Progress in sol-gel autocombustion synthesis of metals and alloys

This paper is an overview of the progress of sol-gel autocombustion synthesis of metals and metal alloys. Sol-gel is a convenient method to synthesize a variety of oxides by mixing of different elements at an atomic level. Autocombustion synthesis is a self-sustaining process caused by the heat generated from its exothermic reaction. By combining these two methods, the sol-gel autocombustion method is introduced in the synthesis of metals and metal alloys. The experimental principle and technological route are introduced in detail in this review. By using metal nitrate, citric acid etc. as starting materials, the dried gels are prepared through sol-gel routine. Under the protection of inert gas, the autocombustion could be activated at low temperature in a tube furnace. After the autocombustion was activated, the gel burned violently, and a large amount of white gas was released. During heating the gel, mass spectrum shows that the H-2, CO and CH4 are evidently identified near the combustion temperature. They are well known reducing agents, which can be used in the redox reaction for synthesizing metals from oxides. Based on the data obtained from the TG-DTA and mass spectrum analysis, it is speculated that there are mainly five reactions appearing during the burning of the gel at high temperature: exothermic reaction between fuel and oxidant; metal oxide(s) formation by decomposition of the nitrate(s); generation of CH4, CO and H-2 by the decomposition of CHx containing groups of complexing agent; exothermic reaction between CH4/ CO/ H-2 and oxidant; the reduction of metals from their corresponding metal oxides by CH4 and H-2 in nascent product. The application of this method to the synthesis of metals and metal alloys is shown by realized examples. This method shows many advantages in the synthesis of metals, such as simple apparatus, inexpensive raw materials, a relatively simple preparation process, and fine powder products with high homogeneity. Moreover, very low temperature is required to activate the reaction, and then the combustion can continue to take place without needing additional energy supply. This method has potential applications in experimental material researches.