Preparation and application of ultrafine-bubble water for metal matrix composite powders

Hetero-agglomeration assisted by ultrafine bubbles (UFBs) is promising for fabricating high-concentration, impurity-free nanoceramic/metal composite powders. However, the influence of intrinsic UFB characteristics (e.g., concentration, bubble size, and zeta potential) on the electrostatic self-assembly of the powder remains poorly understood. In this study, the effects of processing times, types of gases, and types of water on UFB characteristics, as well as the role of UFBs in composite powder fabrication during the hetero-agglomeration process, were systematically investigated. Nanoparticle tracking analysis revealed that the concentration of UFBs increased, whereas their size decreased with increasing processing times, resulting in an increase in the absolute value of the negative zeta potential. Additionally, compared with air and Ar, the dissolution and ionization of CO₂ molecules produced H⁺ ions, resulting in a lower absolute zeta potential. The impurity ions in tap water promoted the formation of a double electric layer on the surface of the UFBs, which contributed to their long-term stability, resulting in smaller average sizes and higher concentrations. However, ethanol tended to accumulate at the bubble–water interface, reducing the surface tension and making the bubbles more prone to expansion and bursting in ethanol-added water. More importantly, the quality of the composite powders fabricated via hetero-agglomeration was strongly influenced by the characteristics of the UFBs. Significant heterogeneous decoration with ZrO₂ nanoparticles was observed using deionized water; however, uniform composite powders were obtained using UFB water. The UFBs in tap water exhibited a high concentration and a weaker surface charge induced by the absorption of impurity ions, resulting in a lower quantity of nanoceramic decoration compared with that in deionized water. The results of this study provide new insights into the control of UFB characteristics and highlight the great potential of designing high-quality composite powders using the UFB-assisted hetero-agglomeration process.