W and two-dimensional WO3/W nanocrystals produced by controlled self-sustaining reduction of sodium tungstate

The influence of calcium fluoride (CaF2) on combustion characteristics of Na2WO4 + 3 Mg system and microstructure of the produced W and WO3/W crystals is investigated. The results of thermodynamic analysis and experimental investigations show that CaF2 simultaneously enhances the conversion of Na2WO4 toward tungsten and binds sodium through the formation of NaF phase. The examination of the microstructure of quenched combustion products and differential scanning calorimetry analysis indicate that at early stages of combustion, a part of Na2WO4 is reduced by Mg to tungsten, whereas another part reacts with CaF2 forming CaWO4 and NaF. Subsequent magnesium reduction of CaWO4 significantly increases the overall temperature of the combustion process. Such modification in reaction mechanism coupled with postcombustion processing (e.g., acid/basic treatment) of the product allows us to produce either pure tungsten nanocrystals or tungsten oxide—tungsten nanostructures consisting of two-dimensional WO3 nanoflakes assembled on a W core. It is found that CaF2 does not influence the sizes of tungsten nanocrystals. However, since the addition of CaF2 leads to the increase of overall reaction temperature, it facilitates the formation of W particles with equilibrium crystal shape by faceting process.