Novel Process for Solid State Reduction

Recently, the reductive expansion synthesis (RES) method was introduced as a means to create nano- and sub-micron metal particles and alloys by rapid heating of physical mixtures of urea with metal nitrates. In this work, the generality of the RES method was demonstrated by producing metal micron and sub-micron particles from oxide and hydroxide precursors, outlining the impact of temperature, precursor ratio, and gas flow rate on the product formation. Findings showed that precursor selection significantly influenced the temperature required for complete reduction, the amount of carbon present in the final product, and the size of the metal particles produced. For complete reduction of NiO to micron scale particles, a high urea content and elevated temperature of approximately 1073 K (800 °C) were necessary, while Ni(OH)₂ could be reduced at considerably lower temperatures to produce sub-micron particles of approximately 200 nm in size and carbon encapsulated. The research demonstrated that variations in parameters significantly affected the reduction process and the characteristics of the resultant metal particles, supporting the notion that the primary mechanism of reduced metal particle formation is the reduction of metal oxide particles by gases produced from urea decomposition. These findings suggest that the RES method can be tailored to synthesize a variety of metal particle architectures suitable for diverse applications.