Flame-based synthesis of multicomponent metal nanoparticles and nanostructured coatings
Sharma, Munish Kumar
MetadataShow full item record
The dissertation focuses on synthesis and applications of multicomponent metal nanoparticles and coatings for printable and flexible microelectronics applications such as radio frequency identification tagging (RFID), printed circuits, and conductive patterns on flexible substrates. At present, silver and copper dominate as conductive ingredients in metal-based inks. A key bottleneck in the growth of the printable electronics market is the high cost of silver and poor oxidation resistance of copper. Research aimed at finding better alternatives to pure silver and copper in metal-based inks is urgently needed. One of the solutions lies in creation of alloy nanoparticles or mixtures of single component metal nanoparticles to give rise to improved property combinations, such as high conductivity, low cost, and oxidation resistance in a single formulation. These multi-component nanoparticles must ultimately be produced at high volume and low cost, must exhibit high electrical conductivity (>100 S/m) at low sintering temperatures (< 300°C), and must remain oxidation resistant over the life cycle of the products in which they will be used. This synthesis route used in this research work is a flame based aerosol reactor which is a continuous, scalable, and economic route towards mass production of these engineered nanoparticles. The metal nanoparticles are synthesized via a high temperature reducing jet (HTRJ) reactor developed by Scharmach et al. (2011). This synthesis method is based on thermal decomposition of an aqueous solution of inorganic metal salt precursors using low cost energy from hydrogen combustion. The goals of the research described here were to: (1) Synthesize new bimetallic and multi-component metal nanoparticles (such as core-shell/hybrid morphology) and nanostructured films; (2) Study their electrical conductivity and other properties to assess their potential for use in silver replacement nano-inks for printable electronics applications as well as other applications; (3) Elucidate the effects of process parameters on the crystallinity and particle size of metal nanoparticles produced in the HTRJ reactor; (4) Develop computational models to aid in understanding the flow, heat, and mass transfer inside the HTRJ reactor. The HTRJ reactor employs a converging-diverging nozzle that produces a high-velocity, well mixed gas stream of combustion products, such as unreacted hydrogen and water vapor. The hot gas stream transfers energy to the aqueous precursor, which evaporates and decomposes into reactive gas phase species. Nanoparticles nucleate and grow from these reactive gas-phase species. A key feature of the HTRJ process is that it provides exceptionally rapid heating and mixing of a cold (liquid or gas) stream with the hot combustion product gases. In our nanoparticle synthesis application, this allows us to decouple the flame chemistry, which occurs upstream of the nozzle, from the nanoparticle formation chemistry, which occurs downstream of the nozzle. Excess hydrogen rapidly reduces any metal-oxides to the corresponding metals. The particles grow by collision and coalescence during their ~50 ms residence time in the reactor. A large flow of cool nitrogen quenches this process at the reactor exit. The particles are collected on a polymer membrane filter. Pure silver, pure copper, copper-silver, copper-silver-tin, and copper-nickel nanoparticles and coatings were synthesized and characterized using scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-Ray diffraction (XRD), and atomic force microscopy (AFM). Palladium and palladium alloy nanoparticles and coatings such as Pd-Ag, and Pd-Ag-Cu were also synthesized and were deposited on porous substrates such as sintered SS-316 and alumina (Al 2 O 3 ) for potential applications in hydrogen purification membranes. In the last chapter of this dissertation, we modeled aspects of the HTRJ process in FLOW-3D. FLOW-3D is a powerful commercial CFD package. In it, the fluid dynamics and heat transfer in the HTRJ process were modeled as a one component, compressible, and turbulent flow. Gas velocity, gas temperature, pressure, wall to fluid heat flux, and solid wall temperatures were computed. The modeling effort presented here demonstrates and tests the capabilities of FLOW-3D to model such complex flows. The model was simplified using axisymmetric and single component assumptions.