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18 SMT Magazine • October 2014 scale-up A critical issue for nanomaterials manufac- turing is scalability. The controlled fabrication of metal particles in the size range of interest has only been demonstrated previously with gold and silver after a decade of research. In solu- tion-phase nanoparticle synthesis, there are two competing processes: nucleation and growth. Particle nucleation dominates the reaction in the beginning but then levels off over time, and particle growth takes over as the dominant pro- cess. However, nucleation does continue, pro- viding a constant flux of very small particles. This leads to an increasing size distribution that is volume dependent. At a small scale (e.g., a few hundred milliliters) the two processes can be controlled quite well, allowing for a suit- ably narrow size distribution. However, with increasing volumes (e.g., thousands of gallons) controlling nucleation and growth can be more difficult, yielding a very broad size distribution. This situation was overcome by designing a process that, for the first time, completely separates nucleation and growth without any nucleation additives. The concept works so well nanOcOPPer-baseD sOLDer-Free eLectrOnic assembLY materiaL continues that all scale-up steps attempted to date have worked on the first try. This gives a strong in- dication that this process is fully scalable with little modification. In order to demonstrate scalability, our facil- ities were upgraded with 5 L and 100 L chemical reactor systems (Figure 3). The 100 L reactor is now used routinely at the 80 L batch size to pro- duce over 500 g of high quality material. nanocopper characterization As-synthesized nanocopper exhibits a num- ber of bulk characteristics including color, lus- ter, and consistency that can be used as an ini- tial indicator of material quality. A high quality material is typically dense, exhibits a copper color with metallic luster, and has a paste-like consistency. A more fluffy, powder-like, dull, and brown-to-black appearance warns of an in- ferior product. NanoCopper has also been characterized by a variety of microscopy and analytical tech- niques including scanning electron micros- copy (SEM), energy dispersive spectroscopy (EDS), X-ray diffraction (XRD), thermogravi- FEATurE Figure 3: nanocopper reaction at the 5 L scale (A) and 80 L scale (B).