Kinetically controlled boron carbide composite sub-micrometer particle formation pathways during continuous-flow laser melting in liquids with high-power lasers

Monodisperse, spherical sub-micrometer particles with clean, smooth surfaces are key materials used as composite reinforcements, abrasives, and tribological additives, but remain synthetically challenging to produce. Pulsed laser melting in liquids (LML) is an environmentally friendly, emerging route for producing such particles. However, demonstrations have so far been limited to batch reactors, which are difficult to scale up to industrial production. Here, we extend laser melting in liquids to a continuous flat jet flow-through reactor in which boron carbide composite nanoparticles (B 4 CC) dispersed in ethanol are fed perpendicular to a nanosecond pulsed high-power ultraviolet laser beam; this is complemented by additional experiments employing pristine boron carbide. We elucidate how flow conditions and the application of modern high-repetition-rate lasers alter particle formation pathways. Characterization of B 4 CC, irradiated at fluences of 45–135 mJ·cm −2 , by transmission electron microscopy and dynamic light scattering, evidences the formation of a bimodal size distribution of spherical particles with mean diameters of ≈50 and ≈100 nm, verifying the parallel occurrence of isochoric reshaping and an aggregation–melting–fusion mechanism. While the smaller particle size fraction formed in accordance with thermal equilibrium calculations, the second size mode, derived from a multistep aggregation–melt–fusion pathway, was significantly smaller than predicted. Systematic variation of the number of laser pulses per unit volume (PPV) from 1 to 16 by tuning the interpulse delay (τ IP ) from 1.4 to 0.1 ms revealed an initial increase of the mean diameter of the second mode with PPV, reaching maximum size at 4 PPV. Interestingly, at higher PPV, further particle growth is arrested, probably due to an interplay between low particle mass concentration and shorter τ IP limiting the number of aggregation–melt–fusion steps, indicating kinetically controlled particle formation pathways. If the laser fluence is increased to 180 mJ·cm −2 , laser fragmentation in liquid prevails, and small mass fractions of ultrasmall nanoparticles (approx. 2 nm) are formed, which constitutes the upper limit of the LML process window. Overall, at the example of the ultrahard material boron carbide, difficult to process by other methods into perfect spheres, kinetically controlled LML in a flat jet flow-through reactor gives access to nanostructures in a size range previously unattainable by laser processing, establishing the basis for upscaling production in liquid flow with matched laser pulse numbers.