Synthesis of Refractory Ceramics via Rapid Metathesis Reactions between Solid-State Precursors

Abstract

Chemical exchange (metathesis) reactions are used in many syntheses of important solids. While metathesis reactions in the liquid and gas phase are well-known, metathesis reactions from solid-state precursors have received much less attention. This review details work on the synthesis of refractory ceramics via rapid metathesis reactions between solid metal halides and alkali (or alkaline earth) metal main group compounds (e.g., Li3N or MgB2). The discussion includes thermodynamic considerations in choosing appropriate precursor couples. Through a careful choice of precursors, rapid, highly exothermic reactions can reach high temperatures (>1000 °C) on very short time scales (<1 s). The products are often crystalline and single phase with crystallite sizes varying from tens of angstroms to a few microns, depending on the refractory nature of the material and the reaction conditions (i.e., scale and the use of inert additives). The utility of this metathesis route is demonstrated by metal nitride, boron nitride, and metal boride systems. Additionally, metastable cubic zirconium oxide and phosphide phases are mentioned. Since these reactions can be considered pseudoadiabatic, maximum reaction temperatures (Tad) can be calculated using thermodynamic relationships. The calculated Tad's agree well with experimental measurements carried out using in situ thermocouples. Calculating the value of Tad at an intermediate point in a reaction (for salt and element formation only) is found to be a useful tool in predicting reaction propagation. Self-propagating reactions generally occur after localized initiation when the intermediate Tad value is greater than the melting point of the byproduct salt.