Microwave-assisted citrate gel-combustion synthesis of nanocrystalline urania

Abstract A systematic investigation on the microwave-assisted citrate gel-combustion synthesis of nanocrystalline urania powders is being reported for the first time. Nanocrystalline urania powders were synthesized by microwave-assisted citrate gel-combustion method by using citric acid as a fuel and nitrate as an oxidizer. The fuel to nitrate ratio (R) was varied from 0 to 1 (0, 0.1, 0.25, 0.5, 0.75 and 1.0). It was observed that at a particular stoichiometric ratio (R = 0.1), auto ignition reaction takes place resulting in a porous product. The as-prepared powders were calcined and further reduced under flowing hydrogen. All these powders were characterized for the phases present viz., X-ray diffraction (XRD), size distribution of pores, transmission electron microscopy (TEM), specific surface area (SSA), thermogravimetry-differential thermal analysis-mass spectrometry (TG-DTA-MS) and infrared spectroscopy. It was observed that the microwave-derived powders with an R-value of 0.1 had the lowest bulk density, highest SSA and highest total pore volume. Upon calcination and reduction, the total pore volume decreased. The specific surface area of the calcined powders was the lowest and that of the as-prepared powders was the highest while that of the hydrogen-reduced powders was in between. An opposite trend was observed with the X-ray crystallite size (XCS). The microwave-derived nanocrystalline urania powders showed a lower SSA, higher XCS, higher lattice strain, higher residual carbon and lesser total pore volume than their hot plate derived counterparts. This is probably due to the influence of microwave which accelerates the reaction leading to faster grain coarsening.