Non-transferred DC plasma arc method for Al2O3 nanoparticles synthesis

• A new real-time monitoring approach enables Al 2 O 3 synthesis controlled via NTDC plasma arc. • Post-synthesis analysis revealed the atomization mechanisms in the plasma synthesis of Al 2 O 3 . • Reproducible γ/δ/θ -Al 2 O 3 NPs phases emerge across multiple cycles in a single synthesis round. This study presents a new real-time monitoring approach for the non-transferred direct current (NTDC) plasma arc production of Al 2 O 3 nanoparticles (NPs). Observations of plasma ignition, ember dispersion, and plasma surface temperature during aluminum feeding demonstrate successful thermal decomposition, driven by substantial energy absorption that enables particle fragmentation and underpins atomization. Spectral emission study verifies the generation of gaseous Al-O, with excitation wavelengths for Al–O formations detected in the 450–550 nm range, surpassing the aluminum emission at 396.1 nm, indicating complete oxidation, corroborated by consistent findings from EDX and XRD investigations. During the feeding process, the emission temperature attains 5640 K, whilst the surface temperature is 503 K, indicating cooling rates exceeding 10 2 K/s, enabling initial phase estimation. Post-synthesis characterization (SEM and TEM) verifies the effective transformation of spherical Al 2 O 3 NPs (20 to 130 nm). Observed NPs behaviors—such as expansion, shell breakup, internal growth, and electrostatic adhesion—underline the nanoscale atomization pathway. EDX and XRD elemental analyses indicate that the minor presence of Cu (2.07–3.33 %) suggests electrode erosion. The phase fractions obtained from Rietveld refinement calculations reveal the presence of γ-Al 2 O 3 (26.6 %), δ-Al 2 O 3 (60.8 %), and θ-Al 2 O 3 (12.6 %). These variations are attributed to localized temperature gradients that result in a non-uniform cooling surface of the NPs, thereby promoting the formation of these metastable phases. These Al 2 O 3 phases consistently emerge across multiple cycles within a single synthesis round with minor discrepancies, emphasizing the method's potential for reproducible, controlled and scalable nanoparticle production and its possible extension to other metal oxides.