Limited grain growth and chemical ordering during high-temperature sintering of PtNiCo nanoparticle aggregates.

High-temperature sintering of ternary Pt(x)Ni(100-x-y)Co(y) (x = 28-44%, y = 40-54%) nanoparticles of interest in catalysis was studied in situ and in real-time with synchrotron-based x-ray diffraction. For the first time we were able to experimentally capture the early stage of the thermal treatment, and found the nanoparticles to undergo an unusual two-step coalescence process that involves transient growth and restructuring of the nanoparticles. The coalescence process is accompanied by lattice contraction, likely due to composition evolution towards a random alloy. In the late stage of sintering, evidence was found for self-limited grain growth and L1(0) chemical ordering. The order-disorder transition temperature was found to be around 800 °C in all four ternary alloy compositions studied. Fitting of the experimental data with the model for grain growth with size-dependent impediment leads to an activation energy for mass transport of about 100 kJ mol(-1), and may be used as a predictive tool to estimate particle size as a function of heat treatment temperature and duration.