Figure 2: Challenge and our strategy for fast-growing phase control.

(a) Phase diagram of a typical immiscible alloy Al-Bi. (b) Schematic of the structure evolution during the cooling of an immiscible alloy. Above the miscibility gap, the alloy components are completely miscible as a single phase. During cooling, the minority phase liquid droplets nucleate and grow rapidly by diffusion, then coalesce and settle down to bottom under gravity and/or move to the center driven by thermocapillary force in a short time. (c) Our strategy for growth control of minority phase droplets by nanoparticles during cooling of immiscible materials. Nanoparticles are dispersed uniformly in the single solution above miscibility gap. During the cooling through the miscibility gap, they spontaneously assemble at the interface between the nucleated minority droplets and the liquid matrix, rapidly forming a dense, thin nanoparticle coating on the minority droplets to significantly block the diffusional transport to greatly limit their growth. The rapid and effective diffusional growth restriction by nanoparticles enables a continuous nucleation of new droplets during the whole period of cooling, because of effectively limiting the consumption of supersaturated solute by the droplets nucleated first and significant reduction of latent heat released. In addition, the thin nanoparticle coating will also effectively resist the coalescence by introducing a high capillary pressure in the liquid film between the approaching nanoparticle-coated droplets. Since the driving forces (gravity and thermocapillary) for segregation are not significant on very small droplets, a uniform distribution of very small droplets can be readily obtained.