Scalable Manufacturing of Metal Micro/Nano Wires and Particles by Thermal Fiber Drawing from a Preform
The objective of this study is to significantly advance the fundamental understanding of the process of metal-core thermal drawing from a preform for the scalable manufacturing of metal micro/nano-particles and wires. Metal micro/nano-wires and particles, with controlled size, aspect ratio and spatial distribution, exhibit unusual mechanical, electrical, magnetic, thermal, and optical properties that are desirable for numerous applications in industry as well as for fundamental research. Thermal fibre drawing from a preform, capable of mass production of continuous and uniform glass and polymer fibers, has emerged as an advanced scalable manufacturing tool for such metal micro/nano-wires and particles. It is of tremendous scientific and technical interest to gain fundamental knowledge through theoretical and experimental studies and to explore and break the fundamental limits of the metal-core thermal drawing process.
Throughout this study, material combinations of the metal Tin (Sn) core and thermoplastic Polyethersulfone (PES) cladding are used as a model system. Because the viscosity ratio between molten metal and polymer (or glass) melt is much smaller than one during normal process conditions, results obtained in this study based on Sn/PES system are expected to have useful implications for other metal/glass or metal/polymer combinations as well.
Despite the fact that thermal drawing from a preform is capable of the high volume production of continuous metal microwires for numerous applications, no theoretical model can yet satisfactorily provide effective predictions of core diameter and continuity from process parameters and material properties. A long wavelength model is therefore derived aiming to fill this gap and describe the dynamics of a molten metal micro-jet entrained within an immiscible, viscous, nonlinear free-surface extensional flow. The model requires numerical data (e.g. drawing force and cladding profile) be measured in real-time. Examination of the boundary conditions reveals that the diameter control mechanism is essentially volume conservation. The flow rate of molten metal is controlled upstream while the flow velocity is controlled downstream realized by solidification of the molten metal. The dynamics of the molten metal jet is dominated by interfacial tension, stress in the cladding, and pressure in the molten metal. Taylor’s conical fluid interface solution (Taylor 1966) can be considered as a special case in this model. A dimensionless capillary number Ca=2Fa/(γA(0)) is suggested to be used as the indicator for the transition from continuous mode (i.e. viscous stress dominating) to dripping mode (i.e. interfacial tension dominating). Experimental results showed the existence of a critical capillary number Ca_cr, above which continuous metal microwires can be produced, providing the first-ever quantitative predictor of the core continuity during preform drawing of metal microwires.
With the new understanding obtained through the fundamental studies on single-core preform drawing, experimental design and analysis on thermal drawing of micro metal wires are carried out to optimize the drawing conditions for multi-core preforms. To minimize the experimental cost and time, we made an unreplicated 23 factorial design and evaluated the effects and interactions of drawing parameters (e.g. draw-down ratio, stress at melt front, and aspect ratio) on the capillary instability and core continuity. Two numerical indicators, namely Relative Standard Deviation (RSD) and Deviation from Draw-down Ratio (DDR), were proposed as measures of the extent of growth of capillary instability and the core continuity respectively. The results indicate that all main factors considered significantly affect the RSD and DDR and the interactions between main factors are not negligible. Comparison between models fitted respectively based on RSD and DDR suggests that capillary instability may not be the only cause of core break-up, which led to the discovery of solid-state break-up. Empirical rules for optimizing parameters are derived based on the surface plots of RSD and DDR. At least at microscale domain, maximizing the stress at the melt front is believed to minimize the growth of capillary instability while maximizing the draw-down ratio tends to maximize the chance of obtaining a continuous core, given all other parameters unchanged.
While previous studies focus on maintaining the continuity of the metal core during preform drawing, i.e. preventing fluid instabilities, exploratory studies were also conducted to control the wavelength of break-up by electric fields and to harness the fluid instabilities for the production of metal nanoparticles. We conducted a feasibility study on utilizing a radial electric field to control the break-up wavelength of an initially continuous microwire during preform drawing. It was hypothesized that the radial electric field significantly affects the break-up wavelength of the metal core during preform drawing. The result of an unpaired two-sample t-test confirmed the effect thus shed new light on the controlled emulsification of molten metal in a viscous dielectric medium. We then experimentally show the scalable manufacturing of metal Sn nanoparticles (<100 nm) in Polyethersulfone (PES) fibers. The underlying mechanism for the particle formation is revealed, and a strategy for the particle diameter control is proposed. This process opens a new pathway for scalable manufacturing of metal nanoparticles from liquid state facilitated solely by hydrodynamic forces, which may find exciting photonic, electrical, or energy applications.