of synthesized C₄AF, and impact of urea to metal nitrates (UR/MN) molar ratio on synthesis effect were investigated with the aid of differential thermogravimetric analysis, X-ray diffraction and Fourier transform inferred spectrometry. It is found that pure C₄AF can be prepared by the SPCR method in 2 h at 500 °C. The UR/MN molar ratio plays a significant role in the thermal decomposition behavior of dried gel, purity, crystallinity and crystallite size of synthesized C₄AF. Ignition temperature should be not lower than 500 °C but higher temperatures were unnecessary. No trace of free lime in synthesized C₄AF is detected and calcium carbonate is the transition phase. Further calcining the synthesized C₄AF at high temperatures is beneficial for increasing crystallinity, purity and crystallite size. Reaction activation energy of the further calcination process is 119.6 kJ/mol. It is more efficient to improve the synthesis effect by increasing UR/MN molar ratio than further calcination at high temperatures.

Keywords

C₄AF / combustion synthesis / self-propagating / brownmillerite / purity / low temperature

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