Synergistic spray pelletizing-microwave pyrolysis for morphology-controlled ruthenium powder synthesis: Kinetics and precursor engineering

To address the inherent issues of severe particle agglomeration and difficult morphology control in traditional pyrolysis of ammonium hexachlororuthenate ((NH₄)₂RuCl₆) for ruthenium (Ru) powder preparation, this study developed a spray pelletizing-microwave pyrolysis coupled process to fabricate micron-sized spherical Ru powder with excellent dispersibility. By optimising spray pelletizing conditions (stirring for 2 h, slurry concentration of 0.02 mol/L, atomization temperature of 200 ℃, feeding speed of 20 r/min), irregular (NH₄)₂RuCl₆ precursor particles (600–1900 nm) were successfully transformed into smooth solid spheres (400–730 nm), providing homogeneous precursors for microwave pyrolysis. Through single-factor experiments and response surface methodology (RSM) optimisation, a pyrolysis rate of 99.52 % was achieved under microwave pyrolysis conditions of 470 °C for 95 min with 22 g precursor, yielding micron-sized spherical Ru powder with controlled morphology: uniform particles of 150–300 nm featuring excellent dispersibility and sphericity, which meets the YS/T 1068–2015 industry standard (particle size < 5 μm for sputtering targets). Spark plasma sintering (SPS) of the as-prepared powder produced target materials with 97.4 % relative density, fine-grained structure (average grain size 2.97 μm), and texture-free characteristics, effectively suppressing abnormal grain growth and enhancing the strength, plasticity, and sputtering stability of the targets. Non-isothermal kinetics analysis showed two pyrolysis stages: the first stage (apparent activation energies ($E_a$) = 254.5 kJ/mol) followed the volume contraction (R3) model, with gas release as the rate-controlling step; the second stage ($E_a$ = 222.6 kJ/mol) conformed to the nucleation and growth (A2) model, governed by Ru crystal nucleation and growth. This coupled process synergistically regulated Ru powder morphology, particle size, and distribution: spray pelletizing shaped precursors into uniform spheres, while microwave pyrolysis inhibited agglomeration via rapid homogeneous volumetric heating. The kinetic analysis provides theoretical guidance for process optimisation, and the obtained Ru powder demonstrates significant potential for high-performance sputtering target applications.